Mount Hood Earthquake Activity: Volcanic or Tectonic Origins?

Abstract

On 29 June 2002, a M D 4.5 earthquake occurred 4.6 km south of Mount Hood, Oregon. More than 200 small aftershocks ( M D ≤3.8) occurred between 29 June and 15 August 2002, by which time seismicity returned to background levels. We analyze well-constrained earthquakes from the summer 2002 sequence near Mount Hood, recorded by the Pacific Northwest Seismic Network (pnsn), and well-constrained, small earthquakes ( M D ≤3.5) recorded by the pnsn between 1986 and 2002. We apply waveform cross-correlation to selected events, establish a one-dimensional P -wave velocity model, and relocate the entire catalog by using the double-difference algorithm of Waldhauser and Ellsworth (2000). We find that earthquakes before 2002 occur in four distinct clusters: (A) a well-defined linear feature, striking N15E, about 5 km south of the summit, (B) a linear feature, trending northwest–southeast a few kilometers south of group A, (C) a few tight clusters located about 9 km south-southwest of the summit, and (D) a small cluster beneath the summit. Earthquakes in group D do not appear to lie on a fault and may relate to volcanic activity. However, almost all earthquakes in the 2002 swarm occur in group A, south of the summit, at depths similar to earthquakes from previous swarms. First-motion fault-plane solutions from the mainshock and largest aftershocks have normal mechanisms, and solutions from well-constrained aftershocks have normal to oblique normal mechanisms. This is consistent with the apparent strike of the feature south of Mount Hood’s summit and is similar to focal mechanisms of older earthquakes in group A. b - and p -values for the summer 2002 earthquakes resemble a tectonic mainshock-aftershock sequence. We interpret these earthquakes as the northernmost example of Basin-and-Range seismicity yet recorded and conclude that the Mount Hood earthquakes do not yet suggest potential for volcanic unrest.