Hypocenter Relocation and Source Parameters of the 2006 Kiholo Bay, Hawaii Earthquake Sequence

We studied aftershock distribution of the Kiholo Bay earthquake (MW 6.7), which occurred on October 15, 2006 (UTC) beneath the NW part of the Island of Hawaii, to determine the fault plane from the two nodal planes. To more precisely locate aftershocks relative to one another, we used the cross correlation technique [e.g., Wolfe et al., 2003] and double-difference hypocenter determination [Waldhauser and Ellsworth, 2000]. The relocated cloud of aftershocks has a NW-SE trend with a SW dip, which is consistent with one of the nodal planes given in the Harvard moment tensor solution. The hypocenter of the main shock is located close to the eastern edge of the activity, suggesting that the rupture was almost unilateral to the NW. We also estimated corner frequencies of 31 earthquakes (2.5 < ML < 5.0) which occurred in 2006 and 2007 near the source region of the Kiholo Bay earthquake to investigate the spatial variation and scaling relationship of stress drop. We used strong-motion and short-period seismic stations operated by the USGS Hawaiian Volcano Observatory (HVO). We picked earthquakes with ML 3.7 and 2.0 as empirical Green"fs functions for strong-motion and short-period waveforms, respectively. We fitted the source spectra with the omega squared model and estimated corner frequencies of P waves from the deconvolved waveforms. We then calculated stress drops from the model of Madariaga [1976] assuming that the local magnitude determined by HVO is equal to the moment magnitude. The stress drops range from 0.040 to 23 MPa. We found that earthquakes close to hypocenters of the main shock and the largest aftershock (ML 5.0) have smaller stress drops. No significant relationship between stress drop and seismic moment was observed but it is difficult to argue that the stress drop is constant because of the large variability. Two of the 31 earthquakes occurred before the Kiholo Bay main shock and their stress drops have no systematic differences compared to the values of the 29 aftershocks.