HIGH SEISMICITY SEQUENCE IN THE IZVOARELE AREA (GALATI COUNTY) - ROMANIA
References: 15th International Multidisciplinary Scientific GeoConference SGEM 2015, www.sgem.org, SGEM2015 Conference Proceedings, ISBN 978-619-7105-33-9 / ISSN 1314-2704, June 18-24, 2015, Book1 Vol. 3, 1043-1050 pp
A sudden seismological unrest started on mid-September 2013 in the Izvoarele village area, earthquakes and subterranean noises occurring almost continuously for a month. The seismological monitoring provided more than 370 seismic events, with magnitude ranging between 0.3 and 4.0 and depth ranging from 0.3 km to more than 30 km.
Considering the historical seismicity, studied during the end of the XIXth century and the first half of the XXth century by means of seismic intensity, the Izvoarele–Galati area represents the southern end of a N-S high seismicity sector developed between Barlad and Galati. Based on these observations regarding epicentral areas and destructive effects, several NW-SE to N-S trending lines of “seismic sensitivity” were interpreted at that time.
The study of the 2013 seismic sequence showed that most earthquakes provided magnitudes between 0.3 and 1.5, ca 10% having higher magnitudes (16 more than 2.5, 11 more than 3.0 and 8 more than 3.5).
The relationship between Magnitude and Depth for the recorded seismic events displays a scattered distribution of the 2013 seismic events with low magnitude values (0.3 to 2.0), most of them included in the 0.1–10.0 km depth interval. Higher magnitude earthquakes (2.0 to 4.0) were generated in a smaller range of depth values (3.0 to 7.0).
The relationship between Magnitude and Time illustrates three main stages of high seismicity for the studied 2013 seismological event: September 25-27, September 30 to October 7 and October 12-15, with exceptionally dense seismic activity during October 2-5.
Considering the spatial development of the active seismic zone, two parallel lines trending NE-SW in the area were observed when plotting the earthquakes epicentres
with magnitude higher than 3.0. Active segments of a regional NE-SW trending fault system were interpreted this way.
The seismic events that occurred at depths higher than 10 km illustrated several distinct episodes during the intense seismological activity: September 23 and 28-29; October 4-5, 15 and 20-21.
A quite close spatial association was found between the 2013 high seismicity area and an oil exploitation facility, the extracting works dating here from the second half of the XXth century. The oil reservoirs are shallow, located between 0.5 and 0.7 km depth, the sandy deposits being sealed by faults or compact crystalline rocks. Induced seismicity might be expected as brine is being injected during time to replace the extracted oil.
A characteristic of this geological structure, that might be important in understanding the unusual seismicity and subterranean noises, is represented by the thin marl beds separating the sandy beds, which collapse in case of a sudden pressure decrease and determine the flooding of the oil reservoir.
Keywords: seismological unrest, active fault systems, induced seismicity, Izvoarele area, Romania
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