Breaking Badly
Dr. Morgan Page talks California earthquakes.
If a recent Hollywood blockbuster has you concerned about California’s next big quake, take some reassurance from the fact that a new U.S. Geological Survey (USGS) earthquake hazard map would probably see it coming. As USGS Research Geophysicist Dr. Morgan Page explained at Tuesday’s first Sierra Nevada Aquatic Research Laboratory (SNARL) Spring Lecture, the USGS recently created a new and more accurate forecast model for California earthquakes.
In her lecture, “Breaking Badly: Forecasting California Earthquakes,” Dr. Page noted that we currently cannot predict the precise time, location, and size of future damaging earthquakes. But that doesn’t mean we don’t know anything. In fact, Dr. Page said, we know quite a lot: where earthquakes have occurred in the past, major earthquake faults, and where and how frequently aftershocks have followed quakes.
For those unfamiliar with plate tectonics, a quick crash course: California is sandwiched between the Pacific and North tectonic plates. The former migrates northwest at about two inches per year compared to the latter. The sliding between the plates plays out in fits and starts, with periods of quiet followed by sudden “slip” along cracks in the Earth. These “fault ruptures” are what cause the ground to shake.
Dr. Page and her colleagues recently completed the a third, innovative earthquake model called UCERF3 (Uniform California Earthquake Rupture Forecast, Version 3) to estimate the magnitude, location, and likelihood of potentially damaging earthquakes throughout the state.
The biggest change from UCERF2 to UCERF3 was an increase in the faults Version 3 included, she said. Scientists have begun to observe that a network of faults, rather than a single fault, work together to alleviate strain being placed on these faults as the plates come together.
Modeling this network of faults is a new tactic, Dr. Page said. The first earthquake hazard map, created in 1988 specifically for the San Francisco area, “divided faults into segments,” she said. The model then predicted possible magnitudes of earthquakes based on the history of that particular segment. A Japanese hazard map did the same, but then the magnitude 9 Tohoku earthquake (the scale, which quantifies the energy released by an earthquake, goes to 10) struck Japan in 2011, coming from one segment of a fault never considered to pose that large a threat.
“The segments linked up to create a magnitude 9,” Dr. Page explained. “We’ve experienced something like this with the Landers earthquake [magnitude 7.3] in 1992; several faults ruptured simultaneously.”
Scientists have begun to consider how one fault’s release of energy might move that energy through another fault, and another, generating greater releases of energy than a single fault could manage on its own. Accordingly, while the UCERF2 model included about 10,000 faults, the UCERF3 model “has a quarter of a million,” Dr. Page said. “We’re taking into account all the ways in which ruptures could occur, given the complex fault system we have in California.”
Dr. Morgan Page.
The biggest projected earthquake? Not surprisingly, one along the San Andreas Fault, which is the fastest moving fault in the State at about 34 millimeters per year. Dr. Page said the worst-case scenario the UCERF3 model predicted was the possibility of a magnitude 8.3 earthquake along the fault from Mendocino down almost to the bottom of the State. But that particular quake isn’t predicted to happen within the next 30 years, at least.
A great deal of data goes into these predictions, Dr. Page explained. Scientists use both geologic and GPS data to estimate how fast faults are moving, as well as paleoseismic data (which requires digging trenches to find evidence of where past earthquakes ruptured a given fault) to create an earthquake chronology. This allows scientists to trace earthquakes back thousands of years, rather than the 150 years we’ve been keeping historical records. To that, modelers add existing, known earthquake locations.
But there is always an element of surprise when it comes to earthquakes. “Sometimes there will be earthquakes that happen on faults we don’t know about,” Dr. Page said. This happened with the 1994 Northridge earthquake, for instance.
She added that there are also inconsistencies at times between geologic and GPS data on fault “slip rates” (meaning how fast faults are moving). In order to take that into account, about 1,400 different models went into creating the final UCERF3 forecast model.
Dr. Page said yet one more element factored into the model is how recently earthquakes have occurred in a given location. Seismologists have determined that a major earthquake relieves stress where the earthquake originated from, although surrounding areas suffer short-term clusters of aftershocks. (In one case in Japan, however, small aftershocks are still continuing 100 years later.)
Because Mono County and Inyo County have experienced major quakes so recently, geologically speaking, in 1872, 1980, and 1986, our major faults are “less ripe” for damaging activity, Dr. Page said. The percent of probability of a larger magnitude earthquake in our area in the next 30 years is about 1 percent or less, according to the UCERF3 model.
Meanwhile “the southern San Andreas is looking bad,” she said. “The very southern San Andreas is one of the most hazardous [faults] because it hasn’t gone since about 1680.” This section of the San Andreas has about a 20 percent chance or greater of a magnitude 6 earthquake or larger in the next 30 years, she said.
For those interested in earthquake probabilities in their area, Dr. Page said to check out the UCERF3 website at http://wgcep.org/UCERF3. The site offers a Google maps earthquake map.
One of the eventual goals of the earthquake forecast is to run in real time. “We want to dynamically change our hazard map for a specific location on a specific day, based on earthquake events,” Dr. Page said. In the meantime, the UCERF3 model will continue to inform the State’s early warning system, as well as determining earthquake insurance rates, State building codes, and risk-mitigation efforts.
