Lori Dengler | Two interesting earthquake sequences: a reminder of our unique geological environment – Times-Standard

This year is still a relatively quiet year from an earthquake and tsunami perspective. Four earthquakes reached the magnitude 7 range, the largest of which was a magnitude 7.5 on March 24 in the Tonga Islands region in the southwestern Pacific Ocean. The quake was felt on two of Tonga’s islands, but its depth of about 150 miles below the surface and far from populated areas meant the shaking was modest and posed no tsunami threat.

Five earthquakes were close enough to populated areas to cause deaths, and the death toll from the 2026 quake currently stands at 18 people. It’s always a loss when people die in earthquakes, but the effects have been modest compared to other years. The deadliest was M5.8 in Afghanistan on April 3 which claimed 12 lives due to the collapse of buildings not designed to withstand side-to-side movements of seismic waves.

Two successive earthquakes that struck the western United States this week may have raised some eyebrows but caused no damage. These phenomena were not surprising to seismologists; they are only related to each other because they reflect the complex tectonic environment of the region. They tell an interesting story and may dispel some of the misinformation in the media.

The first sequence was centered about 500 miles north of us on the Juan de Fuca Range, 230 miles west of the central Washington coast. I started with M2.9 on April 12th just after midnight. Eighteen earthquakes followed over the next nine hours, including a 4.2 and a 4.1, the largest in the series. No earthquakes have been reported in this area since then, and it appears to have closed as suddenly as it began. We call this a swarm where all earthquakes are similar in size and no larger earthquake stands out as a major shock.

None of the earthquakes in this swarm were reported, and they were all too small to trigger any tsunami alerts. The Juan de Fuca Ridge is the spreading center that separates the Juan de Fuca Plate from the Pacific Plate, similar to the Gorda Ridge off the coast of northern California. The earthquakes were not large enough to be recorded well enough on ground-based seismic stations to determine fault characteristics. The epicenters appear to be trending in the ENE direction in the map view, but offshore locations have greater uncertainty and are not good enough to pinpoint the causative fault.

It certainly appears to be connected to the chain, an active spreading center where volcanic and geothermal activity are common. If you’ve been following volcanic news reports, you might think this earthquake swarm is a sign that Axial Seamount is finally about to erupt. An axial seamount is a gentle bulge in the Pacific sea floor that straddles a rift zone. Classified as a shield volcano, it extends about 30 miles long and rises 3,600 feet above the sea floor. It is the site of frequent, small, flowing eruptions that can only be detected by sensitive instruments on its sides. Volcanic eruptions were detected on the axial seamount in 1998, 2011 and 2015, each lasting no more than a few weeks.

In December 2024, a group of scientists led by William Chadwick of Oregon announced that a network of axial seamounts showed that the volcano was swelling and had reached 95% of its value before the 2015 eruption. At the time, they concluded that a new eruption was likely next year. Volcanoes can be fickle, and a pivotal volcano is still biding its time. The seamount is still swelling, and an eruption could occur at any time. See the latest eruption forecast at https://axial.ceoas.oregonstate.edu/axial_blog.html.

This week’s earthquakes were centered 60 miles northeast of the Axial Seamount. No changes have been detected in the deep ocean instrumentation since the earthquake swarm began and seismologists and marine volcanologists agree that there does not appear to be a connection between the two. The axial volcano will eventually erupt again, but don’t worry. When basalt lava seeps to the seafloor, it will raise the temperature of the deep ocean around the ocean by a few degrees and can only be observed by a network of instruments and marine residents on the nearby seafloor.

The second sequence began on April 13 with an M5.7 aircraft at 6:29 PM PDT in western Nevada about 275 miles southeast, near the town of Silver Springs. This was a typical earthquake sequence with a mainshock followed by numerous aftershocks. The main shock tallied nearly 6,500 reports, most in an area stretching from Carson City to San Francisco, but some as far away as Salt Lake City and Las Vegas. It was powerful enough to knock items off the shelves of grocery stores in the central area, but no injuries or major damage were reported.

5.7 was centered just a few miles below the surface on an ENE-trending strike-slip fault. It has been followed by 245 aftershocks so far, clearly marking the rupture of the 10-mile-long fault. Most of the aftershocks were too small to be felt, but 24 of them reached a M3 range and were felt by residents of nearby towns. The sequence is by no means over. Four small earthquakes appeared on my computer Saturday morning while I was writing this column. The USGS estimates a 50% chance of more M3s this week and a small chance of an M4.

The Nevada earthquake was also an opportunity for the USGS and the Nevada Seismological Laboratory at the University of Nevada to test the expanded ShakeAlert earthquake early warning alert system in Nevada. Earlier this year, Congress appropriated $34.85 million to develop hardware and surveillance infrastructure and fully integrate the Nevada grid with California’s grid.

In the past, a lack of data in Nevada has caused problems for ShakeAlert in eastern California. The increase in funding may have been triggered in part by a false alert sent out in December 2025 that notified many Californians that shaking was expected after the system detected a false M5.9 near the California-Nevada border. To detect an earthquake and estimate its size accurately, you need stations surrounding the epicenter. ShakeAlert cannot work well for near-boundary earthquakes if the data coming from one side is weak.

Nevada’s full integration into the ShakeAlert system is not expected until 2028, but the improvement may already be noticeable from this week’s ShakeAlert notice. The initial alert was sent 17 seconds after the quake struck, with a magnitude estimated at 5.3 and the epicenter located 2 miles from the final location reviewed. As hardware improves, the notification time should drop to less than 10 seconds and will soon provide greater accuracy in size estimation.

Earthquakes in this area of ​​Nevada are not surprising. It lies in the transition zone between the eastern California shear zone and the Basin and Range topography, both of which developed to accommodate western North American extension caused by plate movement between the Pacific and North American plates. Unlike many maps, this plate boundary is not a simple line where the San Andreas Fault separates the two regions. It is a wide area whose deformation extends at least into Utah.

Nevada has seen its share of earthquakes recently. The M5.7 struck just 11 miles south of this week’s earthquake in December 2024. The M6.5 Monte Cristo earthquake in May 2020 and the M6.0 Antelope Valley earthquake in July 2021 were about 50 and 60 miles away, respectively. Larger earthquakes have occurred in earlier times. 1954 was a particularly fragile era for the region with earthquakes of magnitude 6.2, 6.5, 7.1 and 6.9 all occurring within a 60-mile radius of this week’s activity.

Most Nevada earthquakes have been concentrated in remote areas with few people. But Nevada’s population is growing, and a repeat of the larger 1954 earthquakes would cause major losses. There are also several faults designated as active in the greater Las Vegas area that have not produced strong earthquakes since the area boomed in the 1940s. I wouldn’t be at all surprised if the next devastating earthquake in the United States comes from Nevada.

Lori Dengler 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 the “Living on Shaky Ground” preparedness magazine? Leave a message at 707-826-6019 or email [email protected].

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