Core-collapse supernovae and the Younger Dryas/terminal Rancholabrean extinctions     

G. Robert Brakenridge 《Icarus》2011,215(1):101-106

Early predictions that some supernovae release large quantities of prompt high energy photons are now corroborated by optical identification of core-collapse supernovae associated with extragalactic GRBS (beamed γ-ray bursts) and XRFS (beamed or un-beamed X-ray flashes). Given the in-galaxy supernova frequency and GRB and XRF recurrence statistics, significant Earth-incident events during the past several million years very likely occurred and nearby events should have affected the Earth and other planetary atmospheres, including terrestrial surface solar UV, the Earth’s climate, and its ecology. The Younger Dryas Stadial (∼12,900 to 11,550 calendar yr BP) began with sharply cooler temperatures in the Earth’s northern hemisphere, regional drought, paleoecological evidence compatible with increased UV, and abrupt increases in cosmogenic ¹⁴C and ¹⁰Be in ice and marine cores and tree rings. In North America, stratigraphic and faunal sequences indicate that a major pulse of mammalian extinctions (at least 23-31 genera) began very close to 12,830 calendar yr BP and was sudden: deposits one century younger are devoid of diverse extinct fauna remains. A 10 s beamed GRB within 2 kpc of the Earth delivers 100 kJ m⁻² fluence to the Earth’s atmosphere, where it causes spallation and catalytic reactions depleting 35-50% O³, and producing excess NO_x species (which favor cooling, drought, and surface fertility), ¹⁴C, and ¹⁰Be. An un-beamed, 10⁵⁰ erg hard photon impulse at ∼250 pc produces similar terrestrial atmospheric effects. A well-characterized massive star supernova, the unusually close Vela event (d = 250 ± 30 pc; total energy of 1-2 × 10⁵¹ erg; age constrained from remnant nebula shock velocities considerations at 13,000-16,000 yr and from the pulsar characteristic age at ∼11,400 yr) may have initiated the Younger Dryas climate change, and caused the extinction of the terminal Rancholabrean fauna.  相似文献   

CMT data inversion using a Bayesian information criterion to estimate seismogenic stress fields     

Toshiko Terakawa  Mitsuhiro Matsu'ura 《Geophysical Journal International》2008,172(2):674-685

We developed an inversion method to estimate the stress fields related to earthquake generation (seismogenic stress fields) from the centroid moment tensors (CMT) of seismic events by using Akaike's Bayesian information criterion (ABIC). On the idea that the occurrence of an earthquake releases some part of the seismogenic stress field around its hypocentre, we define the CMT of a seismic event by a weighted volume integral of the true but unknown seismogenic stress field. Representing each component of the seismogenic stress field by the superposition of a finite number of 3-D basis functions (tri-cubic B-splines), we obtain a set of linear observation equations to be solved for the expansion coefficients (model parameters). We introduce prior constraint on the roughness of the seismogenic stress field and combine it with observed data to construct a Bayesian model with hierarchic, highly flexible structure controlled by hyper-parameters. The optimum values of the hyper-parameters are objectively determined form observed data by using ABIC. Given the optimum values of the hyper-parameters, we can obtain the best estimates of model parameters by using a maximum likelihood algorithm. We tested the validity of the inversion method through numerical experiments on two synthetic CMT data sets, assuming the distribution of fault orientations to be aligned with the maximum shear stress plane in one case and to be random in the other case. Then we applied the inversion method to actual CMT data in northeast Japan, and obtained the pattern of the seismogenic stress field consistent with geophysical and geological observations.  相似文献   

Numerical modelling of rise and fall of a dense layer in salt diapirs   总被引：4，自引：0，他引：4  

Z. Chemia  H. Koyi  H. Schmeling 《Geophysical Journal International》2008,172(2):798-816

Numerical models are used to study the entrainment of a dense anhydrite layer by a diapir. The anhydrite layer is initially horizontally embedded within a viscous salt layer. The diapir is down-built by aggradation of non-Newtonian sediments ( n = 4, constant temperature) placed on the top of the salt layer. Several parameters (sedimentation rate, salt viscosity, perturbation width and stratigraphic position of the anhydrite layer) are studied systematically to understand their role in governing the entrainment of the anhydrite layer. High sedimentation rates during the early stages of the diapir evolution bury the initial perturbation and, thus, no diapir forms. The anhydrite layer sinks within the buried salt layer. For the same sedimentation rate, increasing viscosity of the salt layer decreases the rise rate of the diapir and reduces the amount (volume) of the anhydrite layer transported into the diapir. Model results show that viscous salt is capable of carrying separate blocks of the anhydrite layer to relatively higher stratigraphic levels. Varying the width of the initial perturbation (in our calculations 400–800 m), from which a diapir triggers, shows that wider diapirs can more easily entrain an embedded anhydrite layer than the narrower diapirs. The anhydrite layer is entrained as long as rise rate of the diapir exceeds the descent rate of the denser anhydrite layer. We conclude that the four parameters mentioned above govern the ability of a salt diapir to entrain an embedded dense layer. However, the model results show that the entrained blocks inevitably sink back if the rise rate of the diapir is less than the rate of descent of the anhydrite layer or the diapir is permanently covered by a stiff overburden in case of high sedimentation rates.  相似文献   

A multidisciplinary study of a slow-slipping fault for seismic hazard assessment: the example of the Middle Durance Fault (SE France)     

E. M. Cushing  O. Bellier  S. Nechtschein  M. Sébrier  A. Lomax  Ph. Volant  P. Dervin  P. Guignard  L. Bove 《Geophysical Journal International》2008,172(3):1163-1178

Assessing seismic hazard in continental interiors is difficult because these regions are characterized by low strain rates and may be struck by infrequent destructive earthquakes. In this paper, we provide an example showing that interpretations of seismic cross sections combined with other kinds of studies such as analysis of microseismicity allow the whole seismogenic source area to be imaged in this type of region. The Middle Durance Fault (MDF) is an 80-km-long fault system located southeastern France that has a moderate but regular seismicity and some palaeoseismic evidence for larger events. It behaves as an oblique ramp with a left-lateral-reverse fault slip and has a low strain rate. MDF is one of the rare slow active fault system monitored by a dedicated dense velocimetric short period network. This study showed a fault system segmented in map and cross section views which consists of staircase basement faults topped by listric faults ramping off Triassic evaporitic beds. Seismic sections allowed the construction of a 3-D structural model used for accurate location of microseismicity. Southern part of MDF is mainly active in the sedimentary cover. In its northern part and in Alpine foreland, seismicity deeper than 8 km was also recorded meaning active faults within the crust cannot be excluded. Seismogenic potential of MDF was roughly assessed. Resulting source sizes and estimated slip rates imply that the magnitude upper limit ranges from 6.0 to 6.5 with a return period of a few thousand years. The present study shows that the coupling between 3-D fault geometry imaging and accurate location of microseismicity provides a robust approach to analyse active fault sources and consequently a more refined seismic hazard assessment.  相似文献   

The reflection seismic survey of project TIPTEQ—the inventory of the Chilean subduction zone at 38.2° S     

K. Groß  U. Micksch  TIPTEQ Research Group  Seismics Team 《Geophysical Journal International》2008,172(2):565-571

We describe results of an active-source seismology experiment across the Chilean subduction zone at 38.2°S. The seismic sections clearly show the subducted Nazca plate with varying reflectivity. Below the coast the plate interface occurs at 25 km depth as the sharp lower boundary of a 2–5 km thick, highly reflective region, which we interpret as the subduction channel, that is, a zone of subducted material with a velocity gradient with respect to the upper and lower plate. Further downdip along the seismogenic coupling zone the reflectivity decreases in the area of the presumed 1960 Valdivia hypocentre. The plate interface itself can be traced further down to depths of 50–60 km below the Central Valley. We observe strong reflectivity at the plate interface as well as in the continental mantle wedge. The sections also show a segmented forearc crust in the overriding South American plate. Major features in the accretionary wedge, such as the Lanalhue fault zone, can be identified. At the eastern end of the profile a bright west-dipping reflector lies perpendicular to the plate interface and may be linked to the volcanic arc.  相似文献   

Quasi-static analysis of strike fault growth in layered media     

Nobuki Kame  Shuji Saito  Kenji Oguni 《Geophysical Journal International》2008,173(1):309-314

We study the effects of structural inhomogeneity on the quasi-static growth of strike-slip faults. A layered medium is considered, made up of an upper layer bounded by a free surface and welded to a lower half-space with different elastic property. Mode III crack is employed as a mathematical model of strike-slip fault, which is nucleated in the lower half-space and then propagates towards the interface. We adopt FEM-β, newly proposed analysis method for failure, to simulate the quasi-statistic crack growth governed by the stress distribution in layered media. Our results show that along planar traces across interfaces a compliant upper layer has significant effects on promoting/suppressing crack growth before/after its extension into the layer and vice versa for a rigid one. This proposes a possibility that surface breaks due to strike-slip faulting could be arrested by deposit layers at the topmost part of the Earth's crust.  相似文献   

Note on rain-triggered earthquakes and their dependence on karst geology   总被引：2，自引：0，他引：2  

S. A. Miller 《Geophysical Journal International》2008,173(1):334-338

Recently reported rain-triggered seismicity from three separate storms occurred exclusively in karst geology. In this paper, I discuss how the hydrogeology of karst controls rain-triggered seismicity by channeling of the watershed after intense rainfall directly into the karst network. Such channeling results in very large increases in hydraulic head, and more importantly, substantially increases the vertical stress acting on the underlying pore-elastic media. Rapid loading upon a pore-elastic media induces seismicity by increasing pore pressure at depth in a manner similar to that observed from reservoir impounding. Using a simple 1-D model of a pore-elastic medium, it is shown that the instantaneous fluid pressure increase at depth is a substantial fraction of the pressure step applied at the boundary, followed by time-dependent pore pressure increases associated with the typical linear diffusion problem. These results have implications for the change in fluid pressure necessary to trigger earthquakes, and leads to the following hypothesis to be tested: Unambiguous rain-triggered seismicity will only occur in karst regions.  相似文献   

Effect of margin curvature on plate deformation in a 3-D numerical model of subduction zones     

M.-A. Bonnardot  R. Hassani  E. Tric  E. Ruellan  M. Régnier 《Geophysical Journal International》2008,173(3):1084-1094

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Resolving three-dimensional anisotropic structure with shear wave splitting tomography     

David L. Abt  Karen M. Fischer 《Geophysical Journal International》2008,173(3):859-886

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Crustal thickness estimation beneath the southern central Andes at 30°S and 36°S from S wave receiver function analysis     

Benjamin Heit  Xiaohui Yuan  Marcelo Bianchi  Forough Sodoudi  Rainer Kind 《Geophysical Journal International》2008,174(1):249-254

We have used the S wave receiver function (SRF) technique to investigate the crustal thickness beneath two seismic profiles from the CHARGE project in the southern central Andes. A previous study employing the P wave receiver function method has observed the Moho interface beneath much of the profiles. They found, however, that the amplitude of the P to S conversion was diminished in the western part of the profiles and have attributed it to a reduction of the impedance contrast at the Moho due to lower crustal ecologitization. With SRF, we have successfully detected S to P converted waves from the Moho as well as possible conversions from other lithospheric boundaries. The continental South American crust reaches its maximum thickness of ∼70 km (along 30°S between 70°W and 68.5°W) beneath the Principal Cordillera and the Famatina system and becomes thinner towards the Sierras Pampeanas with a thickness of ∼40 km. Negative phases, possibly related to the base of the continental and oceanic lithosphere, can be recognized in the summation traces at different depths. By comparing our results with data obtained from previous investigations, we are able to further constrain the thickness of the crust and lithosphere beneath the central Andes.  相似文献