The 1992 Cape Mendocino Earthquake and Tsunami

The Earthquake

On April 25, 1992, a large earthquake (MW7.2) and powerful aftershocks rocked the Cape Mendocino area of Humboldt County in California. The shaking was felt throughout much of northern California and in southern Oregon.

The epicenter was onshore near Petrolia at the south end of the Cascadia subduction zone, a tectonic plate boundary where the Gorda and North America plates converge (move toward each other). This area is one of California's most geologically complex and seismically active areas.

Following the earthquake, the USGS found that twelve benchmarks within 60 miles (100 km) of the epicenter had moved measurably, some by as much as sixteen inches (40 cm) horizontally and six inches (15 cm) vertically.

Locations and magnitudes of the 1992 Cape Mendocino earthquake and two largest aftershocks. Sources: Mattison and others, 1992.
Date Time (PDT) N. Lat. W. Lon. Depth MW
4/25/92 11:06:04 40.37 124.31 15km 7.2
4/26/92 00:41:40 40.44 124.58 18km 6.5
4/26/92 04:18:26 40.40 124.56 21km 6.6
Map of the Cascadia subduction zone.
Map of the Cascadia subduction zone and related features. The epicenter for the 1992 Cape Mendocino earthquake is shown as a yellow star. Note cross-section A-A', which is discussed in the next figure. Click/tap the image to open a high-resolution version. Source: Modified from Nelson et al., 2004.
Cascadia subduction zone cross-section.
A cross-section showing typical tectonic plate interaction in the Cascadia subduction zone. This is cross-section A-A' from the map above. Between earthquakes, tectonic stresses cause subsidence in some areas and uplift in others. When the stresses release during an earthquake, the subsided areas 'rebound' up, while the uplifted areas sink down. Click/tap the image to open a high-resolution version. Source: Modified from Savage, 1983.

The earthquake and aftershocks damaged buildings, roads, and bridges, and triggered numerous landslides. Structural damage was concentrated in Ferndale, Fortuna, Petrolia, Rio Del, and Scotia. President George H.W. Bush declared Humboldt County a major disaster area.

The Tsunami

The 1992 Cape Mendocino earthquake produced a small tsunami that arrived at the coast within 20 minutes of the earthquake (Oppenheimer and others, 1993; Dengler and others, 1995). The tsunami was recorded on tide gages (or gauges) along the coast of California and Oregon. The National Oceanic and Atmospheric Administration (NOAA), which operates these tide gages, reported a 1.6 foot (0.5 m) tsunami at Crescent City (Mattison and others, 1992). At Eureka, wave heights were about 1 foot (0.3 m) (Oppenheimer and others, 1993; Dengler and others, 1995) and at Point Arena, about 0.6 feet (18 cm) (Mattison and others, 1992). The tsunami was even detected in Hawaii, measuring about 0.3 feet (9 cm) there (Mattison and others, 1992).

Learn more about your exposure to tsunami hazards at the California Tsunami Preparedness Guide, and explore our Tsunami Hazard Area Maps and Data.

Marigrams of the 1992 Cape Mendocino tsunami.
Marigrams (tide gage records) of the 1992 Cape Mendocino tsunami from NOAA tide gages in California and Oregon. The marigrams show that the tsunami lasted more than eight hours and that the first wave is not necessarily the largest wave. Click/tap the image to open a high-resolution version. Source: Oppenheimer et al., 1993.

The following map shows locations of tide gages that detected and recorded the tsunami. Explore the wave data by clicking/tapping on the triangle symbols. If you're on a smartphone, you should Open the map in a new window 

What Causes a Tsunami?

Tsunamis are produced by large-scale displacement of water during an earthquake, volcanic eruption, or underwater landslide. The most common source of tsunamis are subduction zone earthquakes that cause a portion of the sea floor to lift up, along with the vast amount of water above it. Conversely, strike-slip earthquakes cause horizontal movement of the sea floor which rarely results in a significant tsunami; however, the ground shaking produced by strike-slip earthquakes may induce underwater landslides, which can generate damaging tsunamis. Visit our Tsunami Education page for more information about how earthquakes generate tsunamis.

Significance and Lessons Learned

The 1992 Cape Mendocino earthquake was a significant event for several reasons:

  • This was just five years after Dr. Brian Atwater wrote an article that provided the first evidence that the Cascadia subduction zone was capable of generating large magnitude earthquakes. The 1992 earthquake was on a fault related to the Cascadia megathrust subduction zone fault, so it reinforced the messages presented by Dr. Atwater in 1987.
  • The important lessons from both the paper and the earthquake were that the Pacific Northwest is vulnerable to locally-produced earthquakes and tsunamis, and that people should prepare for the impact from this newly confirmed source of seismic hazard.
  • This event inspired the formation of the National Tsunami Hazard Mitigation Program, which helps support state and local tsunami preparedness efforts. It is also largely responsible for the creation of the California Tsunami Program.
  • From a policy perspective, the 1992 Cape Mendocino earthquake and tsunami supported the State Mining and Geology Board's earlier decision to include tsunami in the Seismic Hazards Mapping Act (SHMA). Under the SHMA, the CGS maps hazard zones and develops guidance for site-specific analysis and mitigation strategies.

Regional Partners

The Redwood Coast Tsunami Work Group is a volunteer-based organization that was formed in the years following the 1992 Cape Mendocino earthquake. Created by geologists at Cal Poly Humboldt (formerly Humboldt State University) to help people across the state become more prepared to respond to and recover from earthquakes and tsunamis, this group has been actively engaged with the community, with local and regional stakeholders, and with other organizations ever since. Visit the RCTWG's web site commemorating the 1992 Cape Mendocino earthquake and tsunami.

References

Atwater, B.F., 1987, Evidence for Great Holocene Earthquakes Along the Outer Coast of Washington State in Nature, v. 236, no. 4804, p. 942-944.

Dengler, L.A., Moley, K., McPherson, R., Pasyanos, M., Dewey, J.W., and Murray, M., 1995, The September 1, 1994 Mendocino Fault Earthquake in California Geology, v. 48, no. 2, p. 43-53.

Mattison, E., Tabilio, L., and Walker, P., 1992, The Cape Mendocino Earthquakes in California Geology, v. 45, no. 2, p. 56-57.

Nelson, A.R., Asquith, A.C., Grant, W.C., 2004, Great Earthquakes and Tsunamis of the Past 2000 Years at the Salmon River Estuary, Central Oregon Coast, USA in BSSA, v. 94, no. 4, p. 1276-1292.

Oppenheimer, D., Beroza, G., Carver, G., Dengler, L., Eaton, J., Gee, L., Gonzalez, F., Jayko, A., Ki., W.H., Lisowski, M., Magee, M., Marshall, G., Murray, M., McPherson, R., Romanowicz, B., Satake, K., Simpson, R., Somerville, P., Stein, R., and Valentine, D., The Cape Mendocino, California, Earthquakes of April, 1992: Subduction at the Triple Junction in Science, v. 261, no. 5120, p. 433-438.

Savage, J.C., 1983, A Dislocation Model of Strain Accumulation and Release at a Subduction Zone in JGR, v. 88, no. B6, p. 4984-4996.

The California Geological Survey Tsunami Unit

When tsunamis or coastal flood events occur or are forecast to occur in California, the California Geological Survey Tsunami Unit deploys teams to make direct observations and collect perishable data. This Coastal Flood Event Response also includes digital forms of data collection and web interfaces to share data in real time.

California Geological Survey Tsunami Unit. The California Geological Survey is a division of the Department of Conservation.
  • Preparation: Tsunami Hazard Area Maps and Data
  • Response: Tsunami and Coastal Flood Event Response Program
  • Recovery: The CGS is working with partners to develop tsunami recovery and debris guidance documents for coastal communities.
  • Mitigation: The CGS has prepared Harbor Improvement Reports so that maritime infrastructure can be designed with tsunami hazard mitigation in mind.

Contact the CGS Tsunami Unit


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California Seismic Hazards Program

Last modified: April 24, 2022

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