1. Investigations
This contract supports continued research focusing on earthquakes in the eastern Sierra Nevada and western Basin and Range region. During the past year we have finished the analysis of stress triggering of aftershocks by the 1994 Double Spring Flat, Nevada, earthquake pertaining to Element II: Evaluating Urban Hazard and Risk and Element III: Understanding Earthquake Process. We have also completed a forensic investigation of the Kean Canyon, Nevada explosion, an industrial accident. Local and federal goverment agencies initiated the investigation and asked the UNRSL for a seismic analysis.
2. Results
A Test For Static And Dynamic Stress Changes On Triggered Aftershocks Caused By The 1978 Diamond Valley, California And 1994 Double Spring Flat, Nevada Earthquakes
We studied the 1994 Mw 5.8 Double Spring Flat earthquake sequence in the context of plausible prior stresses and the evolution of Coulomb failure stress, CFS, as the aftershock sequence evolved (Fig. 1). The timing of the Double Spring Flat earthquake could have been delayed by static stress changes due to the 1978 M 5.2 Diamond Valley earthquake (Somerville et al., 1980). The magnitude of the CFS decrease, and a time of 16 years for the stresses to reaccumulate between 1978 and 1994, implies a minimum regional stressing rate of 0.013 bars/yr. Aftershocks from the 1994 earthquake migrated south-eastward from the Genoa fault zone towards the Antelope Valley fault zone along a series of conjugate fault pairs (Ichinose et al., 1998a).
We compare static CFS and dynamic CFS to aftershock locations and rake angles to possibly determine if nearby aftershocks are more likely triggered by near field pulse or by the passing of seismic waves. The static CFS and dynamic CFS was computed for planes of expected failure surrounding the 1994 mainshock plane. The dynamic CFS is a time series which we chose the peak to peak values for our comparisons. The static CFS locations are consistent with an increase of CFS from the 1978 and 1994 mainshocks, if the mechanisms are left-lateral slip on northeast striking planes and normal slip on north-south striking planes dipping east. The conjugate northwest strike-slip plane is also moved toward failure if the mechanisms are right-lateral slip (Fig. 1). The December 1995 aftershock cluster would have also increased CFS southward along the northern Antelope Valley fault, where three M > 4 aftershocks occurred in 1996.
The peak to peak dynamic CFS was greatest for high angle strike-slip faults and about 1/2 smaller for rakes around -90° (Fig. 2). The dynamic positive CFS lobes are also occur in the same location of static positive CFS lobes. Distance fall-off of aftershocks from the mainshock fault plane are also compared to R-3 and R-1 models which are characteristics of stress decay for intermediate- and far-field terms. The spatial density of aftershocks that are triggered by static stress changes may, like the stress itself, decay at rates greater than or equal to R-3, while aftershocks triggered by dynamic stress changes might, like the dynamic strain amplitude, decay as R-1. Eight aftershock sequences including the Double Spring Flat earthquake favor a R-1 distance decay, suggesting that dynamic stress changes are an important factor as an aftershock triggering mechanism.
Synthetic dynamic CFS time histories show radiation patterns similar to the static CFS and a distance decay of R-1, thereby making it difficult to differentiate either triggering mechanisms by spatial correlation. We did observe larger dynamic CFS especially at farther distances and others have suggested that transient loads can also advance the time to instabilities using simple spring-slider model (Gomberg et al., 1997), based on Dieterich's (1994) rate- and state-dependent friction constitutive laws. We feel that conditions exist for dynamic stress changes to drive Dieterich's (1994) triggering model by hysteresis motions as well as by discrete stress steps. This may be a plausible mechanism for delayed triggering like remote triggering seen after the 1992 Landers earthquake (Anderson et al., 1994).
For future studies we are interested in how stress concentrations occur in regions of normal fault step-over segments and their correlations to zones of high modern seismicity. The 1978 and 1994 earthquakes between the Genoa and Antelope Valley fault zones provide a good example and we have identified other possible sites in the Western Great Basin to apply this analysis.
Seismic Analysis of an Industrial Accident
On the morning of January 7, 1998, an unfortunate fatal accident occurred at the Kean Canyon chemical plant east of Reno, Nevada. The recently organized US Chemical Safety and Hazard Investigation Board (CSB), with the cooperation of several state agencies, initiated an investigation into the accident with the long range goal of improving the safety at explosive manufacturing facilities. An important aspect of the investigation was determining the chronology of events. John Piatt at the CSB reported that the accident consisted of two explosions that occurred within about 3.5 seconds and were separated horizontally by approximately 250 feet (76.2 meters), along a direction of S33°E The CSB requested that the Seismological Laboratory independently look into the explosion. Using an high precision cross-correlation method applied to both seismic and air-waves recorded at several seismic stations in northern Nevada, we are able to resolve the relative locations, and azimuth between the sources and the chronology of two explosions.
The difference in moveout of air-waves between the two explosions, measured at several stations, associates the southern site with the second explosion. The separation of explosions, based on an analysis of these air-wave arrivals, at 3 stations is about 73 meters with an uncertainties ranging from ± 7 to 21 meters. We obtained only a single estimate of source separation using P-waves which is 80 meters with a larger uncertainty of ± 78 meters. We did a simultaneous determination of the separation and the azimuth of the explosions which combines the moveout at different stations. The best solution occurs with a separation of 73.2 meters with the second explosion occurring at azimuth of S35°E from the first (Fig. 3). These estimates are well within uncertainties of investigation by the US Chemical Safety and Hazard Investigation Board.
From the relative spectral amplitudes of P- and air-waves, we suggest that explosion B had downward directivity, while A may have been more upwards directed. The corner frequency of the P-waves is much smaller than expected for the physical dimension of the explosions, indicating that attenuation is exerting a major influence on the P-wave spectrum at high frequency.
The scientific forensic results of this analysis has provided useful information in the US Chemical Safety and Hazard Board's investigation. We confirm that the relative separation of sources can be determined precisely using only a pair of regional seismic stations which can be helpful in investigating accidents. We are encouraged that this approach may also be applied to earthquakes.
3. Publications
These papers and abstracts were submitted or published during the last year with full or partial support from the USGS NEHRP contract.
Ichinose, G. A., J. G. Anderson, and K. D. Smith, Static stress changes caused by the 1978 Diamond Valley, California and 1994 Double Spring Flat, Nevada earthquakes (abstract), EOS Transactions, American Geophysical Union, Vol. 78, 1997.
Ichinose, G. A., K. D. Smith, and J. G. Anderson, A test for dynamic and static stress changes caused by the 1978 Diamond Valley, California and 1994 Double Spring Flat, Nevada earthquakes, (Submitted to BSSA), 1998b.
Anderson, J. G., K. D. Smith, and G. A. Ichinose, Seismic analysis of an industrial accident, (Submitted to Bull. Seismo. Soc. Am.), 1998c.
References Cited
Anderson, J. G., J. N. Brune, J. N. Louie, Y. Zeng, M. Savage, G. Yu, Q. Chen, and D. dePolo (1994). Seismicity in the western Great Basin apparently triggered by the Landers, California earthquake, 28 June 1992, Bull. Seism. Soc. Am., 84, 863-891.
Dieterich, J. H. (1994). A constitutive law for rate of earthquake production and its application to earthquake clustering, J. Geophys. Res., 99, 2601-2618.
Gomberg, J., M. L. Blanpied, and N. M. Beeler (1997). Transient triggering of near and distant earthquakes, Bull. Seism. Soc. Am., 87, 294-309.
Ichinose, G. A., K. D. Smith, and J. G. Anderson, Moment Tensor inversions of the 1994 to 1996 Double Spring Flat, Nevada Earthquake Sequence and implications for local tectonic models, Bull. Seismo. Soc. Am., In Press, 1998a.