THE CORNER FREQUENCIES AND STRESS DROPS OF INTRAPLATE EARTHQUAKES IN THE NORTHEASTERN UNITED-STATES

This article presents the estimation of stress drops for small to midd le-sized intraplate earthquakes in the northeastern United States. The vertical-component Sg and Lg waves of 49 earthquakes were analyzed, a nd their seismic corner frequencies and seismic moments were determine d. For these events, both short-period and broadband records were obta ined from stations in the region. There are eight events each of which has an aftershock good enough to be treated as its empirical Green's function, and their corner frequencies were estimated from empirical G reen's function methods. For the other events, the corner frequencies were directly estimated by the spectral fitting of the vertical compon ent of the Sg- or Lg-wave displacement spectrum with the omega-square source spectral model, using the available broadband and high-frequenc y short-period data and a frequency-dependent Q correction. The static stress drops, Delta sigma, were then calculated from the corner frequ ency and seismic moment. From our study, the source corner frequencies estimated by fitting the Lg displacement spectrum with the assumed om ega-square source model are more consistent with the corner frequencie s measured from empirical Green's function deconvolution method than t hose estimated from the intersection of horizontal low-frequency spect ral asymptote and a line indicating the omega(-2) decay above the corn er frequency. The source corner frequencies we estimated proved to be most appropriate for the small to middle-sized earthquakes. The static stress drops calculated from these corner-frequency estimates tend to be independent of seismic moment for events above a certain size. For earthquakes with size less than about 2 x 10(20) dyne-cm, the stress drop tends to decrease with decreasing moment, suggesting a breakdown in self-similarity below a threshold magnitude. A characteristic ruptu re size of about 100 m is implied for these smaller earthquakes.