MOMENT-TENSOR ANALYSIS USING REGIONAL DATA - APPLICATION TO THE 25-MARCH-1993, SCOTTS-MILLS, OREGON, EARTHQUAKE

In this paper we outline a procedure we use for routine moment-tenser analysis of regional data from broadband seismic stations in northwestern North America and apply it to the moment magnitude 5.5, March, 1993, Scotts Mills, Oregon, earthquake. The results compare favorably with those obtained from teleseismic data. Sire found that the earthquake occurred at a depth of 13-15 km and had a mechanism with approximately equal amounts of reverse and right-lateral strike-slip components. The estimated stress drop of 40 bar is average on a world-wide basis, supporting the view that the rather large damage was caused primarily by poor construction and not by exceptional properties of the source. The Scotts Mills earthquake is most likely related to the Mt. Angel Fault, This fault is a part of the Gales Creek-Mt, Angel structural lineament (GCMAL) extending about 150 km across the Willamette Valley, At present data are not sufficient to estimate the likelihood of an earthquake involving the entire GCMAL, but given its length an earthquake of magnitude 7 is conceivable. The results of this study, together with investigations of other earthquakes, suggest that sparse broadband networks can be used efficiently for determining source parameters of earthquakes of magnitude greater than 4.0 in regions with infrequent seismicity.