Seismicity and lithospheric stress in New York and adjacent areas

Abstract

The earthquake distribution obtained by a relatively dense network of short-period high-gain stations in New York and adjacent areas from 1971 to mid-1976 shows nearly all of the features of seismicity maps from standard listings from 1928 to 1975. In addition, the network data show a region of significant activity in the Adirondack Mountains of northern New York and several sizeable earthquakes in western Lake Ontario. In both areas, few or no earthquakes were previously reported. Both epicentral maps show a region of low seismicity in the Vermont section of the control-versial Boston-Ottawa seismic zone. Well-located earthquakes in the northeastern United States and southeastern Canada range in depth from near surface to about 25 km. Deeper earthquakes (>10 km) have not been found throughout the region, but may be localized in those areas where large earthquakes have occurred. They may also be indicative of deep fault zones which have a greater potential for the occurrence of future large earthquakes. Single events, main shock-aftershock sequences, and swarms are found in this region. We do not yet understand why particular types of earthquake sequences occur in specific areas. In many cases, aftershock sequences or earthquake swarms can be related to mapped faults or inferred extensions of faults. Recent stress indicators in New York and adjacent areas support the presence of an ENE-trending maximum compressive stress in the lithosphere west of the Appalachian Mountains in New York, Ohio, and southern Ontario. The eastern boundary of this stress domain is suggested by limited data in northern New England, southeastern New York, and New Jersey. We postulate that variations in the stress field, which may exist near the eastern boundary of this stress domain, and the general lack of suitably oriented faults in Vermont may explain the region of low seismicity in Vermont along the proposed Boston-Ottawa seismic zone.