Results

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 << Back

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.

Figure1. Map with arrays locations and arrays stations distributions

The noise variations in Bucharest area have been studied at URS stations in two frequency bands: the first one corresponding to the microseisms (0.05 – 0.5 Hz) and the second one corresponding to the domain where the seismic noise has mainly anthropogenic origin (0.5 – 25 Hz). In the microseismic band the observed noise variations are due to natural factors (such as storms in the Black Sea), while in the high frequency band they are generated by anthropic activities. Once the variations identified, we investigated how they influence the amplitude and frequency of the spectral peaks of H/V ratios (Figure 2).

Figure 2. The H/V ratios computed for two different days at station URS21. An increase of the noise level during the winter day (julian day 023) affects the spectral peak observed at larger periods

The polarization analysis of the microseisms recorded by a three-component (3C) seismic station allows, to a certain degree, to constrain the incoming direction of the microseisms at the station (Stutzmann et al., 2009; Schimmel et al., 2011). We applied the approach introduced by Schimmel et al. (2011) which measures the degree of polarization of the seismic noise in the microseisms frequency band. The analysis has been conducted for each 3C broadband station of the seismic arrays as well as for the stations located close to the Black Sea (TIRR, EFOR, MANR). Figure 3 shows, as an example, the results of the polarization analysis, expressed in terms of back azimuths (BAZ), obtained at BURAR station.

Figure 3. Rose diagrams with station particle motion back azimuths in the frequency band 0.05 – 0.1 Hz obtained for five years of data (2011-2015)

In case of seismic arrays a powerful tool for determining the backazimuth and the slowness of coherent seismic waves is the f-k analysis. This approach offers a much higher resolution in terms of backazimuth determination and source location than the polarization analysis and can discriminate several simultaneously active microseismic sources, which the 3-component polarization analysis cannot do. The four seismic arrays used in the project (for SCP array we selected a subset of stations) have different configurations and different number of elements, so that we computed the array transfer function in order to identify the sensitivity and resolution of each array. To identify the directions to the noise sources we applied two methods: the fk analysis (Capon, 1969) and IAS Capon method (Gal et al., 2014) (Figure 4) which represents a better solution than fk in case of broader frequency range of interest.

Figure 4. Results of IAS Capon analysis for two different time periods (23rd of January 2004 and 4th of July 2004)

References

Capon, J., 1969. High-resolution frequency-wavenumber spectrum analysis, Proc. IEEE, 57(8), 1408–1418.

Gal, M., Reading, a. M., Ellingsen, S. P., Koper, K. D., Gibbons, S. J., & Nasholm, S. P. (2014). Improved implementation of the fk and Capon methods for array analysis of seismic noise.

Schimmel M, Stutzmann E, Ardhuin,F, and Gallart J (2011) Polarized Earth's ambient microseismic noise. Geochemistry, Geophysics, Geosystems, 12(7).

Stutzmann E, Scimmel M, Patau G, and Maggi A (2009) Global climate imprint on seismic noise, Geochemistry Geophysics Geosystems, doi: 10.1029/2009GC002619

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)