Characterization of fault zones by analysis of aftershock waveform data

Abstract

Large property contrasts between materials in a fault zone and the surrounding rock are often produced by repeating earthquakes. Fault zones are usually characterized by fluid concentration, clay-rich fault gouge, increased porosity, and dilatant cracks. Thus, fault zones are thought to have reduced seismic velocities than the surrounding rocks. In this article, we first investigated the synthetic waveforms at a linear array across a vertical fault zone by using 3D finite difference simulation. Synthetic waveforms show that when sources are close to, inside, or below the fault zone, both arrival times and waveforms of P- and S-waves vary systematically across the fault zone due to reflections and transmissions from boundaries of the low-velocity fault zone. The arrival-time patterns and waveform characteristics can be used to determine the fault zone structure. Then, we applied this method to the aftershock waveform data of the 1992 Landers M7.4 and the 2008 Wenchuan (汶川) M8.0 earthquakes. Landers waveform data reveal a low-velocity zone with a width of approximately 270-370 m, and P- and S-wave velocity reductions relative to the host rock of approximately 35%–60%; Wenchuan waveform data suggest a low-velocity zone with a width of approximately 220–300 m, and P- and S-wave velocities drop relative to the host rock of approximately 55%.

Keywords

waveform characteristics / seismic-wave propagation / fault zone structure

References

[1] Aki K.. Characterization of Barriers on an Earthquake Fault. J. Geophys. Res., 1979, 84: 6140-6148.

[2] Ben-Zion Y.. Properties of Seismic Fault Zone Waves and Their Utility for Imaging Low-Velocity Structures. J. Geophys. Res., 1998, 103(B6): 12567-12585.

[3] Ben-Zion Y., Sammis C. G.. Characterization of Fault Zones. Pure Appl. Geophys., 2003, 160(3–4): 677-715.

[4] Ben-Zion Y., Peng Z., Okaya D., . A Shallow Zone Structure Illuminated by Trapped Waves in the Karadere-Duze Branch of the North Anatolian Fault, Western Turkey. Geophys. J. Int., 2003, 155: 1021-1041.

[5] Chester F. M., Evans J. P., Biegel R. L.. Internal Structure and Weakening Mechanisms of the San Andreas Fault. J. Geophys. Res., 1993, 98: 771-786.

[6] Chester F. M., Chester J. S.. Ultracataclasite Structure and Friction Processes of the Punchbowl Fault, San Andreas System, California. Tectonophysics, 1998, 295(1–2): 199-221.

[7] Eberhart-Phillips D., Stanley W. D., Rodriguez B. D., . Surface Seismic and Electrical Methods to Detect Fluids Related to Faulting. J. Geophys. Res., 1995, 100(B7): 12919-12936.

[8] Evans J. P., Chester F. M.. Fluid-Rock Interaction in Faults of the San Andreas System: Inferences from San Gabriel Fault Rock Geochemistry and Microstructures. J. Geophys. Res., 1995, 100(B7): 13007-13020.

[9] Graves R.. Simulating Seismic Wave Propagation in 3D Elastic Media Using Staggered-Grid Finite Differences. Bull. Seismol. Soc. Am., 1996, 86: 1091-1106.

[10] Kanamori H.. Mechanics of Earthquakes. Annu. Rev. Earth Planet. Sci., 1994, 22: 207-237.

[11] Klimentos T.. The Effects of Porosity-Permeability-Clay Content on the Velocity of Compressional Waves. Geophysics, 1991, 56(12): 1930-1939.

[12] Lee, W. H. K., 1999. Digital Waveform Data of 238 Selected Landers Aftershocks from a Dense PC-Based Seismic Array. Tech. Rep., US Geol. Surv.

[13] Levander A. R.. Fourth-Order Finite-Difference P-SV Seismograms. Geophysics, 1988, 53(11): 1425-1436.

[14] Li H., Zhu L., Yang H.. High-Resolution Structures of the Landers Fault Zone Inferred from Aftershock Waveform Data. Geophys. J. Int., 2007, 171(3): 1295-1307.

[15] Li Y. G., Leary P. G.. Fault Zone Trapped Seismic Waves. Bull. Seismol. Soc. Am., 1990, 80: 1245-1271.

[16] Li Y. G., Aki K., Adams D., . Seismic Guided Waves Trapped in the Fault Zone of the Landers, California, Earthquake of 1992. J. Geophys. Res., 1994, 99(B6): 11705-11722.

[17] Li Y. G., Vernon F. L., Aki K.. San Jacinto Fault-Zone Guided Waves: A Discrimination for Recently Active Fault Strands near Anza, California. J. Geophys. Res., 1997, 102(B6): 11689-11701.

[18] Mooney W. D., Ginzburg A.. Seismic Measurements of the Internal Properties of Fault Zones. Pure Appl. Geophys., 1986, 124(1–2): 141-157.

[19] Peng Z., Ben-Zion Y., Michael A. J., . Quantitative Analysis of Seismic Fault Zone Waves in the Rupture Zone of the Landers, 1992, California Earthquake: Evidence for a Shallow Trapping Structure. Geophys. J. Int., 2003, 155(3): 1021-1041.

[20] Randall C. J.. Absorbing Boundary Condition for the Elastic Wave Equation: Velocity-Stress Formulation. Geophysics, 1989, 54(9): 1141-1152.

[21] Rovelli A., Caserta A., Marra F., . Can Seismic Waves be Trapped inside an Inactive Fault Zone? The Case Study of Nocera Umbra, Central Italy. Bull. Seismol. Soc. Am., 2002, 92: 2217-2232.

[22] Scholz C. H.. The Mechanics of Earthquakes and Faulting, 1990, New York: Cambridge Univ. Press