Array measurements of deep tremor signals in the Cascadia subduction zone

Preliminary analysis of deep tremor recorded during July, 2004, in the Cascadia Subduction zone shows that small aperture arrays can resolve the slowness and back azimuth of seismic waves with a useful resolution. Data were collected by three dense arrays of short-period seismometers specifically deployed in the Puget Sound area under an US-Italy-Canada cooperative effort. Slowness analyses at the three arrays indicate that the 2 - 4 Hz tremor wave-field is composed of waves propagating with apparent velocities higher than 4 km/s. Combining this with polarisation analysis show these waves to be transverse (SH) waves. However, P-waves, though smaller in amplitude, can be detected by different slowness values obtained for the radial and transverse components. The intersection of wave vectors determined by the back azimuth and slowness values measured at the three arrays provides a preliminary estimate of source location for a sample of the recorded deep tremor.