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A JERS 1 differential L-band synthetic aperture radar (SAR) interferogram of the March 26,1997 Kagoshima-ken-hokuseibu earthquake (Mw=6.1) in southwestern Japan shows about 9 cm peak-to-peak coseismic surface displacement in the radar line-of-sight (LOS) direction. A permanent GPS array detected 1 to 2 cm horizontal displacements from this earthquake. By inverting the SAR and GPS data together, we estimated a fault mechanism without any seismological data. A theoretical radar LOS displacement pattern from a single fault model of the earthquake motion matches the SAR and GPS observations closely. The model assumes left lateral slip of 0.46 cm with rake angle of 19° on a rectangular fault plane of dimensions 11 km (width) by 12 km (length). We demonstrate that L-band SAR interferometry can describe several cm surface displacement in detail and construct a fault model. However, despite the acquisitions being during the cold season, there are apparent water vapor signatures in the interferogram with equivalent path delays of up to 1.5 cm.

[1] Evolutionary history of Nojima Fault zone is clarified by comprehensive examinations of petrological, geophysical, and geochemical characterizations on a fault zone in deep-drilled core penetrating the Nojima Fault. On the basis of the results, we reconstruct a whole depth profile of the architecture of the Nojima Fault and identify the primal slip layer activated by 1995 Kobe earthquake. The deepest part (8- to 12-km depth) of the fault zone is composed of thin slip layers of pseudotachylite (5 to 10 mm thick each, 10 cm in total). Middle depth (4- to 8-km depth) of the fault zone is composed of fault core (6 to 10 m thick), surrounded by thick (100 m thick) damage zone, characterized by zeolite precipitation. The shallow part of the fault zone (1- to 4-km depth) is composed of distributed narrow shear zones, which are characterized by combination of thin (0.5 cm thick each, 10 cm in total) ultracataclasite layers at the core of shear zones, surrounded by thicker (1 to 3 m thick) damage zones associated with carbonate precipitation. An extremely thin ultracataclasite layer (7 mm thick), activated by the 1995 Kobe earthquake, is clearly identified from numerous past slip layers, overprinting one of the shear zones, as evidenced by conspicuous geological and geophysical anomalies. The Nojima Fault zone was 10 to 100 times thicker at middle depth than that of shallower and deeper depths. The thickening would be explained as a combination of physical and chemical effects as follows. (1) Thickening of “fault core” at middle depth would be attributed to normal stress dependence on thickness of the shear zone and (2) an extreme thickening of “damage zone” in middle depth of the crust would result from the weakening of the fault zone due to super hydrostatic fluid pressure at middle depths. The high fluid pressure would result from faster sealing with low-temperature carbonate at the shallower fault zone.

[1] The zircon fission-track analysis of pseudotachylyte answered how old the Nojima fault is, which was activated during the 1995 Kobe earthquake in Japan. The time of seismic fault motions at the crustal interior can be constrained by dating pseudotachylyte formation. We succeeded in separating zircons from the pseudotachylyte layer of the fault as well as in dating them by the fission-track method. Here we show that the layer has a significantly younger age (56 ± 4 Ma) than the initial cooling age of the host rock, suggesting that the fault was already initiated at ∼56 Ma, much older than the previous estimate based on the spatial distribution of sediments around the fault.

[1] To understand better heat generation and transfer along faults, fission track (FT) thermochronologic analysis was performed on zircon separates from Cretaceous granitic rocks around the Nojima fault, activated during the 1995 Kobe earthquake. Samples were collected from the Geological Survey of Japan (GSJ) 750 m borehole and nearby outcrop. Zircon FTs showed reduced mean lengths and ages for samples less than ∼10 m away from the fault at depth. This is interpreted as representing an ancient thermal anomaly by heat transfer or dispersion via fluids at the deep crustal interior because the degree of FT annealing is correlated to that of deformation/alteration of borehole rocks. From extrapolation of data in the mean FT age-length plot as well as from model age calculation, it is suggested that last cooling took place at ∼30–40 Ma.