The M w 7.8 Kokoxili earthquake of 14 November 2001, which ruptured a 450-km-long stretch of the sinistral Kunlun strike-slip fault, at the northeastern edge of the Tibet plateau, China, ranks as the largest strike-slip event ever recorded instrumentally in Asia. Newly available high-resolution satellite HRS images (pixel size Յ1 m) acquired in the months following the earthquake proved a powerful tool to complement field investigations and to produce the most accurate map to date of the coseismic displacements along the central Kusai Hu segment of the rupture. The coseismic rupture geometry south and west of Buka Daban Feng, near the earthquake epicenter, was also investigated in detail. Along the Kusai Hu segment, slip partitioning is for the first time observed to occur simultaneously during a single event, with two parallel strands, ϳ2 km apart, localizing almost pure strike-slip and normal faulting. In all, 83 new HRS coseismic offset measurements, some of which calibrated by field work, show large, well-constrained variations (Ն100%) of the slip function over distances of only ϳ25 km. Tension cracks opening ahead of the shear dislocation and later offset by the upward propagating strike-slip rupture were observed, demonstrating that the rupture front propagated faster at depth than near the surface. The triple junction between the central Kusai Hu segment, the Kunlun Pass fault, where the rupture ended, and the Xidatan-Dongdatan segment, which could be the next segment to fail along the main Kunlun fault, acted as a strong barrier, implying that direct triggering of earthquake rupture on the Xidatan-Dongdatan segment by Kokoxili-type earthquakes may not be the rule.
We have conducted a paleoseismic investigation of serial fault rupture at one site along the 110-km rupture of the North Anatolian fault that produced the M w 7.4 earthquake of 17 August 1999. The benefit of using a recent rupture to compare serial ruptures lies in the fact that the location, magnitude, and slip vector of the most recent event are all very well documented. We wished to determine whether or not the previous few ruptures of the fault were similar to the recent one. We chose a site at a step-over between two major strike-slip traces, where the principal fault is a normal fault. Our two excavations across the 1999 rupture reveal fluvial sands and gravels with two colluvial wedges related to previous earthquakes. Each wedge is about 0.8 m thick. Considering the processes of collapse and subsequent diffusion that are responsible for the formation of a colluvial wedge, we suggest that the two paleoscarps were similar in height to the 1999 scarp. This similarity supports the concept of characteristic slip, at least for this location along the fault. Accelerator mass spectrometry (AMS) radiocarbon dates of 16 charcoal samples are consistent with the interpretation that these two paleoscarps formed during large historical events in 1509 and 1719. If this is correct, the most recent three ruptures at the site have occurred at 210-and 280-year intervals.
Motivation
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