On the state of stress in the near-surface of the earth's crust

Five models for near-surface crustal stresses induced by gravity and horizontal deformation and the influence of rock property contrasts, rock strength, and stress relaxation on these stresses are presented. Three of the models—the lateral constraint model, the model for crustal stresses caused by horizontal deformation, and the model for the effects of anisotropy—are linearly elastic. The other two models assume that crustal rocks are brittle or viscoelastic in order to account for the effects of rock strength and time on near-surface stresses. It is shown that the lateral constraint model is simply a special case of the combined gravity-and deformation-induced stress field when horizontal strains vanish and that the inclusion of the effect of rock anisotropy in the solution for crustal stresses caused by gravity and horizontal deformation broadens the range for predicted stresses. It is also shown that when stress levels in the crust reach the limits of brittle rock strength, these stresses become independent of strain rates and that stress relaxation in ductile crustal rocks subject to constant horizontal strain rates causes horizontal stresses to become independent of time in the long term.     

Cyanide Chemistry in Comet Hale-Bopp (C/1995 O1)

Observations of comet Hale-Bopp (C/1995 O1) have been carried out near perihelion (1997 March) at millimeter wavelengths using the NRAO 12 m telescope. The J=1-->0, 2-->1, and 3-->2 lines of HCN at 88, 177, and 265 GHz were measured in the comet as well as the J=3-->2 lines of H13CN, HC15N, and HNC. The N=2-->1 transition of CN near 226 GHz was also detected, and an upper limit was obtained for the J=2-->1 line of HCNH+. From the measurements, column densities and production rates have been estimated. A column density ratio of [HCN]/[HNC] = 7+/-1 was observed near perihelion, while it was found that [HCN]/[HCNH+] greater, similar 1. The production rates at perihelion for HCN and CN were estimated to be Q(HCN) approximately 1x1028 s-1 and Q(CN) approximately 2.6x1027 s-1, respectively, resulting in a ratio of [HCN]/[CN] approximately 3. Consequently, HCN is sufficiently abundant to be the parent molecule of CN in Hale-Bopp, and HCNH+ could be a source of HNC. Finally, carbon and nitrogen isotope ratios of 12C/13C = 109+/-22 and 14N/15N = 330+/-98 were obtained from HCN measurements, in agreement with previous values obtained from J=4-->3 data. Such ratios suggest that comet Hale-Bopp formed coevally with the solar system.     

A study of small aftershocks of the Oroville California, earthquake sequence of August 1975

The August 1, 1975 earthquake near Oroville, California, occurred along the Sierra foothills in a region characterized by occasional moderate earthquakes. Several earthquakes in the general region, including those in 1869, 1875, and 1909, appear to have had significant aftershock sequences. The general character of the aftershock sequence of the Oroville earthquake thus does not appear to be anomalous when measured against the known seismic history of this area.

Four smoked-paper micro-earthquake recorders were deployed immediately following the occurrence of the main earthquake to attempt to define the structural associations of the principal earthquake by location and analysis of aftershocks. Focal locations for 243 micro-earthquakes in the magnitude range of 1–3 were selected from the 30-day period (August 2–September 1), during which monitoring was continued. The aftershocks clearly define a planar surface striking north–south and dipping west at 62° from the surface to a depth of about 12 km. Aftershocks during the first two days of monitoring defined a surface of active faulting of approximately 100 km². Extension of this surface both to the north and south began on August 5 at focal depths of 5–10 km, resulting in a total ruptured area of approximately 125 km². The number of aftershocks per day decreased at the rate oft^−1.1, but the decay curve was punctuated by several secondary aftershock sequences. No. direct relationship between the aftershock sequence and the presence of Oroville Reservoir was observed.     

Spatial distribution,variation and trend of five-day antecedent rainfall in Singapore

ABSTRACT

This paper presents the spatial distribution, variation and trend of 5-day antecedent rainfall in Singapore based on rainfall data from 22 meteorological stations. The effect of climate was analyzed by dividing the study period into three decades i.e. Decade 1: 1982–1991, Decade 2: 1992–2001 and Decade 3: 2002–2011. Kriging interpolation was used for rainfall mapping. The results show that spatial distribution of 5-day antecedent rainfall does not coincide with that of the annual rainfall. The maximum annual rainfall occurred in the northwestern side of Singapore. On the other hand, the maximum 5-day antecedent rainfall occurred in the north and northeastern sides. The results of this study suggest that the climate change has increased the amount of 5-day antecedent rainfall quite significantly, i.e. from 420.5 (1987) to 592.9 mm (2006), thus affecting the vulnerability of the area with respect to rainfall-induced slope failure. The analyses also showed that most of slope failures were located in the north and northeast of Singapore between December and earlier March. More slope failures were observed in Decade 3 as compared to Decades 1 and 2. In other words, the analysis confirmed that 5-day antecedent rainfall had affected the slope stability in Singapore.     

Imaging the Hikurangi subduction zone, New Zealand, using teleseismic receiver functions: crustal fluids above the forearc mantle wedge

We image the Hikurangi subduction zone using receiver functions derived from teleseismic earthquakes. Migrated receiver functions show a northwest dipping low shear wave feature down to 60 km depth, which we associate with the crust of the subducted Pacific Plate. Receiver functions (RF) at several stations also show a pair of negative and positive polarity phases with associated conversion depths of ∼20–26 km, where the subducted Pacific Plate is at a depth of ∼40–50 km beneath the overlying Australian Plate. RF inversion solutions model these phases with a thin low S -wave velocity zone less than 4 km thick, and an S -wave velocity contrast of more than ∼0.5 km s⁻¹ with the overlying crust. We interpret this phase pair as representing fluids near the base of the lower crust of the Australian Plate, directly overlying the forearc mantle wedge.     

Ambient noise Rayleigh wave tomography of New Zealand

We present the first New Zealand-wide study of surface wave dispersion, using ambient noise observed at 42 broad-band stations in the national seismic network (GeoNet) and the Global Seismic Network (GSN). Year-long vertical-component time-series recorded between 2005 April 1 and 2006 March 31 have been correlated with one another to yield estimated fundamental mode Rayleigh wave Green's functions. We filter these Green's functions to compute Rayleigh wave group dispersion curves at periods of 5–50 s, using a phase-matched filter, frequency–time analysis technique. The uncertainties of the measurements are estimated based on the temporal variation of the dispersion curves revealed by 12 overlapping 3-month stacks. After selecting the highest quality dispersion curve measurements, we compute group velocity maps from 7 to 25 s period. These maps, and 1-D shear wave velocity models at four selected locations, exhibit clear correlations with major geological structures, including the Taranaki and Canterbury Basins, the Hikurangi accretionary prism, and previously reported basement terrane boundaries.     

Impacts of saturation-dependent anisotropy on the shrinkage behavior of clay rocks

Geomaterials such as soils and rocks can exhibit inherent anisotropy due to the preferred orientation of mineral grains and/or cracks. They can also be partially saturated with multiple types of fluids occupying the pore space. The anisotropic and unsaturated behaviors of geomaterials can be highly interdependent. Experimental studies have shown that the elastic parameters of rocks evolve with saturation. The effect of saturation has also been shown to differ between directions in transversely isotropic clay rock. This gives rise to saturation-dependent stiffness anisotropy. Similarly, permeability anisotropy can also be saturation-dependent. In this study, constitutive equations accommodating saturation-dependent stiffness and hydraulic anisotropy are presented. A linear function is used to describe the relationship between the elastic parameters and saturation, while the relative permeability–saturation relationship is characterized with a log-linear function. These equations are implemented into a hydromechanical framework to investigate the effects of saturation-dependent properties on the shrinkage behavior of clay rocks. Numerical simulations are presented to demonstrate the role of saturation-dependent stiffness and hydraulic anisotropy in shrinkage behavior. The results highlight that strain anisotropy and time evolution of pore pressures are substantially influenced by saturation-dependent stiffness and hydraulic anisotropy.

     

Modeling Ocean Currents Through Complex Random Fields Indexed in Time

Surface ocean currents are often of interest in environmental monitoring. These vectorial data can be reasonably treated as a finite realization of a complex-valued random field, where the decomposition in modulus (current speed) and direction (current direction) of the current field is natural. Moreover, when observations are also available for different time points (other than at several locations), it is useful to evaluate the evolution of their complex correlation over time (rather than in space) and the corresponding modeling which is required for estimation purposes. This paper illustrates a first approach where the temporal profile of surface ocean currents is considered. After introducing the fundamental aspects of the complex formalism of a random field indexed in time, a new class of models suitable for including the temporal component is proposed and applied to describe the time-varying complex covariance function of current data. The analysis concerns ocean current observations, taken hourly on 30 April 2016 through high frequency radar systems at some stations located in the Northeastern Caribbean Sea. The selected complex covariance model indexed in time is used for estimation purposes and its reliability is confirmed by a numerical analysis.