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Newly Discovered Continental Collision Could Trigger Massive California Tsunami

Newly Discovered Continental Collision Could Trigger Massive California Tsunami

“We’re dealing with continental collision,” states geologist Mark Legg at Legg Geophysical in Huntington Beach, California. “That’s fundamental. That’s why we have this mess of a complicated logjam.”

What has basically been discovered is that fragments of blocky crust have lifted upward while also slipping sideway along the Santa Cruz-Catalina Ridge Fault, just like the plates along the San Andreas Fault do.

Then, out sea a bit is the Ferrelo Fault zone, which is also thrusting upwards of one side of the fault. All this vertical movement causes blocks of crust to be compressed as well as sliding horizontally relative to each other–what Legg calls as “transpression.”

Compression comes from the blocks of the Borderland being dragged northwest, but then slamming into the roots of the Transverse Ranges – which are east-west running mountains north and west of Los Angeles. In fact, the logjam has helped build the Transverse Ranges, Legg explained.

“The Transverse Ranges rose quickly, like a mini Himalaya,” Legg said. The real Himalaya arose from a tectonic-plate collision in which the crumpled crust on both sides piled up into fast-growing, steep mountains rather than getting pushed down into Earth’s mantle as happens at some plate boundaries.

As Southern California’s pile-up continues, the plate movements that build up seismic stress on the San Andreas are also putting stress on the long Santa Cruz-Catalina Ridge and Ferrelo Faults. And there is no reason to believe that those faults and others in the Borderlands can’t rupture in the same manner as the San Andreas, said Legg.

“Such large faults could even have the potential of a magnitude 8 quake,” said geologist Christopher Sorlien of the University of California at Santa Barbara, who is not a co-author on the new paper.

“This continental shelf off California is not like other continental shelves – like in the Eastern U.S.,” said Sorlien. Whereas most continental shelves are about twice as wide and inactive, like that off the U.S. Atlantic coast, the California continental shelf is very narrow and is dominated by active faults and tectonics. In fact, it’s unlike most continental shelves in the world, he said. It’s also one of the least well mapped and understood. “It’s essentially terra incognita.”



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