← Back to Gallery

Relocation of the 1975 Oroville ML 5.7 Earthquake Sequence and Insights Into Its Origin

On 1 August 1975, a M_L 5.7 earthquake occurred along the Cleveland Hill fault in the vicinity of Oroville Dam and its reservoir Lake Oroville in northern California. The event occurred soon after refilling of the reservoir from the largest summer drawdown recorded up to that time. Because of the proximity of the 1975 sequence and its timing relative to refilling, it was immediately suggested that the earthquakes were triggered by the reservoir. Soon after the mainshock, seismic stations were deployed by several organizations to record and locate the aftershocks. The aftershocks defined a 15-km long, 60 degree-dipping normal fault with events located to a depth of 10 km. To gain greater insight into the rupture process and geometry of the Cleveland Hill fault and the cause of the earthquakes, a double-difference relocation of the sequence was performed. A velocity model for the area was developed using the program VELEST. About 1000 well-recorded events from July 1975 to June 2018 were relocated using the updated velocity model and the program HypoDD. The relocated sequence shows similar trends to previous locations, but with tighter clustering in map and cross-section views. Seismicity aligns along a plane dipping approximately 55 to 60 degrees west, with seismicity extending to a depth of 14 km. Although the majority of events are located several kilometers south-southwest of the reservoir, the northern extent of the aftershock zone reaches the southern edge of Lake Oroville. The relatively shallow nature of the foreshock and possibly mainshock, assuming they were co-located, might suggest that the sequence was initiated in an area influenced by pore pressure increases due to Lake Oroville. There does not appear to be any clear spatial migration of seismicity after the mainshock. Although there were no earthquakes located near Lake Oroville in the years prior to 1975, the area appears to have been reactivated with events occurring up to present.

Session: Earthquake Source Processes at Various Scales: Theory and Observations [Poster]

Type: Poster

Room: Evergreen Ballroom

Date: 4/21/2022

Presentation Time: 08:00 AM Pacific

Presenting Author: Sarah Smith

Student Presenter: No

Additional Authors