abstract

An analysis is made of seismic waves recorded at some 40 stations in California (a) from the explosion CHASE V located in the ocean south of the Mendocino escarpment and (b) from a contemporaneous earthquake preceding it located deep in the crust near the west flank of the Sierra Nevada. The epicenter of CHASE V estimated by the routine Berkeley Station method is 20 km from the supplied shot-point; unfortunately, evidence from the traveltime analysis raises doubts on the supplied position and caution is necessary in using this result as a standard for locations of off-shore earthquakes in this region.

Two seismic refraction profiles, CHASE V to Barrett (in southern California) and the Chico earthquake to Barrett, show strikingly different Pn average velocities. The average apparent Pn velocity along the coastal profile is 8.0 km/sec, in agreement with previous studies. The more easterly profile indicates a smaller speed of 7.8 km/sec; a number of lines of evidence point to subcrustal P velocities which decrease from the Coast Ranges toward the Sierra Nevada. The Pn travel-times from CHASE V show significant relative delays at Shasta and Mineral, interpreted as a consequence of a root to the Klamath and Cascade ranges, and at Tinemaha, a consequence of the Sierra root.

The focal depth of the Chico earthquake exceeds 20 km, deeper than coastal California earthquakes; confirmation of the depth comes from a P wave fault-plane solution which indicates dominant strike-slip motion parallel to the tectonic trend (N30°W approximately). The analysis of travel-times from the two sources substantiates the earlier finding of Dehlinger and others that anomalously low Pn velocities in the upper mantle extend from California into Oregon east of the Coast Range.

The frequency spectrum of the seismic signature of the explosion differs markedly from that of the earthquake, although both have been assigned a Richter magnitude of about 4.6. The CHASE V signal was highly energetic near the bubble-pulse frequency (∼ 2 c/s); measured first P motions were uniformly compressional.

Amplitude measurements from CHASE V from short-period vertical component instruments yield an absorption coefficient for Pn of κ = 0.00183 ± 0.0022 per kilometer for propagation across the calibrated telemetry array in the Coast Ranges (350 < Δ < 580 km). The corresponding Q is 320.

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