ANOMALOUSLY DEEP CRUSTAL EARTHQUAKES IN THE VENTURA BASIN, SOUTHERN CALIFORNIA

Earthquakes at depths of 20-30 km occur beneath the Ventura Basin in southern California. The epicentral distribution of deep seismicity outlines an east trending ellipse which corresponds closely with the mapped Santa Clara Syncline, the main structural element of the Ventura basin with 15 km thickness of sediments. Clear Pn and Pg phases are seen for even the deepest earthquakes which demonstrate that these earthquakes are occurring within the crust. Travel time curves from shallow and deep earthquakes in the western Transverse Ranges require that the depth to the Moho under the Ventura basin must be depressed by 7-10 km relative to the surrounding area, indicating crustal thickening of 20-35%. Other researchers have shown that the Ventura basin has the lowest heat flow in western California. Thus, low heat flow, exceptionally thick (15 km) sediments, rapid shortening of the crust, the deep earthquakes, and a depressed Moho all coincide within a small area, strongly suggesting a common cause for all five phenomena. The confinement of the deep earthquakes and depressed Moho to the Ventura region suggests that the rapid shortening of the Ventura basin is anomalous and not representative of the rate of shortening across all of the western Transverse Ranges.