John M. Rekoske | Searcharxiv

arXiv2024

Reduced-order modeling for complex 3D seismic wave propagation

Elastodynamic Green's functions are an essential ingredient in seismology as they form the connection between direct observations of seismic waves and the earthquake source. They are also fundamental to various seismological techniques including physics-based ground motion prediction and kinematic or dynamic source inversions. In regions with established 3D models of the Earth's elastic structure, 3D Green's functions can be computed using numerical simulations of seismic wave propagation. However, such simulations are computationally expensive which poses challenges for real-time ground motion prediction. Here, we use a reduced-order model (ROM) approach that enables the rapid evaluation of approximate Green's functions. The ROM technique developed approximates three-component surface velocity wavefields obtained from numerical simulations of seismic wave propagation. We apply our ROM approach to a 50 km x 40 km area in the greater Los Angeles area accounting for topography, site effects, 3D subsurface velocity structure, and viscoelastic attenuation. The ROM constructed for this region enables rapid computation (0.001 CPU hours) of complete, high-resolution, 0.5 Hz surface velocity wavefields that are accurate for a shortest wavelength of 1.0 km. Using leave-one-out cross validation, we measure the accuracy of our Green's functions in both the time-domain and frequency-domain. Averaged across all sources and receivers, the error in the rapid seismograms is less than 0.01 cm/s. We demonstrate that the ROM can accurately and rapidly reproduce simulated seismograms for generalized moment tensor sources in our region, as well as kinematic sources by using a finite fault model of the 1987 Mw 5.9 Whittier Narrows earthquake as an example. We envision that our rapid, approximate Green's functions will be useful for constructing rapid ground motion synthetics with high spatial resolution.

physics.geo-ph↗

arXiv2023

Instantaneous physics-based ground motion maps using reduced-order modeling

Physics-based simulations of earthquake ground motion are useful to complement recorded ground motions. However, the computational expense of performing numerical simulations hinders their applicability to tasks that require real-time solutions or ensembles of solutions for different earthquake sources. To enable rapid physics-based solutions, we present a reduced-order modeling (ROM) approach based on interpolated proper orthogonal decomposition (POD) to predict peak ground velocities (PGVs). As a demonstrator, we consider PGVs from regional 3D wave propagation simulations at the location of the 2008 Mw 5.4 Chino Hills earthquake using double-couple sources with varying depth and focal mechanisms. These simulations resolve frequencies $\leq$ 1.0 Hz and include topography, viscoelastic attenuation, and S-wave speeds $\geq$ 500 m/s. We evaluate the accuracy of the interpolated POD ROM as a function of the approximation method. Comparing the radial basis function (RBF), multilayer perceptron neural network, random forest, and $k$-nearest neighbor, we find that the RBF interpolation gives the lowest error ($\approx$ 0.1 cm/s) when tested against an independent dataset. We also find that evaluating the ROM is $10^7-10^8$ times faster than the wave propagation simulations. We use the ROM to generate PGV maps for one million different focal mechanisms, in which we identify potentially damaging ground motions and quantify correlations between focal mechanism, depth, and accuracy of the predicted PGV. Our results demonstrate that the ROM can rapidly and accurately approximate the PGV from wave propagation simulations with variable source properties, topography, and complex subsurface structure.

physics.geo-ph↗