**4. Estimation of the model parameters**

The source and earthquake parameters have been obtained from the pervious seismic hazard study which has been conducted for Tombak area. The most of the models based on the stochastic method are fundamentally point-source models. Although it is true that near and intermediate-field terms are lacking, in most applications the frequencies are high enough that the far-field terms dominate, even if the site is near the fault. Furthermore, the effects of a finitefault averaged over a number of sites distributed around the fault (to average over radiation pattern and directivity effects) can be captured in several ways: 1) using the closest distance to faulting as the source-to-site distance; 2) using a two-corner source spectrum; 3) allowing the geometrical spreading to be magnitude dependent. The material properties described by density *ρ*, and shear wave velocity *β*, are estimated to be 2.8 gr/cm3 and 3.5 km/sec, respectively. All parameters used for simulation are summarized in Table (1).

In the methodology of Beresnev and Atkinson (1997,1998), modelling of finite source requires information of the orientation and dimensions of fault plane, as well as information of the dimensions of sub-faults and the location of hypocenter. The trends of epicenteral and hypocenteral distribution are in accordance with the strike and dip angle of the focal mecha‐


#### **Table 1.** Model parameters

West of the Makran coast, where oceanic crust is subducting beneath Eurasia, the collision of the Arabian shield with Iran has uplifted the Zagros Mountains. The Zagros Mountains belt represents the early stage of a continental collision between the Arabian plate and the central Iran continental blocks. The Zagros Mountains are a seismically active region. Seismicity is restricted to the region between the Main Zagros Thrust and the Persian Gulf. Strong earth‐ quakes are thought to occur on blind active thrust faults, which do not reach the surface. Fault plane solutions of these earthquakes indicate displacement mainly on low to high-angle reverse faults at depth of 6-12 km in the uppermost part of the basement. Most of the earth‐ quakes for the region have generally M = 5.0 to 6.5, and have originated on sources beneath the decollement (Berberian 1995). Subduction on the main Zagros thrust has now ceased and it is seismically inactive (Ni and Barazangi 1986) except for the northern Zagros, where the surface trace of the thrust has been reactivated as right-slip main recent fault. The Zagros active fold-thrust belt lies on the north-eastern margin of the Arabian plate, on Precambrian (Pan-African) basement. It is composed of Cambrian to Neogene's folded series and is the result of five major tectonic events (Berberian and King 1981; Berberian 1983). The Zagros fold-thrust belt is composed of five units. The folds are parallel to the thrust faults. The axial part of the folds, striking NW SE, appears as broad asymmetrical folds with axial planes dipping to the NE and North. Their north-eastern limbs gently dip (20°) to the NW whereas their southwestern limbs are steeper (40°) to the SE reaching 60 to 80°down slope and in some cases are nearly vertical, overturned or thrusted. The Main Zagros Thrust Fault (MZTF) indicates a fundamental change in sedimentary and structural evolution and seismicity. It marks the geosuture between the two colliding plates of the Eurasia and the Arabia. The global zone taken into account lies between 32°N and 26°N in latitude and 50°E and 58°E in longitude.

66 Engineering Seismology, Geotechnical and Structural Earthquake Engineering

The source and earthquake parameters have been obtained from the pervious seismic hazard study which has been conducted for Tombak area. The most of the models based on the stochastic method are fundamentally point-source models. Although it is true that near and intermediate-field terms are lacking, in most applications the frequencies are high enough that the far-field terms dominate, even if the site is near the fault. Furthermore, the effects of a finitefault averaged over a number of sites distributed around the fault (to average over radiation pattern and directivity effects) can be captured in several ways: 1) using the closest distance to faulting as the source-to-site distance; 2) using a two-corner source spectrum; 3) allowing the geometrical spreading to be magnitude dependent. The material properties described by

In the methodology of Beresnev and Atkinson (1997,1998), modelling of finite source requires information of the orientation and dimensions of fault plane, as well as information of the dimensions of sub-faults and the location of hypocenter. The trends of epicenteral and hypocenteral distribution are in accordance with the strike and dip angle of the focal mecha‐

and 3.5 km/sec, respectively.

**4. Estimation of the model parameters**

density *ρ*, and shear wave velocity *β*, are estimated to be 2.8 gr/cm3

All parameters used for simulation are summarized in Table (1).

nism (strike, dip, slip) = (175, 85, 153) of the mainshock (Yamanaka 2003). The source dimension is therefore roughly estimated to be 20 *km* x 16 *km* (Yamanaka 2003).

Source parameters can be classified into two types (Irikura 2000): global source parameters and local source parameters. They represent different features of the fault source and are determined by different methods. The global source parameters characterize the macro feature of the entire source area and include spatial orientation of fault (location, attitude, buried depth), fault size (length, width, area), and both average slip and average rupture velocity on the fault plane. In the global source parameters both the slip type and spatial orientation are determined by seismogeology investigation and geophysical exploration; while the moment magnitude of the scenario earthquake caused by an active fault is estimated from its seismic hazard assessment. Fault size and average slip on the fault plane are also estimated by seismic scaling laws. In this study, the information for generating near-field strong ground motion such as magnitude related to each return period and the epicentral and hypocentral distances for stochastic method have been extracted from the seismogeology investigation that have been presented in Table (2). Table (3) lists the basic parameters used in the strong ground motion predictions.


**Table 2.** Parameter values of finite fault source model


to the earthquake magnitude and to distance from the source. This stochastic method is particularly useful for simulating the higher-frequency ground motions, and it is used to predict ground motions for regions of the world in which recordings of motion from damaging earthquakes are not available. This simple method has been successful in matching a variety of ground motion measures for earthquakes with seismic moments spanning more than 12 orders of magnitude. SMSIM (StochasticModel SIMulation or Strong Motion SIMulation) is a set of programs for simulating ground motions based on the stochastic method. Programs are included both for time-domain and for random vibration simulations. In addition, programs are included to produce Fourier amplitude spectra for the models used in the simulations and to convert shear velocity versus depth into frequency-dependent amplification. The necessary parameters in these models are distinguished for defining the theoretical relationships. In this study, the near-field strong motion time histories, obtained from the stochastic method, are presented for both levels of earthquakes (475y and 5000y) as shown in Figure 4. The long period

Using the magnitude for each return period, the duration of pulse is defined for both levels of earthquakes. The other parameters for the pulse are extracted from Mavroeidis and Papageorgiou (2003). The data has been obtained by the calibration procedure of

> **Oscillatory character of the signal**

6.5 475 50 1.5 134° 5 7.0 5000 90 2 100° 6.4

Simulation of Near-Field Strong Ground Motions using Hybrid Method

A simple and powerful method for simulating ground motions is based on the assumption that the amplitude of ground motion at a site can be specified in a deterministic way, with a random phase spectrum modified such that the motion is distributed over a duration related to the earthquake magnitude and to distance from the source. This stochastic method is particularly useful for simulating the higher-frequency ground motions, and it is used to predict ground motions for regions of the world in which recordings of motion from damaging earthquakes are not available. This simple method has been successful in matching a variety of ground motion measures for earthquakes with seismic moments spanning more than 12 orders of magnitude. SMSIM (StochasticModel SIMulation or Strong Motion SIMulation) is a set of programs for simulating ground motions based on the stochastic method. Programs are included both for time-domain and for random vibration simulations. In addition, programs are included to produce Fourier amplitude spectra for the models used in the simulations and to convert shear velocity versus depth into frequency-dependent amplification. The necessary parameters in these models are distinguished for defining the theoretical relationships. In this study, the near-field strong motion time histories, obtained from the stochastic method, are presented for both levels of earthquakes (475y and 5000y) as shown in Figure 4. The long period pulse has been calculated and presented in Figure 5

In this stage, the pulse acceleration superimposes to the synthetic acceleration time history. The near-source pulse is shifted in time so that the peak of its envelope coincides with the time of rupture front of station. The final acceleration time histories for both levels of earthquakes are shown in Figure 6. The response spectra, obtained from simulated strong ground motion analysis, for 5% damping ratio are shown in Figure 7 for return periods 475 and 5000 years. As it is seen in Figures 7(a) and 7(b), the response spectrum has a sudden increasing for period ranges of 1 to 4 and 2 to 6 for 475y and 5000y, respectively. Therefore, structures which their periods settle in these ranges are influenced from near-field due to the long period pulse ground motion. In Figure 8, the smoothed response spectrum, obtained for return period of 475 years, is compared with IBC 2000 and Standard No. 2800. This figure shows that the response spectra are close to each other in the period range of 0 to 1 second. When the period exceeds than 1

> **0 5 10 15 20 25 30 TIME (s)**

**Phase of Pulse** 

 **(degree)** 

69

**Time shift t0 (second)** 

http://dx.doi.org/10.5772/55682

actual near-fault strong ground motion records. The chosen parameter values have been summarized in Table (4).

**Amplitude of pulse A (cm/s)** 

In this stage, the pulse acceleration superimposes to the synthetic acceleration time history. The near-source pulse is shifted in time so that the peak of its envelope coincides with the time of rupture front of station. The final acceleration time histories for both levels of earthquakes are shown in Figure 6. The response spectra, obtained from simulated strong ground motion analysis, for 5% damping ratio are shown in Figure 7 for return periods 475 and 5000 years. As it is seen in Figures 7(a) and 7(b), the response spectrum has a sudden increasing for period ranges of 1 to 4 and 2 to 6 for 475y and 5000y, respectively. Therefore, structures which their periods settle in these ranges are influenced from near-field due to the long period pulse ground motion. In Figure 8, the smoothed response spectrum, obtained for return period of 475 years, is compared with IBC 2000 and Standard No. 2800. This figure shows that the response spectra are close to each other in the period range of 0 to 1 second. When the period exceeds than 1 second, the effect of long period pulse becomes apparent in the response spectra.

second, the effect of long period pulse becomes apparent in the response spectra.

Figure 4. Synthetic acceleration time histories for return periods: (a) 475 years, and (b) 5000 years

**Figure 4.** Synthetic acceleration time histories for return periods: (a) 475 years, and (b) 5000 years

(a) (b)

**-400 -300 -200 -100 0 100 200 300 400**

**ACCELERATION (cm/s2**

**)**

pulse has been calculated and presented in Figure 5 for both return periods.

Table 3. Basic parameters used in the strong ground motion predictions

**Return period (year)** 

**w**

Table 4. Input parameters for defining long period pulse

**5. Results and discussion** 

for both return periods.

**-300 -200 -100 0 100 200 300**

**ACCELERATION (cm/s2**

**)**

**0 5 10 15 20 25 30 TIME (s)**

**Table 3.** Basic parameters used in the strong ground motion predictions

Using the magnitude for each return period, the duration of pulse is defined for both levels of earthquakes. The other parameters for the pulse are extracted from Mavroeidis and Papa‐ georgiou (2003). The data has been obtained by the calibration procedure of actual near-fault strong ground motion records. The chosen parameter values have been summarized in Table (4).


**Table 4.** Input parameters for defining long period pulse

### **5. Results and discussion**

A simple and powerful method for simulating ground motions is based on the assumption that the amplitude of ground motion at a site can be specified in a deterministic way, with a random phase spectrum modified such that the motion is distributed over a duration related

figure shows that the response spectra are close to each other in the period range of 0 to 1 second. When the period exceeds than 1

**Oscillatory character of the** 

**Phase of Pulse** 

**Time shift t0 (second)** 

Using the magnitude for each return period, the duration of pulse is defined for both levels of earthquakes. The other parameters for the pulse are extracted from Mavroeidis and Papageorgiou (2003). The data has been obtained by the calibration procedure of

actual near-fault strong ground motion records. The chosen parameter values have been summarized in Table (4).

**Amplitude of pulse** 

to the earthquake magnitude and to distance from the source. This stochastic method is particularly useful for simulating the higher-frequency ground motions, and it is used to predict ground motions for regions of the world in which recordings of motion from damaging earthquakes are not available. This simple method has been successful in matching a variety of ground motion measures for earthquakes with seismic moments spanning more than 12 orders of magnitude. SMSIM (StochasticModel SIMulation or Strong Motion SIMulation) is a set of programs for simulating ground motions based on the stochastic method. Programs are included both for time-domain and for random vibration simulations. In addition, programs are included to produce Fourier amplitude spectra for the models used in the simulations and to convert shear velocity versus depth into frequency-dependent amplification. The necessary parameters in these models are distinguished for defining the theoretical relationships. In this study, the near-field strong motion time histories, obtained from the stochastic method, are presented for both levels of earthquakes (475y and 5000y) as shown in Figure 4. The long period pulse has been calculated and presented in Figure 5 for both return periods. 7.0 5000 90 2 100° 6.4 Table 4. Input parameters for defining long period pulse **5. Results and discussion**  A simple and powerful method for simulating ground motions is based on the assumption that the amplitude of ground motion at a site can be specified in a deterministic way, with a random phase spectrum modified such that the motion is distributed over a duration related to the earthquake magnitude and to distance from the source. This stochastic method is particularly useful for simulating the higher-frequency ground motions, and it is used to predict ground motions for regions of the world in which recordings of motion from damaging earthquakes are not available. This simple method has been successful in matching a variety of ground motion measures for earthquakes with seismic moments spanning more than 12 orders of magnitude. SMSIM (StochasticModel SIMulation or Strong Motion SIMulation) is a set of programs for simulating ground motions based on the stochastic method. Programs are included both for time-domain and for random vibration simulations. In addition, programs are

Table 3. Basic parameters used in the strong ground motion predictions

**Return period** 

**w**

**Parameters Values** Fault orientation Strike 122°, Dip 40°

Fault dimensions along strike and dip (*km*) 28 × 16 Burial depth of upper limit of the fault (*km*) 5.0

Subfault dimensions along strike and dip (*km*) 1 × 1

Moment magnitude (*MW*) 6.7

Stress drop (bar) 50

Geometrical spreading 1/*R*

Crustal shear wave velocity (*km*/*s*) 3.7

**Table 3.** Basic parameters used in the strong ground motion predictions

68 Engineering Seismology, Geotechnical and Structural Earthquake Engineering

**Amplitude of pulse A (cm/s)**

(4).

**ω**

**Return period (year)**

**5. Results and discussion**

**Table 4.** Input parameters for defining long period pulse

Crustal density (*g*/*cm3*) 2.8

Using the magnitude for each return period, the duration of pulse is defined for both levels of earthquakes. The other parameters for the pulse are extracted from Mavroeidis and Papa‐ georgiou (2003). The data has been obtained by the calibration procedure of actual near-fault strong ground motion records. The chosen parameter values have been summarized in Table

> **Oscillatory character of the signal γ**

6.5 475 50 1.5 134° 5 7.0 5000 90 2 100° 6.4

A simple and powerful method for simulating ground motions is based on the assumption that the amplitude of ground motion at a site can be specified in a deterministic way, with a random phase spectrum modified such that the motion is distributed over a duration related

**Phase of Pulse ν (degree)**

**Time shift t0 (second)**

Windowing function Saragoni-Hart Kappa 0.05

Rupture velocity (*km*/*s*) 0.8 × shear wave velocity

*Q(f)* 150*f* 0.5

In this stage, the pulse acceleration superimposes to the synthetic acceleration time history. The near-source pulse is shifted in time so that the peak of its envelope coincides with the time of rupture front of station. The final acceleration time histories for both levels of earthquakes are shown in Figure 6. The response spectra, obtained from simulated strong ground motion analysis, for 5% damping ratio are shown in Figure 7 for return periods 475 and 5000 years. As it is seen in Figures 7(a) and 7(b), the response spectrum has a sudden increasing for period ranges of 1 to 4 and 2 to 6 for 475y and 5000y, respectively. Therefore, structures which their periods settle in these ranges are influenced from near-field due to the long period pulse ground motion. In Figure 8, the smoothed response spectrum, obtained for return period of 475 years, is compared with IBC 2000 and Standard No. 2800. This figure shows that the response spectra are close to each other in the period range of 0 to 1 second. When the period exceeds than 1 second, the effect of long period pulse becomes apparent in the response spectra. included to produce Fourier amplitude spectra for the models used in the simulations and to convert shear velocity versus depth into frequency-dependent amplification. The necessary parameters in these models are distinguished for defining the theoretical relationships. In this study, the near-field strong motion time histories, obtained from the stochastic method, are presented for both levels of earthquakes (475y and 5000y) as shown in Figure 4. The long period pulse has been calculated and presented in Figure 5 for both return periods. In this stage, the pulse acceleration superimposes to the synthetic acceleration time history. The near-source pulse is shifted in time so that the peak of its envelope coincides with the time of rupture front of station. The final acceleration time histories for both levels of earthquakes are shown in Figure 6. The response spectra, obtained from simulated strong ground motion analysis, for 5% damping ratio are shown in Figure 7 for return periods 475 and 5000 years. As it is seen in Figures 7(a) and 7(b), the response spectrum has a sudden increasing for period ranges of 1 to 4 and 2 to 6 for 475y and 5000y, respectively. Therefore, structures which their periods settle in these ranges are influenced from near-field due to the long period pulse ground motion. In Figure 8, the smoothed response spectrum, obtained for return period of 475 years, is compared with IBC 2000 and Standard No. 2800. This

Figure 4. Synthetic acceleration time histories for return periods: (a) 475 years, and (b) 5000 years

second, the effect of long period pulse becomes apparent in the response spectra.

**Figure 4.** Synthetic acceleration time histories for return periods: (a) 475 years, and (b) 5000 years

**Figure 5.** Long period pulses for return periods: (a) 475 years, and (b) 5000 years (a) (b)

Figure 5. Long period pulses for return periods: (a) 475 years, and (b) 5000 years

**Figure 8.** Comparison between response spectra (Blue Curve: 475y, Red Curve: Standard No. 2800, Gray Curve: IBC 2000)

Simulation of Near-Field Strong Ground Motions using Hybrid Method

http://dx.doi.org/10.5772/55682

71

Soil investigation and borings are carried out for the detail design of storage tank foundations and spill basin structures of NIOC LNG Project. The site is located at the Persian Gulf coast in Tombak region, approximately 60 *km* away from Assaluyeh city, in Bushehr province. Standard penetration test has been running since initial stages of drilling operation. SPT tests have been performed approximately in each 1.5 *m* advance of drilling. Thus, there is a full set of data covering whole area with SPT results. Seismic tests of downhole are done for full depth (80 *m*). Shear and compression waves velocities (*Vs* & *Vp*) are determined, accordingly. The results are presented in Table (5). The longitudinal wave velocity increases due to water table. The water table in borehole is approximately 9.0 *m* below the ground. Generally, the shear wave velocity increases versus depth due to an increase of soil density. According to Standard No. 2800, the shear wave velocity more than 760 *m*/*s* is assigned as rock; therefore the seismic bed rock is located at 10 *m* below the ground. The mentioned standard is used to classify the soil type which is type II and class C in this project. One-dimensional ground response analysis of the site is carried out

by the equivalent linear approach using SHAKE 91 program (Idriss et al. 1992).

**6. Site response analysis**

Figure 6. Final acceleration time histories for return periods: (a) 475 years, and (b) 5000 years

**)**

**1100 1000 )Figure 6.** Final acceleration time histories for return periods: (a) 475 years, and (b) 5000 years

**800 900 1000**

**Figure 7.** Response spectra of final acceleration time histories for return periods: (a) 475 years, and (b) 5000 years

**6. Site response analysis** 

**700 800 900**

Figure 8. Comparison between response spectra (Blue Curve: 475y, Red Curve: Standard No. 2800, Gray Curve: IBC 2000)

Soil investigation and borings are carried out for the detail design of storage tank foundations and spill basin structures of NIOC LNG Project. The site is located at the Persian Gulf coast in Tombak region, approximately 60 *km* away from Assaluyeh city, in Bushehr province. Standard penetration test has been running since initial stages of drilling operation. SPT tests have been performed approximately in each 1.5 *m* advance of drilling. Thus, there is a full set of data covering whole area with SPT results. Seismic tests of down-hole are done for full depth (80 *m*). Shear and compression waves velocities (*Vs* & *Vp*) are determined,

**Figure 8.** Comparison between response spectra (Blue Curve: 475y, Red Curve: Standard No. 2800, Gray Curve: IBC 2000)

#### **6. Site response analysis**

**Figure 5.** Long period pulses for return periods: (a) 475 years, and (b) 5000 years

70 Engineering Seismology, Geotechnical and Structural Earthquake Engineering

Figure 5. Long period pulses for return periods: (a) 475 years, and (b) 5000 years

Figure 6. Final acceleration time histories for return periods: (a) 475 years, and (b) 5000 years

(a) (b)

**ACCELERATION (cm/s2**

**)**

(a) (b)

**-400 -300 -200 -100 0 100 200 300 400**

**)**

**Figure 6.** Final acceleration time histories for return periods: (a) 475 years, and (b) 5000 years

**Response Accelaration(cm/s2**

(a) (b)

**Figure 7.** Response spectra of final acceleration time histories for return periods: (a) 475 years, and (b) 5000 years

**Response Accelaration(cm/s**

**2**

**)**

Figure 7. Response spectra of final acceleration time histories for return periods: (a) 475 years, and (b) 5000 years

(a) (b)

**0 1 2 3 4 5 6 7 8 9 10 Period(sec.)**

> **0 1 2 3 4 5 6 7 8 9 10 Period(sec.)**

**0 10 20 30 40 TIME (s)**

Figure 8. Comparison between response spectra (Blue Curve: 475y, Red Curve: Standard No. 2800, Gray Curve: IBC 2000)

Soil investigation and borings are carried out for the detail design of storage tank foundations and spill basin structures of NIOC LNG Project. The site is located at the Persian Gulf coast in Tombak region, approximately 60 *km* away from Assaluyeh city, in Bushehr province. Standard penetration test has been running since initial stages of drilling operation. SPT tests have been performed approximately in each 1.5 *m* advance of drilling. Thus, there is a full set of data covering whole area with SPT results. Seismic tests of down-hole are done for full depth (80 *m*). Shear and compression waves velocities (*Vs* & *Vp*) are determined,

**6. Site response analysis** 

**Response Accelaration(cm/s2**

**)**

**2**

**Response Accelaration(cm/s**

**)**

**-400 -300 -200 -100 0 100 200 300 400**

**ACCELERATION (cm/s2**

**)**

**0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 Period(sec.)**

**0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 Period(sec.)**

**0 5 10 15 20 TIME (s)**

> Soil investigation and borings are carried out for the detail design of storage tank foundations and spill basin structures of NIOC LNG Project. The site is located at the Persian Gulf coast in Tombak region, approximately 60 *km* away from Assaluyeh city, in Bushehr province. Standard penetration test has been running since initial stages of drilling operation. SPT tests have been performed approximately in each 1.5 *m* advance of drilling. Thus, there is a full set of data covering whole area with SPT results. Seismic tests of downhole are done for full depth (80 *m*). Shear and compression waves velocities (*Vs* & *Vp*) are determined, accordingly. The results are presented in Table (5). The longitudinal wave velocity increases due to water table. The water table in borehole is approximately 9.0 *m* below the ground. Generally, the shear wave velocity increases versus depth due to an increase of soil density. According to Standard No. 2800, the shear wave velocity more than 760 *m*/*s* is assigned as rock; therefore the seismic bed rock is located at 10 *m* below the ground. The mentioned standard is used to classify the soil type which is type II and class C in this project. One-dimensional ground response analysis of the site is carried out by the equivalent linear approach using SHAKE 91 program (Idriss et al. 1992).


(a) (b)

(c) (d)

(a) (b)

**Figure 10.** Spectral accelerations obtained by response analyses for return periods: (a) 475 years, and (b) 5000 years

The present study has been focused on simulating near-field strong ground motions using hybrid method for Tombak area in south-eastern part of Iran. The simulation of ground motion has been carried out using the stochastic method proposed by Boore (2003). Afterwards, the analytical model proposed by Mavroeidis and Papageorgiou (2003) is applied to consider the impulsive character of near-fault ground motion. Then, the response of mentioned site under simulated ground motion has been studied by conducting one dimensional ground response

**Figure 9.** Acceleration time histories: (a) simulated on bedrock for return period = 475 years, (b) obtained on ground surface for return period = 475 years, and (c) simulated on bedrock for return period = 5000 years, (d) obtained on

ground surface for return period = 5000 years

**7. Conclusions**

)

Simulation of Near-Field Strong Ground Motions using Hybrid Method

http://dx.doi.org/10.5772/55682

73

#### **Table 5.** Soil properties

According to the geotechnical site investigation, the soil type and the thickness of each layer are defined. From depth 0.0 *m* to 8.0 *m*, there are very diverse layers including boulder, gravel with some silty sand or sandy silt. SPT tests for these layers are refused because of coarse size of grains. From 8.0 *m* to 41.6 *m*, a very dense light brown sandy gravel with some cobbles is observed. SPT values are very high for this layer and some trace silt is seen from depth 32.0 *m* to 32.5 *m*. After this layer, a very dense light brown silty sand with some gravel is exist with 2.0 *m* thick. From 43.5 *m* to 50.0 *m*, we can see a very dense light brown sandy gravel with some cobbles and trace silt and clay. This layer changes to very dense gray silty sand with trace boulder (at depth: 50.0 *m* to 52.0 *m*). From 52.0 *m* to 52.6 *m*, borehole drilling machine interfaces to a piece of rock presenting by "boulder" phrase in borehole log. From 52.6 *m* to 54.0 *m*, borehole log shows a very dense to medium dense of gray silty sand. This layer is become very dense and its color changes to light brown from depth 54.0 *m* to 61.0 *m*. From depth 61.0 *m* to 64.0 *m*, some gravel and trace silt is added to last mentioned layer, so we can see a silty sand with some gravel and trace silt. From 64.0 *m* to 73.6 *m*, borehole log shows a very dense light brown sandy gravel with trace cobbles and clay. At the end of borehole, to depth 80.0 *m*, borehole log shows a very dense light brown silty sand with trace gravel and clay. Considering the results, particularly the seismic down-hole and SPT data, it is found that the average shear wave velocity equals approximately to 650 *m*/*s* and the SPT number is more than 50. For site specific response analysis, it is recommended to use shear wave velocity measured in the field. The average shear wave velocity of soil within 30 *m* was found to be around 650 *m*/*s*. The simulated ground motion is used as the input motion for ground response analysis. The results of ground response analysis are presented in Figure 9. The variation of maximum acceleration with depth as shown in Figure 9 indicates that the increase of PGA at the surface is about 1.06 and 1.11 times higher than those on the bed rock for return periods of 475 years and 5000 years, respectively. The response spectra obtained from ground response analysis for 5% damping ratio are shown in Figure 10.

**Figure 9.** Acceleration time histories: (a) simulated on bedrock for return period = 475 years, (b) obtained on ground surface for return period = 475 years, and (c) simulated on bedrock for return period = 5000 years, (d) obtained on ground surface for return period = 5000 years

**Figure 10.** Spectral accelerations obtained by response analyses for return periods: (a) 475 years, and (b) 5000 years

#### **7. Conclusions**

**Depth Density** *Vp Vs E G K*

72 Engineering Seismology, Geotechnical and Structural Earthquake Engineering

**Table 5.** Soil properties

ratio are shown in Figure 10.

*m gr/cm3 m/s m/s MPa MPa MPa*

0-1.3 2.1 570 330 571 229 377 0.25 1.3-4 2.1 850 500 1297 525 817 0.24 4-5.5 2.1 1040 600 1891 756 1263 0.25 5.5-10 2.1 1500 700 2801 1029 3353 0.36 10-20 2.25 1520 760 3466 1300 3466 0.33 20-30 2.3 1600 800 3925 1472 3925 0.33 30-42 2.15 1600 835 3936 1499 3505 0.31 42-52 1.96 1650 870 3879 1484 3358 0.31 52-62 2.2 1600 820 3911 1479 3660 0.32 62-78 2.17 1650 900 4529 1758 3564 0.29

According to the geotechnical site investigation, the soil type and the thickness of each layer are defined. From depth 0.0 *m* to 8.0 *m*, there are very diverse layers including boulder, gravel with some silty sand or sandy silt. SPT tests for these layers are refused because of coarse size of grains. From 8.0 *m* to 41.6 *m*, a very dense light brown sandy gravel with some cobbles is observed. SPT values are very high for this layer and some trace silt is seen from depth 32.0 *m* to 32.5 *m*. After this layer, a very dense light brown silty sand with some gravel is exist with 2.0 *m* thick. From 43.5 *m* to 50.0 *m*, we can see a very dense light brown sandy gravel with some cobbles and trace silt and clay. This layer changes to very dense gray silty sand with trace boulder (at depth: 50.0 *m* to 52.0 *m*). From 52.0 *m* to 52.6 *m*, borehole drilling machine interfaces to a piece of rock presenting by "boulder" phrase in borehole log. From 52.6 *m* to 54.0 *m*, borehole log shows a very dense to medium dense of gray silty sand. This layer is become very dense and its color changes to light brown from depth 54.0 *m* to 61.0 *m*. From depth 61.0 *m* to 64.0 *m*, some gravel and trace silt is added to last mentioned layer, so we can see a silty sand with some gravel and trace silt. From 64.0 *m* to 73.6 *m*, borehole log shows a very dense light brown sandy gravel with trace cobbles and clay. At the end of borehole, to depth 80.0 *m*, borehole log shows a very dense light brown silty sand with trace gravel and clay. Considering the results, particularly the seismic down-hole and SPT data, it is found that the average shear wave velocity equals approximately to 650 *m*/*s* and the SPT number is more than 50. For site specific response analysis, it is recommended to use shear wave velocity measured in the field. The average shear wave velocity of soil within 30 *m* was found to be around 650 *m*/*s*. The simulated ground motion is used as the input motion for ground response analysis. The results of ground response analysis are presented in Figure 9. The variation of maximum acceleration with depth as shown in Figure 9 indicates that the increase of PGA at the surface is about 1.06 and 1.11 times higher than those on the bed rock for return periods of 475 years and 5000 years, respectively. The response spectra obtained from ground response analysis for 5% damping

**υ**

The present study has been focused on simulating near-field strong ground motions using hybrid method for Tombak area in south-eastern part of Iran. The simulation of ground motion has been carried out using the stochastic method proposed by Boore (2003). Afterwards, the analytical model proposed by Mavroeidis and Papageorgiou (2003) is applied to consider the impulsive character of near-fault ground motion. Then, the response of mentioned site under simulated ground motion has been studied by conducting one dimensional ground response analysis. The results can be used for estimating the probable ground motion acceleration timehistories to be used in the hazard analysis of specific sites in the region under study, particu‐ larly for performance analysis of exiting structures. The ability of this hybrid method in simulating strong motions is also shown in this study. The simulation parameters, obtained in this study, can be used to asses the strong-motion level at a much larger number of sites, where no record is available, to investigate how different characteristics of motion affect the damage distribution in the Tombak region. Based on the above study the following conclusions are derived:

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Simulation of Near-Field Strong Ground Motions using Hybrid Method

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