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CORE CONCEPTS Tectonic tremors could offer insights into the big shakers Danielle Venton, Science Writer Early in 1995, an earthquake struck Japan measuring 7.3 on the Richter scale (1). The shaking lasted for 20 seconds. Asphalt roads looked like shattered peanut brittle and whole neighborhoods crumpled to the ground. Nearly 6,400 people lost their lives, and more than 300,000 people were left homeless. Until then, Japan had viewed itself as an earthquake- prepared nation. Although their earthquake engineering was and still is some of the best in the world, the Kobe disaster prompted an increased investment in research, such as the installation of a new network of sensors, the “High Sensitivity Seismograph Network Japan” or “Hi-net. ” These extremely sensitive sensors, installed at the bottom of bore- holes, were at depths of more than 100 meters. The depth and sensitivity of the roughly 800 sensors around Japan revealed something seismologists had never seen before. Deep in the earth, the researchers discovered, the ground trembles weakly in slow, stuttering, episodic earthquakes lasting for minutes, hours, or even weeks. Depending on the fault, these events can occur daily, roughly annually, or more or less frequently. Seismol- ogists now call these events “tectonic tremors.” [Initially they were called “nonvolcanic tremors” by the team of Japanese researchers who discovered them, because their signal is similar to the shaking that occasionally emanates from volcanoes as they fill with magma (2).] A Whole Lot of Shaking Going On Once researchers knew how to look for tremors— their signals can now be detected even with surface sensors—they were found to be common. “The more places that we look, the more we find these things,” says Aaron Wech, research geophysicist at the United States Geological Survey (USGS). “It’s actually more surprising when we don’t find them.” Wech has set up the Pacific Northwest Seismic Network (https://www. pnsn.org/tremor) to automatically detect and catalog tremors in Cascadia, a region of the US Pacific North- west; its subduction zone separates the North American Plate from three Pacific Ocean plates. How the tectonic tremors are related to surface quakes is still an open question, but one that many seismologists are eager to study and answer. “I think it’s one of the most exciting discoveries in earthquake science in the last several decades,” says David R. Shelly, a seismologist at the USGS who studies tectonic tremors along the San Andreas Fault in California. The discovery of these really deep, slow-slipping, nondamaging earthquakes changes researchers’ views of faults in a few key ways. First, faults are potentially much deeper than was first thought. For example, in the case of the San Andreas, the earthquakes felt on the surface typically rupture to a depth of only 12–15 kilo- meters (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 “a broad zone of 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 coast Cascades range, are due (or overdue) for a major destructive earthquake. Some researchers hope that tectonic tremors will eventually help forecast major A giant crane pulls crushed cars out of the debris January 18, 1995, after the Hanshin Expressway in Kobe, Japan was devastated during the country’s worst earthquake in nearly 50 years. Image courtesy of © Kimimasa Mayama/Reuters Pictures. 7930–7931 | PNAS | July 19, 2016 | vol. 113 | no. 29 www.pnas.org/cgi/doi/10.1073/pnas.1610000113 CORE CONCEPTS Downloaded by guest on July 16, 2020
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CORE CONCEPTS
Tectonic tremors could offer insights into thebig shakersDanielle Venton, Science Writer
Early in 1995, an earthquake struck Japan measuring7.3 on the Richter scale (1). The shaking lasted for 20seconds. Asphalt roads looked like shattered peanutbrittle and whole neighborhoods crumpled to theground. Nearly 6,400 people lost their lives, and morethan 300,000 people were left homeless.
Until then, Japan had viewed itself as an earthquake-prepared nation. Although their earthquake engineeringwas and still is some of the best in the world, the Kobedisaster prompted an increased investment in research, suchas the installation of a new network of sensors, the “HighSensitivity Seismograph Network Japan” or “Hi-net.” Theseextremely sensitive sensors, installed at the bottom of bore-holes, were at depths of more than 100 meters. The depthand sensitivity of the roughly 800 sensors around Japanrevealed something seismologists had never seen before.
Deep in the earth, the researchers discovered, theground trembles weakly in slow, stuttering, episodicearthquakes lasting for minutes, hours, or even weeks.Depending on the fault, these events can occur daily,roughly annually, or more or less frequently. Seismol-ogists now call these events “tectonic tremors.” [Initiallythey were called “nonvolcanic tremors” by the team ofJapanese researchers who discovered them, becausetheir signal is similar to the shaking that occasionallyemanates from volcanoes as they fill with magma (2).]
A Whole Lot of Shaking Going OnOnce researchers knew how to look for tremors—their signals can now be detected even with surfacesensors—they were found to be common. “The moreplaces that we look, the more we find these things,”says Aaron Wech, research geophysicist at the UnitedStates Geological Survey (USGS). “It’s actually moresurprising when we don’t find them.” Wech has set upthe Pacific Northwest Seismic Network (https://www.pnsn.org/tremor) to automatically detect and catalogtremors in Cascadia, a region of the US Pacific North-west; its subduction zone separates theNorth AmericanPlate from three Pacific Ocean plates.
How the tectonic tremors are related to surfacequakes is still an open question, but one that manyseismologists are eager to study and answer. “I think it’sone of the most exciting discoveries in earthquakescience in the last several decades,” says David R.Shelly, a seismologist at the USGS who studies tectonictremors along the San Andreas Fault in California.
The discovery of these really deep, slow-slipping,nondamaging earthquakes changes researchers’ viewsof faults in a few key ways. First, faults are potentiallymuch deeper than was first thought. For example, inthe case of the San Andreas, the earthquakes felt on thesurface typically rupture to a depth of only 12–15 kilo-meters (3). Kobe’s earthquake initiated at a spot on thefault 18 kilometers down (4).
“Before this, it wasn’t clear what happened to thefault as you went deeper into the earth,” Shelly says.Many geologists thought that, in the deep crust,plates just sort of floated along in “a broad zone ofcontinuous deformation.”
Tectonic tremors offered up another insight: theEarth’s crust isn’t slipping continuously, it’s slippingepisodically. Some places along the deep fault arestuck most of the time and slip every few months.Some are less stuck and slip every few days (5). Shellysays in his work they observe some tectonic tremor atsome point along the San Andreas every single day.
These slow-slip earthquakes can be found aroundthe globe, primarily in subduction zones, places whereone tectonic plate is diving beneath another (6). Themost destructive earthquakes occur in subductionzones located in places such as Chile, Japan, Alaska,Washington state, and British Columbia. Some of theseareas, such as in the Canadian and United States westcoast Cascades range, are due (or overdue) for a majordestructive earthquake. Some researchers hope thattectonic tremors will eventually help forecast major
A giant crane pulls crushed cars out of the debris January 18, 1995, after the HanshinExpressway in Kobe, Japan was devastated during the country’s worst earthquakein nearly 50 years. Image courtesy of © Kimimasa Mayama/Reuters Pictures.
7930–7931 | PNAS | July 19, 2016 | vol. 113 | no. 29 www.pnas.org/cgi/doi/10.1073/pnas.1610000113
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earthquakes, offering an informative new signal for themuch sought-after early warning system.
Promises for Prediction?However, thus far, researchers have not characterizedthe connection between the harmless deep, slow,“swarms” of earthquakes called tectonic tremors, andthe shallow, sudden, destructive earthquakes that top-ple buildings and take lives. “The big earthquakes areso infrequent that there just aren’t many examples tostudy,” says Shelly. But theymust be related, he reasons.“We know that the tremors are being caused by slip onthe deep part of the fault and that it’s the shallower part
that tends to generate the big earthquakes.” Thus, anytime the deeppart of the fault slips, it adds an incrementof 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 seis-mologist at the Lamont–Doherty Earth Observatory,Columbia University. His current fascination is study-ing the Cascadia Subduction Zone near VancouverIsland (7). The last tremor Armbruster analyzed oc-curred several months ago: from late December 2015into January 2016. But, because there hasn’t been anymajor surface shaking during or since that time, hecan’t use that collection of data to predict when thenext 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 since2002, we don’t have a lot of practice in detecting themand then looking at what else is happening.”
In Cascadia, Armbruster observes a tremor about oncea year. However, amajor earthquake hasn’t occurred sinceabout 1700. So, he says, although there have been hun-dreds of these smaller signals in the past 300 years, eachone incrementally increasing the pressure on the fault,there have been too few big quakes to detect a link.
What will it take to establish that link? Wech hopeshis “Interactive Tremor Map,” a feature of the PacificNorthwest Seismic Network website (https://www.pnsn.org/tremor), will help establish a baseline for tremoringin Cascadia. “If you play around on the map interfaceand expand the timeline, you can see 300,000 epicen-ters in there,” he says, “there are hundreds of swarms oftremor events, yet we’ve not had any big earthquakes.”
Even thoughWech created the online system, whichsends out email alerts when tremor activity crossescertain thresholds, he confesses that its utility is unclear.“To be honest, we don’t know what to do with thatinformation,” he says. “If I got an email from my codethat said all of Washington, Oregon, and VancouverIsland is tremoring like crazy, I would be alarmed, but Idon’t really know what it means. We just haven’t ob-served it long enough to know if that’s normal or not.”
One of the greatest potential benefits of studyingtremors, Wech believes, is their ability to reveal earth-quake rupture processes in general. “When a slowearthquake happens you can go to the webpage andwatch the tremor migrate; you can see it move severalhundred kilometers over several weeks,” he says. Thatoffers a slow-motion, high-definition image of an earth-quake nucleating, growing, moving, and stopping. Thatmight, Wech believes, change how seismologists seeregular earthquakes, which happen too quickly to ob-serve with any precision. And,Wech says, it could make aplace 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. GeochemGeophys Geosyst 14(12):5371–5392.
Visualization of a recent swarm of tremors accompanyingslow slip showing the migration of a slow earthquake overthe course ofmonths along the Cascadia subduction zone.Image courtesy of Aaron Wech (Alaska Volcano Obser-vatory, US Geological Survey).
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