38 January 2012 Physics Today www.physicstoday.org

Earthquakes have been understood concep-tually for about 100 years. Strain and stressbuild until the rock fractures, sliding alonga fault surface, with the fracture propaga-tion moderated by elastic waves. Although

some details about friction and the ensemble behav-ior of evolving fault networks still have to beworked out, the earthquake cycle follows an unchal-lenged master paradigm with an uneven two-stepof rapid sliding and slow reloading.1 Or so wethought 10 years ago.

Earthquakes mark the relative motion betweentectonic plates, large and small. Since the 1960s, thetheory of plate tectonics has recognized that Earth’slithosphere is broken into pieces of various sizes,ranging from much of the vast area under the PacificOcean down to as small a rock mass as one is ableto consider. The definition of lithosphere—the shal-lower volume of rock that behaves in a plastic man-ner compared with the underlying viscous flow ofthe so-called asthenosphere—sets the stage for thedichotomy, now recognized as overly simplistic, ofshallow, earthquake-ridden plate boundaries atop adeeper region of silent and steady deformation.

The discovery of episodic tremor and slip(ETS), which is thought to be tectonic fault slipmany orders of magnitude slower, and generallyjust a bit deeper, than regular earthquakes, hasadded a new dimension to that picture. It has in-jected new ideas about tectonic plate boundariesand even raised the possibility of greater pre-dictability of large earthquakes.

“Slow slip” is a more accurate name than ETS.Still widely used, the term “ETS” refers to the initialobservation of large episodes that radiate high- frequency energy (1–10 Hz, tremor), show low- frequency motion (movement over days to weeks,

slip), and sometimes recur like clockwork(episodic). But closer observation revealed that ETSmotion spans a continuum from frequencies of 20 Hz to periods of years and that the events areoften aperiodic. Furthermore, numerous smallerand shorter episodes have been observed along withthe large ones. What the new phenomena all havein common is that slip advances much more slowlythan in regular earthquakes,2,3 so we use the term“slow slip” throughout the rest of this article.

The tremor in slow-slip events has been termed“nonvolcanic” tremor to distinguish it from tremorsignals generated by magmatic fluids in volcanoes.Several results suggest that as in regular earth-quakes, the tremor is directly generated by shearslip on or near the plate interface. But high-pressurefluids (water and carbon dioxide) are widelythought to be present in slow-slip regions and mayplay a yet unknown role.

Many other aspects of slow slip remain unex-plained. For example, it’s not yet known why slow-slip earthquakes are orders of magnitude more pro-longed than traditional earthquakes. In this articlewe examine their many observed differences andoffer speculation about the underlying geology andphysics.

Tectonic settingSlow slip has been seen most clearly in subductionzones, where pairs of tectonic plates converge. (Fora brief introduction to plate tectonics, see the box onpage 39.) To be more specific, we’ll consider the partof the Cascadia subduction zone near our home-town of Seattle, Washington. Slow slip in Cascadiais similar to that seen in other subduction zones inJapan, Mexico, Alaska, and Costa Rica. We couldhave told this story with details from several re-gions—observations in Japan are now impressivelydetailed, for example—but we’ll cite the studieswith which we are most familiar. The subductionzone’s geometry is shown in figure 1.

John Vidale and Heidi Houston are professors of Earth and space sciencesat the University of Washington in Seattle.

Slow slip:A new kind of earthquake

John E. Vidale and Heidi Houston

Sandwiched between the shallow region of sudden, infrequent earthquakes and the

deeper home to continuous viscous motion lies anintermediate realm of intermittent sliding and rumbling. Discovered in recent years, it still harbors many secrets.

Slow slip appears at intermediate depths,roughly 35–55 km beneath the Puget Sound, be-tween the shallow regular earthquakes and deeper,more continuous motion. Down to 35 km, the plateboundary generally obeys the traditional expecta-tions, locking for decades to millennia before slidingrapidly. Slip over a 1000-km length of fault can reachtens of meters in the greatest earthquakes. Plenty ofsmaller earthquakes also pepper the interface andthe surrounding deforming volume. At depthsbelow 55 km, motion is still thought to be smoothand constant.

Tremor, probably accompanied by slow slip,has also been observed at shallower depths on so-called crustal transform fault zones such as the SanAndreas Fault in California, but most slow slip de-tected so far occurs on the subduction interface.

The motion in slow-slip episodes seems to fallmainly or entirely on the primary fault plane in theregion. The relative motion on the fault is in the di-rection of expected plate motions based on regularearthquakes and geodetic measurements in the re-gion. Some controversy remains whether significantoff-fault activity is also present.

Seconds to weeks and beyondSlow slip in Cascadia is some of the loudest, mostperiodic in recurrence, and best studied in theworld. The average rate of motion along the plateboundary is 4 cm/yr, but as GPS measurements ofsurface deformation revealed, rather than slidingsteadily at that rate, the plate boundary from 35 to55 km is stuck for 11–15 months, then moves rela-tively quickly for several weeks.4 The cycle is re-markably close to periodic: Twenty or so docu-mented repetitions show only 10% variance inrecurrence interval.5

In the early 2000s, at the same time that slowslip was coming into view in Cascadia, an evenmore surprising phenomenon was noticed in Japan.Sections of the plate interface beneath the lockedboundary in several subduction zones were radiat-ing intermittent prolonged bursts of tremor.6

Tremor is high-frequency vibration that continuesfor minutes to days, fluctuating in amplitude, with-out the abrupt onset characteristic of normal earth-quakes. It’s most clearly observed in the frequencyrange 2–10 Hz, but observing it at all requires adense, high-quality network of seismometers. Japanhad recently installed the Hi-net network, compris-ing hundreds of seismometers emplaced in bore-holes 100–2000 m deep. With Hi-net’s station den-sity and quiet surroundings, scientists could see thetremor signals previously obscured by such Earthnoise as the pounding of ocean waves and wind inthe trees. From the common amplitude modulationsseen across many stations, it was evident that thetremor was generated deep beneath Earth’s surface.

Guided by the GPS observations from Casca-dia, scientists in Japan quickly noticed slow defor-mation in the same time and place as their tremor.Likewise, researchers in North America foundtremor accompanying their slow deformation. It be-came clear that tremor and slip often occur togetheron the same patch of fault. Figure 2a shows the par-

ticularly well-resolved area of tremor for Cascadianevents, and figure 2b shows the slip that was ob-served in the same place at the same time.

When the instrumentation and noise are most fa-vorable, motion on a slow-slip patch can be seen withperiods of 0.03 s (for 30-Hz tremor) to weeks. Tremordetected on seismometers and slip so slow it is onlyvisible with geodetic techniques such as GPS turn outto represent the end members of a single slow-slipprocess. Tidal records, GPS, strainmeters, tiltmeters,and seismometers of various types, because of theirindividual sensor designs and noise peculiarities,each glimpse slow slip only in a limited passband.The overall shape and variability of the slow-slipspectra remain subjects of study.

Sometimes only tremor or slow slip is de-tectable, and sometimes both are detectable simul-taneously in overlapping but not identical areas.Sometimes tremor appears in scattered patches thatappear to be part of a more steadily moving under-lying disturbance. Part of the difference is thattremor can be detected with much greater spatialand temporal resolution than slow slip can, butthere also appear to be differences in the relativeamounts of the two happening between variousplaces on the fault. Recent observations demon-strate that slow slip also sometimes occurs at depthssimilar to those of regular earthquakes—and evenin the region of subduction zones between regularearthquakes and the surface—and that a singleslow-slip event can span a wide depth range.

More unique featuresTo recap the story so far, slow-slip episodes arenews because they strike the deeper portion offaults previously thought to move boringly steadily,they take a long time to complete, and they can recuralmost like clockwork. Traditional earthquakesshare none of those characteristics.

Slow-slip events differ from regular earth-quakes in several other ways. The stress released isorders of magnitude smaller, and small changes in

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The upper 100 km or so of Earth’s crust and mantle are broken into tec-tonic plates. Within each plate, the rock is relatively strong and does notdeform much. The relative motion of the two rubbing plates, averagedover thousands of years, ranges from a few to 20 cm/yr.

Boundaries between plates are divided into three types, according tothe plates’ relative motion. Subduction zones are the geological bound-aries at which tectonic plates converge, with one plate overriding and theother diving deep into Earth’s mantle. At mid-ocean ridges, the platesmove apart, with new rock upwelling in between. And at transformboundaries, plates shift laterally relative to each other.

Subduction zones are of great scientific and societal interest becausethey harbor a host of valuable landscapes and geophysical threats. Thedownwelling of cold crust and mantle creates large volumes of brittle rockthat pave the way for the largest earthquakes. Big earthquakes near coast-lines spawn tsunamis. The dragging of water-saturated crust into the fierydepths leads to magma upwelling through volcanic edifices. The highmountains near many subduction zones are susceptible to landslides.

The majority of global subduction occurs around the aptly namedPacific Ring of Fire.

Plate tectonics, faults, and subduction zones

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Slow slip events

stress become important. The speed and direction ofslow-slip propagation follows its own distinct pat-tern, as does the relationship between event dura-tion and total fault motion. Clearly, different physicsis involved.

The small stress release for slow-slip episodesis apparent in several ways. Most directly, the mo-tion is small and spreads out over a large area. Forexample, each of the four slow-slip events summedto produce figure 2b involved only 1–2 cm of slip,but that motion was enough to make the fault lockup again. A normal earthquake with a 100-km-longrupture would show meters of slip, hundreds oftimes the drop in shear stress.

Slow slip is so sensitive to small stress changesthat it can be triggered by tidal stresses and by sur-face waves from distant earthquakes.2 Slow-sliptremor amplitude ebbs and wanes with tidal stress-ing, whereas normal earthquakes show almost nocorrelation with the tides. Strong earthquake sur-face waves have been observed to incite bursts oftremor in phase with the arrival of stresses thatwould encourage slip on the tremoring faults. Fig-ure 3 shows an example in the case of some extra -ordinarily strong surface waves. Regular earth-quakes are sometimes also triggered by suchstressing, but they are generally delayed from thestressing pulses.

An intriguing feature is the variety of speedsand directions in which tremor migrates over thecourse of a slow-slip episode. Major episodes, suchas the one shown in figure 4, migrate along the plateboundary at about 10 km/day. The progress is fitfulhour by hour, but fairly steady day by day. Thatprogress is punctuated, however, by periods ofrapid reversal, during which the activity jets back-wards at about 200 km/day.7 On the time scale of

minutes, tremor tends to streak along a nearly per-pendicular path, in the direction of the plates’ rela-tive motion, at about 50 km/hour.8 In contrast, therupture in a regular earthquake generally propa-gates at about 2–3 km/second, the speed at whichstresses travel through the rock.

Like regular earthquakes, slow-slip events canbe small or large. They can last anywhere from afraction of a second for slow microearthquakes toseveral weeks for full-blown ETS events. And aswith regular earthquakes, slow-slip episodes aremore likely to be small than large, even sharing theGutenberg–Richter log-linear distribution, devel-oped for regular earthquakes, of number of eventsas a function of magnitude. The relationship be-tween duration and moment, however, is dramati-cally different for the two types of events. (Seismicmoment is the product of the area of faulting andthe amount of slip.) Slow-slip activity radiates energy and accumulates slip at a fairly steady rate,both during individual events and between eventsof dramatically different duration. As a result, theduration is directly proportional to the moment. Incontrast, for regular earthquakes the affected faultlength is proportional to the event duration, the rup-tured area is proportional to the square of faultlength, and the amount of slip is proportional tolength, so the duration is proportional to the cuberoot of the moment.9 Slow-slip events and regularearthquakes therefore form two distinct continua ofevents.

Figuring out the physicsConceptually, the simplest view of the physics of aslow-slip event is that the overriding and down-ward-plunging plates rub against each other alonga well-defined interface, with their motion gov-

Slow slip

Figure 1. Slow slip

appears most prominentlyin subduction zones, onthe plate boundary between the shallow,locked section and thedeeper, freely slipping section. The illustrationshows an east–west crosssection beneath Seattle,Washington, and Vancouver,British Columbia, Canada,in the Cascadia subductionzone. The Juan de FucaPlate is pushed under theNorth American Plate asthe two plates move to-gether. Marine sedimentsscraped off the subductingplate build up in the accretionary prism.

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erned by frictional forces between their brittle sur-faces. However, the physical and chemical environ-ment at the subduction zone interface is not knownin detail and may differ between subduction zones.The “interface” may actually broaden with thehigher pressures and temperatures at greaterdepths, and it may behave ductilely rather than brit-tlely, in a so-called subduction channel. The miner-als present undergo several poorly understoodphase transitions at slow-slip depths. Fluids (H2O orCO2) are probably present and may be important.Any of these conditions could greatly complicatethe physics of slow slip, so much remains to be understood.

Rate-and-state friction laws are empirical lawsdesigned to account for the full range of behaviorobserved during regular earthquakes. They de-scribe how friction varies with material properties,slip speed, and the history of the state of the contactsurface. To describe slow slip as well, the frictionlaws would need to include mechanisms to keep theslip speed in check so that it doesn’t accelerate tothat of a regular earthquake. The effective normalstress must be very low, consistent with the pres-

ence of pressurized fluids, and the stress release in-volved must be several orders of magnitude smallerthan for regular earthquakes. It appears that suit-ably designed rate-and-state friction laws can de-scribe slow-slip pulses, such as those in Cascadia,and can reproduce the faster propagation of tremoron shorter time scales.10 But what is still lacking is aclear connection between the empirical laws and theactual physics and chemistry—for example, thephase transitions in the source region, which can’tbe easily studied due to rock heterogeneity and theinaccessible location.

Healing due to mineral precipitation on timescales of months could moderate the periodic break-ing in slow slip.10 Alternatively, accelerating slip maylead to a competition between two opposing effects:Heating of the rock reduces friction, and dilatancy—expansion of fluid-filled pores as the rock issheared—increases it.11 In that model, fluid-mediatedchanges in friction control whether slip is slow or fast.

Further evidence for the key role of fluids is thediffusion-like migration of slow-slip activity. In thepropagation of some slow-slip events, as determinedby tremor measurements, the distance traveled is

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SFigure 2. Tremor and slip are usually observed on the same patch of fault during a slow-slip episode. (a) Tremor, observed byseismometers and quantified as the number of tremor epicenters per 0.1° × 0.1° bin, summed over four slow-slip episodes inthe Cascadia subduction zone from 2004 to 2008. (b) Total slip, measured by GPS, for the same four episodes. The contours inboth panels show the depth of the plate interface. (Adapted from ref. 15.)

Figure 3. Seismic waves from distant earthquakes can trigger slowslip. (a) High-frequency tremor under Vancouver Island in southwesternCanada in the wake of the 2002 Denali earthquake in Alaska. (b) Low- frequency seismic record showing thesurface waves that triggered thetremor. (Adapted from ref. 16.)

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Slow slip events

proportional to the square root of time. It appearsthat something, perhaps fluid or shear stress, maydiffuse during slow slip.12

An anomalously high Poisson’s ratio in the un-derlying subducting crust and high conductivity,consistent with ample fluid content, are observed inseveral slow-slip regions. That the expected depthsof dewatering reactions in subducting rocks coin-cide with zones of slow slip—and the tendency ofslow slip to appear above younger slabs, which aremost likely to still be losing water—strengthens thisargument.

Open questionsNot every plate boundary that’s been monitoredwith good instrumentation shows signs of slow slip,and it’s not known why. Looking for patterns amongobservations may help to reveal the geological fea-tures that enable slow slip. But it’s not clear whetherthe physical properties below, above, or at the inter-face are most important.

Several lines of evidence, as described above,suggest that fluids play a role in slow slip. Fluidsmay also be important in weakening rock to allowregular earthquakes, and they could be even morecritical to the creation of the weaker conditions pos-tulated for slow slip. A key result is that tremorstreaks and lines of concentrated tremor tend toalign with the direction of relative fault motion.That coincidence suggests that smearing of favor-able geological units or corrugations in the plate interface contributes to the pattern of slow-slip behavior.

Can slow slip be helpful in assessing the threatof great earthquakes? It’s possible, and that possibil-ity is one of the motivations for the current studies.But no one has yet systematically monitored slow-slip-prone zones before and after great earthquakes,so the postulated patterns remain unconfirmed.

If slow-slip zones are releasing most of their

stress, as many researchers assume, the locations ofslow slip may mark the edges of an area of fault thatis stressed enough to break. But we don’t yet knowwhether tremor locations, long-term geodetic meas-urements, or other measurements best outline theedge of the dangerously locked zone.

A simple pattern would be if great earthquakesare most likely to occur when nearby slow slip is ac-tive. Slow slip on a given patch is generally activeless than 10% of the time, so a correlation could nar-row the time of greatest earthquake threat. But inCascadia, for example, the zone that could break ina great earthquake is adjacent to several slow-slippatches that are all active at different times. So es-tablishing a correlation between slow slip andnearby great earthquakes would require long-termobservation over at least decades.

More powerful but more speculative possibili-ties involve the idea that slow-slip patterns mayevolve during the cycle of stress recharge before anearby great earthquake. Some numerical modelsindicate that great earthquakes might nucleate inslow-slip zones as normal slow slip that runs awayover the course of minutes to months. An exampleis shown in figure 5. Or slow-slip episodes leadingup to a great earthquake might cover larger orsmaller areas, recur more frequently, or have morevigor with a recognizably accelerating pattern.

Most unconstrained, and perhaps most un-likely, is the possibility that we can directly observegeophysical changes revealing lubrication or stressbuilding between events, and they can tell us whenthe next magnitude 9 event is imminent.

Two recent observations are tantalizing. Closeobservation of the time before the Izmit earthquake,which killed approximately 20 000 people in Turkeyin 1999, showed an hour of accelerating seismic ac-tivity beneath where the fault ruptured,13 a findingconsistent with triggering by a slow-slip episode.Similarly, in the month and especially two days just

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Figure 4. Evolution of a single slow-slip episode in the Cascadia subduction zone.The episode started with thedeep blue dots near the southern end, spread bilaterallyfor a few days, and continuedto the north for the rest of the 2 weeks shown. (Courtesy ofAaron Wech.)

before the Tohoku-oki earthquake, which killedabout 20 000 people in Japan in 2011 (see the articleby Thorne Lay and Hiroo Kanamori in PHYSICSTODAY, December 2011, page 33), slow slip and otherseismic activity were observed near the main-shockhypocenter.14 Just how often such slow-slip precur-sors appear, and how often they are followed by de-structive earthquakes, remains to be seen. But thoseresults may indicate progress in the supremely frus-trating challenge of earthquake prediction.

The discovery of slow slip has inspired geo-physicists and remains the focus of intense atten-tion. Dense arrays of seismometers are beingplanted in several places, which we hope will re-solve many of the questions framed above. As YogiBerra said, “You can observe a lot by watching”.

References1. National Research Council, Committee on the Science

of Earthquakes, Living on an Active Earth: Perspectiveson Earthquake Science, National Academies Press,Washington, DC (2003).

2. Z. Peng, J. Gomberg, Nat. Geosci. 3, 599 (2010).3. J. L. Rubinstein, D. R. Shelly, W. L. Ellsworth, in New

Frontiers in Integrated Solid Earth Sciences, S. Cloetingh,J. Negendank, eds., Springer, New York (2010), p. 287.

4. M. M. Miller et al., Science 295, 2423 (2002).5. H. Dragert, K. Wang, G. Rogers, Earth Planets Space 56,

1143 (2004).6. K. Obara, Science 296, 1679 (2002).7. H. Houston et al., Nat. Geosci. 4, 404 (2011).8. A. Ghosh et al., Geochem. Geophys. Geosys. 11, Q12010

(2010).

9. P. M. Shearer, Introduction to Seismology, 2nd ed., Cam-bridge U. Press, New York (2009).

10. Y. Liu, A. M. Rubin, J. Geophys. Res. 115, B10414 (2010).11. P. Segall et al., J. Geophys. Res. 115, B12305 (2010).12. S. Ide et al., Nature 447, 76 (2007).13. M. Bouchon et al., Science 331, 877 (2011).14. R. Ando, K. Imanishi, Earth Planets Space 63, 767

(2011). 15. A. G. Wech, K. C. Creager, T. I. Melbourne, J. Geophys.

Res. 114, B10316 (2009). 16. J. L. Rubinstein et al., Nature 448, 579 (2007).17. Y. Kaneda, K. Hirahara, T. Furumura, J. Disaster Res. 4,

61 (2009).18. T. Hori et al., Earth Planet. Sci. Lett. 228, 215 (2004). ■

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Figure 5. Numerical simulation of possible slow-slip nucleation inJapan’s Tokai region, southwest of Tokyo. Slow slip in the area marked by the circle grows into meters of coseismic slip—that is, a large earth-quake. The cabled 25-element DONET array is now installed in the re-gion to monitor deformation in real time.17 (Figure adapted from ref. 18.)