Moment Tensor Inversion of Regional Phases: Application to a Mine Collapse

—?The aim of our study consists of analyzing potentially non-double-couple seismic events recorded at regional distances. In order to define the nature of the seismic source, a moment tensor inversion is carried out as this method is general enough not to initially constrain the source mechanism. In this paper we present an application to a seismic event induced by a mine collapse which occurred near the town of Halle in Germany. Because of its induced nature, many parameters such as the location and geometry of this seismic source are known. This information allows us to test the influence of inadequate propagation modeling on the moment tensor obtained from the inversion. Green's functions have been computed with the reflectivity method in a flat layered medium, using the European model EurID (Du et? al., 1998; Dufumier et al., 1997). From the inversion of P-wave seismograms recorded by the German Regional Seismic Network will, we obtained a source time function which can be decomposed into two subevents. The first one has a large isotropic part and a deviatoric mechanism with near vertical nodal planes. No volume change is observed for the second subevent, but a deviatoric component opposite of the first one. The addition of S-waves does not change the results of the inversion which are stable. Surface waves were not used because of their poor dispersion curves. Based on the moment tensor obtained from these inversions, the physical process at the source is compatible with a large cavity collapse.     

Recent crustal movement studies in Aswan region, Egypt: introduction

After the November 1981 earthquake in the Aswan area along the Kalabsha fault, Nubian Desert, studies on its origin were initiated in the Helwan Institute of Astronomy and Geophysics [now National Research Institute of Astronomy and Geophysics (NRIAG) in Helwan]. As a first step an array of seismic stations were established in cooperation with the Lamont-Doherty Geological Observatory and the correlation between the number of seismic events and the water level in Lake Nasser was investigated. In order to understand the eartqquake mechanism, additional information on the crustal deformation due to water load was required, and the International Center on Recent Crustal Movements (ICRCM) was invited to consult in the necessary procedures aimed at recent crustal movement studies around part of the western bank of Lake Nasser. The first local network of horizontal and vertical geodetic measurements was established and measured in 1984. Simultaneously, with respect to the extreme meteorological conditions in the desert, additional investigations as refraction studies, stability of bench marks etc. were initiated. The extension of the network and an increase of repeated measurements first brought results during the last few years. The results of these studies will be reported in the following papers in detail as a homogenous block of scientific communication.     

Present state of geodynamic properties of Kalabsha area, nortwest of Aswan Lake, Egypt

Aswan Lake is the second largest man-made lake in the world. Its filling started 1964 and reached the maximum water level in 1978. An earthquake of magnitude 5.5 took place in 1981 along the most active fault near the lake (Kalabsha fault). This earthquake was follwed by a tremendous number of smaller events that continue till now.Seismicity and the underground water table around the lake are monitored continuously through a radio-telemetered network. A local geodetic network was established around parts of the active faults in the northwestern part of the High Dam Lake, for monitoring vertical and lateral movements. The Kalabsha local geodetic network (the first one) was established around an active part of the Kalabsha fault in 1983. Precise geodetic measurements have benn carried out twice a year since 1984.On the basis of the repeated geodetic measurements, seismicity of the area and geophysical as well as geological data, the present state of the geodynamical properties of the Kalabsha area is studied.Remarkable horizontal movements were detected; they are correlated with the seismicity of the area and are attributed to the differential loading by the lake. The Kalabsha fault is a right-lateral strike-slip motion on an E-W plane. The magnitude of the movements detected along the fault is variable for the different epochs of measurements and is correlated with both seismicity and water loading in the lake.     

Expansions of the double product of the global gravity field

Summary Four fundamental forms of expansion of the double product of the gravity function into an infinite series are derived. Two isotropic parts are separated, one is expressed by Legendre polynomials and the other by invariants of Wigner's D-matrices.

---

¶rt; m n¶rt;mau ¶rt; au ¶rt; nu¶rt;uaumau uu ¶rt;. ¶rt; ¶rt; umn amu, na aam n nu a¶rt;a u ma n uauam amu ua.

     

The map of vertical crustal movements in Czechoslovakia and its interpretation

In addition to the measurements carried out in the framework of the 1973–1978 international relevellings, further levelling lines have been remeasured on the territory of Czechoslovakia till now. Considering all new relevellings on the region under study, an independent adjustment of annual velocities of vertical movements in whole network was performed. The values of vertical movements were determined relatively to the fundamental benchmark Želešice in the central part of Czechoslovakia, monumented in the bedrock of the southeastern border of the Bohemian Massif. The results are presented in a map of vertical movements and preliminarily interpreted in connection to the geological features of the territory under study.     

The dynamics of the northwestern border of the Carpathians

The border area between the Bohemian Massif and the Carpathians arc is covered by dense networks of repeated levellings as well as horizontal (triangulation, trilateration) measurements. The results were adjusted in separate vertical or horizontal networks respectively. Derived rates of vertical and horizontal movements are presented. The properties of these movements are discussed as a background to the tectonic development of the area under study.     

Earthquakes in the Czech Republic and Surrounding Regions in 1995–1999

In the time span from January 1995 to December 1999 the Czech National Seismological Network (CNSN), consisting of ten permanent digital broadband stations, several local networks and two data centers, detected and recorded 9530 regional natural seismic events, 27 greater than magnitude 2. Most of these events were located by the Czech Seismological Service (CSS), and the most prominent of them were analyzed in detail. A large number of quarry blasts were recorded as well but were not included in the analysis. We provide basic information on the configuration of the CNSN and on the way of routine data processing employed by the CSS in this paper. The over-all regional seismicity monitored by the CNSN in 1995–1999 is briefly reviewed. The main results of observations and evaluation of the local (NW-Bohemia/Vogtland, South Bohemia, Sudeten) and induced (Kladno, Píbram, Upper Silesia, Lubin/Poland) seismic activity within this period are presented in a condensed form. Finally, a summary on macroseismic observations on the territory of the Czech Republic in 1995–1999 is also presented.     

On a method of computing the gravitational fields of inhomogeneous bodies

Summary A solution of the direct gravity problem for a finite body with variable density is given. The method is based on Green's formula and is applicable when a particular solution of Poisson's equation is known. The attraction due to the body is expressed by integrals over its surface The exact solution of the direct gravity problem, as known from the theory of two-dimensional fields [1–3], is closely connected with the problem of the analytic continuation of the exterior field of the attracting mass system into its interior. In the first place, this is a problem of determining the singularities of the exterior field, their distribution within the system and their nature. This approach to the solution of the direct problem is also meaningful from the point of view of determining the characteristics of the attracting system and, therefore, also of solving the inverse problem. In the case of two-dimensional fields the methods of analytical continuation were widely developed in a series of well-known papers by V. N. Strakhov, and they are mainly based on the methods of the theory of the functions of the complex variable. These methods were also successfully applied by Tsirulskii and Golizdra [1, 2] in treating the homogeneous and inhomogeneous, two-dimensional direct problem by means of Cauchy's integrals. However, as regards three-dimensional fields a number of fundamental problems has not been solved in this respect.Dedicated to 90th Birthday of Professor Frantiek Fiala