Lake deposits reveal directional shaking during devastating 1976 Guatemala earthquake
image:
Lake sediment core showing the background sedimentation in the lake (laminations) and the disruption generated by a turbidite (light gray layer with no internal structure).
view moreCredit: Jonathan Obrist-Farner
Sediment cores drawn from four lakes in Guatemala record the distinct direction that ground shaking traveled during a 1976 magnitude 7.5 earthquake that devastated the country, according to researchers at the Seismological Society of America’s Annual Meeting.
The earthquake, which killed more than 23,000 people and left about 1.5 million people homeless, took place along the Motagua Fault, at the boundary between the North American and Caribbean tectonic plate boundary.
Severe ground shaking from the 1976 earthquake caused landslides and sediment-laden turbidity currents that can be seen clearly in cores taken from the lakebeds. Normally, researchers might expect that this shaking would produce the thinnest sediment deposits in lakes furthest away from an earthquake, since seismic waves weaken as they travel away from an earthquake epicenter.
But in the Guatemalan lakes, the cores with the thickest sediment traces of the earthquake occur at the end of the fault rupture, said Jonathan Obrist-Farner, a geologist at Missouri University of Science and Technology. “What we see is lakes that are actually the closest to the epicenter but just away from the rupture path have very thin deposits.”
Jeremy Maurer, a geophysicist also at Missouri University, suggested that the unusual pattern had in this case recorded the directivity of the 1976 shaking.
It’s not unusual for scientists to find evidence of past earthquakes in lake sediment cores, Maurer added, noting examples from New Zealand to Turkey that offer a glimpse at how far away a particular earthquake could have an impact.
“What hasn’t been done as much is looking at where these lakes are located in relationship to the fault,” said Maurer. “Are they off-axis or on-axis? Does the direction of the rupture have an effect on sediment deposits?”
When the U.S. Geological Survey collected field data after the 1976 earthquake, “they found, for example, adobe houses that were 10 kilometers south of the main rupture path that were still standing, yet those that were actually on the fault trace and towards the propagation direction all collapsed,” said Maurer. “I think there’s a lot of evidence that points to the directivity of the rupture and now we’re just looking at it sedimentologically from the lakes.”
The researchers began recovering and analyzing cores from the lakes in 2022. “We thought it would be a very interesting opportunity to not just look at the 1976 earthquake, but actually learn a little bit more about the paleoseismic history of the plate boundary, which we know very little of,” said Obrist-Farner, who is originally from Guatemala.
Although there was a brief rush of seismologists to the region after the 1976 earthquake, the impacts of a 36-year civil war and sparse instrumentation have left the plate boundary poorly monitored. Paleoseismic data like the lake records are important for building a more complete picture of the country’s seismic risk.
Last year Obrist-Farner’s team retrieved their largest cores yet from the lakes, with lengths of sediment that may represent up to four thousand years of lake history. Their initial analysis shows evidence of the 1816 earthquake of at least magnitude 7.5 that is known mostly from historical documents.
Native American names extend the earthquake history of northeastern North America
In 1638, an earthquake in what is now New Hampshire had Plymouth, Massachusetts colonists stumbling from the strong shaking and water sloshing out of the pots used by Native Americans to cook a midday meal along the St. Lawrence River, according to contemporaneous reports.
When Roger Williams, founder of the Rhode Island colony, talked with local Native Americans, he reported that the younger tribe members were surprised by the earthquake. But older tribe members said they had felt similar shaking four times in the past 80 years.
In his talk at the Seismological Society of America’s Annual Meeting, Boston College seismologist John Ebel urged his colleagues to collect more information about past earthquakes in eastern North American from Native American stories and languages.
Although it might not feel like earthquake country to a Californian, for example, northeastern North America experiences regular seismic activity and has hosted large earthquakes in the past. Written records of these earthquakes include the past 400 years, but Ebel said extending this record further into the past with the help of Native American knowledge can help scientists better understand earthquake hazard in the area.
Sometimes the clues to past seismic activity are in Native American place names, Ebel said. There’s Moodus, Connecticut, for instance. Moodus comes from an Algonquin dialect and means “place of noises.” For hundreds of years, people have heard “booms”—as if echoing in an underground cavern—in the area. Ebel said the Moodus noises are similar those he heard as a graduate student camping in the Mojave Desert following a magnitude 5.1 earthquake.
“The Moodus noises sounded like distant thunder of a boom coming up from the ground, very similar to what I heard from the California aftershocks several years before,” said Ebel, who noted that modern seismic instruments have recorded earthquake swarms in Moodus. “So the ‘place of noises’ means that they were hearing earthquakes long before Europeans came to that locality.”
Then there’s the regular small earthquake activity in the northwest suburbs of Boston, where Ebel and his colleagues have been monitoring since the mid-1970s. “I was going through books one day looking for information on historical earthquakes there, and I come across this WPA guide from the 1930s, and it's talking about Route 2, which runs right through that area, and it goes right near a hill called Mount Nashoba,” he recalled.
The guide included “a little translation that said Nashoba is from an Indian word that means ‘hill that shakes.’ So now I've got all of these little earthquakes, and right in the center of it is a place with an ancient name that means hill that shakes,” Ebel said.
Researching which tribes in the region have a word for earthquake could be useful, “because that would suggest that earthquakes were a rather repetitive thing,” he noted. His early searches indicate that the Seneca, Cayuga, Natick and Mi’kmaq tribes all have a word for earthquake.
Ebel said interdisciplinary research with ethnologists with more detailed knowledge about Native American languages and narratives could be very helpful to seismologists looking to extend the northeastern North America earthquake record into pre-colonial times. “If there are legends that preserve information about probable earthquakes, for instance, it might be possible to define some sort of estimate of [shaking] intensity from the descriptions in the stories,” he suggested.
How wide are faults?
At the Seismological Society of America’s Annual Meeting, researchers posed a seemingly simple question: how wide are faults?
Using data compiled from single earthquakes across the world, Christie Rowe of the Nevada Seismological Laboratory at the University of Nevada, Reno and Alex Hatem of the U.S. Geological Survey sought a more comprehensive answer, one that considers both surface and deep traces of seismic rupture and creep.
By compiling observations of recent earthquakes, Rowe and Hatem conclude that from Turkey to California, it’s not just a single strand of a fault but quite often a branching network of fault strands involved in an earthquake, making the fault zone hundreds of meters wide.
“So that suggests that significant parts of the broad array of fractures that develops over many earthquakes can be activated in a single earthquake,” said Rowe, who noted that this width sometimes roughly corresponds to the width of Alquist-Priolo zones established for safe building in California.
“We want to know how this might change things like the shaking patterns that you would expect, or how much radiated energy you get from an earthquake,” Rowe explained. “Because it’s not the same if you have slip distributed on many strands as when it is all on one strand of the fault.”
At the same time, the researchers found that the width of creep zones at these earthquakes are much narrower, both near the surface and 10-25 kilometers deep in the earth. The creep zones, between 2 and 10 meters wide, “may be the most localized behavior a fault does,” Rowe said.
The study emphasizes the importance of thinking of faults in a more three-dimensional manner, said Rowe.
“As a geologist, it's always kind of been a cognitive disconnect for me when I talk to earthquake modelers who have these two-dimensional features that they model earthquakes on,” she said. “Because the sheer resistance, the strength or the friction, comes from a volume of rock that's deforming during an earthquake or in between earthquakes. So the size of that volume controls the strength of the fault in some really tangible ways.”
The researchers used a variety of data in their study, including rupture maps, creeping zone width from surveys of slowly shifting monuments along faults and satellite observations, the locations of earthquake aftershocks, low velocity damage zone widths, and the zones delineated by certain types of rock such as pseudotachylyte, ultramylonite and mylonite that are a signature of creep and deformation.
The findings also have implications for how scientists study past earthquakes to calculate earthquake recurrence intervals on faults, Rowe noted.
Slip rates and recurrence intervals can be constrained using localized measurements, but it can be difficult to disentangle the slip that occurred during an earthquake and aseismic slip that occurred after the event. The 2014 Napa, California earthquake is a good example of this phenomenon, said Rowe, noting that almost half of the slip measured after that event occurred slowly after the earthquake.
But if the Napa earthquake occurred thousands of years ago and researchers came across its traces in the rock record, “you would just see a bigger earthquake. You might lump all of that slip as a single event,” Rowe said.
Creep isn’t always accounted for in calculating recurrence intervals, “so finding out that creep zones are quite narrow means that we should be aware that we could be convolving creep with seismic slip when we look at those paleoseismic records,” she added.
