Aftershocks of the March 9, 1994, Tonga earthquake: The strongest known deep aftershock sequence

Abstract

The March 9, 1994 (Mw 7.6, depth 564 km), deep Tonga earthquake produced an exceptionally strong aftershock sequence for a deep earthquake. Using eight temporary broadband stations deployed on the Fiji, Tonga, and Niue islands, we identify 144 aftershocks with mb ranging from 3.2 to 6.0 during the first 41 days following the mainshock. The number and seismic moment of the March 9 aftershocks lie within the range of aftershock productivities of typical shallow earthquakes of similar moment. The aftershocks show a power law decay with time following the mainshock, with an exponent (p value) of −1.0, similar to shallow aftershock sequences. Fifty aftershocks can be accurately located using a hypocentroidal decomposition method; these aftershocks align along a near‐vertical ENE striking plane similar to one of the mainshock nodal planes. Sixteen of the 18 well‐located aftershocks during the first 2.5 hours are located to the NNE of the mainshock epicenter, consistent with NNE rupture propagation derived from waveform analysis. Later aftershocks are located south of the epicenter, suggesting aftershock expansion beyond the region of substantial moment release in the mainshock. Several of the aftershocks are located outside of the normal bounds of the seismically active slab, consistent with evidence that the mainshock rupture terminated outside the active slab. Five possible foreshocks occurred along the preferred fault plane within the 22 days prior to the mainshock. At least three “triggered” aftershocks occurred at distances of 50 to 110 km within the first six hours following the mainshock; triggered aftershocks in adjacent slab material may be common for large deep earthquakes. While there is some variation in the focal mechanisms of the aftershocks, most resemble the mainshock focal mechanism. All the mechanisms show compressional axes dipping to the west, consistent with background seismicity in this region. Overall, most of the aftershock characteristics of the 1994 Tonga event are similar to those found for shallow earthquakes, suggesting similarities between shallow and deep earthquakes in the mechanical processes that produce aftershocks.