HR: 1340h
AN: S43A-1052    [Abstracts]
TI: Apparent Stress and Rupture Speed of Small Earthquakes in a South African Gold Mine : Constraints on Fracture Energy
AU: * Yamada, T
EM: takuji@eps.s.u-tokyo.ac.jp
AF: Department of Earth and Planetary Science, Graduate School of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-0033 Japan
AU: Mori, J J
EM: mori@eqh.dpri.kyoto-u.ac.jp
AF: Earthquake Hazards Division, Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji, Kyoto, 611-0011 Japan
AU: Ide, S
EM: ide@eps.s.u-tokyo.ac.jp
AF: Department of Earth and Planetary Science, Graduate School of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-0033 Japan
AU: Kawakata, H
EM: kwkt@drs.dpri.kyoto-u.ac.jp
AF: Research Center for Disaster Reduction Systems, Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji, Kyoto, 611-0011 Japan
AU: Iio, Y
EM: iio@rcep.dpri.kyoto-u.ac.jp
AF: Research Center for Earthquake Prediction, Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji, Kyoto, 611-0011 Japan
AU: Ogasawara, H
EM: ogasawar@se.ritsumei.ac.jp
AF: Faculty of Science and Engineering, Ritsumeikan University, 1-1-1 Noji Higashi, Kusatsu, 525-8577 Japan
AB: Nine tri-axial borehole accelerometers were installed within 200 m along a 2,650-m-deep haulage tunnel in the Mponeng gold mine in South Africa. We analyzed the high sample rate recordings (15 kHz) to determine source parameters of small earthquakes in the mine. We analyzed radiated seismic energies and static stress drops of 28 earthquakes (0.0 < M < 1.4) that occurred within 200 m of the stations to investigate their apparent stresses. Apparent stresses of the 28 events were from 0.2 to 3 MPa and static stress drops were 0.71 to 29 MPa. These values are similar to those for larger (M > 6) earthquakes. To study the source processes, we also carried out multiple time-window waveform inversions for the five largest events (0.8 < M < 1.4) among the 28 earthquakes. From the inversion results, we could determine the fault planes for all five events and estimate the range of rupture speed. We can conclude that rupture speeds were faster than 2.5 km/s (65 % of the shear wave velocity). The radiation efficiency is written as a function of the rupture speed and becomes greater with increasing rupture speed. This study indicates that radiation efficiencies of small earthquakes in the South African gold mine are almost equal to those of larger natural earthquakes. We found that the source parameters (apparent stress, rupture speed, radiation efficiency, and static stress drop) did not largely differ from values for larger earthquakes. This suggests that the dynamic rupture processes of these small events were similar to those of the larger earthquakes.
DE: 7203 Body waves
DE: 7209 Earthquake dynamics (1242)
DE: 7215 Earthquake source observations (1240)
SC: Seismology [S]
MN: Fall Meeting 2005