Tectonic implications of the large shioya-oki earthquakes of 1938

The tectonic processes taking place along the southern part of the Japan trench are discussed on the basis of the focal mechanism of the 1938 Shioya-Oki event which consists of the five large earthquakes of M_s = 7.4, 7.7, 7.8, 7.7 and 7.1. Detailed analyses of seismic waves and tsunamis are made for each of these earthquakes, and the dislocation parameters are obtained. The total seismic moment amounts to 2.3 · 10²⁸ dyn.cm. The five earthquakes are grouped into either a low-angle thrust type or a nearly vertical normal-fault type. These mechanisms are common with other great earthquakes of the northwestern Pacific belt, and can be explained in terms of the interaction between the oceanic and continental plates. The vertical displacement inferred from the seismic results is in approximate agreement with the precise level data over the period from 1939 and 1897. This agreement suggests that the rate of the strain accumulation at the preseismic time is very small in the epicentral area. Repeated levelings at the postseismic time reveal a large-scale recovery of the coseismic subsidence. The postseismic deformation is one-third to one-half of the coseismic displacement. The time constant of the recovery is estimated to be 5 years or less. This type of deformation may be a manifestation of viscoelasticity of a weak zone underlying the continent. The amount of dislocation, together with the longterm seismicity, suggests a seismic slip rate of about 0.4 cm/year, which is one order of magnitude smaller than that for the adjacent regions. This suggests that a large part of the plate motion is taking place aseismically in this region. The tectonic process now taking place in the southern Japan trench can be considered to represent a stage just prior to a complete detachment of the sinking portion of the oceanic plate.     

Quantification of major earthquake tsunamis of the Japan Sea

The size of major tsunamigenic earthquakes which occurred in the Japan Sea is quantified on the basis of seismic and tsunamigenic source parameters. The tsunami magnitude M_t is determined from the instrumental tsunami-wave amplitudes. The M_t values thus obtained are on average 0.2 units larger than the values of moment magnitude M_w, though the M_t scale has originally been adjusted to agree with M_w. Moreover, the volume of displaced water at the source is on average 2.3 times as large as that for the Pacific events with a comparable M_w. Nevertheless, the observed height of the sea-level disturbance at the source is found consistent with the amount of crustal deformation computed for the seismic fault models. These results indicate that the tsunami source potential itself is large for M_w in comparison with the Pacific events. The large source potential is explained in terms of the effective difference both in the rigidity of the source medium and in the geometry of the fault motion. For the Japan Sea events, the M_t scale still provides the physical measure of the tsunami potential, and M_t minus 0.2 corresponds to M_w. This predicts that the maximum amplitude of tsunami waves from Japan Sea earthquakes is at least two times as large as that from Pacific earthquakes with a comparable M_w.     

Static and dynamic fault parameters of the Saitama earthquake of July 1, 1968

The source mechanism of the Saitama earthquake (36.07°N,139.40°E, M_s = 5.4) of July 1, 1968, is studied on the basis of P-wave first motion, aftershock, long-period surface-wave data and low-magnification long-period seismograms recorded in the nearfield. A precise location of the aftershocks is made using P and S—P time data obtained by a micro-earthquake observatory network. The synthetic near-field seismograms based on the Haskell model are directly compared with the observed near-field seismograms for wave form and amplitude to determine the dynamic fault parameters. The results obtained are as follows: source geometry, reverse dip slip with considerable right-lateral strike-slip component; dip direction, N6°E; dip angle 30°; fault dimension, 10 × 6 km²; rupture velocity, 3.4 km/sec in the direction S30°E; average dislocation, 92 cm; average dislocation velocity, 92 cm/sec; seismic moment, 1.9 · 10²⁵ dyn-cm; stress drop, 100 bar. The effective stress is about the same as the stress drop. For major earthquakes in the Japanese Islands, the dislocation velocity, .D, is found to be proportional to the stress drop, σ. This relation can be expressed by .D - (β/μ)σ, where β is the shear velocity and μ is the rigidity. This result has an importance in engineering seismology because the stress drop scales the seismic motion in the vicinity of an earthquake fault.     

High nutrients in the coastal area after heavy rain observed in the central Seto Inland Sea in July 2012

Temporary enhancement of the nutrient concentrations in the coastal area was observed after heavy rain in the central Seto Inland Sea in July 2012. After passage of a stationary front accompanied by heavy rain, the river outflow was enhanced, and low salinity and high nutrient concentrations were detected near the mouths of rivers. The offshore salinity and nutrients increased and decreased, respectively, which suggested that a snapshot event, such as heavy rain, could influence the short-term variation of the coastal marine environmental conditions, such as the salinity and nutrient distributions.