The March 9, 1994 (Mw 7.6), deep Tonga earthquake: Rupture outside the seismically active slab

Abstract

We investigate the rupture process of the March 9, 1994, Mw 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 duration indicate that the rupture propagated to the NNE and extended well beyond the background seismicity. We inverted 47 P and SH waveforms, including regional broadband waveforms from the Southwest Pacific Seismic Experiment, using a method that solves for the focal mechanism change 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 zone and that the rupture terminated 10–20 km beyond the bounds of the previous seismic activity. A significant change in focal mechanism occurred when the rupture propagated into the previously aseismic region. Rupture along the near‐vertical NNE striking nodal plane provides a somewhat better fit to the body waveforms than rupture along the near‐horizontal nodal plane. This result, combined with the planar alignment of aftershocks and the general NNE directivity of the waveforms, provides strong evidence that the rupture occurred on the near‐vertical plane. Thermal modeling of the Tonga slab indicates that the rupture terminated in material about 200°C warmer than the temperature that normally limits the occurrence of smaller earthquakes. Additionally, aftershocks seem to be suppressed in the outer regions of the rupture, which contain about half of the moment release but only 1 of the 15 well‐located aftershocks. We suggest that slabs may be composed of an inner cold core, where seismic rupture initiates and small earthquakes occur, and a thermal “halo” of warmer material, which can sustain rupture 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 core such as transformational faulting, and propagation through the warmer material may occur through ductile 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, accounting for the lack of background seismicity and the dearth of aftershocks.