Late Quaternary systematic stream offsets caused by repeated large seismic events along the Kunlun fault, northern Tibet

The Kunlun fault is one of the largest strike-slip faults in northern Tibet, China. In this paper, we focus upon the Kusai Lake–Kunlun Pass segment of the fault to understand the geomorphic development of offset streams caused by repeated large seismic events, based on tectono-geomorphic analysis of high-resolution satellite remote sensing images combined with field studies. The results indicate that systematic left-lateral stream offsets appear at various scales across the fault zone: Lateral offsets of small gullies caused by the 2001 M_w 7.8 Kunlun earthquake vary typically from 3 m to 6 m, meanwhile streams with cumulative offsets of 10 m, 25–30 m, 50–70 m, 250–300 m and 750–1400 m have resulted from repeated large seismic events during the late Quaternary. An average slip rate of 10 ± 1 mm/year has been estimated from the lateral stream offsets and ¹⁴C ages of alluvial fan surfaces incised by the streams. A three-dimensional model showing tectono-geomorphic features along a left-lateral strike-slip fault is also presented. The Kusai Lake–Kunlun Pass segment provides an opportunity to understand the relationship between geomorphic features produced by individual large seismic events and long-term geomorphic development caused by repeated large seismic events along a major strike-slip fault.     

Evaluation of change in subsurface thermal environment due to groundwater flow in the Tokyo Lowland,Japan

Information on the distribution of subsurface temperature and hydraulic heads at 24 observation wells in and around the Tokyo Lowland, the eastern part of the Tokyo Metropolitan area, were examined to make clear the relationship between groundwater and the subsurface thermal environment in the urban area. Minimums in temperature–depth logs due to subsurface temperature increasing at shallow parts were recognized in 21 wells. This fact shows subsurface temperature is affected by ground surface warming in almost all of this area. Deeper than minimums, where the effects of surface warming became relatively small, regional variation is observed as follows: high temperatures are shown in the central part to the southern part, and low temperatures shown in the inland to eastern part. The high temperature area corresponds to an area where the lower boundary of groundwater flow is relatively shallow. This area corresponds also to an area with severe land subsidence resulting from excessive groundwater pumping. It is considered that this high temperature area is formed by the effects of upward groundwater flow affected by hydrogeological conditions and pumping. On the other hand, a comparison between past data (1956–1967) and present data (2001–2003) revealed widespread decreasing temperature in the inland area. This is explained by downward groundwater flow based on an analysis of temperature–depth logs. This fact suggests that subsurface temperature is not only increasing from the effects of surface warming but also decreasing from the effects of groundwater environment change due to pumping.     

Empirical green's function corrected for source effect

One of the severe problems in the semi-empirical method for the prediction of strong ground motions is that there is no objective criterion for choosing empirical Green's functions. It is undesirable that synthesized strong ground motions are affected by the source process of an earthquake whose record is adopted as an empirical Green's function. Through the analysis of strong motion accelerograms of two aftershocks of the 1983 Japan Sea earthquakes, it is found that characteristics of the accelerograms are dependent on the moment rate function derived from teleseismic observations. A procedure is presented for removing the effect of the source process from observed strong motion accelerograms. The thus obtained empirical Green's function expresses approximately the impulse response of the medium between the earthquake source and the observation site.     

Solubility of FeO in (Mg,Fe)SiO3 perovskite and the post-perovskite phase transition

Phase relations in Mg_0.5Fe_0.5SiO₃ and Mg_0.25Fe_0.75SiO₃ were investigated in a pressure range from 72 to 123 GPa on the basis of synchrotron X-ray diffraction measurements in situ at high-pressure and -temperature in a laser-heated diamond-anvil cell (LHDAC). Results demonstrate that Mg_0.5Fe_0.5SiO₃ perovskite is formed as a single phase at 85–108 GPa and 1800–2330 K, indicating a high solubility of FeO in (Mg,Fe)SiO₃ perovskite at high pressures. Post-perovskite appears coexisting with perovskite in Mg_0.5Fe_0.5SiO₃ above 106 GPa at 1410 K, the condition very close to the post-perovskite phase transition boundary in pure MgSiO₃. The coexistence of perovskite and post-perovskite was observed to 123 GPa. In addition, post-perovskite was formed coexisting with perovskite also in Mg_0.25Fe_0.75SiO₃ bulk composition at 106–123 GPa. In contrast to earlier experimental and theoretical studies, these results show that incorporation of FeO stabilizes perovskite at higher pressures. This could be due to a larger ionic radius of Fe²⁺ ion, which is incompatible with a small Mg²⁺ site in the post-perovskite phase.     

Ecologic and taxonomic diversification in the Mesozoic brackish-water bivalve faunas in Japan, with emphasis on infaunalization of heterodonts

Mesozoic brackish-water bivalve faunas in Japan diversified in three steps: at the beginning of the Early Jurassic, Early and Late Cretaceous. The Hettangian Niranohama Fauna in northeastern Honshu represents the establishment of a heterodont-dominated brackish-water fauna that persisted until the early Late Cretaceous. No similar composition is known from the Triassic. The infauna consists mostly of non-siphonate and some short-siphonate heterodonts, while the epifauna is represented by diverse pteriomorphian families. In the Early Cretaceous Tetori Group in central Honshu, the long-siphonate heterodonts Tetoria (Corbiculidae) and the semi-infaunal soft-bottom oyster Crassostrea appeared. The evolutionary diversification of the latter, known as the most important element of modern brackish-water faunas, may thus originate at that time. In the early Late Cretaceous (Cenomanian) of the Goshoura and Mifune Groups in west Kyushu, several euryhaline deep-burrowing heterodont families, such as Veneridae and Tellinidae, further diversified in the brackish and marine environments. The Late Cretaceous is characterized by massive shell biolithic beds in which large Crassostrea species are common, a feature common for Cenozoic brackish-water faunas. The long-term changes in the composition of the brackish-water faunas in Japan represents thus an evolutionary record, irrespective of the severe physiological and environmental conditions imposed on the highly conservative nature of the fauna.