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:

birgore@boun.edu.tr2 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