It is important to distinguish between natural earthquakes and those induced by CO₂ injection at carbon capture and storage sites. For example, the 2004 Mw 6.8 Chuetsu earthquake occurred close to the Nagaoka CO₂ storage site during gas injection, but we could not quantify whether the earthquake was due to CO₂ injection or not. Here, changes in pore pressure during CO₂ injection at the Nagaoka site were simulated and compared with estimated natural seasonal fluctuations in pore pressure due to rainfall and snowmelt, as well as estimated pore pressure increases related to remote earthquakes. Changes in pore pressure due to CO₂ injection were clearly distinguished from those due to rainfall and snowmelt. The simulated local increase in pore pressure at the seismogenic fault area was much less than the seasonal fluctuations related to precipitation and increases caused by remote earthquakes, and the lateral extent of pore pressure increase was insufficient to influence seismogenic faults. We also demonstrated that pore pressure changes due to distant earthquakes are capable of triggering slip on seismogenic faults. The approach we developed could be used to distinguish natural from injection-induced earthquakes and will be useful for that purpose at other CO₂ sequestration sites.

Fine-scale thermohaline structure within ocean column can be mapped seismically in the Kuroshio Current, off the Muroto Peninsula of Shikoku Island, Japan. In this paper, we present the application of automatic sound speed picking analysis to the multi-channel seismic reflection data acquired in a different period to estimate time-lapse sound speed distribution across the Kuroshio Current. This method is based on an optimal velocity trajectory solving by the eikonal equation with a finite-difference algorithm. In contrast to the seismic inversion technique, this automatic analysis enables us to obtain contrast sound speed profiles without heavy dependency on sound speed or temperature data directly measured at discrete locations. As a result, this method can visualise sound speed profiles of fine-scale thermohaline structure developed at interleaving or diapycnal mixing processes of different water masses in the Kuroshio Current. The images of all profiles mapped from automatic sound speed analysis distinguish water masses and their fine-scale internal structure such as cold and warm water eddies, thermohaline staircases and internal waves revealing acoustic contrasts at interfaces across where sound speed and temperature change. Applying our approach for individual seismic line acquired in different time-steps for 3D seismic data can provide time–space variant images of fine-scale thermohaline structure for studies of oceanographic processes as well as large-scale ocean current and climate systems.

It is important to distinguish between natural earthquakes and those induced by CO2 injection at carbon capture and storage sites. For example, the 2004 Mw 6.8 Chuetsu earthquake occurred close to the Nagaoka CO2 storage site during gas injection, but we could not quantify whether the earthquake was due to CO2 injection or not. Here we simulated changes of pore pressure during CO2 injection at the Nagaoka site and compared them with estimated natural seasonal fluctuations of pore pressure due to rainfall and snowmelt as well as estimated pore pressure increases related to remote earthquakes. We clearly distinguished changes of pore pressure due to CO2 injection from those due to rainfall and snowmelt. The simulated local increase in pore pressure at seismogenic fault area was much less than the seasonal fluctuations related to precipitation and increases caused by remote earthquakes, and the lateral extent of pore pressure increase was insufficient to influence seismogenic faults. We also demonstrated that pore pressure changes due to distant earthquakes are capable of triggering slip on seismogenic faults. The approach we developed could be used to distinguish natural from injection-induced earthquakes and will be useful for that purpose at other CO2 sequestration sites.