The Greenland earthquake of 11 July 1987 and postglacial fault reactivation along a passive margin

Abstract

AbstractIntraplate earthquakes, in many cases, occur near continental margins. This observation applies very well to the case of Greenland, a stable continental region with low seismicity. The seismicity of Greenland is nearly totally concentrated along the coastlines of the island, where it is relatively free of ice cover. On 11 July 1987, the largest Greenland earthquake of instrumental record (mb = 5.5, Mw = 5.4) occurred in continental crust off the northeastern coast in the Wandel Sea. Waveforms of long- and short-period teleseismic P and long-period teleseismic SH waves have been modeled to estimate the source parameters for this earthquake. The focal mechanism determined is a normal faulting with a strike-slip component (strike = 148°, dip = 59°, and rake = −131°). The strike direction of one of the nodal planes was found to be consistent with the direction of the dominant structural trend observed in the area. A previous normal faulting mechanism with very different strike directions for this event was found to have inferior waveform fit and is not consistent with the fault strike directions from the local geological setting. The seismic moment of the July 1987 earthquake is 1.75 × 1024 dyne-cm. The focal depth and stress drop are 5 km and 389 bars, respectively. The high stress drop may relate to the relatively long earthquake repeat time in the region. The observed normal faulting of this earthquake can be interpreted as a rupture due to an extensional stress field at the forebulge outside of the ice sheet because of deglaciation flexure in the region. Another major earthquake (26 November 1971; mb = 5.1) 311 km to the south of the 11 July 1987 event was likewise determined to be normal faulting and can be explained as another event triggered by deglaciation. In addition, we find that the sparse general seismicity of Greenland can also be related to postglacial rebound.