Some Earthquake Zones Create Focusing Points Which Cause Rule-breaking Tsunamis
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April Flowers for redOrbit.com – Your Universe Online
An international collaboration has resulted in a new study showing that the earthquake zones off certain coasts, such as Japan and Java, make them especially vulnerable to tsunamis. These zones can produce a focusing point that creates massive and devastating tsunamis capable of breaking the rules by which scientists used to think tsunamis worked.
Previously, scientists largely believed that the maximum onshore height for tsunamis could not exceed the depth of the seafloor. The new study, published online in the Proceedings of the Royal Society, Series A, reveals that when focusing occurs, the scaling relationship breaks down. Flooding can be up to 50 percent deeper with waves that do not lose height as they get closer to shore.
“It is as if one used a giant magnifying lens to focus tsunami energy,” said Utku Kanoglu, professor at the Middle East Technical University (METU). “Our results show that some shorelines with huge earthquake zones just offshore face a double whammy: not only they are exposed to the tsunamis, but under certain conditions, focusing amplifies these tsunamis far more than shoaling and produces devastating effects.”
This effect was observed in both the Tohoku tsunami of 2011 in Northern Japan, and the 2006 tsunami that struck Central Java.
“We are still trying to understand the implications,” said Costas Synolakis, director of the Tsunami Research Center at the University of Southern California’s Viterbi School of Engineering. “But it is clear that our findings will make it easier to identify locales that are tsunami magnets, and thus help save lives in future events.”
Sections of the sea floor lift during an earthquake while others sink, creating tsunamis that propagate trough-first in one direction and crest-first in the other. The research team discovered that on the trough-first side of the earthquake zone there is a location where focusing occurs. This strengthens the tsunami before it hits the coastline with an unusual amount of energy that is not seen by the crest-first wave. That focal point can concentrate the tsunami´s power right on to the coastline based on the shape, location, and size of the earthquake zone.
Before this study, scientists thought that tsunamis usually decrease in height continuously as they move away from the earthquake’s epicenter and grow closer to shore, just as wind waves do. Instead, the study’s authors suggest that the crest of the tsunami remains fairly intact close to the source.
“While our study does not preclude that other factors may help tsunamis overgrow, we now know when to invoke exotic explanations for unusual devastation: only when the basic classic wave theory we use does not predict focusing, or if the focusing is not high enough to explain observations,” said Vasily Titov, a researcher at NOAA´s Pacific Marine Environmental Laboratory.
This study represents the collective efforts of scientists and researchers from METU, NOAA, Akdeniz University, CMLA Ecole Normale Superieure Cachan, University College Dublin, HCMR, and the University of Southern California.