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[en] Whether large earthquakes are triggered by other large earthquakes at very large distances, and at time scales where the triggered event is delayed by more than several days or weeks, is an open question. Here I provide statistical evidence to show that large earthquakes near the subduction zones of the Pacific plate boundary lessen significantly the average time until the next large earthquake that occurs near the boundaries of the smaller and adjacent Nazca and Cocos plates in the eastern Pacific Ocean. In addition, the existence and importance of this relationship is enriched by the finding that the larger the magnitude of the Pacific plate earthquake, the more likely it becomes that the time until the next earthquake in the distant triggered zone is less than the time since the last earthquake in the triggered zone. There are also specific regions along the western and northern Pacific plates where these relationships appear to be particularly prominent. The findings call into question the notion that the hazard of large earthquakes in distant regions is not raised following other large earthquakes, and they provide statistical evidence for relationships between earthquakes occurring months apart on different plates.
[en] The Aki-Utsu method of Gutenberg-Richter (G-R) b value estimation is often misapplied so that estimations not using the G-R histogram are often meaningless because they are not based on adequate samples. We propose a method to estimate the likelihood Pr(bǀbm, N, M1, M2) that an observed bm estimate, based on a sample of N magnitudes within an [M1 − ≤ ΔM/2, M2 + ΔM/2) range, where ΔM = 0.1 is the usual rounding applied to magnitudes, is due to a “true” source b value, b, and use these likelihoods to estimate source b ranges corresponding to various confidence levels. As an example of application of the method, we estimate the b values before and after the occurrence of a 7.4-magnitude earthquake in the Mexican subduction zone, and find a difference of 0.82 between them with 100% confidence that the b values are different.
[en] On 11 March 2011, a giant interplate earthquake of magnitude Mw 9.1 struck Tohoku-Oki area in eastern Japan. The earthquake is acknowledged to have permanently altered the stress field in and around the focal region. To capture the temporal change of the overall stress field, I examined 1251 focal mechanisms, from the NIED MT catalog, that occurred before and after the Tohoku-Oki earthquake from January, 2006 to January, 2017 in the focal region near the subducting slab. The examined observations were grouped into six periods, and based on the selected NIED MT catalog, the stress regime in each period was obtained by using the damped stress tensor inversion method. Based on the temporal evolution of stress rotation, the corresponding deviatoric stress level was estimated using a simplified 2D model. Results of the 10-year seismic stress cycle show that several years before the Tohoku-Oki earthquake, the stress accumulation level seems to have experienced an acceleration process. Studies suggest that this increasingly critically stress state combined with the sufficiently reduced coupling rate off the Tohoku area finally resulted in the unprecedented megathrust event. The coseismic process was violent and released almost all of the deviatoric stress that presented before the main shock. The resultant stress state even reached frictional overshoot. Thus, the postseismic stress pattern in the source region was reshaped significantly, especially for the upper plate and updip portion of the lower plate. After the main shock near the rupture surface, a surprisingly rapid and high-level stress reloading occurred within several postseismic years. To reconcile the classical subduction zone earthquake generation cycle model, the event may be described as an instantaneously decoupled stress state between the upper and inner plates.
[en] This correction stands to correct Figure 7c listing a low minimum slip of 12 m for Case 2 instead of the correct value of 8 m, as stated in the body of the text and depicted on the chart of cumulative slip. The corrected chart explanation and caption are shown below. This error did not affect any of the findings of the paper or the chart itself. This is a correction to the original article.
[en] Complete text of publication follows. The Kyushu district is a typical high angle subduction zone in Japan, at which the Shikoku basin and the Philippine sea plate subduct beneath the Eurasian plate, and many quaternary active volcanoes, such as Aso, Kirishima and Sakurajima volcanoes, are located along the volcanic front in Kyushu. Network-MT observations, which use telephone line networks as long baseline telluric measurements (Uyeshima, 1990), were carried out in the Kyushu district from 1993 to 1998. We analyzed these data sets to determine regional scale electrical conductivity structure. As a preparatory step for three-dimensional imaging of the electrical resistivity structure beneath the Kyushu district, we applied several two-dimensional inversion analyses to the Network-MT impedance responses across the characteristic geology, tectonics and volcanoes. Here we used the REBOCC inversion code (Siripunvaraporn and Egbert, 1999), which adjusts appropriate the horizontal and vertical smoothing factors according to the intervals of the observation sites. In addition, we considered several tens kilometers electrode spacing for the measurement of voltage differences. And we were able to get much clearer overall resistivity structure to explain the observed Network-MT data set and have a rough grasp of the resistivity structure beneath whole Kyushu. One of the resistivity models, whose profile goes along around the Kirishima volcano group, we obtained a remarkable conductor beneath the Kirishima volcano which shows a good agreement with the previous result of ULF MT survey (Ichiki et al., 2000). Further, we found that the bottom of this conductor extends to the subducting Philippine Sea Plate. Then we carried out a three-dimensional inversion analyses to take account of the effects of the three-dimensional geographical features, especially, the ocean surrounding the Kyushu district. In this presentation, we would like to explain details of our reanalysis and obtained two-dimensional and three-dimensional models, and introduce the future direction of this study.
[en] The subduction related, island arc Pleistocene volcano of Solander Island in Foveaux Strait is composed of adakite, a new rock type first described from Adak Island in the Aleutian arc. Adakites are formed by the partial fusion of young oceanic crust under eclogitic facies conditions. Comparison with other adakite localities suggests that the oceanic crust presently subducting at the nearby trench may be <25 Ma old. (author). 23 refs., 5 figs., 2 tabs
[en] Complete text of publication follows. On 26 December 2004, a large earthquake of Mw=9 occurred in the Sumatra area located in the western part of subduction zone, Indonesia. The Sumatran subduction is where the convergence between the Indo-Australian plate and the Sundaland plate occur at 4-5 cm/yr. Several multidisciplinary studies have been involved to investigate the cause of the earthquake such as seismic and geodynamic studies. Through research collaboration among Nagoya University, Tokyo Institute of Technology, Bandung Institute of Technology and Syiah Kuala University, We were carried out Magnetotelluric and GPS surveys. Magnetotelluric measurement was done at 12 sites crossing the Sumatra fault and Seulimeum fault . Two components of horizontal electric field, two components of the horizontal magnetic field and one component of the vertical magnetic field were used in this survey. In this profile, we have carried out MT measurement along a 65 km long SW-NE profile. MT data have been modeled using two dimensional inversion including sea to became reality. The 2D final model has shown an extreme resistivity value has been found around the fault at a depth of 5 km from the surface. Western part of Sumatra fault is characterized by low resistivity. In contrast just beneath the Sumatra fault characterized by high resistivity. By comparison to GPS result, there is a good agreement between resistivity structure and relative motion based on GPS measurement.
[en] Subduction, the slow natural submersion of oceanic plates beneath continental plates of the earth, seems an ideal approach for permanent disposal of highly radioactive nuclear waste. A single borehole drilled into the Juan de Fuca plate off Vancouver Island, 3 ft in diameter and 1 km deep, would cost $21 million, and would only be half-filled with the total spent uranium fuel produced in Canada in one year, less so with true waste, with ample space for cladding and capping. The deposited waste would then submerge at 5cm/year with the plate underneath the Northamerican continent, untouched for millions of years. (author)
[en] We attempt to estimate the long-term probability of a Magnitude (M) 8 earthquake along the Sagami Trough in the Kanto subduction zone, central Japan. A Brownian passage time model is applied to sets of historical earthquakes identified in previous studies. An optimal model is obtained by the maximum likelihood method for each data set. The optimal parameters are not well constrained since each data set includes a small number of earthquakes. To obtain reliable probabilities, two weighting methods are introduced. First, we apply the weighted log-likelihood method, where the model parameters are estimated from the log-likelihood function, summed up with each log-likelihood weighted in proportion to the reliability of the data set. Second, probabilities are estimated as the weighted average of every alternative model. The weight of each model represents the normalized relative likelihood of the model. The probabilities of the weighted log-likelihood function are within the ranges of those obtained for each set. In averaging the proposed sequence and over probable parameter values, the probability of an M 8 earthquake occurring in the next 30 years is estimated to be 2.0 to 4.6%, depending on the cutoff value of the weight.
[en] In order to explain the seismic–aseismic slip patterns observed on megathrust faults, numerical simulations were carried out using the quasi-dynamic asperity fault model with the slip-dependent friction and stress-dependent healing. Two friction law parameters, strength and slip-weakening distance, are interpreted in the subduction channel context. The parameters are treated as random fields with specified characteristics. Their distributions define heterogeneities of the interplate frictional coupling. The higher strength regions accumulate stresses, whereas the slip-weakening distance lengths control the stress release rates. The simulation results indicate that the slip-dependent asperity model reproduces key features of real megathrust fault behavior. First, stable and unstable slip movements can occur at the same locations, even if the friction parameters are fixed. Second, two rupture styles, single asperity breaks and wide, smooth, propagating rupture fronts, can be distinguished. The latter style is responsible for large slips near the free surface, where lower fault strengths are expected. The reason for these effects is that slip instabilities depend both on local friction and on the system stiffness, which is related to the slipping area size and distribution of slips. It is also shown that the high-strength interplate patches, such as subducted seamounts, can both promote and restrain large earthquakes, depending on the slip-weakening distance lengths.