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[SSS07-10] Relocation of hypocenters just around the ICDP DSeis boreholes using the cross-correlation and the Double Difference
Keywords:Seismicity streaks, 2014 M5.5 Orkney earthquake, ICDP DSeis project, Double-Difference earthquake location
On August 5, 2014, an M5.5 earthquake (The Orkney Earthquake) occurred directly beneath the Moab Khotsong gold mine on the outskirts of Orkney, South Africa. Fifteen surface strong-motion seismographs (Council for Geoscience (CGS); continuous A/D conversion at 200 Hz) recorded the mainshock and aftershocks within an epicentral distance of a few tens of kilometers. At a depth of 2-3 km, several dozens of the mine's geophones A/D-converted at 6 kHz the seismograms for more than ten times the number of aftershocks within epicentral distances of several km. These indicated that the seismogenic zone was within the feasible range of drilling.
The International Continental Scientific Drilling Program (ICDP) approved the DSeis project (Drilling into Seismogenic zones in South African gold mines), which completed the full-core drilling with a total length of 1.6 km from three holes in 2017-2018.
Tadokoro et al. (2021 JpGU) have already reported the successful relocation of 2609 major aftershocks over an area of several kilometers from August 4 to October 31, 2014 using the Double-Difference method (DD; Waldhauser and Ellsworth, 2000). The relocation, with a resolution of tens of meters, elucidated the aftershocks consisting of "streaks" as reported, e.g., in studies of the San Andreas or Hayward faults (e.g., Waldhauser et al., 2000; Rubin et al., 1999).
The 3D seismic reflection survey suggests that there are some geological structures around the DSeis boreholes that disrupt strong seismic reflectors (Suzuki et al. JpGU 2022). However, around the DSeis boreholes, the larger aftershocks were not relocated without attempts of the smaller aftershocks relocation.
In this study, we used waveform cross-correlation method to accurately determine the difference in arrival times for the 1800 smaller aftershocks from August 2 to 20, 2014, in a 2 km x 1 km x 1 km volume around the DSeis holes.
The DD relocated smaller aftershocks elucidated streaks around the DSeis holes in accordance with the structures suggested by the disruption of the reflectors in the 3D seismic cube.
However, it turned out that the DSeis holes didn't intersect the aftershock cloud, even though the clouds were within a few hundred meters of each other.
This study is part of the activities of the DSeis team consisting of seismologists, geologists, geomicrobiologists, rock mechanists and mining engineers from Japan, South Africa, USA, Switzerland, Germany, India and Australia. ICDP, JSPS, US NSF, SA NRF, German DFG, Ritsumeikan Univ., MEXT 2nd Earthquake and Volcano Hazard Reduction Research, and Kochi Core Center have supported the project.
The International Continental Scientific Drilling Program (ICDP) approved the DSeis project (Drilling into Seismogenic zones in South African gold mines), which completed the full-core drilling with a total length of 1.6 km from three holes in 2017-2018.
Tadokoro et al. (2021 JpGU) have already reported the successful relocation of 2609 major aftershocks over an area of several kilometers from August 4 to October 31, 2014 using the Double-Difference method (DD; Waldhauser and Ellsworth, 2000). The relocation, with a resolution of tens of meters, elucidated the aftershocks consisting of "streaks" as reported, e.g., in studies of the San Andreas or Hayward faults (e.g., Waldhauser et al., 2000; Rubin et al., 1999).
The 3D seismic reflection survey suggests that there are some geological structures around the DSeis boreholes that disrupt strong seismic reflectors (Suzuki et al. JpGU 2022). However, around the DSeis boreholes, the larger aftershocks were not relocated without attempts of the smaller aftershocks relocation.
In this study, we used waveform cross-correlation method to accurately determine the difference in arrival times for the 1800 smaller aftershocks from August 2 to 20, 2014, in a 2 km x 1 km x 1 km volume around the DSeis holes.
The DD relocated smaller aftershocks elucidated streaks around the DSeis holes in accordance with the structures suggested by the disruption of the reflectors in the 3D seismic cube.
However, it turned out that the DSeis holes didn't intersect the aftershock cloud, even though the clouds were within a few hundred meters of each other.
This study is part of the activities of the DSeis team consisting of seismologists, geologists, geomicrobiologists, rock mechanists and mining engineers from Japan, South Africa, USA, Switzerland, Germany, India and Australia. ICDP, JSPS, US NSF, SA NRF, German DFG, Ritsumeikan Univ., MEXT 2nd Earthquake and Volcano Hazard Reduction Research, and Kochi Core Center have supported the project.