Fault ‘scratches’ offer clues to rupture direction

Researchers have discovered a pattern in slickenlines — scratches on fault surfaces — that can be used to determine which way a fault ruptured in a past earthquake.
 

By Jennifer Schmidt, Ph.D., Temblor Earthquake News Director (@DrJenGEO)
 

Citation: Schmidt, J., 2020, Fault ‘scratches’ offer clues to rupture direction, Temblor, http://doi.org/10.32858/temblor.119
 

Within minutes following an earthquake, scientists around the world can pinpoint the near-exact location of the rupture, how deep it originated and what type of fault slipped. This rapid-fire analysis is possible because of a global network of seismometers and other ground-motion monitoring equipment. Prior to modern instrumentation, knowledge of where earthquakes occurred came mostly from records of damage and written accounts of shaking. But these records provide a coarse view of the events and offer little certainty about what transpired at depth.

Fortunately, earthquakes leave behind fingerprints of their destructive power. In some cases, those fingerprints are as clear as a painting on canvas: Structures are flattened, Earth’s surface buckles and cracks, streams or fences that cross a fault are offset. Clues about where an earthquake originated and in which direction it propagated during the rupture can also be more subtle, though. A new study outlines one such line of evidence — scratches in the rock record called “slickenlines” — that can be used to determine how a rupture propagated during past earthquakes.

 

An old record with new revelations

Just as dragging a piece of heavy furniture across a hardwood floor can leave a scratch, small bumps on one side of a fault surface can scratch the other side of the fault during an earthquake. Scientists have known about slickenlines for a long time and have used them to determine the distance and orientation that the fault slipped. A floor scratch reveals how someone dragged a heavy chair 5 feet between the window and the door, whereas a slickenline reveals how a fault slipped horizontally 5 feet. But until now, scientists couldn’t use slickenlines to reveal where an earthquake started or which way it ruptured.

 

 

Earthquakes start at a point

Slickenlines record where a rupture starts — its “nucleation point” — and how the seismic waves spread out in all directions like ripples on a pond, says Jesse Kearse, a structural geologist and doctoral student at the Victoria University of Wellington, lead author of the new study in the Journal of Geophysical Research: Solid Earth.

As the rupture progresses, the area of the fault surface that slips, or moves, increases. The leading edge of the slipping portion of the fault — the “rupture front” — releases energy in its wake. This energy influences the way the fault slips and continuously pushes the fault offset slightly off course, according to Kearse. Slickenlines record this push in their curvature. Imagine the heavy chair being dragged in an arc across the floor, rather than a straight line.

The orientation of a curved slickenline, according to this study, depends on where the slickenline formed relative to the nucleation point of the earthquake rupture. These records would therefore reveal how the rupture propagated from the origin.

 

 

Uncovering earthquake rupture direction

For any surface-rupturing earthquake, scientists can now make sense of the orientation of curved slickenlines on fault scarps or excavated portions of faults. With knowledge of the relative offset on the fault — right lateral strike-slip or thrust for instance — scientists can determine where this location is relative to the nucleation point of the quake. By documenting slickenline orientation in several places along the fault trace, scientists can roughly locate the earthquake origin.

This kind of record is essential for historic or modern earthquakes for which there is no instrumental record, such as the 1857 Fort Tejon earthquake in Southern California. As Kearse and co-author Yoshihiro Kaneko of GNS Science in New Zealand suggest in this article, by using slickenlines, scientists could now determine whether the rupture propagated along the San Andreas in one direction or radially from the nucleation point. Curved slickenlines that were uniformly oriented along the rupture length would suggest that the earthquake originated at one end and the rupture propagated in one direction from that point.

 

Testing longstanding theories

This study outlines a way for scientists to test ideas about how earthquakes propagate. Earthquakes often propagate along a fault in one main direction, as in the 2002 magnitude-7.9 Denali quake in Alaska.

“[This study] has a potential high impact,” says Ruth Harris, a research geophysicist at the U.S. Geological Survey Earthquake Science Center. Harris says she envisions scientists testing long-held theories about why earthquakes rupture and propagate in one direction versus another along certain faults. Some scientists think that the rock type variability across the fault surface plays a role. One could document the orientations of slickenlines along a number of fault scarps and compare this with instrumental data to determine if quakes always rupture in a certain direction for the same rock type. This study gives scientists a new way to look at these ideas, Harris says. With a better understanding of fault behavior comes better earthquake hazard models.

 

 

This new method is not necessarily a way to use slickenlines from previous earthquakes to forecast which way a specific fault will rupture in the future, however. Scientists couldn’t, for example, look at slickenlines on an old San Andreas Fault scarp and say that the fault will rupture in the same direction again. It is still unclear whether faults are consistent in the way that they rupture over time, according to Harris. Additionally, “you don’t know where earthquakes are going to start; that’s part of our problem,” Harris says. But Kearse says he hopes that scientists will start using this tool to look at slickenlines around the world. With a more complete catalog of rupture directions for previous earthquakes, scientists would have a better grasp of how faults typically rupture.

 

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Further Reading

Kearse, J., & Kaneko, Y. On‐fault geological fingerprint of earthquake rupture direction. Journal of Geophysical Research: Solid Earth, e2020JB019863.

Kearse, J., Kaneko, Y., Little, T., & Van Dissen, R. (2019). Curved slickenlines preserve direction of rupture propagation. Geology, 47(9), 838-842.