A map of earthquakes for North America reveals geological evidence and earthquake hazards

The new North American Generation Stress Map includes the first demonstration of the rift across the continent as well as more than 300 new measures of the direction from which the greatest pressure in the crust occurs. The background color indicates the relative stress size, or cracking pattern. (Photo credit: Jens-Erik Lund Snee and Mark Zoback) Credit: Jens-Erik Lund Snee and Mark Zoback

How are mountains formed? What are the forces needed to sculpt the basin? Why does the earth tremble and shake?

Earth scientists follow these basic questions to gain a better understanding of our planet’s past and its present works. Their discoveries also help us plan ahead by preparing us for earthquakes, locating oil and gas exploration, and more. Now, in a new, expanded map of tectonic pressures that affect North America, Stanford researchers offer the most comprehensive presentation yet of subterranean powers.

The results, published in Nature Communications on April 23, have implications for understanding and mitigating problems associated with the resulting earthquake – human-caused earthquakes – from unconventional oil and gas extraction, especially in Oklahoma, Texas and other areas targeting energy exploration. But they also ask a whole new set of questions that researchers hope to stimulate a wide range of modeling studies.

“Understanding the forces in the Earth’s crust is a basic science,” said co-author of the study, Mark Zubac, Professor Benjamin M. Page of Geophysics at Stanford School of Earth, Energy and Environmental Sciences (Stanford Earth). “In some cases, it has immediate application, and in other cases, it may be applied after decades to practical questions that do not exist today.”

The first continental synthesis of data

The new research provides the first quantitative synthesis of the rift across the entire continent, as well as hundreds of measurements of the directions of compression pressure – the direction from which the largest pressure in the crust occurs. The map was produced by compiling new and previously published measurements from wells as well as inferences about fault types or “patterns” based on earthquakes in the past.

The three possible patterns of rift include extended or normal fissures, where the crust extends horizontally; the slip rift, where the Earth slides behind itself, as in the case of San Andreas error; and the opposite, or orientation, of the fault in which the Earth moves over itself. Each one causes a completely different vibration from the point of view of danger.

“In our risk maps at the moment, in most places, we have no direct evidence of the type of earthquake mechanisms that could happen,” said Jack Baker, a professor of civil and environmental engineering who was not involved in the study. “It is interesting that we have gone from this blind assumption of anything possible to having some site-specific conclusions about the types of earthquakes we might expect.”

Zoom in

In addition to providing a continent-wide view of the processes governing the North American plate, the data – which includes nearly 2,000 stress trends, 300 of which are new in this study – provide regional clues about the behavior of the subterranean layer.

Lead author Jens said: “If you know the direction of any error and the stress situation nearby, you know how likely it will fail and whether you should be concerned about it in both the naturally occurring earthquake scenarios caused by the industry”, Eric Lund Seny, PhD 20, now a Postdoctoral Fellow at the American Geological Survey (USGS) in Lakewood, Colorado. “We’ve detailed some places where previously published geodynamic models match well with the new data, and other places where the models don’t fit very well.”

In the eastern United States, for example, the rift revealed by the study is the exact opposite of what might be expected as the surface “bounces” slowly after the ice sheets melting over most of Canada and the northern United States thaw about 20,000 years ago, according to Lund Snee. The discovery that recoil pressures are much lower than those actually stored in the crust from plate tectonics will enhance scientists’ understanding of the earthquake potential in that region.

In the western United States, researchers were surprised to see changes in pressure types and trends over short distances, with significant rotation only within tens of miles – a feature that current models of Earth’s dynamics do not reveal.

“It is now clear how pressure can vary systematically across a sedimentary basin in some areas,” Zubak said. “We see things we haven’t seen before that require a geological explanation. This will teach us new things about how Earth works.”

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more information:
Nature Communications (2020). Doi: 10.1038 / s41467-020-15841-5

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