The effect of normal stress on the real area of contact during frictional sliding in rocks

The real area of contact during frictional sliding has been determined as a function of changing normal stress in triaxial experiments through the use of thermodyes. Utilizing the technique, described by Teufel and Logan in 1978, with saw-cut surfaces inclined 35° to the load axis, determinations were made for monolithologic sliding of Tennessee sandstone and Indiana limestone and dilithologic sliding of the same rocks. Confining pressures to 200 MPa were investigated at a constant shortening rate of 10^–2 mm/sec and at room temperature. Direct measurements were made of single-asperity areas and the asperity density. The product of these measurements gives the percent area of real contact across the sliding surface. Single-asperity area and density are found to remain relatively constant during the displacement. Single-asperity areas are in the ranges of 0.4 to 6×10^–2 mm² for sandstone, 0.8 to 2×10^–2 mm² for limestone, and 0.2 to 24×10^–2 mm² for sandstone sliding against limestone. These values are smaller than the grain size of either rock. The values increase with increasing normal stress for both monolithologic and dilithologic sliding. In sandstone the asperity density increases from about 0.8 to 2.75 contacts per square millimeter in a logarithmic fashion. Monolithologic limestone has values of about 0.9 contacts per square millimeter and does not show significant change with increasing normal stress. The percent area of real contact increases in all cases, with average maximum values of 16% of the apparent area at a normal stress of 374 MPa in sandstone, 18% at 25 MPa in limestone, and 22% at 123 MPa in the dilithologic specimens. The normal stress recalculated for the real area of contact approaches the unconfined compressive strength for sandstone and limestone.     

New terrestrial heat flow values from Hungary

Summary Temperature and conductivity measurements show, that in the Southern part of Transdanubia (the part of Hungary which lies Westwards from Danube) the heat flow is about 2–2.4·10^–6 cal/cm² sec. Eastward from the Danube, in the Hungarian Plain estimates are even higher, and vary between 2.3·10^–6 and 2.8·10^–6 cgs. The gradient of temperature is everywhere quite high, 5.0 resp. 5.8·10^–4 deg. C/cm on the average. Thus, at a depth of 1000 m, the virgin rock temperature is about 60–70 deg. C, at 2000 m about 110–130 deg. C.     

The effects of sliding velocity on the frictional and physical properties of heated fault gouge

The frictional properties of a crushed granite gouge and of gouges rich in montmorillonite, illite, and serpentine minerals have been investigated at temperatures as high as 600°C, confining pressures as high as 2.5 kbar, a pore pressure of 30 bar, and sliding velocities of 4.8 and 4.8×10^–2 m/sec. The gouges showed nearly identical strength behaviors at the two sliding velocities; all four gouges, however, showed a greater tendency to stick-slip movement and somewhat higher stress drops in the experiments at 4.8×10^–2 m/sec. Varying the sliding velocity also had an effect on the mineral assemblages and deformation textures developed in the heated gouges. The principal mineralogical difference was that at 400°C and 1 kbar confining pressure a serpentine breakdown reaction occurred in the experiments at 4.8×10^–2 m/sec but not in those at 4.8 m/sec. The textures developed in the gouge layers were in part functions of the gouge type and the temperature, but changes in the sliding velocity affected, among other features, the degree of mineral deformation and the orientation of some fractures.     

A fracture mechanics study of subcritical tensile cracking of quartz in wet environments

Load relaxation and cross-head displacement rate-change experiments have been used to establish log₁₀ stress intensity factor (K) versus log₁₀ crack velocity (v) diagrams for double torsion specimens, of synthetic quartz cracked on thea plane in liquid water and moist air.For crack propagation normal toz and normal tor at 20°C,K _Ic (the critical stress intensity factor) was found to be 0.852±0.045 MN·m^–3/2 and 1.002±0.048 MN·m^–3/2, respectively.Subcritical crack growth at velocities from 10^–3 m·s^–1 to 10^–9 m·s^–1 at temperatures from 20°C to 80°C is believed to be facilitated by chemical reaction between the siloxane bonds of the quartz and the water or water vapour of the environment (stress corrosion). The slopes, of isotherms in theK-v diagrams are dependent upon crystallographic orientation. The isotherms have a slope of 12±0.6 for cracking normal tor and 19.9±1.7 for cracking normal toz. The activation enthalpy for crack propagation in the former orientation in liquid water at temperatures from 20°C to 80°C is 52.5±3.8 kJ·mole^–1.A discussion is presented of the characteristics of theK-v diagrams for quartz.     

Thermal Structure of the Ionian Slab

We propose a thermal model of the subducting Ionian microplate. The slab sinks in an isothermal mantle, and for the boundary conditions we take into account the relation between the maximum depth of seismicity and the thermal parameter L_th of the slab, which is a product of the age of the subducted lithosphere and the vertical component of the convergence rate. The surface heat-flux dataset of the Ionian Sea is reviewed, and a convective geotherm is calculated in its undeformed part for a surface heat flux of 42 mW m^–2, an adiabatic gradient of 0.6 mK m^–1, a mantle kinematic viscosity of 10¹⁷ m² s^–1 and an asthenosphere potential temperature of 1300°C. The calculated temperature-depth distribution compared to the mantle melting temperature indicates the decoupling limit between lithosphere and asthenosphere occurs at a depth of 105 km and a temperature of 1260°C. A 70–km thick mechanical boundary layer is found. By considering that the maximum depth of the seismic events within the slab is 600 km, a L_th of 4725 km is inferred. For a subduction rate equal to the spreading rate, the corresponding assimilation and cooling times of the microplate are about 7 and 90 Myr, respectively. The thermal model assumes that the mantle flow above the slab is parallel and equal to the subducting plate velocity of 6 cm yr^–1, and ignores the heat conduction down the slab dip. The critical temperature, above which the subduced lithosphere cannot sustain the stress necessary to produce seismicity, is determined from the thermal conditions governing the rheology of the plate. The minimum potential temperature at the depth of the deepest earthquake in the slab is 730°C.     

Temporal and spatial variation on codaQ −1 associated with the North Palm Springs earthquake of July 8, 1986

A temporal and spatial change of codaQ ^–1 associated with the occurrence of the North Palm Springs earthquake of July 8, 1986 was studied by using 242 small local earthquakes in the vicinity of the mainshock. We found that the codaQ ^–1 of earthquakes which occurred before the mainshock was significantly higher than that of the aftershocks in the mainshock area while the codaQ ^–1 for the surrounding area remained almost constant throughout 1986. CodaQ ^–1 was determined separately for the lapse time windows of 10 to 20 sec. and 15 to 40 sec. for the period from 1981 to 1987. The result for the time window 10 to 20 sec. showed a peak in codaQ ^–1 before the time of mainshock at all frequencies. The peak appeared earlier at lower frequencies. There was no significant change in codaQ ^–1 for the time window 15 to 40 sec., probably because the change was restricted to a small area.     

Mid-latitude atmospheres,winter and summer

Summary Winter and summer Mid-Latitude (45^oN) atmospheres to 90 km, two of a family of nine atmospheres supplemental to the U.S. Standard Atmosphere (1962), provide information on atmospheric structure by seasons rather than the mean annual data shown in the Standard, which is described for reference. Principal data sources for constructing these atmospheres consisted of summaries of Northern Hemisphere radiosonde observations at stations near, 45^oN, and observations made from rockets and instruments released by rockets, from nearly a dozen Northern Hemisphere launching sites.Winter and summer temperature-height profiles begin with surface temperatures of –1° and +21 °C, respectively, and contain three isothermal layers: –58°C at 19 to 27 km in winter and –57.5°C at 13 to 17 km in summer; –7.5° and +2.5°C at 47 to 52 km; and –79.5 and –99°C at 80 to 90 km, respectively. The temperature-height curve for the U.S. Standard has a surface temperature of +15°C with isothermal regions at 11 to 20 km (–56.5°C), 47 to 52 km (–2.5°C), and 80 to 90 km (–92.5°C). In all three atmospheres, temperature gradients for various segments are linear with geopotential, height. Humidity is incorporated into the lowest 10 km of the Supplemental Atmospheres, whereas the Standard is dry. Figures and tables depict temperature, relative humidity, pressure, and density for winter and summer, and temperature, pressure, density, speed of sound, and dynamic viscosity for the U.S. Standard Atmosphere.The Supplemental Atmospheres are mutually consistent; zonal wind profiles, computed from the geostrophic wind equation and selected pressure heights, compare favorably with existing radiosonde and rocket wind observations.     

Magnetic properties,self-reversal remanence and thermal alteration products of smythite

Summary This study is a follow up of the investigation of some magnetic properties and metastability of greigite in samples obtained from Miocene claystones in the Kruné hory (Erzgebirge) Piedmont basins (Bohemia). Three different methods of upgrading the smythite were applied; the magnetic properties of the concentrates are compared. The thermal conversion of smythite sets in at 200°C while greigite converts at 250°C. The first intermediate products to be formed are iron sulphides, marcasite clearly dominating over pyrite and pyrrhotite. Apart from a Fe³⁺ sulphate with a composition of Fe₂(SO₄)₃, oxidation of these sulphides gives rise to -Fe₂O₃. The result of the subsequent decomposition of the mentioned sulphate is the formation of -Fe₂O₃, which retains the sulphate structure. The final product of the thermal decomposition at 800°C is -Fe₂O₃. In the smythite concentrate the conversion to Fe³⁺ sulphate and -Fe₂O₃ is about twice as intensive as in greigite. No direct conversion to -Fe₂O₃ was found. During the thermal process self-reversals of remanence were observed, in various samples as many as four reversals in the temperature interval from 340 to 590°C. The occurrences of self-reversals of remanence were only observed at high degrees of thermal demagnetization, of the order of 10^–2 down to 10^–3 in the temperature interval of sulphide origin (below 400°C), and of the order of 10^–4 down to 10^–6 in the temperature interval of Fe-oxides origin (above 400°C).Presented at the 3rd Conference on New Trends in Geomagnetism, Castle of Smolenice, Czechoslovakia, June 22–29, 1992     

The influence of plants on atmospheric carbon monoxide and dinitrogen oxide

It is shown by laboratory experiments and extensivein-situ measurements that higher plants (Vicia faba, Platanus acerifolia, Fagus silvatica, andPinus silvestris) produce carbon monoxide. The measurements were carried out under natural conditions with respect to the concentrations of O₂ and H₂O, and temperature. The CO₂- and CO-mixing ratios were varied in the ranges 350 to 530 ppm and 3–270 ppb, respectively. The CO-production rates were found to be light dependent with an average value per cm² of leaf area of 3×10^–13 g/sec for a radiation intensity of 5×10⁴ erg/cm² sec. The production rates are independent of the CO₂- and CO-mixing ratios employed in the test atmosphere. Considering the production rate of 3×10^–13 g/cm² sec to be representative for global conditions the total CO-production by plants is estimated to be 0.5–1.0×10¹⁴ g/year. In contrast to carbon monoxide atmospheric dinitrogen oxide is not influenced by plants in the same manner.     

Fault dimensions,displacements and growth

Maximum total displacement (D) is plotted against fault or thrust width(W) for 65 faults, thrusts, and groups of faults from a variety of geological environments. Displacements range from 0.4 m to 40 km and widths from 150 m to 630 km, and there is a near linear relationship betweenD andW ². The required compatibility strains (e _s) in rocks adjacent to these faults increases linearly withW and with and ranges frome _s=2×10^–4 toe _s=3×10^–1. These are permanent ductile strains, which compare with values ofe _s=2×10^–5 for the elastic strains imposed during single slip earthquake events, which are characterised by a linear relationship between slip (u) andW.The data are consisten with a simple growth model for faults and thrusts, in which the slip in successive events increases by increments of constant size, and which predicts a relationship between displacement and width of the formD=cW ². Incorporation of constant ductile strain rate into the model shows that the repreat time for slip events remains constant throughout the life of a fault, while the displacement rate increases with time. An internally consistent model withe _s=2×10^–5, giving repeat times of 160 years and instantaneous displacement rates of 0.02 cm/yr, 0.2 cm/yr, and 2.0 cm/yr when total displacement is 1 m, 100 m, and 10 km, and slip increasing by 0.5 mm with each event, gives a good approximation of the data. The model is also applicable to stable sliding, the slip rate varying with ductile strain rate and withW ².     

Seismicity and Seismic Hazard in Alexandria (Egypt) and its Surroundings

— Alexandria City has suffered great damage due to earthquakes from near and distant sources, both in historical and recent times. Sometimes the source of such damages is not well known. Seismogenic zones such as the Red Sea, Gulf of Aqaba-Dead Sea Hellenic Arc, Suez-Cairo-Alexandria, Eastern-Mediterranean-Cairo-Faiyoum and the Egyptian costal area are located in the vicinity of this city. The Egyptian coastal zone has the lowest seismicity, and therefore, its tectonic setting is not well known. The 1998 Egyptian costal zone earthquake is a moderate complex source. It is composed of two subevents separated by 4 sec. The first subevent initiated at a depth of 28 km and caused a rupture of strike (347°), dip (29°) and slip (125°). The second subevent occurred at a shallower depth (24 km) and has a relatively different focal parameter (strike 334°, dip 60° and slip 60°). The available focal mechanisms strongly support the manifestation of a complex stress regime from the Hellenic Arc into the Alexandria offshore area. In the present study a numerical modeling technique is applied to estimate quantitative seismic hazard in Alexandria. In terms of seismic hazard, both local and remote earthquakes have a tremendous affect on this city. A local earthquake with magnitude M_s = 6.7 at the offshore area gives peak ground acceleration up to 300 cm/sec². The total duration of shaking expected from such an earthquake is about three seconds. The Fourier amplitude spectra of the ground acceleration reveals that the maximum energy is carried by the low frequency (1–3 Hz), part of the seismic waves. The largest response spectra at Alexandria city is within this frequency band. The computed ground accelerations due to strong earthquakes in the Hellenic Arc, Red Sea and Gulf of Aqaba are very small (less than 10 cm/sec²) although with long duration (up to 3 minutes).     

Temperature of an active lava channel from spectral measurements,Kilauea Volcano,Hawaii

A narrow band spectroradiometer was used to determine the characteristic temperatures of a very active channeled lava flow for the phase 50 eruption of Pu'u 'O'o on the East Rift Zone of Kilauea Volcano, Hawaii. During the twilight of 19 February 1992, 14 spectra of this activity were acquired over a 51 minute interval [18.29 to 19.20 Hawaiian Standard Time (HST)], from which the thermal distribution of energy of two 18 m² areas, one near the center and one near the margin of the flow, may be investigated. A twocomponent thermal mixing model applied to the data taken of the center of the channel gave, in the most powerful instance (1.8x10⁵ W/m²), a crust temperature of 940° C, a hot component temperature of 1120°C and a hot radiating area of 60% of the total area. A simultaneous spectrum acquired near the channeled flow margin yielded a crust temperature of 586° C and a hot area of only 1.2% of the total area radiating at 1130° C. Average radiant flux densities recorded for the center of the lava channel (1.3x10⁵ W/m² average) are much greater than previous measurements of lava lakes (4.9x10³ W/m²) or recently emplaced lava flows (maximum of 7.2x10⁴ W/m²). The energetic nature of this eruption is shown by satellite measurements made at 02.33 HST on 22 February 1992 by the Advanced Very High Resolution Radiometer in Band 2 (0.72–1.10 m). These show the utility of using existing satellites with moderate resolution (1 km x 1 km pixels) and high temporal coverage (eight overpasses each day for Hawaii) as potential thermal alarms for rapidly assessing the hazard potential of large volcanic eruptions.     

Evaluation of gases,condensates, and SO2 emissions from Augustine volcano,Alaska: the degassing of a Cl-rich volcanic system

After the March–April 1986 explosive eruption a comprehensive gas study at Augustine was undertaken in the summers of 1986 and 1987. Airborne COSPEC measurements indicate that passive SO₂ emission rates declined exponentially during this period from 380±45 metric tons/day (T/D) on 7/24/86 to 27±6 T/D on 8/24/87. These data are consistent with the hypothesis that the Augustine magma reservoir has become more degassed as volcanic activity decreased after the spring 1986 eruption. Gas samples collected in 1987 from an 870°C fumarole on the andesitic lava dome show various degrees of disequilibrium due to oxidation of reduced gas species and condensation (and loss) of H₂O in the intake tube of the sampling apparatus. Thermochemical restoration of the data permits removal of these effects to infer an equilibrium composition of the gases. Although not conclusive, this restoration is consistent with the idea that the gases were in equilibrium at 870°C with an oxygen fugacity near the Ni–NiO buffer. These restored gas compositions show that, relative to other convergent plate volcanoes, the Augustine gases are very HCl rich (5.3–6.0 mol% HCl), S rich (7.1 mol% total S), and H₂O poor (83.9–84.8 mol% H₂O). Values of D and ¹⁸O suggest that the H₂O in the dome gases is a mixture of primary magmatic water (PMW) and local seawater. Part of the Cl in the Augustine volcanic gases probably comes from this shallow seawater source. Additional Cl may come from subducted oceanic crust because data by Johnston (1978) show that Cl-rich glass inclusions in olivine crystals contain hornblende, which is evidence for a deep source (>25km) for part of the Cl. Gas samples collected in 1986 from 390°–642°C fumaroles on a ramp surrounding the inner summit crater have been oxidized so severely that restoration to an equilibrium composition is not possible. H and O isotope data suggest that these gases are variable mixtures of seawater, FMW, and meteoric steam. These samples are much more H₂O-rich (92%–97% H₂O) than the dome gases, possibly due to a larger meteoric steam component. The 1986 samples also have higher Cl/S, S/C, and F/Cl ratios, which imply that the magmatic component in these gases is from the more degassed 1976 magma. Thus, the 1987 samples from the lava dome are better indicators than the 1986 samples of degassing within the Augustine magma reservoir, even though they were collected a year later and contain a significant seawater component. Future gas studies at Augustine should emphasize fumaroles on active lava domes. Condensates collected from the same lava-dome fumarole have enrichments ot 10⁷–10² in Cl, Br, F, B, Cd, As, S, Bi, Pb, Sb, Mo, Zn, Cu, K, Li, Na, Si, and Ni. Lower-temperature (200°–650°C) fumaroles around the volcano are generally less enriched in highly volatile elements. However, these lower-termperature fumaroles have higher concentration of rock-forming elements, probably derived from the wall rock.     

Crustal thermal models for South Siberia

Summary The results of geothermal research carried out in South Siberia (West-Siberian and Siberian Platforms, Altai-Sayan folded area and the Baikalian arched uplift zone) were employed to calculate the Earth's crustal temperatures. It is shown that temperature is a function of the heat flow value. The maps of the surface heat flow and the temperatures at the Moho discontinuity are presented and characteristic temperature-depth profiles were compiled for specific tectonic areas. The West-Siberian and Siberian Platforms, as well as the Altai-Sayan folded region are characterized by rather low heat flow, 42–50mW m ^–2 (1.0–1.2 µcal/cm ² s), and low temperatures at the Moho boundary.400–500 °C. The thermal conditions in the area of Lake Baikal are extreme, the heat flow reaches here100–142 mW m ^–2 (2.4–3.4 µcal/cm ² s), and the temperature at the crust's bottom may exceed1000 °C. The Moho-surface according to our calculations is nonisothermal.     

The effect of water on stress relaxation of faulted and unfaulted sandstone

A series of stress relaxation experiments have been carried out on faulted and intact Tennessee sandstone to explore the influence of pore water on strength at different strain rates. Temperatures employed were 20, 300 and 400°C, effective confining pressure was 1.5 kb and strain rates as low as 10^–10 sec^–1 were achieved. Most samples were prefaulted at 2.5 kb confining pressure and room temperature. This is thought to have secured a reproducible initial microstructure.The strength of the dry rock was almost totally insensitive to strain rate in the range 10^–4 to 10^–10 sec^–1. In contrast, the strength of the wet rock decreased rapidly with strain rate at rates less than 10^–6 sec^–1. Brittle fracture of the quartz grains which constitute this rock is the most characteristic mode of failure under the test conditions used.The experimental data are discussed in terms of the possible deformation rate controlling processes, and it is suggested that in the wet experiments at intermediate to high strain rates (10^–7 to 10^–4 sec^–1) the observed deformation rate is controlled by the kinetics of water assisted stress corrosion, whilst deformation at low strain rates (ca. 10^–9 sec^–1) is controlled by a pressure solution process.The results have implications for the rheology of fault rocks at depths of perhaps 10 to 15 km in sialic crust.     

Changes of Ozone Content at Middle Latitudes During Forbush Decreases in Cosmic Rays

The variations of total ozone at Alma-Ata (43°N, 76 °E) and ozone profiles obtained by balloon sounding at Tateno (36°N, 140°E), Wallops Island (38°N, 75°W) and Cagliari (39°N, 9°E) in the periods of Forbush decreases (FD) in galactic cosmic rays have been analysed. A decrease of total ozone was observed in the initial stage of the FD and an increase 10–11 days later. The average total deviations calculated using the superposed epoch method for 9 FD events are equal to 30 D. U. in the positive and to –18 D. U. in the negative phase. The changes of average ozone profiles, associated with 26 FD events, are more significant in the lower stratosphere and upper troposphere. The decrease of the partial ozone pressure at a height of 12–15 km is about 30 mb. These vertical variations of ozone coincide with the average changes of the respective temperature profiles. A cooling, on the average, of 3°C was observed at 12–15 km, and a heating of 4°C below this level.     

Monitoring Colima Volcano,Mexico, using satellite data

The Colima Volcanic Complex at the western end of the Mexican Volcanic Belt is the most active andesitic volcano in Mexico. Short-wavelength infrared data from the Landsat Thematic Mapper satellite were used to determine the temperature and fractional area of radiant picture elements for two January data acquisitions in 1985 and 1986. The 1986 data showed four 28.5 m by 28.5 m pixels (picture elements) whose hot subpixel components had temperatures ranging from 511–774° C and areas of 1.8–13 m². The 1985 data had no radiating areas above background temperatures. Ground observations and measurements in November 1985 and February 1986 reported the presence of hot fumaroles at the summit with temperatures of 135–895° C. This study demonstrates the utility of satellite data for monitoring volcanic activity.     

Mantle heat flow and geotherms for major tectonic units in central Europe

The results of seismic measurements along the deep seismic sounding profile VII and terrestrial heat flow measurements used for construction of heat generation models for the crust in the Paleozoic Platform region, the Sudetic Mountains (Variscan Internides) and the European Precambrian Platform show considerable differences in mantle heat flow and temperatures. At the base of the crust variations from 440–510°C in the models of Precambrian Platform to 700–820°C for the Paleozoic Platform and the Variscan Internides (Sudets) are found. These differences are associated with considerable mantle heat flow variations.The calculated models show mantle heat flow of about 8.4–12.6 mW m^–2 for the Precambrian Platform and 31 mW m^–2 to 40.2 mW m^–2 for Paleozoic orogenic areas. The heat flow contribution originating from crustal radioactivity is almost the same for the different tectonic units (from 33.5 mW m^–2 to 37.6 mW m^–2). Considerable physical differences in the lower crust and upper mantle between the Precambrian Platform and the adjacent areas, produced by lateral temperature variations, could be expected. On the basis of carbon ratio data it can be concluded that the Carboniferous paleogeothermal gradient was much lower in the Precambrian Platform area than in the Paleozoic Platform region.