cut-off frequencies (Fig. 4.10left and Fig. 4.10right). For the discretized solution of sub-fault
4×4km (Fig. 4.10), the ME in the triangle area behind rupture propagation for the case of a cut-off frequency of 0.33 Hz (Fig. 4.10 left) area, is larger than that in the same region for the case of a cut-off frequency of 0.25 Hz (Fig. 4.10 right) which means the larger the cut-off frequency, the larger is the misfit introduced by the discretizing of the fault plane.
Figure 4.10: Cut-off frequency effect on the ground motion accuracy from discretized sources. Misfit energy (ME) between the seismogram (low-pass filtered with different cut-off frequen- cies) of one discretized solution (i.e., 2×2 km, 3×3 km, 4×4 km, vertical axis) and the “continuous” solution is shown over the study area. Left. Cut-off frequency of 0.33 Hz. Right. Cut-off frequency of 0.25 Hz. They-component of velocity is adopted.
4.1.4 Sub-fault Size
The key point about the discretization of the fault plane is how big the sub-fault could possibly be. To get that value, how the misfit introduced by the discretization of the fault plane (into elemental sub-faults) changes with the sub-fault size should be investigated. We can observe that trend from Fig. 4.8, Fig. 4.9and Fig. 4.10. With the increase of the sub-fault size, the ME of the entire working area is getting larger. Therefore we conclude that the growth of the sub-fault size will lead to more misfits to the synthesized seismic motions.
4.2
Conclusions and Hints to the Future Work
In order to simulate an earthquake or fault system with a larger number of rupture processes, we developed a new method named Numerical Green’s function method. First we test the accuracy of this method with a few of hypothetic earthquake simulations.
A pilot study is carried out for a homogeneous medium to investigate how different pa- rameters will affect the accuracy of our method comparing to the true solution, in our work we take the quasi-continuous solution. Several parameters could affect the simulation of the finite fault earthquakes. First we investigate how the wave form is affected by these parame- ters. The misfit energy between two seismograms (discretized vs. “continuous” solutions) are
adopted to calibrate the accuracy. Our conclusions are: (1) the bigger the rupture velocity, the more accurate is the synthesis; (2) the accuracy of the synthesis at different receivers is affected by the directivity effect; (3) the accuracy of the synthesis decreases with the growth of the sub-fault size; (4) the accuracy of the synthesis decreases with the cut-off frequency used to filter the ground motions.
This quantitative research shows us the basic trends of how the misfit introduced by the discretization of fault plane varies with different parameters in a homogeneous media. With these results we can move to a real 3D media and choose a seismically active fault or fault system. We choose the Los Angeles basin as our major working area and investigate the optimal sub-fault size for this area in the following chapter.
Chapter 5
NGF Data-Base
There are lots of seismically active regions in the world which could be used as our working area. The Los Angeles basin has high population and industrial density and is known as an area of high seismic risk. Plenty of studies have been carried out for the basin structure investigation for a better earthquake risk assessment. Also many studies have been done to reveal the fault and fault system buried in this region. Thus we choose this region as our target for the trial of our method. In this chapter we present the necessary information about the seismicity, the velocity structure and the target fault geometry in 3D. The necessary verification in terms of sub-fault size is redone for our 3D target area. Some theories about the wave propagation in the near-source region are shown here to provide some basic knowledge for people unfamiliar with this topic.
The physics about the wave propagation in the near-source is briefly described to provide some fundamental knowledge for a better presentation and discussion of the results.
5.1
Los Angeles Basin
The greater Los Angeles region is tectonically complex and seismically active. From the geo- logical point of view, the Los Angeles basin is a deep sedimentary basin. This basin is located at the junction of the northern Peninsular Ranges and the central Transverse Ranges. Dur- ing Miocene time, the basin started to build up through a pull-apart process (Wright,1987). Five million years ago, the opening Gulf of California migrated the boundary between the Pacific and the North America plates to the San Andreas fault (SAF) east of the basin. This geological process created a left step (known as the Big Bend) along a right-lateral strike slip SAF north and east of the basin (Atwater,1989). As moving in the NW direction toward the Big Bend, the region started to undergo compression. Following basin evolutions are involved volcanism, uplift, extension, block rotation, pulling apart, shear faulting, compression, and folding (e.g., Campbell and Yerkes,1976; Wright,1987). At the present time the sediments in the central basin are more than 10 km thick which form a northwest-southeast elongated synclinorium whose flanks are folded and cut by a group of Quaternary active faults (Ziony and Yerkes,1985).
Present day crustal deformation, south of the basin, is dominated by NW-trending strike slip faults (e.g., the SAF, the San Jacinto fault (SJF), the Elsinore fault, and the Newport- Inglewood fault). East of the basin, the SJF merges with the SAF. The Whittier-Elsinore fault system, the Newport-Inglewood fault, and the Palos Verdes fault cut through the east and
west flanks of the basin (Ziony and Yerkes,1985). In the north of the basin, a frontal thrust fault system has pushed up the San Gabriel Mountains. This fault system is defined by the Sierra Madre-Cucamonga fault system along the SE and the Santa Susana fault along the SW. The Santa Monica Mountains have been uplifted by the E-W trending Malibu-Santa Monica- Raymond Hill fault system (Davis et al., 1989). Beneath most of the central and western Transverse Ranges, horizontal detachment in the lower crust or at the Moho boundary has been suggested (e.g., Bird and Rosenstock,1984;Weldon and Humphreys,1986;Namson and Davis,1988;Huftile and Yeats,1995). Furthermore, six major thrust or oblique fault systems have been identified in and around the Los Angeles basin area by recent geomorphological and trenching studies (Dolan et al.,1995): the Sierra Madre-Cucamonga system, the Los Angeles basin fault system, the Santa Monica Mountains fault system, the Oak Ridge fault system, the San Cayetano fault, and the Palos Verdes fault. Dolan et al. (1995) have presented the detail description of the locations and scale of thrust and horizontal detachments occurring in the region.
Earthquakes also help illuminate the tectonics of the Los Angeles basin. Earthquake focal mechanisms show a mixture of NW dextral strike slip and N-S convergence (Hauksson,1990). Earthquakes of right lateral strike slip faulting represented by the 1933 M 6.4 Long Beach earthquake dominate the seismicity south of the basin. North of the basin, many of the earthquakes have thrust mechanisms, often coupled with left-lateral displacements. Examples include the 1971 San Fernando M 6.6, the 1987 Whittier Narrows M 5.9, the 1991 Sierra Madre M 5.8, and the 1994 Northridge M 6.7 earthquakes (Working Group on California Earthquake Probabilities [WGCEP], 1995).
Across the basin from San Pedro to Mt. Wilson, about 10 mm/yr convergence is revealed by early geodetic studies in the Los Angeles basin (Cline et al., 1984). However, the data available at that time were not good enough to make the estimate very accurate. North of the basin,Cheng et al.(1987) investigated more than ten years of trilateration measurements. They estimated 6 mm/yr convergence normal to the SAF along a NW profile from the Malibu fault to the White Wolf fault. Little convergence within the San Gabriel Mountains normal to the SAF was found byLisowski et al.(1991) when adopting the updated trilateration data. Shen (1991) revealed about 3 mm/yr convergence along the southern frontal fault system. Taking the advantage of combining the Very Long Baseline Interferometry (VLBI) and GPS measurements,Feigl et al.(1993) reported 5.0 ±1.2 mm/yr shortening from Palos Verdes to JPL which is located in the SW foothills of the San Gabriel Mountains. West of the basin, 7-10 mm/yr N-S convergence across the Ventura Basin was determined by Donnellan et al. (1993) using GPS methods. Recently, Sany et al.(1995) found that the SAF is located near the center of a deformation band.
One of the most recent significant earthquake event affecting southern California was the January 17th1994 Northridge Earthquake. At 4:31 A.M. on Monday, January 17, a moderate but very damaging earthquake with a magnitude of 6.7 struck the San Fernando Valley. In the following days and weeks, thousands of aftershocks occurred, causing additional damage to affected structures. Fifty-seven people were killed and more than 1,500 people seriously injured. For days afterward, thousands of homes and businesses were without electricity; tens of thousands had no gas; and nearly 50,000 had little or no water. Approximately 15,000 structures were moderately to severely damaged, which left thousands of people temporarily homeless. 66,500 buildings were inspected. Nearly 4,000 were severely damaged and over 11,000 were moderately damaged. Several collapsed bridges and overpasses created commuter havoc on the freeway system. Extensive damage was caused by ground shaking, but earthquake
5.2 Newport-Inglewood Fault 33