Calculating Tsunami Risk for the US East Coast
Apr. 19, 2013 — The greatest threat of a tsunami for the U.S. east coast from a nearby offshore earthquake stretches from the coast of New England to New Jersey, according to John Ebel of Boston College, who presented his findings today at the Seismological Society of America 2013 Annual Meeting.
The potential for an East Coast tsunami has come under greater scrutiny after a 2012 earthquake swarm that occurred offshore about 280 kilometers (170 miles) east of Boston. The largest earthquake in the 15-earthquake swarm, most of which occurred on April 12, 2012, was magnitude (M) 4.0.
In 2012 several other earthquakes were detected on the edge of the Atlantic continental shelf of North America, with magnitudes between 2 and 3.5. These quakes occurred off the coast of southern Newfoundland and south of Cape Cod, as well as in the area of the April swarm. All of these areas have experienced other earthquake activity in the past few decades prior to 2012.
The setting for these earthquakes, at the edge of the continental shelf, is similar to that of the 1929 M7.3 Grand Banks earthquake, which triggered a 10-meter tsunami along southern Newfoundland and left tens of thousands of residents homeless.
Ebel's preliminary findings suggest the possibility than an earthquake-triggered tsunami could affect the northeast coast of the U.S. The evidence he cites is the similarity in tectonic settings of the U.S. offshore earthquakes and the major Canadian earthquake in 1929. More research is necessary, says Ebel, to develop a more refined hazard assessment of the probability of a strong offshore earthquake along the northeastern U.S. coast.
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The above story is reprinted from materials provided by Seismological Society of America, via EurekAlert!, a service of AAAS.
Note: Materials may be edited for content and length. For further information, please contact the source cited above.
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Big Earthquakes Might Calm the Earth
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 A map of the earthquakes triggered around the globe (shown as black dots) within a week of the April 11, 2012 earthquake off the coast of Sumatra (shown as a white star).CREDIT: Fred Pollitz, USGS |
The biggest earthquakes make the Earth ring like a bell. But do earthquakes that rock the world increase the risk of new temblors?
Not by much, according to two reports presented today (April 19) at the Seismological Society of America's annual meeting in Salt Lake City. In fact, one recent quake near Sumatra, Indonesia, seemed to actually quiet global earthquakes.
The April 11, 2012, Indian Ocean earthquake was the largest strike-slip shake-up ever recorded. A strike-slip fault moves horizontally, as California's San Andreas Fault does. The magnitude-8.6 quake triggered earthquakes worldwidefor up to six days, said Fred Pollitz, a geophysicist at the U.S. Geological Survey's (USGS) Menlo Park, Calif., office.
But once the triggered quakes stopped, Pollitz was surprised to find a sharp drop in moderate earthquakes for more than three months. Seismic monitors detected no earthquakes bigger than magnitude 6.5 for 95 days. Normally, quakes of this size hit every 10 days, Pollitz said.
"That is quite a rare occurrence," he told OurAmazingPlanet. "The chance of that happening is about 1 in 10,000."
Energetic earthquake
Pollitz thinks the Indian Ocean earthquake's unusually energetic seismic waves, which traveled shallowly through Earth's crust for long distances, could have shifted stresses on faraway faults, delaying earthquakes.
"Usually, these dynamic seismic waves only increase the chance of an earthquake at any distance," Pollitz said. "We've documented, for the first time, that they can also decrease the chance of an earthquake."
Because it doesn't necessarily take a massive earthquake to trigger new shocks — the 2002 Denali, Alaska, earthquake, a magnitude 7.9, set off quakes throughout the northwestern United States — Pollitz wants to comb through past earthquake records in hopes of finding more quiet periods.
"We need to go back and look at these other earthquakes and see if this is a general pattern of a temporary increase and a longer-term decrease," Pollitz said.
Global risk
Although the Indian Ocean earthquake may have had a rare calming effect after some time had passed, it did immediately spark a global furry of quakes, as many massive earthquakes often do. Tom Parsons, a USGS geophysicist also at Menlo Park, wants to know how often dangerously large temblors follow strong earthquakes.
"Every time we have a magnitude 7 or something larger, should the whole rest of the planet be concerned about the increase in hazard?" Parsons told OurAmazingPlanet.
Parsons analyzed hundreds of earthquakes of at least magnitude 7.0 in the past 30 years. Only 24 of the 260 big earthquakes triggered large earthquakes globally.
The risk of earthquakes bigger than magnitude 5.0 translates to 1 or 2 percent in the hours following a large earthquake, the study found. Parsons and other scientists are looking into why some large earthquakes seem to trigger other temblors and other don't.
"That is far less frequent than anticipated," Parsons said. "It's still in the range of random chance, but it's not a zero chance."
These aftershocks were delayed by up to nine hours compared to their smaller magntiude-2 and magntiude-3 counterparts, which may rupture as soon as 45 minutes after the first big quake.
http://www.livescience.com/28874-big-earthquakes-might-calm-earth.html~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Measuring the Hazards of Global Aftershock
Apr. 19, 2013 — The entire world becomes an aftershock zone after a massive magnitude (M) 7 or larger earthquake -- but what hazard does this pose around the planet? Researchers are working to extend their earthquake risk estimates over a global scale, as they become better at forecasting the impact of aftershocks at a local and regional level.
There is little doubt that surface waves from a large, M≥7 earthquake can distort fault zones and volcanic centers as they pass through Earth's crust, and these waves could trigger seismic activity. According to the Tom Parsons, seismologist with the U.S. Geological Survey, global surveys suggest that there is a significant rate increase in global seismic activity during and in the 45 minutes after a M≥7 quake across all kinds of geologic settings. But it is difficult to find strong evidence that surface waves from these events immediately trigger M>5 earthquakes, and these events may be relatively rare. Nevertheless, seismologists would like to be able to predict the frequency of large triggered quakes in this global aftershock zone and associated hazard.
Studies of hundreds of M≥7 mainshock earthquake effects in 21 different regions around the world has provided some initial insights into how likely a damaging global aftershock might be. Initial results show that remote triggering has occurred at least once in about half of the regions studied during the past 30 years. Larger (M>5) global aftershocks appear to be delayed by several hours as compared with their lower magnitude counterparts. Parsons suggests that local seismic networks can monitor the rate of seismic activity immediately after a global mainshock quake, with the idea that a vigorous uptick in activity could signal a possible large aftershock.
Parsons presented his research at the annual meeting of the Seismological Society of America.
Story Source:
The above story is reprinted from materials provided by Seismological Society of America, via EurekAlert!, a service of AAAS.
Note: Materials may be edited for content and length. For further information, please contact the source cited above.
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