In Japan, seismic waves slower after rain, large earthquakes

March 5, 2012

An earthquake is first detected by the abrupt side-to-side jolt of a passing primary wave. Lagging only slightly behind are shear waves, which radiate out from the earthquake's epicenter and are seen at the surface as a rolling wave of vertical motion. Also known as secondary or S waves, shear waves cause the lifting and twisting motions that are particularly effective at collapsing surface structures. With their capacity to cause damage, making sense of anything that can influence shear wave vertical velocities is important from both theoretical and engineering perspectives.

In Japan the Kiban-Kyoshin network (KiK-net) is made up of 700 seismic detection stations spread across the country. Each station has two separate seismic detectors, one at the surface and one buried in a . Analyzing the KiK-net data for the nearly 112,000 earthquakes that hit Japan between 2000 and 2010, Nakata and Snieder identify a number of relationships that seem to affect shear wave vertical velocities in the near surface.

The authors find that in the months following a major earthquake, shear wave velocities at nearby stations were cut down by 3 percent to 4 percent. Further, the authors find that the shear waves that propagate fastest oscillate in the direction of motion of the underlying tectonic plate. Finally, they find that the shear wave velocity changed with the season. In the southern reaches of Japan the summer months are marked by heavy precipitation. Comparing rainfall records with the data derived from KiK-net's observations, the authors find that shear wave velocities were significantly reduced following periods of heavy rainfall. They suggest that groundwater infiltration would fill any cracks in the subsurface, increasing pore pressure and reducing seismic wave velocities.

Explore further: Seismic experiments provide new clues to earthquake wave directionality and growth speed

More information: Estimating near-surface shear wave velocities in Japan by applying seismic interferometry to KiK-net data, Journal of Geophysical Research-Solid Earth, doi:10.1029/2011JB008595 , 2012

Related Stories

For earthquakes 'speed kills'

August 17, 2007

High-speed ruptures travelling along straight fault lines could explain why some earthquakes are more destructive than others, according to an Oxford University scientist.

Gravity Waves Make Tornados

March 19, 2008

Did you know that there's a new breakfast food that helps meteorologists predict severe storms? Down South they call it "GrITs."

Recommended for you

Climate ups odds of 'grey swan' superstorms

August 31, 2015

Climate change will boost the odds up to 14-fold for extremely rare, hard-to-predict tropical cyclones for parts of Australia, the United States and Dubai by 2100, researchers said Monday.

0 comments

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.