Description of construction of Bormann/Hammond predicted velocity field for UCERF3


We built a block model from the fault geometries provided by Kaj Johnson for the purposes of the UCERF3 block modeling exercise.  All faults are locked at the surface to a depth of 12 km. We did not adjust the locking depths in our inversion. The pacific plate is a large block that encompases GPS sites distributed around the plate so it imposes an effective rigid body motion on the west side of the model.  

We constrained motion of the blocks by solving for block rotations and fault slip rates in a regularized inversion. We used as input the velocity field provided by Kaj for the purposes of the exercise.  We performed no editing of the velocity field, no outlier removal, no adjustment of uncertainties and no adjustments for the effects of viscoelastic relaxation.

We allowed the blocks to experience small degree of internal deformation. Terms for a uniform horizontal tensor strain rate were estimated as damped parameters for each block.   In most cases the inferred strain rates were less than the uncertainties in the strain rate, but for 6 (out of 13) blocks strain rates of less than 15 nanostrains/yr were inferred.

The velocity field is provided on a 0.1˚ x 0.1˚ grid for the extent of the block model, i.e. from 121.9 to -114.1 longitude, 32 to 37.2 latitude. The file format is longitude, latitude, veast, vnorth (degrees, degrees, mm/yr, mm/yr).