摘要:

We have made a statistical study of the spatial distribution of low frequency waves (∼0.01–0.1 Hz) in the region upstream of the pre‐dawn to dawn side bow shock (−50 Re

ISSN:0094-8276    

DOI:10.1029/94GL02918

年代:1995

数据来源: WILEY

摘要:

Past and recent observations indicate the presence of lightning at altitudes in excess of 30 km. The phenomenon is manifested as a high altitude optical flash, correlated with the presence of giant thunderstorms in the atmosphere below. This letter presents the first physical model of the process. The model is based on low frequency RF breakdown of the upper atmosphere, ignited by the upward propagating electromagnetic pulses due to conventional low altitude lightning. Horizontal intercloud lightning strokes form the optimal configuration. Horizontal lightning discharges with cloud‐to‐cloud moment charge ∼ 6,000–8,000 C‐km account for the observed level of optical

ISSN:0094-8276    

DOI:10.1029/94GL02733

年代:1995

数据来源: WILEY

摘要:

We have determined the black carbon concentration in three sets of ice core samples from the GISP2 (Greenland Ice Sheet Project 2) ice core. The major peaks in black carbon concentration between 320 and 330 A.D. correlate well with peaks in ammonium concentration and with the dips in electrical conductivity measurements (ECM), which allows us to identify extensive forest fires in this time period. The average black carbon concentration during the 320 to 330 A.D. decade is found to be 2.1 µg of black carbon per 1 kg of snow melt water. The current snow (1989 and 1990) from the GISP2 site shows an average black carbon concentration of about 2.0 µ/kg suggesting that the rate of black carbon deposition at the GISP2 Greenland site during 1989–1990 was about the same as 1670 years

ISSN:0094-8276    

DOI:10.1029/94GL02841

年代:1995

数据来源: WILEY

摘要:

Gas transfer velocities for two gases, SF6(sulfur hexafluoride) and ³He, were determined in a small pond by injecting a mixture of these gases into the water and monitoring the decline of their concentrations over the next eight days. For wind speeds between 1.5–2.5 m s−1, no variations of gas transfer velocity with wind speed could be resolved with our data. Gas transfer velocities at wind speeds greater than 3 m s−1were substantially larger and consistent with other lake tracer experiments. From the ratio of gas transfer velocities for SF6and3He we calculated the Schmidt number exponent to be 0.5

ISSN:0094-8276    

DOI:10.1029/94GL02410

年代:1995

数据来源: WILEY

摘要:

Altimeter profiles of the ERS‐1 and Topex‐Poseidon satellites have been used to compute a geoid surface from which we have extracted medium‐wavelength geoid anomalies over the central Pacific. In this region, the geoid shows prominent elongated anomalies of 20–40 cm in amplitude, with a spacing of ∼1000 km and oriented N60°W. In the south central Pacific, the Polynesian hotspot swells seem to be located on the linear geoid highs. However, the latter extend much farther eastward, preceeding the active hotspots. To the north, other geoid lines are visible but they do not coincide with known tectonic features. The wavelet transform applied to raw geoid data clearly detects a strong signal at the 1000–1200 km wavelength. The amplitude of the lineations increases with age, by a factor of 3 between 10 and 50 my. Analysis of seafloor topography corrected for age and sediments reveals topography anomalies correlated with the geoid lineations. Admittance and coherence calculations give high coherence (0.9) in the 1000–1200 km waveband and admittance values of 1.5 m/km at 10 my and 3 m/km at 60 my. The mechanism producing the lineations is unclear. Their characteristics however are not incompatible with a con

ISSN:0094-8276    

DOI:10.1029/94GL02858

年代:1995

数据来源: WILEY

摘要:

Long‐period (1–10 sec) ground motions recorded in the Los Angeles basin region during the Northridge earthquake show complex waveforms, extended durations and multiple sets of arrivals which cannot be attributed solely to source processes or wave propagation within a plane‐layered medium. These features suggest a strong interaction between the propagating seismic wave field and the laterally varying subsurface geologic structure of this region. Recorded motions at a hard rock site in the Santa Monica Mountains (scrs) are smaller by nearly a factor of three in peak velocity compared to recordings at more distant sites in the adjacent, northwest portion of the Los Angeles basin. In addition, although the rock site recording has a relatively simple wave shape, the basin site recordings are dominated by late arriving, large amplitude pulses of energy. We interpret these arrivals to be surface waves, which are generated by body waves interacting with the thickening margin of the basin. A preliminary modeling analysis of these data indicates that a combination of both large‐scale (deep basin) and small‐scale (shallow micro‐basin) structures are needed to explain the observe

ISSN:0094-8276    

DOI:10.1029/94GL02894

年代:1995

数据来源: WILEY

摘要:

We use regional broadband seismograms to obtain seismic moment‐tensor solutions of the two September 20, 1993, Mw=6, Klamath Falls, Oregon earthquakes, their foreshock and largest aftershocks (MD>3.5). Several sub‐groups with internally consistent solutions indicate activity on several fault segments and faults. From the estimated moment‐tensors and depths of the main shocks and from the aftershock distribution we deduce that both main shocks occurred on an east‐dipping normal fault, possibly related to the Lake of the Woods fault system. Rotation of T‐axes between the two main shocks is consistent with the two dominant trends of the aftershocks and mapped faults. We propose that a change in fault strike acted as temporary barrier separating the rupture of the main shocks. Empirical Green's function analysis shows that the first main event had a longer rupture duration (half‐duration 1.7 s) than the second (1.2 s). In December, vigorous shallow activity commenced near Klamath Lake's western shore, 5–10 km east of the primary aftershock zone. It appears a Mw=5.5 aftershock occurring the day before, though within the primary aftershock zone, triggered

ISSN:0094-8276    

DOI:10.1029/94GL02844

年代:1995

数据来源: WILEY

摘要:

The Ayu Trough lies on the southern boundary between the Philippine Sea Plate and Caroline Plate. Although this trough may be the best place to study the evolution and kinematics of the Philippine Sea Plate, the origin and evolution of this trough are poorly understood. Our geophysical and morphological surveys in the northern part of the Ayu Trough revealed that the trough shares morphological similarities with slow‐spreading mid‐ocean ridges. The seafloor ages and an average spreading rate of the trough were inferred from the average length of the ridge segments, distribution of sediment thickness, and basement subsidence. Based on the seismic section at 3°30′N, the opening of the trough started about 25 Ma with an average half‐spreading rate 4.1 mm/yr. This spreading rate is much slower than some previous estimates. The present magma production rate and activity of the trough is estimated to be even lower than the 25 m.y. average. The seafloor depth of the trough axis and relief of the axial valley are approximately twice as deep as that of active mid‐ocean ridges. The trough axis valley is covered with sediment in contrast with the axial valleys of active mid‐ocean ridges. Sedimentation at the trough axis shows that the spreading rate has been quite slow for the

ISSN:0094-8276    

DOI:10.1029/94GL02719

年代:1995

数据来源: WILEY

摘要:

Indentation experiments were performed with a pyramidal Vickers indenter on glass samples to observe the dynamic process of indentation. Successive picture images of indentation were taken by a CCD camera through a microscope. The change in diagonal length of the indentation was measured as a function of time of the order of several tens of milliseconds. The results show that the initial process of indentation is highly dependent on the collision velocity of the indenter when it comes into contact with the sample. Higher collision velocities yield larger rates of initial increase in diagonal length. An empirical equation was obtained that is similar to a visco‐elastic Voigt model. It is suggested that local heating due to friction between the indenter and the sample or adiabatic compression reduced the viscosity of the sample material. The dynamic effect observed may be a cause of the change in resistance that is seen in the rate‐dependent rock frict

ISSN:0094-8276    

DOI:10.1029/94GL02843

年代:1995

数据来源: WILEY

摘要:

The water content in modified spinel, synthesized at 15.5 GPa under hydrous conditions, has been measured by means of secondary ion mass spectrometry (SIMSrpar;. We found that the modified spinel crystals contain up to 3.1±0.4 wt % H2O, which is consistent with the amounts estimated from the deficits in the oxide totals of the microprobe analysis. X‐ray diffraction analyses for a single crystal showed that the sample containing 2.5±0.3 wt % H2O is of the modified spinel structure with the lattice parameters a=5.663(1), b=11.546(2), c=8.247(4)Å and V=539.2(5)ų. The present results suggest that a considerable amount of H2O may exist as hydrous modified spinel in the mantle transition zone, which could have important implications for the constitution and dynamics of the

ISSN:0094-8276    

DOI:10.1029/94GL02965

年代:1995

数据来源: WILEY