Although these very-low-frequency quakes get their name from seismic waves detected on land, the researchers discovered these events are actually rich in high-frequency waves as well. High-frequency waves tend to weaken with distance as they go through matter, which is why land seismometers did not detect these waves but ocean seismometers closer to the quakes did. The long duration of the quakes and the high-frequency waves now seen from them suggest these events may be caused by fluid seeping into fractures in the rock, making it easier for parts of the earth to slip past each other and generate tsunami earthquakes.
These findings suggest that authorities should keep a closer eye on the shallow areas of subduction zones. For instance, the huge tsunamis generated by the magnitude 9.0 quake that struck Japan in 2011 might be due in significant part to a slip in the shallow parts of the Japan Trench lying east of the country's main island.
"It is very important for us to monitor continuously seismic activities close to the trench," researcher Hiroko Sugioka, a seismologist at the Japan Agency for Marine-Earth Science and Technology at Yokosuka, told OurAmazingPlanet. "It is mitigation against unexpectedly large tsunami disasters."