**1. Introduction**

Earthquake disaster investigations have shown that numerous strong earthquakes are caused by remobilization of active faults. Many casualties and severe damages to structures as well as huge economic losses have resulted from ground motions of strong earthquakes caused by active faults buried under urban areas. Recently, both potential hazard and defenses of active faults concealed under urban area has become a grand research subject paid highly attention to by the seismologists. Near-field strong ground motions, especially their high frequency content, are intensively affected by both slip heterogeneity on fault plane and rupture process of an earthquake fault. In the simulations of near-field strong ground motions, modeling effective finite fault source is very important. The gradually increasing number of recorded near source time histories has recently enabled strong motion seismologists to analyze more precisely the character of the near-fault ground motions and therefore contribute to the physical understanding of those features that control them (Malagnini et al. 2002, 2011; Akinci et al. 2010; D'Amico et al. 2010). Mavroeidis and Papageorgiou (2002) presented a compre‐ hensive review and study of the factors that influence the near-source ground motions.

The stochastic method of synthesizing ground motion based on seismology interests engi‐ neers specifically in simulating higher-frequency ground motions (Akinci et al. 2001). The method is widely used to predict ground motions for regions, in which ground motion record‐ ings from past earthquake are not available (Boore 2003). For far field, the point source model of Boore (1983)isveryeffective;however,fornear-field,themethodcannotincorporate the factors whichhave significant effectonthenear-fieldstronggroundmotions, andyields anoverestima‐ tion of such ground motions. The stochastic ground motion modelling technique, also known as the band limited white-noise method, has been first described by Boore (1983). Ever since, manyresearchershaveappliedthemethodtosimulategroundmotions frompoint sources (e.g., Boore and Atkinson 1987; Atkinson and Boore 1995; Zafarani et al. 2005; D'Amico et al. 2012).

© 2013 Ebrahimian; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 Ebrahimian; licensee InTech. This is a chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 Ebrahimian; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

On the other hand, realistic acceleration time-histories should be employed in structural analysis to reduce the uncertainties in estimating the standard engineering parameters (Hutchings 1994), particularly for non-linear seismic behavior of structures. Thus, designers need to know the dynamic characteristics of predicted ground motion consistent with source rupture for a particular site to be able to adequately design an earthquake-resistant structure. Hall et al. (1995), Makris (1997), Chopra and Chintanapakdee (2001), Zhang and Iwan (2002) have experimentally as well as analytically studied the elastic and inelastic response of engineering structures subjected to actual near-fault records or simplified waveforms intend‐ ing to represent the typical ground motion pulses observed in near-field regions.

At high frequencies (*f* > 1 Hz), ground motions become increasingly stochastic in nature. The stochastic methods are generally capable of matching the spectral amplitudes of high frequen‐ cy ground motions, but are generally not capable of matching the recorded waveforms (Somerville 1998). Firstly, ground motions are estimated by identifying the major regional faults and propagating seismic waves generated at these potential sources to the site of interest. The two commonly used techniques, finite-fault and point source methods of Boore and Atkinson (1987) and Beresnev and Atkinson (1997, 1998) are used for simulation of earth‐

Simulation of Near-Field Strong Ground Motions using Hybrid Method

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

57

The main objective of the chapter is to simulate the near-fault strong motion records. Simula‐ tion of ground motions is carried out using the hybrid method proposed by Mavroeidis and Papageorgiou (2003) and the stochastic model of Boore (2003). Due to unavailability of strong recorded ground motion, the stochastic method proposed by Boore (2003) is applied to simulate the acceleration time histories. The ground motion spectrum is generated by Atkinson and Boore model (1995). Firstly, macro-source parameters characterizing the whole source area, i.e. global source parameters such as fault length, fault width, rupture area, and average slip on the fault plane are estimated; secondly, slip distributions characterizing heterogeneity or roughness on the fault plane, i.e. local source parameters are reproduced by the hybrid slip model; finally, the finite fault source model, which is developed based on the global and local source parameters is combined with the stochastic method. A simple, yet effective, analytical model proposed by Mavroeidis and Papageorgiou (2003) is also used to adequately describe the impulsive character of near-fault ground motions both qualitatively and quantitatively. The calculated response spectra are compared with those, mentioned in International Building Code (IBC 2000) and Iranian Code of Practice for Seismic Resistance Design Building (Standard No. 2800) to validate the availability and practicability of the proposed method for near-field Tombak site at south-eastern part of Iran. This site includes massive LNG storage plants near to fault. Then, the response of mentioned site under simulated ground motion has been studied by conducting one dimensional ground response analysis. According to the above study, the bed rock and ground surface accelerations of the site are provided. The results can be used in hazard analysis of specific sites in the considered region, particularly for the performance

A simple and powerful method for simulating ground motions is to combine parametric or functional descriptions of the ground motion's amplitude spectrum with a random phase spectrum modified such that the motion is distributed over a duration related to the earthquake magnitude and to the distance from the source. This method of simulating ground motions often goes by the name ''Stochastic method''. It is particularly useful for simulating the higherfrequency ground motions of most interest to engineers (generally, *f* > 1 *Hz*), and it is widely used to predict ground motions for regions of the world in which recordings of motion from potentially damaging earthquakes are not available. One of the essential characteristics of the

quakes. Both techniques have an omega-squire spectrum.

analysis of structures.

**2. Simulation method**

During the past decades, much effort has been given in reliable simulation of strong ground motion from finite faults through methodologies that include theoretical or semi-empirical modeling of the parameters affecting shape, duration and frequency content of the strong motion records. Due to unavailability of strong recorded ground motion, simulation of ground motion has been carried out using the stochastic method proposed by Boore (2003). The ground motion spectrum has been generated by Atkinson and Boore model (1995). Even though the success of the point-source model has been pointed out repeatedly, it is also well known that it often breaks down, especially near the sources of large earthquakes. Recently, Beresnev and Atkinson (1997) have proposed a technique that overcomes the limitation posed by the hypothesis of a point source. Their technique is based on the original idea of Hartzell (1979) to model large events by the summation of smaller ones. In Beresnev and Atkinson (1997), the high-frequency seismic field near the epicentre of a large earthquake is modeled by subdivid‐ ing the fault plane into a certain number of sub-elements and summing their contributions, with appropriate time delays, at the observation point. Each element is treated as a point source. A stochastic model is used to calculate the ground motion contribution from each subelement, while the propagation effects are empirically modeled. Combining the stochastic method with the finite fault source model, Silva (1997), Beresnev and Atkinson (1998), Motazedian and Atkinson (2005) have proposed different methods, which could be effective for simulating or predicting near-field ground motions.

Two Californian earthquake events may be characterized as historical milestones related to near-source ground motions: the 1966 Parkfield and the 1971 San Fernando earthquakes. The 1966 Parkfield, California, event provided the now famous Station 2 (C02) record at a distance of only 80 m from the fault break (Housner and Trifunac 1967). Modern quantitative analysis of strong ground motion observations was started with this record. Aki (1968) and Haskell (1969) demonstrated that the observed transverse (i.e., fault-normal) displacement component of this ground motion record, which exhibited a simple impulsive form, was precisely what is expected for a right-lateral strike-slip rupture propagating from northwest to southeast. The 1971 San Fernando, California, earthquake provided the equally well-known Pacoima Dam (PCD) record. The strike-normal velocity component of this record also exhibited an impulsive character that several investigators attempted to model (e.g., Boore and Zoback 1974; Niazy 1975; Bouchon 1978). In addition, this record was the one that made earthquake engineers recognize the severe implications of the impulsive characteristics of near-source ground motions on flexible structures.

At high frequencies (*f* > 1 Hz), ground motions become increasingly stochastic in nature. The stochastic methods are generally capable of matching the spectral amplitudes of high frequen‐ cy ground motions, but are generally not capable of matching the recorded waveforms (Somerville 1998). Firstly, ground motions are estimated by identifying the major regional faults and propagating seismic waves generated at these potential sources to the site of interest. The two commonly used techniques, finite-fault and point source methods of Boore and Atkinson (1987) and Beresnev and Atkinson (1997, 1998) are used for simulation of earth‐ quakes. Both techniques have an omega-squire spectrum.

The main objective of the chapter is to simulate the near-fault strong motion records. Simula‐ tion of ground motions is carried out using the hybrid method proposed by Mavroeidis and Papageorgiou (2003) and the stochastic model of Boore (2003). Due to unavailability of strong recorded ground motion, the stochastic method proposed by Boore (2003) is applied to simulate the acceleration time histories. The ground motion spectrum is generated by Atkinson and Boore model (1995). Firstly, macro-source parameters characterizing the whole source area, i.e. global source parameters such as fault length, fault width, rupture area, and average slip on the fault plane are estimated; secondly, slip distributions characterizing heterogeneity or roughness on the fault plane, i.e. local source parameters are reproduced by the hybrid slip model; finally, the finite fault source model, which is developed based on the global and local source parameters is combined with the stochastic method. A simple, yet effective, analytical model proposed by Mavroeidis and Papageorgiou (2003) is also used to adequately describe the impulsive character of near-fault ground motions both qualitatively and quantitatively. The calculated response spectra are compared with those, mentioned in International Building Code (IBC 2000) and Iranian Code of Practice for Seismic Resistance Design Building (Standard No. 2800) to validate the availability and practicability of the proposed method for near-field Tombak site at south-eastern part of Iran. This site includes massive LNG storage plants near to fault. Then, the response of mentioned site under simulated ground motion has been studied by conducting one dimensional ground response analysis. According to the above study, the bed rock and ground surface accelerations of the site are provided. The results can be used in hazard analysis of specific sites in the considered region, particularly for the performance analysis of structures.
