**3. The seismotectonic and seismicity of Tombak region**

spectrum of the synthetic time history produced in step 2.

Assuming that the duration of the pulse is independent of the source–station distance for stations located within ~10 km from the causative fault, the pulse period and the moment magnitude are related through the following empirical relationship obtained by least-squares

64 Engineering Seismology, Geotechnical and Structural Earthquake Engineering

In this section, we propose a very simplified methodology for generating realistic synthetic ground motions that are adequate for engineering analysis and design. We exploit the simple analytical model introduced in the present work to describe the coherent (long-period) component of motion and the stochastic (or engineering) approach to synthesize the incoherent (high-frequency) seismic radiation (for a review of the stochastic approach of ground motion synthesis; see Boore (1983) and Shinozuka (1988)). For the latter component of motion, due to the proximity of the point of observation to the source, it is necessary to use a source model that provides guidance as how to distribute the available seismic moment of the simulated event on the fault plane. Such a source model is the specific barrier model of Papageorgiou and Aki (1983). According to this model, an earthquake is visualized as a sequence of equalsize sub-events uniformly distributed on a rectangular fault plane. At the present time, the proposed mathematical model along with its scaling laws can take into account (with confi‐ dence) only for the forward directivity effect. Even though the analytical expression can replicate near-fault ground motion records that manifest the permanent-translation effect as well, the limited number of recordings with permanent translation does not permit the derivation of appropriate scaling laws. Therefore, the proposed analytical model should be utilized with caution for the generation of synthetic long-period ground motions that intend to incorporate the permanent translation effect. In these cases, the permanent offsets of the synthetic displacement time histories should be compatible with the tectonic environment and earthquake magnitude of the simulated event. The proposed methodology is written in

**1.** Select the moment magnitude, *MW*, of the potential earthquake and calculate the prevail‐ ing frequency, *fP*, by *fP*=1/*TP*. For selected values of the parameters *A*, *γ* and *ν* (or for a suite of values of these three parameters), generate the coherent component of acceleration time

**2.** For the selected fault–station geometry, generate the synthetic acceleration time histories for the moment magnitude, *MW*, specified previously, using the specific barrier model.

**3.** Calculate the Fourier transform of the synthetic acceleration time histories generated in

**4.** Subtract the Fourier amplitude spectrum of the synthetic time history generated in step 1 from the Fourier amplitude spectrum of the synthetic time history produced in step 2.

**5.** Construct a synthetic acceleration time history so that (a) its Fourier amplitude spectrum is the difference of the Fourier amplitude spectra calculated in step 4; and (b) its phase

history (or a suite of time histories) using equation (19).

log 2.2 0.4 *T M p W* =- + (24)

fit analysis:

MATLAB with the following steps:

steps 1 and 2.

The Zagros region is one of the most seismically active regions in Iran. The Tombak LNG terminal is located along the Persian Gulf northern coast, south of the Zagros Mountains, which mark the deforming zone separating Arabia (Arabian plate) and Central Iran (Eurasian plate) (Figure 3(a)). Location of Tombak area is presented in Figure 3(b). The massive LNG storage tanks exist in this terminal. These tanks have high importance from engineering and econom‐ ical point of view so seismic loads should be considered in their analysis and design. The relevant codes of LNG storage containers emphasize that a comprehensive seismic hazard investigation should be conducted for regional seismicity and earthquake events of known near-fault. amplitude spectra calculated in step 4; and (b) its phase coincides with the phase of the Fourier transform of the synthetic time history generated in step 2. 6. Superimpose the time histories generated in steps 1 and 5. The near-source pulse is shifted in time so that the peak of its envelope coincides with the time that the rupture front passes in front of the station. **3. The seismotectonic and seismicity of Tombak region**  The Zagros region is one of the most seismically active regions in Iran. The Tombak LNG terminal is located along the Persian Gulf

investigation should be conducted for regional seismicity and earthquake events of known near-fault.

West of the Makran coast, where oceanic crust is subducting beneath Eurasia, the collision of the Arabian shield with Iran has **Figure 3.** Location map: (a) Zagros folded zone, and (b) Tombak area

and 26°N in latitude and 50°E and 58°E in longitude.

**4. Estimation of the model parameters** 

Figure 3. Location map: (a) Zagros folded zone, and (b) Tombak area

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.

nism (strike, dip, slip) = (175, 85, 153) of the mainshock (Yamanaka 2003). The source dimension

**Parameters Values**

ρ*s*, β*s*, *V* , < *R*ΘΦ >, *F* , *R*<sup>0</sup> 2.8, 3.5, 0.707, 0.55, 2.0, 1.0

*Q, cQ 180f0.45, 3.5 km/s*

Site amplification Boore and Joyner (1997) generic rock

Source duration 0.5/*fa*

Path duration 0.05 *R*

Site diminution parameters (*fmax*,κ) 100.0, 0.03

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

**Magnitude Epicentral**

OBE (475 years) 6.5 5 11 12 SSE (5000 years) 7.0 5 14 15

**Tombak**

**distance (km) Depth (km) Hypocentral**

*r*<40 km: 1/*r r*≥ 40 km: (1/40)(40/*r*)

Simulation of Near-Field Strong Ground Motions using Hybrid Method

0.5

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

67

**distance (km)**

is therefore roughly estimated to be 20 *km* x 16 *km* (Yamanaka 2003).

Geometrical spreading (including factors to insure continuity of function)

motion predictions.

**Table 1.** Model parameters

ZFF-C

**Zone Seismic source**

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