Fakultät für Geowissenschaften - Digitale Hochschulschriften der LMU

Seismic databases and processing tools currently available are mainly limited to classic three-component seismic recordings and cannot handle collocated multi-component, multi-disciplinary datasets easily. Further, these seismological databases depend on event-related data and are not able to manage state of the art continuous waveform data input as well. None of them allows for automated request of data available at seismic data centers or to share specific data to users outside one institute. Some seismic databases even depend on licensed database engines, which contradicts the open source character of most software packages used in seismology.

This study intends to provide a suitable answer to the deficiencies of existing seismic databases. SeisHub is a novel web-based database approach created for archiving, processing, and sharing geophysical data and meta data (data describing data), particularly adapted for seismic data. The implemented database prototype offers the full functionality of a native XML database combined with the versatility of a RESTful Web service. The XML database itself uses a standard relational database as back-end, which is currently tested with PostgreSQL (http://www.postgres.org) and SQLite (http://www.sqlite.org). This sophisticated structure allows for the usage of both worlds: on the one hand the power of the SQL for querying and manipulating data, and one the other hand the freedom to use any standard connected to XML, e.g. document conversion via XSLT (Extensible Stylesheet Language Transformations) or resource validation via XSD (XML Schema). The actual resources and any additional services are available via fixed Uniform Resource Identifiers (URIs), where as the database back-end stores the original XML documents and all related indexed values. Indexes are generated using the XPath language and may be added at any time during runtime. This flexibility of the XML/SQL mixture introduced above enables the user to include parameters or results as well as meta data from additional or yet unknown monitoring techniques at any time. SeisHub also comprises features of a “classical seismic database” providing direct access to continuous seismic waveform data and associated meta data. Additionally, SeisHub offers various access protocols (HTTP/HTTPS, SFTP, SSH), an extensible plug-in system, user management, and a sophisticated web-based administration front-end. The SeisHub database is an open source project and the latest development release can be downloaded via the project home page http://www.seishub.org.

The SeisHub database has already been deployed as central database component within two scientific projects: Exupéry (http://www.exupery-vfrs.de), a mobile Volcano Fast Response System (VFRS), and BayernNetz, the seismological network of the Bavarian Seismological Service (Erdbebendienst Bayern; http://www.erdbeben-in-bayern.de).

Abstract

Exploration activities outlined a shear zone-hosted gold and base metal occurrence in low-grade metasedimentary rocks near the town of Workamba, Tigray State, northern Ethiopia. Gold resources are estimated at ~2 t Au with concentrations reaching up to 8 g/t Au in the ore. The geology of the study area is dominated by foliated or sheared, lower greenschist facies metavolcanic and metasedimentary rocks, which are correlated to the lower island-arc metavolcanic rocks of the ~860 to 750 Ma Tsaliet Group and upper carbonate and metasedimentary rocks of the ~740 Ma Tambien Group, respectively. The Tsaliet and Tambien groups represent the southern parts of the Arabian-Nubian Shield. The Tsaliet Group, which is exposed in the north-western parts of the study area, consists mainly of metavolcaniclastic rocks. The overlying slate, phyllite, sericite-chlorite schists and marbles of the Tambien Group occupy the south-eastern part of the area. A steeply dipping NE trending ductile shear zone occurs in the metasediments along the contact with the Tsaliet Group. It is intruded by monzogranite and lamprophyre dikes or sills. The shear zone is correlated with an early regional deformation event (D1), which also caused NE foliations and ENE trending folds. A second regional deformation phase (D2) resulted in NNE oriented weakly developed dextral shear bands and N-S trending parasitic folds. NW striking brittle faults postdate D1 and D2 ductile structures.

Trace element patterns of the metasedimentary rocks support findings of previously published works suggesting that they are derived from the rocks of the Tsaliet Group. Major, trace, and rare earth element characteristics of the monzogranite dikes/sills permit the conclusion that they are related to the ~620 to 520 Ma post-tectonic magmatism in the Tigray region. The post-tectonic granitoids are resulted from mantle-derived magmas, which are modified by fluid components of the subducted slab or involvement of continental crust. Petrographic and geochemical results of the lamprophyres show that they are calc-alkaline in nature. The monzogranite dikes/sills have 208Pb/204Pb, 207Pb/204Pb, and 206Pb/204Pb values between 37.176 to 37.310, 15.508 to 15.525, and 17.799 to 18.310, respectively. These lead isotope ratios are compatible with a significant mantle component in these rocks. Lead isotope ratios of the metasedimentary and metavolcanic rocks at Workamba are heterogeneous, suggesting that not all samples did retain their original Pb isotope signatures. However, the original Pb isotope composition of these rocks tends to be less radiogenic as those of the post-tectonic dikes/sills.

Field and petrographic observations show that the metavolcanic rocks are locally affected by propylitic alteration and sericitization. The metasedimentary rocks suffered from pervasive silicification, sericitization, and carbonatization. The monzogranite and lamprophyre dikes/sills locally experienced pervasive sericitization, carbonatization, epidotization, and chloritization. Two generations of quartz veins occur cutting and/or parallel to D1 foliation on the Tsaliet and Tambien groups. The first generation quartz veins are folded and less abundant as compared to the unfolded second generations. The metasedimentary rocks and the dikes/sills are also cut by calcite ±quartz veinlets in the mineralized zone. Mass balance calculations of the metasedimentary rocks suggest that Na2O, and Sr are removed by hydrothermal fluids from the original composition of the rocks. K2O, MnO, CaO, Ba, and Pb are variably added or removed. The dikes/sills retained near original geochemical composition.

The shear zone hosted metallic mineralization occurs within the metasedimentary rocks, in close spatial proximity to the dikes/sills. It took place under brittle conditions. Pyrite, sphalerite, galena, and chalcopyrite are the main sulfide phases, whereas pyrrhotite, arsenopyrite, and chalcocite are minor components. Gold

Theoretically, to fully describe the change in the medium around a point one needs three components of translation, six components of strain, and three components of rotation. It is expected that collocated measurements of translations and rotations may help (1) correcting translation signals recorded by classical seismometers for contamination by ground rotations, (2) extracting additional information on earthquake source properties, soil-structure interactions, and properties of the subsurface, and (3) providing additional ground motion information to earthquake engineers for seismic design. Thus, in addition to translations and strains, the rotational part of ground motions should also be recorded. However, the lack of instrumental sensitivity did not allow seismologists to observe rotational motions for decades. Recently, ring laser technology has provided the means to develop instruments that allow in principle the observation of rotational motions in a wide frequency band and epicentral distance range.

Here we present the observations of rotational ground motions around a vertical axis in the P coda (the section between the onsets of direct P- and S- waves) of tele-seismic signals on a ring laser sensor at the Fundamental Observatory Wettzell, southeast Germany. The studies focus on finding the explanation for the observed P coda rotations as well as the way to extract additional information from the use of co-seismic rotational motions. First, the effects of co-seismic tilts on ring laser measurements are quantified based on magnitude-amplitude relations and translation derived tilts. Then the phenomenon of scattering assuming three dimensional random media and topography that may generate the observed P coda rotations is investigated through analysis of observations and forward modelling. The partitioning of P and S energy indicated by the stabilization of the ratio of energies of the two is used to constrain scattering properties. Finally, an analytical approach focusing on the solution of plane waves in linear elastic anisotropic media is used to quantify the anisotropic behavior through the variations of rotational wavefield. The focus is on quasi-P waves and transverse isotropic media. Kelvin-Christoffel equation and the Thomson parameters, descriptive of the degree of anisotropy, are used. The obtained results show that 1) P-SH scattering in the random crust is the main cause of the P-coda observations; and 2) rotational motions contain additional information (at least) about scattering properties and anisotropic coefficients and that joint measurements of translational and rotational motions at only one point allow the extraction of additional information. The results demonstrate the potential benefit not only of measurements of co-seismic rotational ground motions but also of the use of the amplitude content of seismic signals.