A nonlinear spider spring with an initial displacement offset is used to suspend a mass with a coil or permanent magnet and to restore the force spring in order to miniaturize a borehole vertical seismometer with a low natural frequency. We investigate the response properties of the nonlinear spider spring, which remain unknown even though the seismometer adopting nonlinear spider spring is widely used. The relation between the initial offset and the spring constant is clarified for the first time: the spring constant is weakened with increasing initial offset. We confirmed that the flat spider spring has a nonlinear property (as does the spider spring with an offset), although it has previously been considered a linear spring. An analysis of limitations in the use of the seismometer revealed that the nonlinearity can be ignored if the seismometer is used under the condition of small displacement that is recommended by manufacturer.
Matsushiro earthquake swarm is a most well known and well studied earthquake swarm in the history of seismology. From viewpoints of geophysics, geology and geochemistry, various observations, field surveys and analyses had been made to reveal characteristics of the swarm. On the basis of these studies, several models had been proposed to explain why and how the swarm originated and developed. However, majority of studies had focused on the activity around the climatic stage of the swarm in 1966 and so far, studies on the initial stage of the swarm in mid 1965 had remained few due to lack of sufficient observation data. The exact area where the swarm was born had not been known and the swarm was vaguely believed to originate from the area around Mt. Minakami in former Matsushiro town (now belongs to Nagano city). Considering that the manner of initial development of swarm activity represents an important characteristic of the swarm earthquake, we tried to get more clear view about initial stage of the swarm activity in this study. We re-investigated seismograms obtained by routine observation and studied seismograms of temporal stations for the first time which had not been processed yet. By scanning analogue seismograms, we made a complete data set of S-P times with high precision for each station. Although the data set of S-P times from two stations are not sufficient for conventional hypocenter location, we were able to narrow down a possible source area of the swarm activity under the reasonable assumptions. By considering direction of initial motion of P waves and assuming local velocity model of the upper crust in the region and plausible focal depth of 4.5km of the swarm earthquake, we found that the area of swarm in the very beginning in August, 1965 is located about 4km north-east of the Matsushiro earthquake fault (MEF) in former Wakaho town near its border with former Matsushiro town. Size of the initial swarm area was 3-4km in diameter. While swarm activity in the initial region was gradually decaying in September new swarm activity appeared separately from the initial swarm area around southern and south-west part of the MEF. Activity in the new swarm area had been increasing and it was developed to more intense swarm after October, 1965 when establishment of temporal seismic station network of the Earthquake Research Institute, University of Tokyo enabled detailed hypocenter location. The new swarm area coincides with the area where large amount of ground water moved upward and was released on the ground surface in the climatic stage of the swarm. It was well known that source area of the swarm was split and expanded toward north-east and south-west after March, 1967 in its climatic stage. Present study on the initial development of swarm area suggested that characteristics of the Matsushiro earthquake swarm such as splitting and expansion of its source area toward northeast and southwest were inherent in their early stage of the activity in August and September, 1965.
We examined GEONET GPS data to clarify the contemporary deformation in and around the Echigo plain, Niigata prefecture where located in the Niigata-Kobe tectonic zone and the deformation zone of the eastern margin of the Japan Sea. Compressional strain in a direction of ESE-WNW is concentrated in a narrow zone where strain rate is about 0.2ppm/yr along the coast of the Japan Sea. This strain concentration zone which is about 25km wide accommodates more than 5mm/yr of compression and geographically corresponds to the Echigo plain. The zone significantly subsided whereas surrounding regions was uplifted. These characteristics of the deformation are concordant with that measured by conventional geodetic surveys spanned more than several decades. The strain rate observed by GPS does not change significantly during more than a decade in the strain concentration zone except for coseismic periods, which is contrast with a large temporal change of a strain rate in the eastern region affected by a subduction of the Pacific plate. We proposed a simple model to explain the characteristics of the deformation. The model consists of stable sliding on a deep part of reverse faults which are extended to the east and west rims of the Echigo plain. Fault geometry penetrating the crust and calculation with gravitational viscoelastic asthenosphere is essential to reproduce the observed subsidence in the strain concentration zone.
We present mathematical expressions to compute body-wave response function for dipping anisotropic multi-layer structure. The expression is derived on the basis of ray theory incorporating reflection-transmission coefficients on velocity boundary interfaces. By using the expression, synthetic seismograms are computed for some seismic structures consisting of dipping anisotropic layers. The resultant seismograms are transformed into receiver functions. We verify that the receiver functions are approximately in agreement with those synthesized by the layer-matrix method based on wave theory. As an example, we show receiver functions calculated for an anisotropic velocity structure of the subduction zone where the oceanic plate is obliquely descending.
The rupture process of the April 11, 2011 Fukushima Hamadori earthquake (Mj 7.0), which is one of the induced earthquakes by the 2011 Tohoku earthquake (Mw 9.0), is investigated using near-field strong motion waveforms. After the Fukushima Hamadori earthquake, distinct normal fault type surface ruptures were recognized in the source area. Those surface ruptures line up along the previously-known fault traces, Idosawa fault and Yunotake fault, which exist about 10km apart each other in echelon. We first apply the iterative decomposition method to confirm that these faults acted at the almost same instant whether or not. Additionally, relocation of hypocenters of aftershocks using double-difference method is performed. These results show that the two normal faults slipped with the delay of about 10 s. Two fault segments are assumed according to those results and the multi time window inversion method is applied to determine detailed rupture process on the faults. As the result, total moment of 1.1×1019Nm (Mw 6.6) and the maximum slip of 2.5m on the Idosawa segment are recovered. The result shows that the Idosawa segment acted first and the Yunotake segment slipped after that. Finally, the static Coulomb failure stress change (ΔCFF) on the Yunotake segment gained by the slip on the Idosawa segment is calculated. The ΔCFF in the large slip area on the Yunotake segment is positive and it suggests that the stress change gained by the Idosawa segment induced the slip on the Yunotake segment.
Review of the previous tsunami analyses for the 2011 Tohoku-oki earthquake indicates that the large slip of more than 40m was estimated on the plate interface near the Japan Trench. The plate interface near the Japan Trench was previously ruptured by the tsunami earthquakes such as 1896 or 1611 Sanriku tsunami earthquakes. Along the subduction zone off Sumatra, the 2004 great Sumatra earthquake ruptured the plate interface near the trench with a large slip amount of 29m. There were also tsunami earthquakes such as the 1907 Sumatra earthquake or 2010 Mentawai earthquake which ruptured the plate interface close to the trench. These evidences suggest that occurrence of the large earthquakes which rupture the plate interface near trench such as tsunami earthquakes is more common than that we thought previously. Variability of the great earthquakes along the plate interface makes us difficult to evaluate future great interplate earthquakes along subduction zones.