Nat. Hazards Earth Syst. Sci., 11, 2817–2820, 2011www.nat-hazards-earth-syst-sci.net/11/2817/2011/doi:10.5194/nhess-11-2817-2011© Author(s) 2011. CC Attribution 3.0 License.

Natural Hazardsand Earth

System Sciences

The stress field of Vrancea region from fault plane solution (FPS)

L. Telesca1, V. Alcaz2, and I. Sandu2

1Consiglio Nazionale delle Ricerche, Istituto di Metodologie per l’Analisi Ambientale, C. da S. Loja, 85050 Tito (PZ), Italy2Moldavian Academy of Sciences, Institute of Geology and Seismology, Academiei str. 3, Chisinau, Moldova

Received: 7 June 2011 – Revised: 29 July 2011 – Accepted: 30 August 2011 – Published: 24 October 2011

Abstract. The fault plane solutions (FPS) of 247 seismicevents were used for stress field investigation of the region.The eigenvectorst,p,b, and moment tensorM componentsfor each FPS were defined and computed numerically. Theobtained results confirm the hypothesis of subduction-typeintermediate depth earthquakes for the Vrancea seismic re-gion and this may be considered the first approximation ofthe stress field for the whole of the Vrancea (intermediatedepth) region.

1 Introduction

The Vrancea seismic zone is located in the Southeast of theCarpathian region and is characterized by maximal seismicactivity at intermediate depths, with epicenters distributedwithin the ellipse domain 60× 30 km2 rotated by azimuthθ ≈ 45◦ according to the major axis (Radu and Polonic, 1982;Constantinescu and Enescu, 1984) (Fig. 1).

The recent relocation of several hypocenters of seismicevents has increased the accuracy of the ellipse domain to90× 20 km2 (Radulian et al., 2007; Hurukawa et al., 2008).

To explain the high seismic activity of this region, differ-ent seismotectonic models were developed (Constantinescuet al., 1972; Radu, 1974; Constantinescu and Enescu, 1984;Oncescu, 1984; Enescu and Enescu, 1992; Linzer, 1996;Sperner, 1996; Matenco et al., 1997; Enescu and Enescu,1998; Wortel and Spakman, 2000; Gvirtzman, 2002), butall of them are not in full accordance with the observationaldata (Bala, 2000). Due to the complexity of the problem, theinterpretation of deep Carpathian geo-tectonic processes isstill an open scientific debate. At the moment, more often,the dynamic variance of Carpathian seismotectonic model

Correspondence to:L. Telesca(luciano.telesca@imaa.cnr.it)

(Constantinescu and Enescu, 1984) is used, which showsa time-evolution from active to relict subduction. Here,the Carpathians genesis is related to the hypothetical Tethysoceanic plate consumption (Dewey et al., 1973), and its de-tachment from the continent-continent contact region of tec-tonic plates (Matenco et al., 1997).

The subject of this investigation is the regional seismicstress field. It has been described earlier (Oncescu, 1987)based on a relatively small amount of crustal and subcrustalfault plane solutions (FPS). Later, for the whole Romanianterritory, an analysis of crustal stress field has been donethrough the zoning procedure of the investigated regions,based on a limited dataset of FPS (Radulian et al., 1996). Inthe present paper, we used a statistical method and the highdensity dataset of FPS (for the Vrancea intermediate depthfoci zone), to confirm or to correct the previous statementson regional subduction. The statistical method has been ap-plied for the FPS parameters:p, b, t .

2 Data and methodology

To estimate the Vrancea region stress field, a statistical anal-ysis of the fault plane solutions FPS dataset was performed.This method requires the high dense and omogeneous datasetof FPS for the investigated region. All the FPS data from theavailable regional catalogues, starting from 1967 up to 2006,has been merged into one unique database (Sandu and Zai-cenco, 2008) and used in this study.

According to the concept forshallowdepth andinterme-diatedepth events (Radu et al., 1982; Radulian et al., 1996),we divided our dataset into two parts, with 18 % (h< 60 km)and 82 % (60< h< 200 km) from the total number of events,respectively.

Published by Copernicus Publications on behalf of the European Geosciences Union.

2818 L. Telesca et al.: The stress field of Vrancea region from the fault plane solution (FPS)

The stress field of Vrancea region from fault plane solution (FPS)

Luciano Telesca1, Vasile Alcaz2, Ilie Sandu2 1Consiglio Nazionale delle Ricerche, Istituto di Metodologie per l’Analisi Ambientale, C.da S.Loja, 85050 Tito (PZ), Italy

2 Moldavian Academy of Sciences, Institute of Geology and Seismology, Academiei str. 3, Chisinau, Moldova Abstract

The fault plane solution (FPS) of 247 seismic events were used for stress field investigation of the region. The eigenvectors t, p, b, and moment tensor M components for each FPS were defined and computed numerically. The obtained result confirm the hypothesis of subduction-type intermediate depth earthquakes for Vrancea seismic region and this may be considered the first approximation of the stress field for whole Vrancea (intermediate depth) region.

Keywords: fault plane solution; earthquake; Vrancea; stress field Introduction The Vrancea seismic zone is located in the South-East of Carpathian region and is characterized by maximal seismic activity at intermediate depths, with epicenters distributed within the ellipse domain 60×30 km2 rotated by azimuth θ ≈ 45° according to the major axis (Radu and Polonic 1982; Constantinescu and Enescu, 1984) (Fig.1).

Figure 1. Vrancea zone

Fig. 1. Vrancea zone.

The recent relocation of several hypocenters of seismic events has increased the accuracy of the ellipse domain to 90×20 km2 (Radulian et al., 2007; Hurukawa et al., 2008). To explain the high seismic activity of this region, different seismotectonic models were developed (Constantinescu et al., 1972; Radu, 1974; Constantinescu and Enescu, 1984; Oncescu, 1984; Enescu and Enescu, 1992; Linzer, 1996; Sperner, 1996; Matenco et al., 1997; Enescu and Enescu, 1998; Wortel and Spakman, 2000; Gvirtzman, 2002), but all of them are not in full accordance with the observational data (Bala, 2000). Due to the complexity of the problem, the interpretation of deep Carpathian geo-tectonic processes is still an open scientific debate. At the moment, more often, it is used the dynamic variance of Carpathian seismotectonic model (Constantinescu and Enescu, 1984), which shows a time-evolution from active to relict subduction. Here, the Carpathians genesis is related to the hypothetical Tethys oceanic plate consumption (Dewey et al., 1973), and its detachment from the continent-continent contact region of tectonics plates (Matenco et al., 1997). The subject of this investigation is the regional seismic stress field. It has been described earlier (Oncescu, 1987) based on a relatively small amount of crustal and subcrustal fault plane solutions (FPS). Later, for whole Romanian territory, an analysis of crustal stress field has been done trough the zoning procedure of the investigated regions, based on a limited dataset of FPS (Radulian et al, 1996). In the present paper we used a statistical method and the high density dataset of FPS (for Vrancea intermediate depth foci zone), to confirm or to correct the previous statements on regional subduction. The statistical method has been applied for FPS parameters: p, b, t. Data and Methodology To estimate the Vrancea region stress field, a statistical analysis of the fault plane solutions FPS dataset was performed. This method requires the high dense and omogeneous dataset of FPS, for the investigated region. All the FPS data, from the available regional catalogues, starting from 1967 up to 2006, has been merged into one unique database (Sandu and Zaicenco, 2008), and used in this study.

Figure 2. The hypocenter distribution of selected events.

According to the concept for shallow depth and intermediate depth events (Radu et al, 1982; Radulian et al, 1996) we divided our dataset into two parts, with 18% (h<60km) and 82% (60<h<200km) from total number of events respectively. The very concentrated spatial distribution of the earthquake epicenters in the area (Fig.2) makes the algorithm of numerical computation of the stress field quite simple; as a result, it becomes easier to select the more appropriate coordinate system for each subgroup of intermediate events (Gephart, 1984). This allow us to undestand how the mechanic stress field changes according to the event depth for the investigated region. The same words we can not say for crustal events,

Fig. 2. The hypocenter distribution of selected events.

The very concentrated spatial distribution of the earth-quake epicenters in the area (Fig. 2) makes the algorithmof numerical computation of the stress field quite simple;as a result, it becomes easier to select the more appropriatecoordinate system for each subgroup of intermediate events(Gephart and Forsyth, 1984). This allows us to undestandhow the mechanic stress field changes according to the eventdepth for the investigated region. We can not say the same forcrustal events, which are spread on a much larger area, andaccording to (Radulian et al., 2002) are characterized by ran-dom orientation of the principal stress axes and nodal planesof FPS.

which are spread on much larger area, and according to (Radulean et al., 2002) are characterized by random orientation of the principal stress axes and nodal planes of FPS.

Figure 3. Depth distributions for Vrancea region dataset events (Sandu I., Zaicenco A., 2008)

The highly dense dataset allows us to construct the high resolution image of the stress field, also to obtain a stable FPS due to a large number of intermediate depth events on the investigated region. Therefore, we focused only on the intermediate earthquake dataset (202 events) to define the regional seismic stress field.

From the theory, a fault plane it is completely specified by two angles: the fault strike φ defined as the azimuth of the strike direction and the dip δ defined as the angle between a horizontal plane and the fault plane (Koyama, J., 1997). With respect to a reference system in which the x-axis points north, the y-axis points east and the z-axis points upwards (Fig.4), the outward normal n to the fault plane is described by its three components:

nx = - sin δ*sin φ , ny = sin δ*cos φ , nz = -cosδ

The direction of a slip on a fault plane is conveniently described by the rake, which is the angle λ between the slip and strike directions. The unit slip vector s is then given as:

sx=sinλ*cosδ*sinφ+cosλ*cosφ, sy=-sinλ*cosδ*cosφ+cosλ*sinφ, sz=-sinλ*sinδ

The tension axis t, pressure axis p, null axis b, can be easily defined through n, s vectors:

)(2

1 snt += , b=n×s , )(2

1 snp −=

The t, p, b vectors are the eigenvectors of moment tensor M (Lay and Wallace, 1995), which for non-trivial solutions satisfy the condition (secular equation):

M*a = m*a, det (M – m*I) = 0

where m1, m2, m3 correspond to the eigenvectors; a1, a2, a3 define the principal axes of the stress (t, p, b) (Shearer, 2004), and I is the identity matrix.

Fig. 3. Depth distributions for the Vrancea region dataset events(Sandu and Zaicenco, 2008).

The highly dense dataset allows us to construct a high res-olution image of the stress field, also to obtain a stable FPSdue to a large number of intermediate depth events on theinvestigated region. Therefore, we focused only on the in-termediate earthquake dataset (202 events) to define the re-gional seismic stress field.

From the theory, a fault plane it is completely specifiedby two angles: the fault strikeφ defined as the azimuth ofthe strike direction and the dipδ defined as the angle be-tween a horizontal plane and the fault plane (Koyama, 1997).With respect to a reference system in which thex-axis pointsnorth, they-axis points east, and thez-axis points upwards(Fig. 4), the outward normaln to the fault plane is describedby its three components:

nx = −sin (δ) sin (φ),ny = sin(δ) cos(φ),nz = −cos(δ)

The direction of a slip on a fault plane is conveniently de-scribed by the rake, which is the angleλ between the slipand strike directions. The unit slip vectors is then given as:

sx = sin(λ) cos(δ) sin(φ)+ cos(λ) cos(φ),

sy = −sin(λ) cos(δ) cos(φ)+ cos(λ) sin(φ),

sz = −sin(λ) sin(δ)

The tension axist , pressure axisp, null axisb, can be easilydefined throughn, s vectors:

t =1

√2(n+s),b = n×s,p =

1√

2(n−s)

The t , p, b vectors are the eigenvectors of moment tensorM (Lay and Wallace, 1995), which fornontrivial solutionssatisfy the condition (secular equation):

M ×a = m×a,det(M −mI) = 0

wherem1, m2, m3 correspond to the eigenvectors;a1, a2,a3 define the principal axes of the stress (t , p, b) (Shearer,2009), andI is the identity matrix.

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L. Telesca et al.: The stress field of Vrancea region from the fault plane solution (FPS) 2819

Figure 4. Geometrical illustration of fault plane, slip direction, the normal direction to the fault plane. The slip vector is the movement of the hanging wall block relative to footwall block.

Results and Discussion The eigenvectors, which specify the direction of the tension axis t, the pressure axis p, the

null axis b, and moment tensor M components for FPS of all 202 intermediate events were computed numerically. Using the vertical components for p, t, s (slip) axes, we found that 91% of the source mechanisms of intermediate depth earthquakes have the same subduction-type features. On x-z profile it is shown clearly the downward of t axis, and the uniform distribution for both t, p axes in horizontal plane projection, shown on x-y and x-z profiles (Fig.5). The arguments for supporting the subduction process in the Vrancea region may be the vertical tension axis distribution according to the events depth. The maximum value for the vertical tension component is more characteristic for intermediate depth, which is represented through the high density plots (Fig. 6). Here, the 81% of the 202 intermediate depth events have the downward slip vector projection distribution (Fig. 6).

Figure 5. The t and p axes projection for horizontal (left) and vertical (right) profiles.

Fig. 4. Geometrical illustration of fault plane, slip direction, thenormal direction to the fault plane. The slip vector is the movementof the hanging wall block relative to the footwall block.

Figure 4. Geometrical illustration of fault plane, slip direction, the normal direction to the fault plane. The slip vector is the movement of the hanging wall block relative to footwall block.

Results and Discussion The eigenvectors, which specify the direction of the tension axis t, the pressure axis p, the

null axis b, and moment tensor M components for FPS of all 202 intermediate events were computed numerically. Using the vertical components for p, t, s (slip) axes, we found that 91% of the source mechanisms of intermediate depth earthquakes have the same subduction-type features. On x-z profile it is shown clearly the downward of t axis, and the uniform distribution for both t, p axes in horizontal plane projection, shown on x-y and x-z profiles (Fig.5). The arguments for supporting the subduction process in the Vrancea region may be the vertical tension axis distribution according to the events depth. The maximum value for the vertical tension component is more characteristic for intermediate depth, which is represented through the high density plots (Fig. 6). Here, the 81% of the 202 intermediate depth events have the downward slip vector projection distribution (Fig. 6).

Figure 5. The t and p axes projection for horizontal (left) and vertical (right) profiles.

Fig. 5. Thet andp axes projection for horizontal (left) and vertical(right) profiles.

3 Results and discussion

The eigenvectors, which specify the direction of the tensionaxist , the pressure axisp, the null axisb, and moment tensorM components for FPS of all 202 intermediate events werecomputed numerically. Using the vertical components forp,t , s (slip) axes, we found that 91 % of the source mechanismsof intermediate depth earthquakes have the same subduction-type features. On the x-z profile, the downward oft axis,and the uniform distribution for botht , p axes in horizontalplane projection are shown clearly, on the x-y and x-z profiles(Fig. 5). The arguments for supporting the subduction pro-cess in the Vrancea region may be the vertical tension axisdistribution according to the event’s depth. The maximumvalue for the vertical tension component is more characteris-tic for intermediate depth, which is represented through thehigh density plots (Fig. 6). Here, 81 % of the 202 interme-diate depth events have the downward slip vector projectiondistribution (Fig. 6).

Figure 6. The tension (t) and slip (s) axes distribution profile according to the event depth.

As it is possible to argue, the p and t axes are randomly distributed, showing no preferential stress regime for crustal depths. This behavior has been explained, in the mentioned references, by existence of a series of active faults in the fractured crust. At intermediate depths, the stress field is well defined statistically by clear vertical and horizontal orientations for t, s and p axes. The difference between crustal and subcrustal stress regime on Vrancea zone was previously studied (Oncescu and Trifu, 1987) with stress inversion method applied on a relatively small FPS dataset. We confirmed such results by using a statistical method for more complete and homogeneous FPS database. In the context of the obtained results, the following question is relevant: how can be interpreted the Vrancea intermediate depth subduction process in terms of the regional scale tectonics? Different stress regimes on crustal and subcrustal depth domain give arguments in favor of a break-off plate subduction processes in the region. So, the tectonic model, defined by the stress field, can be a partial or complete detached slab of continental crust, involved into a weak sinking in the upper-part mantle (Oncescu and Trifu, 1987; Wenzel et al, 1999; Radulian M et al, 1999, 2004, 2007). Conclusions

On the basis of catalogue of regional earthquakes, covering the time interval from 1967 to 2006, a statistical analysis of the seismic stress field characteristics in Vrancea region was performed. The stress field characteristics were deduced from fault plane solutions dataset. Statistical approach requires highly dense and homogeneous dataset for the investigated region and only intermediate Vrancea events comply with this requirement. For Vrancea intermediate depth region it was found that the slip axis and tension axis have the preferential downward orientation. The compressive axis shows the preferential tectonic cumulative stress. The horizontal cumulative stress and vertical (downward) stress release correspond to the subduction source mechanism (reverse faulting). Statistical analysis shows that almost all (about 91%) of the source mechanisms of Vrancea intermediate depth earthquakes have the same subduction-type features. The regional seismic stress field is the main factor for the moment, and this can contribute to a better understanding of the inner processes of the seismogenic zone. Our results support the hypothesis of existence a break-off plate subduction process at intermediate depths for the SE Carpathians region.

Fig. 6. The tension (t) and slip (s) axes distribution profile accord-ing to the event depth.

As it is possible to argue, thep andt axes are randomlydistributed, showing no preferential stress regime for crustaldepths. This behavior has been explained, in the mentionedreferences, by the existence of a series of active faults in thefractured crust.

At intermediate depths, the stress field is well defined sta-tistically by clear vertical and horizontal orientations fort , s

andp axes.The difference between the crustal and subcrustal stress

regime on the Vrancea zone was previously studied (Oncescuand Trifu, 1987) with the stress inversion method applied ona relatively small FPS dataset. We confirmed these results byusing a statistical method for a more complete and homoge-neous FPS database.

In the context of the obtained results, the following ques-tion is relevant: how can the Vrancea intermediate depth sub-duction process be interpreted in terms of the regional scaletectonics?

Different stress regimes on crustal and subcrustal depthdomains give arguments in favor of a break-off plate sub-duction processes in the region. Hence, the tectonic model,defined by the stress field, can be a partial or complete de-tached slab of continental crust, evolved into a weak sinkingin the upper-part mantle (Oncescu and Trifu, 1987; Wenzelet al., 1999; Radulian et al., 1999, 2004, 2007).

4 Conclusions

On the basis of a catalogue of regional earthquakes coveringthe time interval from 1967 to 2006, a statistical analysis ofthe seismic stress field characteristics in the Vrancea regionwas performed. The stress field characteristics were deducedfrom a fault plane solutions dataset.

A statistical approach requires a highly dense and homo-geneous dataset for the investigated region and only interme-diate Vrancea events comply with this requirement.

For the Vrancea intermediate depth region, it was foundthat the slip axis and tension axis have a preferential down-ward orientation. The compressive axis shows the prefer-ential tectonic cumulative stress. The horizontal cumulativestress and vertical (downward) stress release correspond tothe subduction source mechanism (reverse faulting). Statisti-cal analysis shows that almost all (about 91 %) of the source

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2820 L. Telesca et al.: The stress field of Vrancea region from the fault plane solution (FPS)

mechanisms of Vrancea intermediate depth earthquakes havethe same subduction-type features.

The regional seismic stress field is the main factor for themoment, and this can contribute to a better understanding ofthe inner processes of the seismogenic zone. Our results sup-port the hypothesis of the existence of a break-off plate sub-duction process at intermediate depths for the SE Carpathianregion.

Acknowledgements.The present study was supported by theCNR/ASM 2011–2012 Project “Characterization of seismicityof Moldova Republic territory. Contribution to seismic hazardassessment”. One of the authors (I. Sandu) expresses his sinceregratitude to ICTP for awarding him the STEP fellowship.

Edited by: M. E. ContadakisReviewed by: I. Paskaleva and another anonymous referee

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