Unusual Fault Pattern Surfaces in Earthquake Study

Like scars that remain on the skin long after a wound has healed, earthquake fault lines can be traced on Earth's surface long after their initial rupture. Typically, this line of intersection is more complicated at the surface than at depth. But a new study of the April 4, 2010, El Mayor–Cucapah earthquake in Baja California, Mexico, reveals a reversal of this trend. Superficially, the fault involved in the magnitude 7.2 earthquake appeared to be straight, but at depth, it’s warped and complicated.

The study, which was led by researchers at the California Institute of Technology with NASA Jet Propulsion Laboratory geophysicist Eric Fielding serving as a coauthor, is available online in the journal Nature Geoscience.

In a standard model, transform plate boundary structures -- where two plates slide past one another -- tend to be vertically oriented, which allows for lateral side-by-side shear fault motion. However, as the study found, the 75 mile (120 kilometer) long El Mayor–Cucapah rupture involved angled, non-vertical faults and the event began on a connecting extension fault between the two segments.

The new analysis indicates the responsible fault is more segmented deep down than its straight surface trace suggests. This means the evolution and extent of this earthquake's rupture could not have been accurately anticipated from the surface geology alone, says the study’s lead author Shengji Wei. Anticipating the characteristics of earthquakes that would likely happen on young fault systems (like the event in the study) is a challenge, since the geologic structures involved in the new fault systems are not clear enough.

Jean-Philippe Avouac, director of Caltech's Tectonics Observatory and principal investigator on the study, says the data can be used to illustrate the process by which the plate boundary -- which separates the Pacific Plate from North America -- evolves and starts connecting the Gulf of California to the Elsinore fault in Southern California.

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