P-wave velocities in the earth's core: An interim report

Recent observations of core phases made at conventional stations and seismograph arrays are summarized and evaluated to produce a series of conclusions concerning the P-wave velocity structure of the earth's core. Limits are suggested for allowable variations in P velocity in various parts of the core. The prime conclusion is that observations that previously demanded velocity discontinuities in the lower part of the outer core may now be explained adequately on a scattering hypothesis, and that in models where parametric simplicity is desired, the earth's core may be approximated by a two-layered model, with the P-wave velocity varying continuously in each layer.     

Seismological studies of the Raoul Island eruption, 1964

Raoul Island, the largest of the Kermadec Group, is about 8 km across and is situated about 1100 km north-east of Auckland, New Zealand. An eruption of steam and mud occurred on the island on 1964 November 21 (local date); this was preceded by an earthquake swarm which started on 10 November. Records made with the Willmore seismograph at the Meteorological Station on the north coast of the island show that within four hours of the start of the swarm, the frequency of shocks had risen to a peak of more than 80 an hour. Most shocks had sharp beginnings with S-P intervals of 1 to 2 seconds. Tremor was noticeable on 11 November and increased until by 12 November it was continuous, masking all but the largest of the discrete shocks. The level of tremor and the number of recorded shocks then decreased, until by the time the largest earthquake occurred on November 14, only 30 to 40 shocks were being recorded per hour. The largest earthquake was assigned a magnitude of 5.7 from recordings made in New Zealand, and was felt on Raoul Island at intensity 7 on the Modified Mercalli scale. Further shocks and tremor were recorded, and on 15 November the tremor was particularly active for several hours. On 21 November, an eruption occurred, throwing steam, mud, and rocks to an estimated height of 800 m from a crater on the edge of Green Lake, about 2 km from the Meteorological Station. During the eruption the seismograph recorded a peak vibration 30 to 40 times the amplitude of the normal background level. By two hours after the eruption, the level of vibration had stabilised at double that before the eruption. The island was evacuated from 23 November until 6 December, during which time the seismograph was inoperative. From 6 December to 11 December three portable seismographs were recording in addition to the permanent station. By this time the frequency of recorded shocks had dropped to about two per hour. The earthquakes were located in the vicinity of Denham Bay, some kilometres to the west of the main crater. Volcanic activity has been observed previously in Denham Bay, and it is thought that the Bay may be a former crater.     

Differential rotation of short-lived solar filaments

We have measured the rotation rate of short-lived solar filaments as a function of their latitude. The resulting rotation curve appears to be somewhat flatter than the corresponding curve for long-lived filaments.     

Freshwater requirements of a semi-arid supratidal and floodplain salt marsh

When rivers are impounded, the reduction in downstream flow can produce important and often adverse effects, especially in the estuarine environment. One or more dams have been proposed for the Olifants River system in the Western Cape, South Africa. This estuary has an extensive area of salt marsh that was examined to see whether it required occasional flooding with freshwater to wash out accumulated salts. The dominant salt marsh species,Sarcocornia pillansii, occurred in supratidal and floodplain areas where the water table was shallowest, the soil moisture highest, and the soil electrical conductivity lowest. Aerial photographs and simulated runoff data showed that no flood had covered the floodplain during the previous 80 years. The data indicate that salt marsh plants use saline groundwater during the dry months of the year in order to survive, but use the short season winter rainfall period with low salinity conditions to grow and reproduce. This study demonstrated that live roots ofS. pillansii reached the water table during the dry season. Tissue and soil water potentials, the relationship between vegetation cover, depth to the water table, and electrical conductivity of the groundwater support the conclusion that saline groundwater is the only source of water during the drier months of the year. Freshwater flooding of the river in winter may be important because it covers the supratidal area with less saline water and reduces the depth to the water table on the floodplain. This makes the groundwater more accessible to the halophytes growing on the floodplain.     

Long-term impacts of forest treatments on water yield: a summary for northeastern USA

Long-term changes in annual water yield are summarized and compared for 11 catchment studies in the northeastern USA. Substantial increases in water yield of up to 350 mm year⁻¹ were obtained in the first year by clearing forest vegetation and controlling regrowth with herbicides. Commercial clearcutting with natural regrowth resulted in initial increases in water yield of 110–250 mm year⁻¹. This range in response was due to differences in precipitation and configuration of cuttings. Unless regrowth was controlled with herbicides, yield increases declined quickly after cutting, seldom persisting for more than 10 years. However, yield increases were readily extended over 20 years or more with intermediate cuttings and/or repeated control of regrowth with herbicides. Nearly all increases in water yield occur during the growing season as augmentation of baseflow. Changes in species composition after forest cutting on several study catchments eventually resulted in decreased water yields compared with those from uncut, control catchments. Results are discussed in terms of implications for surface water supplies, global climate change, nutrient cycling, hydrological modeling, and long-term research.     

Measurement of biogenic sulfur emissions from soils and vegetation using dynamic enclosure methods: Total sulfur gas emissions via MFC/FD/FPD determinations

Metal foil collection/flash desorption/flame photometric detection (MFC/FD/FPD) was one of the analytical methods used to measure emissions of gaseous, sulfur-containing compounds from several terrestrial natural sources during a cooperative field program in the summer of 1985. Nonspeciated, total sulfur gas emissions were determined by using the MFC/FD/FPD technique in combination with a Nafion Perma-Pure drying device to sample air from three designs of dynamic enclosure chambers. These enclosures were placed over various soil orders and vegetation in the vicinity of field sites in Iowa and Ohio previously examined during the 1977–80 SURE study of biogenic sulfur fluxes. Because of the sensitivity and detection characteristics of the MFC/FD/FPD technique, it was possible to obtain measurements on enclosure air samples that were collected for relatively short time periods,. e.g., 1 to 5 min. The magnitudes of these time-resolved, total sulfur gas emissions are correlated exponentially with internal enclosure air temperatures. Potential errors and uncertainties associated with this application of the MFC/FD/FPD methodology are assessed.The total sulfur gas flux values obtained from this study and the SURE program are compared. Unquantified sources of error in the current two parameter extrapolation model used to calculate regional and global terrestrial source strengths of biogenic sulfur emissions are also summarized and are shown to prevent a reliable estimate of overall uncertainty limits in the resultant inventory.