Coseismic drop of seismic velocity caused by the 2023 Turkey–Syria earthquakes

The Mw 7.8 earthquake in Turkey on 6 February 2023 was extraordinary for various reasons. It originated in depth of only 10 km, ruptured along a fault plane around 300 km long and the surface was covered by an extensive network of high-quality seismic instruments. The strong motions resulted in a vast number of tragic casualties and huge material losses in Turkey and Syria. However, abundant and proximate seismic observations of this event and numerous aftershocks give an opportunity to deepen the understanding of earthquake processes. In this study, we carried out an assessment of coseismic changes of seismic velocity using Passive Image Interferometry. We used data from one strong-motion and twenty-four broadband sensors. We observed coseismic drops of seismic velocity, which reached up to -1.79 per cent at a location directly at the ruptured East Anatolian Fault Zone. Along the Mw 7.8 earthquake fault, we observe frequency dependence of the velocity changes. At frequencies above 0.5 Hz, the velocity drops seem to be higher at locations close to the ruptured faults than in the more distant areas

Comparison of multiple lapse time window analysis and qopen to determine intrinsic and scattering attenuation

This study compares the results of Multiple Lapse Time Windows Analysis (MLTWA) and full envelope inversion (Qopen) to determine intrinsic and scattering attenuation of the crust using the region around the central part of the Leipzig–Regensburg fault zone in Germany as an example. We use 18 of the region’s strongest earthquakes from 2008 to 2019 with a magnitude between 1.4 and 3.0 in the frequency band range between 3 and 34 Hz. The determined attenuation values of both methods are similar within their error bars. The inverse quality factors of the shear wave are relatively low compared to other regions, with values of 3.2 × 10−4 to 8.7 × 10−4 for Q−1i and 1.4 × 10−4 to 2.8 × 10−4 for Q−1sc⁠, respectively. As a by-product of Qopen, we also obtain the energy site amplification of the stations used in the inversion as well as source displacement spectra and moment magnitudes of the inverted earthquakes. Several combinations of inversion parameters were tested for MLTWA, with Q−1i and Q−1sc providing the lowest trade-off. Likewise, we investigated the influence of window length on the results of Qopen. We found a dependency of the results on the length, if the windows are shorter than 30 s. For longer time windows, the dependence disappears, and the result becomes independent of window length