Northern California Offshore Earthquake – June 15, 2005

This earthquake, magnitude 7.2, was a rare event for anywhere in California. It was caused by subduction on the part of the Gorda Tectonic Plate.

On June 15, 2005, a 7.2 magnitude earthquake struck the center of the Gorda Tectonic Plate about ninety miles offshore from the northern coast of California. This plate lies relatively close to the coast and can be considered a southward extension of the large Jan de Fuca Plate. Both of these plates are subducting beneath the North American Plate. The Gorda is a very active one. It produces earthquakes of magnitude 7 about every twenty years, a rare level of activity for any place in California. This particular earthquake was felt widely in Oregon, Washington, and California, and it had a significant impact as far north as Seattle and southwards to the Mexican border.

This earthquake occurred in a deformed section of the southernmost part of the Gorda Plate. Light shaking from it was widely felt along the northern Californian coastline all the way to the Oregon boundary. Preliminary analysis of the quake indicated that it resulted from slip on a left-lateral, strike-slip fault. This type of fault has been documented for other earthquakes with epicenters in the interior of the Gorda Plate. There was no tsunami. Earthquakes with strike-slip mechanisms are less likely to produce tsunamis because they cause relatively little vertical ground movement.

While this earthquake was very strong and exceptional, it must be noted that in general, lesser quakes are very common in the Gorda Plate because it is subjected to north-south compression as the Pacific Plate moves toward the Northwest and collides with the southern boundary of the Gorda Plate. A tsunami warning was automatically triggered from the West Coast Tsunami Warning System for the entire U.S. West Coast because of the strength of the quake. It was withdrawn soon afterward.

Crescent City, a place that historically always suffered greatly from tsunamis on the northern Californian coast, sounded its warning sirens shortly after 8 P.M. and thousands of residents and visitors were evacuated. In many other areas, communication problems prevented the tsunami warning from reaching the public until after it had been canceled. The tsunami warning had been based on the earthquake’s large magnitude, initially calculated at a higher figure than the one that was finally determined. Information that takes longer to gather is used to determine whether a tsunami warning should be expanded, continued, or canceled. Most important is direct measurement of water levels by NOAA’s ocean bottom sensors and shoreline tide gauges that relay data by satellite to the tsunami-warning centers. Well after the predicted arrival times no tsunami had been clearly detected at the three tide gauges nearest the epicenter and even in Crescent City the additional height of the tidal gauges was only four inches. After careful consideration of all available information, the National Weather Service canceled the tsunami warning at 9 P.M.

Analysis of earthquake data helped seismologists explain why only a tiny tsunami had been generated: the earthquake occurred on a strike-slip fault, now believed to be a left-lateral fault within the Gorda Plate, a fault ridden piece of oceanic crust being compressed and deformed by the northward- moving Pacific Plate. Earthquakes on strike-slip faults produce much less vertical movement of the sea floor than do earthquakes on thrust faults. While experts knew that there was little likelihood of a tsunami in this instance, their minds were still very much occupied by the tsunamis of December 2004 in Indonesia and they were not going to take any risks. NOAA uses two types of water-level sensors to detect tsunamis: shoreline tide stations and offshore ocean-bottom sensors. NOAA operates a number of tide stations on Pacific coastlines and six buoys in the Pacific Ocean. The memory of the devastating Indonesian tsunami of 2004 is still strong and it has inspired a joint project involving USGS and NOAA to install additional buoys not only along the northern Californian coast but also in selected Atlantic and Caribbean sites.

The June 15, 2005, earthquake occurred twenty miles west of another 7.2 quake, the one that hit off the coast of Humboldt Bay on November of 1980. Between 1980 and 2005, in the same general area, there were ten others that measured between 6 and 7 in magnitude and one, that of Cape Mendocino in 1992, that was of magnitude 7.2. One has to go back a very long distance in time to find three Californian earthquakes of this magnitude occurring in the same general area in and east of the Gorda Plate. The November 1980 quake, the largest in the previous twenty-five years, injured six people and caused property damage estimated at $2 million. Most of the damage occurred east of Fields Landing, where two sections of an overpass on U.S. Highway 101 collapsed onto the railroad tracks below. At Fields Landing, two houses were displaced from their foundations, one non-reinforced chimney fell, and gas, water and sewer lines were broken. This shock and most of its aftershocks occurred on a large, left-lateral, strike-slip fault that extends northeastward from Cape Mendocino. The earthquake was felt over a large area, including parts of Oregon, western Nevada, and northern California. Many aftershocks occurred.

The Cape Mendocino 7.2 earthquake of April 1992 was located on land near Petrolia at a depth of seven miles below ground. This location is very near the inferred position of the Cascadia subduction zone boundary between the Gorda Plate and the North American Plate; that is, the epicenter is very close to the point where the subducting plate begins to move down into the asthenosphere. It is thus the first major historic earthquake on the subduction zone. Motion was along a north-south oriented fault plane dipping gently down to the east. The North American Plate was thrust up and over the Gorda Plate. It produced measurable coastal uplift in the vicinity of Cape Mendocino on the order of 4–5 feet and a tsunami which was recorded at tide gauges from Port Orford, Oregon, to Port San Luis near San Luis Obispo. Maximum recorded wave heights were just under 2 feet in Crescent City. The main shock was followed by two magnitude 6.6 aftershocks, both intraplate Gorda earthquakes located about fifteen miles off shore. The main shock together with the large aftershocks caused $60 million worth of damage and led to a federal disaster declaration.

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