Road workers in Hayward, California, repaired a displaced curb that was a prime example of Hayward Fault activity. Seismologists have been monitoring this curb for over 45 years.
The famous curb was located at the intersection of Rose and Prospect Streets. It gradually shifted relative to another curb, which aroused great interest among scientists. From 1974 to 1979, the curb along with a part of the road moved about two centimeters. Over time, the curb slabs lying next to it completely stopped touching.
However, local authorities decided to repair the road and installed a wheelchair ramp at the site of the shift. It turned out that the city administration simply did not know about the importance of this place. “If we knew about the shift, we would probably look at the curb differently, and even try to help scientists document it,” said Assistant Mayor Kelly Makadu.
"It is sad. It was a real disappointment. Indeed, it was unusual to have such evidence of a rift right here. Now the scientists are left with only photographs that document the sidewalk sliding over the years, "Auckland science journalist Andrew Alden wrote in a blog post.
This shift most clearly demonstrated the movement of the Pacific and North American tectonic plates, but there was other evidence of underground activity. In addition to displacement of curbs, seismologists observed cracks in the asphalt, divergence of columns in the colonnade of the sports stadium, and other signs. Scientists collected more accurate data using high-precision sensors installed at the boundaries of the fault.
The study of the displacement of the curb was very important for scientists, since the movement of the Pacific and North American tectonic plates in the near future may cause a strong earthquake. In addition, the displacement of these plates caused a much more dangerous fault - the San Andreas fault. While the Hayward Fault is part of the overall movement between the plates, the San Andreas Fault is a major transformation of the Pacific / North American plate boundary.
San Andreas Fault
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This fault extends 1,300 kilometers along the coast of California, mostly overland. The fault goes deep into about 16 kilometers. The thickness of the lithospheric plates is about 100 kilometers. They drift on liquid lava, crawling one on top of the other, causing earthquakes and other cataclysms.
San Andreas Fault. Aerial photograph
Photo: Ikluft /wikipedia.org/GFDL
The edges of the two slabs in the San Andreas Fault resemble poorly fitted cogwheels. They rub against each other, and the frictional energy generated along their boundaries does not find a way out. In places where the movement of the plates is relatively free, the accumulated energy is released in thousands of small shocks. They cause almost no damage and are recorded only by sensitive instruments.
In other places, the plates are pressed quite tightly, and when they move, they immediately release powerful energy. Then earthquakes occur with a magnitude of at least 7 on the Richter scale. Such an earthquake could occur in the next 50 years, seismologists say. It can cause billions of dollars in damage and take up to 20,000 lives.
Double break
San Andreas is considered the most likely site for the next seismic event in the next few decades. However, the disaster could be more devastating if San Andreas activity interferes with the San Jacinto Fault, which runs through San Bernardino, Riverside, San Diego and Imperial County in Southern California.
Juliana S. Lozos, associate professor of geological sciences at the University of California at Riverside, found that a similar event happened about 200 years ago. It caused a powerful shock that was felt over a large area from Los Angeles to San Diego. The 7.5 magnitude San Juan Capistrano earthquake on December 8, 1812 was the result of the simultaneous rupture of two faults, Lozos said.
California coast. Lines on the right represent the San Jacinto Fault and on the left the Elsinore Fault
Photo: NASA
Previously, it was believed that the tremor was caused by the San Andreas Fault. However, computer simulations showed that the earthquake began to the south - in the San Jacinto area, and then involved San Andreas in the disaster. The activity of two faults simultaneously can be very dangerous for California. The state's infrastructure has been built to account for the tremors caused by one fault. The consequences of a simultaneous rupture can be unpredictable.
Cascadia Rift
The Cascadia Fault, which stretches 900 kilometers from Vancouver Island to Northern California, also poses a serious threat to the United States. Cascadia is located at the junction of the oceanic plate and the plate of the North American continent. A plate from the ocean squeezes the continental one, as a result of which it contracts by 30–40 mm annually.
According to seismologists, sooner or later, the pressure between the plates will accumulate to the limit, after which a strong shock will occur, leading to a mega-earthquake of magnitude from 8.7 to 9.2. The push will cause a giant wave, some of which will even reach Japan. The wave can rise to a height of 30 meters, seismologists say. According to the US Emergency Management Agency (FEMA), Cascadia could cause the death of 13 thousand people.
Cascadia zone joints. A volcanic ridge formed after the fracture of the tectonic plate is marked in red
Photo: NASA
Seismologists consider Cascadia more dangerous than San Andreas, since the movement of Cascadia will entail not only an earthquake, but also a giant tsunami. In addition, 45 years ago, scientists did not know about the existence of this fault. Therefore, the United States is not ready for such destructive events. The country's authorities have begun to conduct large-scale exercises in the event of a disaster in the Cascadia subduction zone. FEMA plans to do them in the future.
New Madrid fault
North of the American state of Alabama is in the zone of influence of the New Madrid Fault. This fault is about 20 times the size of San Andreas. The last earthquake in this seismic zone occurred in 1812. Recently, however, activity along the fault line has begun to increase.
Engraving depicting the New Madrid earthquake
Photo: wikipedia.org / Public Domain
“I think most people know that an earthquake can happen here, but they just can't remember the last time they were shaken,” said Gary Patterson, a geologist at the Center for Earthquake Research and Information in Memphis. The earthquakes that occurred earlier in this region were felt at a distance of 1000 to 1200 kilometers from the epicenter, the scientist noted.
According to the FEMA scenario, more than 900 people in Alabama could be affected by a 7.7 magnitude earthquake. In general, 86 thousand residents can suffer in the United States. Computer simulations based on the 1812 earthquake showed that a repetition of the same seismic event is possible in the next 50 years.