2015 Phase 1
The aim of this phase was to characterize the ambient seimc noise recorded at two seismic arrays that are part of the Romanian Seismic Network and installed in Romania, one in the Northern part of the country - Bucovina (BURAR) array and one in the Vrancea seismic zone - Plostina (PLOR) array (Figure 1). The noise level was analyzed in different frequency bands as a function of time of day and season. Investigation of the influence of noise variations on the stations detection capability of the two networks was also performed. Only part of the results obtained in this phase are presented bellow.
2016 Phase 2
The second phase focussed on the analysis of seismic noise as recorded by the arrays used in the project - BURAR, PLOR, SCP and URS (Figure 1) - and aimed, on one hand, to estimate the influence of the noise variations on the H/V ratios in Bucharest area and, on the other hand, to identify the directions to the sources responsible for generating the seismic noise in different frequency bands. The data used in the analysis covered different time intervals: November 2003 – August 2004 for URS array, July 2009 – June 2011 for SCP array and January 2011 – December 2015 for BURAR and PLOR arrays.
2017 Phase 3
The third phase focused on several aspects related to the seismic noise recorded by the stations used in the project: i) analysis of correlations between background seismic noise and sea level data ii) analysis of the characteristics of the noise cross-correlations obtained at ‘small scale’ (interstation distances between 2 and 60 km) and ‘large scale’ (interstation distances between 100 and 500 km) iii) noise based monitoring of two seismic areas (Vrancea and Galati)
i) We used four seismic stations (EFOR, MANR, MFTR and TIRR) and three sea level stations (TSCT, TSMN and TSSL) to investigate the correlations between seismic noise and sea level data. The analysis was performed for the time interval December 2015-March 2017 (Figure 1)
Figure 1. Stations locations (red triangle – sea level station, green square – seismic station) and comparison between the sea level data and the level of seismic noise obtained for the period 5.12 s
ii) We investigated the characteristics of the noise cross correlations computed between: the seismic stations deployed in Bucharest area during a temporary experiment (URban Seismology - URS stations), seismic stations in installed in Vrancea (BISRR, COVR, MLR, NEHR, PANC, PETR, PLOR si VRI) and Galati (CFR, IZVR, SCHL, SCTR, SLCR, TATR, TUDR, VLDR) areas, seismic stations deployed within the Southern Carpathian Project (SCP stations) and the following seismic stations of the Romanian Seismic Network: CFR, DOPR, IAS, ISR, MLR, OZUR, PETR, PLOR$, TESR, TIRR, VRI (Figure 2).
Figure 2. Map with the stations locations used to compute the noise cross correlations (blue squares – URS stations, yellow diamonds – SCP stations, red triangles – RSN stations)
The analyses were performed in different frequency bands: 0.3 – 1 Hz and 1 – 5 Hz for the stations with interstation distances between 2 and 60 km, 0.05 – 0.5 Hz for stations with interstation distances larger than 100 km. We studied the influence of the anthropogenic and seasonal noise variations on the cross –correlations and their corresponding dispersion curves.
Figure 3. The interferogram and daily cross-correlation functions obtained for the URS station pair URS29-URS31 in the frequency band 1 – 5 Hz
iii) We have implemented the MSNoise package (Lecocq et al., 2014) to monitor two active seismic areas (Vrancea and Galati) using ambient seismic noise. We used the data recorded between 2013 and 2017 by the stations located around the Vrancea (BISRR, COVR, MLR, NEHR, PANC, PETR, PLOR si VRI) and Galati (CFR, IZVR, SCHL, SCTR, SLCR, TATR, TUDR, VLDR) areas.
Lecocq, T., Caudron, C., & Brenguier, F. (2014). MSNoise, a python package for monitoring seismic velocity changes using ambient seismic noise. Seismological Research Letters, 85(3), 715-726.