Determination of the Earth's structure by Local Earthquake Tomography method

Postgraduate Study

Author: Josipa Kapuralić, PhD mag. ing. geol.

Knowledge of the Earth's interior is key to understanding geological structures and their relationships observed on the surface. In the last two decades, many regional studies of the European lithosphere have been carried out, while in the past ten years local geophysical researches have been intensified in the Dinarides. Recent geophysical efforts significantly contributed to the clarification of the crustal and lithospheric geological model in this region. This investigation is a continuation of geophysical studies focused on the Dinarides and its adjacent areas. The study area represents the boundary zone between the African and European plate, i.e. the contact between the Adriatic microplate as part of the African plate and the Pannonian basin as part of the European plate (Figure 1).

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Figure 1 Tectonic map of the wider area (Schmid et al., 2008). The superimposed boundaries of the Pannonian crust, Transition Zone, Dinaridic crust and Moho fragmentation, that is contact between the Adriatic microplate and Pannonian tectonic segment, are based on gravity modelling (Šumanovac, 2010).

Local Earthquake Tomography method (LET) was applied for the first time in this area, in order to advance our understanding of the crustal structure and its relationship to the upper mantle in the contact area between the Adriatic microplate and European plate. The LET method enabled the construction of a three-dimensional (3D) P-wave seismic velocity model. P-wave travel-times are calculated from earthquakes, which were recorded by temporary and permanent seismic stations placed in the survey area. Data were collected in two phases. After the first phase of data collection and processing, a 3D velocity model was constructed in the area of the northern Dinarides. The model was verified and calibrated along the existing detailed 2D model in the northern Dinarides (Šumanovac et al., 2016), thus improving the achieved resolution of the new 3D model (Figure 2).

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Figure 2 Vertical cross-section through the LET model along the profile in the northern Dinarides – comparison with 2D model along the same profile. The topography is shown on the top, and the triangles denote seismic stations. The solid blue and red lines are crustal and Moho discontinuities inferred from the velocity model established by controlled-source experiments along the 2D model (šumanovac et al., 2009). The grey solid line represents the Moho inferred from receiver function modelling (Šumanovac et al., 2016). The dashed blue and red lines are discontinuities interpreted on the basis of LET model. The dashed black line denotes the estimated mean depth for the Moho obtained by combining all three models. Note the vertical exaggeration. Numbers in green colour denote Vp seismic velocities from the 2D seismic refraction model, while HDB and crustal faults are represented by solid green and red dotted lines, retrospectively (Šumanovac et al., 2009)

The second phase of data collection covered a wider area, and 3D P-wave velocity model was constructed in the area of the entire Dinarides and the transition zone towards the Pannonian Basin. The data enabled high resolution, and the model confirmed a heterogeneous distribution of seismic velocities. The inverted velocity model shows that the crust under the Dinarides is characterized by relatively stronger lateral and vertical velocity changes when compared to the crust in the Pannonian basin area. The strong velocity increase in the crust below the Dinarides indicates that the Dinaridic crust could be interpreted as two-layered, while the Pannonian crust is probably one-layered, which confirmed the previously observed main characteristics of these two crusts. Beneath the Pannonian Basin, in the area of main depressions, there are low-velocity anomalies caused by thick deposits of sedimentary rocks. There is a high-velocity anomaly between the Sava and Drava depressions. In a tectonic framework, this high-velocity anomaly is within the ophiolitic zone.

The most reliable feature in the model concerns the structure of the lower crust and uppermost mantle. The Moho shape can be determined in vertical cross-sections based on the highest vertical velocity gradient in the lower crust (Figure 3).

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Figure 3 Vertical cross-sections through the P-wave inversion model along P1 and P2 profiles, orientation W – E. The dashed black line denotes the slightly smoothed Moho discontinuity interpreted based on the velocity in this LET model.

There is a deep low-velocity zone beneath the Dinarides, which extends to a depth of more than 55 km and has characteristic NW – SE trending (Figure 4). This anomaly is interpreted as the fragmentation in the uppermost mantle and it is the first direct indication of the contact of two tectonic units. High velocity and low-velocity alteration in the narrow area below the Dinarides could be the first geophysical evidence of the contact of Adriatic and Pannonian mantles.

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Figure 4 Depth slices across the P-wave inversion model. White dashed lines denote the low-velocity zone in the upper mantle.

Reference:

Schmid, S. M., Bernoulli, D., Fügenschuh, B., Matenco, L., Schefer, S., Schuster, R., Tischler, M. & Ustaszewski, K. (2008). The Alpine-Carpathian-Dinaridic orogenic system: correlation and evolution of tectonic units. Swiss Journal of Geosciences, 101(1), 139-183.

Šumanovac, F. (2010). Lithosphere structure at the contact of the Adriatic microplate and the Pannonian segment based on the gravity modelling. Tectonophysics, 485(1-4), 94-106.

Šumanovac, F., Orešković, J., Grad, M. & ALP 2002 Working Group. (2009). Crustal structure at the contact of the Dinarides and Pannonian basin based on 2-D seismic and gravity interpretation of the Alp07 profile in the ALP 2002 experiment. Geophysical Journal International, 179(1), 615-633.

Šumanovac, F., Hegedűs, E., Orešković, J., Kolar, S., Kovács, A. C., Dudjak, D. & Kovács, I. J. (2016). Passive seismic experiment and receiver functions analysis to determine crustal structure at the contact of the northern Dinarides and southwestern Pannonian Basin. Geophysical Journal International, 205(3), 1420-1436.

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Josipa Kapuralić, PhD, mag. ing. geol. is a research assistant at the Department of geophysical exploration and mine surveying, at Faculty of Mining, Geology and Petroleum engineering, University of Zagreb. She got her PhD title in June 2020, with thesis Crustal and lithospheric mantle structure beneath the Dinarides and south-western Pannonian basin from Local Earthquake Tomography.

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