San Andreas Fault stress hits 1,000-year high, raising quake risk

Tectonic stress along Southern California’s San Andreas and San Jacinto fault systems has reached the highest levels seen in the last 1,000 years and has exceeded them in some areas, according to new research led by geoscientists. University of Hawaii at Mānoa.

The researchers say the system shows no signs of imminent rupture, but is operating under unusually high stress in a long-term seismic cycle that could support large earthquakes, including multiple fault events.

Published in the study Journal of Geophysical Research: Solid Earthsuggests that the region is in a “critically loaded state” with stress build-up in multiple fault segments. One key area of ​​focus is the Cajon Gap, a junction between two fault systems that can act as an “earthquake gate,” either preventing ruptures from passing between faults or allowing them to connect to a single, larger event.

More than 160 years since the last major rupture, the system is highly stressed, said lead author Liliane Burkhard, a research fellow at the Institute of Geophysics and Planetology at the University of Hawaiʻi and a scientist at the University of Bern.

“Currently, the system is critically stressed with stress across the region at historically high levels and more than 160 years since the last major rupture,” Burkhard said.

Because of its size and proximity to major population centers such as Los Angeles, San Bernardino, Riverside and the Coachella Valley, a rupture involving both fault systems could cause much more damage than a single-fault earthquake, researchers say.

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Can the San Andreas Fault break completely?

The San Andreas Fault is not a crack that could “rip open” and pull most of California away from the continent. This is a strike-slip plate boundary, meaning the Pacific Plate and North American Plate are sliding horizontally without separating from each other. US Geological Survey.

In other words, even in a very large earthquake, lateral movement along the fault occurs, not disintegration of the land mass. Parts of California may suddenly shift by feet or even tens of meters during a major rupture, but both sides remain part of the same crustal system.

There are other tectonic settings on Earth (such as the East African Rift, which is in the process of splitting into two tectonic plates) where continents are pulling apart and eventually forming new oceans, but the San Andreas system is not one of them. It is a strike-slip boundary where plates slide over each other rather than moving away from each other.

But continued movement along the fault over millions of years could bring Los Angeles and San Francisco closer and closer. geological models suggest.

What happens if the San Andreas Fault ruptures?

Surface rupture occurs when movement along a fault deep within the Earth pushes the surface to the surface. according to Pacific Northwest Seismic NetworkIt is one of the clearest physical expressions of an earthquake, showing the permanent deformation that occurs when two sides of a fault cross each other.

Most earthquakes do not cause surface rupture. US Geological Survey. Some faults do not reach the surface, and even if they do, the rupture does not always propagate upward during a given event.

When surface rupture occurs, horizontal or vertical slides may occur, depending on the type of fault. Strike-slip faults, such as the San Andreas Fault, typically cause horizontal displacement, while dip-slip faults can cause vertical displacement. Some earthquakes involve a combination of both.

In a large rupture, strong shaking can last from tens of seconds to a minute. The most severe damage will occur near the fault and in areas built on soft or water-saturated soils; This can increase shaking and increase the risk of liquefaction. Scientists note that infrastructure built along active faults is particularly vulnerable because surface rupture can directly offset roads, buildings and other structures spanning the fault trace.

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San Andreas Fault map

What causes earthquakes?

The Earth has four layers: inner core, outer core, mantle and crust. The crust and upper part of the mantle form another area called “”.lithosphere“acting like a skin surrounding the Earth’s surface”.

But the lithosphere is not a single piece, but is divided into puzzle-like pieces called tectonic plates, according to the USGS. These parts of the lithosphere are not stable and move slowly.

As tectonic plates pass each other, they occasionally bump or collide. This puts pressure on the edges of the plates. When the stress becomes too great, cracks called “faults” form. The point where these faults move against each other is called the “fault line”.

When there is too much friction between fault lines, energy is suddenly released, triggering seismic waves that cause earthquakes.

Why are earthquakes so common in California?

California is located at the boundary between two major tectonic plates, the Pacific Plate and the North American Plate. These plates are in constant motion and slide over each other at different speeds.

The state is also home to more than 500 active faults, making it one of the most earthquake-prone areas in the United States, according to geologists. The most famous is the San Andreas Fault, a nearly 800-mile-long system that runs through much of the state and forms the primary boundary between two plates.

Other important fault systems include the Hayward Fault in the Bay Area, the Calaveras Fault in Central California, and the San Jacinto and Elsinore faults in Southern California.

But earthquakes are not limited to land. Many occur offshore, including near the Mendocino Triple Junction where the Pacific, North American, and Gorda plates meet. This junction forms one of the most seismically active offshore areas along the West Coast.

Can earthquakes be predicted?

Earthquakes are not predicted. Both of them US Geological Survey nor have scientists predicted a major earthquake, and there is no expectation that this will change in the near future.

What is California’s Earthquake Early Warning system?

California’s Earthquake Early Warning system uses seismic sensors, ground motion monitoring and warning technology to offer people alerts via cellphones before the strongest tremors hit. official website.

Officials say even a brief warning can allow people to take protective actions such as “Drop, Cover and Hold” or put devices and systems into safe mode. In some cases, especially for those closer to the epicenter, warnings may come just as shaking begins or after the quake has passed.

The system is delivered through multiple channels, including Android Earthquake Alerts, Wireless Emergency Alerts, and the MyShake app.

Developed by the UC Berkeley Seismology Laboratory, MyShake is a free smartphone app that provides audio and visual alerts for earthquakes, typically magnitude 4.5 or greater, or when weak shaking is expected. It is available on select computer platforms as well as iPhone and Android devices.

What to do during an earthquake

The classic evergreen box that fits well into any earthquake story.

• Get on your hands and knees

• Cover your head and neck under sturdy furniture if possible

• Hold until the shaking stops

• Stay indoors unless you are near a known coastal tsunami risk area

Stay informed. Get weather alerts via text

Brandi D. Addison covers weather across the United States as the USA TODAY Network’s Weather Connect Reporter. He can be reached at baddison@gannett.com..

This article first appeared on USA TODAY: The San Andreas Fault is at its highest stress in 1000 years and what does it mean?