Contemporary kinematics of the western United States determined from earthquake moment tensors, very long baseline interferometry, and GPS observations

Abstract

Abstract. Using moment tensors from earthquakes between 1850 and 1995, we determine the horizontal velocity gradient tensor field associated with the seismic deformation in the western U.S. plate boundary zone. The velocity vectors obtained from the integration of the seismic strain rates across the entire plate boundary lie within 5 ø of the NUVEL-1A PacificNorth American plate motion direction. The magnitude of the earthquake-related velocity is 62% of the NUVEL-1A total Pacific-North American plate motion. If earthquake occurrence is a random, Poisson-like process, then the large formal errors associated with the model velocity estimates are an indication that the 144-year interval is too short to define the longterm seismic moment release rate to within an uncertainty less than the deficit between the observed seismic moment release rate and the total long-term moment release rate associated with both the seismic and aseismic deformation. To spatially resolve the deficit of the earthquake moment release in the last 144 years, we have also estimated the total long-term horizontal velocity gradient tensor field using seismic strain rate tensors and recent geodetic velocities, both with and without the NUVEL-1A Pacific-North American rigid plate motion constraint. We find a systematic difference between the Southern California Earthquake Center (SCEC) geodetic velocity map and NUVEL-1A North American reference frames with a pole at (38.8øN, 124.2øW, 0.33 ø m.y.-1). This difference is significant at the 99% confidence level and indicates that 4-7 mm/yr of NE-SW convergence is absorbed in offshore areas of southern California if both the SCEC velocity map and NUVEL-1A correctly describe the relative motion between the Pacific and North America plates. Deficit strain rates calculated from comparison of observed seismic strain rates over the last 144 years with long-term model strain rate tensors are largest in areas east of San Francisco Bay, in the Transverse Ranges, and along the southern San Andreas fault and San Jacinto fault. The deficit in each 100 by 50 km grid area within these regions is equivalent to an accumulation of strain in the last 144 years that would be released by an Mw > 7 earthquake. Elsewhere the accumulated strain in the last 144 years is small, except in northern Mojave Desert, along the Garlock fault, and in the region southwest of the southern San Andreas-San Jacinto fault zone. Within each 100 by 50 km grid area in these regions the accumulated strain is equivalent to that released by an Mw -6.5-7.0 earthquake. Introduction Deformation in the western

Plate boundary deformation between the Pacific and North America in the Explorer region

“…The latter is caused by the fact that recurrence times of earthquakes on individual faults are typically much longer than the time interval for which quantifiable seismological data are available. The contribution in the total error associated with focal mechanism uncertainties is small in comparison with our conservative error estimate of šM 0 m i j for each moment tensor to account for the random process of earthquake occurrence (Shen-Tu et al, 1998) and so it is ignored here. Bi-cubic spline functions have been used to determine a continuous strain rate field for various regions (e.g.…”

“…1 are defined by assigning a conservative standard error of šM 0 m i j for each event plus an incompleteness factor for each area . The treatment of errors accounts for the randomness of earthquakes in space and time as well as for the possible incompleteness of the earthquake catalogue (Shen-Tu et al, 1998). The latter is caused by the fact that recurrence times of earthquakes on individual faults are typically much longer than the time interval for which quantifiable seismological data are available.…”

Approximating the Distribution of Pareto Sums

“…McCaffrey (1997), Shen-Tu et al (1998) and Holt et al (2000) considered the statistical bounds for the sums of uppertruncated Pareto rvs, for the case when several earthquakes approaching the maximum size are observed. In these cases the distribution of the sum can be approximated using the Central Limit Theorem (see more in Section 5).…”

Methods of determining weight scaling factors for geodetic–geophysical joint inversion

“…GPS data and seismic moment tensor data have been used to invert for the crustal motion velocity field and strain rates through bicubic Bessel interpolation (Shen-Tu et al, 1998;Holt et al, 2000). The relation between the horizontal velocity field u(x) and the rotation vector function W(x) can be described as wherex is the unit radial position vector of points on the surface of the Earth and r is the Earth radius (6.371 × 10 9 mm).…”

“…Since then, a rapid and great development has occurred, with the continuum, linear form and single dataset replaced by the discrete, non-linear form and various datasets. Examples include the joint inversion of electronic distance-measuring instrument (EDM), global positioning system (GPS) and very long baseline interferometry (VLBI) (Lisowski et al, 1990), the joint inversion of leveling, GPS and gravity (Zhao and Sjöberg, 1993;Zhao, 1995;Li et al, 2002;Li and Xu, 2005), the joint inversion of geodetic and seismic data, and the joint inversion of geodetic, seismic and geologic data Haines, 1993, 1995;Holt et al, 2000;Williams et al, 1993;Tinnon et al, 1995;Shen-Tu et al, 1998;Shen-Tu and Holt, 1999;Xu et al, 2000Xu et al, , 2003Segall and Matthews, 1997;England and Molnar, 1997;Wu et al, 2001;Kreemer et al, 2000;Wan et al, 2004).…”