#sesismic

[earthstory]

In this video you can watch as sensitive seismograms around the United States detect the up and down motion triggered by waves from the California Earthquake moving around the planet.

[sciencenewsforstudents]

Powerful storms may be causing offshore ‘stormquakes’

A perfect-storm mixture of hurricane, ocean and seafloor topography can create distinct seismic signals called “stormquakes.”

Powerful hurricanes can whip the ocean into a frenzy — and that wave energy can be strong enough to hammer the seafloor, producing a novel kind of quake.

These stormquakes, as described online October 14 in Geophysical Research Letters, are a newly identified type of interaction between Earth’s atmosphere, ocean and crust. Unlike earthquakes, which are triggered by subsurface shifting within the solid Earth, the driving force behind these seismic signals are ocean waves that have been whipped into deep swells by a hurricane or nor’easter. Stormquakes can be as powerful as a magnitude 3.5 earthquake, a level barely noticeable to people but detectable by seismometers, seismologist Wenyuan Fan and colleagues report.

The work is “a really great first start” at understanding a little-studied part of the seismic record, says physical oceanographer Fabrice Ardhuin of the Ocean Physics and Satellite Oceanography laboratory in Brest, France. “It brings something really new.”

Scientists have long known that the constant sloshing of ocean waves produces seismic signals at frequencies of about once every few minutes, a phenomenon known as “Earth’s hum” (SN: 9/29/04). Waves can also produces high-frequency signals called microseisms, occurring every five seconds or so.

[earthstory]

Tianjin fault and Seismic Gap An Yin et al. recently published in Geology about the Tangshan-Hejian-Cixian (THC) fault in China. They have discovered a "seismic gap" which is without earthquakes. That may sound good, but the lack of earthquakes means energy is only building up for a big one, similar to what happens at famous faults like California's San Andreas fault, which has a locked section near Los Angeles and a different locked section near San Fransisco. The THC fault also is locked near a big city, Tianjin, with 11 million people waiting. At least now they might be able to get more prepared. This fault system has already produced the single deadliest earthquake, in 1976, which killed as many as 750,000 people according to some estimates. -Mr. A Article: A possible seismic gap and high earthquake hazard in the North China Basin, Yin et al., 2014 (http://geology.gsapubs.org/content/early/2014/11/14/G35986.1) Image: Historic seismicity around the THC fault, by Yin et al.