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

Abstract

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.

Active Magmatic Underplating in Western Eger Rift, Central Europe

“…In the upper crust, the V‐shaped bright spot detected along the 9HR profile (Mullick et al, ) is located in the same place. The same V‐shape interface at depths of 3.5–6 km is also identified when interpreting waveforms from local seismicity by Hrubcová et al (). A midcrustal reflector at depths of 18 and 20 km along the CEL09 profile is disrupted in the area of the Cheb Basin (Hrubcová et al, ).…”

Imaging a Crustal Low‐Velocity Layer Using Reflected Seismic Waves From the 2014 Earthquake Swarm at Long Valley Caldera, California: The Magmatic System Roof?

“…For imaging, we apply a wavefield migration technique (reverse time migration; RTM), which is often used for active-source imaging, to the earthquake swarm seismograms recorded at station MLH. The idea of using earthquake waveforms for imaging reflectors with migration has been implemented previously (e.g., Hrubcová et al, 2016;Reshetnikov et al, 2010;Stroujkova & Malin, 2000). Compared to the reflected waves on the west side of Long Valley Caldera used by Stroujkova and Malin (2000), our earthquake sources are dense and waveforms show coherent reflections.…”

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

“…The particle motion of this phase shows a near vertical polarization representing a P ‐type phase (Figures c and f in red). Therefore, the P ‐type secondary phase can be either a PPP reflected wave generated by a shallow subsurface layer as discussed by Hrubcová et al () or a converted/reflected PP phase from an inclined interface. Variations in the incidence angle of the minor secondary phase and the direct P wave on the EW‐NS plane for some clusters, however, indicate that the minor secondary phases are produced rather by reflections from a near‐vertical structure (the Mudurnu fault zone).…”

“…Consequently, waveforms of earthquakes recorded at local seismic networks can provide information that can be utilized for imaging the velocity structure (e.g., Eberhart‐Phillips & Michael, ; Sanford et al, ; Wu & Lees, ) and the interfaces at depths. To perform this analysis, a novel concept for extracting crustal structure from high‐frequency waveforms of local earthquakes was developed by Hrubcová et al (, ) and applied for determining depth and topography of crustal discontinuities. The method has been tested on two local seismic data sets allowing for imaging horizontal interfaces at the site of the Continental Super‐Deep Drilling Project (KTB) in Germany and in the West Bohemia earthquake swarm region in the Czech Republic.…”

“…In this paper, we extend the technique of Hrubcová et al (, ) to near‐fault zone seismic recordings along the North Anatolian Fault Zone (NAFZ) in northwestern Turkey with the aim to image the structure around the strike‐slip Mudurnu segment as a major NAFZ branch. We use local seismicity in the period of 1997–2001 during which the two consecutive 1999 M w > 7 İzmit and Düzce earthquakes occurred (e.g., Gülen et al, ; Tibi et al, ) (Figure ).…”