Focal depths and fault plane solutions of earthquakes under the Tibetan plateau

We compare synthetic and recorded P wave forms to place constraints on the focal depths and fault plane solutions of 16 crustal earthquakes beneath the highest parts (>4000 m) of the Tibetan plateau. Fault plane solutions for all 16 events show combinations of normal and strike slip faulting with T axes oriented approximately east–west. None of these solutions show thrust faulting. Thus the data corroborate previous inferences that the active tectonics are dominated by east–west extension. Focal depths for all 16 events are less than 15 km and appear to be between 5 and 10 km. This style of deformation and these depths of faulting are similar to those in the Basin and Range province of the western United States. Two intermediate depth events below the crust of southern Tibet also show primarily normal faulting with east–west T axes. The solution for one, discussed by Chen et al. (1981), is unambiguous. The solution for the other, the event of August 1, 1973 (27.59°N, 89.17°E, 85±10 km, mb = 4.9) is less certain. Both apparently occurred in the mantle beneath a thick, aseismic lower crust, and their occurrence suggests that brittle deformation occurs there in response to a stress field similar to that operating at shallow depths beneath Tibet.

[1]  B. Burchfiel,et al.  Seismicity, fault plane solutions, depth of faulting, and active tectonics of the Andes of Peru, Ecuador, and southern Colombia , 1983 .

[2]  P. Molnar,et al.  Focal depths of intracontinental and intraplate earthquakes and their implications for the thermal and mechanical properties of the lithosphere , 1983 .

[3]  B. Burchfiel,et al.  Magnitude of crustal extension in the southern Great Basin , 1982 .

[4]  B. Romanowicz.  Constraints on the structure of the Tibet Plateau from pure path phase velocities of Love and Rayleigh waves , 1982 .

[5]  R. Armijo,et al.  Field evidence for active normal faulting in Tibet , 1981, Nature.

[6]  P. Molnar,et al.  Parallel thrust and normal faulting in Peru and constraints on the state of stress , 1981 .

[7]  J. Cipar.  Broadband time domain modeling of earthquakes from Friuli, Italy , 1981 .

[8]  P. Molnar,et al.  Constraints on the seismic wave velocity structure beneath the Tibetan Plateau and their tectonic implications , 1981 .

[9]  B. Romanowicz.  Depth resolution of earthquakes in central Asia by moment tensor inversion of long‐period Rayleigh waves: Effects of phase velocity variations across Eurasia and their calibration , 1981 .

[10]  Peter Molnar,et al.  An intermediate depth earthquake beneath Tibet: Source characteristics of the event of September 14, 1976 , 1981 .

[11]  James Jackson,et al.  Basement faulting and the focal depths of the larger earthquakes in the Zagros mountains (Iran) , 1981 .

[12]  P. Molnar,et al.  Active tectonics of Tibet , 1978 .

[13]  J. Rial.  The Caracas, Venezuela, earthquake of July 1967: A multiple‐source event , 1978 .

[14]  J. Ni,et al.  Late Cenozoic tectonics of the Tibetan Plateau , 1978 .

[15]  Robert B. Smith.  6: Seismicity, crustal structure, and intraplate tectonics of the interior of the western Cordillera , 1978 .

[16]  Robert B. Smith,et al.  5: Fault-plane solutions of the Western United States: A compilation , 1978 .

[17]  P. Molnar,et al.  Seismic moments of major earthquakes and the average rate of slip in central Asia , 1977 .

[18]  Peter Molnar,et al.  Active faulting and tectonics in China , 1977 .

[19]  Michel Bouchon,et al.  Teleseismic body wave radiation from a seismic source in a layered medium , 1976 .

[20]  Hiroo Kanamori,et al.  Mode of the strain release along the Gibbs fracture zone, Mid-Atlantic ridge , 1976 .

[21]  P. Molnar,et al.  Cenozoic Tectonics of Asia: Effects of a Continental Collision: Features of recent continental tectonics in Asia can be interpreted as results of the India-Eurasia collision. , 1975, Science.

[22]  T. Fitch.  Earthquake mechanisms in the Himalayan, Burmese, and Andaman Regions and continental tectonics in central Asia , 1970 .

[23]  D. Helmberger.  The crust-mantle transition in the Bering Sea , 1968 .

[24]  E. W. Carpenter.  TELESEISMIC SIGNALS CALCULATED FOR UNDERGROUND, UNDERWATER, AND ATMOSPHERIC EXPLOSIONS , 1967 .

[25]  Walter I. Futterman,et al.  Dispersive body waves , 1962 .