killer waves: scientists are learning how to predict tsunami risk

3
V. TITOV/NOAA SCIENCE NEWS 152 MARCH 6, 2004 VOL. 165 KILLER WAVES Scientists are learning how to predict tsunami risk BY SID PERKINS L ast Nov. 16, at 8:43 p.m., a magnitude–7.5 earth- quake struck deep beneath the ocean near Alaska’s Little Sitkin Island, far out in the Aleut- ian Islands. Within 25 minutes, National Oceanographic and Atmospheric Administra- tion (NOAA) scientists issued a tsunami warning for U.S. Pacific coastal areas. Forty minutes later, a pres- sure sensor on the seafloor hundreds of kilometers south of Alaska detected the tsunami’s vanguard pulses. Data from that instrument—one of six in a network activated just the previous month—indicated that the wave was only 2 cen- timeters tall there. Simulations previously run on computers sug- gested that such a wave would- n’t be a danger to Hawaii or other distant shores, and NOAA canceled its tsunami warning less than 90 minutes after the quake occurred. A few hours later, the tsunami swept into the harbor in Hilo, Hawaii, and raised water levels there about 21 cm, just 2 cm higher than predicted by the sim- ulations. No damage was done. This successful prediction paid off big for U.S. taxpayers, says Eddie N. Bernard of NOAA’s Pacific Marine and Environ- mental Laboratory in Seattle. He estimates that the canceled tsunami warning saved, in Hawaii alone, at least $68 mil- lion—the economic impact of a statewide coastal evacuation. In comparison, the six seafloor sen- sors in the Pacific network— three south of the Aleutians, two off the U.S. West Coast, and one along the equator between Chile and Hawaii—are part of a tsunami-hazard mitigation effort that has cost only $17.6 million since the program’s inception in 1997. Other parts of the program include assessing tsunami risk for coastal communities. Geophysicists are using digital models of topog- raphy and detailed simulations of fluid flow to estimate which parts of population centers are at most risk from the potentially deadly waves. Some geologists are scouring coastal landscapes to find traces of prehistoric tsunamis, and other scientists are analyzing more- recent records from tidal gauges to see whether they reveal a pat- tern in how often tsunamis of varying sizes strike particular locations. WAVES AHOY Taken from Japanese, the word tsunami typ- ically stimulates thoughts of monstrously tall waves that wipe out coastal communities and kill thousands of people. But tsunamis come in all sizes. Scientists estimate that the death toll of the 141 damaging tsunamis that occurred during the 20th cen- tury exceeds 70,000. During the same period, however, at least 900 smaller tsunamis caused no damage whatsoever. Some, just centimeters high when they hit shore, sloshed right past swimmers and were detected only by instruments. Distinguishing small tsunamis from the large ones before they strike is a matter of life and death, as well as of money. Accurate tsunami simulations, such as the one that NOAA scientists used to cancel last November’s tsunami warning, hold promise for greatly reducing the number of unnecessary evacuations across the Pacific, says Bernard. False alarms cost more than money, he adds, because they erode public confidence in the agencies that broadcast the warnings. That, in turn, could lead people to remain in coastal areas despite accurate warnings. At the fall meeting of the American Geophysical Union in San Francisco last December, Bernard described the National Tsunami Hazard Mitigation Program—a partnership that includes representatives from four federal agencies and the five states that border the Pacific Ocean. He told Science News that he’d like to see the program’s network of tsunami-detecting instruments expand from 6 to around 20 sensors, a number that would permit their spacing at about 400 km apart along the West Coast and the section of Alaska deemed most at risk from tsunamis. A critical part of the mitiga- tion program identifies coastal communities at risk and maps which parts of them would be inundated by a tsunami. About 3 million people are at risk in 512 U.S. cities and towns, says Bernard. So far, maps have been generated for 125 communities that are home to about 1.3 million of those residents. Maps can show tsunami-flood areas in several ways. In the past, such maps have been simple and typically focused on how far inland a large tsunami might reach. Some maps also included information about how fast the flow of water might be over land or how long the inundation might last. Today, says Philip Watts of Applied Flu- ids Engineering in Long Beach, Calif., powerful computer simula- tions enable analysts to calculate many different parameters related SPREADING RIPPLES — This model of a tsunami generated by an earthquake on Nov. 16, 2003, plus data gathered by a seafloor sensor hundreds of kilometers away (star), suggested the wave wouldn’t damage distant coastal areas, such as Hawaii (arrow).

Upload: sid-perkins

Post on 02-Oct-2016

216 views

Category:

Documents


4 download

TRANSCRIPT

V.TI

TOV

/NO

AA

S C I E N C E N E W S1 5 2 M A R C H 6 , 2 0 0 4 V O L . 1 6 5

KILLER WAVESScientists are learning how to predict tsunami risk

BY SID PERKINS

Last Nov. 16, at 8:43 p.m., a magnitude–7.5 earth-quake struck deep beneath the ocean nearAlaska’s Little Sitkin Island, far out in the Aleut-ian Islands. Within 25 minutes, NationalOceanographic and Atmospheric Administra-

tion (NOAA) scientists issued a tsunami warning forU.S. Pacific coastal areas. Forty minutes later, a pres-sure sensor on the seafloor hundreds of kilometerssouth of Alaska detected the tsunami’s vanguard pulses. Data from that instrument—one of six in a network activated justthe previous month—indicated that the wave was only 2 cen-timeters tall there. Simulations previously run on computers sug-gested that such a wave would-n’t be a danger to Hawaii orother distant shores, and NOAAcanceled its tsunami warningless than 90 minutes after thequake occurred.

A few hours later, the tsunamiswept into the harbor in Hilo,Hawaii, and raised water levelsthere about 21 cm, just 2 cmhigher than predicted by the sim-ulations. No damage was done.

This successful predictionpaid off big for U.S. taxpayers,says Eddie N. Bernard of NOAA’sPacific Marine and Environ-mental Laboratory in Seattle. Heestimates that the canceledtsunami warning saved, inHawaii alone, at least $68 mil-lion—the economic impact of astatewide coastal evacuation. Incomparison, the six seafloor sen-sors in the Pacific network—three south of the Aleutians, twooff the U.S. West Coast, and onealong the equator between Chile and Hawaii—are part of atsunami-hazard mitigation effort that has cost only $17.6 millionsince the program’s inception in 1997.

Other parts of the program include assessing tsunami risk forcoastal communities. Geophysicists are using digital models of topog-raphy and detailed simulations of fluid flow to estimate which partsof population centers are at most risk from the potentially deadlywaves. Some geologists are scouring coastal landscapes to find tracesof prehistoric tsunamis, and other scientists are analyzing more-recent records from tidal gauges to see whether they reveal a pat-tern in how often tsunamis of varying sizes strike particular locations.

WAVES AHOY Taken from Japanese, the word tsunami typ-ically stimulates thoughts of monstrously tall waves that wipeout coastal communities and kill thousands of people. Buttsunamis come in all sizes. Scientists estimate that the death tollof the 141 damaging tsunamis that occurred during the 20th cen-tury exceeds 70,000. During the same period, however, at least900 smaller tsunamis caused no damage whatsoever. Some,just centimeters high when they hit shore, sloshed right pastswimmers and were detected only by instruments.

Distinguishing small tsunamis from the large ones before theystrike is a matter of life and death, as well as of money. Accuratetsunami simulations, such as the one that NOAA scientists usedto cancel last November’s tsunami warning, hold promise forgreatly reducing the number of unnecessary evacuations acrossthe Pacific, says Bernard. False alarms cost more than money, headds, because they erode public confidence in the agencies that

broadcast the warnings. That,in turn, could lead people toremain in coastal areas despiteaccurate warnings.

At the fall meeting of theAmerican Geophysical Union inSan Francisco last December,Bernard described the NationalTsunami Hazard MitigationProgram—a partnership thatincludes representatives fromfour federal agencies and the fivestates that border the PacificOcean. He told Science Newsthat he’d like to see the program’snetwork of tsunami-detectinginstruments expand from 6 toaround 20 sensors, a numberthat would permit their spacingat about 400 km apart along theWest Coast and the section ofAlaska deemed most at risk fromtsunamis.

A critical part of the mitiga-tion program identifies coastalcommunities at risk and maps

which parts of them would be inundated by a tsunami. About3 million people are at risk in 512 U.S. cities and towns, saysBernard. So far, maps have been generated for 125 communitiesthat are home to about 1.3 million of those residents.

Maps can show tsunami-flood areas in several ways. In the past,such maps have been simple and typically focused on how far inlanda large tsunami might reach. Some maps also included informationabout how fast the flow of water might be over land or how longthe inundation might last. Today, says Philip Watts of Applied Flu-ids Engineering in Long Beach, Calif., powerful computer simula-tions enable analysts to calculate many different parameters related

SPREADING RIPPLES — This model of a tsunami generated byan earthquake on Nov. 16, 2003, plus data gathered by a seafloorsensor hundreds of kilometers away (star), suggested the wavewouldn’t damage distant coastal areas, such as Hawaii (arrow).

SP BOB 3-6 3/3/04 14:30 Page 152

DEN

GLE

R

W W W. S C I E N C E N E W S. O R G M A R C H 6 , 2 0 0 4 V O L . 1 6 5 1 5 3

to a tsunami strike and to display them in a variety of ways.For example, he and his colleagues can create detailed digi-

tal models of coastal topography and then swamp the area witha cyber-tsunami. In addition to identifying land areas that willbecome inundated, the simulations can characterize the flowsscouring those areas. Well-designed graphic displays caninstantly highlight areas where water flow would be strongenough to knock down a person, wash away an automobile,break a ship’s anchor, or move objects or debris. All these param-eters can help scientists and public-safetyofficials assess tsunami hazards, says Watts.

The same simulations can estimate when atsunami would arrive, when maximum flood-ing would occur, and when the waters wouldrecede. These factors would be of prime inter-est to emergency-response personnel, saysWatts. Those responders would like to knowwhere to place their equipment and person-nel so that they’re out of a tsunami’s reach.They’d also need to know when a tsunami’sfinally spent and it’s safe to move from highground. The simulations could indicate whichroutes in and around the tsunami zone mightbe free of flood debris.

MORE DATA, PLEASE Scientists devel-oping tsunami-risk maps can—and probablyshould—use other data to supplement com-puter simulations, says Lori A. Dengler, a geo-physicist at Humboldt State University inArcata, Calif. The maps that she and her col-leagues are developing for coastal HumboldtCounty in northern California incorporatetsunami-relevant information from the localgeological record.

For example, a marsh just south of Cres-cent City, Calif., contains a layer of sandabout 85 millimeters thick. That materialwas washed inland in 1960 by a tsunamithat was generated by a strong earthquakeoff the coast of Chile. A more recent sheet ofsand about 1.7 cm thick chronicles a 1964tsunami that killed 11 people in CrescentCity. Thick sand layers deeper in the sedi-ments denote earlier tsunamis.

One of those strata, due to a tsunami thatswept through the region in 1700, is 15 cmthick, about nine times as thick as that ofthe killer wave of 1964, Dengler notes.Results of several investigations have sug-gested that a massive earthquake just off thecoast of the Pacific Northwest on Jan. 28,1700, wreaked geological havoc throughoutthe region (SN: 11/29/97, p. 348) andspawned a tsunami that affected ports inJapan and probably elsewhere on that sideof the Pacific. At another California marshnearby, a layer of sand thicker than the onedeposited in 1700 marks an even more mon-strous tsunami that occurred about 2,500years ago.

Analyses of data garnered from modern tidal gauges at somesites in Japan suggest that carefully tracking the pattern ofsmall tsunamis can be useful for determining risk for particu-lar tsunami-prone locations. For 10 communities along Japan’sPacific coast, instruments detected at least 10 tsunamis over sev-eral decades, says Stephen M. Burroughs, a geophysicist atFlorida’s University of Tampa. There was a broad range of peak

heights of individual tsunamis at each site, with the largestwave for each location topping out at least 10 times the heightof the smallest.

For each of the sites, smaller tsunamis occurred more fre-quently than bigger ones, following an equation called a power-law distribution, says Burroughs. Scientists have discoveredthat the sizes and frequencies of many natural phenomena,such as wildfires and earthquakes, show power-law relation-ships (SN: 2/2/02, p. 75).

At Miyako, on Japan’s northeast coast, thepower-law relationship based on tide-gaugedata gathered between 1958 and 1996 sug-gests that 4-m-high tsunamis should occurthere, on average, once each 63 years.Tsunamis about 7 m tall should strike less fre-quently, about once every century. Over thepast 141 years, people have recorded threetsunamis that measured at least 4 m high andone that reached elevations of at least 7 m—rates that are consistent with the ones esti-mated from smaller waves, says Burroughs.

In Tosa-Shimizu, in southwestern Japan,the power-law relationship—developed usingdata on small waves obtained between 1931and 1995—suggests that a tsunami measur-ing 20 m tall might hit the port once every229 years, on average. One such monsterwave did strike the site in 1707, Burroughsnotes.

UNRELIABLE SOURCES What theseanalyses based on on-shore measurementscan’t reveal is the source of past tsunamis orwhat phenomena will cause future ones. Upto 95 percent of tsunamis are caused byearthquakes, and the vast majority of thosewaves are just a few centimeters tall, saysSteven N. Ward of the University of Califor-nia, Santa Cruz. Other phenomena that pro-duce tsunamis less frequently may give riseto the monster waves that can be much largerand more devastating than their earthquake-triggered counterparts.

Consider tsunamis that are caused byunderwater landslides. While some of theseslumps are small or slow, in others, moun-tain-size volumes of sediments shoot downundersea slopes at speeds exceeding 100kilometers per hour. An undersea slide thatoccurred shortly after a major earthquakenear Papua New Guinea in July 1998 cre-ated a tsunami that wrecked a 20-km stretchof coastline. Along those beaches, thetsunami was as much as 15 m tall (SN:8/14/99, p. 100).

One factor that makes undersea landslidesdangerous is that they, unlike earthquakes,don’t seem to have an upper limit on theirmagnitude, says Ward. Scientists estimatethat about 4 cubic kilometers of sediment

moved during the Papua New Guinea slide—an amount of mate-rial that could fill about 1,100 New Orleans Superdomes butnonetheless is small on the geological scale. Imagine the power ofthe largest known submarine landslide, which occurred off thecoast of Norway about 8,000 years ago and moved 8,500 cubic kmof material.

Some of the largest tsunamis, however, may be created byheavenly visitors. After all, comets, asteroids, and meteorites

SANDWASHED SEDIMENT — Coresample taken from a beachside marshnear Crescent City, Calif., shows sev-eral layers of tsunami-deposited sand(arrows) among the brown layers ofpeat that accumulated gradually.

SP BOB 3-6 3/3/04 14:30 Page 153

S C I E N C E N E W S1 5 4 M A R C H 6 , 2 0 0 4 V O L . 1 6 5

that reach our planet’s surface have a 70 percent chance of hit-ting an ocean. Ward and his colleague Steven R. Chesley of NASA’sJet Propulsion Laboratory in Pasadena, Calif., have estimatedthat tsunamis generated by extraterrestrial impacts occur, onaverage, every 5,200 years. Small objects create most of theseimpacts, which usually have only modest effects. On the otherhand, the researchers estimate that a space rock 300 m acrosscould generate a tsunami about 11 m high and inundate areas asfar as 1 km inland.

Scientists may have discovered the impact site of one big spacerock that smacked into the South Pacific just a few hundredyears ago. In eastern Australia, researchers have found jumbleddeposits of rocks more than 130 m above sea level that they pro-pose were left by a tsunami. That debris has been dated to about A.D. 1500—a date that matches when the Maori people inex-plicably moved away from some areas of New Zealand’s coast,says Stephen F. Pekar, a sedimentologist at Queens College inNew York. On New Zealand’s Stewart Island, two sites sportpossible tsunami deposits at elevations of 150 m and 220 m,respectively.

The source locations and heights of waves that could have loftedmaterials to those elevations steered the search for the impact’sground zero to beneath the sea southwest of New Zealand, saysPekar. Sure enough, he and his colleagues have discovered a craterthere that’s about 20 km wide and about 150 m deep. Samples ofsediment taken from the seafloor southeast of the crater, but notthose obtained elsewhere around the crater, contain small min-eral globules called tektites, one hallmark of an extraterrestrialimpact. That pattern suggests that an object may have struckfrom the northwest—a path that would have taken the blazingbolide over southeastern Australia, where aboriginal legends men-tion just such a fireball.

The rock that created tsunamis off New Zealand 500 yearsago may have been around 1 km across, the researchers say. Wardand his colleagues previously estimated that a 1-km-wide aster-oid slamming into the Atlantic Ocean about 600 km off NorthCarolina could send 130-m-tall tsunamis over beaches from CapeHatteras to Cape Cod within 2 hours. In 8 hours, tsunamisbetween 30 and 50 m tall would scour European coasts.

More frequent but possibly less damaging sources for mega-tsunamis include volcanic islands, which can collapse after erup-tions deplete their internal magma reservoirs. Also, the flanks ofsuch islands can catastrophically slide to the ocean floor aftercenturies of weathering has weakened them. Such events, whichcan spawn tsunamis more than 100 m tall, occur somewhere inthe world every 10,000 years or so, says Ward. ■

HIGH AND DRY — This 70-meter-long, multiton construction

barge was swept 75 m inland by an earthquake-spawned tsunami

that struck Japan’s Okushiri Island on July 12, 1993.

D.J

.SIG

RIS

T/IN

TER

NA

TIO

NA

L TS

UN

AM

I IN

FOR

MA

TIO

N C

ENTE

R

SP BOB 3-6 3/3/04 14:31 Page 154