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1.
D. Lange J. Cembrano A. Rietbrock C. Haberland T. Dahm K. Bataille 《Tectonophysics》2008,455(1-4):14-24
A temporal seismic network recorded local seismicity along a 130 km long segment of the transpressional dextral strike-slip Liquiñe-Ofqui fault zone (LOFZ) in southern Chile. Seventy five shallow crustal events with magnitudes up to Mw 3.8 and depths shallower than 25 km were observed in an 11-month period mainly occurring in different clusters. Those clusters are spatially related to the LOFZ, to the volcanoes Chaitén, Michinmahuida and Corcovado, and to active faulting on secondary faults. Further activity along the LOFZ is indicated by individual events located in direct vicinity of the surface expression of the LOFZ. Focal mechanisms were calculated using deviatoric moment tensor inversion of body wave amplitude spectra which mostly yield strike-slip mechanisms indicating a NE–SW direction of the P-axis for the LOFZ at this latitude. The seismic activity reveals the present-day activity of the fault zone. The recent Mw 6.2 event near Puerto Aysén, Southern Chile at 45.4°S on April 21, 2007 shows that the LOFZ is also capable of producing large magnitude earthquakes and therefore imposing significant seismic hazard to this region. 相似文献
2.
Michael Commer Stefan L. Helwig reas Hördt Carsten Scholl Bülent Tezkan 《Geophysical Journal International》2006,167(3):1172-1187
3.
Tadashi Kito reas Rietbrock Christine Thomas 《Geophysical Journal International》2007,169(3):1201-1209
We have developed a new array method combining conventional migration with a slowness-backazimuth deviation weighting scheme. All seismic traces are shifted based on the theoretical traveltime of the scattered wave from specific gridpoints in a 3-D volume. Observed slowness and backazimuth are calculated for each raypath and compared with theoretical values in order to estimate slowness and backazimuth deviations. Subsequently, stacked energy calculated by a conventional migration method is weighted by the slowness and backazimuth deviations to suppress any arrival energy whose slowness and backazimuth are inconsistent with the expected theoretical values. This new method was applied to two P- wave data sets which comprise (1) underside reflections at the 410 and 660 km mantle discontinuities and (2) D" reflections as well as their corresponding synthetic data sets. The results show that the weighting scheme dramatically increases the resolution of the migrated images and enables us to obtain well-constrained, focused images, making upper-mantle discontinuities and D" reflections more distinct by reducing their surrounding energy. 相似文献
4.
Robust estimates of magnetotelluric and geomagnetic response functions are determined using the coherency and expected uniformity of the magnetic source field as quality criteria. The method is applied on data sets of three simultaneously recording sites. For the data acquisition we used a new generation of geophysical equipment (S.P.A.M. MkIII), which comprises novel concepts of parallel computing and networked, digital data transmission. The data-processing results show that the amount of noise on the horizontal components of the magnetic field varies considerably in time, between sites and over the frequency range. The removal of such contaminated data beforehand is essential for most data-processing schemes, as the magnetic channels are usually assumed to be free of noise. The standard remote reference method is aimed at reducing bias in response function estimates. However, this does not necessarily improve their precision as our results clearly show. With our method, on the other hand, we can filter out source field irregularities, thereby providing suitable working conditions for the robust algorithm, and eventually obtain considerably improved results. Contrary to previous concepts, we suggest rejecting as much data as feasible in order to concentrate on the remaining parts of high-quality observations. 相似文献
5.
Frank C. van den Bosch reas Burkert Rob A. Swaters 《Monthly notices of the Royal Astronomical Society》2001,326(3):1205-1215
We compute the specific angular momentum distributions for a sample of low-mass disc galaxies observed by Swaters. We compare these distributions to those of dark matter haloes obtained by Bullock et al. from high-resolution N -body simulations of structure formation in a ΛCDM universe. We find that although the disc mass fractions are significantly smaller than the universal baryon fraction, the total specific angular momenta of the discs are in good agreement with those of dark matter haloes. This suggests that discs form out of only a small fraction of the available baryons, but yet manage to draw most of the available angular momentum. In addition we find that the angular momentum distributions of discs are clearly distinct from those of the dark matter; discs lack predominantly both low and high specific angular momenta. Understanding these findings in terms of a coherent picture for disc formation is challenging. Cooling, feedback and stripping, which are the main mechanisms to explain the small disc mass fractions found, seem unable to simultaneously explain the angular momentum distributions of the discs. In fact, it seems that the baryons that make up the discs must have been born out of angular momentum distributions that are clearly distinct from those of ΛCDM haloes. However, the dark and baryonic mass components experience the same tidal forces, and it is therefore expected that they should have similar angular momentum distributions. Therefore, understanding the angular momentum content of disc galaxies remains an important challenge for our picture of galaxy formation. 相似文献
6.
Geraint J. A. Harker Saleem Zaroubi Rajat M. Thomas Vibor Jeli&#; Panagiotis Labropoulos Garrelt Mellema Ilian T. Iliev Gianni Bernardi Michiel A. Brentjens A. G. de Bruyn Benedetta Ciardi Leon V. E. Koopmans V. N. Pandey reas H. Pawlik Joop Schaye Sarod Yatawatta 《Monthly notices of the Royal Astronomical Society》2009,393(4):1449-1458
Detecting redshifted 21-cm emission from neutral hydrogen in the early Universe promises to give direct constraints on the epoch of reionization (EoR). It will, though, be very challenging to extract the cosmological signal (CS) from foregrounds and noise which are orders of magnitude larger. Fortunately, the signal has some characteristics which differentiate it from the foregrounds and noise, and we suggest that using the correct statistics may tease out signatures of reionization. We generate mock data cubes simulating the output of the Low Frequency Array (LOFAR) EoR experiment. These cubes combine realistic models for Galactic and extragalactic foregrounds and the noise with three different simulations of the CS. We fit out the foregrounds, which are smooth in the frequency direction, to produce residual images in each frequency band. We denoise these images and study the skewness of the one-point distribution in the images as a function of frequency. We find that, under sufficiently optimistic assumptions, we can recover the main features of the redshift evolution of the skewness in the 21-cm signal. We argue that some of these features – such as a dip at the onset of reionization, followed by a rise towards its later stages – may be generic, and give us a promising route to a statistical detection of reionization. 相似文献
7.
Joachim Place Oshaine Blake Daniel Faulkner Andreas Rietbrock 《Pure and Applied Geophysics》2014,171(11):2887-2897
Stress variation and fluid migration occur in deformation zones, which are expected to affect seismic waves reflected off or propagating across such structures. We developed a basic experimental approach to monitor the mechanical coupling with respect to seismic coupling across a single discontinuity between a granite sample in contact with a steel platen. Piezoceramics located on the platen were used to both generate and record the P and S wave fields reflected off the discontinuity at normal incidence. This way, normal (B n ) and tangential (B t ) compliances were calculated using Schoenberg’s linear slip theory (Schoenberg, J Acoust Soc Am 68:1516–1521, 1980) when the roughness, the effective pressure (P eff, up to 200 MPa), and the nature of the filling (gas or water) vary. We observe that increasing the effective pressure decreases B n and B t , which is interpreted as the effect of the closure of the voids at the interface, permitting more seismic energy to be transmitted across the interface. Values of B n are significantly higher than those of B t at low P eff (<60–80 MPa) in dry conditions, and significantly drop under water-saturated conditions. The water filling the voids therefore helps to transmit the seismic energy of compressional waves across the interface. These results show that the assumption B n ≈ B t commonly found in some theoretical approaches does not always stand. The ratio B n /B t actually reflects the type of saturating fluids and the effective pressure, in agreement with other experimental studies. However, we illustrate that only the relative variations of this ratio seem to be relevant, not its absolute value as suggested in previous studies. Consequently, the use of B n against B t plots may allow effective pressure variation and the nature of the pore fluid to be inferred. In this respect, this experimental approach at sample scale helps to pave the way for remotely monitoring in the field the hydro-mechanical state of deformation zones, such as seismogenic faults, fractured reservoirs, or lava conduits. 相似文献
8.
Accurate Location of Synthetic Acoustic Emissions and Location Sensitivity to Relocation Methods, Velocity Perturbations, and Seismic Anisotropy 总被引：1，自引：0，他引：1
G. A. Jones S. E. J. Nippress A. Rietbrock J. M. Reyes-Montes 《Pure and Applied Geophysics》2008,165(2):235-254
Acoustic emission (AE) monitoring is a non-invasive method of monitoring fracturing both in situ, and in experimental rock deformation studies. Until recently, the major impediment for imaging brittle failure within a
rock mass is the accuracy at which the hypocenters may be located. However, recent advances in the location of regional scale
earthquakes have successfully reduced hypocentral uncertainties by an order of magnitude. The least-squares Geiger, master
event relocation, and double difference methods have been considered in a series of synthetic experiments which investigate
their ability to resolve AE hypocentral locations. The effect of AE hypocenter location accuracy due to seismic velocity perturbations,
uncertainty in the first arrival pick, array geometry and the inversion of a seismically anisotropic structure with an isotropic
velocity model were tested. Hypocenters determined using the Geiger procedure for a homogeneous, isotropic sample with a known
velocity model gave a RMS error for the hypocenter locations of 2.6 mm; in contrast the double difference method is capable
of reducing the location error of these hypocenters by an order of magnitude. We test uncertainties in velocity model of up
to ±10% and show that the double difference method can attain the same RMS error as using the standard Geiger procedure with
a known velocity model. The double difference method is also capable of precise locations even in a 40% anisotropic velocity
structure using an isotropic model for location and attains a RMS mislocation error of 2.6 mm that is comparable to a RMS
mislocation error produced with an isotropic known velocity model using the Geiger approach. We test the effect of sensor
geometry on location accuracy and find that, even when sensors are missing, the double difference method is capable of a 1.43
mm total RMS mislocation compared to 4.58 mm for the Geiger method. The accuracy of automatic picking algorithms used for
AE studies is ±0.5 μs (1 time sample when the sampling rate is 0.2 μs). We investigate how AE locations are effected by the
accuracy of first arrival picking by randomly delaying the actual first arrival by up to 5 time samples. We find that even
when noise levels are set to 5 time samples the double difference method successfully relocates the synthetic AE. 相似文献
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10.
Rajat M. Thomas Saleem Zaroubi Benedetta Ciardi reas H. Pawlik Panagiotis Labropoulos Vibor Jeli&#; Gianni Bernardi Michiel A. Brentjens A. G. de Bruyn Geraint J. A. Harker Leon V. E. Koopmans Garrelt Mellema V. N. Pandey Joop Schaye Sarod Yatawatta 《Monthly notices of the Royal Astronomical Society》2009,393(1):32-48