About 25 per cent of the microearthquakes which originate close to Socorro (S minus P ≦ 2.5 secs) produce seismograms with two sharp arrivals about 2.5 and 5.0 seconds after the direct S-phase arrival. The best interpretation of the time-distance data for these late phases is that they are the SxP and the SxS reflections from a crustal discontinuity at a depth of 18 km. The amplitudes of the reflections, which average 0.21 and 0.32 of the direct S-phase amplitude on vertical-component seismograms, are for greater than theoretical amplitudes found by assuming uniform radiation from the focus and reflection of a plane SV wave from a horizontal discontinuity.

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