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PALM LATITUDES

Underground : Life is a Bowl of Sand, Silt and Clay

November 05, 1995|Michael Forrest

Nothing about the junction of the Los Angeles and Rio Hondo rivers in South Gate suggests that it's a remarkable place. An old mattress, a red garden hose and tree branches float upon the brown-green water. You might see a homeless traveler at the water's edge under a bridge, or a kid on a mountain bike, flying down a steep concrete wall for a good scream. The junction looks like any other stretch of flood control engineering, except that it roughly marks the center of Los Angeles' rock-stretching history: It is where the sand, silt and clay of the Los Angeles Basin reach their deepest.

If you grabbed a shovel and a bucket and started to dig your way to China right there, you would have to go down more than 30,000 feet--about as deep as the Himalayas are high--before you hit igneous or metamorphic rock. This is because most of our megalopolis sits on a sand-plugged hole ringed by the Palos Verdes peninsula and the San Gabriel, Santa Monica and Santa Ana mountains. Deep even by geological standards, this bowl of sediment reflects and focuses earthquake waves in unexpected ways, causing, among other things, one city block to shake more than another right next to it.

Fifteen million years ago, the Los Angeles Basin lay under the Pacific. The transverse ranges to the north, including the San Gabriel and Santa Monica mountains, were rotating clockwise like a giant ship spun by a current. The continental crust stretched, smoked and shuddered apart in their wake, and hot molten rock poured up from the sea floor. As the crust of L.A. thinned, a bowl opened up, and silt and clay drifting in the ocean currents and in the ancient rivers that emptied into the Pacific piled up on its bottom. Great layers of microorganisms snowed down into it, eventually becoming L.A.'s more than 10 billion barrels of oil.

Five million years ago the crustal stretching stopped. The Los Angeles area began to shrink and the hole started to fill. Now, much of the contents of the hole are being squeezed skyward. Rock that used to lie at the bottom of the sea can be found at street level, pushed up by the faulting and folding of the earth. Contrary to popular myth, we're not falling into the ocean--we're rising out of it with almost every earthquake.

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