Earthquake and bay: Response of Monterey Bay to the Loma Prieta Earthquake

Abstract

The magnitude‐7.1 Loma Prieta earthquake, which ruptured a segment of the San Andreas fault on October 17, 1989, and caused extensive damage over a large area of central California, also produced substantial motions in nearby Monterey Bay (Figure 1). Earthquake effects included a tsunami, or seismic sea wave, and subsequent surface water oscillations that were detected for about 24 hours following the main shock and widespread, substantial slumping of sediments on the Monterey Bay continental shelf and along the walls of Monterey Submarine Canyon.

The origin of the tsunami excited by the 1989 Loma Prieta Earthquake —Faulting or slumping?

“…Figure 2b shows that the isochron at 18 min is slightly north of Moss Landing. Schwing et al (1990) suggest the possibility of large scale slumping near Moss Landing. Sea level fell by 1 m or more near Moss Landing soon after the earthquake.…”

“…Schwing et al (1990) describe this instrument as a bubble gauge. We digitized and detrended the record.…”

Tsunami Hazards Associated with the Catalina Fault in Southern California

“…Even though submarine landslides may be triggered by large earthquakes offshore, or on coastal or onshore fault systems like the San Andreas, such as occurred in Monterey Bay during the 1989 Loma Prieta earthquake (Schwing et al 1990, McCarthy et al 1993), large possibly catastrophic submarine landslides appear to be relatively rare offshore southern California. For example, the large basement-involved landslide described by Legg and Kamerling (2003) appears to be more than 100 ka, and the large San Pedro Canyon submarine landslide is about 7,500 years old (Fisher 2002).…”

“…It involved a 5-km to 17-km-deep oblique-reverse fault rupture along northwest strike and northeast dip within the Santa Cruz Mountains restraining bend segment of the San Andreas fault zone (Marshall et al 1991). The event caused extensive shaking damage to several cities in the San Francisco Bay area and a small tsunami in Monterey Bay (Schwing et al 1990). Like the proposed Catalina Fault scenario, this event occurred along a restraining bend of a strike-slip fault.…”

“…Furthermore, two large strike-slip earthquakes (M W Ͼ7) have occurred within the past decade on low-slip rate faults of the Mojave Desert region, which has similar geologic structure and tectonic style to the offshore Borderland. The 1989 Loma Prieta earthquake (M W ϭ6.9) demonstrated that oblique reverse earthquakes along restraining bends of the San Andreas Fault system in coastal areas can generate local tsunamis (Schwing et al 1990). The shaking from a large earthquake (M W у7) offshore southern California would damage coastal communities, and its effect would be enhanced were a tsunami triggered from the seafloor displacement.…”