(Oct 6, 2012) Parts of Washington and Oregon are in the midst of silent earthquakes this week. You can’t feel this so-called “slow slip” quake and it doesn’t cause damage. Still, scientists want to learn more about the recently discovered phenomenon. Little is certain so far, but there’s a possibility these deep tremors could trigger a damaging earthquake or serve as a warning bell for the Big One. A bank of computer monitors covers one wall of the University of Washington seismology lab. Some display seismograph readouts that look like jagged mountain ranges stacked one over the other. A big screen shows a current map of tremors under the Pacific Northwest. It is lit up with activity. “Each dot represents the location of a five minute burst of tremor,” says earth scientist Ken Creager. He scrutinizes a dense slash of blue, yellow, green and red dots. The arc stretches south from mid-Vancouver Island, goes under the Olympic Peninsula, Puget Sound and peters out south of Olympia. A separate patch of color radiates out from near Roseburg, Ore.
This current slow slip quake under the Salish Sea has lasted five weeks. Creager says scientists have calculated that a significant event like this releases the equivalent energy of a magnitude 6.5 regular quake. “It’s a lot of energy being released,” Creager says. “It just happens so slowly that you’re not going to feel it. This is the way we like to see energy released.” But there’s a flip side. The grinding and slippage at depth increases the strain closer to the surface where the North American plate and the oceanic plate are stuck together or “locked.” When that offshore fault zone eventually gives way, we get the damaging Big One. University of Oregon Professor David Schmidt makes an analogy to a car teetering partway over a cliff. “And these small slow slip events are somebody standing behind that car giving it a little nudge every several months. So even though the nudge is small, at some point that nudge might be enough to kind of tip us over the edge and cause the car to fall off the cliff.” Or set off the Cascadia mega-quake in this analogy.http://quakes.globalincidentmap.com/
- Warning signs? ‘Silent Earthquakes’ ripple under Cascadia (theextinctionprotocol.wordpress.com)
- Big quakes ‘shake the world’ (stuff.co.nz)