The 8 October 2006 Md 4.5 Cowlitz chimneys earthquake in Mount Rainier National Park

Abstract

An M d 4.5 earthquake located ∼12 km east of Mount Rainier occurred on 8 October 2006 at 02:48 UTC ( figure 1 ). Although not large enough to be damaging or of major tectonic significance, a summary description of the earthquake is warranted because of its proximity to Mount Rainier, and because earthquakes of M d ≥ 4.5 are relatively rare in this region. Previous events of M d ≥ 4.5 have occurred approximately once a decade within a radius of ∼60 km from this mainshock, with the closest and most recent prior earthquake being an M d 4.9 event located 46 km to the southwest in 1989. Magnitudes in this paper refer to the coda duration magnitude determined by the Pacific Northwest Seismic Network (PNSN) ( Crosson 1972 ). We refer to the 2006 event as the “Cowlitz Chimneys” earthquake because it occurred beneath the Cowlitz Chimneys, a prominent ridge in Mount Rainier National Park. This paper describes the mainshock's focal mechanism and its aftershock distribution. The inferred source mechanism, its ordinary aftershock sequence, and the lack of significant triggered seismicity near the volcanic edifice lead us to conclude that this was a regular tectonic crustal earthquake rather than one related to volcanic processes.