Migration of Earth Surface Deformation as a Large Earthquake Trigger

The evolution of the horizontal deformations of the Earths surface in the San Francisco region on a ten-year interval with a daily time resolution is investigated. The deformations of the total shear obtained from the data of permanent GPS observations are analyzed. The initial data are the time series of coordinate changes published by the Nevada Geodetic Laboratory of the University of Nevada, Reno. The evolution of the Earths surface deformations is tracked using kinematic visualizations that demonstrate the change of the deformation field over time. The created animations make it possible to monitor the qualitative nature of the deformation in connection with the seismic process evolvement and fault tectonics. The region under study in recent years has been the subject of special attention due to possible occurrence of a strong earthquake. The visualization of the deformation process demonstrated the zones of activation and the propagation path of the deformation fronts. It is shown that the propagation of shear deformation, initiated from the epicenters of duplicate moderate earthquakes, migrates to the Napa Fault and provokes a major earthquake. This phenomenon is regarded by the authors as a trigger for a seismic event. It is concluded that the Hayward Fault Zone is currently less seismically active as the most mobile one.