Ultra-shallow Earthquakes Caused by Interbed Slip in Global Fold-Thrust Belts

Earthquakes are super shallow when their rupture on the upper crust that of the depth no more than 10 km. In the canonical view, geoscience community often attributes the supper shallow earthquake to the relative crustal motions along the known underlying faults, and repeated earthquake cycles build mountain ranges over millions of years. However, seismic activity and the underlying faults in the margins of evolving orogenic belts exhibit complex spatial relationships in the practice. These events are difficult to locate at depth and to match with the subsurface structures, but they can be highly destructive due to the associated strong ground shaking. Understanding why they occur may provide insights into seismogenic mechanisms and mitigate the hazards, especially in areas in front of mountains that are home to large populations and industries. We herein examine and review the super shallow earthquakes in global fold-thrust belts, based on the subsurface structures, InSAR analysis and Bayesian inversion. We find that much more super shallow earthquakes were not caused by slip along known faults, but by interbed slip due to buckling, demonstrating for the first time that buckling could result in moderate earthquakes (Mw5-Mw7). It can either act as an independent seismogenic structure or be triggered by the mainshock that is often of Mw 7 or higher. This type of seismogenic structure is often overlooked due to its parallel alignment with the layer interface, and its very shallow depth can lead to significant casualties, which must be closely monitored. Additionally, because this seismogenic structure is easily triggered by major earthquakes, locations where such geological records are found may also indicate the presence of potentially larger seismogenic structures, capable of producing earthquakes above magnitude 7. Our findings underscore the buckle folding—a significant mechanism involving bed-parallel contraction—not only produce the short-term intense coseismic deformation on the surface by interbed slip, but also accommodates the long-term long-term mountain building in active fold-thrust belts.