Kinematics of the southwestern U.S. deformation zone inferred from GPS motion data

Abstract

We have estimated the surface deformation field of the southwestern U.S. deformation zone in terms of the velocity gradient field and surface creep simultaneously by inversion of 497 geodetic velocities. The model shows aseismic fault motion consistent with aseismic creep measurements and a sense of motion consistent with geological observations. We deduce that our surface deformation field shows distributed deformation in a zone around the faults containing shear strains and rotations. The eastern California shear zone acts as a distinct fault zone bounded by more rigid blocks. The faults within the zone partition the shear motion, while right‐lateral shear strain rates and clockwise rotations are concentrated between the bounding faults. In the same way, the San Jacinto and southern San Andreas faults act as bounding faults of a fault zone. The Mojave Desert is dominated by right‐lateral shear, whereas the western Transverse Ranges (WTR) is subjected to contraction. We find significant localization of deformation east of the San Andreas fault between the Transverse Ranges and the San Francisco Bay. We attribute this to a relative rigidity contrast across the fault of ∼1.8. Finally, a moment deficit analysis shows an accumulation of moment deficit between 1973 and 2000 corresponding to a Mw = 6.1–6.3 earthquake along the San Andreas fault just north of the Big Bend, around the Imperial and southern San Andreas faults, and in the San Francisco Bay area along the Hayward and southern Calaveras faults.

Response of deformation patterns to reorganization of the southern San Andreas fault system since ca. 1.5 Ma

“…Andreas faults consistent with the results of Bos and Spakman (2005) Bos and Spakman (2005) of ~8-12 degrees/Ma of CW rotation. Differences in verticalaxis rotation rates could arise from the difference in fault connectivity between our models and the block models, as well as from the modeling approach.…”

“…In addition to comparing the time-averaged rotations to the paleomagnetic data, we can compare the patterns of vertical-axis rotation from the present-day model to regional models that calculate vertical-axis rotations from inversion of GPS data (McCaffrey, 2005;Bos and Spakman, 2005). The inversions with simplified geometry for fault networks cannot resolve the level of spatial variability of rotation rates in our models that results from fault interaction.…”

“…Strain energy density patterns serve to identify regions prone to incipient faulting, and provide a spatially distributed measure of the relative mechanical efficiency of fault system. The spatial and temporal variations in vertical-axis rotation obtained from this model can be compared to rates measured from paleomagnetic results in several regions within our study area, and can also be contrasted with spatial variations in rotation or rotation rates estimated by other models (McCaffrey, 2005;Bos and Spakman, 2005) that use more simple fault geometry and invert for block deformation using modern GPS displacement fields. By exploring how changes to boundary loading and fault geometry affect the distribution and style of deformation, we can assess whether or not our current understanding of the evolution of the southern San Andreas fault system is consistent with the geologic data available.…”

Stateline fault system: A new component of the Miocene-Quaternary Eastern California shear zone

“…1B and 1C). Because no equivalent active structures are described in the region immediately northeast of the SFS, and because geodetic motions throughout this region are stable at the limit of detection (~0.2 mm/yr level; Bennett et al 2003), we interpret the SFS as the northeasternmost component of the Eastern California shear zone between latitudes 35° N and 37° N. It has received little attention by workers focusing on understanding the long-and short-term strain distribution in the shear zone (Bos, 2005;Dixon et al, 2003;Dixon et al, 1995;Dokka and Macaluso, 2001;Oldow, 2003), in part because, unlike other major dextral faults across this region, most of its trace is developed in poorly consolidated, late Quaternary lacustrine deposits and is therefore obscured by late Holocene erosion. We partition its trace into three primary segments separated by contractional (left) step-overs, which, from south to north, include the Mesquite, Pahrump, and Amargosa segments (Fig.…”

First estimates of the second‐order ionospheric effect on radio occultation observations

“…The achieved accuracy in geodetic positioning, after removing the ionospheric effect from the GPS observables, is at the centimeter level. However, recent GPS applications in monitoring plate tectonic motion, crustal deformation and atmospheric sounding require millimeter‐level accuracy in receiver and satellite positions [ Bos , 2005]. Hence, the accurate assessment of the potential impact of the second‐order ionospheric effect on GPS applications has gained the interest of the scientific community.…”