NHESSNatural Hazards and Earth System ScienceNHESSNat. Hazards Earth Syst. Sci.1684-9981Copernicus GmbHGöttingen, Germany10.5194/nhess-15-2183-2015Large submarine earthquakes that occurred worldwide in a 1-year period (June 2013 to June 2014) – a contribution to the understanding of tsunamigenic potentialOmiraR.omirarachid10@yahoo.frhttps://orcid.org/0000-0002-6198-7588ValesD.MarreirosC.CarrilhoF.Instituto Português do Mar e da Atmosfera, IPMA, IP, Lisbon, PortugalInstituto Dom Luiz, University of Lisbon, IDL, Lisbon, PortugalR. Omira (omirarachid10@yahoo.fr)7October201515102183220019February201511March201518September201521September2015This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://nhess.copernicus.org/articles/15/2183/2015/nhess-15-2183-2015.htmlThe full text article is available as a PDF file from https://nhess.copernicus.org/articles/15/2183/2015/nhess-15-2183-2015.pdf
This paper is a contribution to a better understanding of the tsunamigenic
potential of large submarine earthquakes. Here, we analyze the
tsunamigenic potential of large earthquakes which have occurred worldwide with
magnitudes around Mw= 7.0 and greater during a period of 1 year, from
June 2013 to June 2014. The analysis involves earthquake model evaluation,
tsunami numerical modeling, and sensors' records analysis in
order to confirm the generation of a tsunami (or lack thereof) following the occurrence
of an earthquake. We also investigate and discuss the sensitivity of tsunami
generation to the earthquake parameters recognized to control tsunami
occurrence, including the earthquake location, magnitude, focal mechanism
and fault rupture depth. Through this analysis, we attempt to understand why
some earthquakes trigger tsunamis and others do not, and how the earthquake
source parameters are related to the potential of tsunami generation. We
further discuss the performance of tsunami warning systems in detecting
tsunamis and disseminating the alerts. A total of 23 events, with magnitudes
ranging from Mw= 6.7 to Mw= 8.1, have been analyzed. This study shows
that about 39 % of the analyzed earthquakes caused tsunamis that were
recorded by different sensors with wave amplitudes varying from a few
centimeters to about 2 m. Tsunami numerical modeling shows good agreement
between simulated waveforms and recorded waveforms, for some events. On the other
hand, simulations of tsunami generation predict that some of the events,
considered as non-tsunamigenic, caused small tsunamis. We find that most
generated tsunamis were caused by shallow earthquakes (depth < 30 km)
and thrust faults that took place on/near the subduction zones. The
results of this study can help the development of modified and improved versions of
tsunami decision matrixes for various oceanic domains.
Introduction
In the aftermath of the 2004 Indian Ocean tsunami, much effort has been
made worldwide to better understand the potential of tsunami generation
following the occurrence of an earthquake. As a result, various research
studies have been carried out in order to (i) identify the source zones most
prone to triggering tsunamis around the world, (ii) understand the generation
mechanism of tsunami sources, (iii) assess tsunami hazard, vulnerability, and
risk along coastal areas, (iv) reinforce the existing tsunami warning
systems (TWSs) and implement new ones and (v) improve the capability of tsunami
warning centers (TWCs) to detect the tsunami well before it hits the coastal
zones. One issue that remains challenging for any TWC, in order to avoid
false warning and to mitigate the tsunami impact on the coastal population, is the
fast confirmation of the generation of a tsunami (or lack thereof) after the occurrence
of an earthquake. Solving such an issue, in particular with the absence of
tsunami detection sensors deployed close to the earthquake epicenter,
remains a hard task and requires robust knowledge on the geology, tectonics
and geodynamics of the region where the earthquakes are generated in
addition to robust tsunami numerical modeling capabilities to simulate the different phases of a tsunami properly.
Locations of the epicenters (red dots), event number
(given in Table 1), date of occurrence and magnitudes of submarine
earthquake events of Mw>= 6.7 that were analyzed during a
1-year period from June 2013 to June 2014.
In general, various parameters are recognized to control the generation of a
tsunami following an earthquake event. These parameters mainly include the
earthquake magnitude, its location, its focal mechanism and its rupture
depth. The available tsunami database from 2000 BC to present (NGDC/WDS, 2014)
shows that among the total of 1425 tsunami historical events, 80 % were
caused by earthquakes of magnitude greater than Mw= 6.5. Most, but not
all, known devastating tsunamis has been caused by earthquakes triggered in
subduction zones. According to Satake and Tanioka (1999), tsunamigenic
earthquakes in subduction zones can be classified into three types:
(i) typical interplate events, which occur at the plate interface;
(ii) intraplate events concerning earthquakes at the outer-rise within the
subducting slab or overlying crust; and (iii) “tsunami earthquakes” that
take place at the shallow extension of the interplate seismogenic zone,
typically beneath the accretionary wedge.
This study aims to contribute to a better understanding of tsunamigenic
potential from sources of tectonic origin by analyzing the tsunami
generation associated with large earthquakes that occurred worldwide during a period of 1 year.
23 submarine earthquake events of magnitudes ranging from Mw= 6.7 up to
Mw= 8.1 have been analyzed (Fig. 1). The period of 1 year seems to be
short to conduct a comprehensive analysis, especially from a statistical
point of view. On the other hand, due to the fact that during the considered
period, 23 events occurred in different regions of the world with
various focal mechanisms and rupture conditions, we assume that a 1-year
period provides enough data to cover the scope of this paper, which does not
include a statistical analysis.
Most considered events occurred in the Pacific Ocean on/near the plate
subduction zones. These events took place with different focal mechanisms of
generation and different earthquake hypocenter depths. The analysis
performed in this study involves source parameters evaluation, tsunami
numerical modeling of generation and propagation, and sensors'
records, including coastal tide gauges (TD), and “Deep-ocean Assessment and
Reporting of Tsunamis” (DART®), that serve to confirm the
generation (or lack thereof) of a tsunami. With this analysis, we attempt to understand why some
earthquakes cause tsunamis and others do not, and how the source parameters are
related to the potential of tsunami generation. Definition of source
parameters includes the earthquake epicenter location (from USGS,
http://earthquake.usgs.gov/earthquakes/), its magnitude, its depth, and its
focal mechanism, which are available by the gCMT (Global
Centroid-Moment-Tensor, http://www.globalcmt.org/) after the event
occurrence. To evaluate the additional parameters, such as the fault
dimensions (L, W) and the average coseismic slip (D), required for tsunami
numerical modeling, we use the earthquake scaling laws Mw-L/W and Mw-D
established by Blaser et al. (2010). To simulate the possible initial
sea-surface perturbation, the earthquake rupture is supposed instantaneous
and the sea-bed displacement is computed using the half-space elastic theory
(Okada, 1985). The tsunami propagation is modeled using the COMCOT
finite-differences shallow water code (Liu et al., 1998).
Earthquake events which occurred in the west Pacific source zone:
(a) overview of the source zone; location of the epicenters (red stars),
focal mechanisms (red beach balls), and subduction zones (yellow lines) for:
(b) the Okhotsk-Russia normal faulting event, (c) the Alaska thrust fault
event, (d) the Honshu normal faulting event, (e) the Philippine reverse
faulting event, and (f) six events, four around the Papua New Guinea, their
focal mechanisms: three are normal faulting and one is thrust faulting, and
two near the Solomon Island, their focal mechanisms: a strike–slip and a
reverse faulting.
Earthquake events which occurred in the east Pacific source zone:
(a) overview of the source zone; (b) location of the California and Mexico
events (red stars), their focal mechanism: thrust faulting and strike–slip
faulting, respectively (red beach ball), the subduction zone (yellow line)
and the spreading centers (red lines); (c) location of four events: the Peru
event and three events in northern Chile (red stars), their focal mechanisms
are thrust faulting (red beach ball), and the subduction zone (yellow line).
Analysis of available sensor records reveals that 39 % of the considered
earthquakes caused tsunamis. Tsunamis caused by non-seismic events, such as
the 24 September 2013 tsunami, possibly associated to submarine
landslide (Heidarzadeh and Satake, 2014), that occurred following the
Mw= 7.7 inland Pakistan earthquake and recorded by TD stations along the
Omani coast, are disregarded in our study. We discuss how various specific
characteristics of the studied earthquakes and their mechanisms of
generation, such as the earthquake magnitude, the location, the type of
focal mechanism and the depth of the rupture determine their tsunamigenic
potential. 67 % of caused tsunamis resulted from thrust faults that occurred
in/near subduction zones. We finally highlight the performance of the
different TWSs around the world related to the triggered tsunami events and discuss,
in light of the paper results, some earthquake characteristics that can be
considered in order to improve the tsunami decision matrixes for various
oceanic domains.
Earthquake events: tectonic setting and focal mechanisms
In this study 23 large submarine earthquakes, which occurred between June 2013 and
June 2014, have been considered. Assessing the tsunamigenic potential of the
studied earthquakes requires an understanding of the tectonic setting of the
regions where these events occurred. Here, we describe these settings
along with the focal mechanism of each earthquake. We consider four main
source zones (SZs) as responsible for the generation of the studied events.
They are: the west Pacific (Fig. 2), the east Pacific (Fig. 3), the South
Atlantic (Fig. 4) and the east Mediterranean (Fig. 5) SZs.
In the west Pacific SZ, 10 events occurred (Fig. 2). In the Sea of
Okhotsk, on 1 October 2013, a Mw= 6.7 earthquake was the result of
normal faulting at a depth of 585 km. Its location was close to the region
where the Pacific Plate subducts beneath the Okhotsk Plate (Fig. 2b). In
the Aleutian Islands region, a Mw= 7.0 earthquake took place on
30 August 2013 at a depth of 26.7 km, as the result of thrust faulting on/near
the Aleutian Trench (Fig. 2c). Offshore of Honshu, Japan, a Mw= 7.1
earthquake occurred on 25 October 2013, associated with normal
faulting in the oceanic crust of the Pacific Plate, east of the Japan Trench
(Fig. 2d). In the Bohol Islands, Philippines, on 15 October 2013, a
Mw= 7.1 earthquake (Fig. 2e) was generated by a previously unmapped
reverse fault (Lagmay and Eco, 2014). The depth of the event (12 km)
indicates that it ruptured a fault within the crust of the Sunda Plate,
rather than on the deeper subduction zone plate boundary. Four events
occurred in Papua New Guinea (Fig. 2f) within two tectonic regions: the
Australian slab and the New Britain Trench. The Australian slab was
responsible for the generation, on 7 July 2013, of a Mw= 7.3
earthquake associated with normal faulting at 385 km depth. Close to the New
Britain Trench, three other earthquakes (Fig. 2e) were the result of thrust
faulting on the Australia–Pacific subduction zone. They took place on
16 October 2013, 11 April 2014 and 19 April 2014 with
Mw= 6.8, Mw= 7.1 and Mw= 7.5 at depths of 45.8, 44.1 and
36 km, respectively. In the Solomon Islands, two earthquakes happened on 12 April 2014,
and 13 April 2014 with Mw= 7.6 and Mw= 7.4,
respectively, along a segment of the Australia–Pacific Plate boundary (Fig. 2f).
The first event was a strike–slip earthquake, while the second was
nearly a pure reverse.
In the east Pacific SZ, six events occurred (Fig. 3). On 10 March 2014,
a Mw= 6.9 earthquake occurred off the coast of northern
California, resulting from a strike–slip motion in a region where the Gorda Plate
subducts beneath the Pacific northwest region (Fig. 3b). On 18 April 2014,
a Mw= 7.3 earthquake happened near the Pacific coast of
Mexico (Fig. 3b) as the result of thrust motion at a shallow depth
(18.9 km). On 25 September 2013, a Mw= 7.0 earthquake occurred off the
coast of Peru at 46.1 km depth because of thrust faulting on/near the interface at
the boundary between the South American Plate and the subducting Nazca Plate
(Fig. 3c). Just off the Chilean coast, three earthquakes resultant of thrust
faulting, at shallow depths of 12, 21.6 and 28.7 km, occurred in the
subduction zone along which the Nazca Plate underthrusts the South American
Plate (Lay et al., 2014) (Fig. 3c). The first event, on 16 March 2014,
with Mw= 6.7, was considered as a foreshock of the 1 April 2014
Mw= 8.1 event, and the 3 April 2014 Mw= 7.7 event was an aftershock.
In the South Atlantic SZ, five events occurred (Fig. 4). At the South
Sandwich Islands one strike–slip event occurred on 15 July 2013 with
Mw= 7.3, at 21.5 km depth, approximately 100 km east of the triple
junction between the South American, Sandwich and Antarctica plates (Fig. 4b).
In the South Scotia Ridge transform plate boundary, two strike–slip events took
place on 16 and 17 November 2013. The first one was a
Mw= 6.9 event that occurred at a shallow depth of 10 km. The second one,
Mw= 7.8, which occurred at a depth of 23.8 km, represents the largest
strike–slip earthquake known to date along this plate boundary (Fig. 4b).
Southwest of the Falkland Islands, a strike–slip faulting Mw= 6.9
earthquake happened on 25 November 2013, near the boundary of the
South American and the Scotia Sea plates (Fig. 4b). On 15 April 2014,
a Mw= 6.8 earthquake occurred in the South Atlantic Ocean to the
east of Bouvet Island resulting from strike–slip motion (Fig. 4b).
In the east Mediterranean SZ, two events occurred (Fig. 5). The
Mw= 6.8 earthquake, which happened on 12 October 2013 about 30 km west
of Platanos, Greece, was associated with a reverse motion near the Hellenic
arc, the region where the African Plate subducts beneath the Aegean Sea
Plate (Fig. 5b). The Mw= 6.9 earthquake, which occurred on 24 May 2014,
was located to the south of Samothraki Island, Greece, near the Saros trough
(Fig. 5b), a region with many similar strike–slip earthquakes.
Earthquake events which occurred in the South Atlantic Ocean
source zone: (a) overview of the source zone; (b) location of five events:
three around Scotia Plate, one near the Falkland Island and another one near
Bouvet Island, their focal mechanisms are strike–slip faulting (red beach
ball), the subduction zone (yellow line), the transforms (magenta lines),
the active spreading center and fracture zones (red lines).
Earthquake events which occurred in the Mediterranean source
zone: (a) overview of the source zone; (b) location of the Greece events
(red star), their focal mechanisms: a strike–slip faulting and a reverse
faulting (red beach ball), and the subduction zone (yellow line).
The studied events have different focal mechanisms. Within the total of
23 analyzed earthquakes, 52 % are thrust, 35 % are strike–slip, and 13 %
are normal faults. Further information and details on the velocity rate of
tectonic plates can be found in DeMets et al. (1994) for the western Pacific SZ,
in Angermann et al. (1999) and DeMets et al. (2010) for the eastern
Pacific SZ, in DeMets et al. (2010) for the South Atlantic SZ, and in
McClusky et al. (2000) for the eastern Mediterranean SZ.
For each analyzed earthquake event we compute a source model including the
fault parameters required for tsunami numerical modeling. For
simplification, we adopt a rectangular shape of the fault rupture,
characterized by a length (L) and a width (W). We consider the Mw
magnitude evaluated by the gCMT for each event and we compute the
corresponding dimensions (L and W) using the scaling laws Mw-L/W
established by Blaser et al. (2010). In comparison with the most frequently
used scaling relations of Wells and Coppersmith (1994), Blaser et
al. (2010)'s study has the advantage of considering the subduction zone events in the
database that they used to establish the earthquakes' scaling relations.
Earthquake parameters for the 23 analyzed events.
EventDateEpicenter location DepthMwLocationFault planeLWSlipno.Lat. (∘)Lon. (∘)(km)(strike, dip, rake)(km)(km)(m)(gCMT)17 Jul 2013-3.923153.920382.97.3Papua New Guinea167, 47, -70∘77271.83215 Jul 2013-60.868-25.14421.57.3South Sandwich Islands271, 85, -18∘96192.21330 Aug 201351.537-175.23026.77.0Aleutian Islands64, 67, 86∘42231.24425 Sep 2013-15.838-74.51146.17.0Peru307, 31, 84∘42231.6151 Oct 201353.200152.786585.56.7Sea of Okhotsk293, 40, -44∘38160.82612 Oct 201335.51423.25215.06.8Greece339, 3, 130∘32180.98715 Oct 20139.880124.11712.07.1Philippines42, 40, 80∘47251.54816 Oct 2013-6.446154.93145.86.8Papua New Guinea307, 43, 85∘32181.04925 Oct 201337.156144.66124.97.1Japan171, 43, -107∘60231.561016 Nov 2013-60.263-47.06210.06.9Scotia Sea96, 66, 2∘53141.131117 Nov 2013-60.274-46.40123.87.8Scotia Sea102, 44, 3∘200283.401225 Nov 2013-53.945-55.00316.06.9Falkland Islands158, 80, -171∘53141.341310 Mar 201440.829-125.13415.06.9California230, 86, -2∘53141.271416 Mar 2014-19.981-70.70212.06.7Chile284, 26, 54∘28170.90151 Apr 2014-19.610-70.76921.68.1Chile355, 15, 106∘177734.00163 Apr 2014-20.572-70.50228.77.7Chile358, 14, 103∘104483.261711 Apr 2014-6.586155.04844.17.1Papua New Guinea310, 42, 87∘47251.401812 Apr 2014-11.270162.14827.37.6Solomon Islands17, 63, 159∘149243.001913 Apr 2014-11.463162.05137.57.4Solomon Islands91, 43, 77∘70352.422015 Apr 2014-53.4978.72216.46.8Bouvet Island128, 81, 5∘46131.192118 Apr 201417.397-100.97218.97.3Mexico303, 18, 98∘62311.712219 Apr 2014-6.755155.02436.07.5Papua New Guinea311, 35, 87∘80392.442324 May 201440.28925.38912.06.9Greece73, 85, -177∘53141.08
Once the earthquake fault dimensions (L and W) are calculated, we use the
seismic moment (Mo) definition of Aki (1972) (Eq. 1) together with
the Mo–Mw relation defined by Kanamori and Anderson (1975) (Eq. 2)
in order to calculate the earthquake slip.
Mo=μLWDMw=23logMo-10.7,
where μ is the shear modulus characterizing the rigidity of the
earthquake rupture region, and D is the average coseismic fault slip.
Table 1 summarizes the fault parameters computed for all the considered
earthquake events. These parameters are used in the next section to compute
the tsunami generation.
Tsunami numerical modeling
In this section, we perform tsunami numerical modeling of both generation
and propagation for all considered earthquake events including those observed to trigger
tsunamis, and those that did not cause tsunamis. Through tsunami
numerical modeling, we attempt to understand why some earthquakes trigger
tsunamis and others do not, and how the earthquake source parameters are
related to the potential of tsunami generation. Also, by comparing the
tsunami observations with the simulations, we aim to discuss how numerical
modeling can contribute to a better performance of TWSs.
In order to model tsunami generation and propagation a set of
bathymetric/topographic grid data is generated for each region of interest
where studied earthquakes took place. 30 arcsec gridded data from the General
Bathymetric Chart of the Oceans (GEBCO, 2015) are used in this study.
Tsunami generation
The initial sea-surface perturbation is generated for all the studied
earthquakes. The earthquake fault parameters derived in Sect. 3.1 (Table 1) were used
to simulate the tsunami generation. The earthquake rupture is supposed
instantaneous and the sea-bed displacement is computed using the half-space
elastic theory (Okada, 1985). The vertical sea bottom displacement is then
transferred to the free ocean surface with the assumption that both
deformations of sea bottom and ocean surface are equal (Kajiura, 1970).
Simulated initial sea-surface perturbations caused by the
studied 23 submarine earthquakes. The number in the top-left corner of each
figure corresponds to the event's number indicated in Table 1.
Figure 6 depicts the simulated tsunami generation for the 23 earthquake
events. Analysis of these results shows a confirmation of tsunami generation
for events with recorded tsunami signals (events 9, 11, 14, 15, 16, 18, 19,
21 and 22 in Fig. 6). On the other hand, Okada's tsunami generation
modeling, using the fault models elaborated in Sect. 3.1, leads also to non-zero
initial sea-surface perturbations for some events with no tsunami observations.
For earthquake events with very deep fault rupture (events 1 and 5),
modeling results show no tsunami generation, which is in accordance with the
observations and also with reasonable expectations. For the other 21 events
that occurred at depths less than 47 km, modeling of tsunami initial
conditions shows that tsunamis were generated, while for only nine of them, a
tsunami was observed. The computed initial tsunami deformations show higher
wave amplitudes for these nine events with tsunami records compared to the
other 12 events for which no tsunamis were observed.
Analysis of tsunami generation results (Fig. 6), clearly indicates that the
strike–slip events (events: 2, 10, 11, 12, 13, 18, 20 and 23) cause
tsunamis with smaller wave amplitudes compared to thrust (events: 3, 4, 8,
14, 15, 16, 17, 21 and 22) and reverse (events: 6, 7 and 19) earthquakes.
Among the studied strike–slip earthquakes, events 11 and 18 have more
potential of tsunami generation than the rest of events (Fig. 6). For these
two events, 11 and 18, tsunami waves of 0.15 and 0.03 m, respectively,
were observed (Table 2).
For the thrust and reverse earthquakes, numerical simulations show tsunami
generation with relatively significant initial wave amplitudes, ranging from
0.1 m up to about 1 m, especially for the events 15, 16, 19, 21 and 22
(Fig. 6). For these earthquakes, tsunami waves of amplitudes between 0.1 m
and about 2 m were recorded (Table 2).
Simulations of tsunami generation for studied normal fault earthquakes
(events 1, 5 and 9) indicate that only event 9 causes a small initial
sea-surface perturbation of about 0.05 m in amplitude (Fig. 6). For the
event 9, tsunami waves were recorded with maximum amplitudes of a few
centimeters near the source area and of 0.4 m at the coast of Japan (Table 2).
Tsunami propagation
Shallow water equations (SWEs) through the COMCOT code (Liu et al., 1998)
are used to simulate the tsunami propagation. This code solves linear and
nonlinear SWEs using an explicit staggered leap-frog finite differences
numerical scheme for linear terms and an upwind scheme for the nonlinear
terms (Wang, 2009). In all considered computation domains, the code employs
radiation (or absorbing) boundary conditions, which have the property that
the wave motion passes from a domain to the other through the boundaries with no
reflections (Broeze and Van Daalen, 1992).
(a) Events with tsunami observations
For all studied tsunamigenic events (39 % of the total events) we perform
numerical simulations of tsunami propagation. Here, we present modeling
results for the Mw= 7.1 Japan event and the Mw= 8.1 Chile event that
occurred on 25 October 2013 and 1 April 2014, respectively. These events offer a good opportunity to test the reliability
of the models considered, including the source model, the bathymetric model
and the tsunami propagation code, by comparing the numerical results against
the tsunami signals recorded by various DART® stations in the Pacific Ocean.
Figures 7 and 8 depict, respectively, the results of tsunami numerical
modeling for the Mw= 7.1 Japan and the Mw= 8.1 Chile events. The
results clearly indicate that a tsunami was generated close to the source
areas, with wave amplitude of a few centimeters (∼ 3 cm) for the
Japan event and up to 1 m for the Mw= 8.1 Chile event (Figs. 7a and 8a).
Comparison of the simulated tsunami waveforms and the recorded signals from
the DART® sensors (Fig. 7b and c for the Japan event, and Fig. 8b–d
for the Chile event) shows relatively good agreement in terms of
tsunami arrival time and maximum wave amplitudes. In Fig. 7b and c we
display the comparison between the simulated waveforms and the ones recorded
by the DART®-21346 and the DART®-21418. The analysis of these results clearly
shows good agreement between simulated and recorded signals in terms of the
tsunami arrival time, wave amplitudes and periods, in spite of the fact
that the generated tsunami is relatively small and also both DART® records
show first-arrival perturbations with maximum wave amplitudes between
0.5 and 1 cm. We interpret these small oscillations as effects resulted from the
filtering process of the DART®-recorded signal. In Fig. 8b we plot both
recorded and simulated waveforms for the DART®-32401 station that is located
about 290 km west from the earthquake epicenter. The analysis of the sensor
record (blue curve) indicates that the DART® station captured, few seconds
after the event occurrence, the seismic signal due to its location
relatively close to the epicenter. A few minutes later, the DART®-32401
recorded the arrival of the first tsunami wave (Fig. 8b). Simulated
waveform (red curve) shows a good agreement with the tsunami recorded signal
for both the tsunami travel time of about 18 min and the maximum wave
amplitude of about 0.25 m. Figure 8c and d depict the comparison between
the recorded and the simulated tsunami signals for both DART® 32412 and 32413,
respectively. These sensors are located 1600 and 2800 km far away
from the earthquake epicenter and therefore recorded only small tsunami
amplitudes (max. amplitude of about 6 cm for DART®-32412, and about 3 cm for
DART®-32413 – blue signals in Fig. 8c and d). The simulated waveforms
at the locations of DART®-32412 and DART®-32413 (red signals in Fig. 8c and d)
also indicate small recorded tsunami amplitudes, relatively in
agreement with the recorded ones.
(b) Events with no tsunami observations
Considering the results of tsunami generation presented in Sect. 3.2.1, we model
tsunami propagation for events with no tsunami observations but for which
Okada's model led to non-zero initial deformations (see Fig. 6). Here, we
consider the Mw= 6.9 northern Aegean earthquake which occurred on 24 May 2014
as an example to present results of tsunami propagation simulations.
Timeline of the actions and activities taken by the
considered TWC's for the 23 earthquakes. The font corresponds to the level
of tsunami alert (information: Roman, advisory: bold, and warning: italic).
MWA is the maximum wave amplitude recorded.
Event, date and origin time (UTC)Tsunami Warning Agency actions and activitiesTsunamioccurrencePapua New Guinea earthquake (Mw= 7.3)PTWC (at 18:42) issues Tsunami Bulletin 1: information.No7 Jul 2013PTWC Hawaii (at 18:42) issues Tsunami Information Statement 1.18:35:30ITEWC (at 18:45) issues Tsunami Bulletin 1.South Sandwich earthquake (Mw= 7.3)PTWC Caribbean Sea (at 14:18) issues Tsunami Information Statement 1.No15 Jul 2013ITEWC (14:19) issues Tsunami Bulletin 1.14:03:43Aleutian Islands earthquake (Mw= 7.0)WC/ATWC (at 16:31) issues Tsunami Information Statement 1.No30 Aug 2013PTWC Hawaii (at 16:34) issues Tsunami Information Statement 1 (Mw= 6.8).16:25:02PTWC issues Tsunami Bulletin 1 (at 16:34): information (Mw= 6.8), and TsunamiBulletin 2: information (at 16:43) (Mw= 7.0).Peru earthquake (Mw= 7.0)PTWC issues Tsunami Bulletin 1 (at 16:48): information (Mw= 6.8), and TsunamiNo25 Sep 2013Bulletin 2 (at 17:11): information (Mw= 7.2).16:42:43WC/ATWC (at 16:49) issues Tsunami Information Statement 1 (Mw= 6.8).Sea of Okhotsk earthquake (Mw= 6.7)ITEWC (at 3:48) issues Tsunami Bulletin 1.No1 Oct 201303:38:21Greece earthquake (Mw= 6.8)NOA (at 13:19) issues Tsunami Message 1: information (Mw=6.3).No12 Oct 2013ITEWC (at 13:20) issues Tsunami Bulletin 1.13:11:53KOERI issues Tsunami Message 1 (at 13:25): regional watch/advisory (Mw=6.8),Tsunami Message 2 (at 13:42): watch/advisory (Mw=6.4), and watch/advisorycancellation (at 13:57).Philippines earthquake (Mw= 7.1)PTWC (at 00:22) issues Tsunami Bulletin 1: Information (Mw= 7.2).No15 Oct 2013WC/ATWC (at 00:23) issues Tsunami Information Statement 1 (Mw= 7.2).00:12:32JATWC (at 00:24) issues Tsunami Bulletin: Information (Mw= 7.3).ITEWC (at 00:24) issues Tsunami Bulletin 1.Papua New Guinea earthquake (Mw= 6.8)PTWC (at 10:37) issues Tsunami Bulletin 1: information (Mw= 7.1).No16 Oct 2013ITEWC (at 10:41) issues Tsunami Bulletin 1.10:30:58JATWC (at 10:46) issues Tsunami Bulletin: information (Mw= 7.2).Japan earthquake (Mw= 7.1)JMA (at 17:14) issues Tsunami Advisories for Fukushima in Japan (Mw=6.8), updatedYes25 Oct 2013Tsunami Advisories for other coastal regions of Japan (Mw=7.1) (at 17:50), and(MWA of17:10:19Tsunami warnings/advisories cancellation message (at 19:05), and information0.4 m)message with Initial Tsunami Observations (at 19:11) Ofunato 0.2 m; Kuji-ko 0.4 m;Ishinomaki-shi Ayukawa 0.3 m; Soma 0.4 m.PTWC (at 17:20) issues Tsunami Bulletin 1: information (Mw=7.5).
Continued.
Event, date and origin time (UTC)Tsunami Warning Agency actions and activitiesTsunamioccurrenceScotia Sea earthquake (Mw= 6.9)PTWC Caribbean Sea (at 03:43) issues Tsunami Information Statement 1 (Mw= 7.4),No16 Nov 2013and Tsunami Information Statement 2 (Mw= 6.8) (at 03:53).03:34:31ITEWC (at 03:48) issues Tsunami Bulletin 1.Scotia Sea earthquake (Mw= 7.8)ITEWC (at 09:16) issues Tsunami Bulletin 1.Yes17 Nov 2013PTWC (at 09:17) Tsunami Bulletin 1: information.(MWA of09:04:55PTWC Caribbean Sea (at 09:18) issues Tsunami Information Statement 1.0.15 m)WC/ATWC (at 09:20) issues Tsunami Information Statement 1.(NOAA reported Tsunami Observations: King Edward TD: 0.15 m).Falklands Islands earthquake (Mw= 6.9)PTWC Caribbean Sea (at 06:41) issues Tsunami Information Statement 1 (Mw= 6.6)No25 Nov 2013ITEWC (at 06:42) issues Tsunami Bulletin 1.06:27:33California earthquake (Mw= 6.9)PTWC (at 05:26) Tsunami Bulletin 1: information (Mw= 7.0).No10 Mar 2014PTWC Hawaii (at 05:27) issues Tsunami Information Statement 1 (Mw= 7.0).05:18:13Chile earthquake (Mw= 6.7)PTWC (at 21:23) issues Tsunami Bulletin 1: information (Mw= 7.0), and TsunamiYes16 Mar 2014Bulletin 2 (at 22:26): information with Tsunami Observations: Patache 0.28 m.(MWA of21:16:29PTWC Hawaii (at 21:23) issues Tsunami Information Statement 1 (Mw= 7.0).0.28 m)Chile earthquake (Mw= 8.1)PTWC (at 23:55) issues Tsunami Bulletin 1: regional Warning/Watch (Mw=8.0),Yes1 Apr 2014Tsunami Bulletin 2: regional Warning/Watch update and Tsunami Observations:(MWA of23:46:47Pisagua 1.92 m; Iquique 1.70 m (at 00:14), Tsunami Bulletin 3: regional2.11 m)Warning/Watch (Mw=8.2) and Tsunami Observations (at 00:34), Tsunami Bulletin 4:regional Warning/Watch and Tsunami Observations (at 01:31), Tsunami Bulletin 5:Warning area reduced (Chile and Peru), Warning/Watch Cancellation for allpreviously named areas and Tsunami Observations (at 02:35), Tsunami Bulletin 6 (at03:44): Warning for Chile and Peru and Tsunami Observations: Juan Fernandez 0.16 m;San Felix 0.68 m; Coquimbo 0.15 m; Caldera 0.11 m; Charnal 0.24 m; Callao La-Punta Pe 0.12 m; Arica 1.83 m; Matarani 0.58 m; Paposo 0.26 m; Pisagua 2.01 m;Mejillones 0.86 m; Tocopilla 0.42 m; DART® 32402 0.05 m; Antofagasta 0.25 m;Patache 1.51 m; Iquique 2.11 m, and Tsunami Bulletin 7 (at 04:43): Warning/WatchCancellation.PTWC Hawaii (at 23:56) issues Tsunami Information Statement 1 (Mw=8.0), andTsunami Advisory for Hawaii with Observations (at 03:46).Chile earthquake (Mw= 7.7)PTWC (at 02:51) issues Tsunami Bulletin 1: Information (Mw=7.4), Tsunami Bulletin 2:Yes3 Apr 2014Warning for Chile and Peru (Mw=7.8) (at 03:13), and Tsunami Bulletin 3 (at 03:59):(MWA of02:43:11Warning Cancellation and Tsunami Observations: Patache 0.68 m; Pisagua 0.19 m;0.74 m)Iquique 0.74 m.PTWC Hawaii (at 02:52) issues Tsunami Information Statement 1 (Mw=7.4).
Continued.
Event, date and origin time (UTC)Tsunami Warning Agency actions and activitiesTsunamioccurrencePapua New Guinea earthquake (Mw= 7.1)PTWC (at 07:15) issues Tsunami Information Bulletin 1 (Mw= 7.3).No11 Apr 2014ITEWC (at 07:17) issues Tsunami Bulletin 1.07:07:23JATWC (at 07:19) issues Information Bulletin (Mw= 6.9).Solomon Islands earthquake (Mw= 7.6)PTWC (at 20:20) issues Tsunami Bulletin 1: regional Warning/Watch (Mw=8.3),Yes12 Apr 2014Tsunami Bulletin 2 (at 20:35): Warning area reduced (Mw=7.8), Tsunami Bulletin 3 (at(MWA of20:14:3920:53): same Warning area (Mw=7.6), and Tsunami Bulletin 4 (at 21:36): Warning0.03 m)Cancellation and Tsunami Observations: DART® 55 012 0.02 m; Lata Wharf 0.03 m.ITEWC (at 20:54) issues Tsunami Bulletin 1.Solomon Islands earthquake (Mw= 7.4)ITEWC (at 12:45) issues Tsunami Bulletin 1.Yes13 Apr 2014PTWC (at 12:46) issues Tsunami Bulletin 1: regional Warning (Mw=7.7), Tsunami(MWA of12:36:19Bulletin 2 (at 13:18): Warning area reduced (Mw=7.4), Tsunami Bulletin 3 (at 13:55):0.16 m)Warning area reduced and Tsunami Warning Cancellation for Vanuatu and PapuaNew Guinea (Mw=7.4), and Tsunami Bulletin 4 (at 14:42): Warning Cancellation andTsunami Observations: Lifou New Caledonia 0.16 m; Honiara 0.02 m; DART® 550120.01 m; Lata Wharf 0.03 m.Bouvet Islands earthquake (Mw= 6.8)PTWC Caribbean Sea (at 04:10) issues Tsunami Information Statement 1.No15 Apr 2014ITEWC (at 04:11) issues Tsunami Bulletin 1.03:57:01Mexico earthquake (Mw= 7.3)ITEWC (at 14:38) issues Tsunami Bulletin 1Yes18 Apr 2014PTWC (at 14:39) issues Tsunami Bulletin 1: Information (Mw= 7.4)(MWA of14:27:24PTWC Hawaii (at 14:40) issues Tsunami Information Statement 1 (Mw= 7.4)0.43 m)NOAA reported Tsunami Observations: Acapulco TD: 0.43 m.Papua New Guinea earthquake (Mw= 7.5)PTWC (at 13:37) issues Tsunami Bulletin 1: regional Warning/Watch (Mw=7.8),Yes19 Apr 2014Tsunami Bulletin 2 (at 14:18): same Warning area (Mw=7.5), Tsunami Bulletin 3 (at(MWA of13:28:0014:39): Warning Cancellation and Tsunami Observations: Tarekukure Wharf 0.05 m.0.05 m)PTWC Hawaii (at 13:40) issues Tsunami Information Statement 1 (Mw=7.8).ITEWC (at 13:40) issues Tsunami Bulletin 1.Greece earthquake (Mw= 6.9)NOA (at 09:30) issues Tsunami Information Message 1 (Mw=6.0).No24 May 2014KOERI (at 09:43) issues Tsunami Message 1: regional Watch/Advisory (Mw=6.6),09:25:02Tsunami Message 2 (at 09:56): Watch Cancellation, and Tsunami Message 3 (at12:39): Watch/Advisory Cancellation.
Tsunami numerical simulation of the 25 October 2013
Japan tsunami event: (a) maximum wave amplitudes distribution in the west
Pacific Ocean and tsunami travel times (dark lines separated each 0.2 h);
(b) comparison between the simulated waveform and recorded signal for the
station DART®-21346; (c) comparison between the simulated waveform and
recorded signal for the station DART®-21418.
Tsunami numerical simulation of the 1 April 2014
Iquique-Chile tsunami event: (a) maximum wave amplitudes distribution in the
east Pacific Ocean and tsunami travel times (dark lines separated each 1 h);
(b) comparison between the simulated waveform and recorded signal for the
station DART®-32401; (c) comparison between the simulated waveform and
recorded signal for the station DART®-32412; (d) comparison between the
simulated waveform and recorded signal for the station DART®-32413.
Tsunami numerical simulation of the Mw= 6.9 northern Aegean
earthquake which occurred on 24 May 2014: (a) tsunami generation;
(b) maximum wave amplitudes distribution in the north Aegean Sea.
(a) Observed tsunami wave amplitudes for the various
magnitudes of the analyzed earthquakes; (b) observed tsunami wave amplitudes
for the different depths of the analyzed events; (c) zoom on the depth
region from 0–50 km and the corresponding observed tsunami wave
amplitudes; (d) proportions of mechanism focal types for the earthquake
events that caused tsunamis.
Figure 9 depicts both the initial sea-surface perturbation (Fig. 9a) and
the maximum wave amplitudes (Fig. 9b) simulated following the occurrence of
the Mw= 6.9 northern Aegean earthquake. In spite of the fact that no tsunami
was recorded in the eastern Mediterranean region after the 24 May 2014
earthquake, modeling the tsunami generation (Fig. 9a) shows the
occurrence of a small tsunami with wave amplitude reaching about 7 cm. Along
the north Aegean Sea coasts, the computed maximum wave amplitudes range from
a few centimeters up to about 0.15 m, reaching a maximum in some coastal zones of the region.
DiscussionTsunami numerical modeling
Tsunami numerical modeling is a key component of any end-to-end tsunami
warning system. Recent progress in numerical modeling allows estimates of
tsunami generation, propagation and coastal impact in a proper way, reaching
good agreement with the recorded and observed data. In this study, we employ
a simple earthquake source model, available bathymetric model and a
validated shallow water code for tsunami simulations.
Tsunami generation
Simulations of tsunami generation are performed using regression laws to
retrieve the fault dimension and the average coseismic slip from the
earthquake magnitude, and then employing the half-space elastic theory of
Okada (1985) to compute the sea-bed deformation. Tsunami initial condition
results (Fig. 6) show relatively significant sea-surface deformations for
events with recorded tsunamis but also lead to small initial perturbation
for some events with no tsunami observations.
Tsunami generation results highlight, on the one hand, the importance of the
earthquake focal mechanism in controlling the tsunamigenic potential, and on
the other hand, the limitation of the used generation model. Comparing the
tsunami initial deformations of both events 11 and 16 of strike–slip and
thrust focal mechanism, respectively, and of relatively similar magnitudes
and depths, clearly indicates that the thrust fault has more potential of
tsunami generation than the strike–slip. However, the tsunami potential is
not only determined by the focal mechanism because some events seem to have
focal mechanisms that favor the result of tsunamis, but for which no waves were
observed. The reverse Mw= 7.1 earthquake that occurred in the Philippines at a
depth of 12 km (event 7) illustrates an example of these events.
Another important factor that seems to affect tsunami generation is the
specific location of some events in oceanic/sea regions characterized by the
presence of islands. The presence of the islands in the co-seismically
deformed region of an earthquake can significantly reduce its tsunamigenic
potential. The 15 October 2013, Mw= 7.1, earthquake in the Philippines
(event 7) and the 24 May 2014, Mw= 6.9, northern Aegean earthquake
(event 23) are two events for which the co-seismically deformed areas
include islands. This may be why no tsunami was recorded for these two
events, even though the numerical modeling shows the generation of small
tsunamis (see Fig. 6).
Simulation results (Fig. 6) led to the generation of tsunamis for
21 earthquake events, while for only nine events, tsunamis were observed. The
results (Fig. 6) also show that the computed initial sea-surface
deformations with relatively significant wave amplitudes correspond to the
cases with observed tsunamis. For almost all the other events, initial waves
of a few centimeters were simulated. For some events, considered as
non-tsunamigenic, the associated earthquakes might trigger small tsunamis,
as simulated in Fig. 6, but they were not recorded due to the absence of
sensors in the near-field areas. Events 2, 10 and 20 are examples of such earthquakes.
The simulation results highlight the relative limitation of the source model
calculations considered in this study that retrieved the fault dimension and
average coseismic slip from the earthquake magnitude using empirical
scaling laws. Moreover, assuming a uniform slip distribution along the fault
plane may result in inappropriate estimates of the tsunami initial wave. A
better constraint of the source model and slip distribution requires inversion
methods of both seismic and tsunami data. Although these methods are robust
for the source model evaluation, they are less appropriate for early tsunami
warning purposes that are usually based on fast estimates of source parameters.
Tsunami propagation
The results of tsunami numerical modeling for both the Mw= 7.1 Japan and
the Mw= 8.1 Chile events that occurred on 25 October 2013 and
1 April 2014, respectively (Figs. 7 and 8), produced relatively good
estimates of wave amplitudes and arrival times when compared with the
recorded signals from DART® stations. These two cases indicate that the
expeditious strategy involving Okada-like modeling of the tsunami initial
condition and numerical modeling of the tsunami propagation can produce
correct results. Nevertheless, the lack of precise source information
(dimension and slip) and detailed bathymetric models leads to some
differences between these signals. Figures 7b, c and 8b–d
highlight these limitations, especially regarding the estimates of
the wave periods and the amplitudes of the second waves. This is
particularly due to the use of empirical scaling law to estimate the
earthquake fault parameters (dimension and slip) as well as adopting a
uniform slip distribution along the fault plane. Appropriate methods to
constrain the fault slip distribution model require inversion of tsunami
data (Fujii et al., 2011; Wei et al., 2013; Satake et al., 2013).
An et al. (2014) assessed the source model of the 1 April 2014
tsunami using the least square inversion of tsunami records from three DART®
stations in the Pacific Ocean. Considering this source model they were able
to properly reproduce the tsunami waveforms at the stations locations using
a shallow water model. Their approach is robust in constraining the
earthquake source model from tsunami observations. However, for early
tsunami warning purposes a fast estimate of the earthquake source is
essential. Tsunami data inversion requires the use of at least the complete
first wave, which leads to delays in tsunami warning dissemination that can
be significant,
especially for local and regional events.
On the other hand, tsunami numerical results for the Mw6.9 north Aegean
earthquake (Fig. 9) show the occurrence of a small tsunami, maximum wave
amplitudes reaching about 0.15 m, in the absence of any record or
observation. This fact clearly highlights the limitation of the regression
laws used in this study to retrieve the fault dimension and the average
coseismic slip from the earthquake magnitude only. Also, the presence of
the islands in the co-seismically deformed region of the north Aegean
earthquake can be one of the factors that determined the tsunamigenic
potential of the event.
In spite of these limitations related to the models used (earthquake source
model, bathymetric model and the shallow water code), they present the
advantage of fast estimates of the earthquake source parameters and then a
fast computation of the tsunami threat, which is relevant for any TWS in
particular for the case of local and regional tsunamigenic hazardous source
zones. However, in some cases the considered models can lead to
inappropriate estimates of tsunami hazards and therefore to inappropriate
tsunami warning decisions.
Tsunamigenic potential and sensitivity to earthquake parameters
In addition to the magnitude, two main parameters are recognized to control
the tsunamigenic potential, namely the focal mechanism and the rupture
depth. Here, we focus only on events with confirmed tsunami and we discuss
the sensitivity of tsunami generation and potential to these three parameters.
The studied events occurred with magnitudes ranging from Mw= 6.7
up to 8.1, at depths between 10 and 585 km, with various focal mechanisms that
include normal, strike–slip and thrust faults. 39 % of these earthquakes
triggered tsunamis that were confirmed from DART® and/or TD records.
In order to highlight the sensitivity of tsunami potential to earthquake
parameters, we plot in Fig. 10 the observed tsunami wave amplitudes for
the different earthquake magnitudes and rupture depths, as well as the
proportion of mechanism focal types for tsunamigenic events. Figure 10a
depicts the observed tsunami wave amplitudes for the various earthquake
magnitudes. This figure indicates that higher wave amplitude (more than
2 m) was observed for higher earthquake magnitude (Mw= 8.1). However,
higher magnitude does not always cause the higher tsunami waves, because the
figure also shows that in some cases, higher magnitude earthquake events (the
Mw= 7.6 Solomon Islands earthquake) cause lower tsunami amplitudes than
smaller magnitude events (the Mw= 6.7 Chile earthquake). This fact
clearly indicates that the event magnitude is not the only factor
controlling the tsunami potential, but there are other parameters of
significant importance. In Fig. 10b and c, we plot observed tsunami
wave amplitudes for the different depths of the analyzed events. These
figures show that significant tsunamis were recorded for low depths. In
Fig. 10d we plot the proportions of mechanism focal types in order to
highlight the contribution of each rupture type to the tsunamigenic
potential. As expected, Fig. 10d shows clearly that most tsunami events
(67 %) were due to reverse/thrust earthquake fault ruptures. This is due
to the fact that the dip-slip faults, including normal and reverse ruptures,
are the favorite earthquake mechanisms for tsunami generation more than the
strike–slip ones as they induce more substantial vertical displacement of
the ocean bottom; although, the strike–slip earthquakes may also trigger
tsunamis, in particular when they present a dip-slip component or when they
interact with irregular sea-bottom topography (Tanioka and Satake, 1996).
A number of research studies have investigated the sensitivity of tsunamigenic
potential to earthquake parameters. Okal (1988) showed that the primary
parameter affecting tsunami generation is the seismic moment. Geist and
Yoshioka (1996) studied the source parameters controlling the generation of
tsunamis along the Cascadia margin. They concluded that the major factors
influencing the generation of tsunamis are the shallowness of the rupture
and the type of the fault. These studies are in relatively good agreement
with our results since most confirmed tsunamis are caused by events of large
magnitude occurring at shallow depths with thrust/reverse faulting ruptures.
It is important to mention here that among the studied events, some of them
are non-tsunamigenic, even though they seem able to cause tsunamis due to
their shallow rupture. Such events can lead to the dissemination of false
alerts especially when the TWS is based upon a pre-defined decision matrix.
The events which occurred in the Mediterranean Sea (Mw= 6.8 Hellenic and
Mw= 6.9 Aegean events) clearly illustrate the limitation of the use of
decision matrix that is usually based only upon earthquake parameters
(magnitude, depth and location) to estimate the severity of the tsunami.
This is in accordance with the study by Tinti et al. (2012) who
investigated the limitation of the decision matrix for the NE Atlantic,
Mediterranean and connected seas (NEAM) region, showing the importance of
considering additional earthquake characteristics such as the focal mechanism.
Tsunami warning
The main goal of a TWC is to provide early alerts to the endangered coastal
population when a possible tsunami is generated. Depending of the severity
of the occurred earthquake and the subsequent tsunami, four types of warning
messages are used by the TWCs around the world including warning, advisory,
watch and information. The tsunami warning message includes, in general,
information on earthquake parameters (origin time, location, depth and
magnitude) as well as an evaluation of the tsunami threat in the surrounding coasts.
For the 23 earthquake events, analyzed in this paper, tsunami alerts were
issued by various TWCs (international, regional, national and/or local)
including the Pacific Tsunami Warning Center (PTWC), the West Coast/Alaska
Tsunami Warning Center (WC/ATWC), the Japan Meteorological Agency (JMA), the
Joint Australian Tsunami Warning Center (JATWC), the Indian Tsunami Early
Warning Centre (ITEWC), and the three candidate tsunami watch providers of
the NEAM region, namely the National Observatory of Athens (NOA), Kandilli
Observatory and Earthquake Research Institute (KOERI) and the tsunami
warning center at the National Institute of Geophysics and Volcanology, Rome
(“Centro di Allerta Tsunami” at “Istituto Nazionale di Geofisica e
Vulcanologia”: CAT-INGV). Alert messages of warning type were issued for
39 % of these events. Warning level messages were disseminated by the PTWC
for five earthquake events that occurred in Chile (Mw= 8.1 and
Mw= 7.7), in the Solomon Islands (Mw= 7.6 and Mw= 7.4), and in
Papua New Guinea (Mw= 7.5). Watch/advisory level messages were issued by
JMA for the Mw= 7.1 earthquake that occurred in Japan and by KOERI for
both the Mw= 6.8 Hellenic and the Mw= 6.9 Aegean events. For the rest
of the events, the issued messages were of information type. In Table 2 we
summarize, for each studied event, the warning issued data including the
disseminated alerts, the agency responsible for alerting, and the type of
the alert message.
On the other hand, some of the issued messages can be classified as “false
warnings” due to the fact that for some non-tsunamigenic events, alert
messages were disseminated indicating possible tsunami generation and
coastal impact. The Mw= 6.8 Hellenic and the Mw= 6.9 Aegean events
occurred in the Mediterranean Sea illustrate such cases well. For both
events, KOERI had issued regional “watch/advisory” warning messages with
possible coastal impact (see Table 2), while it is confirmed later that no
tsunami was generated. These kinds of “false alerts” are mainly due to the
use of a pre-defined decision matrix with limited earthquake
characteristics, as highlighted in Tinti et al. (2012), to estimate the
tsunami potential and its severity. Moreover, there was a relatively
“missed tsunami alarm” for the Mw= 7.3 Mexico event, that caused a 0.43 m
amplitude tsunami recorded by Acapulco TD, while PTWC only issued a tsunami
information bulletin with no report on the tsunami threat.
Reducing the time delay to issue the first tsunami message after the
earthquake occurrence remains challenging for any early TWC. In general, for
the analyzed events in this paper, the TWCs have well performed by
disseminating early tsunami messages within 10 min after the occurrence of
the earthquakes for 75 % of the events. By gathering the tsunami warning
messages from the TWCs for the studied events, the proportions of the first
message time delay indicate that for 9 % of the events, the first warning
message was issued within 2–5 min, for about 70 % of the events within
5–10 min and for less than 22 % of the events, the warning messages were
disseminated between 10 and 15 min.
Conclusions
This study is a contribution to a better understanding of the tsunami
potential from large submarine earthquakes occurring worldwide. The study
considered the preliminary parameters evaluated for the earthquake events
and the tsunami recorded data and used source evaluation models together
with tsunami modeling to investigate the tsunami potential. The analysis of
23 submarine earthquake events which occurred worldwide with magnitudes ranging
from Mw= 6.7 up to Mw= 8.1 leads to the following conclusions:
A significant number of events (39 %) were tsunamigenic.
Most earthquake events that cause confirmed tsunamis have, as expected,
a combination of source parameters that “favor” tsunami generation,
namely a shallow depth and a thrust/reverse faulting focal mechanism that took
place on/near the subduction zones.
The strike–slip events that trigger tsunamis (11 and 18), with relatively
small amplitudes of a few centimeters, are only those of large magnitude
(Mw> 7.5) which occurred at shallow depth (< 30 km).
Most earthquakes that did not trigger tsunamis have fault ruptures that
are too deep (events 1 and 5), they present “non-favoring” earthquake focal mechanisms (almost
pure strike–slip, events 10, 13) or they occurred in areas with the presence of
islands in the co-seismically deformed region (events 7 and 23).
Simulations of the tsunami generation predicted non-zero initial sea-surface
perturbations for some events with non-confirmed tsunamis.
Numerical modeling of tsunamis is an important tool for wave amplitude and
tsunami travel time estimates and then relevant for any TWS, in spite of some
limitations on source evaluation and bathymetric data.
TWCs around the world have performed relatively well for the most analyzed
cases as they provide first warning within 10 min for more than 78 % of the
tsunami events. However for some events, “false alerts” were disseminated, in
particular in the Mediterranean Sea (Mw= 6.8 Hellenic and the Mw= 6.9
Aegean events) where the tsunami warning is mainly based on the use of a
pre-defined decision matrix.
In summary, the present study can help characterize tsunami
decision matrixes for the various oceanic regions. Tsunami decision matrixes
that are based only on limited earthquake parameters (magnitude, depth and
location) should be improved and revised in order to increase the number of
the considered earthquake parameters to be taken into account (the focal
mechanism, for instance).
Acknowledgements
This work is funded by the Join Research Center (JRC) GTIMS project (Global
Tsunami Information Monitoring Service), tender no. JRC/IPR/2013/G.2/13/NC,
and by the EU project ASTARTE – Assessment, STrategy And Risk Reduction for
Tsunamis in Europe Grant 603839, 7th FP (ENV.2013.6.4-3 ENV.2013.6.4-3).
The maps in this work were made using the GMT software package (Wessel et
al., 2013), the main tectonic boundaries were from Bird (2003) and the 30 arcsec bathymetry background was from GEBCO (2015).
Edited by: A. Armigliato
Reviewed by: three anonymous referees
References
Aki, K.: Scaling law of earthquake source time-function, Geophys. J. Int., 31, 3–25, 1972.
An, C., Sepúlveda, I., and Liu, P. L. F.: Tsunami source and its
validation of the 2014 Iquique, Chile, earthquake, Geophys. Res. Lett., 41, 3988–3994, 2014.
Angermann D., Klotz, J., and Reigber, C.: Space-geodetic estimation of the
Nazca-South America Euler vector, Earth Planet. Sc. Lett., 171, 329–334, 1999.
Bird, P.: An updated digital model of plate boundaries, Geochem. Geophy. Geosy.,
4, 1027–1080, 2003.
Blaser, L., Krüger, F., Ohrnberger, M., and Scherbaum, F.: Scaling
relations of earthquake source parameter estimates with special focus on
subduction environment, Bull. Seismol. Soc. Am., 100, 2914–2926, 2010.
Broeze, J. and Van Daalen, E. F. G.: Radiation boundary conditions for the
two-dimensional wave equation from a variational principle, Math. Comp., 58, 73–82, 1992.
DeMets, C., Gordon, R. G., Argus, D. F., and Stein, S.: Effect of recent
revisions to the geomagnetic reversal time scale on estimates of current
plate motions, Geophys. Res. Lett., 21, 2191–2194, 1994.DeMets, C., Gordon, R. G., and Argus, D. F.: Geologically current plate
motions, Geophys. J. Int., 181, 1–80, 10.1111/j.1365-246X.2009.04491.x, 2010.
Fujii, Y., Satake, K., Sakai, S. I., Shinohara, M., and Kanazawa, T.:
Tsunami source of the 2011 off the Pacific coast of Tohoku
Earthquake, Earth Planets Space, 63, 815–820, 2011.GEBCO – General Bathymetric Chart of the Oceans: available at:
http://www.gebco.net/, last access: 1 January 2015.
Geist, E. and Yoshioka, S.: Source parameters controlling the generation
and propagation of potential local tsunamis along the Cascadia
margin, Nat. Hazards, 13, 151–177, 1996.
Heidarzadeh, M. and Satake, K.: Possible sources of the tsunami observed in
the northwestern Indian Ocean following the 2013 September 24 Mw 7.7
Pakistan inland earthquake, Geophys. J. Int., 199, 752–766, 2014.
Kajiura, K.: Tsunami source, energy and the directivity of wave radiation,
Bull. Earthq. Res. Inst. Tokyo Univ., 48, 835–869, 1970.
Kanamori, H. and Anderson, D. L.: Theoretical basis of some empirical
relations in seismology, Bull. Seismol. Soc. Am., 65, 1073–1095, 1975.Lagmay, A. M. F. and Eco, R.: Brief Communication: On the source characteristics
and impacts of the magnitude 7.2 Bohol earthquake, Philippines, Nat. Hazards
Earth Syst. Sci., 14, 2795–2801, 10.5194/nhess-14-2795-2014, 2014.
Lay, T., Yue, H., Brodsky, E. E., and An, C.: The 1 April 2014 Iquique,
Chile, Mw 8.1 earthquake rupture sequence, Geophys. Res. Lett., 41, 3818–3825, 2014.Liu, P. L.-F., Woo, S.-B., and Cho, Y.-S.: Computer programs for tsunami propagation
and inundation, Technical report, Cornell University, USA, available at:
http://tsunamiportal.nacse.org/documentation/COMCOT_tech.pdf (last access: December 2014), 1998.
Hamburger, M., Hurst, K., Kahle, H., Kastens, K., Kekelidze, G., King, R.,
Kotzev, V.,Lenk, O., Mahmoud, S., Mishin, A., Nadariya, M., Ouzounis, A.,
Paradissis, D., Peter, Y., Prilepin, M., Reilinger, R., Sanli, I., Seeger, H.,
Tealeb, A., N. Toksöz, M., and Veis, G.: Global Positioning System constraints
on plate kinematics and dynamics in the eastern Mediterranean and Caucasus,
J. Geophys. Res.-Solid Ea., 105, 5695–5719, 2000.NGDC/WDS – National Geophysical Data Center/World Data Service: Global Historical
Tsunami Database, National Geophysical Data Center, NOAA, 10.7289/V5PN93H7,
available at: https://www.ngdc.noaa.gov/hazard/tsu_db.shtml, last access: 10 December 2014.
Okada, Y.: Surface deformation due to shear and tensile faults in a
half-space, Bull. Seismol. Soc. Am., 75, 1135–1154, 1985.
Okal, E. A.: Seismic parameters controlling far-field tsunami amplitudes: a
review, Nat. Hazards, 1, 67–96, 1988.
Satake, K. and Tanioka, Y.: Sources of tsunami and tsunamigenic earthquakes
in subduction zones, in: Seismogenic and Tsunamigenic Processes in Shallow
Subduction Zones, Birkhäuser, Basel, 467–483, 1999.Satake, K., Fujii, Y., Harada, T., and Namegaya, Y.: Time and space
distribution of coseismic slip of the 2011 Tohoku earthquake as inferred
from tsunami waveform data, Bull. Seismol. Soc. Am., 103, 1473–1492, 2013.
Tanioka, Y. and Satake, K.: Tsunami generation by horizontal displacement
of ocean bottom, Geophys. Res. Lett., 23, 861–864, 1996.Tinti, S., Graziani, L., Brizuela, B., Maramai, A., and Gallazzi, S.:
Applicability of the Decision Matrix of North Eastern Atlantic,
Mediterranean and connected seas Tsunami Warning System to the Italian
tsunamis, Nat. Hazards Earth Syst. Sci., 12, 843–857, 10.5194/nhess-12-843-2012, 2012.Wang, X.: COMCOT user manual-version 1.7, School of Civil and Environmental
Engineering, Cornell University Ithaca, New York, USA,
http://ceeserver.cee.cornell.edu/pll-group/doc/COMCOT_User Manual v1 7.pdf
(last access: December 2014), 2009.
Wei, Y., Chamberlin, C., Titov, V. V., Tang, L., and Bernard, E. N.:
Modeling of the 2011 Japan tsunami: Lessons for near-field forecast, Pure
Appl. Geophys., 170, 1309–1331, 2013.
Wells, D. L. and Coppersmith, K. J.: New empirical relationships among
magnitude, rupture length, rupture width, rupture area, and surface
displacement, Bull. Seismol. Soc. Am., 84, 974–1002, 1994.
Wessel, P., Smith, W. H., Scharroo, R., Luis, J., and Wobbe, F.: Generic
mapping tools: improved version released, Eos Trans. Am. Geophys. Un., 94, 409–410, 2013.