Contribution of radar interferometry to a two-step inversion of the kinematic process of the 1992 Landers earthquake

We study the rupture process of the 1992 Landers earthquake. To limit the tradeoff between slip amplitude and rupture time that affects solutions using only seismological data, we adopt a two-step approach. We first constrain the slip distribution and its uncertainty by independent geodetic data to recover in the second step the temporal details of the rupture propagation. The first step consists of an inversion of interferometric data and Global Positioning System measurements, both independently and together, to constrain slip distribution on a three-segment fault model along both strike and dip direction. We use a genetic algorithm to test the uniqueness of the solution and a least squares formulation to find the model which best fits the data. We conclude from the results of these geodetic inversions that interferometric data are rich enough to access the slip distribution in the case of the Landers earthquake. Since the surface deformations are more sensitive to shallow slip in our configuration, the slip amplitude is better resolved near the surface than at depth. The resulting slip distribution is in agreement with geological observations at the surface and confirms the heterogeneous nature of the Landers earthquake. Most of the slip occurs at shallow depths, on the Homestead Valley fault (second segment), with a maximum value of around 7 m. Another high slip zone is observed on the Johnson Valley fault (first segment) at 8 km depth. In the second step, we invert strong motion data with the a priori final slip amplitude and its uncertainty deduced from geodetic data to constrain the time history of the rupture process. This second step emphasizes a strong variation of the temporal development of the earthquake. Fast rupture front velocities appear within high slip zones, and the rupture slows when it encounters a resistance along the fault. On average, the rupture front propagates with velocities close to the S wave velocity and terminates about 20 s after initiation. The large variations in both slip amplitude and rupture velocity suggest that the rupture process is better described by successively breaking asperities than by a pulse propagating with constant velocity.