For several months now my research and writing has been laser-focused on the complex and intriguing setting of the Maacama fracture zone in Sonoma and Mendocino counties. My last column explored the Maacama fracture zone as the tectonic heart of Mendocino County’s landscapes, ecosystems and culture.
On the morning of June 24, I had just sat down to wrap up the final edits of a presentation on the mighty Maacama that I will be giving this summer in Ukiah when the earthquake hit.
Wednesday’s Redwood Valley earthquake not only shook close to home and coincidentally occurred within a tectonic setting that I’ve been professionally consumed by, but it was also a rare tectonic event within this fracture zone, measuring at magnitude 5.6, believed to be the largest earthquake occurring on the Maacama recorded in the USGS catalog.
The main shock of the June 24 earthquake appears to have emanated from a fault strand of the Maacama fracture zone within the Redwood Valley strands, along Tomki Road. Unfortunately, the epicenter was within a rural residential area, and directly adjacent to the Abhayagiri Buddhist Monastery in upper Redwood Valley. Within the first six hours following the magnitude 5.6 earthquake, there were at least 35 aftershocks over magnitude 1.3.
The Maacama fracture zone is a splintered network of fault strands branching out at angles from and semi-parallel to the mainstem of the Maacama fault. The Maacama system is so fractured that the vast majority of fault strands associated with it have yet to be identified.
Swarms of small earthquakes and interlocking faults along the fracture zone can be so numerous that they form what is known as a mesh, or a fracture mesh.
In 2021, renewed investigations of the northern Maacama’s tectonic geomorphology revealed several new and active fault strands splintering up through Redwood Valley. Wednesday’s earthquake and aftershocks occurred just north of these newly identified strands, which will undoubtedly instigate the identification of yet more fault strands extending up Redwood Valley and within the Maacama fracture zone.
The horizontal side-to-side movement of the oblique transverse mechanics of the San Andreas and Maacama systems involves a lot of collision, resulting in uplift of the land as the plates rub and grind into and past each other. This compression, or transpression, in turn constructs compressional landforms on the tectonic landscape such as thrust faults, linear pressure ridges, and much larger compressional ridgelines within mountain ranges.
The epicenter of Wednesday’s upper Redwood Valley earthquake occurred within the tectonic transpressional foothills of the uplifting Laughlin Range, and it may have been generated by an unmapped thrust fault.
As I introduced in my May 29 column, much of the displacement along the Maacama fault system is the result of fault creep (one study estimates fault creep occurs along 80% of the Maacama), which is described as steady a-seismic movement along a fault zone that is undetectable without instrumentation, unlike the abrupt seismic slippage of an earthquake.
Fault creep can be seen in downtown Willits
The effects of fault creep are usually only visible within urban or built environments, such as areas of pavement and infrastructure disrupted by the steady ground displacement of creep. This makes downtown Willits one of the few hot spots to view fault creep along the Maacama, with the focused mainstem of the Maacama fault slicing through town at an angle from the southeast toward the northwest.
The good thing about fault creep is that it is believed to relieve some seismic energy along fault lines, which lowers the chances of larger earthquake releases. So while the large 5.6 magnitude Redwood Valley quake may be a rare event, larger earthquakes are still a distinct possibility within this active seismic corridor, which expresses an annual displacement rate of 6-9 millimeters a year.
Indications of larger earthquakes have been observed in paleoseismic trench excavations along the Maacama fault near Willits, with the most recent large earthquakes estimated to have occurred between 500 and1200 years ago.
Hearing from those that were closer to the epicenter of Wednesday’s earthquake, the shaking was abrupt and disruptive, and lasted less than a minute. This could indicate the speed/acceleration of the earthquake as well as the geologic environment within which it occurred.
The measure of magnitude is the total energy released by an earthquake, not how the earthquake energy is released. Intensity of an earthquake is how an earthquake is felt, and that is different than the magnitude. Intensity depends on how the energy is released. The energy of an earthquake can be released in different ways, and a very disruptive shaky earthquake may have emanated from harder bedrock with a faster acceleration at release, regardless of the magnitude of the earthquake.
The location of the 5.6 Redwood Valley earthquake is within the geologic Central Belt of the Franciscan Formation, which features harder bedrocks such as old, moderately metamorphosed Franciscan assemblages and peridotites, as well as softer units of serpentinites and sedimentary deposits from the mélange formations.
Further information and investigations of the June Maacama earthquake, fault creep in Willits, basic earthquake behavior and in-depth explorations of California’s living landscapes can be found at CalGeographic.com.
Rowena Forest is a physical geographer and science writer at Cal Geographic.