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Measurements of P and S wave fronts from the dense three-dimensional array at Garni, Armenia

U.S. Geological Survey, Pasadena, California 91106
U.S. Geological Survey, Denver, Colorado 80225
U.S. Geological Survey, Menlo Park, California 94025
National Survey of Seismic Protection, Yerevan, Republic of Armenia
Armenian Academy of Sciences, Yerevan, Republic of Armenia

Abstract

The P- and S-wave arrivals from local earthquakes were studied using an array of 10 three-component instruments in and around a tunnel at Garni Observatory, Armenia. The array has a three-dimensional configuration with lateral dimensions of 300 to 500 m and a depth extent of 100 m. Estimates of the horizontal and vertical components of slowness for P and S wave fronts were used to determine the angles of approach and the propagation velocity. The results showed that the region around the array has low average velocities for both the P (1.43 km/sec) and S (0.61 km/sec) waves, so wave fronts approach the array at steep angles of incidence. Waveforms from one event show clear reflections from the free surface for both P and S waves. The timing of these reflections gives the velocity variation with depth within the array. We estimated a P velocity of 0.33 km/sec within a few meters of the surface, increasing to several kilometers per second for the deepest portion of the array. Local site variations can greatly complicate the high-frequency waveforms, even for tunnel stations in bedrock. The S waves exhibit stronger site dependent waveforms and time delays than do the P waves.

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