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Abstract Seismic hazard assessment, such as the U.S. Geological Survey (USGS) National Seismic Hazard Model (NSHM), relies on estimates of fault slip rate based on geology and/or geodetic observations such as the Global Navigation Satellite System (GNSS), including the Global Positioning System. Geodetic fault slip rates may be estimated within a 3D spherical block model, in which the crust is divided into microplates bounded by mapped faults; fault slip rates are determined by the relative rotations of adjacent microplates. Uncertainty in selecting appropriate block-bounding faults and in forming closed microplates has limited the interpretability of block models for seismic hazard modeling. By introducing an automated block closure algorithm and regularizing the resulting densely spaced block model with total variation regularization, I develop the densest and most complete block model of the western continental United States to date. The model includes 853 blocks bounded by 1017 geologically identified fault sections from the USGS NSHM Fault Sections database. Microplate rotations and fault slip rates are constrained by 4979 GNSS velocities and 1243 geologic slip rates. I identify a regularized solution that fits the GNSS velocity field with a root mean square misfit of 1.9 mm/yr and reproduces 57% of geologic slip rates within reported geologic uncertainty and model sensitivity, consistent with other geodetic-based models in this Focus Section. This block model includes slip on faults that are not included in the USGS NSHM Fault sections database (but are required to form closed blocks) for an estimate of “off-fault” deformation of 3.62×1019 N·m/yr, 56% of the total calculated moment accumulation rate in the model.
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Constraining the Kinematics of the Victoria Microplate and the Northern Western Branch of the East African Rift System
Abstract The fragmentation of continents results in microplates that rotate to accommodate the lateral propagation of bounding rifts. Yet, the relationships between microplate rotation rates, fault slip, and kinematics at propagating rift tips remain unknown. Here, we analyze new Global Navigation Satellite System (GNSS) data and structural geology data from the northern Western Branch of the East African Rift System that defines part of the boundary between the Nubian plate and the Victoria microplate. We resolve 0.0583 ± 0.0293°/Myr (6.48 ± 3.26 mm/yr) counterclockwise rotation of the Victoria microplate, consistent with previous studies, but with significant northwestward shift in the Euler pole relative to earlier work. Strain is largely localized on microplate‐bounding faults with 1.8–2.2 mm/yr slip rates, 7.2 × 10−8–1.28 × 10−7 y−1strain rates, NE‐directed extension, and oblique‐normal fault kinematics. Most GNSS velocities are consistent with block rigidity, but three sites in the NW region of the Victoria microplate indicate possible internal deformation.
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- PAR ID:
- 10645064
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Geophysical Research Letters
- Volume:
- 52
- Issue:
- 20
- ISSN:
- 0094-8276
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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