Cascadia Tremor Located Near Plate Interface Constrained by S Minus P Wave Times
Mario La Rocca,1*
Kenneth C. Creager,2
Danilo Galluzzo,1
Steve Malone,2
John E. Vidale,2
Justin R. Sweet,2
Aaron G. Wech2
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. This lagged correlation represents the time interval
between vertical compressional waves and horizontal shear waves.
Measurements of this interval, combined with location techniques,
resolve the depth of tremor sources within ±2 kilometers.
For recent Cascadia tremor, the sources locate near or on the
subducting slab interface. Strong correlations and steady
S-
P time differences imply that tremor consists of radiation from
repeating sources.
2 Department of Earth and Space Science, University of Washington, Box 351310, Seattle, WA 98195, USA.
* To whom correspondence should be addressed. E-mail: mlarocca{at}ov.ingv.it
