**Earthquakes Precursors**

Dumitru Stanica and Dragos Armand Stanica *Institute of Geodynamics of the Romanian Academy Romania* 

#### **1. Introduction**

78 Earthquake Research and Analysis – Statistical Studies, Observations and Planning

Wiemer, S., Wyss, M. (1994). Seismic quiescence before the Landers (M=7.5) and Big Bear

Zavyalov, A.D. (2006). *Intermediate-Term Earthquake Prediction: Principles, Techniques,* 

*Implementation.* Nauka, ISBN 5-02-033946-6, Moscow (in Russian).

0037-1106.

(M=6.5) 1992 earthquakes. *Bull. of Seismol. Soc. Am.* Vol. 84. No. 3, pp. 900-916, ISSN

Strong earthquake of magnitude 7 or more (on the Richter scale) strikes about once a year somewhere in the world and, several times triggers a cascade of follow-on events, such as tsunamis, floods, landslides, nuclear power plant crisis and public health catastrophes in the affected regions. Thus, during the 2004 Sumatra–Andaman earthquake and Indian Ocean tsunami nearly 230,000 people were killed and more than one million people were left homeless in 13 countries surrounding the Indian Ocean. The May 12th, 2008 earthquake in Western Sichuan, China and January 8th, 2010 earthquake in Haiti caused a death toll well over 75,000 and 320,000 people, respectively. The latest M9 Tohoku earthquake of March 11th 2011 in Japan was the biggest recorded earthquake ever to hit Japan. The earthquake triggered extremely destructive tsunami waves of up to 10 meters that struck Japan minutes after the quakes and caused about 26,000 deaths and 3000 injured. Recent catastrophic earthquakes (2004–2011) occurred in Asia, Europe and America have provided and renewed interest in question of the existence of precursory signals related to earthquakes. In these circumstances, the science community is struggling on how to provide early information related to the occurrence time of such events in order to reduce the loss of human life and property. Previous studies (Gotoh et al., 2002; Fraser-Smith et al., 1990; Freund et al., 1999; Hattori et al., 2006; Hayakawa & Fujinawa, 1994; Hayakawa & Molchanov, 2002; Kopytenko et al., 1994; Liu et al., 2004; Ouzounov et al., 2006; Parrot et al., 2007; Pulinets et al., 2004; Stanica & M. Stanica, 2007; Stanica & D.A. Stanica, 2010; Tramutoli et al., 2005; Tronin et al., 2004; Varotsos, 2005) have shown that there were precursory signals observed on the ground and in space associated with several earthquakes. In the last 10 years, the interdisciplinary group for Electromagnetic Study of Earthquakes and Volcanoes (EMSEV) have demonstrated that the existence of the electromagnetic earthquake precursors by terrestrial and satellite observations is not trivial, and it is necessary a wide international cooperation and several more years of research with primary focus in the following directions: (i) what is the possible generation mechanisms of the electromagnetic phenomena; and (ii) whether electromagnetic precursors systematically precede earthquakes. In this respect, taking into account that the seismic-active Vrancea zone, Romania is one of the "hot" subjects in the Eastern Europe, this paper is focused on the specific methodology able to emphasize the short–term electromagnetic (EM) precursory parameters, associated to intermediate depth earthquakes (70-180Km). We consider that one of the realistic mechanisms for triggering such events in the seismogenic volume can be the dehydration of rocks which make fluid-assisted faulting possible. The changes of electrical conductivity occurred before an earthquake, as a sequence of geodynamic processes

Earthquakes Precursors 81

occurred in the intermediate depth range of 70 to 180 km (in 1940 with moment magnitude Mw7.7, in 1977 with Mw7.4, in 1986 with Mw7.1, and in 1990 with Mw 6.9) and all of them cause destruction in Bucharest, the capital city of Romania, and shake central and eastern

Several geodynamic models related to the triggering mechanism of the intermediate depth earthquakes have been elaborated in this area. Oncescu (1984) and Oncescu at al., (1984) proposed a double subduction model on the basis of 3-D seismic tomographic images: in their interpretation, the intermediate-depth earthquakes are generated within a vertical

Trifu & Radulian (1989), analyzing the seismic behavior of the Vrancea zone, proposed a model based on the existence of two active zones located at depths of 80-110 km and 120-170 km. Both zones are characterized by local stress inhomogeneities capable of generating large

Khain & Lobkosky (1994) suggest that the Vrancea zone results from delamination processes

Linzer (1996) explains the nearly vertical position of the Vrancea slab as the final rollback

Fig. 2. 3D resistivity tomographic image at sub-crustal level in the seismic active Vrancea zone: red circles are intermediate depth earthquakes; blue square delineate the Trans-European Suture Zone (M. Stanica et al., 1999); green line is Peceneaga-Camena fault (P-C fault); pink arrows show the direction of the asthenospheric currents; red arrow shows the

occurred during continental collision and lithosphere sinking into the mantle.

European cities several hundred kilometers away from their epicenters.

surface separating the sinking slab from stable lithosphere.

stage of a small fragment of oceanic crust.

direction of the torsion process of the relic slab

earthquakes.

associated with fluid migration through faulting system developed into and in the close vicinity of the seismogenic volume, could be detected by means of the anomalous behavior of the Bzn parameter taken throughout the frequency range less than 1.66E-2 Hz (Stanica & M. Stanica, 2007; Stanica & D.A. Stanica, 2010). According to the electromagnetic information acquired in 2009-2010 years correlated with seismic events, it is relieved that some days before an EQ occurred, the daily mean variation of the Bzn parameter have an anomalous behavior marked by a significant increase versus its normal distribution identified in non seismic conditions.
