Afterslip on Normal Faults Following Several Moderate-Sized Earthquakes on the Tibetan Plateau Observed Using InSAR

by Isabelle Ryder, University of Liverpool & Roland Bürgmann, UC Berkeley

The relatively frequent occurrence of moderate-sized seismic events makes them a good class of earthquake for exploring coseismic and postseismic deformation fields, and the associated subsurface fault geometry. This paper investigated normal faulting earthquakes that occurred during the past few years in different parts of the Tibetan Plateau. Measurements of the coseismic and post-seismic deformation fields for these events were made using Interferometric Synthetic Aperture RADAR (InSAR) allowing comparison between events. In particular, the spatial relationship between coseismic and postseismic slip was explored, and the time dependence of postseismic displacements. The earthquakes studied had magnitudes between 6.0 and 7.2. A combination of C-band SAR data from the European Space Agency’s Envisat satellite and L-band data from the Japanese Advanced Land Observing Satellite (ALOS) were used in this study.

Clear surface-deformation maps were obtained for the coseismic phase of seven normal faulting earthquakes. In most cases, the coseismic-fault traces are curved, suggesting curvature of the rupture surface at depth. Detailed modeling of the observed displacement using curved faults should yield information on subsurface structure. The figures (Figure 3 and 4) highlight the January 2008 Nima-Gaize event that was modeled using planar surfaces for the mainshock and largest aftershock. Although the obtained slip model gives a very good fit to the five coseismic interferograms (Figure 3), here is a small, but systematic, residual in the images, which is believed to be a consequence of neglecting the curved geometry of the Earth. For five of the seven earthquakes studied, including the Nima-Gaize earthquake, interferograms reveal continued deformation in the months following the coseismic rupture. In every case, the observed images were from the months of June through October, 74% of the slicks were observed during these months, suggesting morefrequent occurrence during summer and fall.

Near-concurrent observations from visible- and infraredremote sensing help to understand the nature of this environmental structure. In the example shown (Figure 5a), the slick marked the outer margin of a thermal front (Label F). Frontogenesis resulted from the flow of old, recently upwelled waters, into the bay where resident waters were much warmer (Figure 5b). The frontal zone was also evident in ocean color (Figure 5c) and the region inshore of the front hosted a red-tide bloom patch (arrow in Figure 5c). A process study during this period has shown that an intense bloom was retained in the northern bay, inshore of the frontal zone, through a period of energetic mixing between the bay and adjacent waters of the California Current System.

Multiple observations support the conclusion that the slicks represent oceanographic processes. In case studies for which a near-concurrent sea-surface temperature (SST) image was available (n=18), the slick coincided with a thermal front having a positive onshore gradient (e.g., Figure 5). Flow of cold upwelled waters into the bay from the NW and warming of the retentive upwelling shadow of the NE bay create the typical NW/SE orientation of thermal fronts in this region, the orientation common to slicks. The relatively narrow minor-axis dimension of the slicks and their curvature over small spatial scales (e.g., Figures 2d and 2j) are consistent with oceanographic dynamics. In contrast, regional winds varied significantly in direction and intensity among the case studies, indicating that direct forcing by a specific wind pattern is not a likely cause of the slicks. One hypothesis for oceanographic processes, supported by previous case studies, is that the slicks are caused by surfactant accumulation in convergence zones. In addition to studying the causes of this common environmental structure, it is important to understand its consequences; for example, how convergent fronts influence lateral mixing, aggregation of plankton and higher trophic levels (Figure 5b), retention and export of blooms that incubate in the northern bay (Figure 5c), and settlement of marine larvae.

This research is ongoing with further examination of: (1) multidisciplinary in-situ data acquired when slicks were observed, and (2) other physical facets revealed by SAR. For example, Label B in Figure 5 indicates where a southward flow of cold waters bifurcated. Immediately north of this point was a wavelike pattern in the SAR image, which was the second most-frequently observed feature in the regional SAR archive. Is this environmental structure directly caused by oceanographic processes such as internal wave generation at a water mass confluence, or atmospheric processes such as bow-shock waves? Time, and more data, will tell.

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