Laurie Dingler | ‘Good’ earthquake swarms in the Pacific Northwest – The Times Standard

Last Tuesday, a 4.2-magnitude earthquake was reported off the coast of central Oregon. Centered in the Blanco Fault Zone (BFZ), it marked the beginning of a swarm of earthquakes, and by the end of the weeks, more than 100 earthquakes were recorded.

A hundred earthquakes in a three-day period seem ominous, so why call it good? Because these earthquakes were too far from shore to cause damage, not large enough to cause a tsunami, they are unlikely to lead to larger and more dangerous earthquakes. The sequence helps with understanding the BFZ, the discussions resulting from the earthquakes, and I hope it has prompted you to take preparedness action.

How can I be sure that Blanco earthquakes are not a problem? I’m never 100% sure of anything, but we do know a lot about the BFZ by its tectonic setting and past behavior, and I don’t think I’m out of my neck when I put it in the benign category.

My guess is that little has heard of the BFZ, although annual seismic production often exceeds the San Andreas fault. In 2021 prior to the current sequence, 27 earthquakes of magnitude 4 and greater were reported on the Blanco fault compared to only seven in the San Andreas fault zone. This week, he raises the 2021 BFZ balance to 101.

The reason for its ambiguity is the location. And the 260-mile fault is completely offshore. The southeastern edge lies about 85 miles west of Bandon, Oregon, and northwest about 300 miles west of Newport. Few BFZ earthquakes have been reported and even the occasional M6 has felt lightly along the coast.

The Blanco fault is an important part of the Juan de Fuca – Gorda panel system. Fifty million years ago, the entire western coast of what is now the United States and Canada was a subduction zone. The Farallon Plate extended over most of the eastern Pacific Ocean. Subduction slowly consumed Farallon until only a small portion remained along northern California to the southern coast of British Columbia. Today, this small remnant consists of three sections: the Explorer Plate (the smaller one) off British Columbia, the Juan de Fuca off the Washington and Oregon (the larger), and the Gorda in the south off northern California and southern Oregon.

The Blanco fault is the shift plate boundary (horizontal movement) through which Juan de Fuca moves toward the coast relative to the Pacific plate. At its western end, it abuts the Juan de Fuca mountain range, an active spreading center with almost continuous submarine volcanic activity. The eastern end connects it to the edge of the Gorda.

Since 1980, 150 M≥5 earthquakes have been recorded on the Blanco Fault. Ten were in the M6 ​​range and a larger 6.5. Few of these earthquakes were reported to be felt because most were more than 100 miles away from populated areas. More than a third of these earthquakes were part of swarms, which are short bursts of seismic activity that lasted no more than a week or two. Swarms differ from the more common aftershock chains in that the largest earthquake occurs early in the sequence and is followed by aftershocks that generally become fewer and smaller over time.

In swarms, earthquakes are of similar size. The current BFZ swarm included 5.8 s, six 5.5 s earthquakes, and nine other M5 earthquakes scattered throughout the sequence. Similar sequences have occurred in the past. The swarm lasted in January 2001 for about a week, producing M6.3s and four earthquakes in the M5 and 14 M4s range. At least 14 swarms have occurred since 2000, all centered along the western half of the rift near the edge.

It’s interesting to compare Blanco’s behavior to the conversion error closest to us. The 150-mile-long Mendocino fault zone (MFZ) that stretches west from Cape Mendocino to the Gorda mountain range is a relative of the Blanco system. Located only 200 to 300 miles south-southeast of the BFZ, it is also a transformation fault, marking the plate boundary between the Gorda and the Pacific Plate.

The MFZ is very seismically active and has produced 48 M≥5 earthquakes since 1980, similar to Blanco’s activity rate. But a closer look reveals significant differences. The Mendocino fault has caused larger earthquakes including 7.1 in 1994 and several 6.6 and 6.7 quakes. Big earthquakes mean more seismic energy is released. In the past 40 years, the Mendocino fault has released nearly twice the earthquake energy as the BFZ. The MFZ earthquake sequences are not swarms; They have major orgasms followed by smaller aftershocks.

Why are these proximal defects so different? Warmer temperatures and fluids significantly weaken the fault zone. The rocks on either side of the Blanco Fault are very small, and the swarm’s behavior on the smaller, warmer half of the Blanco Fault closer to the ridge suggests that it plays a role. The Mendocino fault abuts the relatively young Gorda Plate against the much older Pacific Plate. It may also be stronger due to the pressure exerted by Juan de Fuca in the north. Blanco fault and Mendocino fault are not parallel and the plate motion produces stress in the Gorda plate and along the Mendocino fault.

As I write, it appears that the current BFZ sequence has expired. It is recorded in the books as the most active Blanco swarm of the past two decades, although it is similar in terms of energy release and duration to other sequences. The earthquakes weren’t large enough to affect stresses further away, and seismologists at the USGS Pacific Northwest Seismic Network agree that they had no effect on other fault systems in the area (https://pnsn.org/ blog/2021/12/10/blanco-fracture-zone-swarm-active-unusual-interesting-but-not-regarding).

The Blanco fault and its behavior are important parts of our regional tectonic neighborhood. Although these earthquakes posed no threat, we have plenty of other fault systems that are closer to populated areas and/or capable of causing much larger earthquakes at any given time. The recent earthquakes are a reminder that we live in an earthquake country and the following sequence may not be benign.

Laurie Dingler is Professor Emeritus of Geology at Humboldt State University, and an expert on tsunami and earthquake hazards. Questions or comments about this column, or want a free copy of Preparedness magazine “Living on Shaken Earth”? Leave a message at 707-826-6019 or email [email protected].

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