Earthquake hazards on the cascadia subduction zone

Large subduction earthquakes on the Cascadia subduction zone pose a potential seismic hazard. Very young oceanic lithosphere (10 million years old) is being subducted beneath North America at a rate of approximately 4 centimeters per year. The Cascadia subduction zone shares many characteristics with subduction zones in southern Chile, southwestern Japan, and Colombia, where comparably young oceanic lithosphere is also subducting. Very large subduction earthquakes, ranging in energy magnitude (M(w)) between 8 and 9.5, have occurred along these other subduction zones. If the Cascadia subduction zone is also storing elastic energy, a sequence of several great earthquakes (M(w) 8) or a giant earthquake (M(w) 9) would be necessary to fill this 1200-kilometer gap. The nature of strong ground motions recorded during subduction earthquakes of M(w) less than 8.2 is discussed. Strong ground motions from even larger earthquakes (M(w) up to 9.5) are estimated by simple simulations. If large subduction earthquakes occur in the Pacific Northwest, relatively strong shaking can be expected over a large region. Such earthquakes may also be accompanied by large local tsunamis.     

Visualizing tmRNA entry into a stalled ribosome

Bacterial ribosomes stalled on defective messenger RNAs (mRNAs) are rescued by tmRNA, an approximately 300-nucleotide-long molecule that functions as both transfer RNA (tRNA) and mRNA. Translation then switches from the defective message to a short open reading frame on tmRNA that tags the defective nascent peptide chain for degradation. However, the mechanism by which tmRNA can enter and move through the ribosome is unknown. We present a cryo-electron microscopy study at approximately 13 to 15 angstroms of the entry of tmRNA into the ribosome. The structure reveals how tmRNA could move through the ribosome despite its complicated topology and also suggests roles for proteins S1 and SmpB in the function of tmRNA.     

Murray fracture zone: westward extension

The Murray Fracture Zone is one of the principal east-west rifts in the crust of the northeast Pacific basin. As judged by bathymetric and magnetic surveys, the Murray approaches the Hawaiian Archipelago as a well-defined zone of ridges and troughs accompanied by strong, linear magnetic anomalies. It loses its topographic expression on encountering the Hawaiian Arch but can be traced magnetically to its intersection with the Hawaiian Ridge in the vicinity of Laysan Island (near 172 degrees W). All evidence tends to discount a previously suggested genetic relation between the Murray Fracture Zone and the Necker Ridge.     

Seismic and bathymetric evidence of a fracture zone on gorda ridge

Swarms of seaquakes have been located hydroacoustically on Gorda Ridge near 41.5 degrees N, 127.6 degrees W, where detailed bathymetric charts indicate a slight dextral offset of the rift valley in the center of the ridge. It is suggested that a fracture zone may be nascent in the area.     

Earthquake in a maze: compressional rupture branching during the 2012 M(w) 8.6 Sumatra earthquake

Seismological observations of the 2012 moment magnitude 8.6 Sumatra earthquake reveal unprecedented complexity of dynamic rupture. The surprisingly large magnitude results from the combination of deep extent, high stress drop, and rupture of multiple faults. Back-projection source imaging indicates that the rupture occurred on distinct planes in an orthogonal conjugate fault system, with relatively slow rupture speed. The east-southeast-west-northwest ruptures add a new dimension to the seismotectonics of the Wharton Basin, which was previously thought to be controlled by north-south strike-slip faulting. The rupture turned twice into the compressive quadrant, against the preferred branching direction predicted by dynamic Coulomb stress calculations. Orthogonal faulting and compressional branching indicate that rupture was controlled by a pressure-insensitive strength of the deep oceanic lithosphere.     

Subducted seamount imaged in the rupture zone of the 1946 nankaido earthquake

The Nankai Trough is a vigorous subduction zone where large earthquakes have been recorded since the seventh century, with a recurrence time of 100 to 200 years. The 1946 Nankaido earthquake was unusual, with a rupture zone estimated from long-period geodetic data that was more than twice as large as that derived from shorter period seismic data. In the center of this earthquake rupture zone, we used densely deployed ocean bottom seismographs to detect a subducted seamount 13 kilometers thick by 50 kilometers wide at a depth of 10 kilometers. We propose that this seamount might work as a barrier inhibiting brittle seismogenic rupture.     

Molokai fracture zone: continuation west of the hawaiian ridge

The Molokai Fracture Zone is one of the principal east-west (striking) fracture zones between California and the mid-Pacific. On the basis of bathymetry alone, it has been assumed that the zone ended abruptly east of the Hawaiian Islands. Use of correlation techniques between bathymetric features and their magnetic anomalies in the vicinity of the islands now show that structural elements of the zone cross the islands and continue westward for at least 130 kilometers. Recent volcanism and deep rift zones of the islands parallel the zone, suggesting structural correlation between the strike of the zone and the origin of recent volcanism in the islands.     

Fine structure of the landers fault zone: segmentation and the rupture process

Observations and modeling of 3- to 6-hertz seismic shear waves trapped within the fault zone of the 1992 Landers earthquake series allow the fine structure and continuity of the zone to be evaluated. The fault, to a depth of at least 12 kilometers, is marked by a zone 100 to 200 meters wide where shear velocity is reduced by 30 to 50 percent. This zone forms a seismic waveguide that extends along the southern 30 kilometers of the Landers rupture surface and ends at the fault bend about 18 kilometers north of the main shock epicenter. Another fault plane waveguide, disconnected from the first, exists along the northern rupture surface. These observations, in conjunction with surface slip, detailed seismicity patterns, and the progression of rupture along the fault, suggest that several simple rupture planes were involved in the Landers earthquake and that the inferred rupture front hesitated or slowed at the location where the rupture jumped from one to the next plane. Reduction in rupture velocity can tentatively be attributed to fault plane complexity, and variations in moment release can be attributed to variations in available energy.     

Earthquake hazard after a mainshock in california

After a strong earthquake, the possibility of the occurrence of either significant aftershocks or an even stronger mainshock is a continuing hazard that threatens the resumption of critical services and reoccupation of essential but partially damaged structures. A stochastic parametric model allows determination of probabilities for aftershocks and larger mainshocks during intervals following the mainshock. The probabilities depend strongly on the model parameters, which are estimated with Bayesian statistics from both the ongoing aftershock sequence and from a suite of historic California aftershock sequences. Probabilities for damaging aftershocks and greater mainshocks are typically well-constrained after the first day of the sequence, with accuracy increasing with time.     

地震诱发滑坡的机理研究

在阅读大量相关文献的基础上,归纳了国内外地震诱发滑坡的机理研究现状,分析了毛彦龙等学者提出的累进破坏效应结论,并基于此探讨了地震诱发滑坡的机理研究的发展趋势。     

Management zone delineation using a modified watershed algorithm

Site-specific management (SSM) is a common way to manage within-field variability. This concept divides fields into site-specific management zones (SSMZ) according to one or several soil or crop characteristics. This paper proposes an original methodology for SSMZ delineation which is able to manage different kinds of crop and/or soil images using a powerful segmentation tool: the watershed algorithm. This image analysis algorithm was adapted to the specific constraints of precision agriculture. The algorithm was tested on high-resolution bio-physical images of a set of fields in France.     

Simulation of subduction zone seismicity by dehydration of serpentine

We measured acoustic emission energy during antigorite dehydration in a multianvil press from 1.5 to 8.5 gigapascals and 300 degrees to 900 degrees C. There was a strong acoustic emission signal on dehydration, and analysis of recovered samples revealed brittle deformation features associated with high pore-fluid pressures. These results demonstrate that intermediate depth (50 to 200 kilometers) seismicity can be generated by dehydration reactions in the subducting slab.     

汶川地震滑坡损毁土地复垦关键技术——以北川县陈家坝乡灾后土地复垦项目为例

在对四川地震灾区北川县陈家坝乡损毁土地实地调查的基础上,根据损毁土地成因,分析滑坡损毁土地的基本特征,提出其土地复垦的关键技术主要为截一排一堵。     

地震成因环境污染及其防治对策探讨——以汶川8.0级特大地震为例

通过深入分析震后灾区环境污染状况,提出了相应的环境污染防治对策.