SEISMIC SURVEY TO MONITOR THE MOHO

Press, Volume C, Issue 29635, 4 October 1961, Page 17

SEISMIC SURVEY TO MONITOR THE MOHO

Evidence of the depth of the "Mohorovicic discontinuity” below New Zealand may be discovered next year by the Geophysical Survey, when a seismic traverse is made from Tree Trurtk gorge, north of Waiouru, to Panui Island, in the Hauraki Gulf The Mohorovicic discontinuity, known as the “Moho," is the dividing surface between the rocks of the earth's mantle and the outer layer of the interior. It usually shows up quite distinctly on seismic traverses because the speed of the shock wave is in general markedly greater in the underlying region than the mantle. ■ The traverse will complete a project begun two years ago, when a traverse was made in the reverse direction. A double traverse is necessary for full exploitation of the method, to avoid the many ambiguities that Arise from a one-way traverse. The depth of the discontinuity is of great geophysical interest, since its general depth beneath the oceans is much less than under the continents—three to six miles, as against 20 to 30 miles—and it is not known for certain whether New Zealand is continental or oceanic in this respect. Magnetic surveys indicate that the Mohc may be at a depth of •bout 20 miles, but confirmation is needed. Seismic measurements so far have shown only that the discontinuity is ss 12 miles down. The work is expected to take about a year. A series of explosions is to be let off in the Tree Trunk Gorge and detected at stations 10 miles apart along the line to Panui Island. The measurements cannot all be made at once, since the strength of charge necessary for direction at the various stations increases progressively with distance. The explosives have been provided at a very low’ cost by the Army Department.

Measurements so far made indicate that, below the grey-w-acke and above the discontinuity, there is a layer of basaltic rock, and this is supported by evidence from magnetic surveys. When the project has been completed, a similar traverse will be made from Jackson’s Bay across the South Island to Dunedin. This will be the return half of an outward traverse made about a year ago.

At present, the survey is mapping the depth of the sedimentary basin around Taupo. The work has general importance to the geology of the North Island, but could be rewarding also in connexion with the Wairakei geothermal project.

The deepest sediments located so far are of the order of 5000 ft, but according to the superintendent of the Geophysical Survey (Mr N. Modrintak, there may be depths of 15.000 ft of sediment in the area around the Taupo township. Ute method of probing the sediments has been to measure the speed of the seismic shock wave in the undermass of greywacke rock by taking readings between Tree Trunk Gorge and Whakatane. and also from the gorge to Wellington, both lines being through almost uninterrupted greywacke. The speed through the greywacke has then been compared with the speed through the sedimentary areas to points in or on the other side of the basin.