HR: 0800h
AN: S11A-0994    [Abstracts]
TI: Low Stress Drop Swarm Events in the Yilgarn Craton, Western Australia
AU: Allen, T I
EM: trevor.allen@ga.gov.au
AF: Geoscience Australia, GPO Box 378, Canberra, ACT 2601 Australia
AU: Cummins, P R
EM: phil.cummins@ga.gov.au
AF: Geoscience Australia, GPO Box 378, Canberra, ACT 2601 Australia
AU: Leonard, M
EM: mark.leonard@ga.gov.au
AF: Geoscience Australia, GPO Box 378, Canberra, ACT 2601 Australia
AU: * Collins, C D
EM: clive.collins@ga.gov.au
AF: Geoscience Australia, GPO Box 378, Canberra, ACT 2601 Australia
AB: Since September 2001, the small rural community of Burakin, southwest Western Australia, has been at the focus of seismic activity in Australia. In the six month period following commencement of seismicity, some 18,000 events had occurred, the largest of which having a moment magnitude of M 4.6. At the onset of activity, Geoscience Australia made a concerted effort to deploy a temporary seismic network in the region. The primary objective of this network was to collect high-quality strong-motion data for use in attenuation studies. Levels of seismicity near Burakin have decreased significantly since the 2001-02 swarm, however the region continues to experience a few small earthquakes per month. Earthquake source and path parameters are evaluated for a subset of 67 earthquakes. The dataset comprises some 375 seismograph and accelerograph records for events of magnitude M 2.3-4.6, including strong-motion data for seven earthquakes of M 4.0 and greater recorded at hypocentral distances less than 10 km. Source parameters are evaluated from far-field displacement spectra. Average corner frequencies are typically quite low, chiefly ranging between 2-3 Hz for events M 3.0 and above. Given the small variability in corner frequency, stress drop is observed to increase with magnitude, from very low values of 0.04 MPa to 18 MPa for the largest events in the catalogue. The stress drops for lower magnitude events (M $<$ 4.0) are typically lower than those obtained for southeastern Australian earthquakes of similar seismic moment. Since corner frequency is not observed to vary significantly with seismic moment, it is thought that the spectral content of shallow, small swarm events and consequently, the stress drop, is characteristically different to that of isolated intraplate earthquakes. We suggest that the larger events may be faulting previously unfractured rock or healed fault asperities, while the smaller events are adjustment events or aftershocks and occur on recently faulted surfaces. The work described has provided a useful framework for the development of regional ground-motion relations for Western Australia and will enable a better understanding of the mechanisms driving intraplate seismicity.
DE: 7299 General or miscellaneous
DE: 7212 Earthquake ground motions and engineering
SC: Seismology [S]
MN: 2004 AGU Fall Meeting