THE MARCH 9, 1994 (M-W-7.6), DEEP TONGA-EARTHQUAKE - RUPTURE OUTSIDE THE SEISMICALLY ACTIVE SLAB

WP investigate the rupture process of the March 9, 1994, M-W 7.6 deep Tonga earthquake and its relationship to the background seismicity of the subducted Tonga slab. Variations in observed P and S wave pulse du ration indicate that the rupture propagated to the NNE and extended we ll beyond the background seismicity. We inverted 47 P and SH waveforms , including regional broadband waveforms from the Southwest Pacific Se ismic Experiment, using a method that solves for the focal mechanism c hange during the rupture and the distribution of moment release along the fault plane. The results indicate that significant moment release occurred in previously aseismic regions outside the active seismic zon e and that the rupture terminated 10-20 km beyond the bounds of the pr evious seismic activity. A significant change in focal mechanism occur red when the rupture propagated into the previously aseismic region. R upture along the near-vertical NNE striking nodal plane. This result, combined with the plant alignment of aftershocks and the general NNE d irectivity of the waveforms, provides strong evidence that the rupture occured on the near-vertical plane. Thermal modeling of the Tonga sla b indicates that the rupture terminated in material about 200 degrees C warmer than the temperature that normally limits the occurrence of s maller earthquakes. Additionally, after shocks seem to be suppressed i n the outer regions of the moment release but only 1 of the 15 well-lo cated aftershocks. We suggest that slabs may be composed of an inner c old sore, where seismic rupture initiates and only a few aftershocks. The mechanism by which rupture propagates through the warmer material need not be similar to the process governing rupture nucleation in the cold slab core; nucleation may occur through a process limited to the cold sore such as transformational faulting or plastic instabilities. Isolated deep earthquakes in other subduction zones, such as the 1994 Bolivia event, may occur almost completely within the warmer zone, ac counting for the lack of background seismicity and the dearth of after shocks.