Prefaceстатья из журнала
Аннотация: This special issue consists of contributions from the International Symposium on Slip and Flow Processes in and below the Seismogenic Region, which was held in Sendai, Japan at November 5 through 8, 2001.The papers presented in this special issue cover a variety of subjects regarding the processes at some depth corresponding to the base of the seismogenic zone:1.The geometry of the downward extension of the seismogenic faults in the lower crust 2. Loading process of inland earthquake source faults 3. Rheology of the rocks near the seismogenic region and shear localization 4. Fluids in and below the seismogenic region and their effects on the rheology 5. Faults, fluids and friction 6. Acceleration of slip motion in deep extensions of seismogenic faults.The symposium was mainly focused on the following working hypothesis that downward extensions of seismogenic faults in the lower crust accommodate the localized shear deformation which accumulates stress on the seismogenic faults before large intraplate earthquakes.This model can explain various phenomena related to the crustal deformation, i.e., preslips, seismicity, distribution of active faults, and stress and strain state in the Japanese Island and will play an important role on the forecasting and prediction of large intraplate earthquakes.There are several papers that cover deeper structure on a variety of faults in Japan, New Zealand, and US.Most of the studies used active sources (seismic reflection and refraction) to image faults at depth.Others used local and teleseismic earthquakes or a combination of the two or gravity data.The techniques for processing earthquake data include receiver function, array analysis, travel time tomography, and fault zone trapped wave.These results show that some faults, such as the Nagamachi-Rifu Fault, NE Japan, seem to flatten and merge to low-angle detachment in the middle crust.There are several sub-horizontal reflectors in the depth range from 12 to 15 km and high Vp/Vs ratio lower crust below the Nagamachi-Rifu Fault.In these cases, the faults extend below the seismogenic layer and are probably terminated within the middle crust.On the other hand, images from teleseismic receiver functions show that the San Andreas Fault, which is a large scale transform fault, penetrates the entire crust.New dense GPS networks have been established during the past several years across the active island faults in Japan, and case histories of a number of GPS transects are presented.These studies underscore the importance of a full understanding of earthquake deformation cycle for properly interpreting such results.These dense networks are typically comprised of densely spaced (2-5 km) continuous GPS sites that complement the permanent 1000 GEONET array.Inferred fault slip rates generally agree with the geologically constrained values.Modeling of the earthquake cycle deformation shows that evolution of the pattern and rate of deformation through the cycle depends on both constitutive laws for unstable fault slip and viscoelastic behavior of the sub-seismogenic lithosphere.The strength of the crust is understood based on a simple model, which utilizes the friction law of rocks in the shallower part of the crust and plastic flow law in the deeper part.This model assumes that rocks are uniformly deformed in the ductile region.However this model cannot explain the cut-off depth of seismicity in the intraplate environment.Therefore, a question arises of whether the deformation is localized under mid to lower crustal conditions.Natural shear zones suggest that the deformation is localized under these conditions.For example, the Hatagawa Fault Zone, NE Japan, which was formed at the brittle-plastic transition, shows clear shear localization.Several possibilities, such as shear heating and diffusion creep can be considered as a strain localization mechanism.Experimental study is important for understanding the elemental processes and rheological properties of the crust.Regional data such as temperature distribution is also important for understanding the environment of seismogenic zone.Electromagnetic studies can image fluids in the crust as high conductivity (low resistivity) anomalies and are useful in studying the fluid distribution and the fluid connectivity in the extension of the intraplate fault.The resistivity structure across the Itoigawa-Shizuoka tectonic line was studied, which is one of the most active intraplate faults in Japan.A mid-crustal conductor was found underneath the deformation zone.The conductor implies distribution of connected fluids, which are hosted by porosity-enhanced rocks in shear zones.Current seismicity clusters in the resistive regions near the resistiveconductive block boundary.This suggests the triggering the micro-earthquakes by fluid migration into less conductive (less permeable) crust.A deep crustal conductor was also found at a possible deep extension of the fault.Materials science and field-based structural approaches explore microscale to macroscale fluid distribution and effects of fluids on mechanical behaviour of rocks at elevated temperatures.The results of experimental studies revealed the effects of fluid pressures on the evolution of permeability in marble during progressive deformation.Field-based structural, microstructural and microchemical approaches in ancient exhumed shear zones were used to illustrate coupling between deformation processes, fluid pressures and chemical fluid-rock reaction in both high fluid flux shear systems and low fluid
Год издания: 2014
Издательство: Springer Nature
Источник: Earth Planets and Space
Открытый доступ: gold
Том: 54
Выпуск: 11
Страницы: 999–1000