Publications

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2018
Xu, XH, Sandwell DT, Bassett D.  2018.  A spectral expansion approach for geodetic slip inversion: implications for the downdip rupture limits of oceanic and continental megathrust earthquakes. Geophysical Journal International. 212:400-411.   10.1093/gji/ggx408   AbstractWebsite

We have developed a data-driven spectral expansion inversion method to place bounds on the downdip rupture depth of large megathrust earthquakes having good InSAR and GPS coverage. This inverse theory approach is used to establish the set of models that are consistent with the observations. In addition, the inverse theory method demonstrates that the spatial resolution of the slip models depends on two factors, the spatial coverage and accuracy of the surface deformation measurements, and the slip depth. Application of this method to the 2010 M-w 8.8 Maule Earthquake shows a slip maximum at 19 km depth tapering to zero at similar to 40 km depth. In contrast, the continent-continent megathrust earthquakes of the Himalayas, for example 2015 M-w 7.8 Gorkha Earthquake, shows a slip maximum at 9 km depth tapering to zero at similar to 18 km depth. The main question is why is the maximum slip depth of the continental megathrust earthquake only 50 per cent of that observed in oceanic megathrust earthquakes. To understand this difference, we have developed a simple 1-D heat conduction model that includes the effects of uplift and surface erosion. The relatively low erosion rates above the ocean megathrust results in a geotherm where the 450-600 degrees C transition is centred at similar to 40 km depth. In contrast, the relatively high average erosion rates in the Himalayas of similar to 1 mm yr-1 results in a geotherm where the 450-600 degrees C transition is centred at similar to 20 km. Based on these new observations and models, we suggest that the effect of erosion rate on temperature explains the difference in the maximum depth of the seismogenic zone between Chile and the Himalayas.

2015
Tong, XP, Sandwell DT, Smith-Konter B.  2015.  An integral method to estimate the moment accumulation rate on the Creeping Section of the San Andreas Fault. Geophysical Journal International. 203:48-62.   10.1093/gji/ggv269   AbstractWebsite

Moment accumulation rate (also referred to as moment deficit rate) is a fundamental quantity for evaluating seismic hazard. The conventional approach for evaluating moment accumulation rate of creeping faults is to invert for the slip distribution from geodetic measurements, although even with perfect data these slip-rate inversions are non-unique. In this study, we show that the slip-rate versus depth inversion is not needed because moment accumulation rate can be estimated directly from surface geodetic data. We propose an integral approach that uses dense geodetic observations from Interferometric Synthetic Aperture Radar (InSAR) and the Global Positioning System (GPS) to constrain the moment accumulation rate. The moment accumulation rate is related to the integral of the product of the along-strike velocity and the distance from the fault. We demonstrate our methods by studying the Creeping Section of the San Andreas fault observed by GPS and radar interferometry onboard the ERS and ALOS satellites. Along-strike variation of the moment accumulation rate is derived in order to investigate the degree of partial locking of the Creeping Section. The central Creeping Segment has a moment accumulation rate of 0.25-3.1 x 10(15) Nm yr(-1) km(-1). The upper and lower bounds of the moment accumulation rates are derived based on the statistics of the noise. Our best-fitting model indicates that the central portion of the Creeping Section is accumulating seismic moment at rates that are about 5 per cent to 23 per cent of the fully locked Carrizo segment that will eventually be released seismically. A cumulative moment budget calculation with the historical earthquake catalogue (M > 5.5) since 1857 shows that the net moment deficit at present is equivalent to a M-w 6.3 earthquake.