アイテム詳細

要旨

Seismic stations are operated 24 hours, recording groundmotions by various non-earthquake sources continuously.Meteorological phenomena and human activities areexamples of non-earthquakes. The rapid air expansion bylightning produces a shockwave that creates a cracksound, thunder. Thunder-induced seismic waves recorded atdense seismic stations in Seoul, South Korea are analysedfor inversion of thunder source spectra. Thethunder-induced seismic signals were well identified atdistances of < ~20 km. In the course of source-spectralinversion, the propagation and acoustic-to-seismiccoupling effects are counted. Direct acoustic-to-seismiccoupled seismic waves present apparent phase velocitiesof sound speed in atmosphere. Thunder-induced seismicwaves are dominant at high frequencies (>20 Hz). Thethunder-induced acoustic waves in the atmosphere areobtained by removing the acoustic-to-seismic couplingeffect and site-response effect from the observedthunder-induced seismic waves. The quality factors foracoustic wave attenuation in the atmosphere aredetermined. We determine acoustic thunder source spectraare determined by stacking the inverted acoustic spectra.Human activity is another major source of high-frequencyseismic noise. Analysis with seismic noises in threeconsecutive months of each year enables us to estimatethe noise levels without seasonal effects. The daytimeseismic noise-level changes in major cities of 11countries are assessed using the 3 month records fordecades. The annual seismic noise levels present strongcorrelations with gross domestic product (GDP),particularly with manufacturing and industrial GDP. Theseismic noise levels increase quickly with GDP in low GDPregions but slowly in high-GDP regions. The seismic noiselevels increased by 14 % to 111 % for 5-23 yr dependingon the economic conditions. The high-frequency noiselevel may be a proxy to present the economic condition.