Cascadia tremor located near plate interface constrained by S minus P wave times.

Nonvolcanic tremor is difficult to locate because it does not produce impulsive phases identifiable across a seismic network. An alternative approach to identifying specific phases is to measure the lag between the S and P waves. We cross-correlate vertical and horizontal seismograms to reveal signals common to both, but with the horizontal delayed with respect to the vertical.

Widespread triggering of nonvolcanic tremor in California.

We identified seven locations on or near the transform plate boundary in California where nonvolcanic tremor was triggered by the 2002 Denali earthquake. This result implies that the conditions essential for nonvolcanic tremor exist in a range of tectonic environments. Models explaining tremor typically require conditions endemic to subduction zones, that is, high temperatures and fluid pressures, because previously tremor was nearly exclusively documented in subduction zones.

Tidal modulation of nonvolcanic tremor.

Episodes of nonvolcanic tremor and accompanying slow slip recently have been observed in the subduction zones of Japan and Cascadia. In Cascadia, such episodes typically last a few weeks and differ from "normal" earthquakes in their source location and moment-duration scaling. The three most recent episodes in the Puget Sound/southern Vancouver Island portion of the Cascadia subduction zone were exceptionally well recorded.

Non-volcanic tremor driven by large transient shear stresses.

Non-impulsive seismic radiation or 'tremor' has long been observed at volcanoes and more recently around subduction zones. Although the number of observations of non-volcanic tremor is steadily increasing, the causative mechanism remains unclear. Some have attributed non-volcanic tremor to the movement of fluids, while its coincidence with geodetically observed slow-slip events at regular intervals has led others to consider slip on the plate interface as its cause.

Intraslab earthquakes: dehydration of the Cascadia slab.

We simultaneously invert travel times of refracted and wide-angle reflected waves for three-dimensional compressional-wave velocity structure, earthquake locations, and reflector geometry in northwest Washington state. The reflector, interpreted to be the crust-mantle boundary (Moho) of the subducting Juan de Fuca plate, separates intraslab earthquakes into two groups, permitting a new understanding of the origins of intraslab earthquakes in Cascadia. Earthquakes up-dip of the Moho's 45-kilometer depth contour occur below the reflector, in the subducted oceanic mantle, consistent with serpentinite dehydration; earthquakes located down-dip occur primarily within the subducted crust, consistent with the basalt-to-eclogite transformation.