A new ground motion model that more accurately simulates earthquakes and explosions

Map of the western US study area showing the topography/bathymetry of the WUS256 field (outer thick black line), major physiographic regions named (black lines, Fenneman & Johnson, 1946), plate boundaries (red lines), hotspots (yellow diamonds, Müller et al. , 1993), and Pleistocene and Holocene volcanic centers (yellow squares and circles, respectively, World Volcanism Program (2013)). Feature abbreviations are: BFZ, Blanco Fracture Zone; G.R., Gorda Ridge; JdF, Juan de Fuca hot spot; JdFR, Juan de Fuca Ridge; MFZ, Mendocino fracture zone; R, Ratton Hotspot; RGR, Rio Grande Rift; SAF, San Andreas Fault; SRP, Snake River Plain; and YS, Yellowstone Hotspot. The global inset map (bottom left) shows the WUS256 model field (black) and the waveform simulation Salvus field (blue). Depth to the top of the Cascadia slab in kilometers (Hayes, 2018) is indicated by green dashed lines. Credit: a. Rodgers et al., Journal of Geophysical Research: Solid Earth (2022). doi: 10.1029/2022JB024549

Scientists at Lawrence Livermore National Laboratory (LLNL) have created a new wave tomography model that more accurately simulates the earthquakes and ground motions of an explosion. The paper, published in the Journal of Geophysical Research: Solid Earth, was chosen for the editor’s highlight.

Seismic tomography is a method for estimating the properties of three-dimensional seismic materials inaccessible to Earth, specifically velocities of compression waves and shear waves related to composition and temperature variations. It provides images of 3D structures related to plate tectonic processes as well as models to better represent the propagation of seismic waves through the Earth’s complex structure.

In contrast to typical seismic tomography models, this model uses a full simulation of 3D wave propagation to calculate the sensitivity of the observed seismograms of the Earth’s structure, enabling more accurate simulations and better estimates of seismic source characteristics.

In the new research, scientists have created a new 3D model of the seismic structure of the top 400 km of Earth in the western United States using wave-assisted tomography (AWT). The model is produced by an extensive wave-reversal computational process that updates a subsurface model to better match the observed seismograms. In order to triangulate features in the model, AWT is also data-dense, requiring many seismograms traversing the target region.

The team — made up of scientists from LLNL’s Geophysical Monitoring Program (GMP) and researchers from Mondaic, a small start-up incubator from the Swiss Federal Institute of Technology — used more than 60,000 HPC simulations on the LLNL’s Lassen supercomputer to run 256 iterative models for 72 Earthquake to fit nearly 100,000 earthquakes.

LLNL scientist Artie Rodgers, lead author of the research paper, said: “Although other models exist from the western United States, this model is unique in that it relies on many more reversal frequencies than previous models and provides a much better fit for the recorded seismograms. “. “It could also provide more accurate estimates of seismic source properties by removing the distortion caused by the 3D Earth structure unknown in previous models.”

The researchers found that the seismic structure consists of three-dimensional differences in the velocities and intensity of compressional and shear seismic waves, and the horizontal and vertically polarized waves have different velocities.

While many seismic tomography studies focus on imaging subsurface structure, a primary motivation in the new work was to develop a 3D model to improve waveform fit for periods from 20 to 120 seconds after the Earth’s motion.

“We have produced a more detailed 3D structure of the crust and upper mantle for the purposes of improving the predictive capabilities of 3D waveform simulations for applications such as source characterization and/or simulation of long-range seismic ground motion,” said Rodgers. “The waveform fit significantly with our final model compared to previous models from the same region.”

Nuclear explosion monitoring methods can benefit from 3D models that can accurately simulate short waveforms (20 s), which are strongly influenced by crustal and upper mantle structure. Likewise, studies of the hazards and risks of Earth’s seismic motion require simulating waveforms with a much shorter duration (less than 5-10 seconds) than what was found in the new model. However, Rodgers said the large-scale structure must first fit the longer period data before drilling down to the shorter period waves. Work is underway to solve the structure on a finer scale.

This new method enables LLNL to exploit more seismic waveform information to support national and international monitoring of nuclear testing. Also contributing to the research were UC Berkeley graduate student Claire Doody, and LLNL scientists Andrea Chiang and Nathan Simmons.

Planetary Magnetic Resonance Imaging: High-resolution illumination of Earth’s interior down to the planet’s core. Geophysical Research: Solid Earth (2022). DOI: 10.1029/2022JB024549 Submitted by Lawrence Livermore National Laboratory

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