Discovery sheds new light on cause of earthquakes

Research at the University of Liverpool into a large fault zone in the Atacama Desert in northern Chile has produced new insight into how fluid pressure can cause earthquakes.

Scientists have found how fluids, such as water, become sealed within the earth’s fault planes for a long period of time. This fluid pressure makes it easier for the earth’s plates to move alongside each other, eventually resulting in an earthquake.

Dr Dan Faulkner explains: “The difficulty with predicting earthquakes is that we know so little about how fault planes work. Over the years we have found that even small stresses acting on the earth’s plates can cause large earthquakes. For example the Loma Prieta earthquake in 1989 caused massive devastation, yet there was very little stress acting on the plate boundary to cause the quake in the first place.

“In theory, high stresses are needed to cause slip along a fault plane, but if something like pressurised water or gas gets inside the fault then it should act as a kind of cushion, making movement between plates easier and an earthquake more likely. Until now a problem with this theory was that as fluid pressures increased the rocks would crack and the fluids could escape through the cracks, reducing the ‘cushion’ effect. Our recent study, however, found that much smaller cracks surrounding the fault plane change the stresses acting on the rock, reducing the likelihood of significant cracks forming and allowing the fluid to escape.”

The team measured the density of ‘microcracks’ in the rock near the Chile fault line and applied varying amounts of stress to the rock to see how it responded. They found the ‘microcracks’ changed the elasticity of the rock, which meant stresses that might normally occur at almost right angles to the fault line rotated to a 45 degree angle instead.

Under normal stresses fluid would build up to such as extent that the rock would break and the fluid would escape, reducing the risk of an earthquake. When stress, however, occurs at a 45 degree angle the rock is less likely to break and the low fluid pressures inside can cause earthquakes.

Dr Faulkner added: “We now need to conduct further study into where these fluids and gases are coming from. Scientists are currently drilling of the San Andreas Fault in California, to help us understand more fully the mechanics of fault zones and how earthquakes occur.”

The San Andreas Fault Observatory at Depth (SAFOD) is a deep borehole observatory that will measure the physical conditions under which plate boundary earthquakes occur. Dr Faulkner is one of only two UK scientists who currently have access to rock drilled from the San Andreas Fault, which will be analysed in order to understand fault behaviour.

Dr Faulkner’s research is published in Nature Magazine.

Notes:

What is an earthquake?

• The Earth's lithosphere is made up of plates which are in constant motion. Plate boundaries slide past each other, creating frictional stress. When the frictional stress exceeds a certain point a failure occurs along plate boundaries, known as the fault plane. When the failure results in a displacement of the Earth's crust, seismic waves are radiated causing an earthquake.

Largest earthquakes on record:

• 1960 – Chile: 9.5 (magnitude)
• 1737 – Kamchatka: Russia: 9.3 (magnitude)
• 1964 – Alaska, US: 9.2 (magnitude)
• 2004 – Sumatra, Indonesia: 9.0-9.3 (magnitude)
• 1957 – Alaska, US: 9.1 (magnitude)

Source: University of Liverpool

Related stories:

10 questions shaping 21st-century earth science identified
Ten questions driving the geological and planetary sciences were identified today in a new report by the National Research Council. Aimed at reflecting the major scientific issues facing earth science at the start of the 21st century, the questions represent where the field stands, how it arrived at this point, and where it may be headed.
Helium isotopes point to new sources of geothermal energy
In a survey of the northern Basin and Range province of the western United States, geochemists Mack Kennedy of the Department of Energy's Lawrence Berkeley National Laboratory and Matthijs van Soest of Arizona State University have discovered a new tool for identifying potential geothermal energy resources.
Chemistry of San Andreas Fault may offer clues to earthquake mysteries
Scientists have obtained core samples from deep inside California's San Andreas Fault for the first time, a finding that may lead to a better understanding of the underground molecular events associated with earthquakes, according to an article scheduled for the Oct. 22 issue of Chemical & Engineering News, ACS' weekly newsmagazine.
Geologists recover rocks yielding unprecedented insights into San Andreas Fault
For the first time, geologists have extracted intact rock samples from 2 miles beneath the surface of the San Andreas Fault, the infamous rupture that runs 800 miles along the length of California.
Radioactive Crystals Help Identify and Date Ore Deposits
Reddish-brown crystals of a radioactive mineral called monazite can act as microscopic clocks that allow geologists to date rock formations that have been altered by the action of high-temperature fluids, a process that frequently leads to the formation of rich ore deposits.
Geophysicists reveal new insights into the 'earthquake machine'
The San Andreas Fault Observatory at Depth (SAFOD)—the first underground observatory to provide physical samples and real-time seismological data from deep inside an active fault zone—is yielding surprising new clues about the origin of earthquakes. SAFOD scientists from around the world will discuss these new findings on Dec. 6 at the annual meeting of the American Geophysical Union (AGU) at Moscone Center West in San Francisco.
Sandia underground geo-tools aid in earthquake research
Geothermal researchers at Sandia National Laboratories have developed sensors that can be placed in hotter and higher-pressure underground environments than previous instruments, a capability that is allowing geologists worldwide to make more precise measurements of subterranean conditions before and after large earthquakes occur.
The researchers hope the new sensors will provide geologists with a better understanding of earthquake-related phenomena and possibly provide more sensitive measurements of warning signs for large earthquakes such as the devastating 9.0 magnitude earthquake near Sumatra on Sunday, Dec. 26.
UCSB scientists probe sea floor venting to gain understanding of early life on Earth
New keys to understanding the evolution of life on Earth may be found in the microbes and minerals vented from below the ocean floor, say scientists at the University of California, Santa Barbara. The UCSB scientists are making new contributions to this field of inquiry in their studies of seafloor hydrothermal fluid discharge into the Earth's oceans, which has been occurring ever since the oceans first formed four billion years ago. Conditions below the sea floor may most closely mimic the environment when life began.

News discussion:

Space & Earth science news