THE TŌHOKU EARTHQUAKE OF MARCH 11, 2011: IMPLICATIONS FOR COASTAL CALIFORNIA

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This report is intended to help the Commission place the recent events in Japan in context and to provide perspective for their possible implications to coastal California, based on preliminary information known at this early date.

The discussion below includes the following:

• A description of the Tōhoku Earthquake and tsunami effects
• An evaluation of whether the seismic characteristics of the Tōhoku Earthquake are applicable to the California coast
• A brief description of the earthquake and tsunami risks at California’s three coastal nuclear facilities

References and links to web resources can be found at the end of this report. Some words or phrases are further explained in an appendix. These words or phrases are marked in bold italics where they first occur.

EXECUTIVE SUMMARY

• The vast majority of faults in California, including the San Andreas fault, could not produce a magnitude 9 earthquake.

Most of California is not susceptible to an event of the scale of the Tōhoku Earthquake. Nevertheless, it is important not to become complacent; large earthquakes are inevitable throughout coastal California, and could be devastating in their own right. There is a large population and much infrastructure at risk in central and southern coastal California.

• The Cascadia Subduction Zone could produce a magnitude 9 earthquake similar to the Tōhoku Earthquake.

The northern part of the coastal California, as well as all of coastal Oregon, Washington, and part of coastal British Columbia—the Cascadia Subduction Zone—is susceptible to an earthquake and tsunami event similar to that of the Tōhoku Earthquake. Emergency response scenarios and land use planning must take this into account.

• A nuclear emergency such as is occurring in Japan is extremely unlikely at the state’s two operating nuclear power plants.

The combination of strong ground motion and massive tsunami that occurred in Japan cannot be generated by faults near the San Onofre Nuclear Generating Station and the Diablo Canyon Power Plant. Nevertheless, the geologic conditions near those plants are very likely different than previously believed and ongoing study is warranted. This has been understood for at least the past three years, and some of these studies, and the environmental planning process for other such studies, are underway.

The Tōhoku Earthquake and Tsunami

The magnitude 9.0 Tōhoku Earthquake of March 11, 2011, the fourth most powerful earthquake measured by modern instruments (since about the year 1900), occurred at the interface between the Pacific and North America plates. Two days prior to the earthquake, a magnitude 7.2 earthquake occurred near what would be the epicenter of the Tōhoku Earthquake. It was followed by three aftershocks in the magnitude 6 range later that day. In retrospect, these earthquakes can be viewed as foreshocks to the magnitude 9.0 event that followed. The hypocenter of the Tōhoku Earthquake was located 81 miles off the east coast of the Oshika Peninsula, part of the Tōhoku region of the island of Honshu, near the city of Sendai, at a depth of 20 miles below the seafloor.

Over the next several days, hundreds of aftershocks, the largest of magnitude 6.8, outlined the area of the plate boundary that ruptured. Aftershocks extend to depths of about 340 miles, although most are above a depth of 125 miles. As of March 23, some 726 aftershocks have been recorded, 26 of them magnitude 6.0 or greater, and they are expected to continue, at a decreasing frequency, for the next several years.

The Tōhoku Earthquake is what is termed a “megathrust earthquake:” a major subduction zone earthquake whereby the Pacific plate suddenly lurched beneath the North America plate. The affected portion of the North America plate is a westward and southward extension of the plate from the Alaska region that underlies eastern Siberia and the northern Sea of Japan Some geologists divide this geologically complex region into a number of microplates (for a representation of the geometry of these plates, see http://pubs.usgs.gov/gip/dynamic/slabs.html).

The area of the plate boundary that ruptured during this earthquake was about 190 miles long and 90 miles wide, which is larger than the areas of San Luis Obispo, Santa Barbara, Ventura, Los Angeles, Orange, and San Diego counties combined. At the location of the earthquake, oceanic crust of the Pacific plate is being thrust under oceanic crust of the North America plate at an angle of about 15 degrees, creating the Japan Trench, a bathymetric trough on the seafloor. The average rate of this movement is approximately 83 mm/yr, one of the higher rates of plate convergence in the world. Much of this movement occurs episodically in earthquakes such as the Tōhoku Earthquake.

The Tōhoku Earthquake was accompanied by violent and long-lasting ground shaking. The highest measured ground acceleration was a devastating 2.75 g1, measured over 80 miles from the epicenter, although most stations relatively near the epicenter reported ground accelerations more on the order of 1.0 g. Ground shaking intensity reached a Modified Mercalli Intensity of IX (Violent), and resulted in widespread damage. Damaging secondary effects included liquefaction, lateral spread, and landslides.

The most damaging secondary effect of the earthquake was the resulting tsunami. Megathrust earthquakes of this magnitude occurring beneath the sea at relatively shallow depths always produce large tsunamis and the Tōhoku Earthquake was no exception. About 15 minutes after the earthquake, a tsunami with an amplitude of about 30 feet hit the shoreline. The first wave swept up to six miles inland in flat regions, leveling buildings, and sweeping debris, buildings, ships, vehicles, and airplanes far inland.

The tsunami caused the loss of power, and disabled backup generators, at the Fukushima 1 nuclear power plant. The subsequent loss of coolant in several reactors and spent fuel storage pools resulted in explosions, fires, and partial core meltdowns in at least three nuclear reactors, and the release of radiation to the environment.

The full extent of the damage resulting from the Tōhoku earthquake and tsunami is still unknown and will likely be unknown for some time. Given the thousands of lives that have been lost, the billions of dollars worth of property and infrastructure destroyed, and the ongoing nuclear emergency, it is natural to ask how vulnerable California is to a similar event.

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