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 Harmsen, S. C. Search for Related Content GeoRef GeoRef Citation

The Little Skull Mountain, Nevada, earthquake of 29 June 1992: Aftershock focal mechanisms and tectonic stress field implications

U.S. Geological Survey Denver Federal Center, Denver, Colorado 50225

Abstract

The M_L = 5.6 29 June 1992 Little Skull Mountain earthquake occurred in the southern Nevada Test Site, Nye County, Nevada, within 20 km of a potential national nuclear waste repository at Yucca Mountain, Nevada. While microseismicity in the immediate epicentral vicinity occurred for years prior to the mainshock, foreshock activity (M_L < 2) sharply increased following the Landers, California, earthquake of 28 June, 22 hr before the Little Skull Mountain earthquake. Approximately 3000 aftershocks were located in the period June through September 1992, and their hypocenters suggest activity on two major and several minor faults with different orientations. Seventy aftershocks in June and July 1992 had magnitudes M_L 3.0, and several smaller aftershocks (M_L > 2) were also sufficiently well recorded to provide local network P-wave focal-mechanism solutions. The mainshock and the majority of the aftershocks through September 1992 display predominantly normal-slip focal mechanisms, but unambiguous strike-slip and oblique-slip focal-mechanism solutions are also present in the aftershock series. The mainshock and immediate foreshock depths were 10 ± 1 km below sea level, and depths of focus of aftershocks range from about 6 to 12 km. A large subset of the aftershock hypocenters define a southeast-dipping structure, probably the mainshock fault plane. No variation of mechanism style with depth is apparent. Two mainshock and 84 aftershock focal-mechanism solutions were input into Gephart's (1990) stressfield inversion program, and principal compressional stress directions, _j, j = 1, 2, 3, were determined, as well as the stress difference ratio R = (₁ – ₂)/(₁ – ₃). The results indicate a uniform stress field in which ₃ (minimum stress) is oriented 116°/5° (azi, plunge) and ₁ (maximum stress) is oriented 219°/69° with R = 0.35. The inversion's selection of preferred nodal planes invokes Bott's (1959) criterion, which states that slip occurs in the direction of resolved shear traction on preexisting fault surfaces. For the predominantly normal-slip Little Skull Mountain earthquake mechanisms having northeast-striking planes, the southeast, moderately to steeply dipping (dip > 55°) nodal planes are selected. However, for the dip-slip earthquake mechanisms exhibiting more northerly trending strikes, shallowly to moderately west-dipping (25° < dip < 65°) nodal planes are selected. When a Coulomb-Mohr analysis was applied to the focal mechanisms using the above principal stress directions and an assumed ratio ₁/₃, the ratio of resolved shear stress to normal stress, /_n, on the southeast-dipping nodal plane of the mainshock exhibited a near-maximum level, suggesting a high fault strength. In contrast, a wide range of fault strengths, or apparent coefficients of friction, was suggested by the /_n ratios for Little Skull Mountain aftershock preferred nodal planes. Seismicity on misoriented faults, i.e., those on which shear stress was relatively low, may have been the result of a temporarily elevated fluid pore pressure following the mainshock.

This article has been cited by other articles:

				M. D. Fisk and G. D. McCartor Treatment of Correlation for Clustered Reference Data in Kriging Regional Seismic Discriminants Bulletin of the Seismological Society of America, August 1, 2008; 98(4): 1768 - 1780. [Abstract] [Full Text] [PDF]

				T. Parsons, G. A. Thompson, and A. H. Cogbill Earthquake and volcano clustering via stress transfer at Yucca Mountain, Nevada Geology, September 1, 2006; 34(9): 785 - 788. [Abstract] [Full Text] [PDF]

				Source Effects on Regional Seismic Discriminant Measurements Bulletin of the Seismological Society of America, December 1, 2002; 92(8): 2926 - 2945.

				The 1992 Little Skull Mountain Earthquake Sequence, Southern Nevada Test Site Bulletin of the Seismological Society of America, December 1, 2001; 91(6): 1595 - 1606.

				Normal faults, normal friction? Geology, October 1, 2001; 29(10): 927 - 930.

				Relative Importance of Near-, Intermediate- and Far-Field Displacement Terms in Layered Earth Synthetic Seismograms Bulletin of the Seismological Society of America, April 1, 2000; 90(2): 531 - 536.

				B. Wernicke, J. L. Davis, R. A. Bennett, P. Elósegui, M. J. Abolins, R. J. Brady, M. A. House, N. A. Niemi, and J. K. Snow Anomalous Strain Accumulation in the Yucca Mountain Area, Nevada Science, March 27, 1998; 279(5359): 2096 - 2100. [Abstract] [Full Text]

				M. Meremonte, J. Gomberg, and E. Cranswick Constraints on the 29 June 1992 Little Skull Mountain, Nevada, earthquake sequence provided by robust hypocenter estimates Bulletin of the Seismological Society of America, August 1, 1995; 85(4): 1039 - 1049. [Abstract] [PDF]

				W. R. Walter, K. M. Mayeda, and H. J. Patton Phase and spectral ratio discrimination between NTS earthquakes and explosions. Part I: Empirical observations Bulletin of the Seismological Society of America, August 1, 1995; 85(4): 1050 - 1067. [Abstract] [PDF]