Abstract
The earthquake detection capability of the Matsushiro Seismological Observatory was investigated based on seismograms of shortand long-period components of the World-wide Standard Seismograph in the period from January 1968 to December 1969. The operational constants are shown in Table 2. The results are summarized in the following.
(1) The total number of earthquakes used in this investigation is 8,423, which were registered in the Preliminary Determination of Epicenters, NOAA with assigned magnitude (MB). The frequency of occurrence of the earthquakes according to magnitude and epicentral distance from MAT is listed in Table 1. Of these earthquakes,4,946 shocks were detected by SP component, and 3,902 shocks, by LP component. The detection capability according to magnitude and epicentral distance is listed in Table 4. The corresponding capability at Tsukuba, but for a different period of time, is also listed in the same tabel for reference.
(2) The detection capability by SP is higher than that by LP except in the range of 90° ≤ Δ ≤ 109°, and the detected phase is mostly P or P'. Whereas the detection capability by LP depends mainly on S and L phases, except for local or distant but large earthquakes. Therefore, in the shadow zone of P phase, that is,90° ≤ Δ ≤ 109°, the capability by LP exceeds that by SP.
(3)I Compared with the Tsukuba Seismic Station, where the same investigation was carried out, Matsushiro shows a higher detectability for all magnitudes and distances. This is mainly because of a quieter background noise at MAT, which allows of higher operational sensitivity of seismographs, rather than because of any inherent characteristics of the site. No lower detectability in the winter is seen at Matsushiro as is the case at Tsukuba.
(4) When many aftershocks of a large earthquake occur at a near or intermediate distance (<40°), the detectability rises higher both in SP and LP.
(5) The world-wide detectability maps by SP and LP at MAT are given in Figs.4,5 and 6. Generally speaking, with an incremental detection probability of 90% the Matsushiro Seismological Observatory can detect body waves of earthquakes of 4¾ in MB magnitude over an epicentral distance of 90°. Such a probability of detect surface waves, however, deteriorates by about 10%.
(6) The Matsushiro Seismological Observatory is equipped with many other instruments and its observation window is very large both in dynamic range and frequency band.
