**2. Geodynamic models and possible mechanisms of intermediate depth earthquakes**

The seismic active Vrancea zone is situated at the curvature of the Carpathians and it is bounded on the north-east by the East European Platform, to southward by the Moesian Platform, and on the north-west by Transylvanian Basin (Fig.1).

The hypocenters of the intermediate depth earthquakes are concentrated within a very small seismogenic volume and they are much denser than any other mantle events of intracontinental origin known in the world. In the past century, 4 large seismic events have

Fig. 1. Map of the seismic active Vrancea zone: crustal (white circles) and intermediate depth (red circles) epicenters of the earthquakes taken from ROMPLUS catalogue; pink star represents the Geodynamic Observatory Provita de Sus (GOPS) used as monitoring site of the earthquake precursors

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

**2. Geodynamic models and possible mechanisms of intermediate depth** 

Platform, and on the north-west by Transylvanian Basin (Fig.1).

The seismic active Vrancea zone is situated at the curvature of the Carpathians and it is bounded on the north-east by the East European Platform, to southward by the Moesian

The hypocenters of the intermediate depth earthquakes are concentrated within a very small seismogenic volume and they are much denser than any other mantle events of intracontinental origin known in the world. In the past century, 4 large seismic events have

Fig. 1. Map of the seismic active Vrancea zone: crustal (white circles) and intermediate depth

(red circles) epicenters of the earthquakes taken from ROMPLUS catalogue; pink star represents the Geodynamic Observatory Provita de Sus (GOPS) used as monitoring site of

identified in non seismic conditions.

**earthquakes** 

the earthquake precursors

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 European cities several hundred kilometers away from their epicenters.

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 surface separating the sinking slab from stable lithosphere.

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 earthquakes.

Khain & Lobkosky (1994) suggest that the Vrancea zone results from delamination processes occurred during continental collision and lithosphere sinking into the mantle.

Linzer (1996) explains the nearly vertical position of the Vrancea slab as the final rollback stage of a small fragment of oceanic crust.

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 direction of the torsion process of the relic slab

Earthquakes Precursors 83

through the surrounding rock, what, in conditions imposed by real crustal parameters, may

Another theory supposes the generation of the magnetic signal either by conductive fluid flowing in presence of the magnetic field of the Earth, or by magnetohydrodynamic conversion of the seismic signal into an electric signal during the propagation through a conductive medium (Molchanov et al., 2001). While these mechanisms were proved in laboratory conditions, it is unclear, yet, how this process takes place in conditions rather similar to those specific to the Earth, owing to the lack of measurements in active fault

At the Earth surface the vertical geomagnetic component (Bz) is entirely secondary field and its existence is an immediate indicator of lateral inhomogeneity. For a two-dimensional (2D) structure, the vertical geomagnetic component (Bz) is produced essentially by the horizontal geomagnetic component perpendicular (B) to geoelectric structure orientation and,

Bz(f) Bzn (f) = B (f)

should be time invariant in non geodynamic conditions (Ward et al., 1970), but it becomes unstable due to the geodynamic processes and, therefore, it could be used as a precursory

<sup>2</sup> E (f) 0.2

<sup>2</sup> E (f) 0.2

parameter of the intermediate depth seismic activity (Stanica and D.A. Stanica, 2010). In order to explain cause (earthquake) - effect (anomalous Bzn) relationship, we introduce

=

=

Bzn (f) =

This estimation of Bzn is in error for non two-dimensional geoelectrical structure.

<sup>ρ</sup> (f) <sup>f</sup> B (f)

where: ║ is the resistivity parallel to strike[m], B is the component of the magnetic

From the relations (2) and (3) we may estimate the normalized function Bzn (f), in terms of

ρ (f)

ρz (f)

<sup>ρ</sup>z (f) <sup>f</sup> Bz(f)

where: z is vertical resistivity [m = VmA-1], f is the frequency [Hz] and the E║ is the electric field parallel to strike [Vm-1], Bz is the vertical component of the magnetic induction

(1)

(2)

(4)

, (3)

create surface magnetic fields of a few nT (Fenoglio et al., 1995).

**3.2 Theoretical base of the electromagnetic precursors** 

consequently, the normalized Bzn function defined as:

zones.

the following equations:

[Tesla (T) = V s m-2].

[Tesla (T) = V s m-2].

resistivities as follows:

Also, it is possible to write the relation:

induction perpendicular to strike

Sperner, B., the Collaborative Research Center [CRC] 461 Team, (2005), taking into consideration that the geometry of the subduction zone was not unequivocally defined, proposed four possible configurations for the Vrancea zone: (i) subduction beneath the suture zone; (ii) subduction beneath the fore deep area; (iii) two interacting subduction zones, and (iv) subduction beneath the suture, followed by delamination.

Various types of slab detachment or delamination have been proposed to explain the present-day seismic images of the descending slab (Girbacea & Frisch, 1998; Gvirtzman, 2002; Sperner et al., 2001; Wortel & Spakman, 2000).

Viscous flows due to the sinking seismogenic slab together with dehydration-induced faulting can be considered as possible triggering mechanism explaining the intermediatedepth seismicity in Vrancea (Ismail-Zadeh et al., 2000)

Stanica et al., (2004) show, on the base of the three-dimensional (3D) resistivity tomographic image carried out using magnetotelluric data, that the possible triggering mechanism of the intermediate-depth earthquakes in the Vrancea zone may be the rock response to the active torsion processes sustained by the descending asthenospheric currents and the irregular shape of the relic slab. In their opinion, this torque effect may generate the increase shear stress and drive faulting process within the rigid slab (Fig.2).
