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

<jats:title>Abstract</jats:title><jats:p>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 <jats:italic>M</jats:italic><jats:sub><jats:italic>w</jats:italic></jats:sub>7.0 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 <jats:italic>P</jats:italic> and <jats:italic>S</jats:italic> velocities and Poisson's ratio (<jats:italic>V</jats:italic><jats:sub><jats:italic>p</jats:italic></jats:sub>, <jats:italic>V</jats:italic><jats:sub><jats:italic>s</jats:italic></jats:sub>, and σ) in Kyushu. The most outstanding feature in this region is a Beppu‐Shimabara Graben‐parallel belt of low <jats:italic>V</jats:italic><jats:sub><jats:italic>p</jats:italic></jats:sub> and <jats:italic>V</jats:italic><jats:sub><jats:italic>s</jats:italic></jats:sub> anomalies at depths of the upper crust. We find that within the belt <jats:italic>dV</jats:italic><jats:sub><jats:italic>s</jats:italic></jats:sub>/<jats:italic>V</jats:italic><jats:sub><jats:italic>s</jats:italic></jats:sub> ≈ <jats:italic>dV</jats:italic><jats:sub><jats:italic>p</jats:italic></jats:sub>/<jats:italic>V</jats:italic><jats:sub><jats:italic>p</jats:italic></jats:sub> (<0) in marked contrast to relations in other low‐velocity regions where |<jats:italic>dV</jats:italic><jats:sub><jats:italic>s</jats:italic></jats:sub>/<jats:italic>V</jats:italic><jats:sub><jats:italic>s</jats:italic></jats:sub>| > |<jats:italic>dV</jats:italic><jats:sub><jats:italic>p</jats:italic></jats:sub>/<jats:italic>V</jats:italic><jats:sub><jats:italic>p</jats:italic></jats:sub>|. 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.</jats:p>