Source and aftershock properties of the 1996 Flores Sea Deep Earthquake

Abstract

Abstract. The June 17, 1996 (M w 7.8) Flores Sea event is one of the largest deep earthquakes, and also produced an energetic aftershock sequence, allowing study of the spatial relationships between moment release, aftershock occurrence, and slab geometry. The locations of the mainshock, rupture sub-events, aftershocks, and background slab seismicity are determined using a relative location algorithm. The event occurred at the edge of a 150 km wide aseismic region that represents a tear in the lowermost part of the Indonesian slab. The aftershocks define a 90 km long region parallel to slab strike. Two other aftershocks locate 10-25 km outside the previously active slab.The mainshock propagated about 75 km eastward along an E-W striking, southward dipping plane, with a rupture velocity of 3.5 km/s. There was a substantial change in focal mechanism during the rupture, with the fault plane strike changing by about 25 ø . Overall, the Flores Sea deep earthquake shows numerous large aftershocks, and a moderate rupture velocity (3.5 km/s) and stress drop (-7-20 MPa.) These characteristics are similar to the 1994 Tonga deep earthquake, but different from the 1994 Bolivia event, suggesting they may be typical of large deep earthquakes in colder slabs. Systematic differences in rupture parameters for deep earthquakes in warm and cold slabs suggest that the rupture mechanism is highly sensitive to temperature.

Source characteristics of large deep earthquakes: Constraint on the faulting mechanism at great depths

“…Slow rupture velocities are observed for events in warm subducting slabs, whereas earthquakes in cold subduction zones show fast rupture velocities. This is in conformity with other previous studies [Wiens, 1998b;Tibi et al, 1999;Wiens, 2001]. Such a systematic variation of the rupture velocity is consistent with the observation that ruptures in warm slabs tend to last relatively longer as shown in Figure 22b, and have longer time separations between the rupture initiation and the initiation of the main episode of moment release [Estabrook, 1999].…”

“…Aftershock expansion also has been observed for the 1994 sequence [Wiens and McGuire, 2000], suggesting that this might be a common phenomenon for large Fiji-Tongan deep earthquakes. Event 11 located about 16 km off of the linear trend defined by the sequence (Figure 14, top) likely represents a triggered event as observed for other deep earthquakes [McGuire et al, 1997;Wiens, 1998aWiens, , 1998bTibi et al, 2001]. The 1998 Fiji-Tongan earthquake generated a strong aftershock sequence, and resembles in this respect the 1994 event [Wiens and McGuire, 2000] (Figure 1), the earthquake locations projected into the plan of the cross section define and area that is much larger than the expected width of Wadati-Benioff zone.…”

“…The hypocenter of the earthquakes investigated, in some cases their subevents, and whenever possible the rupture termination points were located using a hypocentroidal decomposition method [ Jordan and Sverdrup , 1981] that minimizes the effect of velocity heterogeneity along the ray paths [ Wiens , 1998b]. The hypocenters were located by inverting teleseismic P , pP , sP and local S arrival times provided by the Preliminary Determination of Epicenter (PDE), whereas picked arrival times were used for inferring the location of the rupture subevents and termination points.…”

Depth dependence of rupture velocity in deep earthquakes

“…Table 1 shows the amplitude ratio of the inferred fault plane to the other. The actual fault planes of 5 earthquakes in the list identified by aftershock distribution [ Wiens et al , 1994; Myers et al , 1995; Wiens , 1998; Tibi et al , 2001; Tibi et al , 2003b] are denoted in Table 1. The selected fault planes coincide with the results of the aftershock analysis.…”

“…Parameters of the Earthquakes InvestigatedMyers et al [1995],Wiens [1998],Tibi et al [2001],and Tibi et al [2003b], respectively.…”