Plates and Blocks

Plate boundary locations are critical for characterizing GPS velocities and the plate kinematics of a particular region. While some plate boundaries in the Indian region are well defined by active fault traces, youthful geomorphology, and abundant local seismicity, others appear more diffuse and ambiguous. We draw on the distribution and kinematics of 20th century seismicity, local geology, and mapped faults, and the GPS velocity itself to define our block model boundaries. Within this paper the term plate (and microplate) refers to the rigid, coherent, lithospheric entity defined by faults, seismicity etc. The term block is the specific implementation of these data into a parameterized set of variables within our block model (e.g. Apel et al., 2006a).

We define our blocks as rigid entities on a spherical earth bounded by dislocations in an elastic halfspace and invert for poles and rates of rotation that minimize the misfit to the GPS velocities using the block modeling code by Meade and Hager (2005). Because our inversion combines rigid block rotation with elastic strain accumulation effects, the parameterization of the block boundary geometry is critical. Geometry of the block boundaries is based heavily on seismicity and adopted from plate reconstructions (Replumaz and Tapponnier, 2003) and prior analyses (Socquet et al., 2006a; Reilinger et al., 2006; Simons et al., 2007; Meade, 2007; Thatcher, 2007) or adjusted as indicated by the geodetic data.