Known as a slow-slip event, or an episodic tremor and slip (ETS), the rumbling being detected isn't necessarily out of the ordinary, as ETS cycles typically occur every 14 months or so. But with the recent sizable shakes in Southern California coupled with the fact that a "big one" is long overdue for the Pacific Northwest (PNW), some are speculating as to what might be brewing.

Though it didn't receive much media coverage, a 4.6 magnitude quake struck the Puget Sound area back in July, right around the time that Southern California experienced a pair of even larger quakes near Ridgecrest. Since this 4.6 was the largest quake to strike the area "in some time," seismic experts are taking notice and investigating.

According to Steve Malone, a professor emeritus with PNSN, this latest ETS cycle near Seattle more than likely began on August 11, with its first hit just south of San Juan Island. And so far, the subsequent tremors appear to be increasing in strength, indicating the potential for larger quakes in the near future.