Research for Non-volcanic Deep Tremors by ushig Seismic Array and Electromapetic Active Monitoring

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An essential point of ACROSS (Accurately Controlled Routinely Operated Signal System), is a noise-robust observation system of acquiring the transfer function between a pair of signal source and receiver by means of continuous transmission of very accurately controlled sinusoidal signals of both seismic and electromagnetic waves. At present, two EM transmitters are being operated ; one at Horonobe in Hokkaido with a set of electric dipoles of 180mx10A continuously for more than 2 years, and another at Shizuoka with a larger dipole moment of 600mx20A for long distance transmission. In this report, the recent results obtained by Shizuoka transmission experiments are presented in order to determine the transfer function and to estimate the delay time of signal propagation.The essential factor for the observation system is to enable us to the acquire the high S/N transfer function data between the source and the receiving site of electromagnetic diffusion waves which are reflected back fro … More m the depth of-20 km just above the subducting plate boundary beneath the Tokai region. The observation system demanded is consistedof (a) the accurately controlled signal transmitters with a large dipole moment of-10^6 Am, (b) the accurately synchronized and sensitive sensor array, and (c) such a technology system that makes noise reduction at every aspect of data management. The most important targets of (c) are the spatial noise in the shallow layers and its temporal change originated from the environmental disturbances (e.g. rain falls leading to the change in conductivity distribution).As the first step of our approach towards realization of the actual monitoring the earthquake and volcanic fields in Tokai region, we have installed a large test transmitter (dipole moment=560mx20A) in Shizuoka University campus to work on the problems (a,b,c). In the trial observation in the frequency range 0.1-20Hz, the S/N of -5 for the transfer functions has been reached by one week stacking of the data acquired at the advanced observation system located up to 20 km from the transmitter. Whereas a number of technical innovations were made and are demanded further as will be shown in the presentation, the results were encouraging and appear as a substantial milestone towards the routine methodology for the active monitoring of our targets. Further we present a prospect towards the monitoring of 'water' in the coming Tokai Earthquake field located deep in the vicinity of contact between the Philippine Sea Plates and Asian Plates in Shizuoka prefecture. ^s a matter of course, we need more powerful transmitters and also an observation network designed specially for our approa.In contrast to EM ACROSS, in which the signal is diffusion wave with large attenuation and dispersion, seismic ACROSS appears easy to grasp the approach by ray-path concept in geometric optics of wave propagation. In addition, the seismic signal source at Toki has been operated already for almost 5 years continuously to accumulate a large amount of data containing the information on the temporal variation in the deep lithospheres. By analyzing seismic data from several Hi-net stations, we found around the Horai Hi-net station)HOUH) in Aichi prefecture shows fairly good S/N and preliminary studies of small seismic array observations around the HOUH indicate relatively strong later phases are clearly observed with slignificant heterogeneities. One of the latest topics is the start of a new seismic transmitter at morimachi(MRI/JMA) in cooperation with Shizuoka Univ.) just above the expected focal plane of Tokai Earthquake. Less