[G-03] Geoscience education from elementary school to university students

[G03-P03] Epicenter determination mapping using teleseismic records and verification of Omori's distance coefficient

*Yoshio Okamoto¹ (1.Kamnoetvidya Science Academy, Thailand)

Keywords:Omori's distance law, epicenter map, TauP , travel-time curve, win2

Omori's distance formula is used for finding epi-center exercises at the middle school level. At first, three sets of S-P times are determined from the seismograms recorded at three observation sites, respectively. Then, they are converted to epicentral distances using Omori's distance formula. Next, students draw three circles having a radius of epicentral distances of each observation site on the map. Finally, they find the epi-center at the crossing circles. This exercise is so fascinating to our students. However, after the digital revolution of recording systems, we can hardly get the local seismograms that are appropriate for classroom use. Although this difficulty, we have already installed my two seismograph systems; one for my house and another for the Thailand science high school, KVIS campus(Okamoto,2018). This system has about 3-5 second free periods pendulums and 3ch integrated amplifiers, similar to the JMA 59-type seismographs, having an advantage for both local and teleseismic waves. Therefore, we can use two seismograms in Japan and Thailand for an epi-center determination. One problem is that such seismograms are teleseismic records more than 1000km apart from the epi-center. Usually, Omori's distance formula must fail out at this long-range because the formula assumes the P-wave and S-wave velocities are constant everywhere beneath the underground. So, we made a conversion diagram between S-P times and far epicentral distances to extend Omori's formula for teleseismic distances. To this purpose, we employ software to caliculate seismic ray paths in the inner earth. This program, TauP (Crotwell et al.,1999), successfully calculates the P and S wave travel times for each epicentral distance. Then we derive S-P times vs. epi-distances to draw a conversion diagram. Using two seismograms recorded in Thailand and Japan, some epi-center finding practices achieved remarkable coincidences with the USGS epi-centers. However, TauP calculation results are somewhat confused for the short distances less than 3000km because multiple paths appear. And less than 1000km, the calculation is failed. Therefore, for the short distances, we use the win2 program (Ito, 2006) to make traveltime curves from paste-up seismograms recorded by Hinet(High Sensitivity Seismograph Network) in Japan. Then S-P times or Omori's coefficients versus distances are easily derived from the P and S travel times. So we can integrate the two results; 1) TauP results(>1000km) and 2) local distances(<1000km) on the diagram like bridging on a missing link. The final results are now compared with the real seismograms.