Southern Californians live in earthquake country, and scientists expect that in the next few decades the southern part of the major San Andreas Fault may experience an earthquake as large as magnitude 7.7 — the “big one.”
In a recently published paper in Geophysical Research Letters, earthquake researchers from the Southern California Earthquake Center (SCEC) Community Modeling Environment (CME) report on large-scale earthquake simulations, known as TeraShake, that produced important insights into how a massive earthquake can be expected to impact the region. The paper was written by Kim Olsen and Steve Day of San Diego State University (SDSU); Jean-Bernard Minster of the Institute of Geophysics and Planetary Physics (IGPP) at UC San Diego; Yifeng Cui, Amit Chourasia, Marcio Faerman, and Reagan Moore of the San Diego Supercomputer Center (SDSC) at UCSD; and Philip Maechling and Tom Jordan of the University of Southern California.
The scientists reported that for earthquake scenarios on the San Andreas Fault in which the rupture begins in the southeast and runs northwest, the chain of sedimentary basins that runs between the city of San Bernardino and downtown Los Angeles forms a “waveguide” that effectively channels large seismic waves along the southern edge of the San Bernardino and San Gabriel mountains and into the Los Angeles area.
“From earlier work we expected a northwest-moving earthquake rupture on the southernmost San Andreas fault to produce stronger and longer-lasting shaking in the Los Angeles area, as compared to a southeast-moving rupture. But we didn't expect this strong 'waveguide' effect for the northwest-moving rupture, which produced intense amplification of seismic energy in local areas,” said lead author and associate professor of geological sciences at SDSU, Kim Olsen. “You can think of the way this generates local earthquake amplification effects as similar to what happens with waves in rapids when a river narrows to a smaller width — the size of the waves increases when the same amount of energy has to pass through a smaller cross-section of the river channel.”
In addition to advancing basic earthquake science, the detailed simulations can eventually help structural engineers design more earthquake-resistant structures. The authors emphasize that future research, in particular earthquake scenario simulations, should take into account the critical role of sedimentary basins in order to more accurately estimate the seismic hazards of major earthquakes in the region.
The highly detailed TeraShake simulations modeled earthquake shaking for alternate scenarios in which the rupture moved northwest or southeast along the same 200-kilometer stretch of the San Andreas Fault. The model area included a vast region 600 km by 300 km by 80 km deep extending from northern Santa Barbara County down to northern Mexico. The simulations, using the Anelastic Wave Model (AWM), a fourth-order finite difference code developed by Olsen, were conducted at a very high spatial resolution of 200 m with about 1.8 billion grid points.
The computations ran for up to four days on 240 processors of SDSC's DataStar supercomputer, the longest runs producing an unprecedented 47 terabytes of output data — more than four times the digital plain text equivalent of the printed collection of the Library of Congress. All this data required a complex choreography of data movement between DataStar, disk, and archival storage. The data collection is archived at SDSC in the SCEC Digital Library, managed by the SDSC Storage Resource Broker (SRB), where it is easily available to researchers for further analysis.
The ground-breaking simulations pushed the envelope of data-intensive computing, and were only possible through a large-scale collaboration among 33 earthquake scientists, computer scientists, and others from eight institutions, including more than 20 SDSC staff with expertise in data cyberinfrastructure spanning areas from computational science and high performance computing and storage to data management and visualization. In addition to SDSC, SDSU, IGPP/UCSD, and USC, participating institutions include the Information Sciences Institute (ISI), the University of California Santa Barbara (UCSB), Carnegie Mellon University (CMU), and the US Geological Survey (USGS).
Working closely with the scientists, SDSC visualization experts produced sophisticated images of the simulations, which helped the researchers make sense of the vast 3-D data collection in order to monitor and improve the simulations and understand the scientific findings.
Olsen, K. B., S. M. Day, J. B. Minster, Y. Cui, A. Chourasia, M. Faerman, R. Moore, P. Maechling, and T. Jordan (2006), “Strong shaking in Los Angeles expected from southern San Andreas earthquake,” Geophysical Research Letters, 33, L07305, doi:10.1029/2005GL025472. (subscription)
Source: San Diego Supercomputing Center; Paul Tooby, SDSC senior science writer.