Short‐term and long‐term tremor migration patterns of the Cascadia 2004 tremor and slow slip episode using small aperture seismic arrays

Tectonic tremor has been recorded at many subduction zones, including the Nankai,
Cascadia, Mexican, and Alaskan subduction zones. This study, the first to use small
aperture seismic arrays to track tremor, deployed three small aperture seismic arrays along
the Cascadia subduction zone during a tremor and slow slip episode in July 2004. The
tremor was active during virtually all (up to 99%) minutes of the analyzed tremor episode
using 5 min sample windows. Individual wave phases were tracked across the arrays and
used to derive slowness vectors. These were compared with slowness vectors computed
from a standard layered Earth model to derive tremor locations. Locations were stable
within a volume roughly 250 km2 in epicenter and 20 km in depth for hours to days before
moving to a new volume. The migration between volumes was not smooth, and the
movement of the sources within the volume followed no specific pattern. Overall
migration speeds along the strike of the subduction zone were between 5 and 15 km/d;
smaller scale migration speeds between volumes reached speeds up to 2 km/min.
Uncertainties in the best locations were 5 km in epicenter and 10 km in depth. For this data
set and processing methodology, tremor does not locate predominately on the primary
subduction interface. Our favored model for the generation of tectonic tremor signals is
that the tremor is triggered by stress and fluid pressure changes caused by slow slip and is
composed, at least in part, of low‐frequency earthquakes broadly distributed in location

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An edited version of this paper was published by AGU. Copyright (2010) American Geophysical Union.