The first surface faulting from a historical intraplate earthquake in North America

Abstract

ALTHOUGH many fault ruptures are known from the boundaries of the Earth's lithospheric plates, the 'stable' interiors of continents are much less active¹. Worldwide, only ten historical intraplate earthquakes are known to have produced surface faulting² and none of these was in eastern North America. Despite the confirmed prehistoric rupture of the Meers fault³, some have wondered if intraplate earthquakes in the stable North American craton might somehow be different from plate-boundary earthquakes, and hence fail to rupture the surface⁴. This is an important consideration for seismic hazard assessments. When the Ungava earthquake (with magnitude (M_s) 6.3 and apparent shallow depth) occurred on 25 December 1989 in northern Canada, we (and, independently, A. Johnston of Memphis State University) concluded that it might have produced surface faulting. Here we report that there was indeed a surface rupture, 8.5 km long, with up to 1.8 m of reverse faulting on a steeply dipping, arcuate fault plane. The faulting confirms the seismological process and indicates that there is a hazard from ground-rupturing earthquakes in eastern North America, but the poor surface expression shows that finding prehistoric fault ruptures in order to constrain seismic hazard estimates will be difficult.