In Japan, seismic waves slower after rain, large earthquakes

Mar 05, 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: Geologist identifies new source of methane for gas hydrates in Arctic

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

Gravity Waves Make Tornados

Mar 19, 2008

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

For earthquakes 'speed kills'

Aug 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.

Recommended for you

NASA image: Fires in Southeastern Australia

1 hour ago

The fires superimposed on the satellite image of southeastern Australia designated by red spots may be indicative of "planned burns" by the Victoria region. This map: http://www.depi.vic.gov.au/fire-and-emergencies/planned-burns/planned-burns-now-and-next-10-days found on the Department of the Environment and Primary Industries for the State of Victoria shows the burns th ...

NASA sees Maysak become a super typhoon

1 hour ago

NASA's Aqua satellite captured an image of Typhoon Maysak as it strengthened into a super typhoon on March 31, reaching Category 5 hurricane status on the Saffir-Simpson Wind Scale. The TRMM and GPM satellites, ...

A new tool to understand volcanic supereruptions

4 hours ago

To understand when and why volcanoes erupt, scientists study the rocks left behind by eruptions past. A method called geobarometry uses the composition of volcanic rocks to estimate the pressure and depth ...

User comments : 0

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.