Pulselike and Cracklike Ruptures in Earthquake Experiments

Dec 03, 2007

Lab experiments that mimic the way the ground moves during destructive earthquakes require some sophisticated equipment, and they yield valuable insights. California Institute of Technology scientists studying how sliding motion spreads along a fault interface conducted a series of experiments involving ultrafast digital cameras and high-speed laser velocimeters to replicate a range of realistic fault conditions.

The Caltech team documented for the first time a systematic variation in earthquake rupture patterns called pulselike and cracklike ruptures. The experiments also revealed that both types of ruptures can transition to a state known as supershear speed, which generates its own characteristic ground shaking. The results appeared in the November 27 issue of the journal Proceedings of the National Academy of Sciences.

The scientists include Xiao Lu, graduate student in aeronautics; Nadia Lapusta, assistant professor of mechanical engineering and geophysics; and Ares Rosakis, the von Kármán Professor of Aeronautics and Mechanical Engineering and director of the Graduate Aeronautical Laboratories.

Simple theoretical models of earthquake ruptures show they slide like a crack--the entire length of the fault slides for just about as long as the earthquake lasts. But slip models used by seismologists to match records of ground motions from past earthquakes have suggested a different mode of rupture, one that moves like a pulse. A pulse of slip would travel down the length of a fault like a ripple passing over the surface of a pond, with all motion contained in the ripple, and the fault surface "healing" in its wake.

The forces that build up on either side of a fault, known as tectonic loading, can vary greatly and lead to different types of fault slip behavior. "Numerical calculations of earthquake ruptures that use friction laws guided by laboratory experiments produce both crack- and pulselike modes, depending on how loaded the fault is," says Lapusta. "We set out to test the predictions of these calculations in our experimental study." Pulse modes are predicted by calculations where faults are less loaded, but to make a fault slip under these conditions, models have to assume that fault friction decreases as the fault slip gets faster. This behavior, called rate-weakening friction, has been of long-standing interest to Lapusta and to Thomas Heaton, professor of engineering seismology, whose influential work on slip pulses demonstrated their short duration, and who proposed rate-weakening friction as a likely explanation.

The experiments began with a 9.5-millimeter-thick photoelastic plate sliced at an angle through its length, simulating a fault in Earth's crust. Pressure on the two sides of the fault was applied incrementally at an angle to build up the different components of loading. To trigger an earthquake rupture, a nickel wire the diameter of a human hair was embedded in the plate interface and then electrically discharged, creating a small explosion followed by a spontaneously spreading rupture. Lasers measured the relative movements on each side of the fault after the shock, and a high-speed camera captured the movements in 5-microsecond intervals. The mini-explosions were repeated for various orientations of tectonic loading.

The experimental setup mimics conditions under which very large earthquakes rupture Earth's crust along major strike-slip faults like California's San Andreas fault or the Kunlun fault in northern Tibet. The initial experimental design was devised by Rosakis; Smits Professor of Geophysics, Emeritus Hiroo Kanamori; and their joint graduate student Kaiwen Xia, who is now a professor at the University of Toronto.

The new experimental results support the models that suggest faults can have pulselike ruptures. "This is the first time we observed this spontaneous pulselike rupture in an experiment that mimics crustal earthquakes. We proved its existence," says Lu.

The experiments also documented under what conditions pulselike ruptures arise. When the plate interface was oriented at a 70-degree angle to the direction of compression, the rupture propagated as a narrow pulse. At smaller angles, the pulses got wider, until they transitioned into cracklike sliding modes. These experimental observations demonstrate the role that tectonic loading plays in how earthquakes rupture, and imply that real faults are governed by rate-weakening friction.

Another experimental result is related to earthquake rupture speeds. Calculations since the 1970s have predicted phenomena known as supershear bursts, which would cause destructive, high-frequency ground motions. Rosakis, Kanamori, and Xia have demonstrated such bursts in their experiments in recent years. Supershear bursts were shown to have caused damage during the 1979 Imperial Valley, 1992 Landers, and 1999 Izmit, Turkey, earthquakes.

In the experiment by Lu, Lapusta, and Rosakis, supershear propagation is seen to arise during both pulselike and cracklike earthquake ruptures. "That's new--nobody has seen before that either of those modes could transition to supershear," says Rosakis. Shock waves generated by supershear propagation generate more ground shaking, he adds, and notes that with more details about exactly how earthquakes rupture, scientists can devise more sophisticated ways for buildings to survive the specific types of shaking that arise.

Source: Caltech

Explore further: NASA balloons begin flying in Antarctica for 2014 campaign

add to favorites email to friend print save as pdf

Related Stories

California gets funding for quake warning system

6 hours ago

California received funding to help begin an earthquake warning system across the state next year that would provide enough time for trains to brake, utilities to shut off gas lines and people to dive under a table until ...

Deep fault drilling project

1 hour ago

It rains a lot in the tiny south Westland town of Whataroa. Every year, this region gets some of the highest rainfall totals recorded anywhere in the World and Whataroa is one of the wetter parts. The town is nestled beneath ...

Erosion may trigger earthquakes

Nov 21, 2014

Researchers from laboratories at Géosciences Rennes (CNRS/Université de Rennes 1), Géosciences Montpellier (CNRS/Université de Montpellier 2) and Institut de Physique du Globe de Paris (CNRS/IPGP/Université Paris Diderot), ...

High earthquake danger in Tianjin, China

Nov 18, 2014

With a population of 11 million and located about 100 km from Beijing (22 million people) and Tangshan (7 million people), Tianjin lies on top of the Tangshan-Hejian-Cixian fault that has been the site of ...

Recommended for you

Scientists make strides in tsunami warning since 2004

21 hours ago

The 2004 tsunami led to greater global cooperation and improved techniques for detecting waves that could reach faraway shores, even though scientists still cannot predict when an earthquake will strike.

Trade winds ventilate the tropical oceans

21 hours ago

Long-term observations indicate that the oxygen minimum zones in the tropical oceans have expanded in recent decades. The reason is still unknown. Now scientists at the GEOMAR Helmholtz Centre for Ocean Research ...

User comments : 0

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.