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2021 and 2022 North Coast California Earthquake Sequences and Fault Complexity in the Vicinity of the Mendocino Triple Junction

Description: 

North of 40N and west of 122W lies the Mendocino Triple Junction (MTJ), one of California’s tectonically most active and complex regions. Damaging earthquakes can occur on the San Andreas and Mendocino faults, within the Gorda and North American plates, and on the Cascadia subduction interface. Recent earthquake sequences in the MTJ region, starting on 2021-12-20 (2021) and 2022-12-20 (2022), highlight the complex interactions of regional faulting. In the catalog, the 2021 foreshock/mainshock (FS/MS) pair, separated by ~10 s in time and 30 km in distance, have catalog magnitudes of 5.7 and 6.2. Detailed analysis indicates they are more likely to be a doublet, both with M~6.1, with the onshore MS triggered by the S-waves of the FS which occurred to its W on the Mendocino Fault. Finite fault analysis shows the FS ruptured to the W, and the MS to the NW, away from the population centers. The 2022 MS (M6.4) occurred just offshore, to the NW of the 2021 MS and produced aftershocks within the Gorda Plate and beneath the False Cape Shear Zone. Its largest aftershock (AS, M5.4) took place ~40 km to the ESE of the main fault rupture. We present finite fault analysis of the 2022 MS which indicates that it ruptured NE, toward the towns in the area and causing damage, which was exacerbated by the 2022 AS. Over the past few years, the instrumentation in the region has been upgraded as part of the implementation of the earthquake early warning system. We explore the seismicity associated with these earthquakes, looking to better define the faults in the MTJ region, as well as their modes of rupture.

Session: Earthquake Source Parameters: Theory, Observations and Interpretations

Type: Oral

Date: 4/18/2023

Presentation Time: 04:45 PM (local time)

Presenting Author: Margaret Hellweg

Student Presenter: No

Invited Presentation: Yes

Authors