Origin of Deep Ground Water in the Matsushiro Earthquake Swarm Area

Abstract

Many small earthquakes occurred in the Matsushiro area in a period from 1965 to 1967. This is called “Matsushiro Earthquake Swarm”. The swarm earthquakes are interpreted as a result of deep ground water intrusion into the focal area. At the hardest time of the crustal movement, water flow from pre-existing springs increased, and many new springs appeared along a hidden fault zone formed by the swarm earthquakes. Some of springs are still alive and have peculiar chemical compositions. The deep ground water must have caused the earthquake swarm. We show the characteristics of the deep ground water through analyzing chemical composition. Our measured data and many published data show that the deep ground water obtains Ca²⁺ from rocks and loses Na⁺ during its upwelling migration through the fault zone of Matsushiro Swarm Earthquakes. The data also show that the major components of Cl^- and (Na⁺+Ca²⁺) have a liner relation in concentrations, although every spring water has different concentrations of Cl^- and (Na⁺+Ca²⁺). It is interpreted in terms of mixing deep ground water with shallow ground water at defferent ratios. The concentrations of the major ions of the deep ground water are estimated from both of our measured data and compiled data using the liner relation, e. g., Na⁺: 310meq/l, Cl^-: 330meq/l. Analysis of our measured data also reveals that the small temporal variation in the concentration of Ca²⁺ in a short period such as several months is attributable to the reaction between the ground water with CO₂ and surrounding rocks.