Japanese scientists detect rare, deep-Earth tremor

Japanese scientists detect rare, deep-Earth tremor
→National Aug. 26, 2016 - 04:03PM JST ( 3 )
Japanese scientists detect rare, deep-Earth tremor
Using seismic equipment on land and on the seafloor that usually detects the Earth's crust crumbling during earthquakes, researchers found something they had not detected before -- a tremor known as an S wave microseism


MIAMI — AFP
Scientists who study earthquakes in Japan have detected a rare deep-Earth tremor for the first time and traced its location to a distant and powerful storm.
The findings, published in the U.S. journal Science, could help experts learn more about the Earth’s inner structure and improve detection of earthquakes and oceanic storms.
The storm in the North Atlantic was known as a “weather bomb,” a small but potent storm that gains punch as pressure quickly mounts.
Groups of waves sloshed and pounded the ocean floor during the storm, which struck between Greenland and Iceland.
Using seismic equipment on land and on the seafloor that usually detects the Earth’s crust crumbling during earthquakes, researchers found something they had not detected before—a tremor known as an S wave microseism.

Microseisms are very faint tremors.
Another kind of tremor, known as P waves, or primary wave microseisms, can be detected during major hurricanes.
P waves are fast-moving, and animals can often sense them just before an earthquake hits.

The elusive S waves, or secondary waves, are slower, and move only through rock, not liquid. Humans feel them during earthquakes.

Using more than 200 stations operated by the National Research Institute for Earth Science and Disaster Prevention in Japan’s Chugoku district, researchers Kiwamu Nishida and Ryota Takagi “successfully detected not only P wave microseisms triggered by a severe and distant North Atlantic storm, known as a weather bomb, but also S wave microseisms, too,” said the study. “The discovery marks the first time scientists have observed… an S wave microseism.”
Microseism S waves are so faint that they occur in the 0.05 to 0.5 Hz frequency range.
The study in the journal Science details how researchers traced the direction and distance to the waves’ origins, and the paths they traveled.  The discovery “gives seismologists a new tool with which to study Earth’s deeper structure,” said Peter Gerstoft and Peter Bromirski of the University of California, San Diego in an accompanying Perspective article.  Learning more about microseismic S waves may “add to our understanding of the deeper crust and upper mantle structure.”
http://www.japantoday.com/category/national/view/japanese-scientists-detect-rare-deep-earth-tremor

Deep Earthquakes
The most common source of damaging earthquakes in Washington and Oregon, are deep earthquakes that rupture faults within the subducting Juan de Fuca plate, "intraplate" earthquakes. These occur beneath Puget Sound at depths from 30 km to 70 km.

Although plate interface earthquakes can be much larger, and crustal earthquakes and can be much closer to our cities, damaging deep earthquakes are the most frequent. They have historically occurred about every 30 years.  The USGS estimates there is an 84% chance of another deep earthquake, of Magnitude 6.5 or greater, striking the region sometime in the next 50 years.
This schematic diagram shows the relative positions of the Juan de Fuca and North American Plates. The pink circle shows the depth of the 1965,  1949, and 2001 earthquakes in cross-section.  The 1949 earthquake caused over $100 million in damage, including damage to the Capitol Building in Olympia. The 1965 earthquake caused over $50 million in damage.  In 2001, the Nisqually earthquake near Olympia shook western Washington and caused over 500 million dollars worth of damage.  Recently there has been growth, development and greater economic activity in the Puget Sound area, increasing the likelihood of costly losses from future earthquakes.
Deep intraplate earthquakes usually are caused by normal faulting, and reliably have very few aftershocks. These earthquakes occur where the plate is bending as it plunges beneath North America, and may also be associated with mineral changes as the plate encounter increased temperature and pressure. Stresses cause faults in the downgoing crustal part of the plate to rupture. While many subduction zones feature deep interplate earthquakes extending to depths exceeding 600 km and with different characteristics, Cascadia's deepest earthquakes are about 100 km deep. Discover more about the Cascadia deep earthquake hazard in a booklet created by the Cascadia Region Earthquake Workgroup.