This past weekend, as the actor called The Rock battled shaking, shattering rocks in the quake movie "San Andreas", geologists added a new danger zone a few hundred miles to the west, with maps of a cracked and crooked seafloor off the California coast. While the real San Andreas fault is unlikely to trigger a giant tsunami like the one that chased Dwayne “The Rock” Johnson in the movie, these maps of a little-known area called the Borderlands pointed to faults that could send dangerous waves towards the fabled beaches of Los Angeles and San Diego.
Seismologists were quick to point out that the underwater faults do not come close to the San Andreas in terms of risk to life and limb and house and home. The lead author of the new research, Mark Legg, said the maps were not a cause for panic. But he did note the faults were something to be concerned about, since they had moved enough in the past to release quakes upwards of magnitude 7.0, and the way they moved could create tsunamis a dozen feet high, enough to inundate beaches near L.A. such as Huntington Beach. Legg Geophysical, the scientist's consulting firm, is located in the town that grew up behind the beach.
The new analysis was published online Friday by the Journal of Geophysical Research: Earth Surface. It is based on a compilation of older sonar surveys and other tests that probed the contours of the sea floor off southern California. In this region, the Pacific plate is moving roughly northwest, dragging rocky blocks of near-coast crust with it. Those blocks make up the Borderlands areas. Just north of the Los Angeles area, they jam up against other rocky slabs that run east-west, the extensions of a coastal mountain range. The collision creates a buildup of tectonic stress, occasionally released as quakes.
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The survey showed that some of the blocks moved not just horizontally but vertically, creating undersea ridges. When blocks are pushed up abruptly, they shove the water above them, and that is how most tsunamis start. The San Andreas rocks move horizontally, for the most part, and that is why they are unlikely to launch a bad wave. Their inland location, away from the water, also lowers the risk. (It does not make the risk zero. San Andreas quakes could send landslides off the coast and into the sea, and that could lead to a small tsunami.) But the rising rocks of the Borderlands makes them an area that California residents will need to watch.