The Significance of Long-Period Ground Motion at Regional to Teleseismic Distances From the 610-km Deep Mw 8.3 Sea of Okhotsk Earthquake of 24 May 2013

The 24 May 2013 earthquake beneath the Sea of Okhotsk (610 km,Mw8.3) producedsignificant ground motion across the whole span of the Japanese islands, from 1,300‐to 4,200‐kmepicentral distance. The largest shaking was concentrated along the back‐arc side of the subductionzone, which is the opposite of the normal pattern for deep earthquakes in the Pacific slab. Observationsfrom the dense Hi‐net and F‐net arrays across Japan show that the largest shaking in northernJapan (near 2,000‐km epicentral distance) was caused by near‐causticSwaves, with triplication ofupgoing and downgoing waves from the deep source and reflected waves from the 660‐km discontinuity.Three‐dimensionalfinite difference method simulations confirm that the antiwaveguide effect of thehigh‐wave speed slab is to push the zone of larger intensity 300 km farther to south than might beexpected. TheSwavefront distorted by the slab has near‐critical incidence at the free surface producinglarge sP and generating shear‐coupled PL (s‐PL) waves with period >3 s. With increasing epicentraldistance theSincident angle exceeds critical, then total sS reflection creates large ground motion atlarge distance (>3,000 km) and even farther (>6,000 km) with sSS. The propagation of sS, sSS linking tosS‐PL, and sSS‐PL wave trains is very efficient in continental structures with thicker crust. The feltreports at large (4,000–8000 km) distances from the 2013 Sea of Okhotsk earthquake can be explained bylengthy, long‐period ground motion in the continental environment with amplification in sedimentarybasins and in tall buildings.This study was conducted with support from Grantsin‐ Aid from the Japan Society of Promotion Sciences (17 K01322)

Significant P wave conversions from upgoing S waves generated by very deep earthquakes around Japan

An important component of the seismic wavefield at moderate epicentral distances from deep earthquakes comes from seismic waves that are radiated upwards from the source. For very deep events, there is a range of distances at which upgoing S can convert into P waves that travel in the crust or in the upper mantle as the sPn phase. For a 600-km-deep event, sPn becomes a precursor to S from about 8°, and can have significant amplitude if the source radiation pattern is favourable. These conversions to crustal P have a very similar travel time property to S, and interfere strongly with S to produce complex wavetrains on both vertical and radial components. Where the locus of conversion falls on thicker continental crust, S waves can be coupled into partially trapped P waves in the crust that produce a long-period shear-coupled PL (s-PL) wave. Such longer period phases generated by large, very deep earthquakes can make a major contribution to sustaining large ground motion for considerable distances from the source. Observations of three very deep (> 575 km) events around Japan demonstrate the range of propagation effects associated with S to P wave (sP) conversion that plays an important role in shaping the later part of the recorded seismograms. The influence of sP conversion on the observed seismograms and the development of the s-PL wave depend on the variation of crustal thickness along the path and epicentral distance, and particularly on the locus of the conversion zone and the properties of the crust at that location.This study was conducted with support from Grants-in-Aid from the Japan Society for the Promotion of Science (No. 17 K01322)

The effect of online exchanges via Skype on EFL learners’ achievements

[EN] This study examined whether direct communication with people from other countries using Skype or Line would affect students’ English test scores in listening and reading as well as the development of their curiosity concerning foreign cultures by comparing the data of an experimental group with that of a control group. The former group conducted online exchanges with foreign students, while the latter group did not. As many Japanese companies engaged in international business require high scores in the TOEIC test, which is one of the multiple-choice English tests of listening and reading often used to show each person’s English proficiency, universities in Japan are making efforts to improve their students’ scores in such an English test. Preparation classes for English tests have been offered. However, students have been likely to lose interest in learning English in the circumstances of this learning style. Results of the study indicated that although the aim of exchange activities is to foster students’ curiosity concerning intercultural matters, students experiencing online exchange with skype significantly raised their scores in TOEIC tests in listening and reading after a programme of synchronous exchanges with foreign students, compared with ones who did not experience such online exchanges.Furumura, Y.; Huang, H. (2019). The effect of online exchanges via Skype on EFL learners’ achievements. The EuroCALL Review. 27(2):13-27. https://doi.org/10.4995/eurocall.2019.11128OJS132727

Interactions of multi-scale heterogeneity in the lithosphere: Australia

Understanding the complex heterogeneity of the continental lithosphere involves a wide variety of spatial scales and the synthesis of multiple classes of information. Seismic surface waves and multiply reflected body waves provide the main constraints on broad-scale structure, and bounds on the extent of the lithosphere-asthenosphere transition (LAT) can be found from the vertical gradients of S wavespeed. Information on finer-scale structures comes through body wave studies, including detailed seismic tomography and P-wave reflectivity extracted from stacked autocorrelograms of continuous component records. With the inclusion of deterministic large-scale structure and realistic medium-scale stochastic features fine-scale variations are subdued. The resulting multi-scale heterogeneity model for the Australian region gives a good representation of the character of observed seismograms and their geographic variations and matches the observations of P-wave reflectivity. P reflections in the 0.5–3.0 Hz band in the uppermost mantle suggest variations on vertical scales of a few hundred metres with amplitudes of the order of 1%. Interference of waves reflected or converted at sequences of such modest variations in physical properties produce relatively simple behaviour for lower frequencies, which can suggest simpler structures than are actually present. Vertical changes in the character of fine-scale heterogeneity can produce apparent discontinuities. In Central Australia a ‘mid-lithospheric discontinuity’ can be tracked via changes in frequency content of station reflectivity, with links to the broad-scale pattern of wavespeed gradients and, in particular, the gradients of radial anisotropy. Comparisons with xenolith results from southeastern Australia indicate a strong tie between geochemical stratification and P-wave reflectivity.Part of this study was supported by JSPS KAKENHI Grant Number 26400443 to K.Y., with additional support from the AuScope AuSREM project. The Earth Simulator Center of JAMSTEC is thanked for providing CPU time on the Earth Simulator

Azimuthal Variation of Lithospheric Heterogeneity in the Northwest Pacific Inferred From Po/So Propagation Characteristics and Anomalously Large Ground Motion of Deep In-Slab Earthquakes

High-frequency oceanic Pn/Sn (Po/So) phases (>2 Hz) recorded at ocean bottom seismometers in the northwest Pacific display strong azimuthal variations in propagation characteristics. In the direction parallel to former Pacific plate motion (N30°W), seismograms show a gentle rise at the onset of Po/So followed by large, long spindle-shaped coda; Po has a low-frequency (<0.25 Hz) precursor and much delayed high-frequency signals, showing weak dispersion with frequency. For orthogonal propagation, the onset of Po/So rises sharply and bursts of Po reverberations in the seawater follow. These differences indicate a strong azimuthal dependence of the scattering waveguide effect of the oceanic lithosphere. Numerical simulations of seismic waves in three-dimensional heterogeneous structures reveal that much of the observed Po/So propagation variability can be explained by laterally elongated fine-scale heterogeneity in the oceanic lithosphere, with a correlation distance of 20 km in the direction parallel to the magnetic anomaly, and a much shorter correlation distance in the perpendicular and depth directions. The longer axis corresponds to the observed Pn/Sn-wavespeed anisotropy in the northwest Pacific, so the heterogeneity pattern was also developed during the formation and growth of the Pacific plate; competing processes produce different styles of fine-scale effects. The elongated heterogeneity distributions in the oceanic lithosphere are carried into the subducting Pacific slab allowing energy from deep-focus earthquakes to propagate to large distances, producing observations of anomalously large ground motions in specific directions. The behavior can be matched with three-dimensional simulation of high-frequency wave propagation with a heterogeneous Pacific slab.This study was conducted with support from Grants-in-Aid from the Japan Society of Promotion Sciences (No. 19H00807), and by the Earthquake Research Institute, the University of Tokyo (ERI JURP 2020-S-04) for using the Fujitsu PRIMERGY CX600M1/CX1640M1 (Oakforest-PACS) in the Information Technology Center, the University of Tokyo. Emeritus support to BLNK from the Research School of Earth Sciences, Australian National University is gratefully acknowledged

Propagation of distinct Love-wave pulses from regional to teleseismic distances in continental and oceanic environments

Surface waves are usually dispersive with long wave trains and steady decay of amplitude with distance. However, if the group velocity is nearly constant for a span of periods a strong pulse is produced that retains its amplitude for large distances. This situation arises for the fundamental mode of Love waves in the period band 40-500 s for crust and mantle structures with a positive gradient of S wave speed in the uppermost mantle. Such a distinct Love-wave pulse with limited dispersion observed at teleseismic distance is termed the G wave in honour of Gutenberg. The long-period G-wave pulse caused by large earthquakes carries a large amount of energy to substantial distances, with significant effects across the globe, for example event triggering. A similar G-type Love-wave pulse with a much shorter-period of 10-20 s is generated for crustal structures without thick sediment. Such pulses produce anomalously large ground displacement at near-regional distances with, for example an overestimate of surface wave magnitude. We investigate the generation and propagation mechanism of the G-type Lovewave pulses in the crust and upper-mantle with the analysis of observed strong motion records from the Mw 6.2 2016 Central Tottori earthquake and the Mw 9.0 2011 Off Tohoku earthquake in Japan, in conjunction with 3-D finite-difference simulation of seismic wave propagation and analysis of dispersion curvesThis study was conducted with support from Grants-in-Aid from the Japan Society for the Promotion of Sciences (Nos. 17K01322, 19H00807)

命令幻聴への服従行動に影響する認知的要因

学位の種別: 課程博士審査委員会委員 : (主査)東京大学教授 石垣 琢麿, 東京大学教授 岡ノ谷 一夫, 東京大学教授 丹野 義彦, 東京大学教授 石浦 章一, 東京大学准教授 四本 裕子University of Tokyo(東京大学