USING THE LOCATIONS OF M≥4 EARTHQUAKES TO DELINEATE THE EXTENTS OF THE RUPTURES OF PAST MAJOR EARTHQUAKES IN INTRAPLATE REGIONS

It has been speculated that localized spatial clusters of earthquakes in intraplate regions may represent late aftershocks of strong earthquakes that took place many hundreds of years ago. In eastern North America, the most active spatial clusters are the New Madrid seismic zone in the U.S. midwest and the Charlevoix seismic zone along the St. Lawrence River in Quebec, Canada. The former was the focal region of several strong earthquakes in 1811-1812 with magnitudes up to about M7.5, and the latter was likely the focus of a major earthquake with M7.2-7.5 in 1663. During the past few decades, most of the stronger (M≥4) earthquakes in both of these seismic zones have taken place near the edges of the current seismicity, consistent with the idea that there remain stress concentrations at the edges of these past ruptures. An examination of the seismicity in the rupture zones of a number of M≥6.5 earthquakes in California since the mid-1900s finds that most of the recent M≥4 earthquakes tend to occur near the ends of those ruptures. Based on this evidence, it is proposed here that M≥4 earthquakes at the edges of spatial earthquake clusters in eastern North America can help delineate the locations of past major ruptures. Furthermore, the focal mechanisms of these recent M≥4 earthquakes can give an indication of the focal mechanisms of the suspected earlier major earthquakes.