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

Abstract

A central question of earthquake science is how far ruptures can jump from one fault to another, because cascading ruptures can increase the shaking of a seismic event. Earthquake science relies on earthquake catalogs and therefore how complex ruptures get documented and cataloged has important implications. Recent investments in geophysical instrumentation allow us to resolve increasingly complex, multi-fault ruptures for even moderate-sized earthquakes. We combine dense seismic and geodetic measurements to reveal an enigmatic rupture in late 2021 at the Mendocino Triple Junction in northern California. We show that rupture was dynamically triggered, yet concurrent, on two distinct faults roughly 30 km apart. Thus, this rupture combines features of complex ruptures usually considered to be single earthquakes, and triggered ruptures considered as multiple earthquakes. This event illustrates that moderate-sized earthquakes can exhibit similar complexity to that more commonly documented for large earthquakes.

Distinct Yet Adjacent Earthquake Sequences near the Mendocino Triple Junction: 20 December 2021 Mw 6.1 and 6.0 Petrolia, and 20 December 2022 Mw 6.4 Ferndale

“…These template-matching catalogs (Fig. S6d) available for the Petrolia sequence from 20 December 2021 to 11 January 2022 (Yeck et al, 2023;Fig. 3b) and the Ferndale sequence from 19 December 2022 to 21 January 2023 (Shelly et al, 2024;Fig.…”

“…The 2021 M w 6.1 and 6.0 Petrolia sequence (Fig. 4a) was a complex, multiple-fault, dynamically triggered rupture (Yeck et al, 2023). It started offshore on 20 December 2021 20:10:20 UTC as an M w 6.1 right-lateral strike-slip earthquake (Figs.…”

“…1, green mechanism), 11 s later at 20:10:31 UTC, ∼30 km away on a fault within the subducting Gorda slab. The second M w 6.0 earthquake had a distributed aftershock zone extending ∼10 km to the north (Yeck et al, 2023). These 2021 Petrolia earthquakes caused limited damage.…”

Subduction intraslab-interface fault interactions in the 2022 M w 6.4 Ferndale, California, earthquake sequence

“…1), generating high rates of seismicity. Faulting in this zone occurs with a variety of orientations, with both seismic and aseismic movement (1)(2)(3).…”

“…Ruptures have also been observed to interact among these faulting regimes; the 1992 reversefaulting event exhibited large strike-slip aftershocks within the Gorda slab (9,11), while a sequence on 20 December 2021 included two temporally overlapping M ~ 6 earthquakes, one offshore on the Mendocino transform fault and the other ~11 s later and ~30 km northeast onshore within the subducting Gorda slab (Fig. 1) (3).…”