MEASUREMENTS OF P AND S-WAVE FRONTS FROM THE DENSE 3-DIMENSIONAL ARRAY AT GARNI, ARMENIA

The P- and S-wave arrivals from local earthquakes were studied using a n array of 10 three-component instruments in and around a tunnel at Ga rni Observatory, Armenia. The array has a three-dimensional configurat ion 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 t he 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 angle s of incidence. Waveforms from one event show clear reflections from t he free surface for both P and S waves. The timing of these reflection s gives the velocity variation with depth within the array. We estimat ed a P velocity of 0.33 km/sec within a few meters of the surface, inc reasing to several kilometers per second for the deepest portion of th e array. Local site variations can greatly complicate the high-frequen cy waveforms, even for tunnel stations in bedrock. The S waves exhibit stronger site dependent waveforms and time delays than do the P waves .