Image courtesy of USGS | Distribution of 2.5+ aftershock events between Dec. 20 and Jan 20.

Aftershock: the nuance of Humboldt geology


by Harrison Smith

Hey – did you feel it? That little jolt? The coffee on your desk is even rippling! I think we just felt an aftershock! 

For some, aftershocks are a fun little jolt that give you something to talk about in line at Los Bagels. For others, they are terrifying reminders of Humboldt’s earthquake vulnerability. 

For almost a month following the Dec. 20 earthquake that devastated parts of the county, over 80 aftershocks of magnitude 2.5 or greater have gently shaken Humboldt- most too small to be felt. Large earthquakes in fault-prone areas like Humboldt may change the stresses on adjacent faults. 

“The kind of earthquake that fault produced, with that kind of slip, can produce a pattern of stresses which, in some instances can promote a nearby fault to be closer to failing,” said Dr Mark Hemphill-Haley, a geology professor at Cal Poly Humboldt.

Some shifts release stress to delay an earthquake, while some increase stress. This can trigger new movement as the fault system searches for a new equilibrium. The pattern of aftershocks can provide important clues into the state of the fault system. 

“From just the 2.5s, you can kind of get an idea of how they ruptured along the subduction zone,” said John Bellini, geophysicist at the USGS National Earthquake Information Center. 

However, accurate triangulation of the Ferndale quake’s epicenter and those of its aftershocks is complicated by the fact that the fault system lies largely offshore. 

“Because all of the stations are onshore and the main shock was just offshore, you don’t get as meaningful distribution,” said Bellini.

Judging from the distribution of the aftershocks, one might think that they followed East-Northeast running faults. However, data is often deceptive. 

“If you look at the trends of the faults, they are Southeast to Northwest,” said Bellini. “This misleading artifact of the map is due to imprecision in triangulation- in order to locate the epicenter of an earthquake, it must be recorded from three different seismic stations.” 

The more surrounded the earthquake is by seismic stations, the greater one’s ability to measure it precisely. Because the closest ‘western’ seismic stations are in Hawaii, precise observation of seismic activity off the Humboldt coast is difficult. 

Image courtesy of USGS | Fault distribution in the North American Plate.

“Most of the stations for most of those quakes are going to be to the east, with nothing to the west for most of them… when that happens, the error ellipsoid for them is going to be stretched in an east-west direction,” said Bellini.

Humboldt County sits at the very south tip of the Cascadia subduction zone, where the Gorda/Juan de Fuca, Pacific and North American tectonic plates come together in a geologically complex region called a triple junction. 

The difficulty of studying the complex tectonic interactions at this triple junction is compounded by the fact that it lies half offshore, however new developments in geology have lifted the veil on offshore tectonics. 

Last summer, researchers from Cal Poly Humboldt’s geology department along with researchers from the US Geological Survey, UC Berkeley, and University of Washington installed over 40 seismometers along an optical fiber cable that runs from Aracata to Eureka. Instruments connected to the optical fiber along with the seismometers provided high resolution about the local seismicity. The instruments were removed after their testing period was complete, but they were reinstalled immediately in the aftermath of the Dec 20, 2022 Ferndale earthquake. 

The use of fiber optic cables for monitoring earthquakes is very recent, but a huge development for geology in areas like Humboldt.

 “We’ve already recorded more than a hundred aftershocks associated with that recent quake,” said Dr. Hemphill-Haley.

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