Apparent Slow Oceanic Transform Earthquakes Due to Source Mechanism Bias

Abstract

Slow earthquakes, characterized as ones whose magnitude increases with period, are often observed occurring on oceanic transform faults (Kanamori and Stewart 1976; Okal and Stewart 1982; Stein and Pelayo 1991; Choy and Boatwright 1995; Newman and Okal 1998; Perez-Campos et al. 2003). Previous work has found that oceanic transform earthquakes often have lower body wave magnitudes compared to ridge and oceanic intraplate earthquakes of comparable surface wave magnitude and seismic moment. This phenomenon might result from the slow events having lower stress drop. Alternatively, the effect could be due to a source mechanism bias resulting from the fact that transform earthquakes are primarily strike-slip events on steeply dipping faults. In this geometry, body waves arriving at teleseismic distances left the source near nodal planes, resulting in smaller amplitudes than for dip-slip events of the same moment. Slow earthquakes are ones that release energy more “slowly” than typical earthquakes and therefore radiate more energy at longer periods. Such earthquakes have been identified in various ways. One is from an anomalous ratio of the body wave magnitude mb , which reflects the energy release at a period of 1 s, to the surface wave magnitude MS , which reflects the energy release at a period of 20 s. Although slow earthquakes have been observed in various tectonic environments, they are especially common for oceanic transform earthquakes that have strike-slip mechanisms, as shown by analysis of a global catalog of oceanic earthquakes (Stein and Pelayo 1991). The transform earthquakes generally had lower body wave magnitudes compared to ridge and oceanic intraplate earthquakes of comparable surface wave magnitude and seismic moment. This effect is illustrated in Figure 1A, where a transform event with a moment larger than the ridge event shown has lower mb than …