9,053 research outputs found
Sampling power-law distributions
No full textPower-law distributions describe many phenomena related to rock fracture. Data collected to measure the parameters of such distributions only represent samples from some underlying population. Without proper consideration of the scale and size limitations of such data, estimates of the population parameters, particularly the exponent D, are likely to be biased. A Monte Carlo simulation of the sampling and analysis process has been made, to test the accuracy of the most common methods of analysis and to quantify the confidence interval for D. The cumulative graph is almost always biased by the scale limitations of the data and can appear non-linear, even when the sample is ideally power law. An iterative correction procedure is outlined which is generally successful in giving unbiased estimates of D. A standard discrete frequency graph has been found to be highly inaccurate, and its use is not recommended. The methods normally used for earthquake magnitudes, such as a discrete frequency graph of logs of values and various maximum likelihood formulations can be used for other types of data, and with care accurate results are possible. Empirical equations are given for the confidence limits on estimates of D, as a function of sample size, the scale range of the data and the method of analysis used. The predictions of the simulations are found to match the results from real sample D-value distributions. The application of the analysis techniques is illustrated with data examples from earthquake and fault population studies
Seismic reflection images of intraplate deformation, central Indian Ocean, and their tectonic significance
No full textMultichannel seismic reflection profiles collected from the intraplate deformation area in the Central Indian Ocean Basin are used to describe brittle structures produced under compressive stress. Reflectors within oceanic basement are divided into four types: north-dipping and south-dipping reverse faults that are interpreted as reactivations of structures formed at the spreading-centre as outward and inward dipping faults respectively; lower-angle, north-dipping reflectors that probably represent new faults or faults just initiating; and sub-horizontal reflectors within the uppermost crtuhsat t are interpreted as hydrothermal alteration fronts. Within the sedimentary cover upwards fault propagation shows the faults steepening from c. 40" just above basement to near vertical and is preceded by sediment folding. A fractal analysis of faulting suggests two fault populations possibly reflecting different criteria for brittle failure. Measurements of north-south crustal shortening indicate a shortening rate of (2+.5 0 .9) mm a-', which is at the lower end of predictions from plate motions, but significant enough to recognize this area as a diffuse plate boundary. The formation of long-wavelength basement undulations and the reactivation on fracture zones and ridge-parallel fault fabrics are linked in a unified tectonic model driven by the high level of intraplate compressive stress in the area. There is little evidence from the seismic profiles for intraplate deformation starting before the widespread unconformity dated as 7 Ma
Faulting and Extension Rate over the last 20,000 Years in the Offshore Whakatane Graben, New Zealand Continental Shelf.
No full textOblique rifting in the offshore Taupo Volcanic Zone, New Zealand, is expressed in widely distributed active normal faulting in the 20 km-wide Whakatane Graben. Active faults are identified along seafloor scarps and displacements of the post-last glacial transgressive ravinement surface (<20ka), using a network of seismic reflection data and multibeam bathymetry. The rifting involves basement blocks back-tilted by 12-16°, controlled by large NW-dipping faults with intersecting antithetic faults within the 3 km-thick sedimentary sequence. Faults along the graben border parallel the rift axis, while those in the centre are moderately oblique to it. We present a novel method to estimate the age of a post-glacial surface (7.5 to 20.5 ka), with consideration to spatially varying subsidence and uplift, and measure fault throw across >400 faults. We derive an extension rate at seismogenic depths (6-10 km) across the graben of 13 ± 6 mm/yr, by summing surface measurements, assuming an average crustal fault dip of 45±15º, and correcting for the discrepancies between surface and deep crustal extension estimates. Structural and kinematic data implies an extension direction 20º oblique to the rift axis, resulting in up to 4.6 ± 2.1 mm/yr of dextral motion parallel to the rift axis. The strike-slip motion is accommodated by dip-slip displacements on oblique faults in the centre of the graben, and oblique-slip faulting along the rift margins. Pure dip-slip in the graben centre represents >50% of the total slip, with the Rangitaiki Fault accommodating 25% of the total extension in the graben
Active faulting on the Ninetyeast Ridge and its relation to deformation of the Indo-Australian plate
No full textThe ~4500 km long Ninetyeast Ridge (NER) in the northeastern Indian Ocean crosses a broad zone of deformation where the Indo-Australian plate is fracturing into three smaller plates (India, Capricorn, Australia) separated by diffuse boundaries whose extents are poorly defined. New multichannel seismic reflection profiles image active faults along the entire length of the NER and show spatial changes in the style of deformation along the ridge. The northern NER (0°N–5°N) displays transpressional motion along WNW-ESE faults. Observed fault patterns confirm strike-slip motion at the western extent of the April 2012 Wharton Basin earthquake swarm. In the central NER (5°S–8°S), deformation on WNW-ESE-trending thrust faults implies nearly N-S compression. An abrupt change in fault style occurs between 8° and 11°S, with modest, extension characterizing the southern NER (11°S–27°S). Although extension is dominant, narrow zones of faults with strike-slip or compressional character also occur in the southern NER, suggesting a complex combination of fault motions. At all sites, active faulting is controlled by the reactivation of original, spreading-center formed, normal faults, implying that deformation is opportunistic and focused along existing zones of weakness, even when original fault trend is oblique to the direction of relative plate motion. Observed faulting can be interpreted as India-Australia deformation in the northern NER and Capricorn-Australia deformation in the southern NER. The India-Capricorn boundary is directly adjacent to the northern NER and this juxtaposition combined with a different style of faulting to the east of the NER imply that the ridge is a tectonic boundary
Optimal Processing of Marine High-Resolution Seismic Reflection (Chirp) Data
No full textChirp frequency-modulated (FM) systems offer deterministic, repeatable source-signatures for high-resolution, normal incidence marine seismic reflection data acquisition. An optimal processing sequence for uncorrelated Chirp data is presented to demonstrate the applicability of some conventional seismic reflection algorithms to high-resolution data sets, and to emphasise the importance of a known source-signature. An improvement of greater than 60dB in the signal- to-noise ratio is realised from correlating the FM reflection data with the transmitted pulse. Interpretability of ringy deconvolved data is enhanced by the calculation of instantaneous amplitudes. The signal-to-noise ratio and lateral reflector continuity are both improved by the application of predictive filters whose effectiveness are aided by the repeatability of the Chirp source.<br/
Imaging wooden artefacts using Chirp sources
No full textThis paper investigates the potential for imaging submerged and buried wooden artefacts in the marine environment using high-resolution seismic reflection techniques. Models to calculate theoretical reflection coefficients of 11 wood species buried in shallow marine sediments are developed. The models use estimates of acoustic impedance contrasts to predict the strength of reflections resulting from buried wooden artefacts. Traditionally, wooden wrecks are composed of oak, with lesser components of mahogany, pine and elm. Experimental results on wood samples from a sixteenth century oak wreck (the Mary Rose) are presented as a control on theory.Results indicate that wooden wrecks may be readily imaged by a suitable sub-bottom profiler operating under appropriate survey conditions. Particular reference is made to Chirp sonar which transmits a frequency-modulated pulse, providing high-resolution digital seismic data. Chirp data acquired over an eighteenth century buried oak wreck (the Invincible, Horsetail, East Solent) is presented. Calculations of reflection coefficients acquired over the site support both the theoretically and experimentally derived data. <br/
Active interplay between strike-slip and extensional structures in a back-arc environment, Bay of Plenty, New Zealand
No full text(paper presented at American Geophysical Union - Fall Meeting, San Francisco, 8-12 Dec 2003
The structural boundary between East and West Falkland: new evidence for movement history and lateral extent
No full textThe contrasting geology of East Falkland with its Lafonian Basin and E–W D1 fold-belt, and West Falkland, with Precambrian basement exposure and NE–SW D2 fold structures, has led previous researchers to infer a steep basement fault beneath the eastern margin of West Falkland. Recent research on the Falkland Islands has revealed further contrasts between East and West Falkland in: (a) the sedimentary, kerogen facies and thermal maturity of the West Falkland and Lower Lafonia groups and; (b) the structural relief of 6–8 km across the Hornby Anticline between West and East Falkland. These factors indicate that East Falkland has been downthrown to the SE along the Hornby Mountain Fault from Devonian until Jurassic times. Interval velocity and Bouguer Gravity data from a seismic line to the SW of the Falkland Islands reveal a similar basement structure to that interpreted under West Falkland and indicate that the fault continues offshore as a series of fault segments for at least 60 km along strike to the SW
Fault and magmatic interaction within Iceland's western rift over the last 9kyr
No full textWe present high-resolution 'Chirp' sub-bottom profiler data from Thingvallavatn, a lake in Iceland's western rift zone. These data are combined with stratigraphic constraints from sediment cores to show that movement on normal faults since 9 ka are temporally correlated with magmatic events, indicating that movements were controlled by episodic dyke intrusion. Sediment depo-centres and the focus of subsidence migrated westwards over 3-4 kyr towards the locus of subsequent brittle failure. We interpret this subsidence as related to dyke intrusion a few km along strike, originating from the Hengill volcanic system, which occurred prior to major dyking, faulting and subsidence within the lake at 1.9 ka
Hood Farm Association bull
No full textHood Farm Association bull kept by J.M. Montgomery, Starkville, Mississippi.https://scholarsjunction.msstate.edu/ext-ua-photos/1052/thumbnail.jp
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