Simulated 800,000 years of California earthquake history to determine hazards

IMAGE: A 3,000-year randomized clip from the California Physics-Based Seismic Simulation Catalog, created on Frontera. Show more

Credit: Kevin Milner, University of Southern California

Fortunately, massive earthquakes are rare events. But this paucity of information blinds us in some ways to its risks, especially when it comes to identifying the risks to a particular site or structure.

“We haven’t observed most of the potential events that could cause significant damage,” explained Kevin Milner, a computer scientist and seismologist at the Southern California Earthquake Center (SCEC) at the University of Southern California. “Using southern California as an example, we haven’t had a really large earthquake since 1857 – this was the last time southern San Andreas had a massive 7.9 magnitude earthquake. The San Andreas earthquake could affect a much larger area than the 1994 Northridge earthquake and others could have.” Also from large earthquakes.

The traditional way to overcome this data shortage involves digging trenches to find out more about past ruptures, gathering information from lots of earthquakes around the world and creating a statistical risk model, or using supercomputers to simulate a specific earthquake in a particular location with a high degree of loyalty.

However, a new framework for forecasting the likelihood and impact of earthquakes on an entire region, which has been developed by a team of researchers associated with SCEC over the past decade, has found common ground and possibly a better way to ascertain the risks.

A new study led by Milner and Bruce Shaw of Columbia University and published in the Bulletin of the American Seismological Society in January 2021 provides results from a state-rate earthquake simulation prototype, or RSQSim, which simulates hundreds of thousands of years of earthquake history in California. Combined with another symbol, CyberShake, the tire can calculate the amount of shaking that would occur for each earthquake. Their results compare well to historical earthquakes and the results of other methods, and show a realistic distribution of earthquake probabilities.

According to the developers, the new approach improves the ability to quantify an earthquake that may occur at a specific site, allowing building code developers, architects and structural engineers to design more flexible buildings that can withstand earthquakes at a specific site.

“For the first time, we have a complete pipeline from start to finish where earthquakes and simulations of ground motion are based on physics,” Milner said. “It can simulate up to 100,000 years on a really complex fault system.”

Apply immense computer power to big problems

RSQSim converts mathematical representations of the geophysical forces that play a role in earthquakes – the standard model of how a core ruptures and propagates – into algorithms, and then resolves it on some of the most powerful supercomputers on the planet. Extensive computational search was enabled over several years by government-sponsored supercomputers at the Center for Advanced Computing in Texas, including Frontera – the most powerful system of any university in the world – Blue Waters at the National Center for Supercomputing Applications, and a summit conference at the Oak facility Ridge Leadership Computing.

“One of the ways we might be able to do better at predicting risk is through physics-based modeling, by harnessing the power of systems like Frontera to run simulations,” Milner said. “Instead of an statistical distribution, we simulate the occurrence and propagation of earthquakes.”

“We have made a lot of progress at Frontera in determining what kind of earthquakes we can expect, which error, and how often,” said Christine Goulet, executive director of applied science at SCEC, who was also involved in the work. “We do not classify the code nor tell it when an earthquake will happen. We run a simulation for hundreds of thousands of years, and let the code transfer pressure from one fault to another.”

The simulations began with the geological topography of California and more than 800,000 hypothetical years have simulated how pressures form and dissipate as tectonic forces act on Earth. Through this simulation, the framework produced an index – a record of an earthquake occurring in a specific location of a certain size and characteristics at a given time. Gault said the catalog produced by the SCEC team in Frontera and Blue Waters was among the largest ever. RSQSim outputs were then entered into CyberShake which again used computer models for geophysics to predict the extent of vibration (in terms of ground acceleration or velocity and duration) as a result of each earthquake.

“The framework produces a complete history of lost time: where the rupture occurs and how it grows,” Milner explained. “We found that they produce realistic ground motions, which tells us that the physics applied to the model is working as intended.” They have more work planned to validate the results, which is critical before design applications are accepted.

The researchers found that the RSQSim framework produces rich and generally variable earthquakes – a sign that it is producing reasonable results – while generating reproducible and trajectory effects.

“For many sites, the risk of vibration is low, for practice case estimates,” Milner said. “But for two sites with special configurations for nearby faults or local geological features, such as near San Bernardino, the risk has increased. We are working to better understand these results and define methods to verify them.”

The work helps determine the likelihood of an earthquake along any of the hundreds of earthquake faults in California, the magnitude of an earthquake to expect, and how it might trigger other earthquakes.

Project support comes from the United States Geological Survey (USGS), the National Science Foundation (NSF), and the WM Keck Foundation. Frontera is a leading national resource for NSF. Computational time was saved on Frontera through a Large Scale Community Partnership (LSCP) award to SCEC which gives hundreds of American scientists access to the instrument to study many aspects of seismology. LSCP Awards provide extended allocations of up to three years to support long-term research efforts. SCEC – established in 1991 and counted on TACC systems for more than a decade – is a prime example of such an effort.

Building the catalog took eight days of continuous computing on Frontera and used more than 3,500 processors in parallel. Simulating Earth’s shake at 10 locations across California requires a similar amount of computing on Summit, the world’s second-fastest supercomputer.

“The adoption of the wider community will be understandably slow,” Milner said. “Since results like this will affect safety, it is part of our due diligence to ensure that these results are technically defensible by the wider community,” added Juliet. But research findings like these are important in order to bypass generalized building codes, which in some cases may not adequately represent the risks faced by the region while in others they are too conservative.

“The hope is that these types of models will help us better characterize seismic hazards, so we are spending our resources building strong, safe and resilient buildings where they are most needed,” Milner said.

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