Simulations predict ground motion of earthquakes on the Bay Area’s Hayward fault

Newswise – The Hayward Fault, part of the larger San Andreas Fault System, extends 74 miles across the East Bay of the San Francisco Bay Area. The fault was too late for an earthquake that could cause serious damage to such a densely populated area.

In a recent study, published in Earthquake Research Letters, scientists at Lawrence Livermore National Laboratory (LLNL) and Lawrence Berkeley National Laboratory (LBNL) simulated earthquake scenarios on the Hayward fault and analyzed how rupture and underground structure affect the intensity of ground motion along the fault.

“Using high-performance computer earthquake simulations, we are interested in knowing what the ground motion of a typical magnitude 7 earthquake in the Bay Area would be, so we can work with engineers to ensure that infrastructure and buildings in the Bay Area can withstand this motion,” said author and LLNL scientist Arben Pitarka. “Thanks to this information, we can also begin preparations to retrofit some vulnerable structures.”

According to geological evidence, magnitude 7 earthquakes occur relatively frequently on the Hayward fault. The USGS estimates a 14.3% chance of a magnitude 6.7 or greater event occurring by 2034 and a 33% chance before 2043, making the Hayward Fault the most dangerous in the Bay Area.

To prepare for the worst, scientists must understand the seismic risk in the region. Under a Department of Energy (DOE) project led by LBNL, the team created a simulation platform that includes two main modeling components: one that simulates the generation of seismic waves during earthquake rupture and another that propagates seismic waves through the Earth.

“Because we do not know exactly how rupture will develop during an expected earthquake — including the total slip of the fault, the location of the collapse epicenter and the speed of rupture — we rely on a large number of simulations in which we vary many modeling parameters, including those of the earthquake rupture model,” Petarca said. “These large-scale simulations allow us to reduce the uncertainty in ground motion estimates associated with earthquake rupture.”

LBNL and LLNL researchers used the Department of Energy’s exascale computing infrastructure to simulate magnitude 50 earthquakes on the Hayward Fault.

“Thanks to this new database, we can not only provide better estimates of the ground motion expected from this type of earthquake, but we can also identify areas susceptible to very strong tremors in the San Francisco Bay Area,” Petarca said.

Sites near the Hayward Fault are at high risk, especially those located within a cone-shaped zone along the fault that begins at the epicenter and spreads out on either side of the fault. Due to the pulse-like nature of ground motion caused by the so-called “rupture direction effect”, this area will experience strong shaking that is much more intense than usual and particularly dangerous for tall or flexible buildings. Empirical ground motion prediction models are not well constrained to account for this effect.

The simulations also revealed other regions vulnerable to increased ground motion intensity: sedimentary basins and small basins. For these bowl-shaped depressions in the shallow crust, such as the Livermore Basin, ground motion is amplified regardless of the location of the earthquake.

Going forward, the team aims to expand its simulations to include models of earthquakes on the San Andreas Fault, which can produce earthquakes of magnitude 7.5 or larger. The larger the earthquake, the more widespread the ground motion is, and the more computational power is required to model it. They also push models to pick up seismic waves of higher frequencies, which requires very detailed knowledge of the Earth’s surface.

Closer to home, LLNL seismologists hope to improve understanding of seismic hazards at the Livermore site, where a large earthquake on the Hayward Fault or a medium-sized earthquake on the nearby Grenville Fault could cause significant damage to equipment sensitive to strong ground vibrations.

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