When boulders the size of refrigerators recently fell on the American highway 395 in the very sparsely populated eastern California, it was not only the drivers who were surprised because they suddenly had to drive around the boulders on a slalom course. Many geologists were also initially surprised at the cause of this rock fall. The trigger was found quickly: Once again, a magnitude 6 earthquake struck the east side of the Sierra Nevada, the vast mountain range that separates California from the rest of the United States over a length of more than 600 kilometers. The quake in the Antelope Valley, south of Lake Tahoe on the border with the state of Nevada, occurred almost two years after a severe series of earthquakes near the desert town of Ridgecrest, also east of the huge mountain range.In fact, more severe earthquakes have occurred in this area in the past three decades than along the famous San Andreas Fault, the “classic” earthquake zone in California.
The landscape of the most populous American state is mainly characterized by the tectonic collision of two huge plates on the earth's surface, which has been going on for more than 30 million years, namely the North American plate, which is slowly drifting west, and the Pacific plate, which is moving much faster in a north-westerly direction. The long-term average of the Pacific moves by five centimeters per year relative to the North American continent.
The 1200 kilometer long San Andreas Fault is commonly considered to be the boundary line between the North American and Pacific plates. It stretches from the Salton Sea, a salty inland lake in the extreme south of California, to Cape Mendocino in the north of the state. Along this largely coastal fault, the Pacific Plate scrapes horizontally past its North American counterpart. As a result, there are repeated earthquakes there.
Although the San Andreas Fault is shown as a solid line on most maps, it is often a zone several tens of kilometers wide with several earthquake faults running parallel to one another.
In the greater San Francisco area, for example, there are other seismically active tectonic boundary lines in addition to the San Andreas Fault, such as the Hayward and Calaveras Faults.
All of California under tectonic tension
Along the flanks of each of these lines, the relative movement of the opposing rocks accurately reflects the drift between the Pacific and North American plates. The western flank moves to the northwest, the eastern flank to the southeast. In detailed geodetic measurements, geoscientists were even able to measure how quickly the clods move on the individual faults. The result was surprising: while the Pacific shifts about five centimeters to the northwest every year, the sum of the shifts along the parallel faults is only about four centimeters per year. What happens to the rest of the plate movement of one centimeter per year?
This drift takes place east of the Sierra Nevada. Geodetic measurements carried out between different points in Nevada and further east on the North American "mainland" and along the mountain range showed that the mountain range shifts by almost ten millimeters to the northwest every year - that is, by exactly the missing centimeter . A research group led by Ian Pierce from the University of Nevada at Reno has now
analyzed these shifts in detail
in the journal
Tectonics
published by the American Geophysical Union
.
On the one hand, the faults lead to the many significant earthquakes in eastern California. These take place along a zone that extends from the Mojave Desert in the south to the latitude of Lake Tahoe and the city of Reno in Nevada. In the past thirty years, a number of earthquakes with magnitudes greater than 7 and dozens of minor tremors have been recorded - such as the recent earthquake series in Antelope Valley. Geologists have now also given this earthquake zone a name, namely "Walker Lane", named after a small river and lake in western Nevada.
This means that not only the western part of California, but the entire state is under the tectonic tension triggered by the plate drift. So it's not just the west of California that is slowly shifting towards the inland. Los Angeles could one day become a suburb of San Francisco. The entire state is also gradually being torn open as the west is pushing northwest at least three times as fast as the east. Like all such large-scale geological processes, this tearing apart takes place extremely slowly, so that no person living today will ever experience this rupture. The frequent severe tremors on the eastern side of the Sierra Nevada are an unmistakable sign that tectonic forces are also constantly pulling on it far in eastern California.