Crustal rheology from focal depths in the North China Basin

In order to investigate earthquakes in the lower crust of North China Basin (NCB), we develop a new method of resolving very accurately the focal depth for local earthquakes in the sedimentary regions by using P and S to P converted wave (Sp) at the sedimentary interface. Theoretical analysis shows clearly that the travel-time difference between the Sp and P wave almost linearly correlates with the focal depth. This finding provides tight constraints on the depth. With this method, we obtain well-constrained depths of 44 events in the NCB, with uncertainties in the depth of about 2 km. Such a fine resolution can have great potential in asking questions regarding the crustal rheology. The depth distribution shows abundance of earthquakes in depth interval of ∼20 km, with some events in the lower crust, but also reveals the absence of seismicity deeper than 25 km. We find a good fit between the depth-frequency distribution in this region and the Yield Strength Envelope (YSE) in the Baikal Rift Systems (BRS). We infer that, the seismogenic thickness is ∼25 km in the NCB and the main deformation mechanism is brittle fracture. We further hypothesize that: (1) the rheological layering of dominant rheology in the NCB is similar to that of the BRS, which can be explained with a quartz rheology at 0-10 km depth and a diabase rheology at 10-35 km depth; (2) the temperature is moderate in the seismogenic zone of crust. We emphasize that many accurately resolved earthquake locations can shed light on the local nature of the crustal rheology, and this strategic method can be employed in other sedimentary regions, which are seismically active.