59. MAGNETIC PROPERTIES AND PALEOMAGNETISM OF VOLCANIC ROCKS AND INTERLAYERED SEDIMENTS FROM THE JAPAN SEA (ODP LEG 127) 1

ABSTRACT Measurements of natural remanent magnetization (NRM), initial susceptibility (K), anisotropy of magnetic susceptibility, frequency dependent susceptibility (Xfd), and viscous remanent magnetization (VRM) are reported from volcanic rocks recovered during ODP Leg 127 in the Japan Sea. The results indicate a significant difference between the basalts drilled in the Yamato Basin (Site 794 and 797) and in the Japan Basin (Site 795). The Koenigsberger ratios (Q) show very low values in the Yamato Basin attesting that the remanence is not dominant over the induced magnetization. This evidence could explain why no magnetic anomaly pattern has been recognized in this basin. Experiments of VRM acquisition and decay show that both the processes are multistage with the acquisition process proceeding more rapidly and deviates more from a log (t) law than the corresponding decay. The sediments interlayered with the basalts in the acoustic basement of the Yamato Basin show processes of remagnetization related to the emplacement of the dikes. Temperatures of heating between 200° and 250° C were estimated from the different unblocking temperatures of the two components of magnetization

Deep-sea borehole seismological observatories in the western Pacific: temporal variation of seismic noise level and event detection

Seismological networks provide critical data for better understanding the dynamics of the Earth; however, a great limitation on existing networks is the uneven distribution of stations. In order to achieve a more uniform distribution of seismic stations, observatories must be constructed in marine areas. The best configuration for oceanic seismic observatories is thought to be placement of seismometers in deep boreholes. Two deep-sea borehole seismological observatories (WP-1 and WP-2) were constructed in the western Pacific and form the initial installations of a 1000 km span network. At present, seismic records of more than 400 total days were retrieved from both the WP-1 and WP-2. Long-term variations in broadband seismic noise spectra (3mHz - 10 Hz) in the western Pacific were revealed from these records, and the data showed that ambient seismic noise levels in borehole observatories are comparable to those of the quietest land seismic stations. In addition, there is little temporal variation of noise levels in periods greater than 10 seconds. Due to this low seismic noise environment, many teleseismic events with magnitudes greater than 5 were recorded. It is confirmed that seismic observation in deep-sea borehole gives the best environment for earthquake observation in marine areas

GEMS: the opportunity for stress-forecasting all damaging earthquakes worldwide

A new understanding of rock deformation allows the accumulation of stress before earthquakes to be monitored by using shear-wave splitting to assess stress-induced changes to microcrack geometry. Using swarms of small earthquakes as the source of shear-waves, such stress accumulations have been recognised with hindsight before some fifteen earthquakes worldwide. On one occasion the time, magnitude, and fault-break of an M 5 earthquake was successfully stress-forecast in a comparatively narrow magnitude/time window. However, suitable swarms of small earthquakes are very uncommon, and routine forecasting requires measurements of controlled-source observations at bore-hole Stress-Monitoring Sites (SMSs). A prototype SMS confirmed that both science and technology are effective for monitoring stress changes before earthquakes, and the sensitivity is such that a network of SMSs, on a 400 km-grid, say, could stress-forecast all M ≥ 5 earthquakes, that is all damaging earthquakes, within the grid. This paper suggests that a Global Earthquake Monitoring System (GEMS) could forecast all damaging earthquakes in both developing and developed countries worldwide

Executive summary: "Mantle Frontier" workshop

The workshop on “Reaching the Mantle Frontier: Moho and Beyond� was held at the Broad Branch Road Campus of the Carnegie Institution of Washington on 9–11 September 2010. The workshop attracted seventy-four scientists and engineers from academia and industry in North America, Asia, and Europe.Reaching and sampling the mantle through penetration of the entire oceanic crust and the Mohorovi�ić discontinuity (Moho) has been a longstanding goal of the Earth science community. The Moho is a seismic transition, often sharp, from a region with compressional wave velocities (Vp) less than 7.5 km s-1 to velocities ~8 km s-1. It is interpreted in many tectonic settings, and particularly in tectonic exposures of oceanic lower crust, as the transition from igneous crust to mantle rocks that are the residues of melt extraction. Revealing the in situ geological meaning of the Moho is the heart of the Mohole project. Documenting ocean-crust exchanges and the nature and extent of the subseafloor biosphere have also become integral components of the endeavor. The purpose of the “Mantle Frontier� workshop was to identify key scientific objectives associated with innovative technology solutions along with associated timelines and costs for developments and implementation of this grandchallenge

The bends on a quantum waveguide and cross-products of Bessel functions

A detailed analysis of the wave-mode structure in a bend and its incorporation into a stable algorithm for calculation of the scattering matrix of the bend is presented. The calculations are based on the modal approach. The stability and precision of the algorithm is numerically and analytically analysed. The algorithm enables precise numerical calculations of scattering across the bend. The reflection is a purely quantum phenomenon and is discussed in more detail over a larger energy interval. The behaviour of the reflection is explained partially by a one-dimensional scattering model and heuristic calculations of the scattering matrix for narrow bends. In the same spirit we explain the numerical results for the Wigner-Smith delay time in the bend.Comment: 34 pages, 21 figure

Discussions of ODP Leg 205 and Drilling of Middle America Seismogenic Zone

Erosional convergent margins, where material is removed from the base of the upper plate and subducted on the lower plate, are fundamental features of the Circum Pacific. The erosional Middle America Trench convergent Pacific margin, remarkable for its broad diversity of dynamic environments, is a natural laboratory for studying convergent margin processes and seismogenesis. These environments include a shallow and deep trench axis, shallow-to-steep-dipping plate interfaces, abnormally hot-to-cold subducting plate temperatures, and a subducting plate with smooth morphology bordering basement ridges and seamounts. The subducting topography accelerates erosion and localizes seismicity

Ocean Seismic Network Pilot Experiment

Author Posting. © American Geophysical Union, 2003. It is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 4 (2003): 1092, doi:10.1029/2002GC000485.The primary goal of the Ocean Seismic Network Pilot Experiment (OSNPE) was to learn how to make high quality broadband seismic measurements on the ocean bottom in preparation for a permanent ocean seismic network. The experiment also had implications for the development of a capability for temporary (e.g., 1 year duration) seismic experiments on the ocean floor. Equipment for installing, operating and monitoring borehole observatories in the deep sea was also tested including a lead-in package, a logging probe, a wire line packer and a control vehicle. The control vehicle was used in three modes during the experiment: for observation of seafloor features and equipment, for equipment launch and recovery, and for power supply and telemetry between ocean bottom units and the ship. The OSNPE which was completed in June 1998 acquired almost four months of continuous data and it demonstrated clearly that a combination of shallow buried and borehole broadband sensors could provide comparable quality data to broadband seismic installations on islands and continents. Burial in soft mud appears to be adequate at frequencies below the microseism peak. Although the borehole sensor was subject to installation noise at low frequencies (0.6 to 50 mHz), analysis of the OSNPE data provides new insights into our understanding of ocean bottom ambient noise. The OSNPE results clearly demonstrate the importance of sediment borne shear modes in ocean bottom ambient noise behavior. Ambient noise drops significantly at high frequencies for a sensor placed just at the sediment basalt interface. At frequencies above the microseism peak, there are two reasons that ocean bottom stations have been generally regarded as noisier than island or land stations: ocean bottom stations are closer to the noise source (the surface gravity waves) and most ocean bottom stations to date have been installed on low rigidity sediments where they are subject to the effects of shear wave resonances. When sensors are placed in boreholes in basement the performance of ocean bottom seismic stations approaches that of continental and island stations. A broadband borehole seismic station should be included in any real-time ocean bottom observatory.This work was sponsored by the National Science Foundation (NSF Grant Numbers: OCE-9522114, OCE-9523541 and OCE-9819439) with additional support from Incorporated Research Institutions for Seismology (IRIS), Joint Oceanographic Institutions, Inc. (JOI Contract No: 12-94), Scripps Institution of Oceanography, a Mellon Grant from Woods Hole Oceanographic Institution, and the Earthquake Research Institute at the University of Tokyo (Visiting Professorship for RAS)