The Erua earthquake cluster and seismic anisotropy in the Ruapehu region, New Zealand

We use seismicity generated from the Erua earthquake cluster (a consistently active area of seismicity about 20 km to the west of Mount Ruapehu) over the last 12 years to study seismic anisotropy in the Ruapehu region. In particular, we search for changes associated with two minor phreatic eruptions on the 4th of October 2006 and the 25th of September 2007. The seismicity rate, magnitude of completeness, focal mechanisms and b-value of the cluster are also examined to investigate whether the characteristics of the seismicity changed over the duration of the study. The hypocenters were relocated, which revealed a westward dip in the shallow seismicity. Shear wave splitting results revealed a decrease in delay time in the 2006-2007 period and a significant variation in the fast shear wave polarization in the same time period. The b-value also increased significantly from 1.0 +/- 0.2 in 2004 to a peak of 1.8 +/- 0.2 in 2007, but no other parameters were found to vary significantly over this time period. We attribute these changes to an increase in pore-fluid pressure in the Erua region due to fluid movement and suggest that this fluid movement may be associated with the eruptions in 2006 and 2007. Citation: Keats, B. S., J. H. Johnson, and M. K. Savage (2011), The Erua earthquake cluster and seismic anisotropy in the Ruapehu region, New Zealand, Geophys. Res. Lett., 38, L16315, doi:10.1029/2011GL049014.