HR: 1330h
AN: S42C-0177    [PDF]
TI: Determination of Fault Planes and Rupture Velocities of Small Earthquakes in a South African Gold Mine: Constraints on Radiation Efficiency
AU: * Yamada, T
EM: takuji@rcep.dpri.kyoto-u.ac.jp
AF: RCEP, DPRI, Kyoto Univ., Gokasho, Uji, Kyo 611-0011 Japan
AU: Mori, J
EM: mori@rcep.dpri.kyoto-u.ac.jp
AF: RCEP, DPRI, Kyoto Univ., Gokasho, Uji, Kyo 611-0011 Japan
AU: Ogasawara, H
EM: ogasawar@se.ritsumei.ac.jp
AF: Fac. Sci. & Engr., Ritsumeikan Univ., 1-1-1 Noji Higashi, Kusatsu, Shi 525-8577 Japan
AU: Iio, Y
EM: iio@rcep.dpri.kyoto-u.ac.jp
AF: RCEP, DPRI, Kyoto Univ., Gokasho, Uji, Kyo 611-0011 Japan
AU: Kawakata, H
EM: kwkt@drs.dpri.kyoto-u.ac.jp
AF: DRS, DPRI, Kyoto Univ., Gokasho, Uji, Kyo 611-0011 Japan
AU: Ide, S
EM: ide@eps.s.u-tokyo.ac.jp
AF: Dept. of Earth and Planetary Science, Graduate School of Science, Univ. of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tok 113-0033 Japan
AB: Analyses of source processes of small earthquakes are important for investigating whether or not there are dynamic differences between small and large earthquakes. However, it is difficult to resolve details of the source of small earthquakes because close station spacing near the hypocenter and data with high sampling rates are necessary. Such observations of mining induced earthquakes are being carried out in a South African gold mine. Nine tri-axial borehole accelerometers were installed within 200 m along a 2,650-m-deep haulage tunnel in the Mponeng gold mine. Many seismic events (-2.7 $<$ M $<$ 3.3) were recorded with a sampling frequency of 15 kHz from February to October, 1996. In this study we focused on the rupture velocity, which is important for investigating characteristics of initiations, arresting mechanisms, and radiation efficiency of earthquakes. We carried out kinematic wave-form inversions for three larger events (M1.4, 1.1, and 0.8) that occurred within 200 m of the stations. First, we determined the velocity structure using arrival time data. Velocities of P and S waves were estimated to be 6.00 km/s and 3.83 km/s, respectively. Next, we determined focal mechanisms from amplitudes of P, SH, and SV waves. Finally we carried out kinematic wave-form inversions for both nodal planes of focal mechanisms assuming various rupture velocities to distinguish the fault plane and the best-fitting rupture velocity. We could determine the fault plane for all events because the model fit to the data for the fault plane was significantly better than for the auxiliary plane. Also, the deduced fault planes were consistent with the results of sub-event locations for these events by Yamada et al. (2002). On the other hand, we could not determine the rupture velocities with complete confidence. One problem is that in general, residuals are likely to be smaller by assuming higher rupture velocities. However, we can conclude that rupture velocities were not less than 50% of the S-wave velocity, on the basis that the slower rupture velocities could not explain wave-forms very well. Therefore, we conclude that rupture velocities of small earthquakes in the South African gold mine are almost the same as those of larger natural earthquakes. The radiation efficiency can be written as a function of the rupture velocity and becomes greater with increase of the rupture velocity. This study indicates that radiation efficiencies of small earthquakes in the South African gold mine are almost equal to those of larger natural earthquakes.
DE: 7209 Earthquake dynamics and mechanics
DE: 7215 Earthquake parameters
SC: Seismology [S]
MN: 2003 Fall Meeting