资源环境科技发展态势分析平台

Great subduction zone earthquakes vary considerably in the updip extent of megathrust rupture. It is unclear if this diversity reflects variations in interseismic strain accumulation owing to the limited number of subduction zones with seafloor monitoring. We use a borehole seismic-geodetic observatory installed at the updip end of the Cascadia fault offshore Vancouver Island to show that the megathrust there does not appear to slip in triggered tremor or slow-slip events when subjected to moderate dynamic stress transients. Borehole tilt and seismic data from recent teleseismic M7.6-8.1 earthquakes demonstrate a lack of triggered slow slip above the M-w 4.0 level and an absence of triggered tremor despite shear-stress transients of 1-10 kPa that were sufficient to trigger tremor on the downdip end of the interface. Our observations are most consistent with a model in which the Cascadia fault offshore Vancouver Island is locked all the way to the trench.

Plain Language Summary Subduction zone thrust faults are generally thought to contain three primary regions in terms of earthquake rupture. Both the shallowest region near the trench (depths similar to 35 km) are thought to fail primarily without earthquakes, while seismic slip is contained primarily within the intervening depth range (similar to 5-35 km). In many subduction zones around the world, the deeper region has been observed to emit small amounts of seismic radiation when nonvolcanic tremor events are triggered by passing seismic waves. Similarly, the shallowest portions of a few subduction zones behave the same way. Here we study the Cascadia subduction zone with newly available subseafloor seismic and geodetic data that indicate a clear difference in behavior between the deep and shallow parts of the fault. This likely indicates that strain is accumulated across most of the breadth of the accretionary prism and that the shallow part of the fault may rupture during great earthquakes.