5:15 PM - 7:15 PM
[SSS10-P14] Calcite-bearing cataclasite and non-double-couple earthquakes in the aftershock area of the 2000 Tottori-ken Seibu earthquake
Keywords:fault rock, tensile shear, geofluids, mineral vein
Generally, fault ruptures occur as shear that moves parallel to the fracture (mode II or mode III fractures), and the equivalent force couple for fault motion is a double couple, represented as slip along a single planar fault. However, non-double-couple(NDC) earthquakes, which cannot be explained by double couple mechanisms, have also been reported in volcanic regions and geothermal areas, suggesting the complexity of fault geometry and rupture processes and/or presence of fluids. Hayashida et al. (2020, GRL) reported NDC earthquakes with magnitudes of 1 to 3 in the aftershock area of the 2000 Tottori-ken Seibu earthquake, observed through hyper-dense seismic monitoring, and suggested that many of these could be explained by tensile-shear rupture mechanisms.
In this study, we analyze calcite-bearing fault rocks from the aftershock area of the 2000 Tottori-ken Seibu earthquake using geological methods to elucidate the relationship with NDC earthquakes. The fault rocks were collected from an outcrop on the western side of Ryokusuiko Lake in Nanbu-cho, Saihaku-gun, Tottori Prefecture (Aizawa et al., 2005, JGSJ; Suzuki et al., 2016, JpGU). The fault rocks consist of a consolidated zone (cataclasite zone) and an unconsolidated, continuous slip zone (fault gouge zone). Based on polished section and thin section observations, the fault rocks are divided into seven components (in order of formation): I. Ultracataclasite zone, II. Weakly foliated cataclasite zone, III. Phyllosilicates-bearing cataclasite zone (fragmented), IV. Fault-parallel calcite veins, V. Illite-bearing fault gouge, VI. Fault-perpendicular calcite veins, VII. Smectite-bearing thin gouge zone. I, IV, and VII exhibit structures indicative of seismic rupture or slip(implosion breccia and principal slip zone). The calcite associated with II, IV, and VI presents a coarse granular, polygonal texture without fibrous or elongation crystal structures. Since the host rock, granite, does not contain calcite, it is inferred that the calcite precipitated rapidly from fluids on fault planes opened during seismic events. The cumulative fault displacement is estimated to be ≧5.4 m from the offset of host rock, and number of deformation events are estimated to be seven from microstructural observations. Assuming a constant displacement per event, the lateral displacement per event is ≧0.77 m. The opening displacements for II and IV are up to 5 mm, yielding an opening to lateral displacement ratio of ≦0.006. This value consistent with the tensile angle reported by Hayashida et al. (2020, GRL) (approximately 0.2 to 6 degree, equivalent to an opening to lateral displacement ratio of about 0.003 to 0.1). Thus, the formation of calcite veins on fault planes could explain the occurrence of NDC earthquakes observed in this region.
In this study, we analyze calcite-bearing fault rocks from the aftershock area of the 2000 Tottori-ken Seibu earthquake using geological methods to elucidate the relationship with NDC earthquakes. The fault rocks were collected from an outcrop on the western side of Ryokusuiko Lake in Nanbu-cho, Saihaku-gun, Tottori Prefecture (Aizawa et al., 2005, JGSJ; Suzuki et al., 2016, JpGU). The fault rocks consist of a consolidated zone (cataclasite zone) and an unconsolidated, continuous slip zone (fault gouge zone). Based on polished section and thin section observations, the fault rocks are divided into seven components (in order of formation): I. Ultracataclasite zone, II. Weakly foliated cataclasite zone, III. Phyllosilicates-bearing cataclasite zone (fragmented), IV. Fault-parallel calcite veins, V. Illite-bearing fault gouge, VI. Fault-perpendicular calcite veins, VII. Smectite-bearing thin gouge zone. I, IV, and VII exhibit structures indicative of seismic rupture or slip(implosion breccia and principal slip zone). The calcite associated with II, IV, and VI presents a coarse granular, polygonal texture without fibrous or elongation crystal structures. Since the host rock, granite, does not contain calcite, it is inferred that the calcite precipitated rapidly from fluids on fault planes opened during seismic events. The cumulative fault displacement is estimated to be ≧5.4 m from the offset of host rock, and number of deformation events are estimated to be seven from microstructural observations. Assuming a constant displacement per event, the lateral displacement per event is ≧0.77 m. The opening displacements for II and IV are up to 5 mm, yielding an opening to lateral displacement ratio of ≦0.006. This value consistent with the tensile angle reported by Hayashida et al. (2020, GRL) (approximately 0.2 to 6 degree, equivalent to an opening to lateral displacement ratio of about 0.003 to 0.1). Thus, the formation of calcite veins on fault planes could explain the occurrence of NDC earthquakes observed in this region.