Nonplanar Fault Source Modeling of the 2008 Mw 6.9 Iwate-Miyagi Inland Earthquake in Northeast Japan

We use crustal deformation data sets acquired by the Phased Array-Type L-Band Synthetic Aperture Radar (PALSAR) and the Japanese nationwide Global Positioning System (GPS) Earth Observation Network (GEONET) to develop a fault source model for the 2008 Iwate-Miyagi inland earthquake (M-w 6.9) that occurred on 14 June 2008 in northeastern Honshu, Japan. The PALSAR data include Interferometric Synthetic Aperture Radar (InSAR), range offset, and azimuth offset acquired from both ascending and descending paths. The range- and azimuth-offset data are the only quantitative deformation signals around the epicenter, and exhibit complicated signals that were not detected by the GPS data. We refine our preliminary model by using Green's functions from triangular dislocation elements to represent nonplanar fault geometries. Although a single segment model can well explain the GPS data, it generates significant misfit residuals in the offset data; thus, both west-dipping and east-dipping segments are necessary, as also suggested from aftershock distribution data. The moment magnitudes of the west-dipping and east-dipping faults are 6.53 and 6.76, respectively, and the total sum, 6.87, is almost identical to the seismological moment magnitude. This result suggests that both segments contributed equally to the total seismic moment, demonstrating a complex rupture process of the inland earthquake.