Modeling Aftershocks of the 17 May 1993 M<sub>w</sub> 6 Eureka Valley, California Earthquake

Modeling Aftershocks of the 17 May 1993 M_w 6 Eureka Valley, California Earthquake

GENE A. ICHINOSE¹, JOHN G. ANDERSON¹, YEUHUA ZENG, and KENNETH D. SMITH

University of Nevada Reno Seismological Laboratory
Mackay School of Mines

¹Also at the Department of Geological Sciences, University of Nevada, Reno
e-mail: ichinose@seismo.unr.edu
Netsite: http://www.seismo.unr.edu/htdocs/WGB/EurekaValley93

Presented at the Fall 1998 American Geophysical Union Annual Meeting in San Francisco, California.

Objectives
1. Preliminary first motion focal mechanisms of the 1993 Eureka Valley mainshock and several aftershocks indicated a shallow west dipping fault plane. The main objective is to resolve these focal mechanism using waveform modeling to investigate the possibility of active low angle faulting.
2. Test non-linear waveform optimization on small earthquakes recorded by short-period 3 component instruments
3. Implications for tectonic models
Earthquake Relocations

We first merged 2833 earthquakes from the Caltech/SCSN, and Univ. of Nevada Reno catalogs with P- and S-wave phase picks from 5 UNR portable sites. We then selected 350 well recorded earthquakes and relocated them using a 1D layered velocity model. The travel time residuals from this subset were then averaged and used as stations delays for the final relocations of earthquakes from January 1993 to July 1998.
Waveform Analysis

The source parameters of Eureka Valley, California aftershocks ranging in size from M_w 1.0 to 4.9 are modeled using a nonlinear inversion process (see Figures 5 through 7). This process minimizes the misfit between single- or multi-station 3 component seismograms and synthetics generated using a fast f-K summation technique of Zeng and Anderson (1995). The inversion process proves effective at regional (Figures 5 and 6) and local distances (Figure 8), with well calibrated Green's functions.
The nonlinear optimization follows the downhill simplex procedure originally developed by Nelder and Mead (1966). We modified the downhill procedure using simulated annealing by adding a small, exponentially decreasing, fluctuation to the input parameters at each iteration. This small fluctuation, controlled by the temperature parameter, prevents the optimization process from being tricked by a local minimum and does well in global optimization. We set an annealing schedule where the "temperature" is decreased over the number of iterations.
The data and synthetics are first aligned by the maximum of the cross-correlation over a controlled window length. The misfit for the 3 components of ground motion are found at each station by taking the squared raw amplitude difference. The objective function is determined by the sum of the misfits at one or more stations.
The Green's functions and data are bandpassed filtered over a range depending on distance and magnitude. For regional earthquakes with ( M > 4.5 ), the bandpass is 100 to 20 seconds, or, 50 to 20 seconds period for ( 3.5 < M < 4.5 ) earthquakes, and 0.5 to 3 Hz for ( 0.5 < M < 2.5 ) local earthquakes recorded by short-period sensors. The local velocity models were modified from the model used in the earthquake relocations and regional models are constructed from simple 1 to 2 crustal layers over a half-space using large nearby earthquakes for calibration.
Conclusions
1. There is no evidence of low angle faulting. The mainshock ruptured along an approximately north-south striking normal fault dipping 51°W (Figure 5). The aftershock relocations in the cross sections shows that the rupture may have started at the epicenter at 12km depth, and only reached an up-dip depth of 8km consistent with a 40-50°W dipping fault plane (Figure 2). These results are in good agreement with Rosen and Peltzer (1995) geodetic model for the mainshock. The mainshock also triggered hanging-wall deformation in the overlying Saline Range along a steeply east and west dipping range fronts with a slip vector varying from normal to strike-slip.
2. The earthquake sequence occurred within the Eastern California Shear Zone (Dokka and Travis) where Dixon et al. (1995) determined that the integrated Basin and Range deformation to be ~ 10 mm/yr directed in a N38°W± 5°. The Eureka Valley earthquake may represent the transfer slip from the Owens Valley fault zone to the Fish Lake Valley-Furnace Creek-Northern Death Valley fault zones (Sawyer and Reheis, 1997) by left-lateral and normal faulting across Eureka Valley and Saline Valley (Dixon et al., 1995; Savage et al., 1990).
3. We apply the non-linear waveform optimization to smaller aftershocks using short-period records and bandwidths between 0.5 to 2 Hz. Waveform inversion of regional earthquakes usually takes advantage of the spectra at low frequency allowing the use of simple velocity models. We show here that smaller earthquakes at near source distance and higher frequencies can also be inverted using 2 or more short-period stations. The seismic moments are in agreement within 10% of the UNR duration magnitudes and the source orientations generally agreed with the distribution of relocated seismicity, CMT solutions, and sometimes with sparse first motion data.

Figure 1. This figure show 2833 relocated earthquakes, (yellow circles), from 1993 to July 1998 including the 17 May 1993 (M_w 6.0) Eureka Valley, California earthquake sequence. Short period, local network and portable instruments are drawn as white circles. Major known dip-slip faults are drawn as solid red lines and strike-slip faults are drawn as dash-dotted red lines.

Figure 2. Same as Figure 2. but without seismicity. Polygons outline cluster of relocated aftershocks. The relocated aftershocks show good correlation with the features of surface topography. The aftershocks are distributed along the eastern edge of the White-Inyo Mountains along small ridges which strike northeast from Saline Valley. One of these ridges, called the Saline Range, is situated over the mainshock. A west-east cross section shows that the mainshock is located at the northern end of the Saline Range where it becomes covered under the alluvium of Eureka Valley. The deepest extent of aftershocks makes an angle of 40-50 degrees and when projected to the surface, would outcrop along the western edge of the Last Chance Range.

Figure 3. Eight west to east vertical cross sectional views arranged in panels from north to south. The cross sections have a depth aspect of 1:1 and the 80m DEM topography have a 10:1 aspect ratio.

Figure 4. Map of Univ. of California Berkeley Digital Seismic Network, Caltech TERRA-scope, and Univ. of Nevada Reno Seismological Lab regional broadband stations used in the waveform analysis. The lower hemisphere projection focal mechanism from the 17 May 1993 (M_w6.0) earthquake points to the location of Eureka Valley, California.

Figure 5. Waveform inversion result for the 17 May 1993 (M_w 6.0) mainshock using 3 component regional broadband stations in Figure 4. See Figure 8 for hypocenter location. Solid traces are observations and dashed traces are synthetics.

Figure 8. The focal mechanisms from Figure 5 and 6 are plotted over the seismicity from 1993 to 1998. he focal mechanisms are in lower hemisphere equal area projection with shaded compressional first motion quadrants (extensional quadrants). The focal mechanisms are rotated in cross sectional views A and B. The cross sections have a depth aspect of 1:1 and the 80m DEM topography have a 10:1 aspect ratio.

Objectives

Earthquake Relocations

Waveform Analysis

Conclusions

Links