Bingming Shen-Tu

Velocity field in Asia inferred from Quaternary fault slip rates and Global Positioning System observations

We perform a joint inversion of Quaternary strain rates and 238 Global Positioning System (GPS) velocities in Asia for a self-consistent velocity field. The reference frames for all geodetic velocity observations are determined in our inversion procedure. India (IN) moves relative to Eurasia (EU) about a pole of rotation at (29.78°N, 7.51°E, 0.353° Myr−1), which yields a velocity along the Himalaya within India that is ∼73–76% of the magnitude of the IN-EU NUVEL-1A velocity and a vector azimuth that is 8–10° clockwise of NUVEL-1A IN-EU vector azimuth. Relative to Eurasia, south China moves at 9–11 mm/yr in the direction 110–120° with a pole position (64.84°N, 156.74°E, 0.12° Myr−1). Amurian block motion has a pole position in a similar location but at a slower rate (64.61°N, 158.23°E, 0.077° Myr−1) and most of the Amurian-Eurasia motion is accommodated by extension across Lake Baikal. Tarim Basin moves relative to Eurasia about a pole of rotation at (39.24°N, 98.2°E, −0.539° Myr−1) and ∼16–18 mm/yr of shortening is accommodated across the west central Tien Shan. There is distributed E-W extension throughout both southern and north central Tibet. Within southern Tibet, between the longitudes of 77°E to 92°E, the deformation field accommodates ∼16–19 mm/yr of E-W extension. We compare predicted seismic moment rates with those observed in this century in Asia. Total observed seismic moment rates within the entire area of central and east Asia (2.2×107 km2) in this century are 2.26±0.7×1020 N m yr−1 as compared with a predicted total rate of 2.03±0.066×1020 N m yr−1. Comparisons between observed and predicted moment rates within 42 subregions reflect the generally unstable process of inferring long-term seismic moment rates from a catalog of limited duration (94 years). An observation period of ∼10,000 years would be required to reduce uncertainties in observed seismic moment rate to the same size as the uncertainties in model tectonic moment rates, inferred from the joint inversion of GPS and Quaternary rates of strain. We show that in general, a better correlation with model tectonic moment rate is inferred from the seismicity catalog by considering the numbers of earthquakes above a cutoff magnitude (mb ≥ 5.0, for the period January 1, 1965, to January 1, 1999).

Deformation kinematics in the western United States determined from Quaternary fault slip rates and recent geodetic data

We estimate the horizontal velocity gradient tensor field from Quaternary fault slip rates, and recent Global Positioning System (GPS) and very long baseline interferometry (VLBI) velocity solutions in the western United States transform plate boundary zone. The total velocity obtained from the Quaternary fault slip rate data across the entire plate boundary is within 1 mm/yr of the NUVEL-1A predicted Pacific (PA)-North American (NA) plate motion velocity, but directions are 5°–6° anticlockwise of directions given by NUVEL-1A. The total velocity obtained from inversion of recent geodetic data is 2°–3° anticlockwise from the NUVEL-1A NA-PA velocity, but the difference between the two is not significant at the 95% confidence level. The discrepancy between the total PA-NA motion obtained from the geological data and NUVEL-1A indicates that a marginally significant amount of NE-SW shortening (possibly as much as 5 mm/yr) is missing overall in the geologic data. Shortening may occur in the long-term in the offshore and coastal areas of California where such shortening is required in the shorter-term geodetic solution. The seismic moment released in the last 148 years is ∼59% of the total moment release rate expected from long-term strain rate field (including both seismic and aseismic deformation) derived from the inversion of geological data with NUVEL-1A far-field PA-NA motion constraints. The accumulated strain in the areas containing the southern San Andreas fault-San Jacinto fault, the San Francisco Bay area, and the area containing the Ventura basin in the Western Transverse Ranges in the last 148 years is the equivalent to that which could be released by an Mw>7.0 earthquake in each 50 × 100 × 15 (km3) crustal volume if strain is to be released seismically in these areas.

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

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° of the NUVEL-IA Pacific-North 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 long-term 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 (38.8°N, 124.2°W, 0.33° 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.

Kinematics of active deformation in the Sulaiman Lobe and Range, Pakistan

The western margin of the Indian plate is highly oblique to the direction of convergence between India and Asia and represents an excellent example of large-scale oblique continent-continent collision. Determining the strain field in western Pakistan and how it relates to the plate motion and plate margin geometry affords an exceptional opportunity for understanding oblique margin processes in general. Through the inversion of regional and teleseismic body waves, we have determined the source parameters of 10 moderate-sized earthquakes that occurred between 1964 and 1985 in and around the Sulaiman Range, Pakistan. The earthquakes are dominantly thrust events with slip vectors that are approximately perpendicular to the lobate Sulaiman mountain front. Slip vector orientations rotate 60°–70° from a N-S to a WNW-ESE direction of compression, consistent with the geometries of the complex, festoon-shaped mountain belts of this margin. We have estimated the spatial variation of the horizontal strain rate and velocity fields within Sulaiman using vertically averaged models that accommodate plate motion constraints within a deforming layer. The most important factors determining the style of strain rotation in the Sulaiman Lobe and Range are the presence of pure strike-slip motion along the Chaman Fault, and the relatively rigid and undeformed Katawaz Basin that is therefore allowed to translate obliquely relative to India. This same conclusion is obtained using either a three-dimensional, frictional, analogue model with significant basal tractions or a thin sheet viscous numerical model without basal tractions. Thrusting in a predominantly NW-SE direction in the Sulaiman Range accommodates 5–14 mm/yr of N-S motion between India-Eurasia and 3–6 mm/yr of E-W shortening. Seismic moment release this century within the India-Eurasia plate boundary zone, west of the western Himalayan Syntaxis, constitutes roughly 40% of the expected total seismic moment release for this time period. Particularly significant moment rate deficits exist within the Sulaiman Range and along the Chaman Fault.

Intraplate deformation in the Japanese Islands: A kinematic study of intraplate deformation at a convergent plate margin

Relative motions within the deforming Japanese Islands with respect to the Sea of Japan are determined using earthquake records over the last 414 years, slip rates on Quaternary faults, and angular change rates obtained from triangulation in the last century. We use a least squares inversion method to match the strain rates on the surface of the Earth with continuous spline functions to recover the velocity gradient tensors. The directions of the principal strain axes obtained from seismic, geological, and geodetic data are in general agreement with each other, with the maximum shortening axis oriented in a WNW direction. Principal strain rate magnitudes obtained from the geodetic data are about 3–5 times larger than the principal strain rates obtained from the seismic and geological data. Intraplate deformation in southwestern Japan determined from the seismic data accommodates a velocity of5.5±2(l σ) mm/yr in a direction parallel to the Nankai trough, which is about 25% of the plate motion velocity component parallel to the Nankai trough between the Philippine Sea and Eurasian plates. In northern Honshu, the velocity vectors from the seismic data are nearly parallel to the plate motion vector at the Japan trench, and the intraplate deformation accommodates about 6% of the total Pacific-Eurasian or North American plate motion velocity. The velocity vector at Kashima, northern Honshu, calculated from the geodetic data is about 9–12 mm/yr in a direction of N55°W-N76°W, which is about half of the very long baseline interferometry velocity calculated in the Eurasian reference frame, suggesting that the Sea of Japan may not be rigidly attached to the assumed rigid Eurasian plate.

Interseismic horizontal deformation in northern Honshu and its relationship with the subduction of the Pacific Plate in the Japan Trench

We investigate the origin of the geodetically observed interseismic horizontal deformation in northern Honshu by comparing shear strain rates, principal strain rates, and velocity fields determined from geodetic data with those calculated from the elastic dislocation models involving interplate motion at the Japan trench. The agreement between the observed and predicted directions of the principal strain axes indicates that the geodetic strain field in northern Honshu is primarily elastic strain transmitted from the Japan trench. In order to match the strain rate tensors and velocity magnitudes obtained from the geodetic data, the dislocation model requires that 35% to 60% of the NUVEL1-A Pacific-North American plate motion is locked at the plate interface along the Japan trench. The down-dip depth limit of the locked zone is inferred to be 55 km, which is consistent with the seismic data in the Japan trench.