The U.S. Geological Survey (USGS) has announced the 2023 release of a 50-state update to its National Seismic Hazard Model (NSHM). Previously, the USGS completed a contiguous update to their NSHM in 2018, which was released to the public via journal publication in Earthquake Spectra at the end of 2019. We detailed these changes in a blog post at the time. The USGS last updated their Hawaii NSHM in 1998 and their Alaska NSHM in 2007.
While the work for the 2023 NSHM update is ongoing, the process is a lengthy and collaborative one. USGS’s overall plan is to update the U.S. NSHM by 2023 with a plan to provide a draft version of the model in 2022, hold a workshop with key stakeholders on the draft version at the beginning of 2023, and update the model at the end of 2023 after time has been given for public comment and peer review. The AIR team attended several workshops held by the USGS over the course of 2021, and we have been providing our feedback and questions throughout the development process. Here we summarize some of the key updates that will be coming to the 2023 NSHM update and that will likely be adopted in AIR’s model update for the contiguous U.S. and Alaska.
Updates to the Source Parameters
The source parameters for NSHM consist of the earthquake location and magnitude along with the associated faults and their geometry, length, and slip rates. For the contiguous update, the USGS has planned a significant update to their crustal fault database, adding 350 crustal faults and bringing the total crustal fault count to approximately 1,000. Fault-specific geologic parameters for the USGS NSHM were last updated for the 2014 model release. Most of these new faults are in the Intermountain West region, encompassing the states of Nevada, Utah, and Idaho (Figure 1). For the Alaska update, the USGS will be moving from using 8 crustal faults with 10 fault sections to include 69 crustal faults with 89 fault sections, where a fault section represents a portion of an active crustal fault with similar geometry and kinematic properties (Figure 1). Alaska will also be including several deforming regions.
For the major subduction zones, the Cascadia Subduction Zone and the Aleutian megathrust will be updated to reflect the latest geometries and historical earthquake studies. This incorporates evidence that the central and southern Cascadian subduction zone contains 4 previously unidentified earthquakes, bringing the total number of earthquakes to 17 megathrust ruptures in the past 6,700 years (Nelson et al., 2021).
Not only is the number of faults in the contiguous U.S. and Alaska changing, but the slip rate or rate at which they deform may also change. Using the latest paleoseismic evidence along with deformation models constructed using high-precision GPS and InSAR data, the fault slip rates in the model will be updated to match the latest available science. In addition, a uniform methodology will be applied for all faults, modeling multi-fault ruptures across all regions, not just California. The epistemic uncertainty related to faults will be better represented by increasing the number of rupture scenarios to accommodate a range of how well the faults are connected to each other.
The process of declustering is a fundamental piece of the source characterization process as it intends to separate the earthquake catalog into mainshocks and aftershocks or dependent and independent events. While the USGS has traditionally used Gardner and Knopoff, (1974) to decluster a historical catalog, it is currently exploring other methodologies such as Reasenberg, (1985), Zaliapin et al., (2008), and Llenos and Michael, (2020). This is because the declustering methodology can be highly subjective and can significantly affect a region’s a- or b-value of the Gutenberg-Richter relationship that determines the magnitude – frequency distribution, and hence can have a large impact on the earthquake rate estimation.
Updates for Sedimentary Basins
In the Western U.S. over the deepest portions of the sedimentary basins, long period shaking for soft soils is higher such that the seismic hazard for tall buildings is greater with increasing basin depth. Amplification is not applied at periods below 1.0s spectral acceleration (SA); at 5.0s SA increases of around 40% can be expected when compared to default values. In addition to adding site-specific basin amplification information for the Los Angeles, San Francisco, Seattle, and Salt Lake City sedimentary basins for USGS’s NSHM 2018, the USGS is exploring the addition of Portland, Reno, Las Vegas, and the Central Valley of California to their 2023 model. The good news is that AIR already includes most of these basins in the model released in 2017, except for Portland, Reno, and Salt Lake City. These three cities might see some increased ground shaking in our upcoming update.
Updates to the Ground Motion Model
The NGA-East and NGA-Subduction projects represent multi-year collaborative efforts to characterize ground motions for tectonically stable continental regions of the Central and Eastern U.S. and for subduction regions of the world, respectively. These areas fit into two of the three broad tectonic categories that ground motion prediction equations (GMPEs) fall into: active crustal regions, stable continental regions, and subduction zones. Significant differences in the distance and site scaling within these broad tectonic categories have been observed due to the availability of data in the past decade.
While USGS’s 2018 update incorporated the results from the NGA-East project, the USGS NSHM 2023 update is looking to include results from the NGA-Subduction project for both the Cascadian and Aleutian megathrusts. Overall, results from the NGA-East project show increased ground shaking in many (but not all) locations across the Central and Eastern U.S. Results from the NGA-Subduction project show reduced ground shaking in many locations across the Cascadian and Aleutian megathrusts, although some variation may exist at different distances from the source.
Some other improvements the USGS is considering for the NSHM 2023 are the inclusion of non-ergodic ground motion models and directivity, neither of which were implemented for AIR’s 2017 release. Non-ergodic models consider regional variations of ground motion in addition to the three broad tectonic categories that GMPEs fall into. This includes a site-specific investigation of how seismic waves propagate along regional geologic structures, such as specific fault systems. Directivity considers the concentration of ground motion along the fault due to the radiation of waves, where the stronger ground motions will be concentrated along the fault in the direction of wave propagation. Especially at long periods, this can increase the hazard at a site in the direction of wave propagation in comparison to the traditional approach.
Concluding Thoughts
The USGS 2023 update is likely to be a major enhancement, incorporating an updated fault model, deformation model, new declustering procedures, and additional sedimentary basins. The USGS is also considering the inclusion of non-ergodic ground motion models and directivity but has not yet decided if they will be included in the update. We are currently working on reviewing the planned NSHM 2018 and 2023 changes for potential implementation into our model framework.
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