Could a Sendai-Sized Quake Hit the US?

The massive tremors and ensuing tsunami that devastated Japan earlier this month was an order of magnitude more destructive than anything that has hit the continental Unites States in historical times. But seismologists say that a similar event could well strike here. In fact, it's only a matter of time. And compared to Japan, we're far less prepared to deal with the consequences.

The danger zone is not California. While Los Angeles and San Francisco suffer frequent damaging quakes, they owe their seismic woes to a relatively shallow phenomenon called a slip-strike fault, caused by two tectonic plates sliding against each other. Sendai was a result of something far more dangerous: a so-called subduction zone, a deep-lying discontinuity caused by one plate slowly burying itself under another.

In both cases, earthquakes are caused by the slow building of pressure as the two plates move relative to one another, but remained locked together at the fault line. The strain increases steadily until the fault gives way, releasing the energy in the form of an earthquake. While strike-slip faults are relatively shallow, a subduction fault is deeper and can release a lot more energy. "One of the signatures of this type of fault," says Mike Blanpied, associate director of the US Geological Survey's Earthquake Hazards Program, "is that they sit quietly until they create a giant quake."And by giant, he means monster. The Sendai event contained more than 30 times the energy of the San Francisco Earthquake of 1906.

Only one such region lies within the Lower 48. It lies off the coast of Oregon and Washington and is called the Cascadia Subduction Zone. Today the Northwest seems seismically calm, but in reality the fault could let go again at any moment. In 1700 the subduction zone spawned an earthquake of roughly the same size as the Sendai event, unleashing a tsunami that may have reached 100 feet high. Based on geological evidence, Chris Goldfinger, director of the Active Tectonics and Seafloor Mapping Lab at Oregon State University, believes that in the past similar-sized quakes have happened about every 300 to 350 years -- meaning we're just about due.

To figure out what would happen if a Sendai-scale event struck today, a consortium called the Cascadia Region Earthquake Workgroup (CREW) has drawn up a detailed scenario of how the quake and its ensuing tsunami would affect buildings, transportation, utilities, and emergency services. The report, entitled "Cascadia subduction zone earthquakes: A magnitude 9.0 earthquake scenario," painted a picture so grim that it read like a bad Michael Bay script. It was pretty hard to believe -- until the Sendai quake brought all its horror vividly to life. It can happen.

Some excerpts from the scenario:

Groundshaking, landslides, liquefaction, tsunamis, fires, hazardous material spills, and building damage are some of the hazards we face from a Cascadia subduction zone earthquake. The ground could shake for four minutes, even more in some places. This will create unprecedented damage and potentially thousands of casualties...

Coastal communities will be subjected to strong shaking, landslides, and tsunamis. Buildings, roads, bridges and utility lines will suffer varying amounts of damage. Some will be destroyed. Extensive injuries and fatalities are likely. Within minutes, a tsunami will arrive, making it essential that residents and visitors understand the need to head for higher ground or inland as soon as the shaking stops.

Coastal Highway 101 will be impassable over large stretches, and landslides through the Coast Range will sever highway travel between the coast and inland areas. Destruction of roads, runways, ports, and rail lines will leave individual cities isolated. Residents and visitors will have to do much of the work of rescuing those trapped in the rubble and will be responsible for the immediate clean-up and organization to distribute relief supplies.

Along the I-5/Hwy 99 corridor, utilities and transportation lines in some areas could be disrupted, perhaps for months. This particular type of earthquake is especially hazardous to tall buildings, which could lead to significant fatalities in downtown areas. Buildings that would be unscathed in a more typical 30-second quake might be severely damaged after several minutes of shaking. Long bridges and utility lines are also at risk, which could create serious longterm economic losses. Landslides could block eastwest travel through the Cascades. As the the center of our regional transportation network, closures at any point here could have far-reaching consequences.

If the projected devastation sounds a lot like what happened in reality in Japan, that's no coincidence. Both catastrophes result from the same time of fault rupturing with the same intensity. They have one well-known precedent in the United States. On Good Friday, 1964, Alaska was hit by an epic five-minute-long quake that registered 9.2, making it the most massive ever recorded in North America. On Kodiak Island, land was raised up 30 feet. Elsewhere, whole villages were destroyed as the ground beneath liquefied and sank. Anchorage suffered devastating landslides, and 30 city blocks were damaged or destroyed.

The magnitude of a quake alone tells you little about its effect on a population. The suffering that results is really a function of two things: the intensity of the shaking and the quality of the preparations that have been made. Japan is by far most earthquake-savvy country on the planet. It lavishes millions on research, its building codes are tough, and its people are psychologically ready. Yet despite Japan's preparations, the 9.0 earthquake wreaked havoc on a scale not seen since World War II.

Americans, on the other hand, spend little time thinking about earthquakes, and even less money. We're just starting to explore technologies that the Japanese have been implementing for decades. One step toward protecting the public, for instance, would be to establish an Earthquake Early Warning (EEW) system. The idea is to detect tremors as a fault begins to slip, and then transmit an alarm signal to population centers on the periphery of the quake zone. Since motion of the quake travels at the speed of sound, and the signal can travel near the speed of light, an efficient system could provide a minute or two of warning -- enough time to stop elevators and let passengers off, or for a teacher to shepherd her students under their desks.

Such a system in Japan gave residents of Tokyo more than a minute's warning that a major temblor was on its way, and may have saved countless lives. For anyone who likes to believe that America stands at the forefront of technology, the sad truth is that we're still years away from developing such a capability -- if indeed we ever succeed.

Right now the USGS is trying to piece together a system that would protect the most vulnerable areas of northern and southern California. Project chief David Oppenheimer says that the system will require the installation of 120 up-to-date seismic monitoring units to provide the necessary density of sensors. In this age of budget cutbacks, the money needed to move forward is in short supply.

Should a magnitude 9.0 earthquake strike the Pacific Northwest tomorrow -- and there's no reason it shouldn't -- the initial damage would in all likelihood be at least as bad as the devastation in Japan, and the aftermath far worse. If there's a bright side to the Sendai quake, it's that it might wake up enough concerned citizens to that fact. The first step in being prepared is understanding that the threat exists.

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