The stopping phase on seismograms

Abstract

Model studies have demonstrated that the stopping phase, an event which originates at the termination of rupture, may be identified on long-period strain recordings. A tentative identification of the stopping phase has been made on ultra-long-period seismograms of three major earthquakes (Montana, 1959; Chile, 1960; and Alaska, 1964). The events chosen lead to reasonable estimates of the length of rupture associated with the earthquake. A second type of stopping event, the breakout phase, must occur when a rupture intersects a free surface. A two-dimensional model study indicates that the breakout phase should be a prominent seismic event, particularly if the first motion is emergent. A review of studies of seismograms of earthquakes which produced surface faulting indicates that a prominent second event is often observed. However, there does not appear to be an adequate criterion to distinguish the breakout phase from the pP phase. Thus no certain identification can be made.