A MAGNITUDE 5 earthquake struck near the junction of three tectonic plates in northern California last week. The offshore quake was centred 13 kilometres west of the town of Petrolia.
鈥淭hese silent earthquakes and episodic tremors could help predict the next 鈥榖ig one鈥欌��
Despite being the state鈥檚 most seismically restless region, scientists are somewhat blas茅 about this activity. They are more interested in 鈥渟ilent鈥� earthquakes and episodic tremors, which could help predict the next 鈥渂ig one鈥� to hit the Cascadia region which includes northern California.
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Surface GPS measurements in Japan and Cascadia show that relatively large 鈥渆arthquakes鈥� 鈥� some equivalent to magnitude 6 or 7 鈥� can occur along subduction zones without setting off a single seismometer. These silent earthquakes release their energy stealthily over weeks rather than seconds. They may be linked to very low frequency tremors that have also been detected in these areas, some with nearly annual recurrence. Episodic tremors last occurred in September 2005 in Cascadia, and more are expected within months.
Studies of silent earthquakes in Hawaii suggest that they can trigger a flutter of small earthquakes, a possible sign that a larger one is imminent. Silent quakes and tremors may help explain plate motions at greater depths, but the role of these phenomena is 鈥渇ar from settled鈥�, says Paul Segall of Stanford University in California.
To study silent quakes and tremors, the west coast of North America is expected to be dotted with more GPS units and strain meters as part of the Plate Boundary Observatory project.
