Tectonic Tremors Could Offer Insights Into the Big Shakers Danielle Venton, Science Writer

OECONCEPTS CORE CORE CONCEPTS

Tectonic tremors could offer insights into the big shakers Danielle Venton, Science Writer

Early in 1995, an earthquake struck Japan measuring A Whole Lot of Shaking Going On 7.3 on the Richter scale (1). The shaking lasted for 20 Once researchers knew how to look for tremors— seconds. Asphalt roads looked like shattered peanut their signals can now be detected even with surface brittle and whole neighborhoods crumpled to the sensors—they were found to be common. “The more ground. Nearly 6,400 people lost their lives, and more places that we look, the more we find these things,” than 300,000 people were left homeless. says Aaron Wech, research geophysicist at the United Until then, Japan had viewed itself as an earthquake- States Geological Survey (USGS). “It’s actually more prepared nation. Although their earthquake engineering surprising when we don’tfindthem.” Wech has set up wasandstillissomeofthebestintheworld,theKobe the Pacific Northwest Seismic Network (https://www. disaster prompted an increased investment in research, such pnsn.org/tremor) to automatically detect and catalog as the installation of a new network of sensors, the “High tremors in Cascadia, a region of the US Pacific North- Sensitivity Seismograph Network Japan” or “Hi-net.” These west; its subduction zone separates the North American extremely sensitive sensors, installed at the bottom of bore- Plate from three Pacific Ocean plates. holes, were at depths of more than 100 meters. The depth How the tectonic tremors are related to surface and sensitivity of the roughly 800 sensors around Japan quakes is still an open question, but one that many revealed something seismologists had never seen before. seismologists are eager to study and answer. “Ithinkit’s Deep in the earth, the researchers discovered, the one of the most exciting discoveries in earthquake ground trembles weakly in slow, stuttering, episodic science in the last several decades,” says David R. earthquakes lasting for minutes, hours, or even weeks. Shelly, a seismologist at the USGS who studies tectonic Depending on the fault, these events can occur daily, tremors along the San Andreas Fault in California. roughly annually, or more or less frequently. Seismol- The discovery of these really deep, slow-slipping, ogists now call these events “tectonic tremors.” [Initially nondamaging earthquakes changes researchers’ views they were called “nonvolcanic tremors” by the team of of faults in a few key ways. First, faults are potentially Japanese researchers who discovered them, because much deeper than was first thought. For example, in their signal is similar to the shaking that occasionally the case of the San Andreas, the earthquakes felt on the emanates from volcanoes as they fill with magma (2).] surface typically rupture to a depth of only 12–15 kilometers (3). Kobe’s earthquake initiated at a spot on the fault 18 kilometers down (4). “Before this, it wasn’t clear what happened to the fault as you went deeper into the earth,” Shelly says. Many geologists thought that, in the deep crust, plates just sort of floated along in “abroadzoneof continuous deformation.” Tectonic tremors offered up another insight: the Earth’s crust isn’t slipping continuously, it’s slipping episodically. Some places along the deep fault are stuck most of the time and slip every few months. Some are less stuck and slip every few days (5). Shelly says in his work they observe some tectonic tremor at some point along the San Andreas every single day. These slow-slip earthquakes can be found around the globe, primarily in subduction zones, places where one tectonic plate is diving beneath another (6). The most destructive earthquakes occur in subduction zones located in places such as Chile, Japan, Alaska, Washington state, and British Columbia. Some of these areas, such as in the Canadian and United States west A giant crane pulls crushed cars out of the debris January 18, 1995, after the Hanshin coast Cascades range, are due (or overdue) for a major Expressway in Kobe, Japan was devastated during the country’s worst earthquake destructive earthquake. Some researchers hope that in nearly 50 years. Image courtesy of © Kimimasa Mayama/Reuters Pictures. tectonic tremors will eventually help forecast major

7930–7931 | PNAS | July 19, 2016 | vol. 113 | no. 29 www.pnas.org/cgi/doi/10.1073/pnas.1610000113 Downloaded by guest on September 23, 2021 that tends to generate the big earthquakes.” Thus, any time the deep part of the fault slips, it adds an increment of stress to the shallow part. “So the fault is con- nected,” he says. “In that sense they’re related.” More seismic data and, indeed, more earthquakes, should help, says John Armbruster, a research seismologist at the Lamont–Doherty Earth Observatory, Columbia University. His current fascination is studying the Cascadia Subduction Zone near Vancouver Island (7). The last tremor Armbruster analyzed occurred several months ago: from late December 2015 into January 2016. But, because there hasn’t been any major surface shaking during or since that time, he can’t use that collection of data to predict when the next big one will be. “People are paying attention to [tremors] and looking for patterns,” Armbruster says. “But given that we’ve only known about them since 2002, we don’t have a lot of practice in detecting them and then looking at what else is happening.” In Cascadia, Armbruster observes a tremor about once a year. However, a major earthquake hasn’t occurred since about 1700. So, he says, although there have been hundreds of these smaller signals in the past 300 years, each one incrementally increasing the pressure on the fault, therehavebeentoofewbigquakestodetectalink. What will it take to establish that link? Wech hopes his “Interactive Tremor Map,” a feature of the Pacific Northwest Seismic Network website (https://www.pnsn. org/tremor), will help establish a baseline for tremoring in Cascadia. “If you play around on the map interface and expand the timeline, you can see 300,000 epicen- ters in there,” he says, “there are hundreds of swarms of tremor events, yet we’ve not had any big earthquakes.” Even though Wech created the online system, which sends out email alerts when tremor activity crosses Visualization of a recent swarm of tremors accompanying certain thresholds, he confesses that its utility is unclear. slow slip showing the migration of a slow earthquake over “To be honest, we don’t know what to do with that the course of months along the Cascadia subduction zone. information,” he says. “If I got an email from my code Image courtesy of Aaron Wech (Alaska Volcano Obser- that said all of Washington, Oregon, and Vancouver vatory, US Geological Survey). Island is tremoring like crazy, I would be alarmed, but I don’treallyknowwhatitmeans.Wejusthaven’tob- earthquakes, offering an informative new signal for the served it long enough to know if that’snormalornot.” much sought-after early warning system. One of the greatest potential benefits of studying tremors, Wech believes, is their ability to reveal earth- Promises for Prediction? quake rupture processes in general. “When a slow However, thus far, researchers have not characterized earthquake happens you can go to the webpage and the connection between the harmless deep, slow, watch the tremor migrate; you can see it move several “swarms” of earthquakes called tectonic tremors, and hundred kilometers over several weeks,” he says. That the shallow, sudden, destructive earthquakes that top- offers a slow-motion, high-definition image of an earth- ple buildings and take lives. “The big earthquakes are quake nucleating, growing, moving, and stopping. That so infrequent that there just aren’tmanyexamplesto might, Wech believes, change how seismologists see study,” says Shelly. But they must be related, he reasons. regular earthquakes, which happen too quickly to ob- “We know that the tremors are being caused by slip on serve with any precision. And, Wech says, it could make a the deep part of the fault and that it’stheshallowerpart place like Cascadia “a laboratory for earthquake physics.”

1 Encyclopaedia Britannica eds (2016) Kobe earthquake of 1995. Encyclopædia Britannica. Available at www.britannica.com/event/ Kobe-earthquake-of-1995. Accessed June 5, 2016. 2 Obara K (2002) Nonvolcanic deep tremor associated with subduction in southwest Japan. Science 296(5573):1679–1681. 3 Nadeau RM, Dolenc D (2005) Nonvolcanic tremors deep beneath the San Andreas Fault. Science 307(5708):389. 4 ISC (2015) ISC-GEM Global Instrumental Earthquake Catalogue (1900-2009), Version 2.0 (International Seismological Centre, UK). 5 Shelly DR, Beroza GC, Ide S (2007a) Non-volcanic tremor and low-frequency earthquake swarms. Nature 446(7133):305–307. 6 Schwartz SY, Rokosky JM (2007) Slow slip events and seismic tremor at circum-pacific subduction zones. Rev Geophys 45(3):RG3004. 7 Rubin AM, Armbruster JG (2013) Imaging slow slip fronts in Cascadia with high precision cross‐station tremor locations. Geochem Geophys Geosyst 14(12):5371–5392.

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