HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS		QUICK SEARCH:   [advanced] Author: Keyword(s): Year:  Vol:  Page:

This Article Full Text (PDF) References Citation Map Services Email this article to a friend Similar articles in this journal Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Brune, J. N. Articles by Anooshehpoor, A. Search for Related Content GeoRef GeoRef Citation

A physical model of the effect of a shallow weak layer on strong ground motion for strike-slip ruptures

Seismological Laboratory/174 University of Nevada, Reno Mackay School of Mines, Reno, Nevada 89557-0141

Abstract

We report results of foam rubber modeling of the effect of a shallow weak layer on ground motion from strike-slip ruptures. Computer modeling of strong ground motion from strike-slip earthquakes has in some cases involved somewhat arbitrary assumptions about the nature of slip along the shallow part of the fault (e.g., fixing the slip to be zero along the upper 2 km of the fault plane). Fault-slip inversion studies indicate that the high-frequency radiation from the shallow part of strike-slip faults is typically less than that for the deeper parts of the fault. In many cases, faults (1) may be weak along the upper few kilometers of the fault zone and may not be able to maintain high levels of shear strain required for high dynamic energy release during earthquakes and (2) may have different constitutive relations for fault slip, for example, slip strengthening. The object of this article is to present results of physical modeling using a shallow weak layer, in order to support the physical basis for assuming a long rise time and a reduced high-frequency pulse for the slip on the shallow part of faults. A weak zone was modeled by inserting weak plastic layers of a few inches in width into the foam rubber model. The long-term strength of the weak layer is about an order of magnitude less than the rest of the model. The transient strength is velocity strengthening with the strength estimated to be about three times higher at slip velocities typical of dynamic slip events. It appears a 2-km-deep, weak zone along strike-slip faults could indeed reduce the high-frequency energy radiated from shallow slip and that this effect can best be represented by superimposing a small-amplitude, short rise-time pulse at the onset of a much longer rise-time slip. For the 15-cm weak zone, the average pulse amplitude is reduced by a factor of about 0.4. The reduction factor for the 20-cm case is about 0.2. For the 30-cm case, it is about 0.1. From these results, we can see that the thicker the weak layer, the more difficult it is for a short rise-time acceleration pulse to push its way through the weak layer to the surface. The velocity strengthening property of the weak layer further damps the slip motion and increases the rise time. These results support reducing the high-frequency radiation from shallower parts of strike-slip faults in modeling studies if it is known that the shallow part of the fault is weak or has not stored up a large shear stress.

This article has been cited by other articles:

				A. Pitarka, L. A. Dalguer, S. M. Day, P. G. Somerville, and K. Dan Numerical Study of Ground-Motion Differences between Buried-Rupturing and Surface-Rupturing Earthquakes Bulletin of the Seismological Society of America, June 1, 2009; 99(3): 1521 - 1537. [Abstract] [Full Text] [PDF]

				K. B. Olsen, S. M. Day, J. B. Minster, Y. Cui, A. Chourasia, D. Okaya, P. Maechling, and T. Jordan TeraShake2: Spontaneous Rupture Simulations of Mw 7.7 Earthquakes on the Southern San Andreas Fault Bulletin of the Seismological Society of America, June 1, 2008; 98(3): 1162 - 1185. [Abstract] [Full Text] [PDF]

				R. Heermance, Z. K. Shipton, and J. P. Evans Fault Structure Control on Fault Slip and Ground Motion during the 1999 Rupture of the Chelungpu Fault, Taiwan Bulletin of the Seismological Society of America, June 1, 2003; 93(3): 1034 - 1050. [Abstract] [Full Text] [PDF]

				B. Shi, J. N. Brune, Y. Zeng, and A. Anooshehpoor Dynamics of Earthquake Normal Faulting: Two-Dimensional Lattice Particle Model Bulletin of the Seismological Society of America, June 1, 2003; 93(3): 1179 - 1197. [Abstract] [Full Text] [PDF]

				Effect of a Shallow Weak Zone on Fault Rupture: Numerical Simulation of Scale-Model Experiments Bulletin of the Seismological Society of America, December 1, 2002; 92(8): 3022 - 3041.

				Precarious-Rock Constraints on Ground Motion from Historic and Recent Earthquakes in Southern California Bulletin of the Seismological Society of America, October 1, 2002; 92(7): 2602 - 2611.

				Displaced Rocks, Strong Motion, and the Mechanics of Shallow Faulting Associated with the 1999 Hector Mine, California, Earthquake Bulletin of the Seismological Society of America, May 1, 2002; 92(4): 1561 - 1569.

				Quasi-Static Slip-Rate Shielding by Locked and Creeping Zones as an Explanation for Small Repeating Earthquakes at Parkfield Bulletin of the Seismological Society of America, April 1, 2001; 91(2): 401 - 403.

				Precarious Rock Evidence for Low Ground Shaking on the Footwall of Major Normal Faults Bulletin of the Seismological Society of America, August 1, 2000; 90(4): 1107 - 1112.

				R. W. Graves Three-dimensional finite-difference modeling of the San Andreas fault: Source parameterization and ground-motion levels Bulletin of the Seismological Society of America, August 1, 1998; 88(4): 881 - 897. [Abstract] [PDF]