Average seismic source-parameter relations for mid-plate earthquakes

Mid-plate earthquakes, which are defined as having their epicenters at least 500 km from plate margins, represent a small portion of the total global seismicity. However, because those occurring within continental interiors are characterized by anomalously large damage areas, they are of particular interest.

Empirical m_b, M_s, and M_o data are used to develop scaling relations for mid-plate earthquakes. From them, it is found that the corner period, T_o2, of their far-field displacement spectra varies as the one-fourth power of the seismic moment, which indicates that average stress drop increases as seismic moment increases. By use of Savage's (1972) rectangular, bilateral dislocation model, the derived spectra of mid-plate earthquakes yield estimates of fault rupture length and width, average fault displacement, and source rise time as a function of seismic moment. These relations show that large mid-plate earthquakes, such as the 1886 South Carolina and 1811 and 1812 New Madrid events, do not require large fault rupture length.