Drop, Cover, Hold

Researchers Create 3D Images that Shed New Light on the Activity Below the Earth's Surface

 
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Left: Diagram of perspective view of basement depths in the Imperial Valley at the southern end of the San Andreas Fault system overlain by faults (black lines) and earthquakes (purple dots). Hot colors show deeper basement, i.e., thicker sediment fill and possibly more severe ground shaking in the event of a large earthquake. Blue-filled area is the Salton Sea. Above: LSU seismologist Patricia Persaud.Photo Credit: April Buffington

3D ImageThe idea of large devastating earthquakes may be hard to fathom for those living outside of California, but for the residents of “Earthquake Country,” ground rumbles are the norm. According to the U.S. Geological Survey, California has about 10,000 earthquakes each year, but most are too small to be felt.

Natural phenomena like these have always fascinated Patricia Persaud. An accomplished seismologist and the newest faculty addition to the Department of Geology & Geophysics at LSU, she is helping to shed new light on the movements below the Earth’s surface.

Persaud is one of the researchers contributing to the Salton Seismic Imaging Project (SSIP), a collaborative effort between U.S. and Mexican institutions, and the U.S. Geological Survey funded by the National Science Foundation and the U.S. Geological Survey. This project has provided detailed, subsurface 3D images of the Salton Trough of southern California and northern Mexico and gives geoscientists a better understanding of the Earth’s structure close to the San Andreas fault.

“The surface of the Earth is made up of many pieces called plates that fit together like a puzzle,” Persaud said. “The plates are constantly moving though very slowly, maybe just a couple of centimeters each year. Over geologic time, this slow and constant motion adds up; mountains grow and ocean basins form and almost all of the action is occurring at the edge of plates. Coincidentally, that’s where humans like to live too.”

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Patricia in one of her field areas in Southern CaliforniaPhoto Credit: Patricia Persaud

In 2011, the SSIP strategically placed 4,000 seismometers in the ground in southern California and New Mexico. The overall goal of the project was to measure the speed and amplitude of seismic waves that traveled from a set of 126 shallow underground explosions to the seismometers, which measure ground motions due to the passage of seismic waves. Seismic waves are produced by earthquakes, volcanic eruptions and other seismic sources. After three weeks, the results were retrieved from the seismometers and processed in a computer program to create an image much like the x-ray of a bone. The dataset allowed the SSIP team to identify important unmapped faults or buried interfaces and will help refine the seismic hazard for parts of Imperial County, California, where the population is projected to almost double by 2060.

“California is earthquake country and the San Andreas Fault is long overdue for a large earthquake, something like a 7.8 in magnitude,” Persaud said.

To put this into perspective, the Sumatra earthquake in 2004, with a moment magnitude of 9.1 to 9.3, is considered one of the deadliest natural disasters in recorded history. This quake triggered a series of deadly tsunamis killing approximately 280,000 people in 14 countries.

Persaud said scientists do not know exactly when the next big earthquake will happen, but they can predict which areas may feel the greatest impact by producing computer models of scenario earthquakes. “There is certainly an earthquake hazard component to this project,” Persaud said. “An estimate of the hazard of an earthquake is related to how much strong ground shaking is going to occur. So, if you have an area of soft sediments, this area is going to shake more than those areas overlying hard rock. Our data will help identify particular areas and this information will help to improve hazard estimates.”

 Shake maps are real-time maps of ground motion and shaking intensity after significant earthquakes. Federal, state and local organizations use these maps for post-earthquake response and recovery and for disaster planning.

 The next phase of the SSIP will include an analysis of the data collected to the north in the Coachella Valley.

 “We know for sure that when the San Andreas fault produces another major earthquake that the Coachella Valley will receive pronounced shaking because it is just north of where the San Andreas is predicted to rupture,” Persaud said.