ON THE RELATION BETWEEN GPS STRAIN FIELD AND ACTIVE FAULTS IN THE EASTERN BALTIC REGION B.A. Assinovskaya 1, V.L. Gorshkov 2, N.V. Shcherbakova 2, N.M. Panas 1 1 Geophysical Service RAS, seismic station Pulkovo, St. Petersburg, 197140, Russia, e-mail: bela.assin@gmail.com; 2 Central astronomical observatory at Pulkovo RAS, St. Petersburg, Russia Abstract. The strain field of Eastern Gulf of Finland-Gulf of Bothnian region has been studied using specially processed GPS-data of Finnish, Sweden, Estonian and Latvian stations. The software GIPSY 6.1.2 (JPL) was used for elaboration. The regional strain field was mapped and analyzed by GRID_STRAIN (Teza et al., 2007). The results were compared with seismic data. The known Osmussaar earthquake (M=4.6) occurred in 1976 in this region. The new digital records of this event were obtained and some source parameters were revalued. The earthquake source zone was studied by COULOMB 3.1 (Toda et al. 2005, Lin, Stein 2004) software in 3D. The stress-strain condition results were compared with GPS-features. In the result, the active fault zones were specified more exactly. The revealed seismotectonic features are very important for seismic hazard assessment of the Baltic Sea region. INTRODUCTION The strain field ΔL/L (nan ostrain/yr) evaluated by GRID_STRAIN software and seismicity around the Lake Ladoga Gulf of Finland region has been previously studied (Assinovskaya, Shchukin, Gorshkov, Shcherbakoba, 2011) using specially processed GPS-data. The GPS-station velocities on long-term observation were estimated by GIPSY 5.0. Analyzing the strain field it was supposed that the borders between areas of different strain-stress state indicate the regional active faults position. The data obtained were compared with regional tectonics and earthquake focal mechanisms. This work covers the area of Gulf of Bothnia and Gulf of Finland joint where significant seismic activity presents and the known Osmussaar earthquake has been occurred. The Osmussaar earthquake (October 25 1976 М =4.8, Н=13 km, I0= 6-7, origin time 08h 39m 47.4s, epicenter 59.36 N (+5.0 km) 23.34 E (+4.6 km)) was one of the first largest events in this region. Instrumentally, the 1976 earthquake source zone was researched by N. K. Bulin (Bulin et al, 1980). Two aftershocks at a depth of 13 km were registered and local deep structure was researched. The main event and two aftershocks formed NW-SE directed line that coincided with one of the focal mechanism plane (see Fig.1). Dr. Slunga (1979) modeled the 1976 earthquake source using surface waves of analog records in the range of distance 1-10º. He found that the earthquake had strike-slip mechanism but contained a part of reverse dip-slip faulting (Fig.1). But it is known that any focal mechanism solution never gives a single answer what fault plane is responsible for earthquake occurrence. In this work, we use independent geodynamic GPS data to model the regional strain field to reveal active structure zones and to point to true earthquake fault plane. Besides, we conducted the special analysis of 1976 focal mechanism data. The results obtained were used for seismic hazard zoning. The Osmussaar earthquake source parameters were calculated also in (Slunga, 1979). The seismic moment was equal 3.5 10 15 n m and the stress drop was equal 150 Mpa. The last value seemed to be too large especially if to compare it with the same parameter of Kaliningrad 2004 earthquakes. The stress drop was 37.4 MPa in the source of the second event of Kaliningrad earthquakes. In this connection some analog seismograms recorded by regional stations were digitized to analyze Sg wave spectrum and to reassess source parameters too. 137
Fig.1. The Osmussaar earthquake position (after R. Slunga, 1979 with our addition). The open circle represents the 1976 epicenter, blue stars note Bulin s aftershocks (see also below). The solid circles denote macroseismic epicenters of earlier shocks in the area. The yellow circles are historical events. The red stars are earthquake epicenters registered instrumentally in 1976-2008. The straight solid lines AA and BB show the orientation of the two possible fault planes. The two heavy dashed lines are curves of constant uplift rate of 0.2 cm/yr (upper line) and 0.4 cm/yr (lower line). The dashed curve lines are the border of crystalline basement. The lines in the Baltic Sea are faults. The nodal planes show stereographic projection of the lower hemisphere. Black regions show compression, white show dilatation. GPS-DATA AND DATA PROCESSING The raw GPS-data of permanent stations of Finland, Sweden (SWEPOS), Latvia (LATPOS) and Estonia (see Tabl. 1) for 1999-2009 were used to elaborate together. The GPS-observation of IGS stations were collected from international archive sites (ftp://igs.bkg.bund.de/euref/obs/ and ftp://cddis.gsfc.nasa.gov/pub/gps/data/daily/). The original GPS-data of SWEPOS net were kindly provided us by Mats Westberg from SWEPOS Control Centre. The data of LATPOS net have been gotten from site http://www.latpos.lgia.gov.lv/spiderweb/frmindex.aspx. A daily positions for each station were estimated by the GIPSY-OASIS software version 6.1.2 (GIPSY 6.1.2 Release Notes, 2012) and then were assessed station velocities. The velocity estimations for Finnish stations OLKI and TOUR (light grey in the Table 1) were only used from (Lidberg at al., 2009) solution. A daily positions and velocities for each station were solved by the GIPSY-OASIS software version 6.1.2 (GIPSY 6.1.2 Release Notes, 2012). Strain fields were modeled using software GRID_STRAIN (Teza et al., 2007). The special software's were used to digitize the analog seismogram and to get the source spectrum of the Osmussaar 1976 earthquake. The source dynamics assessment was carried out with Coulomb 3.1 program (Toda et al 2005; Lin, Stein 2004, http://earthquake.usgs.gov/research/modeling/coulomb). 138
RESULTS Tabl. 1 Positions and horizontal velocities of GPS-station under investigation Station Lonº Latº Ve (mm) Vn (mm) Period (years) LEK0 14.877 60.722-0.10 ± 0.05-0.66 ± 0.06 2007-2009 NOR0 16.246 58.590 0.13 0.05-0.78 0.05 2007-2009 MAR6 17.258 60.595 0.74 0.03-0.70 0.03 2007-2011 UPP0 17.590 59.865 0.21 0.05-1.02 0.04 2007-2009 VIS0 18.367 57.654 0.56 0.03-0.93 0.02 2007-2011 LOV0 17.829 59.338-0.19 0.05-1.08 0.05 2007-2009 METS 24.395 60.217 1.15 0.03-0.63 0.03 2007-2011 SUUR 24.380 59.464 0.85 0.05-1.19 0.05 2007-2011 KURE 22.510 58.256 0.81 0.04-0.99 0.03 2008-2011 LIEP 21.006 56.511-0.51 0.04-1.09 0.03 2006-2009 VENT 21.548 57.391 0.30 0.03-0.99 0.03 2006-2010 VAL0 25.406 57.536 0.61 0.04-1.17 0.04 2008-2011 RIGA 24.059 56.949 0.24 0.03-0.49 0.03 2007-2001 KULD 21.959 56.969 0.14 0.04-0.77 0.03 2006-2009 OLKI 21.473 61.240 0.92 0.04-1.22 0.06 1999-2008 TUOR 22.443 60.416 0.74 0.03-1.31 0.05 1999-2008 The horizontal velocity data were transformed using the ITRF2005 (Altamimi et al., 2007) absolute rotation pole for Eurasia (Tabl. 1). These residual velocities are shown in Fig. 2a. Using all these velocity estimations the strain fields were modeled by software GRID_STRAIN (Teza et al., 2007). The strain fields are shown in Fig. 2b. a b Fig.2. a) GPS-station residual velocities relative to Eurasia plate (blue arrows) and its error ellipses (red). Black circles show GPS station positions. b) Strain fields ΔL/L; nanostrain/yr prepared using Table1 data and software (Teza et al. 2007). Blue and red arrows are extension and compression values respectively. The strain field are calculated on the 50 50 km grid. 139
It is seen that the general NW-SE direction of movement corresponded to local plate movement are broken in some places. This diversity of orientations marked in the velocity of horizontal movement field may be occurring due to the influence of an active fault tectonics in the Gulf of Finland and in the Gulf of Bothnian. The strain field obtained is inhomogeneous in the region of study; there are several blocks with different kind of strain from strike-slip up to full dilatation. However, the interpretation cannot be considered as unique due to the small number of station net. The general orientation of the deformation by GPS coincides with the fault planes in the 1976 Osmussaar earthquake focal mechanism solution, both NW and SW directions can be active. Another method of modeling was used to research the stress field in the area of study. The earthquake Coulomb stress change modeling (Fig. 3) allows getting distribution of parameters on the source level. According to the method used red lobes mean positive Coulomb stress values and outline the area of cracks that could form during the seismic process. This spacious area of cracks with a size about 3 km occurred if only the first NW plane of focal mechanism is realized (Fig.3a). Moreover this NW trend coincides with the direction of the line connecting aftershocks and main shock positions. a b Fig. 3. 1976 Osmussaar earthquake source Coulomb stress change map prepared using software COULOMB 3.1 (Toda et al. 2005; Lin, Stein 2004). Focal mechanism is according to (Slunga, 1979): a) plane I (strike 162º, dip 69º, rake 17º) b) Plane II (Strike 65, dip 74, slip - 158 ). See Fig.1 On the second possible plane the strike slip right lateral motions on the line located along the Gulf of Finland coastline could be realized. There is a geologically found fault in this area. But the depth of that fault is unknown. And it is seen that red lobes aren t such big as it is in the first case (Fig.3b). In this situation we prefer the NW focal mechanism plane. Probably NW fault is active recently. Additionally, we tried to reassess the stress drop value in the 1976 Osmussaar source. We digitized some analog seismogram to get the source spectrum. The processing included data smoothing, response correction and scale corrections and spectrum analysis in terms of velocities and displacements. The corner frequency f c seemed to be ~1.4 Hz, source radius was 900 m. And if M 0 =3.5 10 15 nm (Slunga, 1979), Δσ =2.1 Mpa. This value is less on two orders than that mentioned above. 140
Amplitude, mkm 10 1 displacement 0.1 1 2 Hz Fig.4. Amplitude displacement spectrum, seismic station MOS, seismograph CHARIN. Red lines note flat and sloping parts of spectrum respectively. Blue arrow shows the corner frequency value. CONCLUSIONS The GPS observations of permanent stations located near the Finnish-Bothnian area and around the 1976 Osmussaar earthquake source zone were elaborated. Some data were processed first. The GPS-velocity data were analysed to get an overview about recent geodynamics in the region. The map of horizontal strain was compiled, some areas of different strain type were revealed and borders between these areas were accepted like presumably active faults. The 1976 Osmussaar earthquake focal mechanism data were analysed also and NW-SE direction fault was chosen like an active one. It was established that the GPS-deformation features agree with focal mechanism solution. Some source parameters such as corner frequency and stress drop were specified. References: Altamimi, Z., Collilieux, X., Legrand, J., Garayt, B. and Boucher, C., ITRF2005: A new Release of the International Terrestrial Reference Frame based on Time Series of Station Positions and Earth Orientation Parameters. J. Geophys. Res., 112, B09401, doi:10.1029/2007jb004949. Assinovskaya, B., Shchukin, J., Gorshkov V.and Shcherbakova, N. (2011), On recent geodynamics of the Eastern Baltic Sea region. Baltica, 24 (2), 61-70. Lidberg M., J. M. Jonsson, H.-G. Scherneck and G.A.Milne (2009), Recent results base on continuous GPS observations of the GIA process in Fennoscandia from BIFROST, J. Geodyn. (2010), doi:10.1016/j.jog. 2009.11.010. Slunga R. (1979), Source mechanism of a Baltic earthquake inferred from surface-wave recordings. Bull. Seism. Soc. Am., 69(6), 1931-1969. Teza G., А. Pesci and А. Galgaro (2008), Grid_strain and grid_strain3: Software packages for strain field computation in 2D and 3D environments, Computers & Geosciences, 34, 9, 1142-1153. Toda, S., Stein S. R. and Richards-Dinger K., Bozkurt S. (2005), Forecasting the evolution of seismicity in southern California: Animations built on earthquake stress transfer. J. Geophys. Res., 110, B05S16, doi:10.1029/2004jb003415. 141