Earthquakes in continental Antarctica

No earthquake had been definitely located in the interior of the Antarctic continent before that in Dronning Maud Land, which occurred on 1982 November 4, and was found by the “search” procedure of the International Seismological Centre. A further earthquake of similar magnitude (Mb 4.4) has now been located about 200 km inland from the coast of Wilkes Land, near 69°S, 123°E. The second event occurred on 1983 September 20, and was also found by the ISC search procedure, from readings at four Antarctic seismograph stations, and six farther afield. Reflected phases suggest that the focus of this earthquake is in the upper few kilometres of rock beneath the ice cap. Two later earthquakes occurred on 1984 May 19 about 400 km west of the event of 1983 September 20. They had magnitudes (Mb) of 5.0 and 4.6, and were large enough to be located by the US National Earthquake Information Service.     

Determination of contributors to coal seam structure in Sullivan County, Indiana

The structure of coal seams in Sullivan County, Indiana is subject to local variability controlled by differential compaction. Underlying Silurian reefs contribute to the maximum vertical variation and the underlying Mississippian unconformity surface controls the distribution of the most extensive and mappable structural features. The vertical magnitude, orientation, and lateral extent of the features should be considered during mine layout. Potential underlying Silurian reefs can be distinguished from other contributors to coal seam structure for the purposes of petroleum exploration.     

BLOCK ADJUSTMENT BY THE DIRECTION METHOD

The author explains a simple semi-graphic method of block adjustment of aerial triangulation from transformed strip co-ordinates.     

Sizing up the sub-Tommotian unconformity in Siberia

Sedimentary rocks in the western Anabar region, northwestern Siberia, preserve an exceptional record of evolution and biogeochemical events near the Proterozoic-Cambrian boundary. Carbon isotopic data on petrographically and geochemically screened samples collected at 1 to 2 m intervals support correlation of the lower Anabar succession (Staraya Reckha and lower Manykai Formations) with sub-Tommotian carbonates of the Ust'-Yu-doma Formation in southeastern Siberia. In contrast, the upper Manykai and most of the overlying Medvezhya Formation appear to preserve a sedimentary and paleontological record of an evolutionary important time interval represented in southeastern Siberia only by the sub-Tommotian unconformity. Correlation of the Anabar section with other northern Siberian successions that contain well-dated volcanic rocks permits the estimate that the sub-Tommotian unconformity in southeastern Siberia spans approximately 3 to 6 m.y. Diverse small shelly fossils (but not archaeocyathans) previously thought to mark the base of the Tommotian Stage evolved sequentially throughout this earlier interval.     

A SEMI-GRAPHIC METHOD OF ADJUSTMENT OF HEIGHTS IN A STRIP OF AERIAL TRIANGULATION

Some methods of adjusting the vertical height errors produced in a strip of aerial triangulation are discussed and a semi-graphic method of adjustment suggested.