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.
Bibliographical noteFunding Information:
The study was supported financially by the Strategic Priority Research Program (B) of Chinese Academy of Sciences ( XDB18000000 ), Project of State Key Laboratory of Geodesy and Earth's Dynamics of Chinese Academy of Sciences ( SKLGED2018-4-2-E ), Science for Earthquake Resilience of China Earthquake Administration ( XH17007 ), Project of Hebei Science and Technology Department ( 15275402D ) and Science for Earthquake Resilience of Hebei Earthquake Agency ( DZ20170109002 ). Thoughtful and constructive reviews from anonymous reviewers significantly improved the manuscript. We are grateful to Prof. Timothy M. Kusky and Dr. Xianying Wang, editor Bruce Buffett for helpful comments.
© 2018 Elsevier B.V.
- North China Basin
- crustal rheology
- focal depth
- locally converted wave