Ground motion simulations for İzmir, Turkey: parameter uncertainty

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1 DOI.7/s ORIGINAL ARTICLE Ground motion simulations for İzmir, Turkey: parameter uncertainty Louise W. Bjerrum & Mathilde B. Sørensen & Lars Ottemöller & Kuvvet Atakan Received: 2 May 12 /Accepted: 5 August 13 # Springer Science+Business Media Dordrecht 13 Abstract In ground motion simulation studies based on earthquake rupture scenarios, the absolute ground motion level, frequency content, signal duration and distribution of simulated ground motion is highly dependent on the input parameters used in the calculations. We conduct a predictive study assessing the potential ground motions for İzmir, Turkey based on earthquake scenarios. We calculate ground motions from a reference scenario (Mw6.8) and compare it to 25 test scenarios for the İzmir fault in order to investigate the effect of input parameter uncertainty on the simulated ground motion. In this study, we use a hybrid broadband frequency (.1 Hz) ground motion simulation technique. We find Electronic supplementary material The online version of this article (doi:.7/s ) contains supplementary material, which is available to authorized users. L. W. Bjerrum (*) : M. B. Sørensen : L. Ottemöller : K. Atakan Department of Earth Science, University of Bergen, Allegt. 41, 57 Bergen, Norway louise.bjerrum@outlook.com M. B. Sørensen mathilde.sorensen@geo.uib.no L. Ottemöller lars.ottemoller@geo.uib.no K. Atakan kuvvet.atakan@geo.uib.no Present Address: L. W. Bjerrum OCTIO, Bøhmergaten 44, 557 Bergen, Norway average ground motion levels in the central area of İzmir of more than 6 cm/s 2 and6cm/sforpeak ground acceleration and velocity, respectively, while the standard deviation is found to exceed cm/s 2 and cm/s, respectively. The tested parameters identified to have the largest influence on the absolute ground motion level are seismic moment, average stress-drop, rise time and rupture velocity. Parameters identified to have the least effect on ground motion level are rake and ratio of stress-drop on asperities to background stressdrop. The low-frequency ground motion, critical in terms of damage to the building stock in İzmir, is mostly affected by the location of nucleation point, seismic moment, depth of the rupture area, rise time and the velocity model. It is, therefore, highly recommended that future studies focus on reducing the uncertainties in these input parameters. Keywords Strong motion simulation. Earthquake scenarios. Seismic hazard assessment. Western Anatolia. Ground motion variability. Hybrid broadband simulation technique. Parameter uncertainty. Seismic design code 1 Introduction Ground motion simulation techniques have proven to be useful in retrospect studies for the purpose of understanding the fault rupture complexity and how this complexity influences the simulated ground motion. From ground motion simulations, it is possible to obtain critical

2 information about the frequency content, the absolute ground motion level and the duration of the expected seismic signal. Applying simulation techniques for predictive studies requires a good knowledge of the input parameters, which can be difficult to assess. For this reason, the effect of the uncertainties in the input parameters should be reflected in the simulation results. One way to obtain an estimate of the uncertainties is to simulate several scenarios while varying the input parameters. The problem of variability in simulated ground motions due to uncertainties in source and propagation parameters has been addressed in several studies (e.g. Atkinson and Beresnev 2; Pulido et al. 4; Aochi and Douglas 6; Sørensen et al.7a; Ameri et al. 8; Causseetal.8; Gravesetal.8; Ripperger et al. 8; Wanget al. 8; Olsen et al. 9; Aagaard et al. a; Aagaard et al. b; Cultrera et al. ; Ameri et al. 11; Imperatori and Mai, 12). Most of these studies investigate a smaller set of parameters than what is investigated in this study. In studies in which the effect of the nucleation point location is investigated, a strong effect on the ground motion distribution due to directivity effects are found. Also, magnitude and slip distribution have significant effect on the level of simulated ground motion. In the case of the rupture velocity and rise time, Atkinson and Beresnev (2), Causse et al. (8) and Ameri et al.(11) found a strong dependence, while Aagaard et al. (a, b) found these parameters to have less influence on the variability of the ground motion. Ameri et al. (11) usedthree different simulation approaches in their study. They found that although a general consistency is seen among the median values from the different simulations, for spectral acceleration and peak ground velocity, the effect of parameter uncertainty also depends on the simulation technique. For simulations based on dynamic rupture scenarios, the spatial and temporal variation in the rupture front leads to increasingly more complex rupture paths, which to some extent results in more scatter in the simulated ground motion (Aochi and Douglas 6; Ripperger et al. 8; Olsen et al. 9). Also, Olsen et al. (9) foundareductionofgroundmotionsfrom dynamic simulations compared to simulated ground motions of the same earthquake scenario found through kinematic modelling. Cultrera et al. () applied a discrete wavenumber/ finite element technique to simulate ground motions due to a Mw-6.9 normal fault earthquake, in the low to intermediate frequency band ( 2 Hz). The faulting mechanism and magnitude of the reference scenario in this present study is comparable to their target event. The source parameters tested in the study by Cultrera et al. () include nucleation point location, rupture velocity, slip distribution and source time function. In total, they simulated 4 earthquake scenarios with different parameter combinations. The mean values of all the simulated ground motions from all scenarios were consistent with the empirical predictive model by Akkar and Bommer (7). The sensitivity of the ground motions was found to be significantly affected by the combination of the source parameters. Furthermore, rupture directivity was found to play an important role in the ground motion distributions. In the studies by Pulido et al. (4) and Sørensen et al. (7a), the problem of ground motion variability due to uncertainty in the input parameters was addressed for a scenario earthquake of Mw 7.5 along the Marmara Sea segment of the North Anatolian Fault. Using the same hybrid broadband ground motion simulation technique as applied in this present study, it was found that the simulated ground motions were most sensitive to rise time, rupture velocity and stress-drop (Sørensen et al. 7a). We simulate ground motions for scenario earthquakes along the İzmir fault, striking directly through the city of İzmir (Fig. 1). İzmir is the third largest city in Turkey, with approximately 3.5 mil inhabitants and is expanding rapidly. The city is located near several active faults, which are known to have ruptured several times during history, resulting in severe shaking and damage in the centre of the city (Emre et al. 5). The latest significant earthquake occurred in 1778, with an estimated intensity in İzmir of MMI=X. This earthquake followed two previous significant events, separated in time by approximately 5 years. Since 1778, no large earthquakes have occurred in this region (Ambraseys and Finkel 1995; Papazachos and Papazachou 1997; Papazachos et al. 1997). This apparent low seismic activity during the last 24 years may have resulted in a considerable amount of stress accumulation on the various faults in the area. Stress build-up along these faults is manifested by a local GPS study, focusing on the changes in the velocity field around İzmir (Aktuğ and Kılıçoğlu 6). This study verifies small-scale block rotations as well as differential motion across the faults. Furthermore, the change in the velocity field from north to south confirms N S extension and further opening of the İzmir Bay (Fig. 1). Prospective large

3 E Study area Aegean Sea 4 Manisa 35 N Bay Chios a Urla Fault Tuzla Fault ÿ Gulf of Kusadasi Kusadasi mm/yr Samos Fig. 1 Map of the area around İzmir. The faults mapped by the General Directorate of Mineral Research and Exploration (MTA; Emre et al. 5) are shown as thick black lines; İzmir and Tuzla faults are outlined as stippled white lines. The historic earthquakes (Mw ) in the area are indicated with white circles scaled to size (Ambraseys and Finkel 1995; Papazachos and Papazachou 1997; Papazachos et al. 1997) and the GPS velocity fields from Aktuğ and Kılıçoğlu (6) are shown as white arrows. The inset map shows the location of the study area in a larger regional setting earthquakes in the area are likely to have severe human and economic consequences for the city of İzmir. Previously, probabilistic seismic hazard assessments have been conducted for the area around İzmir (MMI ; Deniz et al. ), but a more detailed map of the seismic hazard is needed for future risk mitigation and city planning. Also, a previous deterministic seismic hazard study, simulating nine hybrid broadband frequency earthquake scenarios, identified the most important faults in the vicinity of İzmir. It was found that a normal faulting earthquake scenario along the İzmir fault (Mw 6.9), which underlies the city centre, gave the highest ground motions in the city centre. This event is considered to be the most critical scenario for the city of İzmir. In addition, a rupture along the Tuzla fault (strike-slip mechanism) located south-west of the city, poses a significant threat to the city. In probabilistic hazard studies, probabilities of exceeding certain ground motions within a given time frame are given. However, in scenario-based hazard assessments, ground motion simulations can provide information on expected frequency content, absolute peak ground motion and duration in case of such an event. Simulating earthquake scenarios is, therefore, important in order to obtain information which is crucial for earthquake resistant engineering applications. In the present study, we follow the approach of Pulido et al. (4) and Sørensen et al. (7a). We investigate the influence of one single input parameter on the simulation results at a time; therefore, the effect on the ground motion due to different combinations of the input parameters is not studied. The parameters tested in these two previous studies include asperity location, attenuation model, rise time, rupture velocity, nucleation point, average stress-drop and stress-drop

4 ratio. In addition, we expand this set of parameters in the present study with: dip angle, rake angle, seismic moment and velocity model. The simulation technique applied in this study is a hybrid broadband simulation technique (Pulido and Kubo 4). We use a kinematic representation of the fault plane, and low-frequency wave propagation is combined with high-frequency stochastic modelling. The hybrid broad-band approach is preferred, since the ground motion calculated from this technique yields spectral information in the frequency range.1 Hz, covering the frequency band for which most structures are vulnerable. The main objective of this study is to investigate the level of expected ground motion in the study area (İzmir, Turkey). Based on the ground motion simulations, we define the variation of expected ground motion as an expression of the uncertainties associated with the input parameters. 2 Methodology We simulate ground motions for 26 earthquake scenarios (one reference scenario and 25 test scenarios with input parameters varied one by one) with a hybrid broadband simulation technique. This allows us to investigate the influence of the individual input parameters on the simulated ground motion. In the following two sections, the simulation technique and the strategy for the study are described. Also, a description of the reference scenario is given. 2.1 Ground motion simulation technique The simulation technique applied in this study follows the approach of Pulido and Kubo (4) and Pulido et al. (4). It is a hybrid broadband simulation technique which combines deterministic low-frequency waveform modelling with a high-frequency semi-stochastic approach. Furthermore, a frequency dependent radiation pattern is applied in the frequency range 1 3 Hz,ensuring a smooth transition from the theoretical double-couple at low frequencies to an isotropic radiation for high frequencies (Pulido and Kubo 4). The ground motion simulations are performed in the frequency range.1 Hz, where the calculations are conducted independently with waveform modelling in the frequency band.1 2 Hz and stochastic modelling for 1.6 Hz. For frequencies between 1.6 and 2 Hz, the two methods overlap and are combined in the frequency domain using a Hanning taper to achieve a smooth transition. Moving the transition frequency range to higher frequency ranges will shift the weight of the effect of the ground motion from the low frequency to high frequency modelling. This is because PGV captures most of the low frequency part of the ground motion dominated by the deterministic method, while the PGA captures the high frequency part of the ground motion dominated by the stochastic simulations (Bouchon 1981; Boore1983). The input for the earthquake rupture scenario is defined as a finite-fault rupture. A focal mechanism is assigned, and the fault is embedded in a flat layered velocity model. The variation of slip across the fault plane during the rupture is accommodated by the location of asperities where a higher stress-drop is assigned. The low-frequency ground motion is calculated numerically using the discrete wave number method (Bouchon 1981), where the fault plane is divided into subfaults, which are treated as point sources. In the high-frequency calculations, we also use a finite asperity model, and ground motion is computed using the stochastic technique of Boore (1983) to generate synthetic Green s functions. For the summation of the high-frequency ground motion, the approach of Irikura (1986) isapplied to the synthetic Green s functions. This hybrid broadband simulation method has proven very efficient for simulating broadband ground motions from finite faults (Pulido and Kubo 4). The total ground motion at a given site is obtained by summing all contributions from the point sources, while assuming a constant rupture velocity within each subfault. Although it is suggested by Boore (3) to use several time-series realisations and calculating an average of the results, we only used a single random realisation from Boore s (1983) technique, in the calculation of the synthetic Green s functions. Our approach is justified as the overall result is obtained by summation of several single random functions from the different subfaults. We use a 1D layered crustal velocity structure in the wave propagation, and simulation sites are located at a bedrock site. The hybrid simulation technique applied in this study has previously been validated in retrospect studies, such as for the Tottori, Japan event (Pulido and Kubo 4), the 4 Sumatra, Indonesia event (Sørensen et al. 7b) and the 8 Wenchuan, China event (Bjerrum et al. ). All ground motions are calculated for bedrock level, and site effects could be accounted for as a postprocessing step.

5 The input used in the scenario models includes a definition of the fault geometry parameters such as the location of the fault, the location of the asperities and location of the nucleation point. Fault rupture parameters such as the seismic moment, stress-drop, rise time and rupture velocity must also be defined. Finally, wave propagation parameters include a model for the high-frequency seismic wave attenuation (Q f a ) in the area and a crustal seismic velocity and density structure. 2.2 Strategy for the sensitivity study and the earthquake reference scenario In order to assess the variation of the simulated ground motions due to uncertainty in the input parameters, we first define a reference earthquake scenario. Within this text, we refer to scenarios with changes in fault geometry, fault rupture or wave propagation parameters as compared with the earthquake reference scenario as test scenarios. We keep the fault dimensions fixed in all tests, and the dimensions are based on a worst case scenario for İzmir. The effect of parameter variation is investigated and quantified by changing one parameter at a time and calculating the difference in the simulated ground motion from a test scenario to the reference scenario. The tests performed in this study will reveal the sensitivity due to both physical effects (i.e. effects in changing the physical parameters) as well as method specific effects. For example, we test the effect of changing the stress-drop and highfrequency attenuation, which are only introduced in the high-frequency stochastic modelling. Likewise, we examine the effect of changing the velocity model, which is introduced in the low-frequency deterministic waveform modelling. The results are presented in maps showing the absolute difference in ground motion between the reference- and test scenario. The parameters investigated in this study are asperity location, dip, rake, location of nucleation point, seismic moment, average stress-drop, ratio between background and asperity-fault stress-drop, rise time, rupture velocity, attenuation model and velocity model. In total, we calculate 25 test scenarios, which are listed in Table 1. The geometry of the reference scenario is shown in Fig. 2a, and the input parameters are summarised in Table 1. The location and dimensions of the fault are based on the fault map from MTA (Emre et al. 5; Fig. 1). The seismic moment is calculated from empirical relations based on the fault area and rupture type (Wells and Coppersmith 1994). The hypocentre is placed in the deeper section of the fault on the lower rim of the asperity. This assumption is based on theoretical and observational evidence (e.g. Mai et al. 5). The focal mechanism solution is based on average values derived from previous smaller events along the fault, where the dip angle is found to be around 6 and rake angles are in the range 9 to in the Global CMT database (Ekström et al. 8). The variation of slip across the fault plane is represented by a simple asperity model with the largest slip across the central part of the fault plane. The area of the asperity corresponds to approximately 22 % of the entire fault area (Somerville et al. 1999). The average stress-drop is set to 3 MPa (Kanamori and Anderson 1975). The rupture velocity and rise time used in the reference scenario on the İzmir fault are set to 2.5 km/s and 1. s, respectively, which are average values from the study by Somerville et al. (1999) as well as for recent similar events. The velocity model used in the simulations is based on Akyol et al. (6; Fig. 3). The attenuation model for the highfrequency decay is based on Akıncı et al. (1995), where the frequency dependence was determined for Lg waves in the frequency range of 1.5 Hz to be Q= 82 f 1.. For the low-frequency modelling we use Q p and Q s calculated from V p in the velocity model following Schön (1996). We use a cutoff frequency (f max ) of Hz and use a subfault size of 2 2 km 2. Calculations were conducted for 468 simulation sites in an area within 38º 38.85ºN and 26.5º 27.75ºE, and with a grid spacing of.5. The simulated peak ground acceleration (PGA) and velocity (PGV) distribution of the reference scenario are shown as the geometric mean of the two horizontal components (Fig. 4a, b). This scenario predicts ground motion, on bedrock level, of more than 5 cm/s 2 and 4 cm/s for PGA and PGV, respectively. The largest simulated ground motions are observed near the surface rupture of the fault. PGA is found to exceed 3 cm/s 2 over an area of approximately 1, km 2. In Fig. 4c, d, the attenuation of the simulated ground motion away from the fault is compared to empirical ground motion relations for Western USA (Boore and Atkinson 8) and Turkey (Akkar and Çağnan ). We find that the ground motion levels for both PGA and PGV fall within the uncertainty of these relations, with a larger spread for the PGV values than for PGA.

6 Table 1 Source parameters for the reference earthquake scenario on the İzmir fault to the left and earthquake scenarios tested in this study to the right together with scenario names Parameter Reference scenario Test scenario Fault geometry parameters (scenarios 1 4) Asperity location Central location Shallow location Scenario 1a Deep location Scenario 1b Divided, central depth Scenario 1c Dip 6 45 Scenario 2a 75 Scenario 2b Rake 9 Scenario 3a 1 Scenario 3b Nucleation point N 27.1 N/38.45 E/16 km Scenario 4a E N/38.46 E/16 km Scenario 4b 11.5 km N/38.43 E/11 km Scenario 4c Fault rupture parameters (scenarios 6 ) Seismic Moment Nm (Mw 6.8) Nm (Mw 6.4) Scenario 5a Nm (Mw 7.) Scenario 5b Average stress-drop 3 MPa Δσ a asp =11 MPa; Δσ b bg =.6 MPa Mpa: Δσ a asp =36 MPa; Δσ b bg =2 MPa Scenario 6a 1 Mpa: Δσ a asp =3.6 MPa; Δσ b bg =.2 MPa Scenario 6b Stress-drop ratio.5.25: Δσ a asp =12 MPa; Δσ b bg =.3 MPa Scenario 7a.: Δσ a asp = MPa; Δσ b bg =1 MPa Scenario 7b Rise time 1. s.5 s Scenario 8a 1.5 s Scenario 8b Rupture velocity 2.5 km/s 2. km/s Scenario 9a 3. km/s Scenario 9b Path related parameters (scenarios 11 12) Attenuation model Q=82 f 1. Q=82 f.5 Scenario a Q=16 f 1. Scenario b Q=16 f.5 Scenario c Velocity model Akyol et al. (6) Western Turkey Kalafat et al. (1987) Scenario 11a Gulf of Corinth (Rigo et al. 1996) Scenario 11b a Δσ asp : Asperity stress-drop b Δσ bg : Background stress-drop J Seismol Also, most of the simulated values lie within one standard deviation of the empirical relation from Akkar and Çağnan (). This serves as an initial reference solution for the ground motion level obtained from the reference scenario. The acceleration and velocity waveforms and the 5 % damped acceleration and velocity spectra for a few stations (together with a map of station locations) are shown in Figs. ES1-ES3. The directivity effect in the rupture of the reference scenario is evident. Signals of longer duration and lower ground shaking (approximately 8 cm/s 2 and cm/s) are simulated east of the fault, in the backward directivity direction. West of the fault, in the forward directivity direction, signals of shorter duration and larger ground shaking (between and 17 cm/s 2 and approximately cm/s) are simulated. Large ground motion is found perpendicular to the rupture, with simulated ground acceleration exceeding 4 and 35 cm/s north of the fault. We filtered the simulated waveforms in three frequency bands (f<1hz, 1 Hz<f<5 Hz, and f>5 Hz) and calculated the geometric mean of the maximum values of the two horizontal components for acceleration and velocity in each frequency band (Fig. ES4). As expected, the higher frequency contribution dominates the acceleration values, whereas the lower frequencies

7 a b Fig. 2 a Geometry of the reference scenario fault plane. The asperity is marked with a gray box. The black star shows the nucleation point for the reference scenario. Open stars indicate alternative nucleation points, tested in scenarios 4a c. The individual lengths of segment and asperity are given. b Locations of the asperities used in the test scenarios 1a c mostly contribute to the velocities. For the interval 1 5 Hz, where both the waveform modelling and the stochastic modelling are applied, large ground motion is obtained for both acceleration and velocity above the entire fault plane. 3 Results We have tested the effect on simulated ground motion due to uncertainty of several different input parameters (Table 1), by varying the input parameters one-by-one with respect to the reference scenario. The technique adopted in this study ignores the parameter interdependency, e.g. the dependence of seismic moment and stress-drop on fault length, and focus of this study is on the implications of varying the individual parameters on the resulting simulated ground motion. For each test we present a shake map showing the difference in peak ground motion with respect to the reference scenario. In the case of the tests where the smallest changes in absolute ground motion are observed (rake, stress-drop ratio and attenuation model), the reader is referred to the electronic supplement for the differential shake maps. Some additional shake maps are also included in the electronic supplement for the tests of location of nucleation point, average stress-drop, rise time and velocity model in order to show the differential ground motion in maps with other colour scaling. In this paragraph the colour scale is kept constant for all differential shake maps for an easier and direct comparison between the different tests.

8 Fig. 3 Crustal velocity models used in this study. The model from Akyol et al. (6) (A6) is for western Anatolia and is used in the reference scenario. The velocity models used in the test scenarios 11a and b are from Kalafat et al. (1987) (K87) and Rigo et al. (1996) (R96), respectively. P-wave and S-wave velocities are shown in black and gray, respectively Depth (km) Velocity (km/s) A6 K87 R Fault geometry parameters The effect of the asperity location on the simulated ground motion is investigated in scenarios 1a c. The effect of moving the asperity upwards and downwards along dip is studied in scenarios 1a and b, while the effect of dividing the asperity into two asperities, placed at either end of the fault plane, is investigated in scenario 1c (see Fig. 2b for the different asperity locations). Moving the asperity to the upper part of the fault (scenario 1a) results in an increase of ground motion above thefaultplaneofmorethan4%and25%forpgaand PGV, respectively (Fig. 5). An increase of ground motion in the direction of rupture directivity is found in the distribution of PGV, in correspondence with the shorter distance from the main energy release (asperity) to the surface. Furthermore, focusing of energy in the direction of rupture directivity results in larger ground motion values. In scenario 1b, the asperity is moved to the deeper part of the fault plane. This does not affect the ground motion significantly, as the changes are less than % and 12 % for PGA and PGV, respectively. When the asperity is divided into two parts, placed at either end of the fault, the changes in simulated ground motion follows the locations of the asperities: Larger ground motion is found towards the ends of the fault, with an increase of more than 4 % and 25 % for PGA and PGV, respectively, and a similar decrease is found in the central part of the fault. This implies a change in the spatial ground motion distribution, rather than change in the maximum PGA and PGV for the scenario. In scenarios 2a and b the sensitivity to the dip angle of the fault plane was investigated. The dip angle in the reference scenario is set to 6º, and the effect of a shallower dipping fault (45º) and a steeper dipping fault (75º) is tested. The results are shown in Fig. 6. Usinga dip angle of 45º (scenario 2a) leads to an increase of the PGA and PGV above the fault plane and to the north of the fault. The increase is more than % for PGA and 25 % for PGV. This is due to the shorter travel path to the ground surface. In scenario 2b, with a dip angle of 75º, a slight change in simulated ground motion is found, and the effect of a steeper dipping fault is minimal. In scenarios 3a and b, we investigated which effect a smaller and a larger rake angle will have on the simulated ground motions. In the reference scenario, we used a rake angle of º, and in the test scenarios the rake angle was set to 9º and 1º for scenarios

9 a b c PGA, cm/s² PGV, cm/s d 1 PGA, cm/s² 2 1 PGV (cm/s) Akkar and nan (), rock site Boore and Atkinson (8), rock site Simulation Akkar and nan (), rock site Boore and Atkinson (8), rock site Simulation 1 2 Distance to fault, km 1 2 Distance to fault, km Fig. 4 Ground motion distribution (taken as the geometric mean of the maximum values of the two horizontal components) for the reference scenario a PGA and b PGV. The surface projection of the ruptured fault plane and asperity are shown as white rectangles, the surface rupture is marked with a thick stippled gray line and the nucleation point is shown as a white star. The grid points for which the simulations are preformed are shown in a regular grid of black points. Comparison of the simulated ground motions from the reference scenario to empirical attenuation relations for the Western USA (Boore and Atkinson 8) and Turkey (Akkar and Çağnan ) for c PGA and d PGV. Solid lines are mean values; stippled lines are ±one standard variation 3a and b, respectively (Fig. ES5). The tests show that changing the rake angle results in small changes in the spatial distribution of the ground motion. The largest changes in PGA and PGV are about % and 7 %, respectively. The smaller rake angle of 9º corresponds to a pure normal faulting event, and an increase in ground motion at the eastern end of the fault is observed, while a reduction in ground motion amplitude is observed at the western end of the fault. The opposite pattern is observed when the rake angle is increased to 1º in scenario 3b. This confirms the focusing of energy due to rupture directivity in the simulated ground motion. The effect of changing the location of the nucleation point was investigated in scenarios 45a c (Fig.7; see Fig. ES6 for another scale). This test changes the location of the nucleation point of the rupture and thereby the

10 Scenario 1a: Asperity located up Scenario 1a: Asperity located up Scenario 1b: Asperity located down Scenario 1b: Asperity located down Scenario 1c: Asperity divided Scenario 1c: Asperity divided PGA difference from reference scenario, cm/s² PGV difference from reference scenario, cm/s Fig. 5 Simulation results for scenarios 1a c where the effect of asperity location is investigated. The plots show the absolute difference in the ground motion values compared to the reference scenario. The surface projection of the ruptured fault plane and asperity are shown as black rectangles; the surface trace is marked with a thick stippled line. The nucleation point is shown as a white star

11 Scenario 2a: Dip = 45º Scenario 2a: Dip = 45º - Scenario 2b: Dip = 75º Scenario 2b: Dip = 75º PGA difference from reference scenario, cm/s² PGV difference from reference scenario, cm/s Fig. 6 Simulation results for scenarios 2a and b, where the effect of dip is investigated. The plots show the absolute difference in the ground motion values compared to the reference scenario; details as in Fig. 5 temporal evolution of the rupture along the fault. In scenario 4a, the nucleation point is placed in the central part of the fault. In scenario 4b, it is placed at the westernmost point of the asperity. In scenario 4c, the nucleation point is shallower, located at the eastern end of the fault, as in the reference scenario (see Fig. 2a for locations). Moving the epicentre to the central part of the fault (scenario 4a) results in a bilateral rupture in the east west direction and small changes in ground motion close to the surface rupture. Scenario 4b is a unilateral rupture towards east. This is the opposite situation of the reference scenario. Decrease in ground motion close to the test nucleation point is observed, while an increase of ground motion close to the reference nucleation point and west of the fault is observed. This distribution is especially pronounced in PGA. When the nucleation point is located at the eastern end of the asperity, as in the reference scenario, at a shallower depth (scenario 4c), a slight change in the spatial ground motion distribution is observed. In this case, a small increase of the ground motion above the deeper lying parts of the fault and decreased values near the surface rupture are found. Changing the location of the nucleation point laterally, along the lower rim of the asperity, results in change of the spatial ground motion distribution, rather than a general increase/decrease of the simulated ground motion. The resulting changes of PGA and PGV close to the fault are less than % and %, respectively.

12 Scenario 4a: Nucleation point Scenario 4a: Nucleation point - Scenario 4b: Nucleation point Scenario 4b: Nucleation point Scenario 4c: Nucleation point Scenario 4c: Nucleation point PGA difference from reference scenario, cm/s² PGV difference from reference scenario, cm/s Fig. 7 Simulation results for scenarios 4a c, where the effect of the location of the nucleation point is investigated. The plots show the absolute difference in the ground motion values compared to the reference scenario. The surface projection of the ruptured fault plane and asperity are shown as black rectangles; the surface trace is marked with a thick stippled line. The nucleation points used in the reference scenario and in the test scenario are shown as a black and a white star, respectively (See Fig. ES6 for another scale)

13 - 3 3 J Seismol Smaller changes are observed across the entire area, especially in the direction of the fault strike (Fig. ES6). However, the comparison of the shake maps of the reference scenario (Fig. 4a, b) with the shake maps of the three test scenarios (Fig. ES7) only shows small changes in the location of the maximum simulated ground motion. The modification of the directivity effect on peak ground motion is more pronounced in scenario 4b. Cultrera et al. () and Spudich and Chiou (8) also observed that change of nucleation point location affects the distribution of simulated ground motion over a larger area. However, in their studies, the change in ground motion was more pronounced. Also, in the study by Sørensen et al. (7a), the change in absolute ground motion due to change in nucleation point is found to have a larger effect over a wider area, than in the present study. This can partly be due to the difference in rupture mechanism, since rupture directivity is known to be of larger importance for strike-slip faulting earthquakes than for normal faulting earthquakes. 3.2 Fault rupture parameters The effect of the seismic moment is investigated in scenarios 5a and b (Fig. 8). Testing seismic moment might seem trivial. However, the magnitude of a potential future earthquake along İzmir fault must be considered uncertain and this must be reflected in a study like this. This test is, therefore, important to include, when the average earthquake scenario with Scenario 5a: Mo = 4.84e+18 Nm Scenario 5a: Mo = 4.84e+18 Nm Scenario 5b: Mo = 3.8e+19 Nm Scenario 5b: Mo = 3.8e+19 Nm PGA difference from reference scenario, cm/s² PGV difference from reference scenario, cm/s Fig. 8 Simulation results for scenarios 5a and b, where the effect of seismic moment is investigated. The plots show the absolute difference in the ground motion values compared to the reference scenario; details as in Fig. 5

14 standard deviation, based on all the earthquake scenarios investigated in this study, is considered in the discussion section. The seismic moment used in our reference scenario is found from the rupture area, by using Wells and Coppersmith s (1994) empirical relations for normal faulting events. In the test scenarios of the effect of seismic moment, the lower value from Wells and Coppersmith (1994), considering the coefficients and standard errors, is used in scenario 5a. This corresponds to a reduction of approximately % of the seismic moment used in the reference scenario, and corresponds to an earthquake of Mw 6.4. For scenario 5b we increased the seismic moment by %, corresponding to a scenario earthquake of Mw 7.. As expected, changing the seismic moment has a clear effect on the simulated ground motion. A lower seismic moment results in a decrease of the simulated ground motion and vice versa. In scenario 5a, a decrease of more than 3 % and 75 % for PGA and PGV, respectively, is simulated above most of the fault plane, while increasing the seismic moment results in a corresponding increase of simulated ground motion. The affected area is widespread and the shape of the contours is very similar in the two tests. When we changed the seismic moment in scenarios 5a and b, no changes were made in the fault dimensions, assuming a change in slip (thus energy release) over the same fault area. This assumption affects the calculated ground motion along the fault plane, while a scaling of the fault length with the applied moment would have influenced the near fault ground motion distribution, e.g. a shorter fault, corresponding to a lower seismic moment, would change the extent of the rupturing segment of the fault and the rupture would thereby not necessarily underlie the densely populated area in İzmir. Also, location of asperity, stress-drop and location of nucleation point would be changed. In scenarios 6a and b, the effect of average stressdrop on the simulated ground motion is investigated. The average stress-drop in the reference scenario is set to 3 MPa. In scenario 6a, the stress-drop is set to MPa, and in scenario 6b, to 1 MPa. When applying a larger average stress-drop across the fault plane, an increase of the simulated ground motion is found and vice versa (Fig. 9; see Fig. ES8 for another scale). The change is largest above the fault plane, and differential PGA of up to 8 % is simulated. Since stress-drop is only introduced in the high-frequency calculations; negligible change in PGV is observed. When assigning the value of 3 MPa for the average stress-drop in the reference scenario, this value was not calibrated against the seismic moment and/or the rupture length used in our test scenarios. As with magnitude, fault area scaling of the average stress-drop could possibly move the area of the largest ground motion away from the centre of İzmir, if the extent of the fault is reduced due to a shorter fault length. In the test scenarios 7a and b, we investigated the effect of changing the stress-drop ratio. The stress-drop ratio is defined as the ratio of the stress-drop across the background fault plane (Δσ bg) to the stress-drop across the asperity (Δσ asp ), and it is in the reference scenario set to be.5. A lower stress-drop ratio (.25 in scenario 7a) implies a larger difference between the stress-drop across the asperity and on the background fault plane. The asperity stress-drop is, therefore, increased while the background stress-drop is decreased. A larger stress-drop ratio (.1 in scenario 7b) corresponds to a higher background stress-drop and a lower stress-drop across the asperities, representing a more uniform rupture across the fault plane. The stress-drop parameter is only used in the high-frequency calculation. This means significant changes are found exclusively in the PGA distributions. The lower stress-drop ratio results in an increase of the simulated ground motion, whereas a high stress-drop ratio will result in lower simulated ground motion (Fig. ES9). This change corresponds to a larger stress release across a smaller area in the case of the small stress-drop ratio and vice versa. The change in PGA in scenarios 7a and b is of the order of 3 %. A larger stress-drop ratio (scenario 7b) affects a larger area, since the energy release is distributed over a wider area of the fault. In scenarios 8a and b, we investigate the effect of the rise time on the simulated ground motion (Fig., see Fig. ES for another scale). In scenario 8a, we use a short rise time of.5 s and in scenario 8b we use a longer rise time of 1.5 s. Decreasing the rise time to.5 s increases the simulated ground motion across most of the study area, with the largest change above the fault plane of more than 8 % for both PGA and PGV. The decrease in PGA and PGV in scenario 8b is found to be approximately % and 25 %, respectively. A rise time of 1.5 s results in larger change over a wider area in the low frequency ground motion, PGV, compared to the high frequency ground motion, PGA. When a rise time of.5 s is used, we obtain a large change in both low and high frequency ground motion over a wide spread area.

15 J Seismol Scenario 6a: Average Δσ = MPa Scenario 6a: Average Δσ = MPa Scenario 6b: Average Δσ = 1 MPa Scenario 6b: Average Δσ = 1 MPa PGA difference from reference scenario, cm/s² PGV difference from reference scenario, cm/s Fig. 9 Simulation results for scenarios 6a and b, where the effect of average stress-drop is investigated. The plots show the absolute difference in the ground motion values compared to the reference scenario; details as in Fig. 5 (See Fig. ES8 for another scale) The effect of the rupture velocity is investigated in scenarios 9a b (Fig. 11). The rupture velocity was reduced to 2. km/s (from 2.5 km/s in the reference scenario) in scenario 9a and increased to 3. km/s in scenario 9b. A lower rupture velocity results in a slight decrease in calculated ground motion of 5 % and % for PGA and PGV, respectively. A larger rupture velocity increases the ground motion above the fault plane (more than % and 5 % for PGA and PGV, respectively) and in the direction of rupture propagation for PGV only. The simulation technique applied in this study is sensitive to high frequency ground motion variations in the vicinity of the fault plane due to the effect of dynamic parameters such as rise time and rupture velocity which are treated kinematically. This is due to the definition of the asperities and hence the slip distribution along the fault where barriers are known to produce high frequency radiation of the seismic waves. The slip model applied as in this study is simplified which in turn will reduce the variation of the high frequency radiation produced by the main shock rupture front. As a consequence acceleration/deceleration of the rupture front due to complexities in the slip distribution is reduced (Madariaga 1977, 1983; Hartzelletal.1996, Bjerrum et al. ). 3.3 Wave propagation parameters In scenarios a c, we investigated the effect of the high frequency attenuation model on the simulation results, and the differential PGA and PGV distributions

16 J Seismol Scenario 8a: Rise time =.5 s Scenario 8a: Rise time =.5 s Scenario 8b: Rise time = 1.5 s Scenario 8b: Rise time = 1.5 s PGA difference from reference scenario, cm/s² PGV difference from reference scenario, cm/s Fig. Simulation results for scenarios 8a and b, where the effect of rise time is investigated. The plots show the absolute difference in the ground motion values compared to the reference scenario; details as in Fig. 5. (See Fig. ES for another scale) from the tests shown in Fig. ES11. The frequencydependent attenuation model is applied in the stochastic modelling. Changing the attenuation model will, therefore, only affect the high-frequency ground motion; thus, in turn, this will result mainly in changes of the acceleration distribution. Q is defined as Q=Q f a, and we tested the frequency dependency of the attenuation coefficient a in scenario a. Lowering a from 1. to.5 implies lower Q values for high frequencies, which means stronger attenuation of the seismic waves, and we observed a decrease in PGA of more than 5 % across large parts of the study area. In scenario b, we changed Q from 82, used in the reference scenario, to 16. A larger value for Q gives a general increase of Q for all frequencies, resulting in less attenuation of the seismic waves. When Q is changed, a slight increase of the simulated acceleration in the direction of rupture propagation is found. The maximum increase of PGA is of the order of 4 %. For comparison we also tested a combination of the two in scenario c (Q=16 f.5 ) finding a decrease in the acceleration. In scenarios 11a and b, we investigate the effect of the crustal velocity model. In the reference scenario, we use a crustal velocity model for the region around İzmir by Akyol et al. (6). In scenario 11a, we changed the velocity model to a velocity model used by Kandili Observatory and Earthquake Research Institute, Istanbul, Turkey for routine location of earthquakes in western Anatolia (Kalafat 1987). In scenario 11b, we used a velocity model for the Gulf of Corinth (Rigo et al. 1996). Both models are shown in Fig. 3.

17 Scenario 9a: Rupture velocity = 2. km/s Scenario 9a: Rupture velocity = 2. km/s Scenario 9b: Rupture velocity = 3. km/s Scenario 9b: Rupture velocity = 3. km/s PGA difference from reference scenario, cm/s² PGV difference from reference scenario, cm/s Fig. 11 Simulation results for scenarios 9a and b, where the effect of rupture velocity is investigated. The plots show the absolute difference in the ground motion values compared to the reference scenario; details as in Fig. 5 The results from these simulations are shown in Fig. 12. In scenario 11a, a decrease of PGA localised above the fault plane is simulated, while an increase in PGV, most pronounced in the rupture directivity direction, is simulated. In scenario 11b, an increase in PGA across the fault surface rupture is simulated. Furthermore, an increase over a wider area in the direction of rupture directivity for PGV is simulated. The changes in PGA and PGVare % and 5 %, respectively. Changes in the velocity model significantly change the low-frequency ground motion through the deterministic wave propagation. A possible explanation for this can be that in scenarios 11a and b, we use velocity models with fewer layers and lower velocities in the shallower (rupturing) part, than in the reference model. These lower velocities result in stronger directivity effects, since the S-wave velocity and the rupture velocity becomes almost equal in the upper layers. However, it is obvious from these tests, that applying different velocity models in the simulation can have a large effect on the simulated ground motion, especially for PGV. The applied velocity model and the rupture velocity should, therefore, be chosen carefully. For further discussion on the effect of changing the crustal velocity model, see the electronic supplement and Figs. ES12 and Discussion We use the simulation results from the reference scenario (see Section 2.2) and the 25 test scenarios to quantify the potential ground shaking levels for the İzmir area

18 Scenario 11a: Velocity model K87 (Western Anatolia) Scenario 11a: Velocity model K87 (Western Anatolia) Scenario 11b: Velocity model R96 (Gulf of Corinth) Scenario 11b: Velocity model R96 (Gulf of Corinth) PGA difference from reference scenario, cm/s² PGV difference from reference scenario, cm/s Fig. 12 Simulation results for scenarios 11a and b, where the effect of velocity model is investigated. The plots show the absolute difference in the ground motion values compared to the reference scenario; details as in Fig. 5 through a scenario-based approach in terms of PGA and PGV. We also account for the uncertainty of the input parameters and their effect on the simulated ground motion by using the average ground motion distribution and the corresponding standard deviation. The largest average PGA and PGV (Fig. 13) are found symmetrically around the surface trace of the fault and in the vicinity of the surface projection of the fault plane. The distribution is similar to the distribution shown for the reference scenario in Fig. 4a, b, although the level of average ground motion is found to be slightly larger and the contours are smoother. The largest standard deviations of PGA and PGV are restricted to a small area along the surface rupture of the fault, close to the asperity for PGA and PGV. The standard deviation exceeds and cm/s close to the surface projection of the asperity (corresponding approximately to 4 % and 5 %) for PGA and PGV, respectively, and decreases rapidly away from the fault. As for the reference scenario (Fig. 4c, d), the simulated ground motion averaged from all the tests to a large degree falls within uncertainty levels of the empirical attenuation relations of Boore and Atkinson (8) and Akkar and Çağnan () well (Fig. 14), and the standard deviations of PGA fall within the standard deviations of the empirical relations while a slightly larger spread in the PGV is observed. The simulations are, therefore, considered to be comparable with the empirical ground motion prediction relations, despite the large uncertainties and variation in the tested input parameters. The larger spread in PGV is associated with the rupture

19 Average PGA Average PGV Standard deviation of PGA Standard deviation of PGV Acceleration, cm/s² 4 6 Velocity, cm/s Fig. 13 Spatial distribution of average ground motion and standard deviation of PGA (left) and PGV (right) based on the reference scenario and the 25 test scenarios. The surface propagation along the fault (i.e. rupture directivity) which is included in such simulations, whereas empirical ground motion prediction equations tend to smoothen these effects. In Figures 15 17, the residuals (test scenario reference scenario) and the ratio of the residual to the reference scenario value are plotted as functions of distance to fault plane for all simulation points and all test scenarios. In general, the absolute residuals show the largest variation for sites close to the fault plane, which is also, depicted in the maps shown in Figs Furthermore, the absolute residuals seem to stabilize for all test scenarios in a fault distance of approximately 3 km. The plots showing the residuals as a function of distance is useful for the understanding of the seismic hazard, and it is evident that the largest projection of the ruptured fault plane and asperity are shown as white rectangles; the surface trace is marked with a thick stippled line. The nucleation point is shown as a white star changes in the ground motions mostly affect the area close to the fault, while the changes in larger distances decreases, independent of which parameter is tested. On the other hand, the plots showing the ratio of the residual to the reference scenario value as a function of fault plane distance remains constant for scaling parameters such as the seismic moment and the average stress drop. For the tests where the fault geometry parameters are tested (scenarios 1 4; Fig. 15), residuals and ratio of residual to reference scenario show no clear distinction between the tests (a, b and c). For this reason, the fault geometry parameters yield a change of the spatial distribution of the resulting ground motion, rather than systematic increase or decrease of the ground motion with respect to the reference scenario.

20 3 2 PGA, cm/s² Distance to fault, km PGV (cm/s) Akkar and nan (), rock site Boore and Atkinson (8), rock site Akkar and nan (), rock site Boore and Atkinson (8), rock site Simulation Simulation Distance to fault, km Fig. 14 Comparison of the average simulated ground motion of the reference scenario and the 25 test scenarios, including the standard deviation, to the empirical attenuation relations for the Western USA (Boore and Atkinson 8) and Turkey (Akkar and Çağnan ). PGA to the left and PGV to the right In Figs. 16 and 17(a, d), the residuals and ratio of the residual to the reference scenario value for test scenarios 5 9 are shown. In these test scenarios the effect on the ground motion while changing different fault rupture parameters is investigated. All plots show a clear separation of the tests (a and b) indicating a systematic increase or decrease in these tests with respect to the reference scenario. While the tests of the seismic moment, average stress drop, stress drop ratio and rupture velocity show almost mirrored results for the increase and the decrease, it is in Fig. 16(d, h) evident that a decrease in the rise time (scenario 8a) has a larger effect on the resulting ground motion than when the rise time is increased (scenario 8b). For the scenarios where the path related parameters are tested (Fig. 17b, c and e, f), we observe a change in ground motion distribution for scenarios and 11 (attenuation and velocity model). In the test of the attenuation the absolute value of the ratio of residual to the reference scenario value increases with fault distance, most pronounced in scenario 1b, where Q is increased from 82 to 16. For scenario 11a, where the more simple velocity model from Kalafat et al. (1987) is tested, we observe an increase in the ratio of the residual to the reference scenario value with increasing distance, meaning that the relative change in ground motion for this test is larger further away from the fault. We calculate the 5 % damped response spectra of acceleration at selected sites (see Figs. ES1 and ES2 for locations) for the reference scenario (see Fig. ES3 in the electronic supplement for both acceleration and velocity spectra). Comparing the simulated 5 % damped acceleration spectra with the Turkish Design Code (Ministry of Public Works and Settlement 7)in Fig 18 reveals that although the simulated ground motions are within the flat part of the spectra (periods.1.3 s). The predicted values exceeds the design spectra for periods larger than.3 s at the station in the centre of İzmir (Station 262) and north of the fault (Station 192) (for station locations, see Figs. ES1 and ES2 in the electronic supplement). These larger periods corresponds to the eigenfrequencies of the majority of the building stock in İzmir. We calculate the 5 % damped response spectra of acceleration (SA) and velocity (SV) at each simulation point from the simulated ground motion time histories for each scenario earthquake. From this we compute the geometric mean of the two horizontal components of SA and SV in all simulation points in four frequency bands: f<1 Hz, 1 Hz<f<3 Hz, 3 Hz<f<5 Hz and f>5 Hz. These frequency bands are important for the İzmir area due to the large variation in building stock. Finally, we calculate the average and standard deviation of the response spectral motion for all the scenarios (Figs. 19 and ). The largest average SA is simulated for