Seismic decoupling for explosions in spherical cavities

Summary A series of paired explosions in a salt mine near Winnfield, Louisiana, has been conducted to test a theory by Dr.A. L. Latter concerning seismic decoupling by underground cavities. The theory predicted a decoupling of about 100. Free-field and surface measurements from an explosion in either a 6-ft-or a 15-ft-radius spherical cavity were compared with similar measurements from a completely tamped explosion. Shot sizes were from 20 Ib up to a few tons. Surface measurements were made out to 100 km and covered the frequency range from 0.5 to 100 cps. The experiment confirmed that decoupling does occur. The actual decoupling factor as a function of frequency is presented and compared with theLatter theory.Presented at the international meeting of the International Union of Geodesy and Geophysics at Helsinki, July 27, 1960.     

Local earthquake quantification

The diversity of noise conditions, and, consequently, the diversity of types of seismographs that are found in different environments, make it unlikely that a single method of determining magnitudes for local earthquakes will be developed. In each area local studies must be carried out to determine the appropriate attenuation-distance function, taking into account known structural features. Each locally determined magnitude scale should be related to either a standard teleseismic scale, or preferably, to Richter's original local magnitude scale, M_L.     

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.