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January 28, 2022

We can usually see the beginning and ending of one earthquake on a seismic record before another one starts. We’d like earthquakes to be well-behaved and happen one at a time... but often they don’t.

Two overlapping earthquakes on a seismic record look like one big messy event. On December 20, 2021, after Petrolia residents felt shaking, the USGS-UC Berkeley Northern California Seismic System processed the data and announced that a magnitude 6.2 had occurred offshore on the Mendocino transform fault. 

map of Gorda Plate area showing two earthquakes and aftershocks from Dec 20, 2021
Map showing the revised location of the M6.2 earthquake (blue circle) that occurred on 12/20/2021.  The magnitude 5.7 and 4.0 foreshocks (dark grey) are located offshore along the Mendocino fault.  Aftershock of this sequence are shown in yellow and historical earthquakes of magnitude 5 and greater in the last 40 years are shown in light grey.

In the subsequent weeks since the event, USGS and UC Berkeley staff have carefully analyzed the data in collaboration with local experts at Humboldt State University. They have concluded that the Petrolia earthquake was really two Petrolia earthquakes 10 seconds apart in two different locations. The first event was a magnitude 5.7 offshore on or near the east-west trending Mendocino transform fault that demarks the plate boundary between the Pacific plate and southern Juan de Fuca plate (alternatively termed the Gorda plate). It was followed 11 seconds later by an M6.2 about 30 km (19 mi) to the ENE, onshore near Petrolia. Preliminary analysis suggests that the M6.2 event occurred in the Gorda plate, based on the depth and the focal mechanism. Most of its aftershocks appear to be in the Gorda plate. Seismologists are interpretating the M5.7 as a foreshock to the M6.2, but more investigation of the data is needed to confirm this hypothesis.  (The M5.7 was itself preceded by a magnitude 4.0 foreshock almost 6 hours earlier).

horizontal lines with squiggly seismic recordings on each overlapping
Seismic waveforms of the December 20, 2021 Petrolia earthquake recorded on 93 different seismic channels (station and channel listed at right).  Phase arrivals are highlighted for the initial offshore M5.7 event in red (P-waves) and blue (S-waves), based on phase arrival times from the Northern California Seismic Network (NCSN).  Many of the prominent arrivals between the red and blue are P arrivals from the subsequent onshore Mw 6.2 earthquake.  Later, large-amplitude waves may be S-waves from this second earthquake.

Because of the logarithmic nature of earthquake magnitudes, M5.7 + M6.2 = M6.24, which is why the magnitude of the initial single event didn’t change when it was split into two events. Seismologists are still working to disentangle the two events more precisely, and the magnitudes of the two events may adjust slightly in the future. 

Earthquakes are common in the region around the Mendocino triple junction, which is the meeting place of the Pacific, Gorda, and North American plates and marks the transition between the right-lateral strike-slip motion dominated by the San Andreas Fault System to the south and the oblique-collisional motion of the Cascadia Subduction zone to the north. In the past century, there were 40 other earthquakes of M6 or larger, including six earthquakes M7 or larger, within 250 km (155 mi) of the December 20 earthquakes. The majority of these events have occurred offshore, within the Juan de Fuca/Gorda plate or along the Mendocino transform fault. 

The December 20, 2021, M6.2 occurred approximately 8 km (5 mi) NW of the April 25, 1992, M7.2 earthquake, which caused significant shaking across northern California, multiple landslides, a rockfall and a tsunami reaching 1.1 meters at Crescent City, California.  The 1992 event, which is generally considered to have occurred on a shallow thrust fault in the North American plate, was followed by two large aftershocks of M6.5 and M6.6 offshore in the Gorda plate – an earlier example of the complex interaction of seismicity from one plate to another in this area.

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