*EPF306 12/29/2004
Text: Web Site Answers Questions about Earthquakes, Tsunamis
(Aftershocks, more earthquakes, tsunami warnings addressed) (2040)
Aftershocks, the possibility of more earthquakes and tsunami warning systems are a few of the issues addressed in a new frequently-asked-questions (FAQ) Web page from the U.S. Geological Survey (USGS) Earthquake Hazards Program.
According to the FAQ, many aftershocks occurred after the December 26, 9.0-magnitude earthquake. In the first two days after the quake, scientists detected 40 aftershocks with magnitudes greater than 5.0. The largest, magnitude 7.5, occurred about three hours after the main shock. None of the aftershocks generated tsunamis. Seismologists cannot yet predict the timing or sizes of individual aftershocks.
The Earth's solid surface floats on a layer of softer rock as a collection of interlocking, movable pieces called tectonic plates.
When the plates move, the Earth's surface reacts: sometimes the plates pull apart (spreading boundary) to create a rift or valley, or they grind past each other (transform boundary) or one plate slides under another (subduction zone) to create earthquakes.
On December 26, beneath the Indian Ocean, pent-up energy from the forces of one plate sliding under another found a weak spot in the overlying rock. The rock thrust upward and the earth shook, sending its vibrations out into the ocean.
This earthquake, according to the USGS, has redistributed tectonic stresses along and near the plate boundaries. In some areas, the redistribution of stresses could shorten the time to the next big earthquake; in other areas, it could increase the time to the next earthquake.
When geologists have mapped the distribution of slippage along the earthquake fault, they will be able to estimate the potential earthquake areas that were affected. But it is not yet possible for scientists to reliably estimate where earthquakes will occur or how large they will be.
There is a system already in place to warn populations along the Pacific Rim of an imminent tsunami. The Pacific Tsunami Warning Center is responsible for tsunami monitoring in the Pacific Basin, but no such system exists for the Bay of Bengal, where the recent disaster occurred.
Information about the Pacific Tsunami Warning Center is available at http://www.prh.noaa.gov/ptwc
This FAQ is available at http://earthquake.usgs.gov/eqinthenews/2004/usslav/neic_slav_faq.html
Information about the USGS Earthquake Hazards Program is available at http://neic.usgs.gov/neis/bulletin/neic_slav_ts.html
Text of the USGS FAQ follows:
(begin text)
U.S. Department of the Interior
U.S. Geological Survey
[Reston, Virginia]
Frequently Asked Questions, December 29, 2004
USGS Earthquake Hazards Program FAQ: OFF W COAST OF NORTHERN SUMATRA
FAQ -- Everything Else You Want to Know About this Earthquake & Tsunami
Magnitude 9.0 OFF W COAST OF NORTHERN SUMATRA
Sunday, December 26, 2004 at 00:58:49 UTC
Question: Can we expect many aftershocks to this earthquake?
Answer: There have been numerous aftershocks detected following the recent magnitude 9 megathrust earthquake. In the first two days following the quake, 40 aftershocks with magnitude greater than five have been detected. The largest occurred about three hours after the main shock and registered a magnitude of 7.5. There have been no reports of tsunamis being generated from the aftershocks. We know from past experience that the number of aftershocks will decrease with time. However, the number of aftershocks can be quite variable.
There might be short episodes of higher activity as well as lulls in activity, but the overall trend will be for fewer aftershocks as time goes by. Seismologists are not able to predict the timing and sizes of individual aftershocks.
Question: How has the occurrence of this earthquake affected the probability of another great earthquake?
Answer: The occurrence of this earthquake will have produced a redistribution of tectonic stresses along and near the boundary between the India plate and the Burma plate. In some areas, this redistribution of stresses will be such as to shorten the time to the next big earthquake compared to what would have been the case if the earthquake had not happened. In other areas, the redistribution of stresses will be such as to increase the time to the next big earthquake. Once the distribution of slip along the earthquake fault has been mapped, it will be possible to estimate the areas that were moved closer to future failure and those that were moved farther from future failure. It is not yet possible, however, to reliably estimate when the future failure will occur in a given area or how large will be the resulting earthquake.
Question: This earthquake occurred within three days of a magnitude 8.1 earthquake in the Macquarie Islands. Is there any relation between the two earthquakes?
Answer: The occurrence of two great earthquakes within such a short space of time is indeed striking. However, there is not a well-established mechanism by which the two earthquakes might be causally related. The two earthquakes are very far from one another and occurred on different plate boundaries. The Macquarie ridge forms part of the Pacific-Australian plate boundary and is a predominantly right-lateral, strike-slip transform fault with a component of convergence that connects two subduction zones to the north and south. The Sumatra megathrust earthquake occurred on the interface of the India and Burma plates and was caused by the release of stresses that develop as the India plate subducts beneath the overriding Burma plate. The occurrence of the two, widely separated, great earthquakes within three days may have been a statistical coincidence.
Question: What was the size of the fault that produced the earthquake?
Answer: An initial estimate of the size of the rupture that caused the earthquake is obtained from the length of the aftershock zone, the dimensions of historical earthquakes, and a study of the elastic waves generated by the earthquake. The aftershocks suggest that the earthquake rupture had a maximum length of 1200 -- 1300 km parallel to the Sunda trench and a width of over 100 km perpendicular to the earthquake source. An early estimate from the study of elastic waves show the majority of slip was concentrated in the southernmost 400 km of the rupture.
Question: What was the maximum displacement on the rupture surface between the plates ?
Answer: The maximum displacement estimated from a preliminary study of the seismic body waves is 20 meters.
Question: What was the maximum displacement of the sea bottom above the earthquake source?
Answer: The displacement of the ground surface will be related to, but somewhat less than, the displacement on the earthquake fault at depth. In places, the block of crust beneath the sea floor and overlying the causative fault is likely to have moved on the order of 10 meters to the west-southwest and to have been uplifted by several meters.
Question: What is the angle of subduction of the India plate beneath the Burma plate?
Answer: At the source of the earthquake, the interface between the India plate and the Burma plate dips about 10 degrees to the east-northeast. The subducting plate dips more steeply at greater depths.
Question: What effect did this earthquake have on the rotation of the earth?
Answer: The redistribution of the earth's crustal mass caused by the earthquake will likely have a tiny effect (a small fraction of a second per day) on the earth's rate of rotation that may be too small to be clearly observed even with astronomical instruments.
Question: Why did the magnitude of this earthquake change?
Answer: While earthquake location can be determined fairly rapidly, earthquake size is somewhat more problematic. This is because location is mainly based upon measurements of the time that seismic waves arrive at a station. Magnitude, on the other hand, is based upon the amplitude of those waves. The amplitude is much more variable than the arrival times, thus causing greater uncertainty in the magnitude estimate.
For larger earthquakes, the problem is compounded by the fact that the larger the earthquake, the lower the characteristic frequency of the seismic waves. This means that surface wave arrivals, which contain lower frequency energy than the body waves, must be used to determine the magnitude. For a great earthquake, several hours of data must be recorded in order to accurately determine the magnitude.
Thus, accurate estimates of the magnitude can follow an accurate estimate of the location by several hours. In the case of the M 9.0 Sumatra-Andaman Islands earthquake, the standard methods were inadequate for measuring the very low frequency energy produced and had to be modified. This delayed the final determination of the magnitude until the next day.
Question: Is there a system to warn populations of an imminent occurrence of a tsunami?
Answer: The Pacific Tsunami Warning Center is responsible for tsunami monitoring in the Pacific Basin. Their website is at http://www.prh.noaa.gov/ptwc/. Tragically, no such system exists for the Bay of Bengal where the recent disaster occurred.
Question: What other great (M > 8) earthquakes have occurred in the region?
Answer: Since 1900 and prior to the December 26 earthquake, the largest earthquake along the subduction zone from southern Sumatra to the Andaman Islands occurred in 2000 and had a magnitude of 7.9. A magnitude 8.4 earthquake occurred in 1797, a magnitude 8.5 in 1861 and a magnitude 8.7 in 1833 . All three ruptured sections of the subduction zone to the south of the recent earthquake. Interestingly, the 1797 and 1833 quakes are believed to have ruptured roughly the same area with only 36 years separating the events. Paleoseismic evidence shows that great earthquakes or earthquake couplets occur about every 230 years (http://www.gps.caltech.edu/~sieh/publications/a10.html).
Question: What other significant tsunamis have occurred in the region?
The following destructive tsunamis are listed on a data base maintained by the Tsunami Laboratory, Institute of Computational Mathematics and Mathematical Geophysics (http://tsun.sscc.ru/tsulab/20041226tsun.htm)
1. 1797/02/10 Central part of the western Sumatra. The quake was most felt near Padang and in the area within +/-2 deg of equator. Padang was flooded by powerful waves. More then 300 fatalities.
2. 1833/11/24 South coast of the western Sumatra, estimated rupture from 1 S to 6 S latitude. Huge tidal wave flooded all southern part of the western Sumatra. Numerous victims.
3. 1843/01/05 Strong earthquake west of the central Sumatra. Terrible wave came from the south-east and flooded all the coast of the Nias Island. Many fatalities.
4. 1861/02/16 Exceptionally strong earthquake affected all the western coast of Sumatra. Several thousand fatalities.
5. 1883 Krakatau explosion 36,000 fatalities
Question: How have tsunamis affected the United States?
Answer: The 1964 great Alaska earthquake (M9.2) tsunami caused damage and loss of life across the Pacific. The West Coast & Alaska Tsunami Warning Center, Palmer Alaska (http://www.wcatwc.gov/64quake.htm) indicates that the 1964 tsunami was the mostdisastrous tsunami to hit the U.S. West Coast and British Columbia in Canada. The largest wave height for this tsunami was reported at Shoup Bay, Valdez Inlet (67 meters). Summary of lives lost and damage for Alaska, Canada, Washington, Oregon and California are: Alaska- 106 deaths and $84 million; British Columbia- $10 million; Washington- minor damage throughout the coast; Oregon- 4 deaths and $0.7 million, with much of the damage away from the coast where rivers overflowed; California- 13 deaths and $10 million damage.
The Pacific Disaster Center reports that tsunamis have accounted for more lost lives in Hawaii than the total of all other local disasters (www.pdc.org). In the 20th century, an estimated 221 people have been killed by tsunamis on the islands of Hawaii. Most of these deaths occurred on the Big Island during the tsunamis of 1946 and 1960, two of the largest tsunamis to strike in the Pacific. The 1946 Aleutian Island earthquake (M8) killed 159 residents and caused more than $26 million in damage. The May 23, 1960 Chile earthquake (M 9.5) generated a 35 foot wave causing 61 deaths and $23 million in damage. Other significant tsunamis in Hawaii include: 1952 (M9) Kamchatka, USSR earthquake ($1 million damage), 1957 (M9.1) Aleutian Islands earthquake ($5 million damage), 1975 (M7.5) earthquake off the Big Island of Hawaii (2 deaths).
This page is brought to you by the Earthquake Hazards Program URL: http://neic.usgs.gov/neis/bulletin/neic_slav_ts.html
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