Measurement of Differential Rupture Durations as Constraints on Source Finiteness of Deep-Focus Tonga Earthquakes

One of the basic issues regarding the physical mechanism for deep earthquakes is the extent to which they occur on pre-existing faults: Do deep earthquakes reactivate faults that formed in the oceanic plate prior to subduction? A previous study of the Tonga subduction zone (Jiao et al., 2000) compared nodal plane distributions of outer-rise events to events as deep as 450 km depth, and found them to be very similar. While this comparison provides some support for the pre-existing fault hypothesis, a more stringent test of the hypothesis is available by identifying the actual rupture planes of the deep-focus events. We are developing the methodology to perform such a test. Between 1990 and 2002, the IRIS FARM database contains approximately 200~earthquakes with m_b ≥~5.5 in the region studied by Jiao et al. The Harvard CMT focal mechanism identifies two possible fault planes, and our goal is to distinguish the true fault plane using observations of source finiteness. Source finiteness is observable on seismograms at different azimuths and distances, for unilateral ruptures, as variations in the apparent rupture duration and, for complex ruptures, as differences in the travel-time delay between subevents. For each earthquake, the rupture duration (or travel-time delay) will be shortest in the direction of rupture propagation and longest in the opposite direction. Rather than measuring the actual rupture duration at each station, we use a cross-correlation technique that includes a stretching factor to measure the differential rupture duration between each pair of stations. These differential measurements then allow us to identify the rupture direction and fault plane for each earthquake. A comparison of the distribution of rupture-plane orientations of deep-focus and outer-rise seismicity allows us to determine whether or not deep seismicity occurs on reactivated faults.