|
|
Figure 1. Interferograms for the three earthquakes considered in this study. Postseismic interferograms are shown for the 1997 Manyi earthquake and the 2001 Kokoxili earthquake. For the Manyi case, ERS-2 SAR data covering an 8-month time interval are used, and for the Kokoxili case, Envisat data covering a 1-year time interval are used. A coseismic interferogram is shown for the 2008 Yutian earthquake, with each fringe representing about 12~cm of line-of-sight displacement.
| Project Summary |
For the last three decades, a rather vigorous debate has been going on
about the most fundamental aspects of the style of continental
deformation in the India-Eurasia collision zone. While one school of
thought envisions Tibet to be a thickened, weak and fluid-like zone
(e.g. Cook and Royden, 2008), others consider the tectonics in
the region as that of essentially rigid microplates bounded by major
lithospheric faults (e.g. Thatcher, 2007). Until this issue is
resolved, it will be difficult to gain a clear idea of what forces
drive the deformation of the upper crust in this region. Large
earthquakes initiate rock mechanics experiments of lithospheric
dimensions in which a sudden stress change leads to stress relaxation
at depth. Surface measurements of the resulting deformation provide a
basis for testing structural and rheological models of the
lithosphere.
Our main objective in this project is the elucidation of lithospheric
rheological structure in northern Tibet, chiefly through InSAR
investigation of postseismic motion following three recent major
earthquakes. These events are the 1997 M 7.6 Manyi earthquake
(Funning et al., 2007; Ryder et al., 2007), the 2001
M 7.9 Kokoxili earthquake (Lasserre et al., 2005) and the
2008 M 7.2 Yutian earthquake. By analysing the spatio-temporal
characteristics of the postseismic signals, and testing the data
against various candidate models, we hope to determine whether
deformation in the mid to lower crust is localized or distributed, and
constrain rheological parameters associated with our preferred
model. Our broader objective is to inform the geophysical debate
concerning the nature of the Tibetan Plateau: specifically, whether it
behaves more like a viscous fluid or a series of rigid blocks.
|
| InSAR Observations |
We use the satellite geodetic technique of Interferometric Synthetic
Aperture Radar (InSAR) to observe postseismic deformation for several
years following each of the three major earthquakes. For the Manyi
case we use data from the European Space Agency's (ESA) ERS-2
satellite, and for the Kokoxili case we use radar scenes from ESA's
Envisat. The Yutian earthquake occurred after the launch of Japan's
L-band ALOS satellite, so we will be able to utilize postseismic data
from this as well as the C-band Envisat. Postseismic interferograms
for both the Manyi and Kokoxili earthquakes reveal several centimeters
of line-of-sight transient displacement in both cases. The wavelength
of the postseismic signal suggests that relaxation processes occur at
a depth of 15-20 km, which is approximately the seismogenic depth in
this area. A deformation time series for the Manyi event can be
constructed from multiple interferograms, and yields a relaxation time
of 0.7~years. It is not possible to make early postseismic
interferograms for the Kokoxili case, and so making a time series is
difficult, but GPS data from a network of 65 sites either side of the
rupture will ultimately give temporal information about the
postseismic transient. We are currently waiting for postseismic data
for the Yutian earthquake; cosesismic interferograms have been
processed, and show clear deformation fringes despite the ice cover in
the area. Figure \ref{ryderfig1} shows interferograms for each of the
three events.
|
| Viscoelastic Models | In order to interpret the observed surface deformation in terms of sub-surface rheological structure, we implement models of postseimic stress relaxation in a viscoelastic medium. The idea here is that stress changes near the fault induced during the earthquake are relaxed due to viscous flow beneath an elastic upper crust. The thickness and viscosity of a Maxwell viscoelastic layer are varied in the models, and we seek parameters which best fit the data. For both the Manyi and Kokoxili cases, the best-fit viscosity is between $5\times10^{18}$ and $1\times10^{19}$~Pa\,s; a single thick viscoelastic layer is preferred, suggesting that the entire mid to lower crust flows, with no stratification in viscous properties. A Maxwell viscoelastic medium is the simplest type of linear viscoelastic rheology, and it is reasonable to run initial models using such a rheology. However, the Manyi time series is not fit consistently throughout the entire 3.5 year observation period by a single Maxwell viscosity; rather, the effective viscosity increases over time. Another linear rheology which could produce this effect is a Burgers body, which consists of a Maxwell and a Kelvin element in series. We are currently running further models, varying the transient and steady-state viscosities as well as the ratio of Maxwell and long term shear moduli. Another candidate relaxation mechanism is localized afterslip on an extension of the coseismic rupture plane at depth. The data will also be tested against models simulating this mechanism. For strike-slip earthquakes such as Manyi and Kokoxili, surface deformation due to distributed viscous flow and localized afterslip can look very similar. The postseismic signature of the normal-faulting Yutian earthquake should be particularly useful for distinguishing between mechanisms, since the surface deformation from the two processes looks very different for dip-slip events. |
| Tools | InSAR processing and analysis, GPS data analysis, forward and inverse elastic and viscoelastic modeling |
| Geographic Location | Northern Tibetan Plateau |
| Group Members Involved |
Isabelle Ryder, Roland Bürgmann |
| Project Duration | In Progress: start of 2008 through 2009 |
|