Evidence of Nonlinear Elasticity of the Crust from the Mw7.6 Manyi (Tibet) Earthquake

Abstract

Satellite synthetic aperture radar (SAR) interferometry shows that the magnitude 7.6 Manyi earthquake of 8 November 1997 produced a 170-kilometer-long surface break with up to 7 meters of left-lateral slip, reactivating a N76 degrees E quaternary fault in western Tibet. The radar interferometric map reveals asymmetric, along-strike displacement profiles between the two sides of the surface rupture, a pattern that cannot be explained with linear elastic theory. This observation suggests that the elastic moduli of the crust in tension and in compression are different because of the presence of cracks in the crust at shallow depth. A model indicates that a ratio of 2 between compressive and tensile elastic moduli can account for the observed asymmetry, a ratio that is consistent with laboratory and borehole measurements.

Broadband source modeling of the November 8, 1997, Tibet (Mw= 7.5) earthquake and its tectonic implications

“…Using the regional mainshock location as the calibration event, the relocations lie on the 180 km trace of the surface rupture that was defined using InSAR [Peltzer et al, 1999] The body wave inversion indicates that most of the moment release occurred early in the rupture near the hypocenter over a relatively small region of the fault. Most of the moment was released above 15 km, and there is little indication of alongstrike directivity in the body waves.…”

“…The largest slip was focused near the epicenter, but the overall rupture probably included additional slip to the west-southwest of the epicenter. The primary slip seems to have been concentrated in a region little more than 50 km wide, but if we include the secondary slip, the total rupture length is of the order of 120-140 km, which is smaller the defined surface rupture [Peltzer et al, 1999]. This increases the likelihood that the subevent is not an early aftershock.…”

“…We used a Joint Epicentral Determination (JHD) [Dewey, 1971[Dewey, , 1983 We show the aftershock relocations in Figure 2, and it is clear that the events align in a northeast-southwest direction, slightly oblique to the east-west fault plane in focal mechanism. This matches the surface rupture defined using interferometric synthetic aperture radar (InSAR) [Peltzer et al, 1999] and occurred on a previously mapped fault [Tapponnier and Molnar, 1977]. However, the locations are biased south of the mainshock since we used the USGS location for the mainshock, which itself is biased 20 km south of the surface trace of the rupture.…”

Crustal deformation along the Altyn Tagh fault system, western China, from GPS

“…The seismicity maps of Ma [1989] and SSBC [1992] showed that numerous M--5-6 earthquakes occurred in the Qaidam basin from mid-1960s to early 1990s, mostly within the Qimen Tagh thrust system and in the region south of Qinghai Lake. The largest earthquake in the region was an M: 7.6 left slip event (the Manyi earthquake) in 1997 along a nearly east-west trending fault at the eastern termination region of the Kunlun fault [Peltzer et al, 1999] (Figure 2). …”

Stress transfer and its implication for earthquake hazard on the Kunlun Fault, Tibet

“…earthquake produced a 170-km-long surface-rupture zone along the westernmost strand of 137 the KF (~86°E to 88°E) with a maximum slip of 7 m (Peltzer et al, 1999; Van The earthquake sequence of these five events provides an excellent chance to decipher 147 how ΔCFS on the KF evolved over the past several decades and how earthquakes 148 communicated with each other by stress transfer. On the other hand, it also provides a good 149 opportunity to outline the segments which have experienced large build-up of stress.…”