Seismic stress in America

This is a new map of the (non-political) tectonic stresses acting on North America.

Created by researchers from Stanford University, US, it provides the first quantitative synthesis of faulting across the entire continent, as well as hundreds of measurements of compressive stress directions – the direction from which the greatest pressure occurs in the Earth’s crust. 

The map was produced by compiling new and previously published measurements from boreholes as well as inferences about kinds or “styles” of faults based on earthquakes that have occurred in the past.

The three possible styles are: extensional or normal faulting, in which the crust extends horizontally; strike-slip faulting, in which the Earth slides past itself, as in the San Andreas fault; and reverse or thrust faulting in which the Earth moves over itself. Each one causes very different shaking from a hazard point of view.

The research, and map, are published in the journal Nature Communications.

“Understanding the forces in the Earth’s crust is fundamental science,” says co-author Mark Zoback. “In some cases, it has immediate application; in others, it may be applied decades later to practical questions that do not exist today.”

In the Western US, the researchers were surprised to see changes in stress types and orientations over short distances, with major rotations occurring over only tens of kilometres – a feature that current models of Earth dynamics do not reveal.

“It’s just much clearer now how stress can systematically vary on the scale of a sedimentary basin in some areas,” Zoback says. “We see things we’ve never seen before that require geologic explanation. This will teach us new things about how the Earth works.”