SCEC Project Details
| SCEC Award Number | 25290 | View PDF | |||||
| Proposal Category | Individual Research Project (Single Investigator / Institution) | ||||||
| Proposal Title | Incorporating Abundant Earthquake Data from Densified Permanent Seismic Networks and Temporary Nodal Array into Local Earthquake Tomography to Improve 3-D Seismic Velocity Structure in the Mendocino Triple Junction Region | ||||||
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| SCEC Milestones | A1-1, A2-1 | SCEC Groups | CEM, Seismology, FARM | ||||
| Report Due Date | 03/15/2026 | Date Report Submitted | 05/05/2026 | ||||
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Project Abstract |
The Mendocino Triple Junction (MTJ) region in northern California is one of the most seismically active regions in the U.S. A high-resolution 3-D seismic velocity model in the MTJ region is essential for assessing seismic hazard. The main goals of this project include: (1) improve an existing 3D velocity model of Guo et al. (2021) by incorporating more local earthquake data to take advantage of the permanent seismic networks densified in recent years and a dense nodal array deployed in 2020, aiming to better characterize downdip variations in material properties from the locked zone to the transition zone; and (2) use the improved model to relocate two recent magnitude 6 earthquake sequences (the 2021/12/20 M6 Petrolia and 2022/12/20 Mw 6.4 Ferndale sequences) to study the seismogenesis and fault structures of these two moderate earthquakes. We have determined new Vp, Vs, and Vp/Vs models using the triple-difference tomography method by incorporating new local earthquake data since 2016 into the tomographic inversion of Guo et al. (2021). The new model shows decrease in Vp/Vs from the locked zone to the transition zone, suggesting significant decrease in porosity and fluid content that may be correlated with the brittle-to-ductile transition. The new model shows very high Vp/Vs in the accretionary wedge, indicating it is a fluid-rich, sedimentary region. Using the new velocity model, we relocated two template matching catalogs and one deep learning catalog of the 2021 and 2022 M6 sequences, which show spatial relationships between mainshock rupture, aftershock distribution, and Vp/Vs structures. |
| Intellectual Merit | We have developed a new high-resolution 3D velocity model in the Mendocino Triple Junction region, one of the most seismically active regions in the U.S., through local earthquake tomography with abundant local earthquake data. Our new model reveals spatial variations in material properties of the subducted slab and the overlying North America plate and their relationships with geologic units, fluid distribution, and megathrust coupling. We relocated the 2021 Petrolia and 2022 Ferndale M6 sequences with our velocity model, achieving a better understanding of the seismogenesis and structures of the ruptured faults that were unknown prior to the M6 sequences. |
| Broader Impacts | Our high-resolution 3D velocity model of the Mendocino Triple Junction region is essential for many research activities in this seismically active region, such as characterizing megathrust fault-zone material properties, estimating basin depths, determining accurate earthquake locations and source mechanisms, developing Community Velocity Model, and simulating ground motions. Our velocity model and high-precision relocations of the 2021 Petrolia and 2022 Ferndale M6 earthquake sequences are valuable for the future update of the Community Fault Model, especially the megathrust fault and the faults ruptured by the 2021 and 2022 M6 earthquakes that were unknown prior to the M6 sequences. |
| Project Participants | David Shelly from USGS provided his phase arrival time data and waveform cross-correlation differential time data for the events in his template matching catalogs of the 2021/12/20 M6 Petrolia and 2022/12/20 Mw 6.4 Ferndale sequences. Clara Yoon from USGS provided her phase arrival time data and waveform cross-correlation differential time data for the events between 2021/12/1 and 2023/6/1 in her deep learning catalog. |
| Exemplary Figure | Figure 2. Along-dip cross-sections of our new Vp, Vs, and Vp/Vs models and relocations of regular earthquake and low-frequency earthquakes, illuminating spatial variations in material properties in the subducted Gorda slab and the overlying North America plate and their relationships with geologic units, fluid distribution, and megathrust coupling. The black line in Fig. 1 trending the NNE direction from the deformation front to the east of low-frequency earthquakes shows the profile of the cross-section. |
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Linked Publications
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