Clues to the next big earthquake are right under this woman's feet
Kate Scharer uses everything from backhoes to trowels to reach deeply for evidence of earthquakes that have occurred over time. Along with colleagues from the University of Oregon and U.S. Geological Survey, this Appalachian State University geologist has documented dozens of earthquakes that occurred along the southern San Andreas Fault since approximately 3,000 B.C.
Their research provides valuable data to help understand risks from earthquakes in the years ahead and to update national seismic hazard maps.
The maps are used by insurance companies to set rates and by engineers when designing structures.
"Data collected by our team and others were used quite extensively in the latest hazard maps, which were released in April," said Scharer, an assistant professor in the Department of Geology.
"It's exciting to see your work used this way. These maps ultimately will guide allocation of earthquake mitigation funds, set insurance rates in California and direct long-term planning for infrastructure."
Scharer said the seismic hazard maps are used by engineers to design buildings that can handle the amount of shaking expected in earthquake prone areas.
Understanding the earthquake odds
"One of the best reasons to study the history of earthquakes is to know what your odds are for earthquakes in the future," Scharer said.
The data she and others have collected along the San Andreas fault indicate that Southern California is due for a major earthquake sooner than many scientists originally thought. Last year she began research on a stretch of the fault north of Los Angeles that is poorly understood. "Although preliminary, the data show the frequency of large earthquakes was much quicker than expected. Because these results would strongly increase hazard estimates, we will continue work there to make sure these initial observations are correct," she said.
The San Andreas fault is a boundary between the Pacific Plate and the North American Plate, two of many plates that form the crust of the earth. The plates move slowly over time and create fault zones where they move past each other. The San Andreas fault runs 800 miles, with its southern section extending from Parkfield, Calif., to the Salton Sea.
To uncover and document evidence of past, large-magnitude earthquakes, researchers use a backhoe to cut long trenches through the sediment, oftentimes 20 feet deep.
"When a rupture happens, the ground surface gets torn and ripped up," Scharer said. “Sediment, such as gravel or sand, later covers and preserves the rips. We look for areas where sediment has been deposited quickly.” Scharer and students in the Department of Geology date the sediments with radiocarbon techniques to determine when the prehistoric earthquakes occurred.
Geological records indicate that the average time between major earthquakes along the southern San Andreas fault is about 100 to 120 years. "Although longer gaps of 200 years have occurred, by all estimates we are beyond the average at this point. That has hazard mitigation and public policy planners alert," she said.
Temblors happen more often than you think
While large-magnitude earthquakes receive close media attention, several million earthquakes occur each year worldwide. According to the U.S. Geological Survey, an estimated 13,000 earthquakes ranging from magnitude 4 to 4.9 occur yearly. Among the more intense earthquakes, there are 1,000 occurring every year at magnitude 5 to 5.9. At least one quake at magnitude 8 or higher occurs each year.
The 7.9 magnitude earthquake that hit China in May occurred along a fault created some 40 million years ago when the continents of India and Eurasia "smashed" into each other, creating the Himalayas and the Tibetan Plateau, Scharer explained. The two continents continue to bump into each other, which shortens the distance between the land masses by about 2 inches each year.
China's earthquake occurred along a thrust fault, which tends to focus high levels of shaking along the rupture. If a 7.9 earthquake were to occur in California, it would affect a larger area because the San Andreas is a slip fault that moves horizontally.
"A 7.9 might be the kind of magnitude we are talking about occurring in the southern San Andreas Fault. Whereas heavy shaking occurred for about 200 kilomters along the fault in China, the same magnitude quake would induce heavy shaking for 400 kilometers along the San Andreas fault," Scharer said.
Based on evidence from geologic deposits exposed in the research team's trenches, the average time between major earthquakes along the southern San Andreas Fault is 100 to 120 years. The historic record includes two earthquakes:
- 1812 - 7.5 magnitude (estimated)
- 1857 - 7.9 magnitude (estimated)
"It's been 150 years since the 1857 earthquake occurred. That span is what concerns geologists," says assistant professor Kate Scharer.