Lori Dengler | The ground roll is real, but we’re still not sure exactly who it is – Times-Standard

Last December, I asked you for stories about unusual events during earthquakes. These reports have contributed to a poster that a group of us will present at the annual meeting of the American Seismological Society next week. I received many observations, including unusual sounds and smells, earthquake lights, strange animal behavior, and trees bending into impossible positions, but the area I decided to focus on for our presentation was seeing the Earth rolling “like waves in the ocean.” I call these reports the Visible Ground Roll (VGR), and it’s time to give them more attention.

I’ve mentioned VGR in previous columns, and your reporting was just the impetus I needed to dig deeper. Ground observations go back to some of the earliest written accounts of earthquakes. The institute of geophysics, Academia Sinica, has compiled a list of earthquakes in China beginning in 1177 BC, and a number of ancient events include descriptions that sound like VGR. Of particular note is the Shaanxi earthquake that occurred during the Ming Dynasty in 1556, which is considered one of the deadliest earthquakes ever. It caused caves where many people lived to collapse, and was followed by widespread crop failure. At least 500,000 people died as a result of a combination of the direct effects of the shaking, famine, and plague that followed. A common description by survivors is seeing people rolling on the ground.

I first became interested in ground rolling as part of the team that re-examined the December 1954 Fickell Hill earthquake (Not My Fault, August 30, 2025). I’ve collected reports from people who were here when the earthquake happened. Some people sent in newspaper clippings or wrote accounts from relatives, but the majority were eyewitness observations from people now aged between their late 70s and 90s who had this problem as children.

My current tally is roughly 40 reports from people who ranged in age from 3 to their early teens when the quake struck. Reports arrived individually via email, letters and phone calls. I didn’t make any prompts about what kind of feedback I was looking for, I just asked them what they could remember. That earthquake occurred just before noon the week before Christmas, and many of my correspondents were abroad at the time. Nearly half reported being startled by the undulating ground position, which I now call VGR.

This wasn’t the first time I’d heard VGR reports. This was something that was introduced in our first seismology class when we learned about intensity scales. Intensity is a qualitative measure of how strong shaking varies, and the Modified Mercalli scale uses “waves appearing on the Earth’s surface” as an indicator of the strongest levels of shaking. Charles Richter’s book “Eliary Seismology” published in 1958 states that “there is almost certainly a real phenomenon of advancing or standing waves” observed by people during some earthquakes. He qualifies his statement that illusions can affect observations.

I’m never surprised when people tell me they see the ground rolling away. I haven’t seen it myself and I’m a little jealous of those who have always considered these observations to be accurate. After the April 1992 Cape Mendocino earthquake, I spent a lot of time developing a survey questionnaire to provide a more quantitative way to measure severity. We asked all kinds of questions, looking for variables that showed an increase with shaking strength, and this questionnaire would eventually become the basis of the USGS’s “Did You Feel It?” web page. It never occurred to me to include a question about VGR. At the time, this was not something that was considered in serious seismology circles.

Reports of the 1954 earthquake made me rethink this phenomenon. 70 years later, many people have vivid memories of the Earth rolling. They often don’t remember many of the things we associate with strong earthquakes such as loud noises or things falling off shelves. What stood out was seeing the chimneys falling, the cracks in the ground, their parents’ fear, and seeing the VGR.

A quick survey of the literature revealed very little. There was a paper written in 1970 by a seismologist who had observed VGR during the Great Chilean Earthquake of 1960. He admitted that most members of the seismological community were not believers and attributed these observations to a disturbed state of mind during an earthquake that distorted perceptions. But he was very clear about his observations and thought it might be due to gravity, which causes solid ground to vibrate, similar to oscillations in bodies of water.

While professional journals were absent, there were a large number of ground reports in newspapers and other media after strong earthquakes. There is no catalog of VGR reviews, so as part of our label, I began collecting these views through online searches and requests through print and social media. There were accounts of other earthquakes before they occurred, such as the Port Royal earthquake in Jamaica in 1692, in which the ground not only rolled but liquefied so violently that people sank into the ground, and the New Madrid earthquakes of 1811-1812 in Missouri and Arkansas that caused the Mississippi River to temporarily flow backwards.

VGR’s list of more recent earthquakes includes some of the largest on record – 1960 Chile (M9.5), 1964 Alaska (M6.4), and 2001 Japan (M9.1). Ten California earthquakes were on my list, three of them on the North Coast. It includes earthquakes in the greater Los Angeles area, Santa Barbara, Coalinga, Loma Prieta, and others. I’m sure a more thorough search would add more events. Most, but not all, reports were of relatively shallow earthquakes with a reverse fault-slip component.

He turned to Dave Wald at the USGS to see if he could find accounts at the USGS “Did you feel it?” Survey responses. The DYFI web page (https://earthquake.usgs.gov/data/dyfi/) was launched in 1999 using a density questionnaire. It does not include any questions about VGR, but participants can add additional comments. A quick search turned up 57 with possible visible wave reports. I haven’t had time to look in detail at the events that sparked VGR’s comments, but there’s no clear connection between how strong an earthquake is felt and seeing the ground rolling.

The big question is what physical phenomenon lies behind these visual reports. We looked closely at two earthquakes with seismic records from the same area as the VGR observations. Powerful motion instruments were operating at Eureka during the 1954 M6.5 earthquake and at Coalinga during the 1983 M6.7 earthquake. The recordings show ground displacements of a few inches with 10-second intervals detectable by the human eye. This is the seismic surface wave portion of the record, and Rayleigh waves that have both vertical and horizontal displacement could be the cause.

The consensus of our group is that the VGR is real for many of the eyewitnesses who report it, and may be caused by Rayleigh waves, but there is still much work to be done. VGR is not related to vibration strength and is not included in current intensity formulations. Our preliminary tabulation of VGR events suggests that shallow earthquake sources may be more likely to produce visible ground rolling. I have not heard of any reports of ground rolling from recent North Coast earthquakes, including the 1994 and 2024 Mendocino fault earthquakes and the 2021 M6.0 Mendocino triple junction event. Eyewitness reports are likely exaggerated, and I doubt the roll height was 20 feet as someone recounted in 1954. Although it is not useful in determining earthquake magnitude, I think the VGR is a very fascinating phenomenon and may be very useful as a tool Educational to attract public interest and awareness of the amazing planet we live on.

Charles Richter was right, as he was about many things seismic – VGR is real, we’re not entirely sure of the exact physical cause, and the earthquake experience may distort people’s perceptions. VGR is certainly worth further exploration.

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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