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: Climate researchers measure the concentration of greenhouse gases above the Atlantic

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

add to favorites email to friend print save as pdf

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 sees last vestiges of Tropical Depression Jack

1 hour ago

Tropical Cyclone Jack had weakened to a tropical depression when NASA and JAXA's Tropical Rainfall Measuring Mission (TRMM) satellite passed above on April 22, 2014 at 1120 UTC/7:20 a.m. EDT.

New discovery helps solve mystery source of African lava

4 hours ago

Floods of molten lava may sound like the stuff of apocalyptic theorists, but history is littered with evidence of such past events where vast lava outpourings originating deep in the Earth accompany the breakup ...

Climate change likely to make Everest even riskier

5 hours ago

Climbing to the roof of the world is becoming less predictable and possibly more dangerous, scientists say, as climate change brings warmer temperatures that may eat through the ice and snow on Mount Everest.

NASA gets two last looks at Tropical Cyclone Jack

Apr 22, 2014

Tropical Cyclone Jack lost its credentials today, April 22, as it no longer qualified as a tropical cyclone. However, before it weakened, NASA's TRMM satellite took a "second look" at the storm yesterday.

User comments : 0

More news stories