Stable Fault Zones May Contribute to Massive Earthquakes

Plate tectonics utterly fascinate me, and this new news out of Caltech University just proves that this particular field of science is one of the most interesting out there.

Researchers at the California Institute of Technology, aka Caltech, along with the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) have found that previous assumptions about relatively “stable” fault connections may not be true, and may in part be responsible for the unexpectedly 2011 magnitude 9.0 Tohoku-Oki earthquake.

New Understanding of Stable Fault Zones
Numerical simulations illustrate that fault segments can move sowly and stably over long periods of time and later host large earthquaks. Dashed lines represent slow slip every 50 years along a cross-section of the fault, with the numbers indicating the simulated time in years. Earthquakes are shown by solid lines plotted every seccond. The area marked Patch B can both slip slowly (e.g., dashed lines above the 4,500 year mark) and participate in large earthquakes (e.g., yellow event).

Earthquakes are caused when two faults slip rapidly past one another. While faults are always moving, scientists had hypothesised that fault connections where the slippage is happening slowly – what is called ‘creep’ – were capable of acting as barriers to fast-slipping shake-producing earthquakes.

Not so, according to Caltech and JAMSTEC.

“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 January 9 in the journal Nature.

Lapusta, along with Hiroyuki Noda, a scientist at JAMSTEC, have found that stable areas will in some cases remain stable in the event of a nearby earthquake, but in some circumstances the seismic wave would enter the stable area in just the right way in which to create an even larger slip. In fact, Lapusta and Noda believe that this is what happened to the 1999 magnitude 7.6 Chi-Chi earthquake in Taiwan, where the largest slip actually happened in the “stable” zone.

“We find that the model qualitatively reproduces the behavior of the 2011 magnitude 9.0 Tohoku-Oki earthquake as well, with the largest slip occurring in a place that may have been creeping before the event,” says Lapusta. “All of this suggests that the underlying physical model, although based on lab measurements from a different fault, may be qualitatively valid for the area of the great Tohoku-Oki earthquake, giving us a glimpse into the mechanics and physics of that extraordinary event.”

This research essentially forces seismologists across the whole planet to re-evaluate their regions, for if a creeping segment can take part in a large earthquake it means that larger events are possible in many areas of the world.

One example is that of the San Andreas Fault. A creeping segment separates the southern and northern parts of the fault, and previous assessments of the area had suggested that this stable zone would prevent the quake propagating from one region to the other, therefore limiting the scope of a San Andreas earthquake. However, the team’s findings cast this assumption into disarray, requiring a new assessment of the area to determine whether a quake could involve Los Angeles and San Francisco metropolitan areas.

“Lapusta and Noda’s realistic earthquake fault models are critical to our understanding of earthquakes—knowledge that is essential to reducing the potential catastrophic consequences of seismic hazards,” says Ares Rosakis, chair of Caltech’s division of engineering and applied science. “This work beautifully illustrates the way that fundamental, interdisciplinary research in the mechanics of seismology at Caltech is having a positive impact on society.”

Next for Lapusta and Noda?  “We plan to further develop the modeling to incorporate realistic fault geometries of specific well-instrumented regions, like Southern California and Japan, to better understand their seismic hazard,” said Lapusta.

Source: Caltech

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