Beyond the Teleseism: Introducing Regional Seismic and Geodetic Data into Routine USGS Finite-Fault Modeling

Abstract

The U.S. Geological Survey (USGS) National Earthquake Information Center (NEIC) routinely produces finite-fault models following significant earthquakes. These models are spatiotemporal estimates of coseismic slip critical to constraining downstream response products such as ShakeMap ground motion estimates, Prompt Assessment of Global Earthquake for Response loss estimates, and ground failure assessments. Because large earthquakes can involve slip over tens to hundreds of kilometers, point-source approximations are insufficient, and it is vital to rapidly assess the amount, timing, and location of slip along the fault. Initially, the USGS finite-fault products were computed in the first several hours after a significant earthquake, using teleseismic body wave and surface wave observations. With only teleseismic waveforms, it is generally possible to obtain a reliable model for earthquakes of magnitude 7 and larger. Here, we detail newly implemented updates to NEIC’s modeling capabilities, specifically to allow joint modeling of local-to-regional strong-motion accelerometer, Global Navigation Satellite System (GNSS), and Interferometric Synthetic Aperture Radar (InSAR) observations in addition to teleseismic waveforms. We present joint inversion results for the 2015 Mw 8.3 Illapel, Chile, earthquake, to confirm the method’s reliability. Next, we provide examples from recent earthquakes: the 29 July 2021 Mw 8.2 Chignik, Alaska, United States, the 14 August 2021 Mw 7.2 Nippes, Haiti, and the 8 July 2021 Mw 6.0 Antelope Valley, California, United States, earthquakes. These examples confirm that jointly leveraging a variety of geophysical datasets improves the reliability of the slip model and demonstrate that such a combination can be leveraged for rapid response. The inclusion of these new datasets allows for more consistent finite-fault modeling of earthquakes as small as magnitude 6. As accelerometer, GNSS, and InSAR observations worldwide become more accessible, these joint models will become more routine, providing improved resolution and spatiotemporal constraints on rapid finite-fault models, and thereby improving the estimates of downstream earthquake response products.

Dense geophysical observations reveal a triggered, concurrent multi-fault rupture at the Mendocino Triple Junction

“…Slip modeling. We used the Wavelet and simulated Annealing SliP inversion (WASP) 23 code to perform the joint inversion using broadband teleseismic waveforms (7 P-waves, 9 SH-waves, and 11 long-period surface waves), in addition to 15 strong-motion accelerometer and 54 static GNSS observations. The inversion uses a one-dimensional local velocity model based on Castillo & Ellsworth 24 , layered on top of the Preliminary Reference Earth Model 25 .…”

“…The tools used to perform these analysis include the Wavelet and simulated Annealing SliP inversion (WASP) 23 (https://github.com/slipinversion/WASP, last accessed Feb 21, 2022), HypoDD 27 for relative earthquake relocations (https://www.ldeo.columbia.edu/~felixw/ hypoDD.html, last accessed Feb 21, 2022), and the python packages pyrocko 19 and obspy 31 . Figures were created using the matplotlib 32 python package and Generic Mapping Tools 33 .…”

Rapid shallow megathrust afterslip from the 2021 M8.2 Chignik, Alaska earthquake revealed by seafloor geodesy

“…The USGS NEIC creates FFMs using teleseismic broadband data and available regional accelerometer and geodetic observations ( 24 , 85 ) to generate event web pages that provide a suite of earthquake source information from the moment a notable event happens. The FFM is generated using a nonlinear inversion approach to modeling slip amplitude, rake, rupture time, and rise time on a discretized fault plane made up of smaller subfaults, finding the solution that best fits the observations in the wavelet domain.…”

“…S4E). All data used in the inversions have been previously published ( 24 ) and are publicly available ( 86 ). The intent of this exercise is to show the spatial resolving power of each component of the inversion and so this example does not include GNSS-A station SEM1.…”

“…As part of its natural hazards program, the U.S. Geological Survey (USGS) responds to and provides rapid assessments of the impacts and evolving hazards of global earthquakes (19)(20)(21). This includes automatically producing slip models ("finite fault models") from teleseismic waveforms (22,23); recent efforts are focused on refining the models by including geodetic surface displacement data (24). For subduction zone earthquakes, finite fault models (FFMs) are used to rapidly assess which portions of the megathrust have slipped and which portions are stressed and/ or retain potentially seismo-and tsunamigenic slip deficits.…”