This predictability most likely stems from the fact that water flowing from below subduction zones may exert significant control over when and where these faults slip silently. As the subducting plate sinks deeper into the earth, it encounters higher and higher temperatures and pressures, which release the significant amount of water trapped in waterrich minerals that exist within the slab. The silent earthquakes may then take place when a batch of fluid from the slab is working its way up—as the fluid passes, it will unclamp the fault zone a little bit, perhaps allowing some slow slip.
What is more, Garry Rogers and Herb Dragert of the Geological Survey of Canada reported last June that these silent tremors might even serve as precursors to some of the region’s large, ground-shaking shocks. Because the slow slips occur deep and at discrete intervals, they regulate the rate at which stress accumulates on the shallower part of the fault zone, which moves in fits and starts. In this shallow, locked segment of the fault, it usually takes years or even centuries to amass the stress required to set off a major shock. Rogers and Dragert suggest, however, that silent slip may dramatically hasten this stress buildup, thereby increasing the risk of a regular earthquake in the weeks and months after a silent one.
Silent earthquakes are forcing scientists to rethink seismic forecasts in other parts of the world as well. Regions of Japan near several so-called seismic gaps—areas where fewer than expected regular earthquakes occur in an otherwise seismically active region—are thought to be overdue for a destructive shock. But if silent slip has been relieving stress along these faults without scientists realizing it, then the degree of danger may actually be less than they think. Likewise, if silent slip is discovered along faults that were considered inactive up to now, these structures will need careful evaluation to determine whether they are also capable of destructive earthquakes.
If future study reveals silent earthquakes to be a common feature of most large faults, then scientists will be forced to revisit long-held doctrines about all earthquakes. The observation of many different speeds of fault slip poses a real challenge to theorists trying to explain the faulting process with fundamental physical laws, for example. It is now believed that the number and sizes of observed earthquakes can be explained with a fairly simple friction law. But can this law also account for silent earthquakes? So far no definitive answer has been found, but research continues.
Silent earthquakes are only just beginning to enter the public lexicon. These subtle events portend an exponential increase in our understanding of the how and why of fault slip. The importance of deciphering fault slip is difficult to overstate because when faults slip quickly, they can cause immense damage, sometimes at a great distance from the source. The existence of silent earthquakes gives scientists a completely new angle on the slip process by permitting the detailed study of fault zones through every stage of their movement.