Shakeless quakes at Kilauea

When the earth moves, experience suggests that there's always some shaking.
Seems reasonable, but new evidence from Hawai'i shows it's not always true.
It seems that if the earth moves slowly enough, it can make significant and measurable changes without anybody actually being able to feel it.
Scientists working at Kīlauea were able to measure such a shudder-less earthquake in an event in 2007. They reported their findings in the journal Science last week, under the headline, “Magmatically Triggered Slow Slip at Kilauea Volcano, Hawai'i. The researchers included Benjamin Brooks, James Foster and Cecily Wolfe of the University of Hawai'i's School of Ocean and Earth Science and Technology, David Sandwell and David Myer of Scripps Institution, and Paul Okubo and Michael Poland of the USGS Hawaiian Volcano Observatory.
Their focus was an event at the volcano in mid-June 2007, in which an underground region of the volcano was subjected to what geologists call a dike intrusion, in which magma pushes into a new region. (Magma is molten rock when it's still underground. It's lava when it hits the surface.)
Using highly accurate global positioning system sensors, volcano
scientists were able to show that the dike intrusion at the rift zone
triggered an earthquake on Kilauea's flank. But they hadn't measured active seismic activity—ground shaking—associated with the movement.
The mechanics of this are something of a mystery, they say.
They call these spontaneous aseismic slip events or slow-slip events.
“The underlying process that generates seismic waves is the rupture of a fault. In typical earthquakes, this fault rupture occurs rapidly (within seconds) and this rapid rupture of a fault generates the seismic waves that people feel and that do all the damage,” said co-researcher Wolfe.
In the June 2007 event, the ground movement occurred very slowly, over hours and days. So slowly, that the ground moved, but did not shake. An earthquake without a seismic wave.
That requires some explanation to those of us who assume that the quaking and the shaking are the same thing. Wolfe said that the earthquake is different from the shake. The quake is the process that involves the originating ground movement. The shaking is the seismic wave that can result from that quake, if we understand this correctly.
“Earthquakes are not themselves seismic waves, rather earthquakes are a process that typically generates seismic waves,” she wrote in an email.
But like a Ninja moving down a path, so slowly that his passage can not be heard, it is possible for the earth to move without the shuddering—and that's a pretty new discovery to science.
"'Slow earthquakes' are a special type/new class of earthquake that has only been recently discovered (within the past decade or so), where the fault rupture occurs so slowly (over hours or days or even months) that the earthquake does not generate any strong seismic waves,” Wolfe said.
© 2008 Jan TenBruggencate