Laurie Dingler | The complex relationship between small and large earthquakes – Times-Standard

Two magnitude 4 earthquakes in three days followed by a 3.7 earthquake five days later. I asked three inescapable questions: What does this mean? Do they relieve stress? Is the big one coming?

I am glad when we felt earthquakes that do no harm. Tickling from mother nature is better at getting your attention than my words. But they also bring up many common misconceptions.

On ShakeOut Day on October 20, the M4.3 earthquake occurred near the Humboldt/Trinity County border. Sixty people submitted poetic reports on the USGS Did You Feel It website from Fort Bragg to Arcata and Inside Redding. 4.1 happened on October 23. Its epicenter was near Fortuna, about 37 miles northwest of 4.3. More than 500 people have submitted reports to Badd, from Fort Bragg to Trinidad. Finally, it was 3.7 last Thursday in triple crossover territory with 32 reports of felt.

Not surprisingly, most people feel M4.1. It was located in a more densely populated area. It was not an aftershock of the previous earthquake. How can I tell? There are three methods for assessing earthquake ‘DNA’ for kinship testing. The first is the location and characteristics. The true aftershock is located within or near the fault zone of the first earthquake. It also has faults consistent with a larger earthquake.

Our four cars were about 40 miles apart. They were also different types of earthquakes. The 4.3 was in plain error, the result of the EW spillover in the Jorda board. It was 4.1 in slip error, as a result of NS pressure. Both were the result of the complex orientation of the forces in the area, but it was just a coincidence that they happened just a few days apart.

The second test is the shades of tension. Seismologists calculate how much an earthquake has changed from the pressure in the surrounding area. Some areas will be compressed or compressed, resulting in increased load and stretching of other areas after the earthquake. These stress shadows do a great job of predicting where most aftershocks will occur (near the ends of the rupture) and explain why earthquakes sometimes occur on adjacent faults. Our group of four didn’t create much shadow of tension, certainly none within 40 miles.

The third is statistics. Was it followed by an earthquake that escalated in earthquake activity much different from normal? The M7.3 Landers earthquake in July 1992 was immediately followed by a group of small earthquakes in the eastern part of California. For over a week after that, eastern California lit up with M2s and 3s. Some were more than 750 miles away, far from where calculations of static stress change show any effect (https://www.science.org/doi/10.1126/science.260.5114.1617). Landers’ proposed explanation is that the distortion associated with the surface waves generated by 7.3 was sufficient to trigger these small earthquakes, especially in regions of geothermal activity. Our recent activity was not associated with an increase in small earthquakes.

Were these “good” earthquakes? They were good in that they did no harm, but it is a common misconception that small earthquakes reduce stress so that large earthquakes are less likely.

It is not difficult to estimate how much energy is released in an earthquake. By knowing the size of the fault, how much it has slipped, and how tightly the ground pushes the two sides of the fault together, the energy can be determined. Large earthquakes produce a lot of energy. The 1992 earthquake sequence released approximately a one-megaton nuclear bomb. Chile’s M9.5 earthquake in 1960 could have provided enough energy for the United States for a year if only that energy could have been harnessed.

Small earthquakes also release energy, but not much. Each step in size means a 32-fold increase in power. It takes 32 its power 5 seconds to equal 6 degrees. It takes about a thousand 5 seconds to equal 7. At this rate, we would need approximately five M5 earthquakes every day for 500 years to take care of how much energy a single Cascadia earthquake would release. Don’t count on small earthquakes to take over.

Is ‘The Big’ Coming? Yes, but the earthquakes in the past two weeks haven’t changed those odds. Could it be earthquakes? It’s hard to say if any particular earthquake might herald something bigger in the next few days or weeks. The USGS compiles earthquake and aftershock statistics and after moderate earthquakes, it publishes aftershock forecasts.

Last Tuesday, a 5.1-magnitude earthquake occurred near the Calaveras Fault east of San Jose. The USGS website (https://earthquake.usgs.gov/earthquakes/eventpage/nc73799091/executive) is a treasure trove of information including felt reports, error features, and coupon distribution. Scrolling through the menu on the left, you will reach the aftershock forecast. The summary gives an 18% chance of a significant aftershock in the next week and a 1 in 500 chance of a larger earthquake.

USGS foreshock/aftershock estimates are based on statistics only – what we usually see after M5s in the San Francisco Bay Area. But there is some additional evidence to look for to rule out whether or not the earthquake is a harbinger. In our region, what worries us is an earthquake in the Cascadia Subduction Zone (CSZ), the interface between the subducting Gorda plate and the North American plate above it. If you live in coastal Humboldt County, you’re only six to eight miles above this bug.

I am sure that last week’s earthquakes have nothing to do with the subduction zone. It was deeper, down the facade. One of the most interesting features of CSZ is how seismically quiet it is as long as we have seismic instruments in the area. Most of the earthquakes that we feel within the Gorda Plate occur either offshore or below the land front. If we had 5 or 6 on or near the front, I’d be more concerned that something bigger might follow soon.

These earthquakes are interesting. They reveal some of the complexity of the North Coast earthquake terrain. They do no harm, but the following damage can.

Laurie Dingler is Professor Emeritus of Geology at Cal Poly Humboldt, and an expert in 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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