Wednesday, July 29, 2026

SoCal Seismic Network Manager Says Global Quake Cluster Has Not Raised Pasadena’s Earthquake Risk

By THERESE EDU

Allen Husker, who manages the Southern California Seismic Network from Caltech, says the local faults beneath Pasadena would deliver the sharpest shaking here

A magnitude 6.8 earthquake collapsed buildings, buckled a section of expressway and started fires in southwestern Japan on Tuesday, the latest large quake in a month that also saw major earthquakes in Venezuela and southern Mexico.

Eleven days earlier, with that run of quakes already drawing attention, a Caltech geophysicist who manages a seismic network monitoring Southern California’s faults told Pasadena Now that widespread seismic activity has not made a damaging earthquake here any more likely.

“So the short answer is no, it hasn’t gone up,” Allen Husker said of the risk that the activity had increased the odds of ‘The Big One’.

Husker, a research professor of geophysics at Caltech and manager of both the Southern California Earthquake Data Center and the Southern California Seismic Network, said the increase is largely a matter of appearance. The number of earthquakes worldwide has not really changed, he said. What changes is how much attention they draw: magnitude 7 events occur in places like the Mariana Trench without being reported at all, whereas the recent quakes struck near large population centers, particularly in Venezuela.

Earthquakes are ongoing, Husker said. Seismologists do not consider the past month s one notably for an increase in earthquakes at all.

Husker spoke with Pasadena Now on July 17, hours after a magnitude 7.3 earthquake struck off the coast of Chiapas, Mexico, and 11 days before Tuesday’s quake in Japan. His assessment addressed the pattern of clustered global activity rather than any single event.

The Mexico quake, he said, was not especially unusual. He previously worked in Mexico and has studied more earthquakes there than he has reviewed in California, and there have been other earthquakes like it in the past.

What happened this month

Tuesday’s earthquake struck near Uto, on Kyushu island, at 4:27 p.m. local time, according to the U.S. Geological Survey, which recorded a magnitude of 6.8 after initially reporting 7.1. Japan’s public broadcaster NHK reported that at least 50 people were taken to hospitals. The second floor of a shopping mall in Kashima collapsed, and the fire department said in a statement that many people were trapped inside. The Japan Meteorological Agency issued tsunami advisories for the Ariake and Yatsushiro seas and lifted them within two hours after no tsunami was detected. Prime Minister Sanae Takaichi said in a post on X that the government had established a task force and urged residents to stay away from the coastline.

The Mexico earthquake on July 17 was centered about 30 miles southwest of Aquiles Serdán, off the coast of Chiapas, at a depth of about nine miles, the USGS reported. It was felt from Mexico City to El Salvador.

On June 24, twin earthquakes of magnitude 7.2 and 7.5 struck northern Venezuela roughly 39 seconds apart, according to the USGS. Venezuelan officials have reported a death toll in the thousands.

A study on fault stress, and what it changes

Research published in June found that tectonic stress on the southern San Andreas and San Jacinto fault systems has reached — and in places exceeded — the highest levels of the past 1,000 years. The study, led by scientists at the University of Hawaii at Manoa and published in the Journal of Geophysical Research: Solid Earth, used a physics-based model fed with a millennium of reconstructed rupture history.

In a statement released by the university, lead author Liliane Burkhard said more than 160 years have passed since the last major rupture on the southern San Andreas and that the system is in a critically loaded state. The study also identified Cajon Pass, where the two fault systems meet northeast of Los Angeles, as a junction that could allow both faults to rupture in a single earthquake.

Asked whether that finding changes the odds, Husker said the odds are always increasing until another earthquake happens, because the plate keeps moving and pushing one side of the San Andreas against the other.

Seismologists already consider a significant Southern California earthquake as a high possibility, he said, and the paper adds to that knowledge rather than overturning it.

It does not change, however,  what he would ask residents to do.

Guidance on what to do before, during and after an earthquake is already published by groups including the Earthquake Country Alliance, he said, at https://www.earthquakecountry.org.

The USGS forecast known as UCERF3 puts the probability of a magnitude 6.7 or larger earthquake somewhere in Southern California at 93 percent over a 30-year window. Asked what a number like that means for someone deciding whether to strap down a water heater, Husker said a giant earthquake is unlikely in any given five-day stretch — but that it could happen at any moment.

“It’s better to do it before the earthquake, than after,” he said.

The faults under Pasadena

Pasadena sits near the Raymond and Sierra Madre faults, two of a number of faults in and around the city. Husker said a damaging earthquake could occur on any of them, and that the shaking here from a local rupture would be different in character from a distant San Andreas earthquake.

In a very large earthquake, he said, it is not the intensity of shaking that grows so much as the wavelength of the seismic waves. Where a deep sedimentary basin sits beneath a city, the natural frequency of that basin can trap those waves and amplify them. Pasadena sits in a good-sized basin, Husker said; Los Angeles sits in a larger one, which is why tall buildings there can swing more in a larger earthquake.

A local fault is a different problem. The shaking would be stronger because the source is so close, and the frequency of that shaking tends to match the natural frequency of the basin Pasadena sits in — a combination Husker described as “damaging.”

The shaking would not last as long as it would in a San Andreas earthquake, he said, but it would deliver sharp jolts and could cause significant damage and injuries.

The damage would also be more concentrated. There is one relative advantage, Husker said: resources could move in and out of a smaller, more narrowly affected area with relative ease.

A large San Andreas earthquake would strike a far wider swath of Los Angeles, forcing government agencies to distribute aid across a much larger area, with damaged roads, heavier reliance on airports and the potential for fires.

A local quake could be smaller in magnitude and still cause a great deal of damage close in, he said, without being as consequential for the region as a whole.

What he recommends

Husker deferred to FEMA for formal preparedness guidance and instead offered his own experience.

“Not all of us want to be preppers,” he said, but moments like this one are worth using to revisit some of the simple things.

He put the one-time jobs first — securing a water heater, a television, shelves and bookshelves — on the reasoning that they are done once and never require attention again. Storing extra water and supplies is harder to sustain, he said, because they eventually have to be replaced.

Another big one people sometimes forget, he said, is knowing their neighbors.

“Strong communities are resilient,” Husker said, describing what he saw after the 2017 earthquake in Mexico, when surrounding communities brought food and shelter to affected neighborhoods. “If we’re good neighbors, we can help each other.”

That one is also a fun one, he said: knock on the neighbor’s door and say hello.

After a nearby magnitude 5 quake — which does raise the near-term chance of something larger — he suggested something smaller: trade the flip-flops for sturdier shoes. It is hard to stay in a state of constant fear, he said, and that is not the goal.

“Smaller steps I think are better,” he said.

If the phone buzzes

In Mexico City, the public alert did not sound during the July 17 quake. The Mexican government said the energy radiated in the first seconds did not exceed the system’s activation thresholds.

Husker said Mexico’s system and the ShakeAlert system used in California are built to do different things.

Caltech helped develop ShakeAlert. Mexico’s system alerts a specific set of cities, including Mexico City and Oaxaca City, all of them far from the Chiapas epicenter — so there was no need to alert them.

ShakeAlert instead reaches people near an earthquake on their cell phones, regardless of whether a major city is involved. He cited the Mendocino earthquake north of the Bay Area, where a sparsely populated region was alerted, as the closer analog. In an equivalent California event, he said, the system would alert.

When an alert arrives: drop, cover and hold on, wait for the shaking to stop, then look for information. After a large enough earthquake, he said, expect aftershocks. After a smaller one, aftershocks are less of a concern.

Information on the ShakeAlert system is at https://www.shakealert.org. USGS event pages and current Southern California seismicity are at https://earthquake.usgs.gov.