Correlation of geomagnetic anomalies recorded at Muntele Rosu Seismic Observatory (Romania) with earthquake occurrence and solar magnetic storms Iren A. Moldovan 1,* , Anica Otilia Placinta 1 , Angela Petruta Constantin 1 , Adrian Septimiu Moldovan 2 , Constantin Ionescu 1 1 National Institute for Earth Physics, Bucharest-Magurele, Romania 2 Azel Designing Group S.R.L., Bucharest-Magurele, Romania ANNALS OF GEOPHYSICS, 55, 1, 2012; doi: 10.4401/ag-5367 ABSTRACT The study presents a statistical cross-correlation between geomagnetic anomalies, earthquake occurrence and solar magnetic storms. The working data are from: (i) geomagnetic field records from Muntele Rosu (MLR) Observatory, and from Surlari (SUA) and/or Tihany (THY) INTERMAGNET Observatories; (ii) seismic data for the Vrancea source zone; and (iii) daily geomagnetic indices from the NOAA/Space Weather Prediction Center. All of the geomagnetic datasets were recorded from 1996 to the present, at MLR, SUA or THY, and they were automatically corrected using a LabVIEW program developed especially for this purpose, highlighting the missing or bad data. Missing data blocks were completed with the last good measured value. After correction of the data, there were a number of issues seen regarding previous interpretations of the geomagnetic anomalies. Some geomagnetic anomalies identified as precursory signals were found to be induced either by increased solar activity or by malfunction of the data acquisition system, which produced inconsistent data, with numerous gaps. The MLR geomagnetic data are compared with the data recorded at SUA/THY and correlated with seismicity and solar activity. These 15 years of investigations cover more than a complete solar cycle, during which time the solar-terrestrial perturbations have fluctuated from very low to very high values, providing the ideal medium to investigate the correlations between the geomagnetic field perturbations, the earthquakes and the solar activity. The largest intermediate depth earthquake produced in this interval had a moment magnitude Mw 6.0 (2004) and provided the opportunity to investigate possible connections between local geomagnetic field behavior and local intermediate seismicity. 1. Introduction Large networks of ground-based instruments [Yumoto et al. 1995, 1996, 2001, Yumoto 2004; e.g. the International Real- Time Magnetic Observatory Network, INTERMAGNET], and even some satellite-based systems [Lagoutte et al. 2006, Parrot et al. 2006], have been dedicated to the monitoring of the geomagnetic field over the last two decades. Several studies have reported the identification of possible anomalous magnetic signals prior to earthquake occurrences [Hayakawa and Fujinawa 1994, Stanica et al. 2006, Stanica and Stanica 2007, 2009, Moldovan et al. 2009, Yumoto et al. 2009, Takla et al. 2011], or increased numbers of seismic events after or during magnetic storms [Hayakawa et al. 2002, Kessel et al. 2006]. Anomalous changes in the geomagnetic field can occur before and during seismic events. As the lithosphere deforms, rock properties can change in response to changes in stress piezomagnetism or to changes in the distribution and composition of fluids in the crust [Freund et al. 1999, Pulinets and Boyarchuck 2004]. The reported expected changes are in the range of a few nT. The problem of identification of seismo-magnetic effects in geomagnetic time series is complicated by the presence of disturbances, which are mainly due to irregular transient time variations that are generated in the terrestrial ionosphere and magnetosphere, and which also depend on the geological structure of the area. The purpose of this study is to examine the dynamics of geomagnetic field variations in relation to the Vrancea (Romania) crustal and intermediate seismic activity and to magnetic storms. The Vrancea seismogenic zone is situated at a bend in the eastern Carpathians, and it is bounded to the northeast by the East European Platform, to the south by the Moesian Platform, and to the west by the Transylvanian Basin. The crustal activity located in the depth interval of 10 km to 40 km is weak, with Mw <5.9 (3.4 in the study period) and an activity rate of 0.514526 for Mw >3.0 [Moldovan et al. 2008]. The intermediate depth seismic zone (60 km to 200 km) is concentrated within a very small area, which is 80 km long Special Issue: EARTHQUAKE PRECURSORS 125 Article history Received August 14, 2011; accepted January 19, 2012. Subject classification: Magnetic anomalies; Magnetic storms; Magnetic and electrical methods; Geomagnetic field variations; 04.06.11. Seismic risk.