A DEEP EARTHQUAKE AFTERSHOCK SEQUENCE AND IMPLICATIONS FOR THE RUPTURE MECHANISM OF DEEP EARTHQUAKES

A distinguishing characteristic of deep earthquakes has been the absen ce of observable aftershock sequences(1,2). Here we report the first e xtensive deep-earthquake aftershock sequence to be observed; it was re corded by an array of eight broadband seismographs following the 9 Mar ch 1994 deep Tonga earthquake. The aftershocks show a power-law decay with time following the main shock, as is typical of shallow events. M ost of the well located aftershocks are concentrated along a steeply d ipping plane consistent with one of the nodal planes of the main-shock mechanism and the mechanisms of three large aftershocks. Assuming the se aftershocks denote the main-shock rupture area, they define a 50 x 65 km fault plane extending across the entire width of the active seis mic zone and into the surrounding aseismic region. The width of the af tershock zone is wider than the expected width of the metastable olivi ne sedge, demonstrating that either the width of the metastable olivin e material exceeds previous estimates, or the aftershocks are not conf ined in such a wedge.