Monday, July 10, 2017

Earthquakes can cause distant undersea landslides months later

[Futurity] Large earthquakes can cause underwater landslides thousands of miles away, weeks or months after the quake occurs, new research suggests.
Researchers analyzing data from ocean bottom seismometers off the Washington-Oregon coast tied a series of underwater landslides on the Cascadia Subduction Zone, 80 to 161 kilometers (50 to 100 miles) off the Pacific Northwest coast, to a 2012 magnitude-8.6 earthquake in the Indian Ocean—more than 13,500 kilometers (8,390 miles) away. These underwater landslides occurred intermittently for nearly four months after the April earthquake.
Previous research has shown earthquakes can trigger additional earthquakes on other faults across the globe, but the new study shows earthquakes can also initiate submarine landslides far away from the quake.
“The basic assumption…is that these marine landslides are generated by the local earthquakes,” says Paul Johnson, an oceanographer at the University of Washington and lead author of the new study. “But what our paper said is, ‘No, you can generate them from earthquakes anywhere on the globe.'”
The new findings could complicate sediment records used to estimate earthquake risk. If underwater landslides could be triggered by earthquakes far away, not just ones close by, scientists may have to consider whether a local or a distant earthquake generated the deposits before using them to date local events and estimate earthquake risk, according to the study’s authors.
The submarine landslides observed in the study are smaller and more localized than widespread landslides generated by a great earthquake directly on the Cascadia margin itself, but these underwater landslides generated by distant earthquakes may still be capable of generating local tsunamis and damaging underwater communications cables, according to the study authors.
The discovery that the Cascadia landslides were caused by a distant earthquake was an accident, Johnson says.
Scientists had placed ocean bottom seismometers off the Washington-Oregon coast to detect tiny earthquakes, and also to measure ocean temperature and pressure at the same locations. When Johnson found out about the seismometers at a scientific meeting, he decided to analyze the data the instruments had collected to see if he could detect evidence of thermal processes affecting seafloor temperatures, such as methane hydrate formation.
Johnson and his team combined the seafloor temperature data with pressure and seismometer data and video stills of sediment-covered instruments from 2011-2015. Small variations in temperature occurred for several months, followed by large spikes in temperature over a period of two to 10 days. Read More