Shin'ichi Sakai

Bio: Shin'ichi Sakai is an academic researcher from University of Tokyo. The author has contributed to research in topics: Epicenter & Induced seismicity. The author has an hindex of 6, co-authored 10 publications receiving 183 citations.

Magma Migration from the Point of View of Seismic Activity in the Volcanism of Miyake-jima Island in 2000

Abstract: From June 26, 2000, an intensive earthquake swarm started under Miyake-jima Island, 180 km south off Japan. This swarm was closely related to the eruption of Miyake-jima Island, probably dominated by underground magmatic activity. The swarm spread toward the northwestern ocean region from Miyake-jima Island, in which a huge number of earthquakes (over 100, 000) including five large events of M>6.0 were detected over about two months. This earthquake swarm was the most active since we started seismic observations in the 1970's.Although there are some telemetered observation stations on the Izu volcanic islands, no offshore instruments were operated in the area of this earthquake swarm. To understand both the spatial and temporal changes of this activity, we conducted a series of ocean bottom seismometer observations. According to the variation in the seismic activity with time, we changed the array configuration of OBSs six times. Furthermore, real-time seismic observations were undertaken using a buoy-telemetering OBS system.Combining the OBS data with those of the island stations, very precise earthquake locations were determined. The epicenter distribution obtained strongly indicates a northwest-southeastern lineament. The vertical cross-section of the events shows two characteristic trends. Deeper (7- 13km) events are forming a very thin (2-km thick) plane, while shallower ones (< 7 km) show a much thicker distribution. These distribution patterns will provide important constraints on the physical mechanism for understaning magma migration.

Correlation between Coulomb stress imparted by the 2011 Tohoku-Oki earthquake and seismicity rate change in Kanto, Japan

Abstract: We show that the seismicity rate increase in the Kanto region around Tokyo following the 2011 Tohoku-Oki earthquake (M w9.0) was well correlated with the static increases in the Coulomb failure function ( ∆CFF) transferred from the Tohoku-Oki earthquake sequence. Because earthquakes in the Kanto region exhibit various focal mechanisms, the receiver faults for the ∆CFF were assumed to be reliable focal mechanism solutions of ̃3,000 earthquakes compiled from three networks (F-net, JMA network, and MeSO-net). The histograms of ∆CFF showed that more events in the postseismic period had positive ∆CFF values than those in the preseismic period (2008 April 1 2011 March 10). Among the 928 receiver faults showing the significant ∆CFF with absolute values≥ 0.1 bars in the preseismic period, 717 receiver faults (77.3 %) indicated positive ∆CFF. On the contrary, 1,334 (88.2 %) out of 1,513 receiver faults indicated positive ∆CFF in the postseismic period. We confirmed that the result is similar for the longer preseismic period, between 1997 October 1 and 2011 March 10. To test the significance of the difference in the distribution of ∆CFF between preseismic and postseismic periods, we used a Monte Carlo method with bootstrap resampling. As a result, the ratio of positive ∆CFF randomly resampled from∆CFF values in the preseismic period never exceeded 83.1%, even after 10,000 iterations. This supports the findings of Toda & Stein [2013]; however, our calculation is more reliable than theirs because we used a much larger number of focal mechanisms compiled from the three networks. It also proves that the static stress changes transferred from the Tohoku-Oki earthquake sequence are responsible for the changes in the seismicity rate in the Kanto region. Earthquakes of focal mechanisms with positive ∆CFF values drastically increased, while those with negative ∆CFFs showed no obvious changes except for immediately after the mainshock. This fault-dependent seismicity rate change strongly supports the contribution of the Coulomb stress transferred from the Tohoku-Oki sequence to the seismicity rate change in the Kanto region. Immediately following the mainshock, earthquakes of all types of focal mechanisms were activated, but the increased seismicity rate of earthquakes with negative ∆CFFs returned to the background level within a few months. This suggests that there might be other contributing factors to the seismicity rate change such as dynamic stress triggering or pore-fluid pressure changes.

Fault Slip Distribution of the 2016 Fukushima Earthquake Estimated from Tsunami Waveforms

Abstract: The 2016 Fukushima normal-faulting earthquake (Mjma 7.4) occurred 40 km off the coast of Fukushima within the upper crust. The earthquake generated a moderate tsunami which was recorded by coastal tide gauges and offshore pressure gauges. First, the sensitivity of tsunami waveforms to fault dimensions and depths was examined and the best size and depth were determined. Tsunami waveforms computed based on four available focal mechanisms showed that a simple fault striking northeast-southwest and dipping southeast (strike = 45°, dip = 41°, rake = −95°) yielded the best fit to the observed waveforms. This fault geometry was then used in a tsunami waveform inversion to estimate the fault slip distribution. A large slip of 3.5 m was located near the surface and the major slip region covered an area of 20 km × 20 km. The seismic moment, calculated assuming a rigidity of 2.7 × 1010 N/m2 was 3.70 × 1019 Nm, equivalent to Mw = 7.0. This is slightly larger than the moments from the moment tensor solutions (Mw 6.9). Large secondary tsunami peaks arrived approximately an hour after clear initial peaks were recorded by the offshore pressure gauges and the Sendai and Ofunato tide gauges. Our tsunami propagation model suggests that the large secondary tsunami signals were from tsunami waves reflected off the Fukushima coast. A rather large tsunami amplitude of ~75 cm at Kuji, about 300 km north of the source, was comparable to those recorded at stations located much closer to the epicenter, such as Soma and Onahama. Tsunami simulations and ray tracing for both real and artificial bathymetry indicate that a significant portion of the tsunami wave was refracted to the coast located around Kuji and Miyako due to bathymetry effects.

Seismic velocity structure in the source region of the 2016 Kumamoto earthquake sequence, Japan

Abstract: We investigate seismic wave velocity structure and spatial distribution of the seismicity in the source region of the 2016 Kumamoto earthquake sequence. A one-dimensional mean velocity shows that the seismogenic zone has a high-velocity and low-Vp/Vs ratio relative to the average velocity structure of Kyushu Island. This indicates that the crust is relatively strong, capable of sustaining sufficiently high strain energy to facilitate two large (Mj > 6.5) earthquakes in close proximity to one another in rapid succession. Three-dimensional tomography of the seismogenic zone around the source of the 2016 Kumamoto earthquake sequence yields Vp = 6 km/s and Vs = 3.5 km/s. Most large-displacement areas (asperities) of the Mj 7.3 event overlap with the seismogenic zone and the overlying surface layer. Aftershock seismicity is distributed deeper than the conventional seismogenic zone, which suggests decreased strength due to fluids or increased stress, both caused by coseismic slip.

The Mechanics Of Earthquakes And Faulting

Abstract: • Stability of steady frictional sliding, inertia and the quasi-static limit • work on ps 4; see course web site

Evidence from the ad 2000 Izu islands earthquake swarm that stressing rate governs seismicity

Abstract: Magma intrusions and eruptions commonly produce abrupt changes in seismicity far from magma conduits that cannot be associated with the diffusion of pore fluids or heat. Such 'swarm' seismicity also migrates with time, and often exhibits a 'dog-bone'-shaped distribution. The largest earthquakes in swarms produce aftershocks that obey an Omori-type (exponential) temporal decay, but the duration of the aftershock sequences is drastically reduced, relative to normal earthquake activity. Here we use one of the most energetic swarms ever recorded to study the dependence of these properties on the stress imparted by a magma intrusion. A 1,000-fold increase in seismicity rate and a 1,000-fold decrease in aftershock duration occurred during the two-month-long dyke intrusion. We find that the seismicity rate is proportional to the calculated stressing rate, and that the duration of aftershock sequences is inversely proportional to the stressing rate. This behaviour is in accord with a laboratory-based rate/state constitutive law, suggesting an explanation for the occurrence of earthquake swarms. Any sustained increase in stressing rate--whether due to an intrusion, extrusion or creep event--should produce such seismological behaviour.

Statistical model for standard seismicity and detection of anomalies by residual analysis

Abstract: A statistical point-process model is derived to describe the standard activity of earthquake occurrences by assuming that general seismicity is given by the superposition of aftershock sequences. The parameters are estimated ty the maximum likelihood method. Using the estimated model, the “residual point process” of the data is defined and used to find the anomalies which are included in the data set but not captured in the considered model for the standard seismicity. For instance, seismic quiescences can be measured quantitatively by using the residual process. Some examples are provided to illustrate such analyses. Furthermore, a time series of the magnitudes on the residual point process is considered, to investigate its dependence either on the time or on the history of the seismicity. By assuming the exponential distribution at each time and modelling of the b- value , we can examine such dependences and estimate them. Two practical examples are shown.

Pretender punishment induced by chemical signalling in a queenless ant

Abstract: Animal societies are stages for both conflict and cooperation. Reproduction is often monopolized by one or a few individuals who behave aggressively to prevent subordinates from reproducing (for example, naked mole-rats1, wasps2 and ants3). Here we report an unusual mechanism by which the dominant individual maintains reproductive control. In the queenless ant Dinoponera quadriceps, only the alpha female reproduces. If the alpha is challenged by another female she chemically marks the pretender who is then punished4 by low-ranking females. This cooperation between alpha and low-rankers allows the alpha to inflict punishment indirectly, thereby maintaining her reproductive primacy without having to fight.

Volcano deformation and eruption forecasting

Abstract: Recent advances in Global Positioning System (GPS), tilt and Interferometric Synthetic Aperture Radar (InSAR) have greatly increased the availability of volcano deformation data. These measurements, combined with appropriate source models, can be used to estimate magma chamber depth, and to provide information on chamber shape and volume change. However, kinematic models cannot constrain magma chamber volume, and provide no predictive