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Seismological Research Letters; March/April 2008; v. 79; no. 2; p. 186-193; DOI: 10.1785/gssrl.79.2.186
© 2008 Seismological Society of America
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The 8 October 2006 md 4.5 Cowlitz Chimneys Earthquake in Mount Rainier National Park

Renate Hartog1, Joan Gomberg2, Seth C. Moran3, Amy Wright1, and Karen L. Meagher2

The first 20% of the full text of this article appears below.


   INTRODUCTION
 
An Md 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 Md ≥ 4.5 are relatively rare in this region. Previous events of Md ≥ 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 Md 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.


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  		Figure 1. Hillshade relief map derived from Digital Elevation Model (DEM; GTOPO3 interpolated to 1 arcsec) showing PNSN catalog locations of all earthquakes with Md > 0.0 from 1 January 1969 up to 8 October 2006. The white star indicates the location of the Md = 4.5 mainshock of 8 October 2006. Also labeled are the western Rainier seismic zone (WRSZ); the St. Helens seismic zone (SHSZ); volcanoes Mount Rainier, Mt. St. Helens, and Mt. Adams; the Goat Rocks volcanic center; and temporary seismic station CAYP. White triangles represent seismic stations.

 
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.


   BACKGROUND
 
The Cowlitz Chimneys epicentral region lies within the Cascades mountain range, . . . [Full Text of this Article]

Pacific Northwest Seismic Network
University of Washington
Department of Earth and Space Sciences
Box 351310
Seattle, WA 98195 USA
renate@ess.washington.edu
(R. H.)






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