DisastersAppearances deceive: supposedly “stable” zone make earthquakes even more powerful

Published 10 January 2013

In an earthquake, ground motion is the result of waves emitted when the two sides of a fault move — or slip — rapidly past each other, with an average relative speed of about three feet per second. Not all fault segments move so quickly, however; new earthquake fault models show that “stable” zones may contribute to the generation of massive earthquakes

In an earthquake, ground motion is the result of waves emitted when the two sides of a fault move — or slip — rapidly past each other, with an average relative speed of about three feet per second. Not all fault segments move so quickly, however — some slip slowly, through a process called creep, and are considered to be “stable,” for example, not capable of hosting rapid earthquake-producing slip.  One common hypothesis suggests that such creeping fault behavior is persistent over time, with currently stable segments acting as barriers to fast-slipping, shake-producing earthquake ruptures. A Caltech release reports that a new study by researchers at the California Institute of Technology (Caltech) and the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) shows, however, that this might not be true.

What we have found, based on laboratory data about rock behavior, is that such supposedly stable segments can behave differently when an earthquake rupture penetrates into them. Instead of arresting the rupture as expected, they can actually join in and hence make earthquakes much larger than anticipated,” says Nadia Lapusta, professor of mechanical engineering and geophysics at Caltech and coauthor of the study, published today (9 January) in the journal Nature.

She and her coauthor, Hiroyuki Noda, a scientist at JAMSTEC and previously a postdoctoral scholar at Caltech, hypothesize that this is what occurred in the 2011 magnitude 9.0 Tohoku-Oki earthquake, which was unexpectedly large.

Fault slip, whether fast or slow, results from the interaction between the stresses acting on the fault and friction, or the fault’s resistance to slip. Both the local stress and the resistance to slip depend on a number of factors such as the behavior of fluids permeating the rocks in the earth’s crust. So, the research team formulated fault models that incorporate laboratory-based knowledge of complex friction laws and fluid behavior, and developed computational procedures that allow the scientists to numerically simulate how those model faults will behave under stress.

The uniqueness of our approach is that we aim to reproduce the entire range of observed fault behaviors — earthquake nucleation, dynamic rupture, postseismic slip, interseismic deformation, patterns of large earthquakes — within the same physical model; other approaches typically focus only on some of these phenomena,” says Lapusta.

In addition to reproducing a range of behaviors in one model, the team also assigned realistic fault properties to the model faults, based on previous laboratory experiments on

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