You are at a classical music concert. There is an orchestra with three main sections. High up at the back, the percussion section has one very loud, large and moody-looking drum that gets struck very rarely. A handful of triangles produce occasional quieter “tings”. Further down, in the middle, there is a small band of violinists, but they are playing the strings so slowly the audience can barely hear them. Down at the front, a family of double bass instruments produces low-pitched, gentler hums from time to time.

This somewhat unconventional orchestra is like a type of tectonic plate boundary known as a subduction zone. Subduction zones delineate the battle lines between the collision of two titanic tectonic plates. Yet, this encounter is rather one-sided. One plate firmly stands its ground; the other sinks into the depths of the Earth. The grinding and sliding of these two plates produces a musical concert that can be detected by sensitive geophysical instruments and by humans during large quakes. The shallow parts of subduction plate boundaries can produce devastating mega-earthquakes with magnitude eight or greater (like the giant drum in the percussion section). In the tens to hundreds of years between these massive quakes, scientists eagerly listen to the signals at subduction zones to estimate whether the plate boundary fault is primed for a future quake, and to forecast what a rupture may look like.



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At the turn of the century, scientists discovered a new type of “sound” produced at subduction zones, known as Episodic Tremor and Slip (or ETS for short). ETS are typically low-pitched rumbling signals (like the double bass instruments) lasting minutes to hours that do not produce the jolts associated with normal earthquakes. These rumbles come from a type of event known as seismic tremor, which is similar to tiny earthquakes. Moreover, in subduction zones in southwest Japan, Mexico and New Zealand, the gap between the part of the fault that hosts large earthquakes and the zone of ETS is filled with an area producing silent ruptures that occur over many months (the virtually inaudible violinists). These cannot be detected using traditional seismometers.



Regions of the plate boundary that produce tremor and slow slip events are of vital interest. They have been used to delineate the total area of fault (and therefore maximum magnitude) that could rupture in a future mega-quake. Until now, the forces driving the zones of deep tremor and slow slip remained unclear.



In a new study using numerical models that re-create the temperature and deformation conditions deep inside a subduction zone, Dr Kelin Wang and Dr Xiang Gao were able to match observations and infer the main factors that control the depth at which seismic rupture, tremor and slow slip occurs. As oceanic plates sink, they transport huge amounts of water into the deep earth – hundreds of metric tons of water every million years. The presence of water causes rocks and their minerals to change their state and behaviour. One such process produces hydrous minerals, such as serpentine, in the mantle, which in turn reduces the number of pathways for fluid to escape. The ponding of highly pressurised water locally reduces the friction of the fault, causing seismic tremor where the subducting plate meets the mantle of the non-subducting (overlying) plate. The transition to this deep portion of the fault with lower friction therefore causes a domino effect of inducing a part-frictional regime that allows slow-slip events to occur at shallower depths.

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Wang, who is based at the Geological Survey of Canada, is one of the lead authors on the study. He says, “this new work brings together very different disciplines of research. We seemed to get stuck in understanding the physical mechanisms of ETS for some time, mainly because we tended to examine one aspect of the complex matter at a time. By integrating different observations and theories, the complex matter became simple”.



The study concludes that the presence of a gap between zones of the fault that are frictionally locked and those which produce ETS means that tremor events cannot rapidly transfer stress to the locked portion. Wang says, “I think the work serves to remind researchers that we cannot readily use slow slip events as precursors of large earthquakes. The relationship between slow slip events and large earthquakes is not as simple as in popular idealized cartoon illustrations”.



However, the authors do not rule out that over time, the cumulative piling up of tremor and slow-slip events could gradually push the locked zone closer toward failure, potentially causing a large and damaging rupture. Fortunately, scientific organisations in many regions, such as in Japan and the Cascadia subduction regions continue to keep a close eye on ETS events. Dense on-land and sea-floor networks of geophysical monitoring instruments able to detect any potential movement of tremor and slow slip toward the locked and primed portion of the plate boundary fault.

