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Recent SCEC Published Research

The following list of recently published papers are based on research sponsored by SCEC. These papers are NOT available from SCEC. Most of the journals containing these papers are available at university libraries, and authors may also have reprints of their papers available by request.

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Other SCEC research papers are listed in an online database.

List of publications announced 8/3/2001

Recent Research as of 9/20/2001:

(SCEC Contribution numbers are in bold)

496. Fuis, G.S., T. Ryberg, W.J. Lutter, and P.L.Ehlig, Seismic mapping of shallow fault zones in the San Gabriel Mountains from the Los Angeles Region Seismic Experiment, Southern California, Journal of Geophysical Research, 106, pp. 6549-6568, 2001

During the Los Angeles Region Seismic Experiment (LARSE), a reflection/refraction survey was conducted along a profile (Line 1) extending from Seal Beach, California, northeastward to the Mojave Desert, crossing the Los Angeles and San Gabriel Valley basins and San Gabriel Mountains. In most shot gathers from the southern and central San Gabriel Mountains, clear secondary arrivals are seen that merge, or appear to merge, with first arrivals at three locations, including the Vincent thrust fault, an exposed late Mesozoic/early Cenozoic megathrust. These secondary arrivals are interpretable as reflections in the shallow crust (less than 5-km depth) from a concave-upward interface that projects to the surface in the north near the Vincent thrust fault, is offset in its central part at the San Gabriel fault (an old branch of the San Andreas fault), and terminates in the south at 1- to 2-km depth at the southern mountain front. The velocity structure above and below this interface strongly suggests it is the Vincent thrust fault: intermediate velocities (6.2 km/s), consistent with mylonites overlying the Vincent thrust fault, are observed above it; lower velocities (5.8 km/s), consistent with the Pelona Schist underlying the Vincent thrust fault, are observed below it. Problems arise, however, in attempting to match this reflector to the exposed Vincent thrust fault, which is seen in outcrops east of Line 1. The Vincent thrust fault is shallower than the reflector in most places. An unmapped structure (monocline, steep fault, or thrust fault) is required between Line 1 and the outcrops that either drops the Vincent thrust fault down to the depths of the reflector or repeats the Vincent thrust fault beneath Line 1 in the footwall of another thrust fault. An alternative interpretation of the reflector is a deep greenstone horizon within the Pelona Schist, although this alternative is not favored by the velocity structure.

519. Day, S.M., and C.R. Bradley, Memory-efficient Simulation of Anaelastic Wave Propagation, Bulletin of the Seismological Society of America, 91, pp. 520-531, 2001.

Realistic anelastic attenuation can be incorporated rigorously into finite difference and other numerical wave propagation methods using internal, or "memory" variables. The main impediment to realistic treatment of anelastic attenuation in 3D is the very large computational storage requirement imposed by the additional variables. We previously proposed an alternative to the conventional memory-variable formulation, the method of coarse-grain memory variables, and demonstrated its effectiveness in acoustic problems. We generalize this memory-efficient formulation to 3D anelasticity and describe a fourth-order, staggered-grid finite difference implementation. The anelastic coarse-grain method applied to plane wave propagation successfully simulates frequency-independent Qp and Qs. Apparent Q,s are constant to within 4% tolerance over more than 2 decades in frequency, and biased less than 4% from specified target values. This performances is comparable to that achieved!! previously for acoustic wave propagation, and is accompanied by the same 8-fold reduction in the memory requirement, relative to the conventional approach. The method closely approximates the wavenunber-integration solution for the response of an anelastic halfspace to a shallow dislocation source, accurately calculating all phases including the surface-diffracted SP phase and the Rayleigh wave. The halfspace test demonstrates that the wavefield averaging concept underlying the coarse-grain method is effective near boundaries and in the presence of evanescent waves. We anticipate that the method will also be applicable to unstructured grid methods, such as finite element and spectral element methods, though additional numerical testing will be required to establish accuracy in the presence of grid irregularity. The method is not effective at wavelengths equal to and shorter than 4 grid cell dimensions, where it produces anomalous scattering effects. This limitation could be s!!ignificant for very high-order numerical schemes under some! circumstances (i.e., whenever wavelengths as short as 4 grids are otherwise within the usable bandwidth of the scheme), but it is of no practical importance in our fourth-order finite difference implementation.

545. Ni, S.D., J.G. Anderson, Y. Zeng, and R.V Siddharthan, Expected Signature of Nonlinearity on Regression for Strong Ground Motion Parameters, Bulletin of the Seismological Society of America, 90, no. 6B, pp. S53-S64, 2001

This study examines the response of soil profiles with nonlinear properties to several hundred synthetic seismograms, generated to represent rock ground motions from magnitude 6.4 and 7.0 scenario earthquakes. Two shear wave velocity models (developed to represent Sit Class CD and D sites with water table at 3 m) are tested. The computed ratios of peak ground acceleration (PGA) between the surface of the soil profile and the bedrock decreases with increasing PGA values. The transition from amplification to deamplification occurs at about 0.2-0.3 g. The spectral acceleration (SA) ratios, defined as the ratios of SA between the surface of the soil profile and the input, vary with the natural period of the oscillators. At short periods less than 0.3 s, the behavior of the SA ratios is similar to the PGA ratios: amplification for lower input SA and deamplification for higher input SA level. At longer periods, the influence of the input SA level on SA ratio decreases, and deamplification is seldom observed. We define the mean trends of these calculations as "Rock Motion Modification" curves (RMM curves). The use of these curves is as follows: a ground motion relation on rock is multiplied by the RMM curve for that ground motion to obtain the expected ground motion relation at the soil surface. This procedure is applied to six sets of empirical ground motion relations. A majority of the empirical ground motion relations are consistent with the RMM curves. In the best case, the comparisons indicate that the empirical soil site PGA and SA ground motion relations are very close to the predicted curves at all epicentral distances. For both PGA and SA, the differences between the empirical and predicted curves are within one standard deviation of the empirical curves. It is quite encouraging that this physical model for soil behavior predicts the average characteristics of the surface motion, given the highly scattered nature of the data set.

564. Rice, J. R., New perspectives in crack and fault dynamics, Proceedings of the 20th International Congress of Theoretical and Applied Mechanics (ICTAM 2000), held 27 Aug - 2 Sept 2000, Chicago, H. Aref and J. W. Phillips, Kluwer Academic Publishers, pp. 1-23, 2001.

Recent observations on the dynamics of crack and fault rupture are described, together with related theory and simulations in the framework of continuum elastodynamics. Topics include configurational instabilities of tensile crack fronts (crack front waves, disordering, side-branching), the connection between frictional slip laws and modes of rupture propagation in earth faulting, especially conditions for formation of self-healing slip pulses, and the rich faulting and cracking phenomena which result along dissimilar material interfaces due to coupling between slippage and normal stress alteration.

565. Rice, J. R., N. Lapusta, and K. Ranjith, Rate and state dependent friction and the stability of sliding between elastically deformable solids, J. Mech. Phys. Solids, 49, no. 9, pp.1865-1898, 2001.

We study the stability of steady sliding between elastically deformable continua using rate and state dependent friction laws. That is done for both elastically identical and elastically dissimilar solids. The focus is on linearized response to perturbations of steady state sliding, and on studying how the positive direct effect (instantaneous increase or decrease of shear strength in response to a respective instantaneous increase or decrease of slip rate) of those laws allows the existence of a quasi-static range of response to perturbations at sufficiently low slip rate. We discuss the physical basis of rate and state laws, including the likely basis for the direct effect in thermally activated processes allowing creep slippage at asperity contacts, and estimate activation parameters for quartzite and granite. Also, a class of rate and state laws suitable for variable normal stress is presented. As part of the work, we show that compromises from the rate and state framework for describing velocity-weakening friction lead to paradoxical results, like supersonic propagation of slip perturbations, or to ill-posedness, when applied to sliding between elastically deformable solids. The case of sliding between elastically dissimilar solids has the inherently destabilizing feature that spatially inhomogeneous slip leads to an alteration of normal stress, hence of frictional resistance. We show that the rate and state friction laws nevertheless lead to stability of response to sufficiently short wavelength perturbations, at very slow slip rates. Further, for the slow sliding between dissimilar solids, we show that there is a critical amplitude of velocity-strengthening above which there is stability to perturbations of all wavelengths.

585. Du, W. and L. R. Sykes, Changes in Frequency of Moderate-size Earthquakes and Coulomb Failure Stress before and after the Landers, California, Earthquake of 1992, Bulletin of the Seismological Society of America, 91, no. 4, pp. 725-738, 2001.

Changes in the frequency of moderate-size events before and after the June 28, 1992 Landers earthquake are investigated, and their implications are discussed in the context of Coulomb Failure Stress (CFS) evolution since 1812 in Southern California. We systematically considered circular regions and equal-area annuli centered on the epicenter of the Landers earthquake.Frequency-magnitude relationships for two 10-year periods before and two 5-year periods around the Landers event are compared. Only events with magnitude, M ≥~4.0 are included; aftershocks are removed. For the larger circular regions with radii of 140 to 160 km, the rate and slope of the frequency-magnitude distribution for moderate-size events just before the main shock appear to be anomalous compared to those for either the preceding or subsequentperiods. For areas closer to the 1992 epicenter, however, the number of events is few and the differences in the distributions are less obvious. When we examined the seismic activity in annuli of equal area, however, the largest changes occurred about 150 km from the epicenter of the main shock, not closer as would be expected for a precursor to the Landers event. We also derive an "Index Value" to better quantify differences in the frequency of occurrence of moderate-size events as a function of time.It and the frequency-magnitude distribution show similar spatialdependence. Since 1812 a large region near Landers has moved closer to failure in terms of changes in Coulomb Failure Stress for faults of San Andreas type. These changes, however, are dominated by co-seismic changes associated with the 1812 and 1857 earthquakes and by tectonic stress buildup related to the San Andreas fault, not by stress buildup associated with the Landers faults themselves, which are characterized by very slowlong-term displacements. Hence, the most pronounced changes in thefrequency of moderate-size earthquakes before 1992 do not appear to be related to stress buildup to the Landers sequence itself.They, along with the Landers sequence, may be indicative of a broad region that is approaching a high stress state prior to an eventual future great earthquake. The failure to find a pronouncedincrease in moderate-size shocks close in to Landers is inaccord with the idea that such increases on a time scale of years to decades are associated with the regional buildup of stress to large earthquake along faults of high (not low) long-term slip rates.

611. Sammis, C. G., and J. R. Rice, Repeating earthquakes as low-stress-drop events at a border between locked and creeping fault patches, Bulletin of the Seismological Society of America, 91, pp. 532-537, 2001.

The source of repeating earthquakes on creeping faults is modeled as a weak asperity at a border between much larger locked and creeping patches on the fault plane. The x-1/2 decrease in stress concentration with distance x from the boundary is shown to lead directly to the observed scaling T proportional to M_o1/6 between the average repeat time and average scalar moment for a repeating sequence. The stress drop in such small events at the border depends on the size of the large locked patch. For a circular patch of radius R and representative fault parameters, Delta sigma = 7.6 (m / R)3/5 MPa, which yields stress drops between 0.08 and 0.5 MPa (0.8 - 5 bars) for R between 2 km and 100 m. These low stress drops are consistent with estimates of stress drop for small earthquakes based on their seismic spectra. However, they are orders of magnitude smaller than stress drops calculated under the assumption that repeating sources are isolated stuck asperities on an otherwise creeping fault plane, whose seismic slips keep pace with the surrounding creep rate. Linear streaks of microearthquakes observed on creeping fault planes are trivially explained by the present model as alignments on the boundaries between locked and creeping patches.

612. Ranjith, K., and J. R. Rice, Slip dynamics at an interface between dissimilar materials, J. Mech. Phys. Solids, 49, pp. 341-361, 2001.

It has been shown recently that steady frictional sliding along an interface between dissimilar elastic solids with Coulomb friction acting at the interface is ill-posed for a wide range of material parameters and friction coefficients. The ill-posedness is manifest in the unstable growth of interfacial disturbances of all wavelengths, with growth rate inversely proportional to the wavelength. We first establish the connection between the ill-posedness and the existence of a certain interfacial wave in frictionless contact, called the generalized Rayleigh wave. Precisely, it is shown that for material combinations where the generalized Rayleigh wave exists, steady sliding with Coulomb friction is ill-posed for arbitrarily small values of friction. Secondly, regularization of the problem by an experimentally motivated friction law is studied. We show that a friction law with no instantaneous dependence on normal stress but a simple fading memory of prior history of normal stress makes the problem well-posed.

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