Accession Number:

ADA278529

Title:

Mapping Crust and Upper Mantle Structure Beneath Southern Eurasia

Descriptive Note:

Annual rept. 1 Sep 1992-31 Aug 1993

Corporate Author:

CALIFORNIA INST OF TECH PASADENA

Personal Author(s):

• Helmberger, Donald V.

Report Date:

1993-08-31

Pagination or Media Count:

46.0

Abstract:

In this study of lateral variations in compressional velocity in the uppermost mantle underneath the Tibetan plateau, 353 Pn travel times were collected for 44 Tibetan earthquakes at 46 seismic stations. The inverse method and procedures in this study differ from previous Pn tomography studies in that corrections were applied to the biases caused by 1 event mislocation by ISC, 2 mantle velocity gradient, and 3 large-scale variations in crustal thicknesses. Main results to date are 1 the average P velocity value for the uppermost mantle in Tibet is 7.93-0.17 kms 2 the average P velocity gradient in the upper 150 km of the mantle is 3.1 x 10exp-31s 3 the 2D P velocity image of the region includes a low velocity zone in the north central Tibet, and two high velocity zones in the western and eastern flanks of Tibet and 4 in much of the area inside Tibet, the crustal thickness exceeds 70 km. Another important finding of this study is that event relocation see the bias 1 plays a very important role in reliably retrieving the detailed lateral variations in 2D P velocity image. Corrections to biases 2 and 3, seem to have a greater effect on the average of the velocity image, causing over-estimations.

Descriptors:

• *EARTH CRUST
• *SEISMIC WAVES
• *EARTHQUAKES
• *SEISMIC VELOCITY
• *EARTH MANTLE
• VELOCITY
• THICKNESS
• TRAVEL TIME
• TECTONICS
• PLATEAUS
• SEISMOLOGICAL STATIONS
• PRIMARY WAVES(SEISMIC WAVES)
• GRADIENTS
• EURASIA
• TOMOGRAPHY
• HIGH VELOCITY
• LOW VELOCITY
• IMAGES

Subject Categories:

• Geology, Geochemistry and Mineralogy
• Seismology

Distribution Statement:

APPROVED FOR PUBLIC RELEASE