Historic Earthquakes

Sometimes non-scientists are better at answering scientific questions than scientists. Up next — geologists work with an historian to learn more about a California earthquake in the year 1857

Historic Earthquakes 1

In January of the year 1857, a large earthquake shook the San Andreas Fault in southern California.

Geologists want to know how strong the quake was, so they can better predict how strong future quakes might be. But no scientific instruments existed back then to measure the quake. Dawn Martindale is an historian at Utah State University in Logan. She’s helping geologists find eyewitness accounts of how earthquakes felt — in old diaries, journals, newspapers and photographs.

Historic Earthquakes 2 Dawn Martindale: We use what’s called the Mercalli index, and its based on how a person feels or what happens to the buildings or the ground, and we assign an intensity, and it’s on a scale of 1 to 12, and from there I can kind of gauge where the epicenter was at.

Martindale makes maps that pinpoint where the quake occured. Geologists compare her maps with fault maps and identify the active fault and estimate the strength.

Dawn Martindale: I’ve always been interested in history since I was a little girl. And I’ve also been interested in science . . . But I never thought you could combine them . . . I had no idea I could even do this for work. So I was really excited.

Letters of Peter TenBroeck

The letters of Peter G. S. TenBroeck, assistant surgeon at Fort Tejon, California were a goldmine, says Dawn Martindale. A colleague stumbled across them in the National Archives in Washington, D.C., and brought copies back to Martindale. They offer eyewitness accounts of the so-called Fort Tejon (TAY hone) earthquake of January, 1857 along the San Andreas Fault. Here are some excerpts from the letters:

January 19, 1857

“At 8:30 am, the principle shock commenced. At the time the earth was in motion for five minutes. The first portion of the shock was much the heaviest and continued for about a minute during which time the trees and the mountains about us were in constant motion and the quarters swayed to and fro. Then came a lull for some two minutes during which the earth sobbed and swelled like a living creature. … At one time it seemed as if a heavy blow had been struck just under my feet so as to lift me into the air. Just above the garrison, oak trees that were 8 to 10 feet in diameter were snapped off near the ground.”

The letter goes on to describe the earthquake direction, as well as the 40 mile long fissure which opened during the quake. After writing this letter, TenBroeck began to keep records of all the aftershocks. He noted the time, and whether they were strong or weak earthquakes. He was helped out by the hospital staff — if one occurred during the night, the staff would tell him about it in the morning. After six months, he took his ledger and analyzed the results. Here’s an excerpt from his analysis letter.

July 3, 1857

“Since the first shock of the earthquake, those which succeeded, as during the month of January, were felt over a large extent of the country. But since then the circle has been constantly diminishing and for the last three months the shocks have been confined almost exclusively to the post and its immediate vicinity. Lately, the majority of the shocks have occurred during the night, and we’ve become so accustomed to them that they do not awaken us as formally.”

One of Martindale’s tasks is to check out the people who write these documents and make sure that they are trustworthy sources of information. She says that Captain Peter TenBroeck seems reputable, and was known for being scientifically minded. Martindale plans to look for more of TenBroeck’s letters in the National Archives in the future.

It’s known that large earthquakes can trigger other earthquakes far away in other parts of the world. So now, Martindale and her colleagues are going to look at records from quakes in Nevada and southern Utah in 1857 to see if they were triggered by this big California quake — Fort Tejon. This could give scientists clues about what might happen in future earthquakes.

Finding “Felt Reports”

Finding information about historical earthquakes is a treasure hunt, and many hours are spent poring over old documents, letters, and diaries.

Dawn Martindale begins many of her searches with state historical societies. These organizations have libraries — most people don’t know they exist — where the historical societies take care of diaries, journals, photographs, and maps and other artifacts of life in the state.

When she goes to these libraries, Martindale gathers any artifact that was written during a certain time period, and then reads it, looking for any information. The information could be tiny — a reference to a building that fell down, or a new crack that appeared and crossed a road. But the clues can also be dramatic. During the Fort Tejon earthquake, someone described how half a circular corral was shifted so much that the corral became an S-shape. Others described how the river sloshed back and forth so much it fell out of its riverbanks.

Martindale also hunts for newspaper articles. Unlike geologists, who might only look in the biggest, well-known newspapers for information, Martindale hunts for newspapers from smaller communities. When she finds them, she reads them for at least a week after the event, sometimes up to 45 days if the earthquake were very large, as was the case in Fort Tejon. Maps, and photographs and old sketches are also important, since they describe what type of houses and buildings the people used, and how these structures were affected in the earthquakes.

According to Dawn Martindale, “I found a felt report from, I believe it was, a journal out towards Blayne county Idaho, it was way out there, and when I showed it to them they said, ‘We didn’t know there were faults there!’ and I said, ‘Well, they felt something, so you gotta go find it now!’ ”

According to Dawn Martindale, “Jim’s asked me,’How do you find this stuff?’ And I say, ‘You know where to look. And if not, you learn to ask.’ I use librarians a lot, they’re usually quite knowledgeable in what they have in the their facilities. Some things they don’t know they have, but they get me on a good start and from there, it’s… just learning to be like a detective and you know and search out anything that might possibly have the tiniest bit of information. I’ve heard some scientists say it’s tedious work. But I guess I like that.”

How this project got started

According to Dawn Martindale, “I started working with Jim Evans when I was a master’s student, at Utah State. He was curious about an earthquake that had happened in Utah in 1884, so he approached the history department. He figured if you need to look at historical records, get a historian! And I heard about the project a year later and volunteered, and it went so well that after that we decided we wanted to try other earthquakes. So we applied for a grant with the USGS, to look at all earthquakes in the basin-range area during the 1800’s. And they said, ‘We’d like you to look at Fort Tejon project.’ So we said OK, and from there it’s taken off.”

According to Dawn Martindale, “I’ve always been interested in history since I was a little girl. And I’ve also been interested in science. My parents and teachers were great. But I never thought you could combine them. And then I even went through my undergraduate — I have a degree in American history, I learned about the revolution like most people. I just didn’t think you could combine it, even though I liked the two fields. And then when I went to Utah State, there was a geologist there who thought, what the heck, let’s give this a try. And I found a historian and they said, well, okay we’ll be there to help you. It’s just evolved since then. I had no idea I could even do this for work. So I was really excited. This is great.”

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