Slow earthquakes happen between the hazardous locked zone and the viscous portion that slips silently. They are found on subduction zones, like Cascadia’s, where a heavy ocean plate sinks below a lighter continental plate. UW
Unknown to most people, the Pacific Northwest experiences a magnitude-6.6 earthquake about once a year. The reason nobody notices is that the movement happens slowly and deep underground, in a part of the fault whose behavior, known as slow-slip, was only recently discovered. A University of Washington seismologist who studies slow-slip quakes has looked at how they respond to tidal forces from celestial bodies and used the result to make a first direct calculation of friction deep on the fault. Though these events occur much deeper and on a different type of fault than the recent catastrophe in Nepal, the findings could improve general understanding of when and how faults break. The new study , published online April 27 , shows that the gravitational pull of the sun and the moon affect the Cascadia fault a few days after it has started slipping. The timing of movement suggests that the friction at this depth on the fault is only 0.1, roughly that of two pieces of lubricated metal. "I was able to tease out the effect of friction and found that it is not the friction of normal rocks in the lab - it's much lower,” said author Heidi Houston , a UW professor of Earth and space sciences.
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