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 M w 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 P and S velocities and Poisson's ratio (V p , V s , and σ) in Kyushu. The most outstanding feature in this region is a Beppu-Shimabara Graben-parallel belt of low V p and V s anomalies at depths of the upper crust. We find that within the belt dV s /V s ≈ dV p /V p (<0) in marked contrast to relations in other low-velocity regions where |dV s /V s | > |dV p /V p |. 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. Key Points:• Seismic tomography was conducted for 3-D distributions of P and S velocities and Poisson's ratio (V p , V s , and σ) in the crust, Kyushu, Japan • The graben actively opening and producing earthquakes is characterized by low V p and V s but little σ anomaly in the upper crust • Water-saturated round pore formation and shear deformation associated with seismic slip equally contribute to graben formation Supporting Information:• Supporting Information S1

The Role of Fluids in the 2008 Ms8.0 Wenchuan Earthquake, China

“…The Sinian‐Paleozoic strata (a set of shallow marine shelf clastic sedimentary rocks) are constrained in <5‐km depth, whereas the low‐V Gaps A and B extend into >15‐km depth; therefore, the slow velocity in the deep part within the two gaps could not be interpreted by the rock compositions alone, which could be relative to fluid intrusion from great depth. Indeed, fluids rising from the middle‐lower crust are likely to intrude into a shallow weakened seismogenic layer either through thin pores (aspect ratio ∼0.01) or by cracks closing, opening, and extending due to high pore pressure accumulation along the western side of the LMSF (O’Connell & Budiansky, 1974; Pei et al., 2019; Rutter & Brodie, 1998; Wang, 2019; Wang Z. et al., 2006). The principal goal of this study was to reveal the role of fluids (including melting and hydrothermal sources) in the 2008 Wenchuan earthquake generation and its rupture process along the LMSF, and we focused on what and how fluids impact these issues.…”

“…Data show that, while pure melt conductivity is ∼1–10 S m −1 for dry rocks with various compositions, electrical conductivity dramatically increases hundreds of times if even 0.01% H 2 O is present at a temperature of 1000 K (Pommier et al., 2008). The fact that fluids are generally able to penetrate into either highly fracture‐damaged faults via thin embedded slippage planes or into rock pores even with aspect ratio of ∼0.1 by cracks opening and closing in the seismogenic layer has been revealed by previous studies (Chester et al., 1993; Sibson, 1992; Wang Z. et al., 2006, 2015b, 2019). To highlight the strong negative anomalies under the western side of the Wenchuan hypocenter, we plotted two vertical cross sections of Vp and Vs anomalies (in percentage) relative to the 1‐D initial velocity model across the 2008 Wenchuan hypocenter (Figure 10b).…”

“…Plots were plotted only when both dVs and dVp were <0. We determined R by minimizing S as follows (Wang Z. et al., 2019): where x n and y n are, respectively, the dlnVp and dlnVs values at the n th node. We evaluated the error associated with the estimates of R as follows: …”

“…In Figure 10b, the low‐V zone (e.g., low‐V1 zone in profile L1) in the upper crust was fitted by a straight line with R = 0.96 (upper plane in Figure 10a), whereas those in the middle‐lower crust (e.g., low‐V2 zone in profile L2) were fitted by a straight line with R = 1.30 (lower plane in Figure 10a). To identify the fluids associated with partial melting from the lower crustal ductile flow or hydrothermal/or aqueous sources in the upper crust (Takei, 2002; Wang Z. et al., 2019), we then searched for the condition in which R ranges (green shade area in Figure 10c corresponding to low‐V1 zone in 10b) between R o ‒ ε /4 and R o + ε /4 where R o = 0.96 and ε = 0.22. From this map view, gas is unlikely to be a stable phase of pore fluid through depths of the upper crust, and hydrothermal fluid/or aqueous water as the pore fluid can explain the observed R in the upper crust if the aspect ratio α of the pore is ∼0.1 (Figure 10c).…”

“…Plots were plotted only when both dVs and dVp were <0. We determined R by minimizing S as follows (Wang Z. et al, 2019):…”