Anisotropic scattering and travel time delay analysis in Kilauea volcano, Hawaii, earthquake coda waves

We study the S wave coda of microearthquake multiplets (earthquakes showing similar seismograms) recorded on the Kilauea volcano, and we analyze the direction of emission for wavelets composing the coda. Our data set consists of 71 small magnitude, well‐relocated, clustered events that occurred in an area of about 1 km 2 in the southern flank of Kilauea, recorded at hypocentral distances of 10 to 20 km. For each station, we apply a cross‐spectral moving‐window technique to each pair of multiplet records to estimate the travel time delays due to source location offsets. These delays are computed for consecutive windows, starting at the P wave arrival time and ending at 4 times the S wave travel time. They are next used to estimate the slowness vector for each window along the seismograms. Our analysis shows (1) that scattered waves are emitted anisotropically along some preferred directions as late as 4 times the S wave travel time, and (2) that for some stations, the azimuthal plane rotates during the seismogram with a clear travel time delay signature. Our analysis shows that travel time delay variation in the coda can be purely due to geometrical propagation effects. This conclusion is of special interest when trying to use travel time delays to infer temporal changes in the crust. Our results do not support a coda diffracted isotropically in the crust, in the magnitude range and at the hypocentral distances we investigated.