Pavla Hrubcová

ALP 2002 seismic experiment

The ALP 2002 was organized as an international seismic experiment whose scientific objective is to further scientific understanding of the structure and evolution of the lithosphere in the Eastern Alps and surrounding areas. The ALP 2002 experiment included passive seismic monitoring and an active source seismic refraction experiment. Furthermore, local high-density deployments were carried out in Austria and Hungary to investigate local geologic problems. All data will be integrated with the goal of better understanding the geodynamic processes currently at work and the complex tectonic history of this region.

Shallow crustal discontinuities inferred from waveforms of microearthquakes: Method and application to KTB Drill Site and West Bohemia Swarm Area

The waveforms of microearthquakes are of high frequency and complicated. They contain many phases secondarily generated at crustal interfaces and at small-scale inhomogeneities. They are highly sensitive to focal mechanisms and thus very different for each station of local networks. However, with a large number of microearthquakes, the scattered waves present in the waveforms can serve for identifying the prominent crustal discontinuities and for determining their depth. In this paper, we develop a new approach for extracting information on crustal structure from such waveforms and apply it for determining depth and lateral variations of crustal discontinuities. We show that strong dependence of microseismic waveforms on radiation pattern requires good station coverage and knowledge of focal mechanisms of the microearthquakes. Analysis of real observations is supported by waveform modeling and by analysis of radiation patterns of scattered waves. The robustness of the inversion for depth of crustal interfaces is achieved by stacking of a large number of waveforms and by applying a grid search algorithm. The method is demonstrated on two microseismic data sets of different origin: microseismicity induced during the Continental Super-Deep Drilling Project (KTB) 2000 fluid injection experiment and natural seismicity in the West Bohemia swarm region. High-frequency conversions at the KTB site indicate a prominent interface at depths of 2.3–4.1 km consistent with previous interpretations. Geologically, it may represent the contact of granitoids with much faster metabasites underneath. Seismicity in West Bohemia indicates a strong-contrast interface at depths of 3.5–6.0 km. This interface is in agreement with previous profiling and might be related to trapping of fluid emanations ascending from the mantle.

Imaging the Mudurnu Segment of the North Anatolian Fault Zone From Waveforms of Small Earthquakes

We analyze waveforms of local earthquakes occurring before, between, and after the two consecutive 1999 Mw > 7 İzmit and Düzce earthquakes in NW Turkey. The waveforms were recorded at three seismic stations DOK, EKI, and GOK located around the Mudurnu segment of the North Anatolian Fault Zone. We focus on the interpretation of a distinct secondary phase contained in the P wave coda that is well separated from the direct P wave. The phase is visible in many waveforms of most seismicity clusters and has a specific constant time delay after the direct P wave arrivals at each station, irrespective of epicentral distance, hypocentral depth, or back azimuth. Based on a polarization analysis of records at station GOK, this secondary phase is interpreted as a PS wave converted at an interface near the stations. Its particle motion is consistent with the direct S wave and displays S wave splitting produced by the anisotropic upper crust. Synthetic modeling indicates that this PS phase can be converted either at a horizontal interface or at a steeply inclined interface. The steep Mudurnu fault zone with the near-surface setting indicating a juvenile pull-apart structure fits well into these interpretations, which are in agreement with the eastward progressing transtensional tectonics known for the region.

Comment on the Seismic Method Depth-Recursive Tomography on Grid (DRTG) Developed by Miroslav Novotný and Recently Published in Three Papers in Surveys in Geophysics

The comment addresses three papers published recently in Surveys in Geophysics. These papers are related, using the same seismic tomography approach developed by the same first author. They deal with modelling of seismic refraction crustal data in the Bohemian Massif and their geological interpretation. Novotny ´ (2011) presents a P-wave velocity model based on tomography along the refraction profile CEL09 of the CELEBRATION 2000 experiment; Novotny ´ (2012) presents a geological interpretation of this model.

Active Magmatic Underplating in Western Eger Rift, Central Europe: Active Magmatic Underplating

The Eger Rift is an active element of the European Cenozoic Rift System associated with intense Cenozoic intraplate alkaline volcanism and system of sedimentary basins. The intracontinental Cheb Basin at its western part displays geodynamic activity with fluid emanations, persistent seismicity, Cenozoic volcanism, and neotectonic crustal movements at the intersections of major intraplate faults. In this paper, we study detailed geometry of the crust/mantle boundary and its possible origin in the western Eger Rift. We review existing seismic and seismological studies, provide new interpretation of the reflection profile 9HR, and supplement it by new results from local seismicity. We identify significant lateral variations of the high-velocity lower crust and relate them to the distribution and chemical status of mantle-derived fluids and to xenolith studies from corresponding depths. New interpretation based on combined seismic and isotope study points to a local-scale magmatic emplacement at the base of the continental crust within a new rift environment. This concept of magmatic underplating is supported by detecting two types of the lower crust: a high-velocity lower crust with pronounced reflectivity and a high-velocity reflection-free lower crust. The character of the underplated material enables to differentiate timing and tectonic setting of two episodes with different times of origin of underplating events. The lower crust with high reflectivity evidences magmatic underplating west of the Eger Rift of the Late Variscan age. The reflection-free lower crust together with a strong reflector at its top at depths of ~28–30 km forms a magma body indicating magmatic underplating of the late Cenozoic (middle and upper Miocene) to recent. Spatial and temporal relations to recent geodynamic processes suggest active magmatic underplating in the intracontinental setting.

Seismological evidence of fault weakening due to erosion by fluids from observations of intraplate earthquake swarms: EVIDENCE OF FAULT WEAKENING

The occurrence and specific properties of earthquake swarms in geothermal areas are usually attributed to a highly fractured rock and/or heterogeneous stress within the rock mass being triggered by magmatic or hydrothermal fluid intrusion. The increase of fluid pressure destabilizes fractures and causes their opening and subsequent shear-tensile rupture. The spreading and evolution of the seismic activity are controlled by fluid flow due to diffusion in a permeable rock (fluid-diffusion model) and/or by redistribution of Coulomb stress (intrusion model). These models, however, are not valid universally. We provide evidence that none of these models is consistent with observations of swarm earthquakes in West Bohemia, Czech Republic. Full seismic moment tensors of microearthquakes in the 2008 swarm in West Bohemia indicate that fracturing at the starting phase of the swarm was not associated with fault openings caused by pressurized fluids but rather with fault compactions. This can physically be explained by a fault-weakening model, when the essential role in the swarm triggering is attributed to degradation of fault strength due to long-lasting chemical and hydrothermal fluid-rock interactions in the focal zone. Since the rock is exposed to circulating hydrothermal, CO2-saturated fluids, the walls of fractures are weakened by dissolving and altering various minerals. The porosity of the fault gauge increases, and the fault weakens. If fault strength lowers to a critical value, the seismicity is triggered. The fractures are compacted during failure, the fault strength recovers, and a new cycle begins. Plain Language Summary The occurrence of earthquake swarms in geothermal areas is usually attributed to a highly fractured rock and/or heterogeneous stress within the rock mass being triggered by magmatic or hydrothermal fluid intrusion. The increase of fluid pressure destabilizes fractures and causes shear-tensile rupture. The spreading and evolution of the seismic activity are controlled by fluid flow due to diffusion in a permeable rock and/or by redistribution of Coulomb stress. This model, however, is not valid universally. We provide evidence that the model is inconsistent with observations of earthquake swarms in West Bohemia, Czech Republic. Microearthquakes in swarms in West Bohemia indicate that fracturing was not associated with fault openings but rather with fault compactions. This can be explained by a fault-weakening model, when the essential role in the swarm triggering is attributed to chemical and hydrothermal fluid-rock interactions in the focal zone. Since the rock is exposed to circulating hydrothermal, CO2-saturated fluids, the walls of fractures are weakened by dissolving various minerals. The porosity of the fault gauge increases, and the fault weakens. If fault strength lowers to a critical value, the seismicity is triggered. The fractures are compacted during failure, the fault strength recovers, and a new cycle begins.