Multiple Directions of Shear-wave Splitting in the 2005 Alpha, Idaho Earthquake Swarm

Abstract:

Most studies of shear-wave splitting identify one dominant fast direction. Two directions of splitting were observed for events in the 2005 Alpha, Idaho earthquake swarm. This tectonic sequence was particularly shallow (most depths < 4 km). S-wave recordings are unusually complex. Identification of S-wave onsets is usually very difficult, even at distances less than 6 km.

The shear-wave window for observing S-wave splitting from shallow events is very short, but at least one station was usually within that distance. Horizontal hodograms indicate that two non-orthogonal directions of splitting exist. Comparison with local topography, mapped faults, earthquake T-axes, and local surface joint azimuths leads to two potential correlations. One direction of splitting may correspond to an older stress direction matching local topography and one peak in joint azimuths. The other may be aligned with the current stress direction, nearby Tertiary faults, and another peak in joint azimuths. The two directions differ by about 45 degrees, in approximate agreement with the hodograms.

The two splitting directions qualitatively support the seismogram complexity. Each leads to a fast and a slow S-wave pulse. The shallowness of the sources is already likely to result in seismograms rich in reflections, and two splitting directions will double the number of S arrivals. At short distances the energy distribution among increased numbers of arrivals is likely to decrease peak ground motions derived from simple models that do not take anisotropy into account.