Crustal Extension and Graben Formation by Fault Slip-Associated Pore Opening, Kyushu, Japan

Abstract

Crustal extension in graben takes place by normal (+strike-slip) faulting, yet the physical processes involved are poorly understood. A series of shallow large earthquakes struck the Kumamoto area of Kyushu, Japan, in 2016. The Mw7.0 main shock was a slip along the southern boundary of the Beppu-Shimabara Graben, where NS-extensional crustal deformation is now taking place. We conducted a tomographic inversion for crustal P and S velocities and Poisson's ratio (Vp, Vs, and σ) in Kyushu. The most outstanding feature in this region is a Beppu-Shimabara Graben-parallel belt of low Vp and Vs anomalies at depths of the upper crust. We find that within the belt dVs/Vs ≈ dVp/Vp (<0) in marked contrast to relations in other low-velocity regions where |dVs/Vs| > |dVp/Vp|. This observation can be interpreted as due to water-saturated, oblate-spheroid pores with either of two very different geometries: one almost round pore and the other very flat pore. We calculate 3-D density anomaly distributions and 2-D gravity anomaly profiles for these two pore models. The gravity anomaly map calculated for spherical pores shows a significant negative anomaly belt that spatially agrees with observed Bouguer anomaly map. The spatial agreement is very poor if pores are modeled as flat. This and extensive seismicity within Beppu-Shimabara Graben imply significance of fault-associated round pore formation in the process of graben opening. Seismic tomography combined with gravity and poroeasticty analyses can constrain the crack/pore state of extensionally deforming crust.