California Revamps Earthquake Maps, Adds 50 New Faults


California residents without earthquake insurance might want to rethink that decision. Why? Because geology officials in the Golden State have recently released an updated seismic activity map of the state – the first such update since 1994 – and it includes more than 50 surface fault lines that have been discovered in the last twenty years. The new map was unveiled during the 150th anniversary celebration of the California Geological Survey, which also presented an updated version of a second map identifying the composition of rock and soil.

According to state geologist John Parrish, the maps can help guide decisions about where to build schools and hospitals, and where construction standards should be higher. “These maps are used to make a lot of other maps, to map landslides … for tsunami coastal mapping. They can tell you what kind of a surface you’re building on, and how close you are to a fault,” Parrish said.

The new maps don’t merely show fifty new faults – they have interactive digital version linked to Google maps, and they also correct distortions leftover from a previous digitizing attempt that left some geologic features more than half a mile from where they were actually located.

As for the fifty new faults added to the roughly 15,000 present in California, while they’re new to the seismic activity map, they have been mapped before. Some of them announced themselves years ago, including the system responsible for 1999’s 7.1 Hector mine quake, which, thanks to the epicenter in the middle of the Mojave Desert, didn’t cause much damage despite its magnitude, though others, like the fault which caused 1994’s Northridge quake are not on the map, even though that system wrecked most of downtown Coalinga. This is because they are “blind thrusts” with no depictable surface ruptures.

Chris Wills, supervising engineering geologist in charge of the project said that the new map, “…really puts the earthquake hazard in context: Where do our earthquakes come from? Where are the most active faults distributed in the state? Where are the most recently active faults that we would tend to worry about most in the state of California? Where are they less common?”

The statewide map is an addition to existing, localized seismic mapping projects where are required by California law. Since 1971, the state geologist has been required to map surface ruptures as an aid to planners in preventing the construction of buildings designed to be used by people on active faults. A newer 1994 law requires the mapping of additional seismic hazards, like landslides and liquefaction.

Since 1998, the National Hazards Disclosure Act has required real estate sellers or their agents to inform potential buyers if a property is located within one of these areas.

As new as the map is, Wills observed, it’s already out of date, because the April 4th quake that struck Baja California at a magnitude of 7.2 produced surface ruptures north of the U.S.-Mexico border. “Anytime you try to do a compilation of everything you know about a state you lag behind studies of an individual area,” he said.

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