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.     

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.     

Non-destructive assessment of polycyclic aromatic hydrocarbon (PAH) exposure by fluorimetric analysis of crab urine

The detection of urinary polycyclic aromatic hydrocarbon (PAH) metabolites by fluorescence spectrophotometry is particularly effective as a practical means to assess PAH exposure in decapod crabs. However, the practical application of this technique has thus far only been tested for the European shore crab (Carcinus maenas) and only a few field studies have been conducted in heavily polluted areas. The present study evaluated the adaptability of this method as a rapid, cost-effective and non-destructive biomonitoring tool for the New Zealand crab species, Macrophthalmus hirtipes (stalk-eyed mud crab). A field gradient could be detected among the sites and different input sources of PAH contamination could be discerned through the differentiation of pyrogenic and petrogenic PAH signatures. The present study shows that the fluorescence screening method is sensitive to relatively low levels of PAH contamination and more broadly applicable to smaller crab species than C. maenas, for which the technique was developed.     

Variation in fold geometry in the Yuso basin, northern Spain: implications for the deformation regime

First generation structures in greywackes of the Yuso Group from the Cantabrian Mountains of northern Spain show a distinct variation in geometry with depth in a regional synclinal structure (Curavacas and Lechada synclines); they are easily distinguished from other deformation events. In the structurally uppermost level we find ‘flap folds’. Flap folds are recumbent structures with the inverted limb preserved. Below this level ‘cascade folds’ are found. These structures have a vergence opposite to that of parasitic folds. The nomenclature adopted is from Harrison and Falcon. Characteristically, these structures have shallowly dipping axial surfaces, in agreement with the shallow dip of the axial plane (regional) cleavage. In the lowermost structural level, upright parasitic folds with a steep cleavage are present. The variation in fold geometry is accompanied by a general steepening of the regional cleavage with increasing depth. In the absence of overprinting relationships the F₁ fold geometries are included in a single deformation event.The steepening of the cleavage with depth reflects the change in orientation of the maximum shortening direction from sub-vertical in the upper part of the syncline to sub-horizontal in the lower part. With increasing depth the deformation regime during F₁ changed from bending to buckling. The deformation regime on the regional scale, however, is associated with basement subsidence and passive formation of the regional synclinal structure. Furthermore, the absence of a distinct microfabric for the different F₁ folds indicates that on a small scale a similar deformation regime was present. We conclude, therefore, that the scale at which we study a structure only reflects the deformation regime at that particular scale. Consequently, the overall deformation regime cannot be determined from single outcrops or microstructural analysis alone.     

The effect of iron on montmorillonite stability. (II) Experimental investigation

Several designs proposed for high-level nuclear waste (HLW) repositories include steel waste canisters surrounded by montmorillonite clay. This work investigates montmorillonite stability in the presence of native Fe, magnetite and aqueous solutions under hydrothermal conditions. Two series of experiments were conducted. In the first, mixtures of Na-montmorillonite, magnetite, native Fe, calcite, and NaCl solutions were reacted at 250 °C, P_sat for between 93 and 114 days. In the second series, the starting mixtures included Na-montmorillonite, native Fe and solutions of FeCl₂ which were reacted at temperatures of 80, 150, and 250 °C, P_sat, for 90-92 days. Experiments were analysed using XRD, FT-IR, TEM, ICP-AES, and ICP-MS. In the first series of experiments, native Fe oxidised to produce magnetite and the starting montmorillonite material was transformed to Fe-rich smectite only when the Fe was added predominantly as Fe metal rather than Fe oxide (magnetite). The Fe-rich smectite was initially Fe(II)-rich, which oxidised to produce an Fe(III)-rich form on exposure to air. The expansion of this material on ethylene glycol solvation was much reduced compared to the montmorillonite starting material. TEM imaging shows that partial loss of tetrahedral sheets occurred during transformation of the montmorillonite, resulting in adjacent layers becoming H-bonded with a 7 Å repeat. The reduced swelling property of the Fe-smectite product may be due predominantly to the structural disruption of smectite layers and the formation of H-bonds. Solute activities corresponded to the approximate stability field calculated for hypothetical Fe(II)-saponite. In the second series of experiments, significant smectite alteration was only observed at 250 °C and the product contained a small proportion of a 7 Å repeat structure, observable by XRD. In these experiments, solute activities coincide with berthierine. The experiments indicate that although bentonite is still a desirable choice of backfill material for HLW repositories, some loss of expandability may result if montmorillonite is altered to Fe-rich smectite at the interface between steel canisters and bentonite.     

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.