Earthquakes can be devastating bursts of destruction, lasting for a few seconds or minutes at most. But not all release of pent-up seismic energy is quick or violent. Slow-motion quakes—also called slow-slip events—can last for days or even months, releasing small amounts of energy over time. Slow-slip events are so gentle they are usually detectable only by GPS networks. Though slow-slip events were first recognized about 2 decades ago, researchers are still learning about the mechanisms that cause them.
A new study published in Science Advances indicates that geological heterogeneity may be at the bottom of at least some slow-slip events. This study focused on the Hikurangi subduction zone off of the coast of New Zealand’s North Island, where the Pacific tectonic plate slides under the Australian plate. The Hikurangi subduction zone is one of the shallowest sites where researchers have observed slow-slip events—at depths of less than 15 kilometers—which opens up some unique research opportunities.
“Previously, many of the areas where we detected slow-slip events were completely inaccessible to direct geological observation,” said Philip Barnes, lead author of the study and a geologist at New Zealand’s National Institute of Water and Atmospheric Research. For example, slow-slip events at the Cascadia subduction zone in North America occur at depths of around 40 kilometers.
The relatively shallow depth of the Hikurangi subduction zone allowed researchers—on two International Ocean Discovery Program (IODP) expeditions—to drill and collect samples from locations in the Pacific plate shortly (in geological terms) before it moves under the Australian plate. By analyzing these core samples, the researchers determined the composition and physical properties of the plate before it moves into areas where slow-slip events have been observed. Then they combined these analyses with previously acquired seismic reflection data—measurements of how sound waves reflect off of different kinds of rocks.
The Heterogeneous Hikurangi
These experiments showed that the area of the Pacific plate located before the Hikurangi subduction zone is extremely heterogeneous. The core samples contained several different kinds of rocks, which had highly variable properties, such as strength, porosity, and texture. “Some rocks were mushy and weak, whilst others were hard, cemented and strong,” Barnes said in a press release.
In some areas, properties such as porosity—a measure of empty spaces within a rock type—varied almost twofold within tens of centimeters. Not only was the Pacific plate made up of heterogeneous rock types, its geology was variable as well, including seamounts rising more than a kilometer above the seafloor.
This geological and physical heterogeneity could explain the slow-slip events observed in the area, according to Barnes. “Many people think faults with slow-slip events are in a transitional frictional state,” he said. “They are very close to failing (which would cause a typical earthquake), but something is holding back that failure.”
The patchwork of rock types and geological features entering the Hikurangi subduction zone could mean that some parts of the fault get periodically stuck and unstuck (what’s called stick slip), whereas other parts are sliding along without much ado (aseismic or fault creep), ultimately yielding an overall area with slow-slip events.
The direct observations from the study will help improve numerical and computer models attempting to solve the enigma of slow-slip events. “Studies of slow-slip environments have either been from various geophysical observations, which tend to be fuzzy, or from ancient geological exposures, which can be difficult to relate to active systems,” said Roland Bürgmann, a geologist at the University of California, Berkley, who was not connected with the study. “This study provides new, directly sampled, and beautifully imaged information about the stuff that is being subducted into a slow-slip zone.”
But of course, this study provides only a couple of data points. “More comprehensive work on multiple subduction zone segments with and without slow-slip zones would need to happen to confirm the association of slow slip with subducted heterogenous rocks and structures,” Bürgmann said.
Other factors, such as fluid pressure changes, probably influence slow-slip events, both at Hikurangi and at other locations. Also, the degree of heterogeneity that the authors found may make interpretations more challenging. “The very heterogeneity and randomness of the Pacific plate means it’s difficult to simply extrapolate that what the researchers see on the incoming plate will be the same when it enters the subduction zone,” said Jean-Philippe Avouac, a geologist at the California Institute of Technology not connected with the study. “While this study is an impressive effort to characterize the incoming seafloor [at the Hikurangi subduction zone], how it plays out in explaining slow-slip events is yet to be fully determined.”
Barnes agrees that more work needs to be done. “This paper is one of the first outputs of a very large research project,” he said. “We are at the beginning stages of what I hope will be a much fuller exploration of slow-slip events.”
—Adityarup Chakravorty ([email protected]), Science Writer