- © 2015 by the Seismological Society of America
Online Material: Table of hypoDD analysis; figure showing waveform fits.
The Cascadia subduction zone offshore Oregon and Washington has been anomalously quiet since the advent of modern seismic networks in the Pacific Northwest. Nonetheless, during the 26 year period from 1989 through 2014, 19 earthquakes with M≥2.8 were detected by seismic networks and located on the continental margin of central Oregon (44.3°–44.7° N) in the region where the Juan de Fuca and North American plates are thought to be locked (e.g., McCaffrey et al., 2007; Burgette et al., 2009; Schmalzle et al., 2014). With increasing evidence that plate boundary earthquakes may undergo a preparatory stage prior to major earthquakes (e.g., Bouchon et al., 2013; Brodsky and Lay, 2014), this seismicity may have broader implications.
In this article, we provide an updated analysis of the seismicity that includes 11 new events (through 20 March 2012) added to the database of high‐precision relative relocations presented by Williams et al. (2011), 5 single‐event locations that include data from ocean‐bottom seismometers (OBSs) deployed as part of the Cascadia Initiative (Toomey et al., 2014), and 1 new moment tensor inversion. We show that several events in March 2012 were located within a cluster of events spatially related to a 2004 Mw 4.9 low‐angle thrust earthquake on or near the plate boundary; moment tensor inversion of waveforms from the largest of these earthquakes (Mw 3.8) indicates a thrust mechanism with similar strike but steeper dip than the 2004 earthquake. In contrast, four earthquakes with magnitude ∼3 on 25 January 2013 represent a new locus of activity on the plate boundary. Comparing depths …