Seismic Velocity Structure of the Greater Mount Rainier Area 1434-95-G-2571 Stephen D. Malone, P.I. Dept. of Earth and Space Sciences, University of Washington Seattle, WA 98195 (206) 543-8020, e-mail: steve@ess.washington.edu
Program Element: II.1 and II.2
Investigations undertaken
This project aims to determine the detailed three-dimensional velocity structure of the central Washington Cascade mountain range by using first arrival data from natural earthquakes recorded on the Pacific Northwest Seismograph Network (PNSN) and 18 temporary stations. These arrival-time data will be used in a non-linear tomographic inversion for velocity, station corrections, and hypocenters.
Results
We are nearing the completion of the field component of this project. The 18 planned temporary seismograph sites either have been or are currently occupied (see Figure 1). Ten sites were occupied between February and October of 1995, and eight sites are currently in operation. These last eight stations will be removed next spring, after the snow-pack in the Cascades melts.
The number of P arrivals recorded by the 10 temporary stations in operation between February and September exceeded our expectations in all but three cases (see Table 1). Preliminary indications are that data from seven of the eight current stations will also exceed expectations. The seismicity level of the central Washington Cascades is slightly higher than in previous years and the additional stations in this region increase the network's sensitivity.
The computational component of our project is in a developmental phase. We are modifying existing code developed by Jonathan Lees to perform a non-linear inversion for P-velocity structure to address the problem of coupling between the hypocenter determination and velocity model. We are pursuing a parameter separation approach similar to Spencer and Gubbins (1980) which allows us to invert for hypocenter parameters and velocity values separately. We believe it is important to proceed in this fashion instead of inverting for both sets of parameters simultaneously, as is commonly done today in local earthquake tomography. In addition, we are developing a method similar to Dziewonski and Anderson (1983) for estimating station corrections that removes as much independent structure as possible from each correction before using it in the velocity inversion.
As a minor part of this project we participated in the major E-W refraction experiment run in September, 1995 through the study area by a consortium of groups including the U.S. Geological Survey, University of Texas at El Paso and Oregon State University. Detailed interpretation of these data is being carried out by those principal institutions; however, we plan to use the resulting two-dimensional model as either a check on our three-dimensional result, or as a constraint if this should be necessary. As a first comparison we have relocated the refraction experiment shots using the regional network and our standard 1-D velocity models for this region (see Figure 2 for a comparison of our relocation epicenters with the true locations).
Dziewonski, A. M., and D. L. Anderson, Travel times and station corrections for P waves at teleseismic distances, J. Geophys. Res., 88 , 3295-3314, 1983.
Spencer, C., and D. Gubbins, Travel-time inversion for simultaneous earthquake location and velocity structure determination in laterally varying media, Geophys. J. R. Astr. Soc., 63 R, 95-116, 1980.
Publications
Moran, S.C., A. Qamar, S.D. Malone, Seismicity at Mount Rainier, Washington, IUGG XXI General Assembly Program , Boulder, CO, (abs) 1995
Malone, S.D., S.C. Moran, Mount Rainier; Washington, USA: tectonics, seismicity, and hazards, Volcanoes in Town , page 49-52 IAVCEI conference on volcanic hazard in densely populated regions, Periodico di Mineralogia, Roma (extended abs), 1995
Moran, S.C., J.M. Lees, S.D. Malone, P-wave tomography at Mount Rainier, Washington: preliminary results, EOS (abs) 1995 in press
TABLE 1
Observed PredictedNote: Predicted P-picks was based on the assumption that eachStation # P-picks # P-picks Dates of Operation ASF 85 46 950320 - 950612 BDM 6 176 (*) 951002 - CMK 20 176 (*) 951006 - DLR 211 127 950407 - 950919 FGM 137 100 950519 - 950919 GRL 40 176 (*) 951002 - MHL 93 58 950802 - 951011 MUP 23 176 (*) 951002 - PAK 132 135 950202 - 950802 QTZ 22 176 (*) 950925 - RAD 37 85 950629 - 950929 RSH 123 91 950613 - 950920 RVN 16 176 (*) 950925 - ULB 108 85 950629 - 950929 WCR 145 91 950612 - 950922 WI2 18 176 (*) 950925 - WPS 72 85 950629 - 950928 XTL 31 176 (*) 951011 -
station would record P-waves only from earthquake with magnitude > 1.0.
(*) = Projected # of P-picks based on number of earthquakes with good locations (BB or better) and magnitudes > 1.0 occurring during the first 8 months of 1995.
