UH researchers report new understanding of deep earthquakes

July 30, 2018

Researchers have known for decades that deep earthquakes - those deeper than 60 kilometers, or about 37 miles below the Earth's surface - radiate seismic energy differently than those that originate closer to the surface. But a systematic approach to understanding why has been lacking.

Now a team of researchers from the University of Houston has reported a way to analyze seismic wave radiation patterns in deep earthquakes to suggest global deep earthquakes are in anisotropic rocks, something that had not previously been done. The rock anisotropy refers to differences in seismic wave propagation speeds when measured along different directions.

Their findings were published Monday, July 30, by the journal Nature Geoscience.

Most earthquakes occur at shallow depths, according to the U.S. Geological Survey, and they generally cause more damage than deeper earthquakes. But there are still substantial questions about the causes of deep earthquakes.

Normal rocks are ductile, or pliable, at these great depths because of high temperature and thus aren't able to rupture in an abrupt fashion to produce deep earthquakes, which occur below subduction zones where two tectonic plates collide at ocean trenches. The plate being pushed under is referred to as the subducting slab. The fact that deep earthquakes occur only in these slabs suggests some unusual process is happening within the slab.

Yingcai Zheng, assistant professor of seismic imaging in the UH College of Natural Sciences and Mathematics and corresponding author for the paper, said seismologists have sought to understand deep earthquakes since the phenomenon was discovered in 1926. Hypotheses include the effect of fluids, runaway thermal heating or solid-phase change due to sudden collapse of the mineral crystal structure.

In addition to Zheng, researchers involved in the work include the first author Jiaxuan Li, a Ph.D. candidate in the Department of Earth and Atmospheric Sciences; Leon Thomsen, research professor of geophysics; Thomas J. Lapen, professor of geology; and Xinding Fang, adjunct professor at UH and concurrently associate professor at the Southern University of Science and Technology China.

"Over the past 50 years, there has been growing evidence that a large proportion of deep earthquakes do not follow the double-couple radiation pattern seen in most shallow earthquakes," Zheng said. "We set out to look at why that happens." The double-couple pattern is caused by a shear rupture of a pre-existing fault.

The work, funded by the National Science Foundation, looked at potential reasons for the differing radiation patterns; Zheng said earlier theories suggest that deep earthquakes stem from a different rupture mechanism and possibly different physical and chemical processes than those that spark shallow earthquakes.

But after studying the radiation patterns of 1,057 deep earthquakes at six subduction zones worldwide, the researchers determined another explanation. They found that the surrounding rock fabric enclosing the deep quake alters the seismic radiation into a non-double-couple pattern. "Both the common double-couple radiation patterns and uncommon patterns of deep earthquakes can be explained simultaneously by shear rupture in a laminated rock fabric," Li said.

Before the subducting plate enters the trench, it can absorb sea water to form hydrated anisotropic minerals. As the slab descends in the Earth's mantle, the water can be expelled due to high pressure and high temperature conditions, a process known as dehydration. The dehydration and strong shearing along the slab interface can make the rock brittle and lead to rupture in intermediate-depth earthquakes, defined as those between 60 kilometers and 300 kilometers deep (37 miles to 186 miles).

"We found at these depths that the anisotropic rock fabric is always parallel to the slab surface, although the slab can change directions greatly from place to place," Li said.

Anisotropy is also found in rocks at even greater depths, which suggests materials such as magnesite or aligned carbonatite melt pockets may be involved in generating the deep ruptures, the researchers said. Because the inferred anisotropy is high -- about 25 percent -- the widely believed meta-stable solid phase change mechanism is not able to provide the needed anisotropy inferred by the researchers.

University of Houston

Related Subduction Zones Articles from Brightsurf:

The connectivity of multicomponent fluids in subduction zones
A team of researchers has discovered more about the grain-scale fluid connectivity beneath the earth's surface, shedding new light on fluid circulation and seismic velocity anomalies in subduction zones.

New fault zone measurements could help us to understand subduction earthquake
University of Tsukuba researchers have conducted detailed structural analyses of a fault zone in central Japan to identify the specific conditions that lead to devastating earthquake.

A review of ridge subduction, magmatism and metallogenesis
Ridge subduction events are very common and important geodynamic processes in modern oceanic plate tectonics (Figure 1), and play an important role in the generation of arc magmatism, material recycling, growth and evolution of continental crust, deformation and modification of overlying plates and metallogenesis.

Scientists review the metallogenesis and challenges of porphyry copper systems above subduction zone
Porphyry copper ± molybdenum ± gold deposits (PCDs) are the most economically important magmatic-hydrothermal metallogenic system above subduction zones, which have supplied nearly 3/4 of the world's copper, 1/2 of the molybdenum and 1/5 of the gold, as well as large amounts of silver, zinc, tin and tungsten, with however their metallogenesis remaining controversial.

Does accelerated subduction precede great earthquakes?
A strange reversal of ground motion preceded two of the largest earthquakes in history.

Is there a technological solution to aquatic dead zones?
Could pumping oxygen-rich surface water into the depths of lakes, estuaries, and coastal ocean waters help ameliorate dangerous dead zones?

Warming climate will impact dead zones in Chesapeake Bay
In recent years, scientists have projected increasingly large summer dead zones in the Chesapeake Bay, areas where there is little or no oxygen for living things like crabs and fish to thrive, even as long-term efforts to reduce nutrient pollution continue.

Could dark carbon be hiding the true scale of ocean 'dead zones'?
The impact of climate change on the world's oceans is becoming increasingly known but new research suggests current computer models could be omitting a crucial piece of evidence when it comes to assessing the scale of ocean dead zones.

SRL publishes focus section on Subduction Zone processes in the Americas
Researchers from around the globe share what they've learned from an unprecedented amount of data collected in the Latin American Subduction Zone over the past two decades.

Researchers from IKBFU find out how to strengthen coastal zones of Baltic Sea
Reconstruction and strengthening of coastal zones are the key issues of many industries that are oriented in the seaside tourism.

Read More: Subduction Zones News and Subduction Zones Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.