Showing papers in "Geophysical Research Letters in 1990"
TL;DR: In this article, a modification of this method is proposed as a more powerful analytical technique, where different coercivity fractions of IRM are remagnetized in successively smaller fields along three orthogonal directions.
Abstract: The common ferromagnetic minerals have distinctive, characteristic coercivities and thermomagnetic properties. The analysis of the acquisition curve of isothermal remanent magnetization (IRM) is a useful but often ambiguous diagnostic technique. For a more conclusive interpretation, IRM acquisition must be combined with subsequent thermal demagnetization of the IRM. A modification of this method is proposed as a more powerful analytical technique. Different coercivity fractions of IRM are remagnetized in successively smaller fields along three orthogonal directions. The thermal demagnetization of each orthogonal component of the composite IRM is then plotted separately. This method often gives a clearer interpretation of the ferromagnetic mineral content of a rock. Examples are described for limestone and sandstone samples.
1,434 citations
TL;DR: An algorithm has been developed to edit automatically Global Positioning System data such that outlier deletion, cycle slip identification and correction are independent of clock instability, selective availability, receiver–satellite kinematics, and tropospheric conditions.
Abstract: An algorithm has been developed to edit automatically Global Positioning System data such that outlier deletion, cycle slip identification, and correction are independent of clock instability, selective availability, receiver-satellite kinematics, and tropospheric conditions. This algorithm, called TurboEdit, operates on undifferenced, dual frequency carrier phase data, and requires the use of P code pseudorange data and a smoothly varying ionospheric electron content. TurboEdit was tested on the large data set from the CASA Uno experiment, which contained over 2500 cycle slips.Analyst intervention was required on 1 percent of the station-satellite passes, almost all of these problems being due to difficulties in extrapolating variations in the ionospheric delay. The algorithm is presently being adapted for real time data editing in the Rogue receiver for continuous monitoring applications.
664 citations
TL;DR: In this paper, the authors report the results of measurements of low frequency magnetic noise by two independent monitoring systems prior to the occurrence of the M{sub s} 7.1 Loma Prieta earthquake of 17 October 1989.
Abstract: The authors report the results of measurements of low frequency magnetic noise by two independent monitoring systems prior to the occurrence of the M{sub s} 7.1 Loma Prieta earthquake of 17 October 1989. Their measurements cover 25 narrow frequency bands in the more than six-decade frequency range 0.01 Hz-32 kHz, with a time resolution varying from a half hour in the ULF range (0.01-10 Hz) to one second in the ELF/VLF range (10 Hz-32 kHz). The ULF system is located near Corralitos, about 7 km from the epicenter. The ELF/VLF system is located on the Stanford campus, about 52 km from the epicenter. Analysis of the ELF/VLF system is located on the Stanford campus, about 52 km from the epicenter. Analysis of the ELF/VLF data has revealed no precursor activity that they can identify at this time. However, the ULF data have some distinctive and anomalous features. First, a narrow-band signal appeared in the range 0.05-0.2 Hz around September 12 and persisted until the appearance of the second anomalous feature, which consisted of a substantial increase in the noise background starting on 5 October and covering almost the entire frequency range of the ULF system. Third, there was an anomalousmore » dip in the noise background in the range 0.2-5 Hz, starting one day ahead of the earthquake. Finally, and perhaps most compelling, there was an increase to an exceptionally high level of activity in the range 0.01-0.5 Hz starting approximately three hours before the earthquake. There do not appear to have been any magnetic field fluctuations originating in the upper atmosphere that can account for this increase. Further, while the systems are sensitive to motion, seismic measurements indicate that there were no significant shocks preceding the quake. Thus, the various anomalous features in the data, and in particular the large-amplitude increase in activity starting three hours before the quake, may have been magnetic precursors.« less
601 citations
TL;DR: The NUVEL-1 model is incorporated into HS2-NUVEL1, a global model for plate velocities relative to hotspots as discussed by the authors, and the results thus obtained are compared with those of the AM1-2 model of hotspot-relative plate velocity.
Abstract: The NUVEL-1 model of current global relative plate velocities is presently incorporated into HS2-NUVEL1, a global model for plate velocities relative to hotspots; the results thus obtained are compared with those of the AM1-2 model of hotspot-relative plate velocities. While there are places in which plate velocities relative to the hotspots differ between HS2-NUVEL1 and AM1-2 by tens of degrees in direction and 15 mm/yr in speed, the hotspot Euler vectors differ with 95 percent confidence only for the Arabian and Indian plates. Plates attached to subducting slabs move faster relative to the hotspots than do plates without slabs.
571 citations
TL;DR: The existence of a threshold pore pressure gradient below which water would not flow through clay-filled fault zones has been studied in this article, which has profound implications for the rheological properties of mature fault zones that contain large thickness of fault gouge.
Abstract: More than twenty-five years ago Miller and Low reported the existence of a threshold pore pressure gradient below which water would not flow through clay. Recent experimental observations of the shear strength of structured water on biotite surfaces have provided a physical basis for understanding this threshold gradient. The existence of this phenomenon has profound implications for the rheological properties of mature fault zones, such as the San Andreas, that contain large thickness of fault gouge. For example, a clay-filled fault zone about 1 km wide at the base of the surface could support core fluid pressure equal to the maximum principal stress over the entire seismogenic zone. As a result, the fault would have near-zero strength and the maximum principal stress measured on the flanks of the fault, would be oriented normal to the fault surface. Another consequence of the threshold gradient is that normal hydrostatic fluid pressures outside the fault zone could coexist with near-lithostatic fluid pressures in the interior of the fault zone without the need for continual replenishment of the overpressured fluid. In addition, the pore pressure at any point should never exceed the local minimum principal stress so that hydrofracture will not occur.
394 citations
TL;DR: In this article, small isolated density depletions are introduced into the magnetotail by an uneven plasma loading process in the far tail, and a significant depletion would move faster than either the gradient-curvature drift speed or the average convection speed.
Abstract: Small, isolated density depletions are postulated to be introduced into the magnetotail by an uneven plasma loading process in the far tail. Such bubbles would be displaced earthward by an interchange process, and a significant depletion would move faster than either the gradient-curvature drift speed or the average convection speed. The dissolution of a bubble into the background results in a local reduction in plasma density, thereby violating ideal MHD and the subsequent conservation of plasma content. If a sufficient amount of magnetic flux in the far tail is contained in bubbles, then the net effect may be a reduction of plasma pressure in the near tail sufficient to resolve the pressure balance inconsistency.
293 citations
TL;DR: In this article, the cross-tail current in the now thin (about 400 km) current sheet is carried by the crosstail serpentine motion of non-adiabatic ions (Speiser, 1965).
Abstract: Throughout most of the growth phase of a substorm, the cross-tail current at x about -10 Re can be supplied by the curvature drift of a bi-directional field aligned distribution of 1 keV electrons. Just prior to its local disruption after substorm onset, the cross-tail current in the now thin (about 400 km) current sheet is carried by the cross-tail serpentine motion of non-adiabatic ions (Speiser, 1965). The instability of this latter current leads to the local disruption of the near-earth current sheet.
267 citations
TL;DR: In this article, the authors show that between 9% and 23% of the total relative plate motion has occurred along the Eastern California shear zone (ECSZ) since its probably inception approximately 10-6 Ma.
Abstract: The newly recognized Eastern California shear zone (ECSZ) of the Mojave Desert-Death Valley region has played a major, but previously underappreciated role in accommodating the dextral shear between the Pacific and North American plates in late Cenozoic time. Comparison of integrated net slip along the shear zone with motion values across the entire transform boundary indicates that between 9% and 23% of the total relative plate motion has occurred along the ECSZ since its probably inception {approximately}10-6 Ma. Long-term integrated shear along the ECSZ (6-12 mm yr{sup {minus}1}) is similar to historic measurements (6.7 {plus minus} 1.3 mm yr{sup {minus}1}). Time-space patterns of faulting suggest that shear was concentrated in the eastern part of the Mojave Desert block and Death Valley during late Miocene and early Pleistocene time, but that the locus of faulting in the south-central Mojave jumped westward between 1.5 and 0.7 Ma.
261 citations
TL;DR: In this paper, a multwavelength airborne lidar system was used to measure polar stratospheric cloud (PSC) distributions in the wintertime Arctic stratosphere and their optical characteristics were measured with a multiview airborne LIDAR system as part of the 1989 Airborne Arctic Stratospheric Expedition.
Abstract: Polar stratospheric cloud (PSC) distributions in the wintertime Arctic stratosphere and their optical characteristics were measured with a multiwavelength airborne lidar system as part of the 1989 Airborne Arctic Stratospheric Expedition. PSCs were observed on 10 flights between January 6 and February 2, 1989, into the polar vortex. The PSCs were found in the 14-27 km altitude range in regions where the temperatures were less than 195 K. Two types of aerosols with different optical characteristics (Types 1a and 1b) were observed in PSCs thought to be composed of nitric acid trihydrate. Water ice PSCs (Type 2) were observed to have high scattering ratios (greater than 10) and high aerosol depolarizations (greater than 10 percent) at temperatures less than 190 K.
251 citations
TL;DR: In this article, the authors found that most large geomagnetic storms during the interval surrounding the last solar maximum (August 1978 - October 1982) were associated with earth-passage of interplanetary disturbances in which the earth encountered both a shock and the CME driving the shock.
Abstract: Previous work indicates that coronal mass ejection (CME) events in the solar wind at 1 AU can be identified by the presence of a flux of counterstreaming solar wind halo electrons (above about 80 eV). Using this technique to identify CMEs in 1 AU plasma data, it is found that most large geomagnetic storms during the interval surrounding the last solar maximum (August 1978 - October 1982) were associated with earth-passage of interplanetary disturbances in which the earth encountered both a shock and the CME driving the shock. However, only about one CME in six encountered by earth was effective in causing a large geomagnetic storm. Slow CMEs which did not interact strongly with the ambient solar wind ahead were particularly ineffective in a geomagnetic sense.
251 citations
TL;DR: In this article, the electron and ion edges of the low-latitude boundary layer of the earth's magnetosphere were found to exhibit large asymmetries parallel and antiparallel, as well as perpendicular, to the local magnetic field.
Abstract: Magnetosheath plasma entering the earth's magnetosphere to populate the low latitude boundary layer, LLBL, is often accelerated to speeds considerably greater than are observed in the adjacent magnetosheath. Measurements made during such accelerated flow events reveal separate electron and ion edges to the LLBL, with the electron edge being found earthward of the ion edge. Plasma electron velocity distributions observed at the earthward edge of the LLBL are often highly structured, exhibiting large asymmetries parallel and antiparallel, as well as perpendicular, to the local magnetic field. These features can consistently be interpreted as time-of-flight effects on recently reconnected field lines, and thus are strong evidence in support of the reconnection interpretation of accelerated plasma flow events.
TL;DR: In this article, the size of particles in water ice clouds cannot be determined because they are much larger than the wavelength of the lidar, and they are not spherical in shape.
Abstract: Lidar observations by Browell et al. (1990) are interpreted using single scattering calculations for nonspherical particles and aerosol microphysical calculations. Many of the lidar observations are consistent with particles containing 10 ppbv of condensed nitric acid vapor and an equivalent mass of water. The lidar observations of these Type 1 clouds identify two subtypes, whose properties are deduced. Type 1b particles are spherical, or nearly spherical, and typically have radii near 0.5 micron; Type 1a particles are not spherical, and have a spherical volume equivalent radius exceeding 1.0 micron. Several factors may cause variations in the size of the particles. The most significant factors are the cooling rate and the degree to which the air parcels cool below the condensation point. Specific examples in which cooling rate and cooling point may have led to variations in particle size are found in the Browell et al. (1990) data set. Condensation of 1 ppmm of water or less is quantitatively sufficient to account for the magnitude of the lidar backscatter observed from water ice clouds. The ice particles are not spherical in shape. The sizes of particles in water ice clouds cannot be determined because they are much larger than the wavelength of the lidar.
TL;DR: In this paper, the first results on the ionospheric ion outflow in the Martian magnetosphere by the ion composition experiment ASPERA on Phobos-2 were presented, showing that Mars is characterized by a strong loss of plasma from its topside ionosphere.
Abstract: This report reviews the first results on the ionospheric ion outflow in the Martian magnetosphere by the ion composition experiment ASPERA on Phobos-2. The measurements show that Mars is characterized by a strong loss of plasma from its topside ionosphere. This loss results from both ion pick-up due to mass-loading of the solar wind in the Martian boundary layer and an acceleration mechanism, quite similar to that observed above the Earth's auroral oval, providing intense ionospheric O+ beams of energies up to several keV.
A preliminary estimate of the ionospheric outflow from Mars indicates that the planet at present is losing oxygen at a rate of ≈3·1025 ions/s. This corresponds to an evacuation of its present total atmospheric oxygen content (contained in CO2 and O2) in less than 100 million years.
TL;DR: An analogue of the magnetosphere developed on the basis of Shaw's (1984) dripping faucet model was used to model the mechanisms of magnetospheric response to energy transfer from the solar wind as discussed by the authors.
Abstract: An analogue of the magnetosphere developed on the basis of Shaw's (1984) dripping faucet model was used to model the mechanisms of the magnetospheric response to energy transfer from the solar wind. It is demonstrated that geomagnetic activity results from nonlinearly coupled physical processes and that the strength and the nature of the coupling changes dramatically as the magnetosphere is driven harder and harder by increasing energy input. Based on initial results obtained from the model, is is suggested that a chaotic transition takes place in the analogue system as the loading rate is increased beyond a critical value. This model is able to explain many of the features in the results of linear prediction filtering techniques.
TL;DR: In this paper, a continuous imbibition/drainage experiment was conducted, where compressive wave velocities were measured at a frequency of 1 MHz; shear wave velocity (Vs) was measured at the frequency of 600 kHz.
Abstract: Laboratory data have been collected during a continuous imbibition/drainage experiment that show a clear dependence of elastic wave velocities on the details of the pore scale distribution of water and air in a sandstone. Compressional wave velocity (Vp) was measured at a frequency of 1 MHz; shear wave velocity (Vs) was measured at a frequency of 600 kHz. During the experiment, Vp showed little variation with the level of water saturation (Sw) during increasing Sw through imbibition until Sw = 0.80, at which point Vp increased dramatically. When Sw was decreased, pronounced saturation-induced hysteresis was observed in the region 03 0.4.
As a simple model, we consider the imbibition process as producing a partially saturated state in all pores; i.e. all pores contain both air and water. The drainage process, in contrast, favors the existence of either air-filled or water-filled pores. As elastic wave velocities are very sensitive to the saturation state in the smaller, “crack-like” pores, these variations in fluid distribution cause related variations in velocities.
TL;DR: In this paper, the root-mean-square roughness is computed in a number of windows of varying length w, and H is measured from the slope of a log-log plot of roughness versus w. The roughness-length method is closely related to the grid fractal dimension, is simple to implement, and can be applied to non-uniformly spaced series.
Abstract: Time/space series of natural variables (e.g., surface topography) are often self-affine, i.e., measurements taken at different resolutions have the same statistical characteristics when rescaled by factors that are generally different for the horizontal and vertical coordinates. Self-affinity implies that the standard deviation measured on a sample spanning a length w is proportional to wH = w2 − D, where H is the Hurst exponent and D is the fractal dimension (1 ≤ D ≤2 for a fractal series). In this paper, a “roughness-length” method based on this property of self-affine series is presented. In practice, the root-mean-square roughness is computed in a number of windows of varying length w, and H is measured from the slope of a log-log plot of roughness versus w. Montecarlo simulations show that the fractal dimension as measured by the roughness-length method is approximately the same as that defined by the power spectrum. The roughness-length method is closely related to the grid fractal dimension, is simple to implement, and can be applied to non-uniformly spaced series.
TL;DR: In this article, high pressure phase relations have been examined for phlogopite + diopside with and without enstatite under vapor absent conditions in the pressure range of 5 to 13 GPa and in the temperature range of 1,000 to 1,300C.
Abstract: High-pressure phase relations have been examined for phlogopite + diopside with and without enstatite under vapor absent conditions in the pressure range of 5 to 13 GPa and in the temperature range of 1,000 to 1,300C Phlogopite in these systems can be stable up to 6-7 GPa and decomposes through pressure-dependent reactions to crystallize phases including potassic amphibole The experimental results suggest that phlogopite, which is one of main hydrous phases in the downdragged hydrated peridotite at the base of mantle wedge, plays an important role in the formation of magmas at the backarc side of a volcanic arc The existence of potassic amphibole at higher pressure regions may imply the involvement of subduction component in magma generation in the region far away from the trench axis
TL;DR: In this paper, the existence of a sharp break in the power spectrum of AE at about 5 hours is demonstrated and several possible explanations of the nonlinear response of AE to the IMF Bs driver are briefly discussed, including variable ionospheric conductivity (increasing with Bs) and several AE saturation mechanisms for the low frequency regime.
Abstract: The existence of a sharp break in the power spectrum of AE at about 5 hours is demonstrated Several possible explanations of the nonlinear response of AE to the IMF Bs driver are briefly discussed, including: variable ionospheric conductivity (increasing with Bs) for the high frequency regime, and several AE saturation mechanisms for the low frequency regime
TL;DR: In this paper, the authors measured strain rates at four geodetic networks in eastern California situated between northern Owens Valley and Transverse Ranges along a small circle drawn about the Pacific-North America pole of rotation.
Abstract: Strain rates measured at four geodetic networks in eastern California situated between northern Owens Valley and Transverse Ranges along a small circle drawn about the Pacific-North America pole of rotation are remarkably consistent. Each exhibits 0.14 {mu}rad/yr simple right-lateral engineering-shear-strain accumulation across the local vertical plane tangent to the small circle. Local faults (e.g., Owens Valley, Garlock, Helendale) traversing these networks are not as closely aligned with the vertical planes of maximum shear-strain accumulation as is the local tangent to the small circle. A fifth network slightly east of the small circle shows no significant strain accumulation. Thus, a shear zone trending N35{degree}W from near the eastern end of the big bend of the San Andreas fault to northern Owens Valley is indicated by these data. This corresponds to the Eastern California shear zone proposed on geological evidence by Dokka and Travis. The shear zone carries {approximately}8 mm/yr of the Pacific-North America relative plate motion from the San Andreas fault north-northwest across the Mojave Desert into Owens Valley and the northern Basin and Range province. The shear zone observed at the surface may be a manifestation of a through-going subcrustal fault.
TL;DR: In this paper, the magnetospheric response to the solar-wind input, as represented by the time-series measurements of the auroral electrojet (AE) index, has been examined using phase-space reconstruction techniques.
Abstract: The magnetospheric response to the solar-wind input, as represented by the time-series measurements of the auroral electrojet (AE) index, has been examined using phase-space reconstruction techniques. The system was found to behave as a low-dimensional chaotic system with a fractal dimension of 3.6 and has Kolmogorov entropy less than 0.2/min. These indicate that the dynamics of the system can be adequately described by four independent variables, and that the corresponding intrinsic time scale is of the order of 5 min. The relevance of the results to magnetospheric modeling is discussed.
TL;DR: In this paper, a laser ceilometer, an acoustic sounder, and a microwave radiometer were used to estimate cloud thickness and the adiabatic and integrated liquid water content of shallow stratocumulus clouds continuously for three days using two-minute averages.
Abstract: A laser ceilometer, an acoustic sounder, and a microwave radiometer were used to estimate cloud thickness and the adiabatic and integrated liquid water content of shallow stratocumulus clouds continuously for three days using two-minute averages. Although the observed liquid water path was close to the theoretical adiabatic value for most of the three days, there was one four-hour period when the liquid water content dropped to about 50% of the adiabatic value. Hourly-averaged values for a 19-day period of intensive observations show that the cloud water content was generally close to the adiabatic value. Occasionally there were clouds greater than 300 m in depth in which the water content was clearly less than adiabatic.
TL;DR: In this paper, the utility of current generation climate models for studying the influence of greenhouse warming on the tropical storm climatology is examined, and a method developed to identify tropical cyclones is applied to a series of model integrations.
Abstract: The utility of current generation climate models for studying the influence of greenhouse warming on the tropical storm climatology is examined. A method developed to identify tropical cyclones is applied to a series of model integrations. The global distribution of tropical storms is simulated by these models in a generally realistic manner. While the model resolution is insufficient to reproduce the fine structure of tropical cyclones, the simulated storms become more realistic as resolution is increased. To obtain a preliminary estimate of the response of the tropical cyclone climatology, CO2 was doubled using models with varying cloud treatments and different horizontal resolutions. In the experiment with prescribed cloudiness, the number of storm-days, a combined measure of the number and duration of tropical storms, undergoes a statistically significant increase in the doubled-CO2 climate. In contrast, a smaller but significant reduction of the number of storm-days is indicated in the experiment with cloud feedback. In both cases the response is independent of horizontal resolution. While the inconclusive nature of these experimeital results highlights the uncertainties that remain in examining the details of greenhouse gas induced climate change, the ability of the models to qualitatively simulate the tropical storm climatology suggests that they are appropriate tools for this problem.
TL;DR: In this paper, a linear analysis is performed to investigate the kinetic cross-field streaming instability in the earth's magnetotail neutral sheet region, and a substorm expansion onset scenario is proposed based on this instability in which the relative drift between ions and electrons is primarily due to unmagnetized ions undergoing current sheet acceleration in the presence of a cross tail electric field.
Abstract: A linear analysis is performed to investigate the kinetic cross-field streaming instability in the earth's magnetotail neutral sheet region. Numerical solution of the dispersion equation shows that the instability can occur under conditions expected for the neutral sheet just prior to the onset of substorm expansion. The excited waves are obliquely propagating whistlers with a mixed polarization in the lower hybrid frequency range. The ensuing turbulence of this instability can lead to a local reduction of the cross-tail current causing it to continue through the ionosphere to form a substorm current wedge. A substorm expansion onset scenario is proposed based on this instability in which the relative drift between ions and electrons is primarily due to unmagnetized ions undergoing current sheet acceleration in the presence of a cross-tail electric field. The required electric field strength is within the range of electric field values detected in the neutral sheet region during substorm intervals. The skew in local time of substorm onset location and the three conditions under which substorm onset is observed can be understood on the basis of the proposed scenario.
TL;DR: In this article, the variance of wind velocity fluctuations for periods from 5 min to 2 hr shows a semi-annual variation with a significant peak in summer centered in July and irregular enhancements in winter.
Abstract: We have regularly monitored mesospheric winds at 60–85km for three years with the MU radar (35°N, 136°E), and have found that the variance of wind velocity fluctuations for periods from 5 min to 2 hr shows a semi-annual variation with a significant peak in summer centered in July and irregular enhancements in winter. The zonal momentum flux induced by gravity waves shows eastward and westward maxima in July–August and December–February, respectively. Typical eastward accelerations at the 70km altitude range from 7 to 13 m/s/day and from −8 to −11 m/s/day in summer and winter, respectively, which agree fairly well with theoretical predictions.
TL;DR: In this article, the influence of changes in the upper stratospheric mean zonal wind on the circulation of the lower atmosphere was investigated, and the results of numerical experiments with a general circulation model were used.
Abstract: An investigation is conducted to determine the influence of changes in the upper stratospheric mean zonal wind on the circulation of the lower atmosphere. In addition to observed data, results of numerical experiments with a general circulation model are used, in which the solar ultraviolet heating rate is varied to force changes in the mean zonal wind in the upper stratosphere.
It is found that when the upper stratospheric mid-latitude westerlies are strong during December, lower stratospheric polar night jet is persistent and the westerlies in the polar region of the troposphere become stronger in the following February. These results are common to both the observations and the numerical experiments.
TL;DR: In this paper, a dextral shear between the overlapping Ghoubbet Asal-Manda Inakir and Manda Hararo-Abhe Bad rifts was found to be a particularly efficient mechanism to accomodate strain between overlapping oceanic rifts.
Abstract: Lateral slip on initially rift-parallel normal faults may be a particularly efficient mechanism to accomodate strain between overlapping oceanic rifts. It occurs in southern Afar, where clockwise block rotations result from distributed dextral shear between the overlapping Ghoubbet Asal-Manda Inakir and Manda Hararo-Abhe Bad rifts. Faulting observed during the 1969, Serdo earthquakes and on SPOT images is consistent with the shear being taken up by left-lateral slip on steep NW-SE striking faults, which formed as normal faults before extensional strain became localized in the two rifts. This bookshelf faulting accounts quantitatively for the 14.5°± 7.5° rotation documented by paleomagnetism in the 1.8 ± 0.4 Ma old Afar stratoid basalts, given the 17.5 ± 5 mm/yr rate of separation between Arabia and Somalia.
TL;DR: The Loma Prieta earthquake occurred on the Sargent fault as discussed by the authors, and the large component of reverse-slip observed in this event agrees with a simple model for slip on a dipping plane within a compressional fault bend.
Abstract: Hypocenters of the Loma Prieta sequence form a dipping zone that rises from the mainshock hypocenter and is parallel to the mainshock nodal plane. Most aftershocks cluster around the perimeter of the zone, surrounding a relatively aseismic center which approximates the region of mainshock rupture. At its southeastern end, the dipping aftershock zone warps into a vertical surface that corresponds to the San Andreas fault. In the central and northwestern parts of the zone at depths above ∼10 km, the aftershocks define numerous disjoint fault structures. The large component of reverse-slip observed in this event agrees with a simple model for slip on a dipping plane within a compressional fault bend. We do not believe that the Loma Prieta earthquake occurred on the Sargent fault. However, we are unable to conclude whether it ruptured the principal plate boundary fault or a less frequently active fault.
TL;DR: In this paper, X-ray diffraction measurements are reported for Ca(OH)2 portlandite as it is compressed to 37.6 GPa in the diamond cell at room temperature.
Abstract: X-ray diffraction measurements are reported for Ca(OH)2 portlandite as it is compressed to 37.6 GPa in the diamond cell at room temperature. Between 10.7 and 15.4 GPa crystalline Ca(OH)2 transforms to a glass, and on decompression the glass recrystallizes between 3.6 and 5.1 GPa. Below pressures of 10.7 GPa the elastic compression of crystalline Ca(OH)2 was measured. A finite strain analysis of these data shows that the isothermal bulk modulus and its pressure derivative are 37.8 + or - 1.8 GPa and 5.2 + or - 0.7 at zero pressure. The change in the unit cell dimensions indicates that the linear incompressibilities of Ca(OH)2 differ by a factor of three.
TL;DR: The region upstream from the Mars subsolar bow shock is surveyed for the presence of MHD wave phenomena using the high-temporal resolution data from the Magma magnetometer as discussed by the authors, and strong turbulence is observed when the magnetic field is connected to the Mars bow shock in such a way as to allow diffuse ions to reach the spacecraft.
Abstract: The region upstream from the Mars subsolar bow shock is surveyed for the presence of MHD wave phenomena using the high temporal resolution data from the Magma magnetometer. Strong turbulence is observed when the magnetic field is connected to the Mars bow shock in such a way as to allow diffuse ions to reach the spacecraft. Also weak waves are observed at the proton gyro frequency. These waves are left-hand elliptically polarized and may be associated with the pick-up of protons from the Mars hydrogen exosphere.
TL;DR: In this paper, the amplitudes of strong ground motions from the Loma Prieta earthquake recorded in the San Francisco and Oakland areas exceeded the levels predicted by standard empirical attenuation relations.
Abstract: The amplitudes of strong ground motions from the Loma Prieta earthquake recorded in the San Francisco and Oakland areas exceeded the levels predicted by standard empirical attenuation relations. Preliminary analysis of accelerograms having known trigger times strongly suggests that the elevation of ground motion amplitudes in the distance range of approximately 40 to 100 km was due to critical reflections from the base of the crust. These reflections, which are identified on the basis of their arrival times and phase velocity, and by comparison with simulated accelerograms, were large and occurred at relatively close range because of the deep focal depth of the earthquake and the strong velocity gradient at the base of the crust. These motions were further amplified, presumably by impedance contrast effects, at soft soil sites in San Francisco and Oakland. The effect of the critical reflections in amplifying peak accelerations of the Loma Prieta earthquake in the San Francisco and Oakland regions was as large as the effect of soft soil site conditions. Focal depth has an important influence on strong motion attenuation at distances beyond about 40 km, and empirical attenuation relations derived from shallow crustal earthquakes may underpredict the ground motions of deeper crustal events in this distance range. Further analyses using an expanded data base that includes recordings of aftershocks are required to rigorously test the proposed explanation of the ground motions recorded in San Francisco and Oakland, and the conclusions drawn from that explanation.