On 11 March 2011, the nation of Japanand geophysicists around the worldreceived a terrible surprise: A hugeearthquake, significantly strongerthan people had anticipated or pre-

pared for in the region, struck off the northeasternshore of Honshu. Shear sliding on the fault wherethe Pacific Plate thrusts below Japan lasted for 150 anxiety-filled seconds, shifted the coast of Japanup to 5 m eastward, and lifted the sea floor by asmuch as 5 m over 15 000 km2, an area comparable tothe state of Connecticut.1,2 Displacements as large as60 to 80 m—the largest ever measured for an earth-quake—occurred near the subduction trench, and atotal strain energy equivalent to a 100-megaton ex-plosion was released during the sliding. This was thegreat 2011 Tohoku-oki earthquake, so-named for theregion it struck, shown in figure 1.

The sudden sea-floor displacement generated amassive tsunami that swept onto hundreds of kilo-meters of coastline along the islands of Honshu andHokkaidō. Tsunami waves 3–15 m high overtopped harbor- protecting tsunami walls and coastal mar-gins and penetrated as far as 10 km inland along thecoastal plain.3 The flood destroyed many smalltowns and villages, killed some 20 000 people, andinitially displaced nearly half a million; six monthslater, tens of thousands were still living in highschool gymnasiums and other temporary quarters(see PHYSICS TODAY, November 2011, page 20). Thehigh waves also disrupted the power supply neededto maintain water circulation for cooling the reac-tors at the Fukushima nuclear power plant, whichprecipitated a catastrophic release of radiation thatcontinues to plague central Japan.

Located along the Pacific Ring of Fire, Japan hasexperienced a long history of earthquakes, but theTohoku-oki quake was a rare giant with no instru-mentally recorded precedent. Researchers estimateits moment magnitude Mw at 9.0. (To appreciate that

measure of radiated seismic energy, see box 1.) Thehuge event abruptly released elastic strain energythat had accumulated in the rocks on either side ofthe shallow-dipping plate boundary—the mega -thrust fault—for more than a millennium since thelast great earthquake in the central part of themegathrust, which occurred in 869 with Mw ≥ 8.3.(Another large earthquake occurred in 1611 near thenorthern part of the 2011 rupture zone, but its de-tails are murky and still debated.)

The strain release generated seismic, tsunami,and atmospheric waves that spread through Earth.Those wave phenomena were captured by thou-sands of geophysical facilities worldwide, includingdense Japanese networks of seismic and GPS sta-tions deployed after the devastating 1995 Kobeearthquake, and by the large numbers of tsunamigauges and deepwater pressure sensors deployednear Japan and across the Pacific Ocean followingthe Sumatra–Andaman earthquake and tsunami in December 2004 (see PHYSICS TODAY, June 2005,page 19). Combined with extensive recordings fromglobal seismic networks, the data from those sta-tions, gauges, and sensors make the 2011 Tohoku-oki event the best-recorded earthquake and tsunamiin history.1,4

Plate-boundary physicsGreat earthquakes are those whose moment magni-tudes are 8.0 or higher. Their activity varies signifi-cantly between different subduction zones. Somesubduction zones—for example, ones in southernChile, Sumatra, and southwestern Japan—producerepeated, great-earthquake ruptures every centuryor two, while others—in the Mariana Islands andTonga, say—produce them rarely, if ever.

www.physicstoday.org December 2011 Physics Today 33

Insights from the great 2011 Japan earthquake

Thorne Lay and Hiroo Kanamori

The diverse set of waves generated in Earth’s interior, oceans, and atmosphere during the devastating Tohoku-oki earthquake

reveal some extraordinary geophysics.

Thorne Lay and Hiroo Kanamori are seismologists at the University of California, Santa Cruz, and the California Institute of Technology in Pasadena,respectively.

That variability is attributable to differences inplate-boundary frictional characteristics, illustratedin figure 2. Dark patches represent seismic portionsof the fault surface where the slip generates earth-quakes, and white patches represent areas wherethe slip is primarily aseismic—slowly sliding with-out earthquakes. At shallow depths, the seismicpatches can rupture as tsunami earthquakes—aname reserved for large, tsunami-generating eventsthat, because of unusually low rupture-expansionrates, occur without generating strong, short-periodseismic waves. Conditionally stable regions nor-mally slip continuously but can also slip seismicallywhen loaded abruptly during the failure of neigh-boring seismic patches.

The ratio of the area of seismic patches to thetotal area of the fault is large for subduction zoneswith frequent great earthquakes and small for zoneswith infrequent great earthquakes. Depending onhow regions of the megathrust with different rup-ture characteristics fail, though, diverse earthquakesequences are possible. A failure of one seismicpatch may produce a single, large earthquake. Butwhen two or more patches fail in a cascade that alsoprompts conditionally stable regions between themto slip seismically, the result is a much larger earth-quake than one would otherwise expect from just

the seismic patches alone. An example of such behavior has been observed for a great earthquakethat occurred along the Ecuador–Colombia coast in 1906.

The Tohoku-oki subduction zone is thought tocomprise a mixture of such seismic and aseismicpatches and a concomitant mix of frictional charac-teristics that can lead to an unusual diversity of rup-ture patterns. Subregions slide aseismically aroundlocalized patches that fail repeatedly in a number ofways—as frequent small earthquakes; as moderate-sized earthquakes accompanied by large amountsof slow, postseismic slip of a few centimeters perday instead of the typical kilometers-per-secondrate of an earthquake rupture; or as shallow tsunamiearthquakes.1,5

The shallow portion of the megathrust justnorth of the 2011 earthquake zone and illustrated infigures 1a and 2b ruptured during the great 1896(Mw about 8.2) tsunami earthquake. The PacificPlate in that region also experienced an internal rup-ture in 1933 from extension near the Japan trench.Similarly large megathrust earthquakes within the2011 rupture zone during the past century all oc-curred near the coast, but the region farther offshorewhere large slip occurred in the 2011 event wasknown to have previously ruptured only in the 869

34 December 2011 Physics Today www.physicstoday.org

Japan earthquake

100 km

Jap

an T

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2011 Modest slip, strongshort-period radiation

2011 Large slip, weakshort-period radiation,strong tsunami generation

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UD

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ku

shim

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anri

ku

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kk

aid

ō Run-upInundation

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wall at Fukushima plant

a b c

Figure 1. (a) The red star marks the epicenter of the 11 March 2011 Tohoku-oki earthquake, which occurredacross the fault where the Pacific Plate subducts underJapan. (Arrows indicate the plate’s direction of motion.) The pink and pale green patches delimit portions of thefault that ruptured with distinct frictional characteristicsduring the 150-s earthquake, and the red dots delimit theone-day aftershock region. Light blue dots encircle rupturezones of historic large earthquakes, and the purple dots encircle the approximate source of a large tsunami in 869.(Adapted from ref. 2.) (b) A map of the Honshu andHokkaidō islands and (c) the relative locations and heightsof the 2011 tsunami inundation (red, the maximum floodlevel reached) and the tsunami run-up (blue, the maximumelevation reached). (Adapted from ref. 3.)

earthquake, which produced a large tsunami inun-dation along the coast near Sendai. Even givenJapan’s exceptionally long historical record of earth-quakes, researchers’ evaluations of the region’searthquake potential are complicated by the rangeof sliding behavior along the megathrust and thewide spans of time—from a few years to many cen-turies—for distinct parts of it to repeatedly slip. Therupture of the 2011 earthquake involved many ofthose parts failing in a cascade that had not been an-ticipated in any of the seismic hazard models for theregion.

Over the 15 years prior to 2011, a network ofGPS stations (now numbering more than 1200)across Japan detected a gradient of westward dis-placements decreasing from east to west across theisland of Honshu. Several groups had modeled thatcompressional crustal strain by assuming that theoffshore plate-boundary fault was locked, unable toslip to varying degrees as the plates converged, assuggested in figure 3. Although the offshore regionto the north experienced little of that locking, thedeeper portion of the megathrust where the 2011event subsequently occurred was found to be notslipping at all. Even so, most studies assumed thatthe “up-dip” part of the fault—that is, the shallowpart of it—was not accumulating strain.6

In recent years new technologies have emergedfor tracking ground positions offshore using ocean-bottom GPS sensors, and data were beginning todemonstrate that the up-dip part of the megathrustwas accumulating strain. Indeed, that surprisinglystrong part of the fault, which had loaded up strain

for more than a millennium without rupturing, sud-denly overcame frictional resistance on 11 March2011. Given a few more years of offshore strainmeasurements before the earthquake struck, it islikely that the potential for so great an event wouldhave been clearer.

Making waves Abrupt sliding of the strongly locked region of themegathrust released elastic strain energy from therock, which drove the rupture, heated the fault, and generated both seismic waves and permanentdeformation in the rock around the fault. As thefault slipped, elastic compressional and transverseseismic waves—known as P and S waves, respec-tively—spread outward in all directions. Reboundof the elastic deformation of the rock, in turn, shiftedthe Japan mainland eastward and uplifted the oceanfloor. The many gigatons of displaced ocean watergenerated tsunami waves, and the seismic andtsunami-wave energy coupled to the atmosphere toproduce yet more waves, all spreading from thesource with different velocities. The entire Earthsystem was perturbed by the energy release, and thetotal mass redistribution reduced the length of anEarth day by about 1.8 microseconds. (For a primeron the physics behind the processes, see the essayby David Stevenson in PHYSICS TODAY, June 2005,page 10.)

Seismic waves shook the ground in Japan witha high frequency of about 10 Hz, ground accelera-tions as large as 2.7 g, and peak ground velocities of80 cm/s across Honshu. At the Fukushima nuclear

www.physicstoday.org December 2011 Physics Today 35

Slow tsunami quake

Seismic

Aseismic

Conditionallystable

Sanriku-oki1933

1896 tsunamiearthquake

Down-dip Up-dip

Normal rupture, short-periodseismic waves

Slow rupture, low-frequencyseismic waves

Stable sliding

Pacific Plate

Pacific Plate

Tohoku-oki

2011

a b

c

JAPAN

Figure 2. A frictionally complex fault. (a) The megathrust off thecoast of Japan comprises regions that slip seismically, regions that slipaseismically, slow- rupturing regions that experience large slip at shal-low depths generating tsunami earthquakes, and conditionally stableregions that slip aseismically unless adjacent slips drive them to slideseismically. (b) Cross-sectional schematic of the Sanriku-oki region shows thatthe great 1896 and 1933 earthquakes ruptured at shallow depths, but thedeeper part of the megathrust appears to be aseismic. (c) By contrast, the en-tire megathrust in the Tohoku-oki region failed—both shallow “up-dip” anddeeper “down-dip” parts of the cross section. The faster-sliding down-dip rup-ture generated high levels of short- period radiation, while the slower-slidingup-dip rupture generated low levels of short- period radiation.

power station, the shaking rose 20% higher than the0.45-g level the station was designed to tolerate, butbecause of the subsequent explosions at the facilityit is unclear how much damage was inflicted fromthe shaking. Despite the strong shaking along thecoast, many structures had only moderate or lightdamage, a testament to Japan’s effective buildingcodes and construction standards.

Several thousand seismometers in Japan’s net-works of short-period and broadband seismic sta-tions recorded the ground vibrations, and continu-ously recording GPS stations made 1200 additionalmeasurements of the transient and static ground mo-tions.1,7 The seismic waves propagated through Earthand were captured by some 1300 broadband globalseismometers, many of which transmitted the data innear-real time to collection centers; the immediateavailability of data facilitated rapid analysis of thesignals by seismologists around the world.1,5,8

The local recordings in Japan formed the basisfor the Japan Meteorological Agency’s tsunami

warning within three minutes from the start of therupture (see box 2). Numerous gauges and tide me-ters measured the tsunami as it approached andrushed over the coast. And though some sensorswere driven off-scale, several complete recordingsdocument the wave, exemplified in figure 4a.

A narrow pulse-like waveform is apparent inthe wave recorded off the coast of Iwate Prefecture(Sanriku), where run-up was greatest. That peak ar-rived about 20 minutes after the faulting began, con-sistent with the sea-floor uplift in a narrow zonenear the Japan trench. Tsunamis travel slowly inshallow coastal water, and many people were ableto reach high ground in time to escape the flooding.

The tsunami spread seaward as well.9 As wavesspread through the northern Pacific, their 20-minuteperiod and meter-high amplitudes were recordedby numerous Deep-Ocean Assessment and Report-ing of Tsunamis buoys operated by the US NationalOceanic and Atmospheric Administration. NOAAanalyzed those signals quickly enough to accuratelypredict the waves’ arrival times and amplitudes atPacific island coastlines. Greatly expanded since2004, the buoy system worked superbly, and thedeepwater signals have been incorporated intostudies of the rupture process.1,3

The tsunami struck Hawaii about 7 hours afterthe earthquake—and the coast of California 3 hourslater still—with enough force to damage harbors,marinas, and coastal resorts. The Philippines, Indonesia, and New Guinea suffered significantly.And minor damage occurred in Peru and Chilewhen the wave struck their coastlines 20 to 22 hoursafter the event.

The combined motions in the rocks and oceanproduced atmospheric waves, which extended thereach of the great earthquake to a planetary scale.Those waves became amplified with altitude as atmospheric density decreased and created largeperturbations in the ionosphere. The coupling ofEarth’s solid, liquid, and gaseous layers is generallywell understood, and the atmospheric waves werewidely recorded by barometric pressure sensors atthe surface. GPS stations around Japan sensed theionospheric perturbations due to their effects on the transmission of satellite radio signals.10 Theionospheric disturbance involved fluctuating elec-tron density up to altitudes of 350 km caused by atmospheric waves expanding from the source at720–800 km/hr.

The main shock of the Tohoku-oki earthquakeproduced extensive aftershock activity in both theupper and lower plates and in regions of themegathrust laterally adjacent to the main rupturezone. Within 24 hours hundreds of aftershocks hadoccurred over an area 500 km long and 300 km wide.Many resulted from extensional faulting in theupper plate, indicative of the large strain relaxationthat occurred when the fault slipped.

Many minor faults within Japan’s crust were activated by the change in stress from the mainevent.11 Extensive regions of the upper 100 m ofJapan’s crust experienced ephemeral reductions inseismic shear velocity, likely as a result of shaking- induced cracking and the movement of ground -

36 December 2011 Physics Today www.physicstoday.org

Japan earthquake

−1 0 1

35°

40°

45°

45°

130°13

5°

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Coupling fraction

Figure 3. The

degree to whichthe plates werelocked togetheralong themegathrustfaults aroundJapan before the2011 earthquakeis inferred from

GPS measurements of crustal strain: Dark red areas indicate plates stucktogether, and dark blue areas indicate regions of the megathrust wherethe plates were sliding past each other aseismically. Latitude and longi-tude are also indicated. (Adapted from ref. 6.)

There are many measures of earthquake size, butoverall faulting significance is best captured by seis-mic moment, Mo = μAD, where μ is the shear rigidity ofrock around the fault, A is the total rupture area thatslides, and D is average slip—that is, the displacementof one side of the fault relative to the other. Expressedin Newton meters, the seismic moment directly deter-mines the strength of low-frequency seismic wavesgenerated by an earthquake. The moment magni-tude, Mw = (log  Mo − 9.1)/1.5, is now a widely usedmagnitude measure based on the seismic moment,which is accurately measured by modeling low- frequency seismic recordings during an event. Theseismic moment increases by a factor of about 31.6 for each unit increase in Mw. The largest recordedearthquake in history was the 1960 Chile earthquakewith Mw = 9.5. The 2011 Tohoku-oki earthquake wasfound to have a seismic moment of 2.9 × 1022 Nm,which gives Mw = 9.0, somewhat lower than the 1964Alaska (Mw 9.2) and 2004 Sumatra–Andaman (Mw 9.2)earthquakes, ranking it the fourth largest event sincethe era of seismological recordings began in the early1900s.

Box 1. Moment magnitude

water.12 Ongoing research is quantifying the broadreach of the stress perturbations from the greatevent.

Rupture physicsArmed with extensive geophysical recordings ofground, ocean, and atmosphere before, during, andafter the 2011 earthquake, researchers can study itsrupture physics closely. Although reports of possi-ble precursory phenomena exist—for instance, IRheating of the atmosphere above the source regionseveral days before the event and anomalous total-electron readings of the ionosphere preceding theearthquake—no clear deformational precursor hasemerged from analyses of the geodetic and seismicdata sets.

There were earthquakes, now identified asforeshocks, for several days before the main shock;the largest was an Mw 7.5 on 9 March 2011. Their im-portance as harbingers is again unclear, but theywere clustered near the point of initiation of themain shock and plausibly represent accelerating de-formation in that region. The weak initial onset ofthe rupture raises many questions about how itgrew so huge.

According to analyses of the data, the mainrupture spread on the fault with relatively low ex-pansion velocity of about 1 km/s, initially expand-ing for about 50 s “down-dip”—that is, deep withinthe fault—toward the coast near Sendai, where nu-merous large earthquakes have occurred this pastcentury and earlier, as outlined in light blue in figure 1a. The rupture then expanded into shallowerdepths for the next 40 s, with the largest slip alongthe fault occurring more than 150 km offshore, closeto the trench. That part of the rupture, in turn, wasfollowed by additional sliding down-dip that mi-grated southward along the coastline at about2.5 km/s.

The slip near the trench was enormous; it aver-aged about 40 m over the upper 100 km of themegathrust and peaked at 60–80 m close to thetrench.1,5,13 Those are the largest fault displacementsdocumented for any earthquake in history. Usingelastic dislocation theory to predict the sea-floor up-lift, one can accurately account for the size of thetsunami from geophysical fault models. But ongo-ing research is evaluating whether any nonelasticdeformations, such as submarine landslides, werealso involved.

Researchers estimate a stress drop of 15–30 MPa in that large-slip region, significantlygreater than the few-MPa stress drops typicallyfound for large interplate ruptures. Seismic imagingof rock around the shallow fault zone reveals rela-tively high P-wave velocities and some seawardarching of the upper plate toward the trench. Thosedata suggest the presence of hard, strong, and brittlerock around the fault, at odds with conventional be-lief that the shallowest regions of subduction zonesare weak and deform mainly anelastically.14

The dense seismic observations of the Earth-Scope Transportable Array in North America revealthat strong coherent bursts of short-period radia-tion, which likely produced the violent shaking in

Honshu, preferentially emanated from the deeperhalf of the megathrust, where total slip was muchsmaller than in the upper part of it.1,5 In that respect,the shallow rupture in the 2011 earthquake behavedmuch like the shallow tsunami earthquake of 1896.But the huge shallow slip, 50–90 s after the earth-quake began, appears to have driven the central,deep portion of the fault to rerupture and slip alongadjacent regions of conditional stability, which ac-centuated the total size and energy.

Thus the 2011 earthquake was essentially acomposite event, as discussed earlier, with attrib-utes of a tsunami earthquake for the shallow portionof the rupture and attributes of a typical megathrustearthquake in the deeper portion of it. The great

www.physicstoday.org December 2011 Physics Today 37

0.0 0.2 0.4 0.6 0.8 1.0

TIME (hours)

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Figure 4. Several tsunami gauges just off Japan’s Pacific coastline record sea-level variations. (a) Measurements fromGPS gauges offshore north and south Iwate in the Sanriku region show the waveform as the tsunami reached each station20–30 minutes after the earthquake struck. The narrow, large-amplitude peak at each station originated near the Japantrench and produced up to 40-m run-up heights along thecoastline. (b) The tsunami also spread out to sea, its peak ampli-tude gradually diminishing across the deep Pacific Ocean, ascalculated for a faulting model derived from seismic waves.(This issue’s cover shows similar calculated tsunami amplitudesfrom modeling buoy data.) The tsunami reached Californiaabout 10 hours after the earthquake. (Adapted from ref. 9.)

2004 Sumatra rupture was similarly enhanced byextending over multiple subregions. The behaviorpoints to a weakness of the “characteristic earth-quake” model often used for probabilistic seismichazard assessment.

The seismic moment estimated for the mainshock is Mo = 2.9 × 1022 Nm, and the total radiatedenergy release is 4.2 × 1017 J. But the most robust as-pect of the earthquake determined from seismologyis the moment rate function, which characterizes thesource strength for seismic wave motions generatedduring the faulting. The comparison in figure 5 ofthe moment rate function from Tohoku-oki withthat of other 20th-century great earthquakes revealsthe 2011 event’s distinctly high peak and short duration. Indeed, by that measure, Tohoku-oki re-

leased a larger peak energy per unit time than anyearthquake ever recorded.

Lessons learned Earth has experienced a high rate of great earth-quakes during the past decade—about 2.5 times theaverage rate for the past century.15 Fortunately, theexpansion and improvement of geophysical record-ings enable geoscientists to learn from each one. The11 March 2011 Tohoku-oki earthquake demon-strated that shallow portions of megathrust faultscan support higher stresses and accumulate largerstrains than conventional wisdom had suggested.Thus tsunami earthquakes, or composite events likethe recent one in Japan, may occur more broadlythan researchers had thought.

38 December 2011 Physics Today www.physicstoday.org

Japan earthquake

2011 Tohoku-oki ( 9.0)1964 Alaska ( 9.2)2004 Sumatra–Andaman ( 9.2)2010 Chile ( 8.8)2003 Hokkaid ( 8.3)

M

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Figure 5. Seismic moment rate functions (MRFs),plotted for several great earthquakes. MRF measuresthe time rate of change in seismic moment—theproduct of the surrounding rock’s rigidity, the faultarea, and the slip—and is roughly proportional tothe rate of energy radiation. The large, smooth, andnarrow MRF for the Tohoku-oki earthquake reflectsthe unusually large slip that occurred over a small localized region. The Sumatra–Andaman earthquake,by contrast, occurred for a much longer duration,and its extended MRF reflects the spatially separatedslip patches along the Sumatra, Nicobar, and Andaman islands. The MRF of the Alaska earthquakehas a character between those extremes. The Chileearthquake lasted nearly as long as the Tohoku-okievent, but with much smaller peak MRF and possiblyshorter-period fluctuations, which suggest differentfrictional characteristics between the regions’ faults.Finally, the far smaller MRF of the Hokkaidō earth-quake illustrates the vast differences in radiated en-ergy and tsunami hazard that distinguish Mw 8 and 9earthquakes.

In recent years the Japan Meteorological Agency (JMA) hasmade significant improvements to its tsunami warning system.On the basis of its preliminary seismic magnitude estimate, 7.9,JMA issued a warning just 2 minutes and 40 seconds after theTohoku-oki earthquake began. The agency warned that 6-m, 3-m, and 3-m tsunamis could be expected along the coast of Miyagi, Iwate, and Fukushima prefectures, respectively.Undoubtedly, that saved thousands of lives because the largesttsunami did not reach the coast until about 20 minutes after theearthquake began.

However, some reports indicate that many people did nottake immediate action out of a belief that the extensive networkof tsunami walls would protect them. Given a more accurateearly estimate of the true enormity of the event, Mw = 9, the JMAwould have been able to issue a warning that might haveprompted more extensive evacuations. Methods have beendeveloped to correctly assess an earthquake’s magnitude and

faulting geometry within minutes of its occurrence; unfortu -nately, those methods had not yet been implemented in Japan’soperational system.

The JMA also operates an earthquake early warning system.Earthquake warning is more challenging than tsunami warningas it relies on the initial seismic vibrations to gauge the likelylevel of ensuing shaking. But the initial 5–10 seconds of shakingproduced by the Tohoku-oki earthquake was weak, comparableto a magnitude 4.9 event, and the JMA’s earthquake early warn-ing system underestimated the expected overall intensity. Nev-ertheless, the information was useful for some. It wasn’t the onlyearly warning system available. The Japanese bullet train net-work, for instance, had its own warning system. More than 20bullet trains were running at high speed in the Tohoku districtwhen the earthquake struck. But within a few seconds of detect-ing the primary seismic wave, the warning system shut down thepower and stopped all the trains without incident.

Box 2. Tsunami and earthquake warning

Moreover, the accumulating examples oftsunami events—documented in the Aleutian Is-lands, Peru, Sumatra, Java, the Kuril Islands, andnow Japan—are persuasive evidence that even rel-atively long seismological records are too limited toadequately assess the hazard from infrequent butdevastating events. Fortunately, the great progressin geodesy to map out regions of strain accumula-tion near subduction zones makes it a major newtool for evaluating seismic potential. But clearly,more off-shore geodetic measurements will be par-ticularly valuable. In any case, the continued inves-tigation of many regions is perhaps the best strategyfor coping with a limited historical record.

The complex variations of slip behavior on theJapan megathrust highlight our ignorance of whatcontrols fault behavior. How pressure, temperature,fluids, rock composition, and other factors influencethe variations in slip rates and seismic radiationover a fault remains to be quantified. Progress re-quires dense onshore and offshore data collectionand concerted investigation of regions of concernsuch as along the Cascadia margin off the coast ofOregon and Washington where a major research effort is under way.

The Tohoku-oki event confirmed the value of applying modern technologies to earthquake andtsunami mitigation efforts. Strain-accumulationmeasurements, offshore fault-zone observations, andearly earthquake and tsunami warning systems allplayed a role in saving lives, as terrible as the eventwas. Extreme events can and do happen, and re-sources may be too limited to fully protect ourselves.Our best prospect for coping with those events’ ef-fects, however, is to draw on our technologies, prepa-rations, and ability to respond when Earth deliversthe unexpected, as it did on 11 March 2011.

We thank many colleagues who have shared informationabout the great Tohoku-oki earthquake summarized here andthank NSF for supporting research on this event with grantEAR0635570.

References1. H. Kanamori, K. Yomogida, eds., Special issue: “First

Results of the 2011 Off the Pacific Coast of TohokuEarthquake,” Earth Planets Space 63, 511 (2011).

2. K. D. Koper et al., Earth Planets Space 63, 599 (2011).3. Japan Society of Civil Engineers Tohoku Earth-

quake Tsunami Information, http://www.coastal.jp/tsunami2011/index.php.

4. M. Sato et al., Science 332, 1395 (2011).5. M. Simons et al., Science 332, 1421 (2011).6. J. P. Loveless, B. J. Meade, J. Geophys. Res. 115, B02410

(2010).7. S. Ozawa et al., Nature 475, 373 (2011).8. S. Ide, A. Baltay, G. C. Beroza, Science 332, 1426 (2011).9. Y. Yamazaki et al., Proceedings of Oceans ’11 MTS/IEEE

Kona, IEEE, Piscataway, NJ (in press).10. A. Saito et al., Earth Planets Space 63, 863 (2011). 11. S. Toda, R. S. Stein, J. Lin, Geophys. Res. Lett. 38,

L00G03 (2011).12. N. Nakata, R. Snieder, Geophys. Res. Lett. 38, L17302

(2011).13. H. Yue, T. Lay, Geophys. Res. Lett. 38, L00G09 (2011).14. D. Zhao et al., Geophys. Res. Lett. 38, L17308 (2011).15. C. J. Ammon, T. Lay, D. W. Simpson, Seismol. Res. Lett.

81 (1), 965 (2010). ■

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