Analysis of the seismic wavefield in the Moesian Platform (Bucharest area) for hazard assessment purposes

Abstract

During large earthquakes generated at intermediate depth in the Vrancea seismic zone, the ground motion recorded in Bucharest (Romania) is characterized by predominant long periods with strong amplification. Time-frequency analysis highlights the generation of low frequency surface waves (<1 Hz) for sufficiently strong and superficial events. This phenomenon has been explained by the influence of both source mechanism (radiation pattern, directivity effects) and mechanical properties of the local geological structure (geological layering and geometry). The main goal of our study is to better characterize and understand the seismic wavefield produced by earthquakes in the area of Bucharest, taking into account its location in the centre of the Moesian Platform, a large sedimentary basin (450 km long, 300 km wide and up to 20 km deep). To this aim, we identify the contribution of different seismic surface waves, such as the ones produced at the edges of this large sedimentary basin or multipath interference waves (Airy phases of Love and Rayleigh waves), on ground motion. The data from a 35 km diameter array (URS experiment) were used. The array was installed by the National Institute for Earth Physics in cooperation with the Karlsruhe Institute for Technology and operated during 10 months in 2003 and 2004 in the urban area of Bucharest and adjacent zones. The earthquake wavefield recorded by the URS array was analysed using the MUSIQUE technique. This technique analyses the three-component signals of all sensors of a seismic array together. The analysis includes 19 earthquakes with epicentral distances from 100 to 1560 km and with various backazimuths with enough energy at low frequencies (0.1-1 Hz), within the resolution range of the array. For all events, the largest portion of energy is arriving from the source direction and the wavefield is dominated by Love waves. The results of the array analyses clearly indicate a significant scattering corresponding to 2-D or 3-D effects of the Moesian Platform. The azimuthal distribution shows that the scattering comes primarily from the southern and northern edges of the basin. The Airy phase of Love waves was clearly identified as the main contributor in the range of the fundamental frequency of resonance of the basin (0.15-0.25 Hz), with directionality along the backazimuth and its opposite direction. Moreover, two further distinct frequency bands around 0.4 and 0.7 Hz with higher amplitudes were identified. Their complex nature is a combination of the higher modes of Rayleigh waves, Airy phases of Love waves and SH waves. Love and Rayleigh wave dispersion curves were successfully retrieved by combining the information of all events and show a good match with the ones obtained using ambient vibrations. Additionally, the first higher mode of Rayleigh waves could be retrieved using data from earthquakes. Also, the prograde and retrograde Rayleigh wave ellipticity was computed.

Ground-motion models for Vrancea intermediate-depth earthquakes

“…In the last century, intensities of VII–VIII MSK were observed at more than 250 km epicentral distances during the 1940 7.7Mw event according to the data published by Kronrod et al (2013). These observations of far-reaching effects could potentially be explained by: (1) unique features of the source properties (Ismail-Zadeh et al, 2012), (2) different attenuation patterns due to the complex tectonic configuration (Radulian et al, 2006), (3) complex seismic response of the geological structure (Manea et al, 2017), and (4) nonlinear behavior of soils during strong earthquakes (e.g. Cioflan et al, 2009; Marmureanu et al, 2016).…”

“…Fäh et al, 2003; Manea et al, 2016; Poggi et al, 2012) and was assessed at all the seismic stations of the Romanian Seismic Network, in several studies: Manea et al (2016, 2019, 2020) and Coman et al (2020). In the aforementioned studies, f 0 was extracted by computing the horizontal-to-vertical Fourier spectral ratio based on ambient noise and not earthquakes as the predominant frequency of Vrancea records varies depending on earthquake magnitude and can be different than f 0 at each site (Manea et al, 2017, 2021); The main advantage of using f 0 is that it does not need additional computation as is required for the sediment depth retrieval and in this way does not include large epistemic uncertainties within the ground-motion computation. The distribution of the number of records with the ARC and f 0 can be seen in Figure 1d and e. The variability of f 0 at all the seismic stations can be seen in the Supplementary Materials (Figure S1).…”

“…This long-periods feature is due to the source complexity (e.g. all types of focal mechanisms, different energy budget between deeper events (130–150 km) and the ones occurred at 90–110 km (Oth et al, 2007); different ray paths depending on the location of the event in the slab (Bokelmann and Rodler, 2014); response of the complex sedimentary structure (response of soft surficial soils (Cioflan et al, 2009), migration of the predominant frequency of resonance (Manea et al, 2021), basin-induced surface waves observed by Manea et al (2017).…”

Dehydration-induced earthquakes identified in a subducted oceanic slab beneath Vrancea, Romania

“…The contact area between the Carpathians and the Moesian Platform (Fig. 1) is highly seismic [1][2][3] . In particular, and most surprisingly, the Vrancea region, located at the SE corner of the Carpathians, is characterized by a puzzling intermediate-depth high-seismicity body [4][5][6] , with the highest seismicity between 60 and 170 km depth (Fig.…”

Variability of Ground Motion Amplitudes Recorded in Bucharest Area during Vrancea Intermediate‐Depth Earthquakes

“…Significant long-period spectral amplifications have been observed in Bucharest, the capital city of Romania, during large magnitude Vrancea intermediate-depth earthquakes occurring in the past 40 years [1,2]. A combination of source and site effects appears to be reason for the occurrence of such amplifications [3][4][5][6] during large magnitude earthquakes.…”