FY 1993

The Cape Mendocino Earthquake: A local tsunami wakeup call?

McCarthy, R.J., E.N. Bernard, and M.R. Legg

In Proceedings of the Eighth Symposium on Coastal and Ocean Management, Vol. III, 2812–2828 (1993)

Sudden displacement of the seafloor due to fault offset or submarine slumping can generate tsunami waves that can travel vast distances across open ocean and damage coastal communities. Local tsunamis generated by faults located in offshore California waters, the sudden uplift of a large land mass due to faulting, or the downslope displacement of large areas due to slumping in submarine canyons can strike the coastline within less than 10 minutes. The 1989 Loma Prieta and 1992 Cape Mendocino earthquakes demonstrated the vulnerability of the California coastline to locally generated tsunamis. The 7.1-magnitude Loma Prieta earthquake induced a large submarine slump along the walls of the Monterey Submarine Canyon. A 20-cm-high tsunami struck Moss Landing minutes after the earthquake. The Cape Mendocino earthquake also had a magnitude of 7.1 and created a .5-m-amplitude tsunami at Crescent City and a 20-cm-amplitude wave at Eureka, California. The time between the earthquake and arrival of the tsunami at Eureka was approximately 20 min. Fortunately, the tsunami struck Crescent City and Eureka at low tide, and no damage to city or harbor facilities occurred. However, if the wave had arrived during high tide or during a storm when meteorological forcing had created an elevated sea level, damage would have occurred and possibly some loss of life. This paper describes the Cape Mendocino earthquake and its uniquely large aftershocks, discusses the potential near-shore tsunami sources for offshore California, and presents some recommendations on how to establish tsunami risk mitigation for coastal California.