The spectral content of Pamir-Hindu Kush intermediate depth earthquakes: Evidence for a high-Q zone in the upper mantle

Abstract

Very high frequencies (5–10 Hz) are recorded at Garm, Tadzhikistan (Δ∼200 km), near Toktogul, Kirgizia (Δ∼600 km) and at other close stations from intermediate depth earthquakes in the Pamir-Hindu Kush region. The seismic phase, Sn, is recorded with large amplitudes and high frequencies at stations in Pakistan and India from intermediate depth earthquakes. Such high frequencies require either extremely high average values of Q(several thousand) or very high stress drops of earthquakes (kilobars) or possibly both. Regardless of the stress drops, we infer that Q ≳ 1000 along paths through a portion of the mantle where Q is usually low. Thus these data indicate a discontinuity, or marked thinning, of the asthenosphere. Even if Q were infinite, the spectral content of the signals at Garm, in addition, could also be interpreted as evidence for relatively high stress drops for the intermediate depth earthquakes. If Q is equal to 3000, the highest calculated stress drops are for events with depths between about 50 and l80 km and range from several tens to several hundred bars. Tables 1 to 3 are available with entire article on microfiche. Order from American Geophysical Union, 1909 K Street, N.W., Washington, D.C. 20006. Document JTT-OOl; $1.00. Payment must accompany order.