Surface rupturing earthquakes produced by intraplate active faults generally have long recurrence intervals of a few thousands to tens of thousands of years. We here report the first evidence for an extremely short recurrence interval of 300 years for surface rupturing earthquakes on an intraplate system in Japan. The Kamishiro fault of the Itoigawa‐Shizuoka Tectonic Line (ISTL) active fault system generated a Mw 6.2 earthquake in 2014. A paleoseismic trench excavation across the 2014 surface rupture showed the evidence for the 2014 event and two prior paleoearthquakes. The slip of the penultimate earthquake was similar to that of 2014 earthquake, and its timing was constrained to be after A.D. 1645. Judging from the timing, the damaged area, and the amount of slip, the penultimate earthquake most probably corresponds to a historical earthquake in A.D. 1714. The recurrence interval of the two most recent earthquakes is thus extremely short compared with intervals on other active faults known globally. Furthermore, the slip repetition during the last three earthquakes is in accordance with the time‐predictable recurrence model rather than the characteristic earthquake model. In addition, the spatial extent of the 2014 surface rupture accords with the distribution of a serpentinite block, suggesting that the relatively low coefficient of friction may account for the unusually frequent earthquakes. These findings would affect long‐term forecast of earthquake probability and seismic hazard assessment on active faults.
Flowering phenology of alpine plants is strongly determined by the timing of snowmelt, and the conditions of pollination of widely distributed plants vary greatly during their flowering season. We examined the reproductive success of the distylous alpine herb, Primula modesta, along the snowmelt gradient under natural conditions, and compared it with the result of artificial pollination experiments. In addition, the compositions and visit frequencies of pollinators to the flower of P. modesta were examined during the flowering period. The pin and thrum plants of P. modesta growing at the same site have an equal ability to produce seeds if a sufficient amount of legitimate pollen grains are deposited on the stigma surface. However, under natural conditions, their seed‐set success was often (even if not always) restricted by pollen limitation, and the functional gender of the pin and thrum plants biased to the female and male, respectively, associated with their growing sites. These variations were not ascribed to resource limitation nor biased morph ratio but to the seasonal changes in pollination situations, a replacement of pollinator types from long‐ to short‐tongued pollinators resulted in unidirectional pollen transfer from long stamens (thrum plants) to long styles (pin plants). The functional gender specialization may enhance the evolution of dioecy from heterostyly, but the severe pollen limitation may cause the breakdown of heterostyly into homostyly. To consider the evolutionary pathway of heterostylous plants, an accumulation of the empirical data is required demonstrating how phenological synchrony between plants and pollinators is decided and to what degree this relationship is stable over years, along with estimates of selection and gene flow in individual plants.
Crush zone structures are heterogeneous due to occurrence of fault steps. This paper provides a geological description of crush zones in the western part of the active Atotsugawa Fault, Hida City, Gifu Prefecture of Japan, to understand the crush zone structure in a compressional step of a strike-slip fault. By means of aerial photograph interpretation and regional distribution of crush zones, we recognize that a compressional step occurs in the Sangawara area, whereas in the Itani area a long strand of the Atotsugawa Fault with ENE-WSW trend is extended. Detailed geological observation suggests that shear planes with NNE SSW and NW SE strikes, oblique to the trend of the Atotsugawa Fault at high angles, are characteristically developed in the Sangawara area, whereas shear planes with ENE WSW strike dominate in the Itani area. The shear planes with NNE SSW and NW SE strikes in the Sangawara area can be a part of the composite planar fabric such as R surfaces which are dominantly developed within compressional steps. Based on the clay mineral composition including smectite and kaolinite, high density of sealed microcracks, and the KAr ages of fault gouge and andesite dike, fault breccia and gouge in the Sangawara area are mainly formed by fracturing at a shallow depth under c.a.after the dike intrusion during Oligocene to early Miocene time. Abstract
Refractive index measurements of volcanic glass shards with known hydration characteristics enable the discrimination of pumice samples from multiple historic eruption events of a single volcano. We analyzed mineral compositions, undertook a morphological classification of volcanic glass shards, and measured the refractive index of glass shards and orthopyroxene from products of the Bunmei, Anei, and Taisho eruptions of Sakurajima volcano. The refractive indices of hydrated and non-hydrated parts within each individual glass shard were measured separately. Concentrations of elements in the glass shards were determined using a femtosecond laser ablation-inductively coupled plasma-mass spectrometry technique, to compare elemental concentrations of the sampled glass shards. The analyses successfully discriminated the Bunmei, An-ei, and Taisho eruption products.Pumice layers in core samples obtained from the southern part of the Miyazaki Plain were identified as the Sakurajima-Bunmei tephra, based on the analyses described above and radiocarbon age data for coaly materials in the pumice layers. The distribution of the pumice layer suggests that the Sakurajima-Bunmei tephra reached the southern part of the Miyazaki Plain as a pumice fall deposit.
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