Common features of the excitation and propagation of strong ground motion for North American earthquakes

The central matter of this investigation is the commonly held belief that eastern North America (ENA) earthquakes are felt to much greater distances, at all levels of intensity, than their western U.S. (WUS) counterparts of the same source strength. By comparing the areas enclosed by Modified Mercalli intensities I to III (felt), IV, V, VI, and VII for ENA and WUS earthquakes at the same source strength, we find that this proposition is mostly false at damaging levels of ground motion (intensities VI to VII and greater). For M ≲ 7, neither should such differences exist, if strong ground motion is confined to R ≲ 150 km and arrives principally as body waves and if ENA and WUS earthquakes share the same average stress drop. In fact, observable differences in the areas enclosed by intensity VI ( A VI) for ENA and WUS earthquakes at the same M point to slight but significant differences in stress drop, the ENA earthquakes having stress drops a factor of about 2 higher. Only for the largest earthquakes, M ≧ 7 ½ as the 1811 to 1812 New Madrid and 1886 Charleston earthquakes seem to be, can strong ground motion propagate to much greater distances as low-loss, higher-mode surface waves in ENA relative to WUS. The strong dependence of high-frequency strong ground motion on stress drop and its weak dependence on M admit the possibility that the 1811 to 1812 events and the 1886 Charleston earthquakes are no larger than M = 6 ½ to 7, provided that their stress drops are higher than average by a factor of 2 or so.