Monday, April 04, 2005

Scientific Reports

Notes From the Field...

USGS Scientists in Sumatra Studying Recent Tsunamis

March 30 - April 26

Photo of international tsunami field survey team taken March 31, 2005.
Leg 1 Team at Saraina Koat Mentawai office in Pandang prior to boarding the MV Seimoa; March 30, 2005.

FIELD STUDY OF THE EFFECTS OF THE MARCH 28, 2005, and DECEMBER 26, 2004, TSUNAMIS IN SUMATRA

The West Sumatra International Tsunami Survey Team is composed of 12 US and 6 Indonesian tsunami scientists. The team is travelling by boat from March 30 to April 26 to collect critical data to determine the size of the March 28, 2005, tsunami. The team will also collect data to document the decay in the December 26, 2004, tsunami from northern to southern Sumatra. Goals of the data collection effort are:

  1. to determine how large a tsunami was created by the March 28, 2005 earthquake;
  2. to fill in the gaps in the December 26, 2004, tsunami measurements of wave height, runup, and inundation distance between Lhoknga (northern Sumatra) and Padang (southern Sumatra);
  3. to do detailed sedimentological investigations, including paleotsunami studies;
  4. to collect nearshore bathymetry, filling a critical data void for tsunami propagation modeling;
  5. to investigate the effectiveness of previous education and mitigation strategies;
  6. to make additional estimates of co-seismic subsidence; and
  7. to measure groundwater salinity in coastal aquifers.

ANTICIPATED PROJECT OUTCOMES:

Anticipated outcomes from this scientific effort include a better understanding of the March 28, 2005, and December 26, 2004, Indian Ocean tsunamis, an improved history of tsunami size and frequency on the west coast of Sumatra, and development of methods for interpreting past tsunami events in Sumatra and elsewhere in the world.

This information can be used by local government officials to help mitigate against future tsunami hazards in Indonesia, and by other coastal communities to help manage coastal development (for example, to decide where to build public buildings or infrastructure). In addition, by sharing research techniques and methodology with scientists from Indonesia, we hope to transfer the skills necessary to allow local scientists to better evaluate tsunami hazards.

DETAILS OF THIS ACTIVITY:

USGS will co-lead a team of 12 US and 6 Indonesian scientists to collect data on the recent tsunamis. The team will take measurements on mainland Sumatra from Banda Aceh to Padang and offshore islands including Nias, Simeulue, and the Banyaks. Because roads are still out and will likely be out for an extended period of time, the team will travel by charter boat from March 30 to April 26 to collect the needed data.

Strategy: Have two 14-day legs to allow more data collection, more scientists to participate, a phased-approach where information from the 1st leg dictates the locations where data is collected in a sediment-oriented 2nd leg.

Map copyright Peter Loud 2004, used with permission; map of western Sumatra showing approximate locations of survey team each day during trip.
Base maps of Indonesia © copyright Peter Loud 2004;
used with permission. Additional maps at
URL http://www.peteloud.co.uk/.
Map of western Sumatra showing approximate location of survey team each day. Long line represents the area of the survey; dates, dots, and stars indicate approximate locations of the survey team when the daily reports were given. For more information, see the daily USGS field reports. See a larger version of the map (145 kb).

Leg 1 (S-1-05-IN) : March 30-April 12; Padang to Banda Aceh

Main focus: Water level measurements. Move quickly up the coast going ashore 2 to 4 times a day to collect as many wave height, runup, and profile measurements as possible. Collect detailed bathymetry in shallow and intermediate water depths at some of the sites. Conduct reconnaissance for easy and safe shore access and sediment transect sites.

Additional focuses: Sedimentation and erosion, paleotsunami deposits, uplift and subsidence, groundwater salinity. These studies would be done in the same time frame (rapid) as the water level and profile measurements.

Leg 1 Scientists now on a boat in Indonesia:

  • Jose Borrero, Leg 1 Leader (USC)
  • Gegar Sapta Prasetya (Tsunami Research Center - BPPT/DKP)
  • Vasily Titov (NOAA PMEL)
  • Widjokongko
    (Tsunami Research Center/Coastal Dynamic Research Center - BPPT)
  • Brian McAdoo (Vassar)
  • Bob Peters (USGS)
  • Rahman Hidayat (Tsunami Research Center/Coastal Dynamic Research Center - BPPT)
  • Bretwood Higman (University of Washington)
  • Etienne Kingsley (USGS Contractor)
  • Lukianto (Natural Resources Inventory technology Research Center - BPPT)

Leg 2: April 13-April 26; Banda Aceh to Padang

Main Focus: Tsunami sediments. Use wave level measurements and reconnaissance from 1st leg to pick sites for detailed sediment work.

Additional focuses: Uplift and subsidence, paleotsunami deposits, vegetation impacts of inundation. These studies would be done in the same time frame (slow) as the sediment transect measurements.

Leg 2 Scientists:

  • Bruce Jaffe (USGS)
  • Guy Gelfenbaum (USGS)
  • Peter Ruggiero (USGS)
  • Bob Morton (USGS)
  • Rahman Hidayat (Tsunami Research Center/Coastal Dynamic Research Center - BPPT)
  • Andy Moore (Kent State)
  • Lori Dengler (Humbolt State)
  • Bretwood Higman (University of Washington)
  • Lukianto (Natural Resources Inventory technology Research Center - BPPT)

The West Sumatra International Tsunami Survey Team:

  • Bruce Jaffe (USGS)
  • Gegar Sapta Prasetya (Tsunami Research Center - BPPT/DKP)
  • Jose Borrero (USC)
  • Guy Gelfenbaum (USGS)
  • Peter Ruggiero (USGS)
  • Bob Morton (USGS)
  • Bob Peters (USGS)
  • Etienne Kingsley (USGS Contractor)
  • Subandono Diposaptono (Subdirectorate Marine Environment Disaster Mitigation - DKP)
  • Rahman Hidayat ( Tsunami Research Center/Coastal Dynamic Research Center - BPPT)
  • Lukianto (Natural Resources Inventory technology Research Center - BPPT)
  • Widjokongko (Tsunami Research Center/Coastal Dynamic Research Center - BPPT)
  • Vasily Titov (NOAA PMEL)
  • Andy Moore (Kent State)
  • Brian McAdoo (Vassar)
  • Lori Dengler (Humbolt State)
  • Bretwood Higman (University of Washington)

ACKNOWLEDGMENTS: Participation of USGS scientists on the team is funded by the Coastal & Marine Geology Program and the U.S. Agency for International Developments Office of Foreign Disaster Assistance. Dr. Bruce Jaffe, USGS, is co-leader of the West Sumatra International Tsunami Survey Team.

DISCLAIMER: This material is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards or with the North American Stratigraphic Code. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.



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Notes From the Field...

USGS Scientists in Sumatra Studying Recent Tsunamis


2 Comments:

Anonymous Anonymous said...

Excerpts from articles about and interviews with Kerry Sieh


Midday yesterday we flew across the Equator to Telukdalam, on the southern coast of Nias Island (itself off the west coast of Sumatra), and near Lagundri bay, a world-famous spot for surfing.

http://today.caltech.edu/today/stor
We made it to our final rendezvous with the Australian 60-Minutes film crew at Sirombu, halfway up the west coast of Nias.

We left Sirombu flying north, reconnoitering the coast all the way to the northern edge of the island. One particularly impressive sight was an entire grove of dead coconut palms sitting out on the reef, seaward of the beach. These obviously had died some time ago, not as a result of the tsunami. They show that the west coast of Nias, like the islands we have been studying farther south, have been sinking during the past several decades. The islands are like a springboard, storing strain for the day when the megathrust below gives way and they spring suddenly back up, producing a great earthquake and tsunami.

Our new fuel-supply boat arrived at about 4 a.m., and we loaded on the extra fuel barrel that had been delivered by truck from Telukdalam the night before. They took off again for Sinabang ( Simeulue Island's capital) around 11 a.m. It will be a 20-hour trip for them at about seven miles per hour from Gunungsitoli to Sinabang. The same trip took us just 1.5 hours by helicopter late this morning.

We were warmly welcomed by Durmili, the local Bupati (government official) of Simeulue.

We went on a car caravan with the whole crowd, led by the Bupati, to view tsunami damage on the southern part of the southwestern coast. We measured heights of the tsunami of about 2.5 meters (eight feet). All the stories we heard still say that the first indication was a recession of the sea. Also of interest to Danny Natawidjaja, my colleague from the Indonesian Institute of Science (LIPI), and myself was the evidence for small amounts of permanent submergence in these towns. Areas that used to be dry now have up to 30 centimeters (nearly a foot) of standing water in them. Local residents insist that the beaches have eroded ten meters (33 feet) or so since the tsunami.

We heard from the Bupati that word has come from the northern coast that the coral reef there is about a meter (three feet) out of the water. This is almost precisely what Danny and I had been guessing would be the case. We will fly there tomorrow morning to see if the reports are true and, if so, to take some measurements. If the northern part of the island has risen, it means that the southern end of the great megathrust rupture that caused the earthquake is under the island

8:46 a.m., Thursday, January 20, en route from Padang, Sumatra, to Jakarta, Indonesia, then on to Singapore

Monday and Tuesday on Simeulue Island were extraordinary, both scientifically and emotionally. On Monday we flew along the southwestern coast, past a score or more of fallen bridges and as many coastal villages, devastated by the tsunami. Near where the island doubles its width we began to see evidence of what looked like an extremely low tide--barren, pale-tan ribbons of coral reef paralleling the coast and extending 100 meters (100-plus yards) and more from the beaches to the waterline.

During our first circling of one of these reef ribbons, we saw striking evidence of emergence--pristine pancake-shaped heads of Porites coral, well above current water level. We landed on the 200-meter wide (700 foot) former shallow reef platform, about halfway between the former sandy beach and the new shoreline. Before we could shut down the engine, 100 or more children and adults swarmed out onto the reef from the trees. We immediately split ourselves into a science team and a relief team; Danny and I began to inspect the corals, while Dayat and Samsir (our pilot and mechanic) began to talk to the villagers and distribute the materials we had brought along as relief aid--clothing, powdered milk, hammers, and fishing equipment.

Even though Danny and I have for the past several years been studying ancient evidence of the slow sinking and fast emergence of the Sumatran coral reefs, we were astonished to find ourselves walking through a pristine marine ecosystem, missing only its multitude of colors, its fish, and its water. Corals of every shape and size rested lifeless on the reef platform--branching corals, massive corals, staghorn corals, fire corals, brain corals, whorls, fans. And here and there a poor crab. Even though the tsunami had raged across the reef, there was scant evidence of any breakage of the delicate whorls and dendritic corals that crunched beneath our feet. But a fishing boat in the trees beyond the shoreline and an overturned, two-ton umbrella-shaped Porites coral heads were testimony to the power of the tsunami. The scene was the marine equivalent of a village on the flank of a volcano after the passage of a nuee ardente (a destructive "glowing cloud")--life quick-frozen in place at the moment of death.

We estimate the emergence here to be nearly a meter and a half. To produce so much uplift, the block above the gently northeast-dipping megathrust, 25 kilometers (15 miles) or so beneath the reefs, must have slipped about ten meters (11 yards) toward the southwest.

We hopscotched our way farther north for the rest of the afternoon, stopping only occasionally to make an additional measurement and to divert around rainstorms. A systematic, detailed survey will have to wait until we can return, hopefully in a few months. Along the northern coast, newly emerged reef ribbons were everywhere. The emergence had doubled the diameter of some of the smaller islets. And along most of these coastlines were old stands of decayed coconut palms and other trees out on the reef, seaward of the old beach--testaments to the fact that the land had been sinking slowly in the decades before the earthquake. In the complexly embayed coastline of the northern coast, muddy flats surrounded by mangrove forest have also emerged above the water. Some of these have muddy brown rectangular fields on them. I think these are very old rice paddies that slowly submerged into the intertidal zone or below in the decades prior to the earthquake. Now they are back above high tide, ready for cultivation again! Some villagers have, in fact, asked us if the water will return soon and submerge the newly dried reef and mud flats. We tell them with confidence that submergence of these new lands will not occur soon. It will take more than 100 years for the water to return to its levels on the day before the earthquake.

On Tuesday it was more of the same. We filled some of the gaps between our measurement sites and reconnoitered the northeastern coast. The former lowest low tide level is now at least 25 centimeters (10 inches) above water. So the island tilted coseismically both from northwest to southeast and from southwest to northeast. The pattern and magnitude of uplift is strikingly similar to what we know happened to the south in the Mentawai Islands during the magnitude 8.7 earthquake of 1833. The half-hour flight back south to Sinabang took us over vast tracts of virgin forest, full of tall, white-trunked dipterocarp trees towering over the lush understory. Inspiring flight, tempered only by the fact that Danny and I were a bit uncomfortable in our pants, soaked from our wading in shallow water.


http://sixtyminutes.ninemsn.com.au/sixtyminutes/stories/2005_02_13/story_1295.asp

Mentawai Islands to monitor the faultline directly below. The most useful archives are underwater. The layers of coral reveal the sea floor's geological history.

DR KERRY SIEH: The coral shows very clearly that the islands are, uniformly along this stretch, the islands are submerging at precipitous rates. All these trees are showing they are being drowned and the beach is encroaching on the land as the land sinks, so it's a very visually dramatic manifestation of strained accumulation.

RICHARD CARLETON: On land, he measures this more precisely. It's not easy. He treks through humid jungle to mountain tops where Dr Sieh has installed 14 satellite tracking stations. GPS readings show the islands are not just sinking, they're being shoved sideways year by year by the two plates grinding against each other below.

DR KERRY SIEH: Every couple of years, all that strain, all that squishing of the spring is released and whoosh! A giant earthquake.

RICHARD CARLETON: He found the fault ruptures in different sections at different times and a massive earthquake was due, because each section erupts about every 200 years. Alarmed by this, Kerry Sieh began sounding a warning.

DR KERRY SIEH: We started just six months ago, handing out the posters and brochures and giving lectures in churches and lectures in classrooms.


http://www.seires.net/content/view/122/52/ Preliminary estimates of the geodetic GPS survey in the Andaman & Nicobar Islands conducted by CESS

The line of Andaman Nicobar Islands shifted about 3 meters to the southwest. They may also have been uplifted or sunken; that data is still being analyzed.

[Kerry Sieh report that the Eurasian plate jumped 10 feet west and 3-10 feet upward.]


http://209.157.64.200/focus/f-news/1357550/posts

No one knew exactly when or where the earthquake would strike. But Sieh knew the boundary was ripe for a major slip. As it turned out, he was focused on an area a bit too far south.

Sieh and his team had determined by slicing into corals, and reading the natural record of water level they preserve, that giant earthquakes along the zone to the south recur about once every 200 years. The last major earthquake to hit offshore of central Sumatra occurred in 1833, was about magnitude 8.7 and produced large tsunamis.

Although Sieh and his group understood the threat Sumatra and its neighbors were facing, a tsunami was not high on the list of concerns for residents and government bodies going into the disaster, Sieh said.

Sieh says he doesn't blame urban planners for not acting on the information he provided to some of them in July. "I don't think the best city planner would have been better,' because it wasn't a priority at the time, he said. "Now it's a major issue.'

That's because only the northernmost portion of the boundary where the two tectonic plates meet ruptured in December. In the December release, Sieh explained that "the fact that most of the other part of the section has generated few great earthquakes in more than a hundred years is worrisome ... other parts within the section of this fault should be considered dangerous over the next few decades.'

Since 2002, Sieh and his colleagues have installed an array of GPS stations to track the sinking of the islands. Following their most recent trip to Sumatra in January, they now have 18 stations stretching to the northern tip of Aceh. They are watching the gradual healing process of the islands in the north and the behavior of the island to the south.

Now, Sieh is trying to inform villagers about what to do when they feel major shaking, urging them to clear paths to high ground and working with government officials and city planners to build adequate bridges and to consider relocating villages in safer locations.

Sieh kept a journal during his January trip to study the earthquake and sent it back to Caltech to share with the public


http://www.nbc4.tv/thurdsayarchive/4037532/detail.html

When this happens in the part of the world that was hit Sunday, the overlying plate jumps westward by some 10 feet, and upward by 3-10 feet, Sieh said.

This raises the overlying ocean, so there is briefly a "hill" of water 3-10 feet high overlying the rupture, which then flows downward, triggering broad ocean waves. When the waves reach shallow water, they slow down and increase greatly in height -- up to 30 feet or so in the case of Sunday's quake, Sieh said.

History reveals that when quakes of this type happen, the rupture at the fault does not happen all at once, Sieh said.

The U.S. Geological Survey reports that the rupture began just north of Simeulue Island in Indonesia.

From the analysis of the seismograms, Caltech's Chen Ji has found that from the origin point, the major rupture went northward about 249 miles. The extent of the major aftershocks suggests the rupture then continued about 620 miles farther north to the vicinity of the Andaman Islands, Sieh said.

He said the section of the subduction fault that runs from Myanmar south across the Andaman Sea, then southeast off the west coast of Sumatra, has produced many large and destructive quakes in the past 200 years.

In 1833, the rupture of a long segment off central Sumatra produced a quake of about magnitude-8.7, and large tsunamis. In 1861, a section just north of the equator produced a magnitude-8.5 temblor and tsunamis.

Sieh said the fact that the recent seismic rupture took place only on the northernmost portion of the Sumatran section of the fault is worrisome, because the other parts of the section have generated few great quakes in more than 100 years -- meaning they could be due for another large one.


http://www.nature.com/news/2005/050117/pf/433183a_pf.html

Indian Ocean fault line poses threat of further earthquakes

Emma Marris

Energy from 26 December quake could hasten the next rupture.

Seismic sensitivity: coral retains a record of changes in sea level, and hence of past earthquakes.

Just weeks after the earthquake that triggered the devastating tsunami in the Indian Ocean, seismologists are turning their attention to nearby sections of the fault line that are also prone to rupture.

Researchers say that there could be further earthquakes both to the north and south of the event that occurred on 26 December, perhaps within decades, and that they might even be powerful enough to cause another tsunami.

The location of the epicentre of last month's earthquake is clear, but seismologists are uncertain where the northern edge of the rupture ended, says Kenji Satake, who studies tsunamis at the National Institute of Advanced Industrial Science and Technology in Tsukuba, Japan.

Some of the northern sections of the fault line may still be under enough strain to cause another earthquake of similar magnitude, says Satake, who hopes to go to the Andaman and Nicobar Islands in February to look for uplift of the land: no uplift would indicate that the fault has not yet ruptured in that area.

Likewise, some sections of the fault line south of the epicentre, which have definitely not yet ruptured, seem to be due for an earthquake.

Kerry Sieh, a geologist at the California Institute of Technology in Pasadena, has been investigating this area for years. He has a network of six instruments on a number of small islands off the coast of Sumatra, the exact locations of which are recorded by global positioning satellites. These reveal that the islands are currently sinking by as much as a centimetre a year, as the continental plate on which they sit is slowly dragged down into the subduction zone by the oceanic plate. When these plates slip during an earthquake, the islands spring back up again as the pressure on the continental plate is released.

Sieh has been investigating historical records of earthquakes in the area by looking at the coral, which retains a record of the low-tide mark — coral can only live under water, so it dies off if the low-tide mark recedes, or grows if the low-tide mark rises. Sieh has used this to estimate changes in elevation to within a couple of centimetres over the past 50 to 100 years, and to look for larger rises associated with past earthquakes.

Sieh says that earthquakes in the region happen about once every 230 years. "We think we are getting close to the next one," he says. The last such event on the Mentawai islands was 145 to 170 years ago, for example. Energy from last month's earthquake will have placed extra strain on the fault, which is likely to hasten the next rupture.

As the Sumatra study progresses, Sieh and his team have been teaching the islanders about earthquake risks. "We have been trying to educate the villagers with posters and brochures, telling them how to build their structures," he says. "I cross my fingers that the people we started to educate took our advice and clambered up the hill when the tsunami hit."

Additional reporting by David Cyranoski.

http://www.scpr.org/programs/airtalk/listings/2005/02/airtalk_20050214.shtml Tues Feb 15, radio interview with Kerry Sieh, two days after his return from Indonesia to gather his info.

My notes

Sieh returned from research in Indonesia, on the changes made by the quake, and on the tsunami. Reports what is going on.

Seismographs tell interesting stories about what earthquake was, what produced it. There are controversies. The rupture was very long, and took a long time to occur, so that early data showed it to be shorter and of lesser magnitude than it was. This caused the magnitude of the quake and the size of the rupture to climb over time.

Changes in the landscape - difficult to sort out from general discussion of what the tsunami did. New swamps were created, and the uplift of coral reefs is spectacular.

Reason little loss of life occurred on Simeule even though housing was wiped out, was that in 1907 there was a severe earthquake with a tsunami, and the story was passed down from grandparents to parents to children over three generations. When the quake hit, everyone ran for high ground. Few people died. This passing down of stories happens more in small villages. Also in big cities it is harder to escape - and the government needs to widen roads and build bridges over impassible swampy sections, to make it easier for people to escape from something like this.

Historically details of major historic earthquakes off Sumatra were poorly known. Coral reefs give the details. The reefs show where and how quakes occur along the subduction zone. They show where quakes are major and where they are minor.

Big quakes tend to occur every 200 years.

The 1200 km long section that just ruptured probably won't triger a magnitude 9 quake again for several hundred years. But the thousand km long segment to the north of it, that runs up the side of Myanmar (traditional Burma), if it ruptured, would affect Burma, Banglodesh, and India. Section to south of it, along Sumatran coast, would affect the Indian Ocean to the southwest, and Sumatra.

7 of 10 biggest quakes this century off Sumatra were 1950-65; intuitively it looks like quakes in the area could tend to be linked.

Navy did its first survey of the sea floor off Sumatra; all Kieh had to say about what they found is massive slumps in the floor of the sea.

The strip that ruptured probably moved 20 m westward and 1 to 1.5 m upward.

The subduction fault is at a very shallow angle - 10 degrees. When the over riding plate moves, it moves up a little and moves out alot.

No geodetic information available yet. There are 5 stations in teh Andamans, put there a few years ago by India. Data not available yet but should be soon. (That is probably the data from within the past month reported above.) It will probably show the islands, which belong to India, moved many meters out toward India.

Sieh's own geodetic data is not analyzed yet.

Singapore moved 2 cm East because of motion of block thousands of miles away.

The geodetic data on the quake is just beginning to be analyzed.

3:39 PM  
Anonymous USGS said...

Source: US Geological Survey
Source Data: 04/27/2005

As soon as the 8.7 magnitude March 2005 earthquake occurred in the Indian
Ocean, the world anxiously awaited word of yet another devastating tsunami
similar to the one that occurred on December 26, 2004, as the result of the
9.0 Indian Ocean earthquake.

But although the tsunami that resulted was certainly large - 13 to 16 feet
is not a tsunami to dismiss - it paled in comparison to the enormous
December event, which had run-ups of 100 feet or higher in Sumatra.

Why such a big difference?

Eric Geist, a U.S. Geological Survey tsunami expert, has been researching
probable reasons for the differences between the two tsunamis. He will
present his preliminary findings at the 2005 Annual Meeting of the
Seismological Society of America meetings in Incline Village, Nevada on
April 27 (8:30 p.m., Lakeside Ballroom).

To Geist, unearthing the answer to the "why the difference" question is
vital because of the many implications such an answer has for designing the
most effective tsunami warning systems.

Geist and his colleagues Vasily Titov and Diego Arcas at NOAA and Susan
Bilek at New Mexico Tech are focusing their scientific scrutiny on four
factors that they believe explain why the March tsunami was so weak in
comparison to the December one. Much of the information scientists have to
determine how these tsunami waves were generated comes directly from
seismic recordings of the earthquake itself, Geist says.

The first factor, obviously, is magnitude ­ in March, it was 8.7, in
December 9.0. As Geist reminded, the magnitude of an earthquake is a
function of the rupture area and the average amount of slip throughout the
rupture. The December earthquake had a higher average amount of slip and
ruptured a longer segment of the fault than the March earthquake.

"Most earthquakes," said Geist, "start with the greatest amount of slip
near the epicenter. The exception to this 'rule' is what happened in
December, where it started with a little slip, then a big burst occurred
later farther away from the epicenter."

The second factor is water depth ­ and is probably the biggest contributor
to the difference in the two tsunamis, Geist said. "In December, a lot of
the energy in the initial earthquake rupture occurred in deep water (about
1 to 2.5 miles), whereas the March earthquake occurred beneath the shelf
just offshore mainland Sumatra at water depths of less than .6 miles or
even beneath Sumatra itself. This difference in water depth over areas
where the greatest energy was released from the earthquake produced a
difference in amplification as the tsunami traveled from the source region
to shore.

Hence, said Geist, the more amplification of the wave, the bigger the
run-ups. Of course, if much of the movement of the surface of the earth
caused by the earthquake is on land, it will not go into generating a
tsunami because there is no water to displace.

The third factor is the depth below the surface of the earth where most of
the fault slip occurred. In March, most of slip occurred at depths of 12
to 25 miles below the surface, whereas for the December earthquake, slip
may have extended all the way to the sea floor at the oceanic trench. This
factor, along with the difference in magnitude, resulted in greater
vertical movement of the sea floor in December. The peak vertical
displacements of the sea floor for the two earthquakes were approximately
16 feet for the December earthquake and about 10 feet for the March
earthquake.

The last factor is that there was a significant difference between the two
events in the primary direction of tsunami-wave "focusing," which affects
the distant or "far-field" impact of the tsunami. In December, the tsunami
energy was focused to the west, toward Sri Lanka and India, and to the
east, toward Thailand. In March, the tsunami energy was focused to the
southwest, away from any nearby land masses (excluding Sumatra itself).
Because the March earthquake occurred beneath the island shelf of Sumatra,
the island itself blocked significant wave activity toward Thailand and
Malaysia.

Geist emphasizes that the knowledge researchers gain about these two
important earthquakes will help scientists design and build more precise
models of tsunami propagation under different conditions -- even in the
same geographic area. Also, he noted, by analyzing these two earthquakes
and comparing them with historical earthquakes of similar magnitude (more
than 8.5), the end result will be a better ability to forecast future
variations in tsunami runup. "This improved understanding will greatly aid
global efforts to provide accurate tsunami warnings and hazard assessments.
The ultimate goal, of course, is to save lives and reduce property damage
from future tsunami disasters."

For more information and updates on field activities, see the USGS Tsunami
& Earthquakes Web Site: http://walrus.wr.usgs.gov/tsunami/.

11:12 AM  

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