Debris pile in Namie City, Japan. Photos: Yoh Kawano/UCLA
The megathrust earthquake that devastated Japan struck four years ago at 10:46 p.m. tonight Los Angeles time — it was just before 3 p.m. on March 11 on the east coast of Japan. In case you have forgotten, the quake was a magnitude 9.0 event — the largest to hit Japan in recorded times — and lasted for several minutes. It triggered tsunami waves that killed almost 16,000 people in Japan and propagated across the Pacific, still detectable when the waves reached the coast of California. The Pacific Tsunami Warning Center today reposted its model of the waves' spread and it remains as awe-inspiring as in 2011.
While much of Japan has recovered, UCLA researchers who visited Fukushima Prefecture in December found an evacuation zone still extends for miles around the disabled Fukushima Daiichi Nuclear Plant, including all of Namie City, which had a population of 21,000 before the tsunami swept through.
Back in California, the U.S. Geological Survey released a report today raising the chances of a magnitude 8.0 earthquake rocking the state. While much less strong than the 2011 Japan quake, it would be the largest earthquake to hit California in recorded times. (The biggie was a 7.9 at Fort Tejon, in the Tehachapi Mountains north of Los Angeles, in 1857.) The recalculated chances of an 8.0 are higher in the Los Angeles area than in Northern California, but still relatively low: one every 532 years in LA. Of course, since there's never been one that we know of, you could argue we're due.
The recalculation upward of the chances is based on new understanding that big quakes can start on one fault and spread to others — indeed, that California's geology is underlain by a network of interconnected faults. This multi-fault involvement occurred in the Japan disaster. The new outlook for the "Big One" in California is labeled by USGS as the third Uniform California Earthquake Rupture Forecast, or UCERF3. Read it here — it's a big PDF file. Excerpt:
Two kinds of scientific models are used to help safeguard against earthquake losses: an Earthquake Rupture Forecast, which tells us where and when the Earth might slip along the state’s many faults, and a Ground Motion Prediction model, which estimates the subsequent shaking given one of the fault ruptures. UCERF3 is the first type of model, representing the latest earthquake-rupture forecast for California. It was developed and reviewed by dozens of leading scientific experts from the fields of seismology, geology, geodesy, paleoseismology, earthquake physics, and earthquake engineering. As such, it represents the best available science with respect to authoritative estimates of the magnitude, location, and likelihood of potentially damaging earthquakes throughout the state…
It has become difficult to identify where some faults end and others begin, implying many more opportunities for multifault ruptures. As a consequence, UCERF3 now considers more than 250,000 different fault-based earthquakes, including multifault ruptures, whereas UCERF2 had about 10,000, and previous models had far fewer. Because we still lack a complete inventory of faults, UCERF3 (and UCERF2 before it) also includes the possibility of earthquakes on unrecognized faults elsewhere in the region.
So what do these changes imply with respect to seismic hazard, the likelihood of ground shaking, as well as for seismic risk, the threat to the built environment with respect to fatalities and economic losses? The answer turns out to be entirely dependent on what you are concerned about. For example, increasing the likelihood of large multifault earth- quakes, which consequently reduces the likelihood of moderate-sized events, may increase the risk to tall buildings or large bridges, but actually lower the risk to residential homes.
As a consequence, it is difficult to make generalizations about the hazard or risk implications of UCERF3 without first specifying both asset types and their locations. Conclusions will vary depending on whether you are designing a single family dwelling in Sacramento, retrofitting the San Francisco–Oakland Bay Bridge, considering the location of a nuclear power plant, laying pipeline across the San Andreas Fault, or considering aggre- gate losses over a large insurance portfolio. The practical implications will need to be considered on a case-by-case basis.
Previously on LA Observed:
Japan hit by 'great' quake and tsunami, Pacific put on watch
Scenes from Japan
Japan toll goes over 10,000 dead
Radiation levels rising in Japan, most workers leave plants
Tsunami surges (gently) up Ballona Creek
Very big earthquakes can be global events, studies find
You can visit the Pacific tsunami debris