Abstract

We observe that broadband seismometers may produce artifact long-period signals that resemble impulse responses, similar to a step in acceleration, in the presence of shaking as moderate as 0.2%g. This observation accords with recent observations in Europe and elsewhere with similar instruments (e.g., Zahradnik and Plesinger, 2005). We present two case studies. For both the 8 October 2006 M 4.5 earthquake near Mt. Rainier in the Pacific Northwest and an M 5.0 event on 29 September 2004 in southern California, artifact signals, possible step tilts, and apparent sensor problems are observed as far as about 200 km from the epicenters. Such long-period artifacts, if not recognized, complicate and degrade estimation of source parameters of moderate and larger earthquakes on regional networks. The exact cause of the artifacts currently remains obscure, but may require alterations to instrument installation and/or design strategies.

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