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 northwest ern North America and apply it to the moment magnitude 5.5, March, 199 3, Scotts Mills, Oregon, earthquake. The results compare favorably wit h those obtained from teleseismic data. Sire found that the earthquake occurred at a depth of 13-15 km and had a mechanism with approximatel y equal amounts of reverse and right-lateral strike-slip components. T he estimated stress drop of 40 bar is average on a world-wide basis, s upporting the view that the rather large damage was caused primarily b y poor construction and not by exceptional properties of the source. T he Scotts Mills earthquake is most likely related to the Mt. Angel Fau lt, This fault is a part of the Gales Creek-Mt, Angel structural linea ment (GCMAL) extending about 150 km across the Willamette Valley, At p resent data are not sufficient to estimate the likelihood of an earthq uake 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 net works can be used efficiently for determining source parameters of ear thquakes of magnitude greater than 4.0 in regions with infrequent seis micity.