Working Groups:

1. Strong Motion (B)
   • Task B3: Site effects-- predictability and reproducibility of site response. Assembly of existing data, collection of new data where needed, statistical analysis of site response data, and data analysis and modeling directed at improved understanding of site response.

2. Subsurface Imaging, Seismicity, and Tectonics (D)
   • Task D1: LARSE. Basin, crustal and mantle structure derived from the new data.

Technical Description

With our recordings of LARSE-2 blasts, we will study ground motion and site conditions from the matched, calibrated 3-component digital stations with 1 Hz seismometers we deployed at the MCS and four sites of precarious and semi-precarious rocks. Precariously balanced rocks that have been found within 20 km of the San Andreas fault (Brune, 1999) suggest lower ground motion during great earthquakes than would be predicted from strong motion recorded at engineering class A rock sites in the western Mojave area. We are also about to record the MCS and the Lovejoy Butte sites with a 24-channel array of vertical instruments, using our existing 1999 project funds. A refraction line run by Gary Fuis provides P-wave velocity estimates over thin alluvium near but not across Lovejoy Buttes. Velocities of the fractured granite below the alluvium may vary by a factor of two, so we will derive additional constraints on shallow velocities at the two sites. We will cross each site with a 350 m refraction array. Each will be recorded with reversed sledgehammer sources.

The same 24-channel instrumentation has been used to passively record microtremor noise in conjunction with broadband arrays by Iwata et al. (1998). P-tau analysis of 4-6 Hz surface waves propagating across these different types of arrays gives similar results for S-wave velocity structure within 100 m of the surface. We will analyze microtremor data on the 24-channel arrays and 3-component stations, for independent confirmation of any velocity differences between the MCS and the precarious rock sites. Field comparison of the ``rock'' at MCS and at the Buttes leads us to believe that the listed class A rock at MCS will prove to have lower shallow S-wave velocity than the much harder rock at the precarious rock sites.

We propose here the costs of analyzing the 3-component records of about 100 LARSE-2 blasts and Hector Mine aftershocks recorded on six stations, and the refraction and microtremor data from two sites. Analysis will take the form of computation of spectral ratios of events recorded simultaneously on our station at MCS and the other stations. Sample spectra show that MCS has significant amplification over any of the precarious-rock sites between 1 and 4 Hz (see our 1999 project report). We propose to document the consistency of this amplification, and plot it with respect to distance for all LARSE-2 and Hector Mine events we previously recorded. We will then see if there is agreement between the lower amplitudes at the precarious-rock sites shown on the spectral ratios, and any differences in 30-m-depth S-wave velocities between MCS and Lovejoy Butte. Jackknife averaging of spectral ratios with and without MCS results will show whether MCS is anomalous when compared to the four Buttes sites. Significant discrepancies from the relations proposed by Borcherdt (1994) would require the proposal of amplification mechanisms for the MCS site not related to shallow S velocities. Our report on these effects can be included in SCEC legacy documents on site responses.

References

• Borcherdt, R. D., 1994, Strong ground motion -- Synopsis: in R. D. Borcherdt, Ed., The Loma Prieta, California, Earthquake of October 17, 1989 -- Strong Ground Motion, U.S. Geological Survey Professional Paper 1551-A, p. A1-A7.
• Brune, James N., 1999, Precarious rocks along the Mojave segment of the San Andreas fault, California: Constraints on ground motion for great earthquakes: Seismological Research Letters, vol. 70, no. 1 (Jan./Feb.), p. 29-33.
• Iwata, T., H. Kawase, T. Satoh, Y. Kakehi, K. Irikura, J. Louie, R. Abbott, and J. Anderson, 1998, Array microtremor measurements at Reno, Nevada, USA (abstract): presented at Amer. Geophys. Union Fall Meeting, Dec. 6-10, San Francisco.

Analysis of Shallow Site Response to LARSE-2 Blasts at Precarious Rock
Sites Near the San Andreas Fault

J. Louie, UNR Seismological Lab 

                                                              04/01/2000
Budget Estimate                                               03/31/2001
------------------------------------------------------------------------
Salaries and Wages
        J. Louie, 3 days @ $450/day                                 1350
        Grad Student Rob Abbott, 3 mo. @ $1100/mo. (20 hrs/wk)      3300
    Total Salaries and Wages                                       $4650

Fringe Benefits
        4.0% of Louie salary                                          54
        2.0% of student stipend                                       66
    Total Fringe Benefits                                            $90

Travel
        Two to 2000 SCEC Annual Meeting (2x$150 air + $150 hotel)    450
        Two to SCEC Workshop (2x$150 air + $50 car rental)           350
    Total Travel                                                    $800

Other
    Computer systems supplies, hardware and software maint.,         300
      administration, network fees to support data dissemination
    Postage, phone toll and line charges                             100
    Copy charges, publication page charges                           600
    Graduate student course fees, 9 @ $92.50                         833
        Total Other                                                $1833

Total Direct Costs                                                 $7373
Indirect Costs: 44.3% of direct costs minus student fees ($6540)    2897
------------------------------------------------------------------------
Total Requested From SCEC                                         $10270

Current Support of John N. Louie

• Shallow fault-reflection and site response recording during LARSE-2 (with J. Brune and R. Anooshehpoor, field costs only), NSF/SCEC, 7/99-6/00:$7,008
• Seismic hazards in the vicinity of Las Vegas and Reno (with J. Anderson, Y. Zeng, F. Su; Louie for 3 days/year), USGS-NEHRP, 4/99-3/01, $100,000.