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Large Stress Release During Normal-Faulting Earthquakes in Western Turkey Supported
by Broadband Ground Motion Simulations
GULUM TANıRCAN,1 HIROE MIYAKE,2,3 HIROAKI YAMANAKA,4 and OGUZ OZEL5
Abstract—This article investigates the stress drop variability of
shallow normal-faulting earthquakes in western Anatolia through
strong-motion simulations. For this purpose, source characteristics
of three moderate to large magnitude events are constrained by the
empirical Green’s function simulation in a broadband frequency
range. Recordings of ten strong-motion stations in 78-km epicentral
distance range are utilized for the simulation. Estimated strong-
motion generation areas (SMGAs) are 22 km2, 66 km2, and
110 km2 where rise times are 0.6 s, 0.7 s, and 0.6 s, respectively, for
the 2011 Simav (Mw 5.8), 2017 Lesvos (Mw 6.3), and 2017 Bodrum-
Kos (Mw 6.6) earthquakes. Those values are found to be consistent
with global source-scaling relationships. One particular observation
is that stress drop ratios between the mainshock and aftershock for
all events are relatively large compared with those previously cal-
culated for strike-slip events in Turkey. Stress drops of SMGAs for
the Simav and Bodrum earthquakes are in the range of 25 MPa, and
this value drops to 19 MPa for the Lesvos earthquake. To further
investigate the stress drop variation of earthquakes in Western
Anatolia, an earthquake source database (fc-Mo) offered by
Yamanaka et al. (IAG-IASPEI 2017, S07-1-03, 2017) is utilized.
Brune (Journal of Geophysical Research, 76:5002, 1971) stress drop
values of [ 360 small to moderate earthquakes (Mw 3.0–6.0) are
calculated with the given corner frequency and seismic moment
information assuming a constant shear wave velocity. Results
indicate that the majority of the earthquakes have a stress drop value
\ 5 MPa. This value changes to between 5 and 57 MPa for the
remaining earthquakes. These high stress drop values support the
former findings stating that normal-faulting earthquakes may release
higher stress than strike-slip earthquakes. This indicates that the
regional stress regime in western Turkey may cause relatively larger
stress release during the normal-faulting mainshocks.
Keywords: Normal faulting, ground motion, empirical
Green’s function method, stress drop.
1. Introduction
It is well known that knowledge on the properties
of stress release during an earthquake is important to
better model the seismic source for both probabilistic
and deterministic seismic hazard assessments. Recent
studies suggest that the variation of stress drop is
dependent on focal depth (Asano and Iwata 2011;
Somei et al. 2014), regional characteristics (Oth
2013), and faulting type (Cocco and Rovelli 1989;
Konstantinou 2014; Thingbaijam et al. 2017). In
addition, Nakano et al. (2015) indicate that the
mainshocks release larger stress drops than their
aftershocks.
Even though there is a vast database of normal-
faulting crustal earthquakes such as the 1985 Borah
Peak, 2009 L’Aquila, 2011 Fukushima-Hamadori,
and 2016 Amatrice earthquakes, reports on ground
motion modeling for normal-faulting earthquakes are
few (e.g., Anderson et al. 2013; Aochi and Miyake
2018) compared with the studies on strike-slip and
thrust faulting. Existing studies on normal-faulting
earthquakes generally target the source characteristics
of the mainshock except for studies in Italy by Cocco
and Rovelli (1989) and in Greece by Margaris and
Boore (1998) and Margaris and Hatzidimitriou
(2002). In this respect, source analyses of normal-
faulting earthquakes in the broadband frequency
range can help the understanding of stress release
during earthquakes in western Turkey, consequently
contributing to seismic hazard assessment studies,
i.e., in the calibration of ground motion prediction
1 Kandilli Observatory and Earthquake Research Institute
(KOERI), Bogazici University, 34684 Istanbul, Turkey. E-mail:
[email protected] Center for Integrated Disaster Information Research,
Interfaculty Initiative in Information Studies, The University of
Tokyo, Tokyo, Japan.3 Earthquake Research Institute, The University of Tokyo,
Tokyo, Japan.4 Department of Architecture and Building Engineering,
Tokyo Institute of Technology, Yokohama, Japan.5 Department of Geophysics, Engineering Faculty, Istanbul
University-Cerrahpasa, Avcilar Campus, Avcilar, 34320 Istanbul,
Turkey.
Pure Appl. Geophys. 177 (2020), 1969–1981
� 2019 Springer Nature Switzerland AG
https://doi.org/10.1007/s00024-019-02357-3 Pure and Applied Geophysics
equations and generation of synthetic ground
motions.
Objectives of the study are twofold, encompass-
ing the calculation of source characteristics of the
recent normal-faulting earthquakes in western Turkey
with an emphasis on stress drop and investigation of
the scaling relationships between the various source
parameters such as strong-motion generation area
(SMGA), rise time, and stress drop with respect to
seismic moment.
2. Normal-Faulting Earthquakes in Western Turkey
The Aegean region is one of the most rapidly
moved and seismically active parts of the world. The
tectonics of the Aegean region is dominated by N–S
directed extensional motions due to subduction of the
oceanic lithosphere under the Aegean Plate in the
Hellenic Arch and westward strike-slip motions along
the North Anatolian Fault Zone (Sengor et al. 1985;
Seyitoglu and Scott 1991). The region contains sev-
eral morphologically prominent active normal faults
with E–W and SW–NE directions at a rate of about
30–40 mm/year (Mc Kenzie 1978; Taymaz et al.
1991). As a result, continuous seismic activity is
observed in the region. Only in the last decade,[40
earthquakes with magnitude[ 5 occurred within the
boundary of 25–31 E 36–40 N (KOERI). Among
those, the 2011 Simav (Mw 5.8), 2017 Lesvos (Mw
6.3), and 2017 Bodrum-Kos (Mw 6.6) earthquakes are
the most noticeable events (Fig. 1). We investigate
ground motion and spectral distributions of the three
above-mentioned prominent normal-faulting earth-
quakes with available strong-motion data from
national and private networks (see http://kyh.deprem.
gov.tr/indexen.htm, Alcık et al. 2017). Geometric
mean horizontal ground accelerations (PGAs) and
spectral accelerations of linear 5% damped single-
degree-of-freedom (SDOF) systems at 0.2 s and 1 s
structural periods (SA) are shown as a function of
distance in Fig. 2. Two GMPEs applicable to Turkey,
Kale et al. (2015) (hereafter, KAAH2015) and Boore
et al. (2014) (hereafter, BSSA14), are also shown in
Fig. 2. GMPEs are calculated using the time-average
shear velocity in the upper 30 m, Vs30 as 760 m/s.
The reported Vs30s of the recording stations are \
760 m/s; hence, the given estimations can be con-
sidered the mean lower bound. It is shown clearly that
overestimation of PGA by GMPEs is[- 1 standard
error at the closest station recordings of the 2017
Bodrum-Kos normal-faulting earthquake. Recently,
Akkar et al. (2018) performed residual analyses of
strong-motion data compiled from the Marmara and
Aegean regions in Turkey and stated that KAAH15
tends to overestimate the PGA and SA at short and
1-s periods. Such a discrepancy can be the combi-
nation of various factors. First, the database used to
derive the GMPE includes more strike-slip events;
therefore, the regional differences in stress drop are
not taken into account in KAAH15. Second,
KAAH2015 does not isolate path and source effects
in their data set used for developing the model, which
may mask some of the crustal features between the
regions (e.g., Moho boundary). Since the GMPEs
include uncertainty at near-source distance, it would
be useful to investigate ground motion amplitude
levels with independent analyses. These indications
motivate us to examine the stress parameters of the
well-recorded normal-faulting earthquakes.
3. Ground Motion Modeling: Empirical Green’s
Function Method
The empirical Green’s function (EGF) method of
Irikura (1986) and Irikura and Kamae (1994) is one of
the fastest and most accurate ways to image the
strong-motion generation area (SMGA) provided that
strong-motion recordings of mainshock-aftershock
couples are available at the epicentral area. The
method essentially uses the small event record as the
EGF and sums them up to follow the omega-squared
source scaling law. The large event is synthesized
from the linear superposition of a small event, which
almost collocates with a large event. The synthetic
motion for the large event is given using the small
event u(t) by the following equation:
UðtÞ ¼XN
1
XN
1
r
rijFðtÞ � ðCuðtÞÞ; ð1Þ
The scaling parameters N and C are the ratios of
SMGA dimensions and stress drops between the large
1970 G. Tanırcan et al. Pure Appl. Geophys.
and small event, respectively. r and rij are the dis-
tance from the hypocenter of the small event and
from (i,j) element to the site. F(t) is the filtering
function. In this model, SMGA is considered a
homogeneous rectangular fault plane in the total
rupture area having large slip velocity and capable of
reproducing near-source strong ground motions in the
broadband frequency range (Miyake et al. 2003 ). The
SMGA is subdivided (N 9 N) into small elements so
as to match the fault size of the small event, which is
used as the EGF. Both main and small events are
assumed to follow the w2 spectral scaling model.
The C and N values can be estimated by spectral
analysis of the large and small event’s waveform data
using the following similarity low:
Uo
uo¼ Mo
mo¼ CN3 Ao
ao¼ CN fca
fcm¼ N ð2Þ
where Uo, uo, Ao, and ao correspond to the flat level
of the displacement and acceleration spectra of large
and small events, respectively. M0=m0indicates the
seismic moment ratio between a large and small
event at the lowest frequency; fcm and fca, respec-
tively, are corner frequencies of the large and small
events. Details of the simulation technique are
explained in Miyake et al. (2003).
4. Strong-motion Data and Data Processing
These normal-faulting earthquakes as well as their
aftershocks are well recorded by vast strong-motion
stations of the Disaster and Emergency Management
Authority (AFAD) at various epicentral distances
(see http://kyh.deprem.gov.tr/indexen.htm). The
Figure 1Map of the study area with epicenters of the target normal-faulting earthquakes and strong-motion stations (red star: mainshock, black star:
aftershock) and focal mechanisms (mainshock only). Red lines are the active fault maps of the region
Vol. 177, (2020) Large Stress Release during Normal-faulting Earthquakes 1971
instruments installed at these stations are three-com-
ponent accelerometers with 100-Hz sampling
frequency.
The present study uses an aftershock recording per
earthquake as the small event (Fig. 1). The mainshock
and aftershock information is listed in Table 1. The
EGF simulation is performed using the strong-motion
data of the 2011 Simav and 2017 Lesvos earthquakes
at three stations and the 2017 Bodrum-Kos earthquake
at four stations. Epicentral distances of stations having
large and small event recording couples are in
between 7 and 78 km (Fig. 1).
As mentioned in above section, EGF simulation
necessitates two essential source scaling parameters:
C and N. These values are calculated beforehand using
the S-wave portion of the horizontal components of
the waveforms at each station. To do that, we calcu-
lated the Fourier amplitude spectrum (FAS) from the
vector summation of horizontal components at each
station and applied ± 10% logarithmic smoothing.
Figure 3 illustrates the FAS and smoothed FAS of
mainshock and aftershock recordings normalized by
their hypocental distances. Their corner frequencies
can also be tracked from the same figure. Flat levels of
FAS of large and small events as well as their corner
frequencies enable us to estimate source scaling
parameters N and C between the large and small
events following the relationships given in Eq. (2).
Estimated N and C values are 4 and 6 for the Simav
event, and they are 5 and 4.4 for the Midilli and 5 and
4.5 for the Bodrum-Kos events, respectively.
5. Simulation
Other source parameters necessary for the simu-
lation are collected from the literature. The 2011
Simav and 2017 Lesvos earthquakes have been well
Figure 2Comparison of 5% damped pseudo-spectral acceleration ordinates of the 2011 Simav, 2017 Lesvos, and 2017 Bodrum-Kos earthquakes with
the most recent GMPE’s applicable to Turkey (BSSA 2014: Boore et al. 2014, KAAH2014: Kale et al. 2015). GMPE medians are shown for
Rjb = 760 m/s. Dotted lines are the ± 1 sigma values
1972 G. Tanırcan et al. Pure Appl. Geophys.
studied by many researchers with different methods
(e.g., Yolsal-Cevikbilen et al. 2014; Demirci et al.
2015; Papadimitrioua et al. 2018), while the dipping
direction of the 2017 Bodrum-Kos earthquake has
been questioned. Just after the earthquake, Kiratzi
(2018) inverted low-frequency strong-motion data of
the event considering both nodal planes, finding that a
south-dipping plane provides a better fit to the data.
Later, Karasozen et al. (2018) modeled the InSAR
and GPS surface displacement to solve the geometry
and slip distribution of the event. The final slip
model, which provides a good match to both the
InSAR and GPS data, is achieved with the north-
dipping fault model. Recently, a similar study was
performed by Konca et al. (2019) to portray the fault
geometry, fault location, seismicity, and slip distri-
bution of the earthquake. They found that the north-
dipping fault geometry best fits the geodetic data.
Even though south-dipping fault geometry also gives
acceptable fitting to the geodetic data, the hypocenter
is 9 km from the fault plane. Re-located seismicity
distribution following the mainshock also implies a
north-dipping fault plane is more likely. Hence, a
north-dipping fault was considered in this analysis.
Since there is no information available for the focal
mechanisms of the aftershocks, the source
mechanisms of the mainshocks and the aftershocks
were assumed to be the same or similar after check-
ing the polarity of aftershock ground motion
waveforms.
The upper frequency of the simulation is limited
to 10 Hz where the lower frequency limit is set based
on the signal-to-noise ratio of the small event, which
ranged from 0.3 to 1.0 Hz for horizontal components.
The average S-wave velocity around the hypocenter
and rupture velocity are 3.5 km/s and 2.8 km/s,
respectively, for the Lesvos and Bodrum regions.
These values are slightly lower at the Simav region
(Cubuk-Sabuncu et al. 2017), 3.2 km/s and 2.56 km/
s, respectively.
Once N and C are determined through the Fourier
amplitude spectral analysis, forward simulations are
performed several times with variable aftershock
source parameters: (1) rise time (0.1–0.2 s), (2)
dimension of the subfault size (1.0–2.5 km), and (3)
all combinations of the rupture starting point in strike
and dip directions. The most appropriate source
model is decided by the smallest average absolute
residual, log(PSAobs)–log(PSAsyn), of observed and
simulated pseudo-spectral acceleration (PSA) in
terms of logarithmic form at 33 structural periods
between 0.01 and 2 s.
Table 1
Earthquake locations and their source parameters. Earthquake location information is collected from KOERI-RETMC catalog (http://www.
koeri.boun.edu.tr/sismo/2/en/)
Event name Date origin time
(GMT)
Mw Mo (Nm)a
E17
fc
(Hz)
Location Vs
Vr
(km/
s)
Focal
MECH.
Station code
Lat.
(N)
Lon
(E)
h
(km)
Strike/dip/
slip
(�)
Simav mainshock 19/05/2011 20:15 5.8 8.70 0.69 39.15 29.09 8 3.2 287/58/-94
(YCTH14)
#4304
#4306
#4504
Simav aftershock 07/06/2011 22:52 4.4 – 2.055 39.08 29.06 5 2.56
Lesvos mainshock 12.06.2017 12:28 6.4 43.1 0.18 38.85 26.35 13 3.5 122/40/-83
(P18)
#1005
#1720
#3535
Lesvos aftershock 12.06.2017 14:19 4.4 – 0.9 38.85 26.38 12 2.8
Bodrum-Kos
mainshock
20.07.2017 22:31 6.6 116 0.23 36.97 27.41 10 3.5 279/37/-75
(K18)
#4809
#4812 #4819
#4817Bodrum-Kos
aftershock
24.10.2017 09:36 4.8 – 1.16 36.98 27.40 10 2.8
aSeismic moment information is taken from the Harvard GCMT Catalogue (www.globalcmt.org). Names of recording stations are also listed.
References of focal mechanisms are: YCTH14: Yolsal-Cevikbilen et al. (2014); P18: Papadimitrioua et al. (2018); K18: Karasozen et al.
(2018)
Vol. 177, (2020) Large Stress Release during Normal-faulting Earthquakes 1973
residualj j¼ 1
NT NstNcomp
XNst
1
XNcomp
1
XNT
1
res T ; comp,stð Þj j
ð3Þ
where NT, Nst, and Ncomp are the number of the
structural period, station, and components.
6. Results and Discussion
A simple comparison can be made through the
peak values and waveforms (Figs. 4, 5, 6). Observed
and simulated waveforms in horizontal directions
(providing the minimum absolute residuals of PSA)
and their PSA are given for each earthquake in the
related figures. In general, synthetic waveforms agree
with the observed ones in the broadband period
range. Peak accelerations, velocities, and displace-
ments are caught by EGF simulations for all events.
Synthetic spectral values at the short period are
overestimated at some stations, though. The good
phase fitting between the observed and synthetic
waveforms as well as acceptable PSA values obtained
from the EGF simulation shows the validity of the
estimated source model. Amplitude differences, on
the other hand, are believed to have arisen from the
focal mechanism assumption of the aftershock due to
the limited available data.
Ideally, earthquakes are expected to have good
azimuthal coverage for modeling. However, this
condition is not fulfilled for the case of the Lesvos
event, since all stations are located in western Tur-
key. Even so, reasonable waveform fittings,
particularly in velocity and displacement, indicate the
accuracy of the source parameters. It is believed that
the resolution in all simulations is kept the same by
using recordings approximately at the same number
of stations, at similar distances, and at similar azi-
muths. Table 2 summarizes the source parameters
obtained by optimal fit of the waveforms. All source
models are composed of one large SMGA with a rise
time of 0.6–0.7 s, which agrees with findings of
Figure 3Fourier amplitude spectra of mainshock and aftershock recordings for the Simav, Lesvos, and Bodrum-Kos events
1974 G. Tanırcan et al. Pure Appl. Geophys.
previous studies mentioned in the previous chapter.
Additionally, the source parameters of the after-
shocks providing the best results in the mainshock
simulation with EGF are presented in Table 3.
To judge the consistency of the source parameters
with global scaling relationships, we compared the
M0-SMGA and M0-rise time of the three normal-
faulting earthquakes in western Turkey with the
relationship proposed by Somerville et al. (1999)
(Fig. 7). We found that scaling is very similar to the
relationship, although some variations are seen in the
M0-rise time scaling for the M6.6 earthquake. It
should be noted that all SMGA and rise time esti-
mates are performed using the same EGF code to
minimize the variability of the method and/or code.
The comparison indicates that the mainshock char-
acteristics of the three normal faultings in western
Turkey do not show significant offsets. It was also
found that the residual function used for comparison
is very sensitive to the rupture starting point and
subfault size, as expected, but not sensitive to small
increments (\ 0.1 s) in rise time; hence, small devi-
ations in rise time are possible.
6.1. Discussion of Stress Drop
The large C values found for three normal-
faulting earthquakes prompt us to examine previous
earthquakes simulated with the same EGF method. A
list of earthquakes, their magnitudes, style of faulting,
and C values is given in Table 4. C values of strike-
slip earthquakes are systematically lower than those
of normal-faulting earthquakes. Pursuing the exis-
tence of large stress release, we also calculate the
stress drop of SMGAs of all events in Table 4
according to Brune (Brune 1970, 1971; Dr = 7/
16(M0/r3), where r is the equivalent radius of small
events’ SMGA assumption. Among them, the highest
stress drop belongs to the 2011 Simav earthquake
with 25 MPa; the 2017 Bodrum-Kos earthquake
follows in second place with 24 MPa. This values
drops to 19 MPa for the Lesvos earthquake.
Figure 4Observed (upper traces) and simulated (lower traces) acceleration, velocity, and displacement waveforms for the horizontal components the
2011 Simav earthquake at stations #4306, #4304, and #4504. The numbers above the waveforms correspond to the maximum amplitudes.
PSAs (5% damped) of observed and simulated horizontal components are also compared in the right panel
Vol. 177, (2020) Large Stress Release during Normal-faulting Earthquakes 1975
Looking at past research on stress estimations in
western Turkey, Margaris and Boore (1998) investi-
gated the source parameters of six normal-faulting
earthquakes in Greece from strong-motion data
analyses and came up with an average stress drop
value of 5.6 MPa. In addition, Margaris and
Hatzidimitriou (2002) expanded the stress drop
estimates for more earthquakes in Greece and con-
cluded the average stress drop is still 5.5 MPa, but
there is a significant difference among the thrust-
faultings with larger stress drops and normal and
strike-slip fault seismic events. Allmann and Shearer
(2009) calculated the stress drop values of 2000
shallow global earthquakes that occurred between
1990 and 2007 using teleseismic recordings. They
found that the stress drop generally varies in a very
wide range, from 0.1 to 100 MPa. The stress drop
values of the normal-faulting earthquakes are
between 1 and 10 MPa with a median value of
3 MPa. Their catalogue covers seven earthquakes
(Mw 5.1–5.7) that occurred in western Turkey. The
stress drop values they found for those events are
between 0.45 and 5.4 MPa (Fig. 8, upper panel).
Konstantinou (2014) analyzed 53 strike-slip and
normal-faulting earthquakes in the Mediterranean
region and reported that stress drop changes were
between 1 and 6 MPa. Their catalogue includes only
one earthquake from western Turkey (the 1995 Dinar
earthquake of Mw 6.3). As for the 2011 Simav
earthquake, Yolsal-Cevikbilen et al. (2014) reported a
stress drop as high as 6.4 MPa. Kinematic inversion
of strong-motion and broadband data by Kiratzi
(2018) found a stress drop of about 3.6 MPa for the
2017 Lesvos event. Stress drop estimations of the
above-mentioned studies are far below our estima-
tion, since those studies could only estimate the
average stress drop over the fault plane. The stress
drop in SMGA is usually at least five times higher
than that in the fault plane (i.e., the stress drop of
SMGA is equivalent to that over the fault multiplied
by the percentage area of SMGA over the fault).
Therefore, the relative values among earthquakes
would be beneficial information for comparison with
our study.
Figure 5Observed (upper traces) and simulated (lower traces) acceleration, velocity, and displacement waveforms for the horizontal components of the
2017 Lesvos earthquake at stations #1005, #1720, and #3535. The numbers above the waveforms correspond to the maximum amplitudes.
PSAs (5% damped) of observed and simulated horizontal components are also compared in the right panel
1976 G. Tanırcan et al. Pure Appl. Geophys.
Independently from the above investigations,
variation of the earthquake stress drop in western
Anatolia was further checked utilizing the source
database provided by Yamanaka et al. (2017)
(Yamanaka personal communication, 2018). They
performed a spectral inversion technique in the broad
frequency range to separate seismic source, propaga-
tion path, and site amplification factors. The strong-
motion recordings they utilized belong to 754 small
Figure 6Observed (upper traces) and simulated (lower traces) acceleration, velocity, and displacement waveforms for the horizontal components of the
2017 Bodrum-Kos earthquake at stations #4809, #4812, #4817, and #4819. The numbers above the waveforms correspond to the maximum
amplitudes. PSAs (5% damped) of observed and simulated horizontal components are also compared in the right panel
Table 2
Parameters of the strong-motion generation area (SMGA) for the
Simav, Lesvos, and Bodrum-Kos mainshocks determined by the
empirical Green’s function (EGF) method
Event name Rise
time
(s)
RSP (str.
9 dip)
N (str.
9 dip)
C SMGA (in km)
(str. 9 dip)
Simav
mainshock
0.60 2 9 3 4 9 3 6 4.8 9 3.6
Lesvos
mainshock
0.70 4 9 4 5 9 5 4.4 11.0 9 6.0
Bodrum-Kos
mainshock
0.60 5 9 2 5 9 5 4.5 11.0 9 10.0
Table 3
Source parameters of the aftershocks providing the best results in
simulation with empirical Green’s function (EGF)
Event name Rise time (s) Subfault length 9 subfault
width (km)
Simav aftershock 0.15 1.2 9 1.2
Lesvos aftershock 0.12 2.2 9 1.2
Bodrum-Kos aftershock 0.12 2.2 9 2.0
Vol. 177, (2020) Large Stress Release during Normal-faulting Earthquakes 1977
to moderate size earthquakes that occurred in Turkey
from 1995 to 2015. We adopt side products (corner
frequency and seismic moment) of 344 (3.0\ML\6.0) western Anatolian earthquakes and calculate the
Brune stress drop (1971) assuming constant shear
wave velocity (3.5 km/s) and the omega-squared
source scaling law. Distribution of the earthquake
stress drop at western Anatolia is given in Fig. 8
(lower panel). Most of the earthquakes (about 80%)
have a stress drop B 5 MPa where the stress drop
values change between 5.0 and 40 MPa for the
remaining earthquakes (Fig. 8). Among them, only
the Simav earthquake has the highest stress drop
value (57.12 MPa). These high stress values support
the former findings stating that normal-faulting
earthquakes may release higher stress than strike-slip
earthquakes.
7. Conclusions
In this article, we investigate the SMGA and
stress drop of three normal-faulting earthquakes using
strong-motion data compiled from western Turkey.
The data used are recorded at ten strong-motion sta-
tions and are from the three moderate to large
earthquakes of Mw 5.8, Mw 6.4, and Mw 6.6 and their
Mw 4 ? aftershocks. The EGF method is performed
Figure 7Strong-motion generation area (SMGA) and rise time versus
seismic moment Moð Þ. White circles in the figures show results of
Somerville et al. (1999). Red circles correspond to results of
Miyake et al. (2003) and Kamae (1998a, b). Results obtained in the
current study are shown by blue circles
Table 4
Parameters of strong-motion generation areas (SMGA) in and
around Turkey
Earthquake date, name M SoF C Dr (MPa)
20170720 Bodrum-Kos 6.6 N 4.5 24
20171024 Bodrum-Kos aftershock 4.8 N 4.2
20170612 Lesvos 6.4 N 4.4 19.5
20170612 Lesvos aftershock 4.4 N 4.5
20110519 Simavc 5.8 N 6 25
20110607 Simav aftershockc 4.4 N 4.2
20100308 Kovancılara 6.1 SS 3.5 2.4
20100308 Kovancılara 5.5 SS 2.5 1.3
19991112 Duzceb 7.1 SS 0.7 12.6
19991112 Duzce aftershockb 5.1 SS 16
SoF style of faulting, N normal, SS strike slip, O obliqueaAfter Baykal et al. (2012); bafter Birgoren, Sekiguchi and Irikura
(2004) and Tanircan et al. (2017); cafter Yamanaka et al. (2017)
cFigure 8Spatial mapping of the stress drop in western Turkey. Upper figure:
earthquakes between 1995 and 2005. Stress drop estimations are
from teleseismic data analyses of Allmann and Shearer (2009).
Lower figure: earthquakes between 1995 and 2015. Stress drop
estimations are from strong-motion data analyses of Yamanaka
et al. (2017). Transparent circles represent earthquakes with stress
drop\5 MPa. Black circles represent the maximum stress drop of
57.2 MPa estimated for the Simav earthquake
1978 G. Tanırcan et al. Pure Appl. Geophys.
Vol. 177, (2020) Large Stress Release during Normal-faulting Earthquakes 1979
to estimate the size and rise time of SMGA. Obtained
values in this study are generally comparable with
those estimated for past global earthquakes. One of
the remarkable points derived from the analysis is the
higher stress drop ratio between the mainshock-
aftershock couples than that of strike-slip faulting
events in Turkey. Brune stress drop estimations for
the small to moderate size earthquakes in western
Turkey also imply the existence of a high stress drop,
albeit at a smaller percentage. This indicates that the
seismic hazard assessment for potential large main-
shocks may need special treatment of stress drop
adjustment when considering GMPEs or source
parameters that are built based on regional small
normal-faulting events. Nevertheless, more normal-
faulting earthquake analyses are required to make a
concrete statement about that.
Acknowledgements
We thank the Disaster and Emergency Management
Authority (AFAD) of Turkey for providing the
strong-motion data used in the study. Some fig-
ures were prepared using the GMT plotting tool of
Wessel and Smith (1995). This study is supported by
the Joint Research Project under the Bilateral
Program of the Japan Society of the Promotion of
Science (JSPS) and the Turkiye Bilimsel ve Teknolo-
jik Arastırma Kurumu (TUBITAK 116Y524).
Publisher’s Note Springer Nature remains neutral
with regard to jurisdictional claims in published maps
and institutional affiliations.
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(Received January 17, 2019, revised October 21, 2019, accepted October 28, 2019, Published online November 11, 2019)
Vol. 177, (2020) Large Stress Release during Normal-faulting Earthquakes 1981