Citation
Vidale, John Emilio (1987) Application of twodimensional finitedifference wave simulation to earthquakes, earth structure, and seismic hazard. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechTHESIS:10132011090721112
Abstract
Although the earth is 3dimensional (3D), numerical simulations of wave propagation through laterally heterogeneous media are easier to formulate and more practical to use in 2D. In this thesis, schemes to model seismic wave propagation through laterally varying structures with 2D numerical algorithms are developed and applied to earthquake and explosion problems.
In Chapter 1, 2D source expressions that have the same radiation patterns as their 3D counterparts are derived which can rectify the following three problems: the use of 2D simulations generally results in "line source tails" on what would be impulsive arrivals in 3D, 1/√R rather than 1/R amplitude decay for body waves, and no decay rather than 1/√R amplitude decay for surface waves. Because this technique approximately transforms waves from a cartesian 2D grid to a cylindrically symmetric 3D world, slightly anisotropic geometrical spreading in 2D better approximates isotropic spreading in 3D than simple isotropic spreading in 2D does. In Section 1.7, a correction to the explosive source expression reduces energy traveling vertically out of the source region, but leaves unchanged the energy traveling laterally out of the source region. In some cases, this correction will significantly improve the results of using a 2D grid to simulate elastic wave propagation from an explosive point source.
In Chapter 2, synthetic seismograms are constructed for the strong motions of the 1968 Borrego Mountain earthquake recorded at EI Centro. A good fit to the data results from using the laterally varying model determined by a detailed refraction survey and the source parameters determined by teleseismic waveform modeling. Shallow faulting is no longer necessary to explain the longperiod surfacewave development.
Synthetic seismograms calculated for the 1971 San Fernando earthquake show strong effects due to lateral variation in sediment thickness in the San Fernando valley and the Los Angeles basin. Using previously determined basin structure and teleseismically determined source parameters, twodimensional SH and PSV finite difference calculations can reproduce the amplitude and duration of the strong motion velocities recorded across the basins in Los Angeles in the period range from 1 to 10 seconds. The edges of basins nearest the seismic source show ground motion amplification up to a factor of three, and tend to convert direct shear waves into Love and Rayleigh waves that travel within the basins. The computed motions are sensitive to the mechanism and location of earthquakes. A strikeslip earthquake on the NewportInglewood fault zone, for example, would produce different patterns of peak velocity and duration of shaking across the San Fernando and Los Angeles basins.
In Chapter 3, the effect of shallow station structure and lateral velocity variation are investigated for records of the Amchitka explosion Milrow. The differences between the MeullerMurphy, HeimbergerHadley, and von SeggernBlandford reduced displacement potential (RDP) source representations are small compared to the differences between using various possible velocity structures.
Creager and Jordan (1986) propose that penetration of subducting slabs under the Kurile Islands and other subduction zones to depths of at least 1000 km is necessary to explain the t ravel time anomalies of deep earthquakes. Such penetration would also affect the amplitudes and waveforms of the body waves from these earthquakes. In Chapter 4, synthetic seismograms appropriate for a record section in a plane perpendicular to the strike of the slab are presented using a coupled finitedifference and Kirchhoff method. An inferred shearwave version of the compressionalwave velocity structure of Creager and Jordan (1986) produces an amplitude decrease up to a factor of four and waveform broadening up to 20 seconds for SH arrivals with a takeoff angle pointing straight down t he slab. Slabs that extend only 300 km below the earthquake but are half as thick and twice as anomalously fast as Creager and Jordan 's (1986) velocity model will roughly preserve the travel time variation pattern, and show less waveform broadening, but produce first arrivals that are emergent. Slabs that become thicker with depth show less waveform broadening. Reconciliation of the amplitude, waveform distort ion, and timing of body waves from deep events is necessary to understand the geometry of slabs near and below the 6.50 km discontinuity.
Item Type:  Thesis (Dissertation (Ph.D.)) 

Subject Keywords:  Geophysics 
Degree Grantor:  California Institute of Technology 
Division:  Geological and Planetary Sciences 
Major Option:  Geophysics 
Thesis Availability:  Public (worldwide access) 
Research Advisor(s): 

Thesis Committee: 

Defense Date:  14 November 1986 
Record Number:  CaltechTHESIS:10132011090721112 
Persistent URL:  http://resolver.caltech.edu/CaltechTHESIS:10132011090721112 
Default Usage Policy:  No commercial reproduction, distribution, display or performance rights in this work are provided. 
ID Code:  6712 
Collection:  CaltechTHESIS 
Deposited By:  Benjamin Perez 
Deposited On:  13 Oct 2011 16:40 
Last Modified:  26 Dec 2012 04:39 
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