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Tohoku quake data 'a global treasure' for seismologists
The Japanese have always lived with the threat of earthquakes as the nation sits on what is known as the "Ring of Fire," a zone of frequent earthquakes and volcanic eruptions that encircles the basin of the Pacific Ocean. Because of their high geographical risk, they have invested heavily in earthquake study and seismic instruments cover the nation.
On March 11, 2011 those instruments detected a magnitude 9.0 earthquake off the coast of northern Japan. The shaking sent buildings crashing and triggered tsunami waves that have claimed tens of thousands of lives.
Seismoligists are now getting a good look at the data from those instruments in the hopes it can help them better understand the shaking so that the information can be used to build strutures that can withstand earthquakes and thus save lives.
One of those seismologists is AAAS member John Anderson, a professor at the University of Nevada, Reno. Anderson studies ground shaking during large earthquakes, known in seismology circles as strong motion seismology.
Below, Anderson discusses the Tohoku quake and what we can learn from it.
AAASMC: First, tell us
about the study of strong motion seismology.
John Anderson, Professor, University of Nevada-Reno: Seismology has two main areas -- exploration, using seismic methods to discover resources like oil. The other, earthquake seismology, the study of earthquake sources and that’s what I do. Within earthquake seismology there is a branch for studying intense ground shaking – known as strong motion seismology. Large quakes can cause major damage and trigger tsunamis like the Tohoku earthquake in Japan on March 11. A strong motion instrument or sensor, records the ground shaking and that information can be used by engineers for evaluating a building's earthquake resistance.
reports had the Tohoku quake in the 8.0 magnitude range. What was your initial reaction?
Anderson: When I first got a text message saying there was a magnitude 8 point-something earthquake in Japan, I thought, I’m not sure I believe it. Anything that big I want to be sure that it really is that big and not some computer glitch. So, I went and looked on the USGS website and it showed aftershocks 400 kilometers south of the epicenter, and I said, I believe it.
AAASMC: Are you analyzing the data collected
from the Japan quake?
Anderson: Yes. Japan is absolutely covered with seismic sensors to record the strong motion of an earthquake. This is the first time that an earthquake of magnitude over 9 has been recorded in anywhere near such detail. In the magnitude 9.0 Sumatra quake of 2004, there was not a single strong motion instrument that recorded the shaking anywhere close to the fault. You have to go back to 1985 after the Mexico quake, which was a magnitude 8.1. At that time it was the best instrumented large earthquake ever recorded. We only get one ≥8.0 earthquake a year on average, if you look at quake statistics. The way I look at it, every time one of these earthquakes happens and we don’t get that data, then we don’t get any records from it and we’ve lost another year in gathering the information that we need to have a high degree of confidence in knowing how to design major structures to resist earthquakes.
In Japan, there will probably be in the high hundreds of instruments that recorded close to the fault, in addition, all the instruments across the country providing records of the strong shaking during this earthquake. The data from this quake is a global treasure.
AAASMC: It is still early on, but what is the data telling you?
Anderson: My preliminary impression is that the ground motion in Japan was stronger than in Mexico in 1985. Which is not a surprise because of the magnitude was a lot bigger. We don’t know if this earthquake is going to be representative of all magnitude 9.0 quakes or not, it is a sample of one.
Also, the duration was long. Not a surprise but a confirmation of our understanding that how long the shaking lasts is related to how big the fault is. Magnitude is also very closely related to how big the fault is - the bigger the fault, the bigger the magnitude, thus the longer the shaking.
I haven’t yet seen any record holders in terms of record high peak acceleration, or record high peak velocity – parameters seismologists typically look for. Most of the amplitudes are unsurprising. One accelerogram has a peak that is about 3 times the acceleration of gravity – which has been exceeded by only one prior measurement that I know about. That may be an upper limit of the shaking of this quake.
will this data be applied to other regions at risk?
Anderson: In 1700 the Cascadia subduction zone that runs from northern California through Oregon and up to Washington had a quake much like the Tohoku quake. The data from Japan is going to be of enormous importance for evaluating the seismic hazard in this area here in the U.S.