Fault Slip Rates and Seismic Moment Deficits on Major Faults in Ordos Constrained by GPS Observation.

The Ordos Block, surrounded by numerous active faults, is a relatively rigid but dangerous area with many strong historical earthquakes. We derive the block rotation velocity and fault slip rates in this area by using GPS data recorded from 1999 to 2007 and implementing an elastic block model. Instead of assuming vertical faults, as did most previous studies in and around Ordos, we use an improved method to invert for the fault dip angles and construct a closed 3-D fault system in our inversion. The predicted slip rates range from <1 mm/yr to ~ 10 mm/yr. Our results are roughly consistent with geological and other geodetic observations. Using the estimated slip rates, we also calculate the cumulative seismic moment due to fault locking and the released moment from historical earthquake catalogues. A comparison of the two quantities indicates that the Hetao Rift has an unreleased seismic moment equal to a Mw 7.9 earthquake, which is also indicated by frequent earthquakes above M6 after 1900.