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Significant progress has been made towards the goal of generating detailed seismic images as an aid to mine planning and exploration at the Kambalda nickel mines of Western Australia. Crosshole and vertical‐seismic‐profiling instrumentation, including a slimline multi‐element hydrophone array, three‐component geophone sensors, and a multishot detonator sound source, have been developed along with special seismic imaging software to map rock structure. Seismic trials at the Hunt underground mine established that high frequency (> 1 kHz) signals can be propagated over distances of tens of meters. Tomographic as well as novel 3-D multicomponent reflection imaging procedures have been applied to the data to produce useful pictures of the ore‐stope geometry and host rock. Tomogram interpretation remains problematic because velocity changes not only relate to differing rock types and/or the presence of mineralisation, but can also be caused by alteration/weathering and other rock condition variations. Ultrasonic measurements on rock core samples help in assigning velocity values to lithology, but geological assessment of tomograms remains ambiguous. Reflection imaging is complicated by the presence of strong tube‐wave to body‐wave mode conversion events present in the records, which obscure the weak reflection signatures. Three‐dimensional reflection data processing, especially three‐component analysis, is time consuming and difficult to perform. Notwithstanding the difficulties, the seismic migrations at the Hunt mine show a striking correlation with the known geology. Combined seismic and radar surveying from available underground boreholes and mine drivages is probably needed in the future to more confidently delineate mineralisation.
A great earthquake occurred at 00:58:49 (UTC) on Sunday, December 26, 2004 off the northwest coast of Sumatra, Indonesia. Its revised moment magnitude was M 9.3 making it in the top four largest earthquakes in the world since 1900 and the largest since the Alaskan 1964 event. The earthquake caused tsunami waves which killed more than 300,000 people in Southern Asia and Africa. There were 31 earthquakes with magnitudes between 5.5 and 7.3 in the 48-h period after the main event, and it seemed that seismicity migrated northwards along the 1200 km fault (http://www.ga.gov.au). Similar size events occurred in that location off Sumatra in the 19th century, but no evidence of written records of their tsunami effects in Australia is found. The devastating megathrust earthquake of 26 December 2004 occurred on the interface of the Indo-Australian and Euro-Asian plates where the first plate subducts beneath the overriding second plate and the Indo-Australian plate begins its descent into the mantle. In the epicentral region, the Indo-Australian plate moves toward the northeast at a rate of about 7 cm/year relative to the Euro-Asian plate resulting in oblique convergence and partitioning into thrust-faulting. From the size of the earthquake, it is likely that the displacement on the fault plane was up to fifteen meters. As with the recent event, megathrust earthquakes often generate large tsunamis that cause damage over a much wider area than is directly affected by ground shaking near the earthquake's rupture. The subduction zone continues further south of the Indonesian archipelago and that area is also a potential risk of producing a megathrust event that may affect coastal parts of northwest Australia. The tragic events of Boxing Day 2004 highlighted the importance of establishing a tsunami warning system for the Indian Ocean like the one for the Pacific. Issues like more and better instrumentation, and a long-term program to educate people in the region about the dangers of tsunamis, were identified as priorities. Of particular interest is the time for identifying and issuing alerts for such devastating earthquakes with possibility to reduce it in future for warning purposes.
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