NHESSNatural Hazards and Earth System SciencesNHESSNat. Hazards Earth Syst. Sci.1684-9981Copernicus PublicationsGöttingen, Germany10.5194/nhess-17-2365-2017Planar seismic source characterization models developed for probabilistic seismic hazard assessment of IstanbulGülerceZeynepzyilmaz@metu.edu.trBuğra SoymanKadirGünerBarışKaymakciNuretdinhttps://orcid.org/0000-0002-7618-0226Department of Civil Engineering, Middle East Technical University,
Ankara, 06800, TurkeyDepartment of Nuclear Safety, Turkish Atomic
Energy Authority, Ankara, 06510, TurkeyDepartment of Geological
Engineering, Middle East Technical University, Ankara, 06800, TurkeyZeynep Gülerce (zyilmaz@metu.edu.tr)22December201717122365238127March201719October20179October20172May2017This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/This article is available from https://nhess.copernicus.org/articles/17/2365/2017/nhess-17-2365-2017.htmlThe full text article is available as a PDF file from https://nhess.copernicus.org/articles/17/2365/2017/nhess-17-2365-2017.pdf
This contribution provides an updated planar seismic source
characterization (SSC) model to be used in the probabilistic seismic hazard
assessment (PSHA) for Istanbul. It defines planar rupture systems for the
four main segments of the North Anatolian fault zone (NAFZ) that are critical for
the PSHA of Istanbul: segments covering the rupture zones of the 1999 Kocaeli and
Düzce earthquakes, central Marmara, and Ganos/Saros segments. In each
rupture system, the source geometry is defined in terms of fault length,
fault width, fault plane attitude, and segmentation points. Activity rates
and the magnitude recurrence models for each rupture system are established
by considering geological and geodetic constraints and are tested based on
the observed seismicity that is associated with the rupture system.
Uncertainty in the SSC model parameters (e.g., b value, maximum magnitude,
slip rate, weights of the rupture scenarios) is considered, whereas the
uncertainty in the fault geometry is not included in the logic tree. To
acknowledge the effect of earthquakes that are not associated with the
defined rupture systems on the hazard, a background zone is introduced and
the seismicity rates in the background zone are calculated using
smoothed-seismicity approach. The state-of-the-art SSC model presented here
is the first fully documented and ready-to-use fault-based SSC model
developed for the PSHA of Istanbul.
Introduction
(a) Major branches of the North Anatolian fault zone, defined
rupture systems and the instrumental seismicity (Mw>4) in the
study area. The buffer zones used for source-to-epicenter matching are shown
around the rupture systems. (b) Simplified active tectonic scheme of
Turkey (modified from Emre et al., 2013). Thick lines are the North Anatolian and
East Anatolian fault zones; thin lines are other active faults.
(c) Distribution of the declustered seismicity used to calculate the
b values. Zone 1, Zone 2 and Zone 3 are the polygons used to calculate the
b values. (d) Slip distribution model for the Çınarcık
segment. Right bending of the northern Çınarcık segment is 28∘
with respect to the central Marmara and Hersek–Gölcük segments. This
results in a 17 mmyr-1 slip along the northern Çınarcık
segment (NÇF) and 9 mmyr-1 normal slip transverse to the
fault. This 9 mmyr-1 slip is the total slip on the northern and southern Çınarcık faults (SÇF). (e) Simplified geometries of
the Çınarcık faults delimiting the Çınarcık Basin based on
seismic profile of Laigle et al. (2008) almost passing through the line XY.
The North Anatolian fault zone (NAFZ), one of the most active fault systems in
the world, extends for more than 1500 km along northern Turkey (Fig. 1b).
NAFZ was ruptured progressively by eight large and destructive earthquakes
(Mw>6.5) in the last century. Earthquakes that occurred
between 1939 and 1967 had ruptured approximately 900 km of a uniform trace
in the east, whereas the 1999 Kocaeli and Düzce earthquakes ruptured
a total fault span of approximately 200 km where the NAFZ is divided into
a number of branches in the west. The northern strand of the NAFZ is submerged
beneath the Marmara Sea to the west of the rupture
zone of the 1999 Kocaeli earthquake, introducing major uncertainties into segment location, continuity, and
earthquake recurrence (Fig. 1a). In 2004, Parsons compiled a catalog of large-magnitude (M>7) earthquakes occurred around the Marmara Sea for the time
period of AD 1500–2000. Based on the rupture zones of these large-magnitude
events, four main segments for the northern strand of the NAFZ around Marmara Sea
were proposed by Parsons (2004): (1) the Ganos segment, which combines the rupture
zones of August 1776 and 1912 earthquakes; (2) the Prince Island segment, which
includes the rupture zones of 1509 and May 1766 earthquakes; (3) the Izmit
segment, defined for the rupture zones of the 1719 and 1999 earthquakes; and
(4) the Çınarcık segment, defined for M∼7 floating earthquakes
(independent normal-fault earthquakes that may have occurred on different
fault segments in or around the Çınarcık Basin). Parsons (2004)
noted that 10 May 1556 (Ms=7.1), 2 September 1754 (M=7.0), and 10 July 1894 (M=7.0) earthquakes were assigned locations in
the Çınarcık Basin or on mapped normal faults in the southern parts
of the Marmara Sea. These events were not allocated to the other segments in
order not to violate the inter-event time calculations, although they could
have occurred on the northern strand of the NAFZ.
The fault segmentation model proposed by Erdik et al. (2004) was similar to
the segmentation model proposed by Parsons (2004) in terms of the fault
geometry; however, smaller segments were preferred. Erdik et al. (2004) noted
that “the Main Marmara Fault cuts through Çınarcık,
central and Tekirdağ basins, follows the northern margin of the basin
when going through the Çınarcık trough in the northwesterly
direction, makes a westwards kink around south of Yeşilkoy until it
reaches the 1912 Murefte–Şarköy rupture”. All of these fault lines
were interpreted as separate fault segments in the segmentation model. Erdik
et al. (2004) considered multi-segment ruptures by assigning lower
probabilities to “cascading ruptures”. Based on the rupture zones
of previous large-magnitude events, multi-segment ruptures involving the
segments in connection with the 1999 Kocaeli earthquake and 1509 earthquake
were included in the rupture forecast. Even though multi-segment ruptures
were considered, the relative probabilities of the multi-segment ruptures vs.
single-segment ruptures were not systematically defined in Erdik
et al. (2004). This seismic source model was updated for the Earthquake
Hazard Assessment for Istanbul project by OYO (2007). The fundamental
differences between the Erdik et al. (2004) and OYO-2007 models are
(1) small segments around Marmara Sea used in the Erdik et al. (2004) model were
combined to form bigger segments in the OYO-2007 model, (2) fault segments that
represent the floating earthquakes were defined. The segmentation model used
in OYO-2007 source characterization is very similar to the segmentation model
proposed by Parsons (2004).
The fault segmentation model used by Kalkan et al. (2009) includes
significant differences in terms of the fault geometry with the Erdik
et al. (2004) model, even though both studies used the active fault maps of
Şaroğlu et al. (1992) for inland faults and the fault segmentation
model from Le Pichon et al. (2003) and Armijo et al. (2005) for the segments
beneath the Sea of Marmara. On the other hand, the magnitude recurrence
models used by Erdik et al. (2004), in the OYO-2007 model, and by Kalkan
et al. (2009) were rather similar. In all of these studies, linear fault
segments were modeled (fully or partially) by the characteristic model
proposed by Schwartz and Coppersmith (1984); therefore, only large-magnitude
events were associated with the fault segments. Additionally, a background
source representing the small-to-moderate magnitude earthquakes (earthquakes
between 5 and 6.5–7 depending on the study) were added to the source model
and the earthquake recurrence of the background source was modeled using
a double-truncated exponential magnitude distribution model. Either the
Poisson (Erdik et al., 2004; Kalkan et al., 2009) or time-dependent renewal
(Brownian Passage Time, Ellsworth et al., 1999) model (Erdik et al., 2004)
was chosen to model the earthquake recurrence rates for linear segments,
whereas the Poisson distribution was used to model the recurrence rates of
the background source in these studies.
Recently proposed SSC models for the western segments of the NAFZ (Gülerce
and Ocak, 2013; Murru et al., 2016) are more detailed in terms of the
segmentation models, magnitude recurrence relations, and estimation of the
activity rates. In the Gülerce and Ocak (2013) SSC model, the length of
segments and the segmentation points were determined and incorporated with
the help of then-available fault maps and traced source lines on the
satellite images. Planar fault segments were defined and the composite
magnitude distribution model (Youngs and Coppersmith, 1985) was used for all
seismic sources to properly represent the characteristic
behavior of the NAFZ without an additional background zone. Unfortunately, the
seismic source model proposed by Gülerce and Ocak (2013) cannot be
directly implemented in the PSHA for Istanbul since the model does not
include the fault segments on the west of the 1999 Kocaeli earthquake rupture
zone. The geometry of the fault segments defined in Murru et al. (2016) is
generally similar to the Erdik et al. (2004) model. Furthermore, Murru
et al. (2016) provided the complete set of parameters required for
a fault-based PSHA analysis (e.g., slip rates, fault widths, rupture models
and rates, parameter uncertainties).
The objective of this study is to provide an updated and properly documented
fault-based SSC model to be used in the PSHA studies in Istanbul.
A significant portion of the tectonic database is acquired from the updated
Active Fault Map of Turkey that was published by General Directorate of
Mineral Research and Exploration (Emre et al., 2013) (accessed through
http://www.mta.gov.tr/v3.0/hizmetler/yenilenmis-diri-fay-haritalari).
The 1/250 000 scale Çanakkale (NK 35-10b), Bandırma (NK 35-11b), Bursa
(NK 35-12), Adapazarı (NK 36-13), Bolu (NK 36-14), and Istanbul (NK 35-9)
sheets of updated Active Fault Map of Turkey were accessed and digitized.
The seismological database is taken from the Integrated and Homogeneous
Turkish Earthquake Catalog published by the Kandilli Observatory and Earthquake
Research Institute (Kalafat et al., 2011). Seismotectonic information related
to the active faults and the fault systems that are available in these
databases and in the current scientific literature are used in combination
with the segmentation models proposed by Gülerce and Ocak (2013) and
Murru et al. (2016) to define the rupture systems. Fault segments, rupture
sources, rupture scenarios, and fault rupture models are determined using the
terminology given in the Working Group of California Earthquake Probabilities
(WGCEP-2003) report and multi-segment rupture scenarios are considered in
a systematic manner. Events in the seismological database are attributed to
the rupture systems and the logic tree weights for the rupture scenarios are
determined by comparing the accumulated seismic moment due to the geological
constraints (rupture dimensions and slip rate) with the seismic-moment
release due to associated seismicity. In contrast to the previous efforts,
the PSHA inputs (e.g., coordinates of the fault segments, logic tree branches
and corresponding weights) are properly documented; therefore, the SSC model
presented here can be directly implemented in the future site-specific PSHA
studies in Istanbul.
Fault segmentation models, rupture systems, and partitioning of slip rates
The SSC model consists of one background source (defined in Sect. 5) and four
distinct (non-overlapping) rupture systems that are defined by considering
the rupture zones of previous large-magnitude earthquakes documented by
Parsons (2004) on the northern strand of the NAFZ. We note that all subsegments
in the defined rupture systems except for northern and southern Çınarcık
segments are assumed to be near vertical with right-lateral slip as suggested
by geological, seismological, and GPS data. The segmentation and the slip-rate partitioning models are not yet well established enough for the fault segments
south of the Marmara Sea; therefore, these segments are not modeled
as planar seismic sources in this SSC model.
Izmit and Düzce rupture systems
Location, geometry, and slip distribution of the rupture zones of the 1999
Kocaeli and Düzce earthquakes have been studied extensively after these
events (e.g., Barka et al., 2002; Langridge et al., 2002; Akyüz et al.,
2002). The surface rupture of the 1999 Kocaeli earthquake extended for almost
165 km and four distinct segments were ruptured (Hersek segment,
Gölcük–Karamürsel–Izmit segment, Sapanca–Akyazı segment, and
Karadere segment as given in Barka et al., 2002). The co-seismic fault was
terminated at the western end of the rupture, very near to the eastern side
of the Marmara Sea (Ergintav et al., 2014). The northern strand of the NAFZ that
delimits the boundary between the Marmara Sea and Çınarcık Block
did not rupture during the 1999 Kocaeli earthquake (Çınarcık segment in
Fig. 1a). Mert et al. (2016) argued that the northern strand of the NAFZ is
observed as a single continuous fault strand along Izmit Bay and at its
entrance to the sea southeast of Istanbul. We included the northern Çınarcık segment (segment 3) in the Izmit rupture system because it is the
western extension of the Hersek–Gölcük segment that was developed in
response to the bending of the main strand of the NAFZ towards NW. This
bending results in a releasing bend and a slip redistribution as dextral
motion parallel to the main strand and normal motion perpendicular to the
Çınarcık segments (Fig. 1d). As seen in Fig. 1e, the vertical throw
of the northern Çınarcık segment is almost twice of the throw of
the southern Çınarcık segment, which is the conjugate fault of the
northern Çınarcık segment. The dip of the northern Çınarcık
segment is assumed to be 70∘ SW as suggested by Laigle
et al. (2008),
while the dip of the southern Çınarcık segment is assumed to be
60∘ NW. The Izmit rupture system proposed here consists of five
(Hersek–Gölcük, Izmit, Sapanca–Akyazı, Karadare and northern Çınarcık) subsegments.
The Düzce earthquake produced a 40 km long surface
rupture zone; however, there is a 4 km releasing step-over around
Eften Lake (Akyüz et al., 2002). Therefore, a two-segment model
(segments D1 and D2) is established for the rupture zone of the Düzce
earthquake (Fig. 1a). The segments and segment lengths for the Izmit and
Düzce rupture systems are given in Table 1. In 1999 earthquakes, these
two rupture systems (Kocaeli and Düzce) were ruptured in two different
episodes. A possible explanation for the separate ruptures in different
episodes would be the development of the restraining bend along the Karadere
segment, which probably locked up the eastern termination of Izmit rupture.
Harris et al. (2002) proposed that the rupture of 1999 İzmit earthquake
was stopped by a step-over at its eastern end (Mignan et al., 2015). In this
study, we assumed the same rupture pattern of 1999 earthquakes and do not
include a rupture scenario that combines these two rupture systems in the
rupture forecast.
The fault segments and rupture systems included in the SSC model.
References given in the last column are (1) Flerit et al. (2004), (2) Murru
et al. (2016), (3) Ergintav et al. (2014), (4) Ayhan et al. (2001), (5) Hergert
et al. (2011). Weights associated with the mean, upper bound and lower bound are
0.5, 0.25, and 0.25, respectively.
The ENE–WSW-trending Ganos Fault is the fault segment at the westernmost
section of the NAFZ that generated the 9 August 1912 Mürefte (Ganos)
earthquake. Magnitudes of this earthquake were estimated from historical
catalogs and field observations as Ms=7.3±0.3 (by
Ambraseys and Jackson, 2000) and Mw=7.4 (by Altunel et al.,
2004), respectively (Aksoy et al., 2010). A second large event occurred
on 13 September 1912 (Ms=6.8±0.35 and the estimated
seismic moment was 2.19×1019 Nm as given in Ambraseys and
Jackson, 2000). Ambraseys and Jackson (2000) suggested a 37 km long
co-seismic rupture for this large second shock. Aksoy et al. (2010) used the
duration of the recorded waveforms to estimate the rupture lengths of 1912
events: assuming the rupture width is 15–20 km, estimated values
were 130±15km and 110±30km for the 9 August and
13 September events, respectively. According to Aksoy et al. (2010),
co-seismic surface ruptures were visible along the 45 km onshore
section of this segment. Supporting the estimations based on waveforms using
aerial photographs, satellite imagery, digital elevation models, bathymetry,
and field measurements, Aksoy et al. (2010) proposed 120±30km long fault rupture for the 9 August 1912 event. Murru
et al. (2016) defined two segments covering the 120±30km long
fault rupture of the 1912 Ganos earthquake: a 74 km long segment that
includes the onshore section and a 46 km long offshore segment
(segments 6 and 7 in Fig. 1a). The maximum seismogenic depth of these
segments was assumed to be 15 km on the basis of the locking depth
suggested by mechanical best fit modeling of GPS data (Flerit et al., 2003)
and by the depth extent of instrumental seismicity (Gürbüz et al.,
2000; Özalaybey et al., 2002; Örgülü and Aktar, 2001; Pınar
et al., 2003). A similar segmentation model is adopted in this study by
implementing minor changes in the subsegment lengths as shown in Table 1.
Central Marmara rupture system
The northern strand of the NAFZ forms a major transtensional NW–SE right
bend under the Sea of Marmara at the Çınarcık trough (Murru et al.,
2016). The fault trace follows the northern margin of the Marmara Sea and
connects the complex central Marmara and Tekirdağ pull-apart basins,
before merging into the NE–SW-striking Ganos Fault on land (Wong et al.,
1995; Okay et al., 1999; Armijo et al., 2002; Le Pichon et al., 2001;
Yaltirak, 2002; McNeill et al., 2004; Murru et al., 2016). Building the
segmentation model for the offshore segments of the NAFZ (also known as the
Central Marmara Fault, CMF) is especially difficult, because the fault traces
are not directly observable (Aksu et al., 2000; Imren et al., 2001; Le Pichon
et al., 2001; Armijo et al., 2002, 2005; Pondard et al., 2007). Murru
et al. (2016) noted that the segments under Marmara Sea are bounded by
geometric fault complexities and discontinuities (e.g., jogs and fault bends)
that can act as barriers to rupture propagation (Segall and Pollard, 1980;
Barka and Kadinsky-Cade, 1988; Wesnousky, 1988; Lettis et al., 2002; An,
1997) and proposed two separate segments for CMF. We adopted the fault
geometry and the segments proposed by Murru et al. (2016) to build the
two-segment central Marmara rupture system (see Fig. 1a for details). As
mentioned by Murru et al. (2016), this model is consistent with the
segmentation model proposed by Armijo et al. (2002) and in good agreement
with the observed Marmara Sea basin morphology and geology (Flerit et al.,
2003; Muller and Aydin, 2005; Carton et al., 2007; Pondard et al., 2007;
Şengör et al., 2014).
Annual slip rates
Past studies based on GPS measurements (McClusky et al., 2000; Meade et al.,
2002; Armijo et al., 2002; Reilinger et al., 2006; Hergert and Heidbach,
2010; Ergintav et al., 2014) suggest a 22±3mmyr-1 dextral
motion along the major block-bounding structures of the NAFZ, with more than
80 % being accommodated along the northern branch. On this branch, the
segments that formed the western and central parts of the Izmit rupture system
(segments 3, 2_1, 2_2 and 2_3 in Fig. 1a) share the total slip rate with
Geyve–Iznik Fault. The slip-rate participation among the northern strand of the NAFZ and the Geyve–Iznik Fault was given as 16 and 9 mmyr-1 in
Stein et al. (1997). However, Murru et al. (2016) have adopted the annual
slip rate of 20±2mmyr-1 for the northern strand based on
the proposals of Flerit et al. (2003) and Ergintav et al. (2014). Similarly,
we achieved a better fit with the associated seismicity of Izmit rupture
system by assigning a 19±2mmyr-1 annual slip rate to the
northern strand of the NAFZ (please refer to Sect. 4 for further details).
Similarly, the total slip rate is distributed over the eastern segment of the NAFZ southern strand (segment 1 in Fig. 1a) and the segments of Düzce
rupture system (D1 and D2). Ayhan et al. (2001) suggested that up to
10 mmyr-1 of the motion is accommodated on the
Düzce–Karadere strand of the NAF. We also utilized the same annual slip
rate of 10±2mmyr-1 for Düzce_1, Düzce_2 and
Karadere segments without any modifications (Table 1).
The catalog completeness analysis for the instrumental earthquake
catalog showing the cumulative number of events for
(a)Mw≥4.0, (b)Mw≥4.5, (c)Mw≥5.0,
(d)Mw≥5.5, and
(e)Mw≥6.0.
The mean slip rates adopted for central and western Marmara subsegments
(19 mmyr-1) are consistent with the neighboring subsegments of
the Izmit and Ganos/Saros rupture systems. Ergintav et al. (2014) noted that
the Prince Island Fault (PIF) (segment 4) is actively accumulating strain and has not
experienced a large event since 1766, making it the most likely segment to
generate a M>7 earthquake. The slip-rate estimate given in Ergintav
et al. (2014) for the PIF and Çınarcık Basin is 15±2mmyr-1. Murru et al. (2016) distributed the annual slip
rate of 17 mmyr-1 among two parallel branches in this zone: 14±2mmyr-1 for the Çınarcık segment and 3±1mmyr-1 for the southern Çınarcık segment based on the
recent works of Ergintav et al. (2014) and Hergert and Heidbach (2010).
Therefore, the slip-rate value that we have used on the horizontal plane
(17 mmyr-1) is identical to these recent estimates (Fig. 1d). In
our analysis, the 6±2mmyr-1 extension is assigned to the
northern Çınarcık segment, while 3±2mmyr-1 is
assigned to the southern Çınarcık segment. Since the northern Çınarcık segment was ruptured during the 17 August 1999 earthquake, we
assumed that all the strike-slip motion was taken up by the northern Çınarcık segment; therefore, the entire 17 mmyr-1 dextral
motion is assigned to the northern Çınarcık segment. The slip rate
given for the Central Marmara Fault by Ergintav et al. (2014)
(2 mmyr-1) is unusually low compared to the previous estimates
and may be suffering from the sparsity of the network and GPS coverage on the
northern shores of Marmara Sea as mentioned by the authors. For this rupture
system, the annual slip rate we adopted (19±2mmyr-1) is
in good agreement with the value given in Murru et al. (2016) (18±2mmyr-1) and with the seismicity rates based on the instrumental
earthquake catalog (Fig. 4b).
The slip rate given in the SSC model of Murru et al. (2016) is directly
adopted for the Ganos subsegment, whereas the slip rate partitioned
between the North Saros and South Saros subsegments in Murru et al. (2016)
is concentrated over the Saros subsegment (Table 1). This is because
the southern segment is developed in response to transtension exerted by the
curvilinear trace of the northern segment (Okay et al., 2004), a mechanism
somewhat similar to the northern and southern Çınarcık segments
proposed above. The slip rate assigned to the Ganos and Saros subsegments is
consistent with the recent GPS velocity profiles given in Hergert and
Heidbach (2010) and Ergintav et al. (2014). Table 1 summarizes the references
for the utilized annual slip rates for each segment and the uncertainty
related to the slip rate included in the logic tree.
Instrumental earthquake catalog and activity rates of earthquakes
The catalog of earthquakes documenting the available knowledge of past seismicity
within the site region is a key component of the seismic source
characterization for the hazard analysis. A very detailed review of the
historical earthquakes and their rupture zones around the Marmara Sea region
was documented by Parsons (2004). These earthquakes and the extension of
their rupture zones are directly utilized in this study to define the
subsegments, rupture systems, and to calculate the mean characteristic
magnitude values. The Integrated and Homogeneous Turkish Earthquake Catalog
published by KOERI (Kalafat et al., 2011), including the events with
Mw>4 that occurred between 1900 and 2010, is employed to
represent the instrumental seismicity in the region. It is notable that areal
source zones (or polygons) are not utilized in the SSC model to estimate the
activity rates; therefore, the maximum magnitude estimates and the PSHA
results are not solely dependent on the collected catalog. The
mainshock–aftershock classification of the catalog (declustering) is
performed and the aftershocks are removed from the data set using the
Reasenberg (1985) methodology in the ZMAP software package (Wiemer, 2001)
with minimum and maximum look-ahead times of 1 and 10 days and an event crack
radius of 10 km.
b values estimated using different methods and corresponding
weights in the logic tree.
Catalog completeness analysis for different magnitude ranges is performed in
order to achieve the catalog completeness levels used in calculating the
magnitude recurrence parameters. Cumulative rates of earthquakes larger than
specific magnitude levels are plotted against years in order to examine the
completeness of the catalog as shown in Fig. 2. For different cut-off magnitudes,
the breaking points for the linear trends in the cumulative rate of events
are examined and a significant breaking point is observed to be at 52 years
from the end of the catalog for magnitudes smaller than 4.5 and 5.0.
Therefore, the catalog was assumed to be complete for 52 years for 4.0≤Mw≤4.5 and 4.5≤Mw≤5.0 earthquakes,
respectively. Although the larger magnitude plots in Fig. 2 suffer from a
lack of data due to the truncation of the catalog, the catalog is assumed to
be complete for the greater magnitudes for the whole time span (110 years).
The catalog completeness intervals used in Şeşetyan et al. (2016)
and in this study for 4.7<M<5.7 earthquakes are consistent even if the
compiled catalogs are different.
Estimated magnitude recurrence parameters for (a) Zone 1,
(b) Zone 2, and (c) Zone 3.
The magnitude–frequency relationship developed for each rupture system and
the background zone is explained in the next section. Only one of the
magnitude–frequency relationship parameters, the slope of the cumulative
rate of events (as known as the b value), is calculated based on the
compiled catalog. We delineated three different zones for estimating the
b value considering the temporal and spatial variability of this parameter
as shown in Fig. 1c. Zone 1 includes the Ganos/Saros and central Marmara
rupture systems, Zone 2 covers the Izmit and Düzce rupture systems, and
Zone 3 is a larger area that includes both Zone 1 and Zone 2. For each zone, the
b value is estimated using the maximum likelihood method provided in the ZMAP
software package. Figure 3a–c shows the completeness magnitudes and the
b values for zones 1, 2, and 3. Analysis results show that the b value
varies between 0.68 and 0.74 for different rupture systems given in the
previous section, whereas the b value for the large area covering the whole
system is equal to 0.76.
Additionally, the b values for each zone are estimated using the modified
maximum likelihood method (Weichert, 1980) that takes into account the
completeness of the catalog for different magnitude bins. The b values
calculated using the Weichert (1980) method are approximately 5 % higher than the
maximum likelihood estimations of ZMAP for zones 1 and 2, but for the larger
zone (Zone 3), estimated b values are almost the same in both methods
(Table 2). To acknowledge the uncertainty in the b value estimations,
30 % weight is assigned to the zone-specific b value calculated by ZMAP
and the zone-specific b value calculated using Weichert (1980) method each,
and 40 % weight is given to the regional b value since the number of
data in this zone are larger and the estimated b value is statistically more
stable. Finally, the b value for the background zone (limits shown in
Fig. 5) is calculated as 0.81 by removing the earthquakes within the buffer
zones. Uncertainty in the b value of the background zone is determined using
the method proposed by Shi and Bolt (1982) and included in the logic tree
(Table 2).
Cumulative rates of earthquakes for the magnitude recurrence model
and associated events (moment-balancing graphs) for (a) Izmit,
(b) Düzce, (c) central Marmara, and
(d) Ganos/Saros rupture systems. Black points are the earthquakes
associated with the rupture system; purple and blue lines show the
single-segment and multi-segment ruptures; the red broken line is the weighted
average of the magnitude recurrence model. In these graphs, the median values
of the slip rates and Mmax and zone-specific b values are
utilized.
Estimated b values are relatively small when compared to the b values
estimated for large areas (b≈1); however, our findings are
consistent with the current literature. Şeşetyan et al. (2016)
provided a thorough analysis of the b value for the Turkish territory
and proposed that b=0.77 for the central Marmara region and b=0.67 for the
North Anatolian fault zone (Fig. 15 of Şeşetyan et al., 2016). The small
differences in the b values proposed by Şeşetyan et al. (2016) and
the b values estimated in this study are due to the geometry of the
selected zones and the differences in the compiled catalogs. The b value
used by Moschetti et al. (2015) for the western United States (b=0.8) is not
very different to our estimates.
Magnitude recurrence models – seismic moments
Seismic sources can generate varied sizes of earthquakes and magnitude
distribution models describe the relative rates of these small, moderate and
large earthquakes. The basic and the most common magnitude frequency
distribution (MFD) is the exponential model proposed by Gutenberg and Richter (1944)
(G–R). Since there is a maximum magnitude that the source can
produce and a minimum magnitude for engineering interest, the G–R
distribution is usually truncated at both ends and renormalized so that it
integrates to unity. The truncated exponential MFD (Cosentino et al., 1977)
is given in Eq. (1):
fmTE(M)=βexp(-β(M-Mmin))1-exp(-β(Mmax-Mmin)),
where β=ln(10)×b value, Mmin is the minimum
magnitude, and Mmax is the maximum magnitude. Youngs and
Coppersmith (1985) proposed that the truncated exponential distribution is
suitable for large regions or regions with multiple faults, but in most cases
it does not work well for individual faults. Instead, individual faults may tend
to rupture at what have been termed “characteristic” size
events and the alternative magnitude distribution for this case is the
characteristic model proposed by Schwartz and Coppersmith (1984). In
characteristic MFD, once a fault begins to rupture in large earthquakes, it
tends to rupture the entire fault segment and produce similar size
earthquakes due to the geometry of the fault. It is notable that the
characteristic model does not consider the small-to-moderate magnitude
earthquakes on a fault. A third model was proposed by Youngs and Coppersmith
in 1985 that combines the truncated exponential and characteristic magnitude
distributions as shown in Eqs. (2) and (3):
fmYC(M)=11+c2×βexp(-β(M‾char-Mmin-1.25))1-exp(-β(M‾char-Mmin-0.25))forM‾char-0.25<M≤M‾char+0.2511+c2×βexp(-β(M-Mmin))1-exp(-β(M‾char-Mmin-0.25))forMmin<M≤M‾char-0.25,
where
c2=0.5βexp(-β(M‾char-Mmin-1.25))1-exp(-β(Mchar-Mmin-0.25)),
and Mchar is the characteristic earthquake magnitude. Coupling
the truncated exponential MFD with seismic sources defined by planar fault
geometries results in unrealistically high rates for small-to-moderate
magnitude events (Hecker et al., 2013), especially in the close vicinity of the NAFZ (Gülerce and Vakilinezhad, 2015). Therefore, the composite MFD
proposed by Youngs and Coppersmith (1985) is utilized to represent the
relative rates of small, moderate and large-magnitude earthquakes generated
by rupture sources defined in this study.
Aleatory variability for style of faulting in the background zone.
Style of faulting WeightsNormalStrike slipReverseNormal-oblique150 km radius background zone0.200.750.05All segments except Çınarcık Fault–1.00–Northern and southern Çınarcık segments–––1.0
Spatial distribution of the activity rates in the smoothed
seismicity source. Red circles are the earthquakes used in the analysis.
Aleatory variability in the rupture scenario weights.
The rupture systems presented in Sect. 2 include more than one subsegment.
We adopted the terminology of WGCEP (2003) and defined the rupture source as
a fault subsegment or a combination of multiple adjacent fault subsegments
that may rupture and produce an earthquake in the future. For Düzce,
central Marmara, and Ganos/Saros rupture systems with two subsegments (as A
and B), three different rupture sources can be defined: single segment
sources (A and B) and a two-subsegment source (A + B). Any possible
combination of rupture sources that describes the complete rupture of the
system is defined as the rupture scenario. Two rupture scenarios for these
rupture systems are (1) rupture of the two subsegments individually and
(2) rupture of the two subsegments together. The rupture model includes the
weighted combination of rupture scenarios of the rupture system. Five
segments defined for Izmit rupture systems form a rupture model with
15 rupture sources and 16 rupture scenarios (Table 5). The minimum magnitude
(Mmin) is set to Mw=4.0 for all rupture sources
considering the completeness magnitude. Mean characteristic magnitudes
(Mchar) for each rupture source are calculated using the
relationships proposed by Wells and Coppersmith (1994) and Hanks and Bakun
(2014). The Mchar values calculated using both equations are
quite close to each other and the absolute value of the difference is smaller
than 0.13 in magnitude units (Table 6). To grasp the epistemic uncertainty,
the average of the Mchar value from both methods are utilized in the
center of the logic tree with 50 % weight and both the Mchar-0.15 and Mchar+0.15 values are included by assigning 25 %
weight. The upper bound for the magnitude PDF (Mmax) is
determined by adding 0.25 magnitude units to Mchar for
each source in each logic tree branch (Table 6).
MFD only represents the relative rate of different magnitude earthquakes. In
order to calculate the absolute rate of events, the activity rate
N(Mmin) defined as the rate of earthquakes above the minimum
magnitude should be used. For areal sources, N(Mmin) may be
calculated by using the seismicity within the defined area. For planar fault
sources, the activity rate is defined by the balance between the accumulated
and released seismic moments as shown in Eq. (4). The accumulated seismic
moment is a function of the annual slip rate (S) in cmyr-1,
area of the fault (A in cm2), and the shear modulus of the crust
(μ=30×1012dynecm-2, Brodsky et al., 2000; Field
et al., 2009). The S for the rupture sources that includes more than one
segment with different S values are calculated using the weighted average
of annual slip rates (weights are determined based on the area of the segment
as shown in Eq. 5).
Rupture sources and rupture scenarios utilized for the Izmit rupture
systema.
a Note: rows show the rupture scenarios and the columns show the
rupture sources. 1 and 0 in a cell indicate that the particular rupture
source is included or excluded in the rupture scenario, respectively.
Scenario weights are given in the last column. For subsegments 3, 2_1,
2_2, 2_3, and 1, please refer to Fig. 1b.
N(Mmin)=μAS∫MminMmaxfm(Mw)101.5Mw+16.05dMSsource=∑all segments for the sourceSsegment×Asegment∑all segments for the sourceAsegment
Ultimately the MFD and the activity rate are used to calculate the magnitude recurrence relation, N(M), as shown in
Eq. (6).
N(M)=N(Mmin)∫MminMmaxfm(Mw)dM
The magnitude recurrence relation given in Eq. (6) and the accuracy of the
model parameters such as the b value or Mmax shall be tested by
the relative frequency of the seismicity associated with the source in the
moment-balanced PSHA procedure. Therefore, a weight is assigned to each
rupture scenario and the cumulative rates of events attributed to that
particular rupture system are plotted along with the weighted average of the
rupture scenarios to calibrate the assigned weights and to evaluate the
balance of the accumulated and released seismic moment. The
moment-balancing graphs for Izmit, Düzce, central Marmara,
and Ganos/Saros rupture systems are provided in Fig. 4 and used to compare
the modeled seismicity rate with the instrumental earthquake catalog. In
these plots, the black dots stand for the cumulative annual rates of
earthquakes and the error bars represent the uncertainty introduced by
unequal periods of observation for different magnitudes (Weichert, 1980). In
Fig. 4, the scenarios that are separated by plus signs in the legend are the
scenarios with multiple rupture sources. When multiple segments rupture
together, these scenarios are separated by a comma sign in the legend. For
example, the “S4, S5” line in Fig. 4c represents the scenario where S4
and S5 subsegments are ruptured individually. This scenario brings in
relatively higher rates for small-to-moderate earthquakes when compared to
the S4 + S5 scenario, which represents the rupture of these two segments
together to produce a larger event.
The best fit between the cumulative annual rate of events and the weighted
average of rupture scenarios (red dashed lines) is established by modifying
the weights of the rupture scenarios by visual interpretation. To achieve
a good fit, the seismic source modeler needs to understand the contribution
of the magnitude recurrence model parameters to the red broken line in
different magnitude ranges. For example, the b value significantly affects
the small magnitude portion of the curve, since the Youngs and Coppersmith
(1985) magnitude PDF is used. Please remember that the b value is calculated
based on the same catalog but for a larger region. Defining a large number
of subsegments for a rupture system also increases the cumulative rate of
small magnitude events. The good fit in the small magnitude range of Fig. 4
shows that (i) the b value calculated using the larger zone is compatible
with the seismicity associated with the planar source, (ii) the utilized
segmentation model is consistent with the relative rates of small-to-moderate
and large events, and (iii) the annual slip rate is compatible with the
seismicity over the fault. The large magnitude rates in Fig. 4 are poorly
constrained since the catalog used herein only covers 110 years and that
time span is obviously shorter than the recurrence rate for the large-magnitude event. Hecker et al. (2013) explained that by “rates of
large-magnitude earthquakes on individual faults are so low that the
historical record is not long enough to test this part of the distribution”
and suggested using the “inter-event variability of
surface-rupturing displacement at a point as derived from geologic data
sets” to test the upper part of the earthquake-magnitude distribution. In
each moment-balancing plot, relatively higher weights are assigned to the
rupture scenarios that combine the individual (single-segment) rupture
sources based on the assumption (and modeler's preference) that
single-segment ruptures are more likely than multiple-segment ruptures. The
weights assigned to each rupture scenario are given in Table 4.
Logic tree representing epistemic uncertainty in maximum magnitudes.
Weights for Mmax1, Mmax2, and Mmax3 are 0.25, 0.5,
and 0.25, respectively (WC94: Wells and Coppersmith, 1994, and HB14: Hanks
and Bakun, 2014, magnitude–rupture-area relation).
Mean and fractals of the single-segment and multi-segment rupture
scenarios with the cumulative rate of earthquakes associated with the rupture
system for (a) Izmit, (b) Düzce, (c) central
Marmara, and (d) Ganos/Saros rupture systems. Solid lines are the
mean rates and the dashed lines show the 5 and 95 % rates for each
rupture scenario.
Background zone – smoothed seismicity
A background source zone of diffused seismicity is utilized to characterize
the seismicity that is not associated with the rupture systems described in
the previous sections. This additional background source zone represents the
seismicity associated with the mapped active faults to the south of Marmara
Sea (orange fault lines in Fig. 1a) and the interpretation that even in areas
where active faults or distinctive zones of seismicity clusters are not
observed, earthquakes can still occur. Figure 1c shows that the spatial
distribution of the earthquakes (outside the buffer zones around the rupture
systems) is not homogeneous: the density of the events increases significantly
around the Geyve–Iznik fault zone. Therefore, defining an areal source zone
with homogeneous seismicity distribution would result in the overestimation
of the seismic hazard in Istanbul. Instead, the background source is modeled
as a source of gridded seismicity where the earthquakes are represented as
point or planar fault sources at the centers of evenly spaced grid cells
(0.05∘ spacing). The truncated exponential magnitude distribution
(Eq. 1) is selected to represent the relative frequency of the different
magnitude events for this source. In the magnitude recurrence model,
spatially uniform Mmax and b values and spatially variable
a values, or seismicity rates, are defined. The minimum magnitude
(Mmin) is again set to Mw=4.0 and the b value is
taken as 0.81. The a value for each grid cell was calculated from the
maximum likelihood method of Weichert (1980), based on events with magnitudes
of 4.0 and larger. The gridded a values were then smoothed by using an
isotropic Gaussian kernel with a correlation distance of 10 km
(Frankel, 1995). The smoothed-seismicity rates overlying the earthquakes
outside the buffer zones are presented in Fig. 5. Tabulated values of the
grid cell coordinates and the seismicity rates are provided in the
Supplement. During the calculations of the smoothed seismicity rates, the
earthquakes in buffer zones are not included in smoothing (and not
double-counted). The buffer zones are only used to “associate” the
earthquakes with the fault zones and collapse the earthquakes to the vertical
fault planes. Therefore, the background source and the fault sources can be
superposed in the PSHA calculations.
The Mmax distribution of the background zone is developed by
taking into account the lack of evidence for surface faulting in the city of
Istanbul. So far, no active fault has been reported from the near vicinity of
the study area. Similarly, the MTA Active Fault Map of Turkey (Emre et al., 2013) does
not contain any active fault in the northern part of the NAFZ between Izmit
and Tekirdağ. Moschetti et al. (2015) mentioned that the development of
the Mmax model for shallow crustal seismicity in the western
United States benefits from the large set of regional earthquake magnitudes
from the historical and paleoseismic records; however, the background
seismicity model accounts for earthquake ruptures on unknown faults;
therefore, the Mmax distribution must reflect the range of
possible magnitudes for these earthquakes. We adopted a similar approach
using the fault segments of the southern strand of the NAFZ documented in
Murru et al. (2016) and calculated the characteristic magnitude for each
segment using Wells and Coppersmith (1994) magnitude–rupture-area relation.
Based on the estimations of characteristic magnitude of earthquakes that may
occur on the southern strand of the NAFZ, the logic tree for Mmax
(centered on Mw=6.8) of the background zone is developed
(Table 6). The focal mechanisms of the background source should reflect the
tectonic style of the parent region; therefore, a weighted combination of
strike-slip (SS, 75 %), normal (N, 20 %), and reverse (R, 5 %)
motions with weights that sum to 1 is assigned to this source (Table 3).
A uniform distribution of focal depths between the surface and 18 km
depth is utilized (Emre et al., 2016).
Discussions on the uncertainty involved in the proposed SSC model
In the proposed SSC model, the uncertainties related to Mmax,
magnitude–rupture-area relations, magnitude recurrence model parameters, and
the annual slip rates are considered and included in the logic tree
(Supplement). On the other hand, the uncertainty related to the fault
geometry such as the uncertainty in segment lengths, fault widths, and dip
angles remained unexplored. All rupture sources within each rupture system
are thought to occur in order to capture the aleatory variability in the
extent and potential size of future ruptures. However, the epistemic
uncertainty in the potential rupture scenarios are not taken into account
since only one set of weights for each rupture scenario is included in the
logic tree. In order to compare the epistemic uncertainty in the proposed SSC
model with the uncertainty in the earthquake catalog, the SSC model
fractals for each rupture source are calculated and the extreme values
represented by the single-segment rupture sources and full-span rupture
source are presented in Fig. 6 with red and blue sets of curves, respectively.
It is notable that the rates of observed earthquakes were used to validate the
rupture scenario weights in Fig. 4, aiming to capture a good fit between
weighted average rates and the mean rates of observed earthquakes. Figure 6
shows that the uncertainty range sampled by the proposed model is consistent
with the rate of earthquakes associated with each rupture system, especially
for Mw<6 events that have a large sample size.
We would like to emphasize that the SSC model presented here is different
to the models proposed by Gülerce and Ocak (2013) and Murru
et al. (2016): differences in the fault geometry are minor but the differences
in the magnitude recurrence models and the time-dependent probabilities of
earthquakes are more significant. Unfortunately, earlier publications did not
provide enough information on earthquake rates to do a case-to-case
comparison of the earthquake rates proposed herein with the previous works.
Our model does not utilize the time-dependent hazard methodologies as in
Murru et al. (2016); however, we believe that ongoing research on the
paleoseismic recurrence periods (National Earthquake Strategy and Action Plan
for 2023, NESAP-2023) will provide a substantial contribution in the PSHA
practice of Turkey and eventually lead to a change in the hazard estimates.
The available paleoseismic data on NAFZ are too few and insufficient to provide
meaningful constraints on the “grand inversion” as used in the UCERF3 model for
California (Field et al., 2014). Therefore, the proposed model does not include
fault-to-fault ruptures that can jump over the boundaries of the defined
rupture systems.
Conclusions
This paper presents details of the SSC model proposed for the PSHA
studies in Istanbul. When compared to the previous SSC models developed for
this region, significant improvements in the proposed model can be listed as
follows: (1) planar seismic sources that account for the most current
tectonic information (e.g., updated fault maps) are built, (2) the
multi-segment rupture scenarios are systematically utilized in the rupture
forecast, (3) buffer zones around the rupture systems are defined to
associate the small, moderate, and large-magnitude events with the rupture
systems, (4) activity rates for the planar rupture systems are calculated
using the geological and geodedic constraints (e.g., slip rate and fault
geometry), (5) a balance of the accumulated and released seismic moment is
considered in building the magnitude recurrence model, and (6) associated
earthquakes are used to test the suitability of the magnitude recurrence
model with the instrumental seismicity rates. Even though the rupture systems
developed in this study account for the relative rates of small, moderate, and
large-magnitude events that can occur on the faults, a background source is
defined to represent the small-to-moderate magnitude earthquakes that may
take place anywhere in the vicinity of Istanbul and the Marmara Sea. Properties
of the rupture systems and background source, the logic tree associated with
both of these components, coordinates of the fault segments, and smoothed
seismicity rates are fully documented throughout the text and in the
Supplement. Therefore, the proposed SSC model can be directly implemented to any
of the available PSHA software for the site-specific PSHA analysis in
Istanbul. We would like to underline that the geometry and the earthquake
rates of the background source may be modified for any application outside
the greater Istanbul area. The hazard analyst can incorporate the full
rupture model and the complete logic tree provided in this paper into most
of the available hazard codes without explicitly calculating the earthquake
rates. In the case that the earthquake rate has to be incorporated to the hazard
code; the earthquake rates for each branch of the logic tree given in the
Supplement can be used.
Most of the data related to the seismo-tectonic database are
available on the institutional websites. Details of the seismic source model
input are provided in the Supplement. No other data set from this article is
publicly available.
The Supplement related to this article is available online at https://doi.org/10.5194/nhess-17-2365-2017-supplement.
The authors declare that they have no conflict of interest.
This article is part of the special issue “Linking faults to
seismic hazard assessment in Europe”. It is not associated with a conference.
Acknowledgements
Authors of this manuscript are grateful for the support provided by the Turkish
Atomic Energy Authority (TAEK). This work was partially supported by the
Pacific Gas & Electric Company Geosciences Department. The authors are
thankful to the guest editors and anonymous reviewers for their insightful
comments. Edited by: Bruno Pace
Reviewed by: two anonymous referees
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