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Za Reni ot Emil Vrancea I-va chast

Deep seismic sounding by microtremor (SSMT) broadband signals for Vrancea seismic zone

Paper structure:

Introduction

Methodology

Results

Discussion and Conclusions

Introduction

Vrancea earthquake source is one of the most specific and curious among different seismic generation regions. Its most important peculiarity is the very narrow intermediate depth zone, able to generate high magnitude earthquakes at relatively high rate. The strong seismic potential, the location of the zone near big cities (Bucharest for example), its relatively deep seismogenic volume, concentrated in a very small volume, make this zone dangerous and put it at high seismic risk. The high occurrence rate and the large macroseismic field, observed in many cases, need very careful and deep investigations to unreveal the specific seismogenesis of the local earthquakes.

The most important purpose in this case could be defined as to establish the seismic generation properties of the Vrancea seismc zone. To do this, it is necessary to study in details the deep interior around the seismogenic source and to try to establish all peculiarities, possibly in high resolution.

Hypotheses review

The most explored hypotheses about the seismogenic properties of Vrancea seismic zone are related to the plate tectonic models, developed for the zone.

In general, the zone can be considered the result of a subducting part of the Thetis ocean slab. There are several indicators about the existence of paleosubduction like findings of ophiolites, flish, glaukofan shale, but up to now all these facts can not be combined in a logically stable ensemble. There are lot of contradictions about the strike and the slip directions of subduction. Some authors (Balla.Z; Csontos et. al) [1,2] consider northward and/or eastward direction (Sandulescu M.) [3], others (Gribacea R. et al) [4] suggest a south southward direction from the Moesian platform. The fact of Neogen magmatism is another argument, supporting the subduction hypothesis. A lot of authors [1,2, 3,4,5] consider that the oceanic lithosphere was joined to the East-European platform and subducted to west and southwestward direction under the Carpatian arc during the Miocene. The ophiolite complex location trough the Transilvanides and Auseni Mountains and the subduction geometry to the north-west, also support a possible subduction of the Moessian platform.

The generalization of all geophysical data gives three alternative models of the geodynamics of Vrancea seismic zone (Fig.1.)

Fig.1. Alternative models of the geodynamics of Vrancea seismic zone: A) the slap is detached and freely down drowns in the mantle; B), the slab is still consolidating with upwarding under the Carpathians lithosphere of the Moessian platform; C) Considers the detached continental lithosphere (Knapp J.H. et al.) [6].For the first model (1A) the slap is detached and freely down drowns in the mantle. Second model - the slab is still consolidating with upwarding under the Carpathians lithosphere of the Moessian platform (Fig 1B). The third model (Fig.1C) considered the detached continental lithosphere.

Each of the models supposes that a relatively cold and dense lithosphere body is located in the upper mantle under the Carpathian arc at the moment, and that it generates seismicity in a certain depth interval. The first two models supposed that subduction of the oceanic lithosphere took place, but they differ in which part of the orogen the process took place and in what direction the subduction developed.

All models try to explain the different peculiarities of the crust-mantle coupling. They aim at explaining the relationships between upper and lower parts of the structures, observed and analyzed by geophysics. Particularly, it is interesting to explore whether the ocean basin has been incorporated during the formation of the East Carpathians, for which there are many signs at the boundaries of the two lithosphere plates. It is expected that they should should be reflected to the geological formations at the surface or in depth.

Each of the models has its drawbacks and many internal contradictions, which means that the geodynamics of the Vrancea zone and its seismogenic properties are still under discussion. In 2005 he patented a new method of passive seismic study based on the analysis of the spatial variation of the spectrum of local micro-seismic area [7]. The method is based on experimentally verified assumption that the vertical component of displacement in micro seismic noise is represented mainly by the vertical component of moving fundamental fashion wave of the Rayleigh. Such an assumption is tenable first because micro seismic field comprises more than surface waves and second surface wave field contains predominantly Rayleigh waves and Love. The wave of Love appears horizontally polarized SH-wave, so no contribution to the vertical component of hesitation. Third, as shown by the numerous field measurements, waves Rayleigh are represented mainly by fundamental (zero) fashion, contributions by first fashion constitutes no more than a few percent in amplitude contribution of higher modes is insignificant in relation to the first.

The possibility of using spatial variations of the energy spectrum of micro seismic field study of deep geological structure of the medium is formulated later as a method of low frequency seismic micro drilling, and was identified by Russian researcher in conducting A.V.Gorbatikov micro seismic surveys on the island. Lantserot Canary Archipelago and later on the island. El Hierro. In further studies of various geological confirms the relevance of the proposed approach.Deep seismic sounding investigations of Vrancea seismic zone have been performed, based on data from 19 broadband seismic stations, located on the territory of Romania (Fig.1). According to the methodology of the seismic sounding, one of the stations was selected to be a basic station (PLOR). Stations data, location and the equipment used are presented in Table 1. The study was done in an area of approximately 63840 km2 (240x266 km). 1. Methodology

The method is based on the inversion of the amplitude-frequency domain of the background seismic noise to depths. The main assumption is that the vertical component of the low frequency part of the noise amplitudes is determined by the fundamental modes of the Raleigh waves representing the main part of the seismic noise. The method is based an the assumption that the inhomogeneities in the earth crust and upper mantle change the spectrum of the low frequency seismic noise as follows: The spectral amplitudes of a fixed frequency f decrease over high seismic waves velocity zones and vice versa they increased over low seismic velocity zones. The technology of measuring and processing provide:

1. Consistent measurement of statistical sustainable microseismic spectra at all points of the network or profile. Reaching statistical sustainable micro seismic signal accumulated over the experimentally determined period of stationarity signal equal to 2 hours.

2. Building on the map or profile of distribution of the amplitude of each frequency in the spectrum.

3. Bind received cards or accounts to the appropriate depth, proceeding from the relationship:

, where H(f) - the depth of the layer of which is being built image, - wavelength of the fundamental fashion relay, f - frequency in the spectrum of micro-seismic signal for which calculations are made, - phase velocity of the fundamental wave mode of relay frequency f, - coefficient of deep attachment estimated in the range 0.4-0.5.

The processing and construction of the image consists in that, for each frequency f in the spectrum is plotted spatial curve (or map) the distribution of the variation of the intensity of the micro-seismic signal. This curve (map) is attached to depth. Based on the totality of the profile curves plotted dimensional image where the horizontal axis are the coordinates and the vertical axis the depth corresponding to the profile (map).

The horizontal resolution of the method is about 4% of the wave length and about 8% determine the depth/vertical resolution of the investigated anomalous inhomogeneity body. The vertical resolution thus can reach 15-16%.

Fig.1. Locations of the seismic stations for this research (Google earth).The frequency f is related to the depth of the inhomogeneity body H and the velocity of the fundamental mode of the Raleigh wave Vr(f) by the relationship [7].

H= 0.5 Vr(f)/fAs a result of the application of this method the velocity contrasts of the seismic waves is the main parameter obtained in 2D and 3D views of the deep Earths interior structures.

The performance of this methodology allow us on the base of the registrations of the microseismic background noise on the above mentioned broadband stations to construct the deep interior around the Vrancea seismic source. (Fig.2.)

The DEM model over the Vrancea deep seismic zone is constructed with a grid 6 6.White dots represent the hypocenters of the earthquakes with magnitude greater than 4.0 for the time interval 2000-2013 generated by Vrancea seismic zone. ( Seismic catalogue of National Institute for Earth Physics Romania).

According to the methodology created for this research for all stations 1 hour seismic noise records are used. The time interval is 00:00:00-01:00:00 on 01.02.2015. For each station the power spectra have been calculated. The variations of the seismic waves velocities are determined according the base station (PLOR). The inversion of the spectral frequencies considered the dispersion of the Raleigh surface waves as the increased velocity is due to

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