Expulsion of geopressured hydrothermal system along active faults and its relation to the occurrence of earthquakes in the Shinanogawa seismic belt, Japan

The 1995 Northern Niigata Earthquake (M 6.0) occurred at a shallow depth in the Niigata seismic gap. The anomaly areas in temperature, electrical conductivity and Cl- concentration of groundwater trend northeast as linear distribution in the epicentral area and are approximately coincident with the area of the seismic intensity 6 (JMA scale). The distributions of seismic intensity 6 and groundwater anomalies convincingly imaged the presence of a buried active fault beneath the epicentral area. The occurrence of this earthquake and the anomalies of groundwater were related to the expulsion of geopressured hydrothermal system (GHS). All epicenters of the destructive earthquakes along the Shinanogawa seismic belt are actually located in the buried active fault zones characterized by the areas of temperature and geochemical anomalies of groundwater. These earthquakes might have been triggered by the activity of GHS. The expulsion of GHS along an active fault in combination with the thermal softening of fault zone attributed to high rock temperature may reduce fracture strength of the rock, and trigger earthquake occurrence. The dimension of the anomaly area of groundwater temperature at the epicentral area reflected the scale of the earthquake fault. The linear anomaly areas of groundwater resulting from the expulsion of GHS and having no historical destructive earthquake are of the predicated areas of future destructive earthquakes. Monitoring of groundwater conditions in these areas may provide useful information regarding the future occurrence of earthquakes.

Research advances in the formation poredynamics of sedimentary basins

In the 21st century,the geodynamics is developing towards quantitative researches.However,due to the irreversible geological processes,it was very difficult to recover the geological process.In particular,the restoration of geological parameter evolution process at the microscopic scale has become a major scientific problem in geology presently.Thereby,a concept of the formation poredynamics is revised and proposed,and the formation poredynamics is a fundamental discipline which focus on the mechanical characteristic of porous media,the pore evolution law,the dynamic genesis and the seepage property of pore fluid during the burial process of clastic rocks.Moreover,it is a new interdiscipline of underground diagenetic dynamics and pore fluid dynamics,and also is as an important part of sedimentary basin dynamics.Research advances were made in both basic theory and applied research.The advances in the basic theory include:(1)the static equilibrium principle of the formation pore,(2)the porosity evolution mechanism and quantitative model of sandstone during the burial diagenetic process,(3)the compaction characteristic and the porosity evolution quantitative model of mudstone,(4)the theoretical relationship between the underground pore fluid temperature and the pore fluid pressure,(5)the influence of the tectonism-induced additional geostress on the pore fluid pressure,and(6)the relationship between the mudstone compaction and the vitrinite reflectance(R_(o))of organic matter.The advances in the applied research include:(1)the geotemperature-geopressure system division of the sedimentary basin and the interpretation of the hydrocarbon distribution dynamic,(2)the modification of the strata pressure prediction model,(3)the construction of the reservoir critical properties and the reservoir dynamics evaluation system,(4)the simulation of the evolution process of the formation fluid pressure,(5)the numerical simulation and physical experimental simulation on the sandstone hydrocarbon charging dynamics,and(6)the dynamic process analysis of the hydrocarbon accumulation in tight sandstone.Through the integration between the pore genesis evolution and the pore fluid dynamic evolution,the formation poredynamics is one of the representative discipline branches that the geological dynamics research had developed toward the underground microscopic scale in recently 20 years,and it also is an inevitable result from the quantitative development of the formation and distribution mechanisms of sedimentary mineral deposits.Based on the formation poredynamics research,eight important research achievements are summarized,and the geological researches are extended from the macroscopic scale to the microscopic scale,to find out the pore parameter evolution law under control of the formation pore evolution during the burial process,and update and improve exploration and production application technologies.