Human physical capacities-our size and the strength of our muscles-determine how we classify the world around us. Us surface dwellers do not have great intuition for its scale or function. “So, I was only a kilometer farther away from it than you were.” “By the Pythagorean theorem, that’s 13 kilometers,” he said. “I’m five kilometers away from you,” Alden says, forming a triangle with his hands to show that his distance from the quake would be the hypotenuse of the triangle formed by five surface kilometers and 12 kilometers of depth. So, really, I was 12 kilometers, or seven-and-a-half miles, from the actual spot the earthquake began. “Your whole premise is that the epicenter is important,” Alden said. But that’s just the point we mark on our human maps by drawing a line from the hypocenter-the actual point in the Earth where an earthquake begins-up to the surface. To the average person on the surface of the Earth, the epicenter means everything. The three-dimensionality of geology is essential for understanding earthquakes. Hayward and Calaveras faults ( Janet Watt) (In fact, these diagrams suggest that the two faults should be considered as one.) Geologists have begun modeling this three-dimensionality, as in the following diagram of an area where the Hayward and Calaveras faults come together, near San Jose. Alden said to think of faults almost like a bedsheet. They might run for tens or even hundreds of miles, and drop miles down into the crust, but they are not very wide at all. That difference will have to be made up for, eventually, and that’s what earthquakes do.įaults are as narrow as the lines we use to paint them onto maps. Troublingly, the recent observed “creep” of the plates past each other along the fault is less than half of the long-term average of about 10 millimeters. On geological timescales-like 100,000 years-a hill that would have once been adjacent to my house could now be half a mile south of it. “And the fault is what separates it.”Ĭonsider that for thousands of years, the areas on either side of the fault have been sliding past each other at a rate of a few millimeters a year. “Oakland is a real upstairs-downstairs city,” says Andrew Alden, a science writer, formerly of the USGS, who focuses on Oakland’s geology in particular. Their geology is far more complicated, with different types of rocks bent and knuckled into place. Most of the flatlands of the East Bay are made of similar stuff, which lies over the top of what’s known as the Franciscan Complex, a heterogeneous collection of rock dating to the time of the dinosaurs tens of millions of years ago.Įast of the Hayward Fault rise the hills, which are considered part of the Diablo Range, a complex set of low peaks that have resulted from the pressure of the tectonic plates coming together on the coast. On the maps, geologists call it “Qhaf”: “alluvial fan and fluvial deposits (Holocene).” The Holocene is the most recent geological era, which includes the present. In geological terms, I live west of the Hayward Fault, on rocks washed down off the hills in recent times. In human terms, I live in north Oakland, near the border with Berkeley. Of all the spots on the Earth reacting to the forces of tectonic motion, this had been the one to go. By the looks of it, the quake had struck on the backside of Alta Bates Summit Medical Center, the place where our kids had been born, and a place that we walk by nearly every day. Geological Survey showing the epicenter of the earthquake. A few days later, she sent me a map from the U.S. My wife ordered MREs from a prepper site. It was strong enough to make us wonder, for a few seconds, if this was the big one.Īfter it passed, we resolved to get another flashlight. It hit in the evening, a couple hours after my wife and I had put the kids to bed. It’s easy to forget that one strand, the Hayward Fault, runs the whole length of the East Bay, cutting under Berkeley and Oakland, just a mile from my house, and that there is a one-in-three chance that it will produce a devastating earthquake before I’m a senior citizen.īut then there are days like January 4, when a magnitude 4.4 quake struck. Most days, it’s easy to forget that coastal California sits at the boundary of two tectonic plates-the Pacific and North American-which are slowly sliding by each other, creating the San Andreas complex of faults.
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