Seismotectonic modelling with the program GoCad; A case study: The swarm quakes in the Vogtland/ NW- Bohemian Region

Transcription

1 Seismotectonic modelling with the program GoCad; A case study: The swarm quakes in the Vogtland/ NW- Bohemian Region Edgar Scheidewig Turmhofstr. 10, Freiberg Abstract The upper crust beneath the Vogtland is a region of active magmatic and tectonic processes, which many geophysical data, long time gps measurements and hydrothermal activity demonstrate. This project deals with the interpretation of geophysical data, more precisely seismic events and processed focal solutions, to get an idea which recent tectonic forces are working in this area and how they correlate with potential fault planes. These were modelled from the geophysical data, with the help of already existing fault maps and geomorphological indicators. For the visualisation of the geological and geophysical data we used the program GoCad. Introduction The region of Vogtland is situated at the southeast boarder of Germany to the Czech Republic. The region of northern Bohemia, which is characterized by carbon-dioxide springs and other hydrothermal settings, is a well investigated area. In contrast to this the central and northern part of the Vogtland don t show that kind of hydrothermal activity. From that we can conclude that the tectonic activity which is shown by the focal solution and the single seismic events must be of different origin. Visualised in GoCad it becomes obvious that most of the seismic events plot as clusters. These clusters are called swarm quakes. The term swarm was firstly introduced by Knett (1899) and Credner (1900) for earthquakes in the Vogtland region, and defined by Sykes (1970) as sequences of earthquakes, in which the number and magnitude of shocks in a cluster gradually increase and decay in time without a distinct main shock.

2 Regensburg-Leipzig-Rostock-Faultzone Three types of earthquake activity have been characteristed by Fischer and Horálek (2003) for the NW Bohemia/ Vogtland region: (a) swarms large sequences of earthquakes, mostly of several thousands of events, clustering in time and space (b) micro swarms swarm like activities usually comprising dozens to hundreds of events (c) background activity variable magnitude, clustering neither in time nor space, inter-event time remains large Hill (1977) explains the origin of earthquakes swarms by a combination of the presence of heterogeneities and the effect of pore fluids. This project briefly summarised seismic data from 1994 until today in the territory of the Vogtland. The analyzed seismic events were detected and recorded by German and Czech stations. The project is focused on natural earthquakes, which were induced by hydrothermal, magmatic or tectonic activity. The aim of this project is to give an idea about recent tectonic processes in the investigated region. For the visual illustration of the geological and geophysical data we used the program GoCad. With this program it is very easy to visualize and combine all kind of relevant data, e.g. seismic profiles, digital elevation models, geological maps or seismic events as point sets of epicentres in a 3 dimensional space. The study area and the tectonic setting N Gera-Jachimov-Faultzone Fig. 1: Geological map of the region Vogtland, Linnemann & Schauer (1999) Red: magmatic intrusive rocks Green: Ordovician/ Silurian schists Brown: Devonian schists

3 Fig. 1 gives a geological overview of the Vogtland, which consists of Devonian schist, some Variszican plutonic complexes like the Eibenstock Pluton, and in the northern part low Quaternary beds with underlying Permian sand- or claystones. In this region two important older fault zones exist. One fault zone runs parallel to the Gera-Jachimov-Fault, which strikes WNW-ESE and was formed in Tertiary during the uplift of the Erzgebirge. The other is the Regensburg-Leipzig-Rostock- Faultzone, which strike N-S and was formed in the Paleaozoic due to the Variszican Orogenesis.. The latter zone is known as the Marianske-Lazsne-Fault zone, which is well investigated by geophysics and other geoscientists. Hypothesis and conventions The hypocentral events plot mostly as clusters. This leads us to the assumption that they accumulate around a zone of weakening in the upper crust. Furthermore we suppose that these zones of energy-release develop on heterogeneities in the upper crust. That means along lithological borders, older fault zones or zones of different rheological behaviour, but without a change in lithology. As already mentioned we used focal solutions for the tectonic interpretation. Therefore we have to establish or rather repeat some conventions about the focal solutions. After the different seismic stations processed their measurements of the first motion of p-, and s-waves, two orthogonal planes are possible fault surfaces. The choice between both will be made with the help of strike values of already known fault zones and with reference of the tectonic setting, in which the earthquake occurred. δ 3 δ 1 δ 1 δ 3 Fig.: 2 top view on a beach ball In this case dextral strike slip

4 Fig. 2 shows the classical view of a focal solution: A beach ball which is subdivided in four quadrants by two orthogonal planes. The yellow (white) quadrants illustrate the compressional regime and the blue (dark) quadrants the tensional regime. From the tectonic point of view this means, that the yellow quadrants represent the axe of principal stress sigma 1 and the blue quadrants sigma 3. With this convention we can make statements, whether the beach ball shows an upward movement (thrust fault), a downward movement (normal fault) or an oblique movement (strike slip fault). The database and methods of work The geophysical data contain 8000 hypocentral events from and 28 focal solutions from the years This data are derived from German and Czech geophysical networks. The dataset of the hypocentral events consists of the attendant xyz-coordinates, the date, the magnitude and the passed time since the first event. The dataset of the focal solutions comprises the appropriate xyz-coordinates, the date, dip, strike and rake of the reference plane and the magnitude. Furthermore we used geological maps with already known fault zones in the investigated region. For the digital elevation model we used SRTM-satellite data from NASA-satellite images with a raster resolution of 90m x 90m. With the program GoCad it is possible to lay the geological map over the digital elevation model, so that the morphology of the DEM corresponds with their attendant geological feature. At first we processed the data with some software like excel to tabulate the data, a small Linux grogram which is called cs2cs to georeference the different types of data and the software Geomatica to produce xyz coordinate data from the SRTM satellite data for the digital elevation model. All the data were georeferenced in the Gauss-Krueger-Zone 4. In GoCad all data are represented in a Cartesian coordinate system, like UTM, so it is important that all of the data s x, y and z coordinates have metric units. Programs for the visualization of focal solutions For the visualization of the focal solutions in GoCad, we used two small pythonprograms. The first program, which is called beach ball, produces the classical view of focal

5 solutions with one difference. The classical view of a focal solution shows the top view on the lower hemisphere, which is divided into four quadrants. With the program we create a real beach ball, that means a 3-dimensional sphere with two orthogonal planes or with 4 quadrants respectively. At first the program starts with a beach ball, whose planes are horizontally and vertically adjusted. The intersection of both planes is directed to the north. First the sphere is rotated rake degrees about the X-axis to produce the correct rake. Then it is rotated (90 - dip) degrees about Y-axis to produce the dip. Finally it is rotated strike degrees about the Z-axis. The second program works with the same algorithm, but it produces a view of the 2 orthogonal planes with the corresponding first motion of the p-, and s-waves as vectors. The radius of the beach balls and the area of the orthogonal planes are proportional to the magnitude with respect to the event. Cluster-analysis of the epicentral data with S-Plus Furthermore we used the statistical concept of the cluster analysis, to get an idea about the temporal distribution of the seismic events. The background of this idea is to find regions, in which different epicentres are active in a small period of time. We assumed that, if these epicentres plot on a more or less plane in the 3-dimensional space and furthermore this plane is situated beneath a known fault-zone, the released stress can be related to it. Therefore we extract on one hand the passed time between the different seismic events, that means the first measured event is timed at 0 min and the time until the following events will be added. On the other hand we extract the difference in min between the seismic events. This is important to find out, which regions in a cluster are active in a period of e.g. 5 or 10 min. We processed this cluster analysis with the program S-Plus. Cluster analysis with GoCAD That kind of cluster analysis is based on the visualization of the different seismic events, so that in this case it is not a statistical cluster analysis, but rather a graphical one. Many events plot in the 3- dimensional space as clusters, which we have already termed swarms. With GoCad we can create regions from the whole dataset, in which only events that form a cluster are gathered. Because every seismic event has a property, which

6 describes the passed time since the very first event, we can now visualize only these events, which happened in the same small period of time, e.g. 5 or 10 min. Fig: 3 Cluster and the regression Plane of the Novy Kostell zone Fig: 4 extrapolated regression plane and DGM of the region These clusters, look like a swarm, so that they can be referred to as swarm quakes. Because of their shape, which is similar an ellipsoid, it is possible to produce a regression plane (Fig. 6, Fig.3) that may be accepted as a potential plane of tectonic movement. These regression planes have been extrapolated with their direction vector in the DGM, so that the intersection between the extrapolated plane and the DGM, can be expected as a potential fault zone (Fig. 4). Now we have a basis to compare these lines with already existing maps of fault-zones (Fig. 5) or geomorphological indicators like steep hillsides or behaviour of rivers in the expected region. At this point it must be mentioned, that this method is of doubtful quality. Because we cannot assume that the behaviour, the strike and dip, respectively, of a potential fault plane in depths of 10 or 15 km is similar to that of a fault on the surface. It is also possible, that the fault dies out in the upper crust and does not reach the surface. But in cases of already known faults this methods possibly helps to get an idea, which one of these might be active, so that these regions can be closer investigated by fieldtrips. Results and Interpretation Most of the reference planes of the focal solutions show approximately the same N strike like the Regensburg-Leipzig-Rostock fault zone (Fig. 7). But there are also local regions like the centre of Vogtland or the region around Novy Kostel, in which the regression planes of the swarm quakes, and therefore the potential fault planes, and the reference planes of the focal solutions show the NW strike of the Gera- Jachimov fault zone (Fig. 5).

7 Furthermore it is possible to make statements about the regional stress field with the created principal stress axis. The red lines show that the direction of most of the maximum principal stress axis, which was extracted from the focal solutions, is the nearly similar to the axis of the world stress map (Fig. 7). With all these indicators we constructed a possible tectonic model of the Vogtland region. Fig.: 5 focal solutions and extrapolated Regression planes White lines: Brown lines: map of faults (geologisches Landesamt) Fig.: 6 Regression planes N Zwic ka u Pla uen δ 1 Fig.: 7 extracted principal stress Axis Yellow marker: maximum Horizontal stress, geological Indicator (from world stress map Rel. 2005) Fig.: 8 Reference planes of the focal solutions In Fig. 9 a possible tectonic model is shown. This interpretation is based on the focal solution. The regional fault zone is characterized by a sinistral strike slip with smaller regions that indicated a normal fault system e.g. the zone around Novy Kostell and the central vogtland. The strike of the central Vogtland normal fault zones is comparable to the Klingenthal-Erlbach-fault, which is a parallel fault to the Gera-Jachimov-fault with a strike of NW-SE. From this we can assume that the stress, which has been created by the regional

8 sinistral strike slip movement on the Regensburg-Leipzig-Rostock fault, is released by these normal fault systems. For the northern territory of the Vogtland, we collected some data in the field. The outcrop is situated near the town Zwickau, in permian sand/claystones. In this outcrop we found some cracks, which look very fresh. So that we can conclude that they must be generated during Quaternary events. The strike of these cracks is shown in Fig.9. These zones of extension are possible in a surrounding regional strike slip system. The data show that the Vogtland is on no account a zone of frozen tectonic activities. Furthermore can we state that the old fault system of the Regensburg-Leipzig- Rostock-Zone and the parallel fault systems to the Gera-Jachimov Fault are reactivated by the regional stress system. Zwic ka u Pla ue n Klingenthal_Erlbach_fault N Fig. : 9 Interpretation of the regional stress system Red dotted line: Regensburg-Leipzig-.Rostock fault zone Yellow line: Klingenthal-Erlbach-fault Blue arrows: normal fault zones

9 References Geologisches Landesamt Sachsen, Fault map of Vogtland and bordering regions Hill, D.P., A model for earthquakes swarms. J. Geophys. Res. 82, Linnemann & Schauer (1999), Geological map of Saxothuringia/Vogtland Program GoCAD Version Earth decision scinces Program beach ball and nodals, after GMT Harvard University, modified by M. Apel Program geomatica Version Sykes, L.R., Earthquake swarms and seafloor spreading. J. Geophys. Res Tomáš Fischer, Joseph Horálek Space-time distribution of earthquake swarms in the principal focal zone of the Bohemia/Vogtland seismoactive region: period , journal of geodynamics 35 (2003)