Research to a Fault
An Appalachian State University geophysicist is part of a team updating data on an area of Southern California recently designated as a special fault study area because of its potential for a significant earthquake.
Scott T. Marshall uses mathematical computer modeling to solve problems related to plate tectonics and associated earthquakes. He is the lead author of a paper recently published in the Journal of Geophysical Research that looks at fault slip rates and patterns of interseismic deformation in the Ventura Basin of Southern California.
He also is one of four co-leaders of a research team funded by the Southern California Earthquake Center to determine the seismic potential of this densely populated region.
Marshall is head of the team’s geodesy group that uses modern satellite technology to track slow and ongoing motions of the ground due to fault movements in the basin. The group also aims to update earthquake data and better understand off-shore faults located in the area. The SCEC recently designated the area as a special fault study area because of newly uncovered geologic evidence of several large earthquakes in the past.
“There is very strong evidence suggesting that the Ventura fault system has experienced several magnitude 8 earthquakes in the past few thousand years,” Marshall said. In comparison, the next “big one” on the San Andreas is expected to be in the magnitude 7.7 to 7.9 range.
Given that the Ventura fault is much closer to major population centers in southern California, Marshall said, “Maybe the San Andreas isn’t the most dangerous fault in town.”
Marshall creates computer simulations of the earth’s movement in the Ventura area by using data transmitted from GPS stations permanently bolted in the ground and digital elevation records of earth movement recorded by InSAR, a type of satellite-based radar technology.
There are at least 45 faults in the Ventura Basin that are being compressed due to the ongoing plate motions. The faults are slow moving — about one to 8 mm per year — and can typically only be detected though changes in the GPS and InSAR data.
“The ground motions in the Ventura Basin are very slow, but we can use them to determine where the faults are and how fast they move,” Marshall said. “You might think, ‘Who cares about small motions like this,’ but that’s what drives huge earthquakes.”
By comparison, the San Andreas Fault moves about 20 to 35 mm per year.
His research shows that strain (squishing) in the fault system is occurring much faster than expected to the east of the city of Ventura. He and others on the research team will use updated satellite data to determine if this is being caused by an existing fault located offshore or the result of a new and previously unidentified fault system.
Advances in computer technology make this type of research possible, Marshall said.
“We couldn’t do anything like this even 10 years ago because computers couldn’t handle the amount of data,” he said. “It is an exciting time to be a geologist.”
The location of the faults in the Ventura is also problematic because of the local geology.
The Ventura Basin contains one of the thickest sedimentary basins in the world, meaning that the subsurface rocks are very loose and soft. Because soft rocks shake more violently and for longer durations, the damage from a major earthquake (more than magnitude 7) in this region could be devastating, he said.
“If the Ventura fault ruptures in a magnitude 8 earthquake, like we think it has several times in the past, it’s not only rupturing closer to population centers, it’s rupturing directly in the sedimentary basin which shakes much longer and stronger than the stiffer rocks that make up the nearby mountain ranges,” Marshall said.
Mathematical models from other SCEC researchers show that shaking from a magnitude 7.8 earthquake on the distant San Andreas fault, about 40 to 100 miles away, could last four minutes or more. Earthquakes generally last from a few seconds to a few minutes. The Loma Prieta earthquake that occurred during the 1989 World Series had a magnitude of 6.9 and the initial quake lasted about 20 seconds.
The last significant earthquake in southern California was the 1994 magnitude 6.7 Northridge earthquake which, at the time, was the nation’s most costly natural disaster. If the Ventura fault does produce a magnitude 8 event, it will release about 90 times more energy than the Northridge event.
“The best way to prevent the loss of life is to study the past and present behavior of faults,” Marshall said. “This gives us an idea of what to expect, so we can make appropriate planning decisions.”