Development of a 3D velocity model for the crust and upper mantle of the Barents Sea and adjacent regions
Abstract
Our study region includes the entire Barents Sea from northern Fennoscandia to the region north of Svalbard, and it extends from the Norwegian Sea and the Fram Strait to the western Kara Sea, surrounding the declared Russian nuclear test sites on Novaya Zemlya. This region consists of Precambrian shields, oceanic and extended continental crust, fold and thrust belts, and deep sedimentary basins. Our main objective is to develop a 3D seismic velocity model of the crust and upper mantle in the study region. This is a two-year project funded by the U.S. DoE and scheduled until September 2005. To compile our 3D model with a nominal resolution of 50×50 km, we use results from previous, crustal-scale 2D seismic wide-angle and normal incidence profiles. Passive seismological recordings, previous receiver function studies, and potential field data provide additional constraints for our model. The upper mantle of our model is based on published regional models. In order to verify our a priori model, we model observed arrival times of several seismic phases with known origins (ground truth events). The final model aims to improve seismic monitoring in this region, which includes improved event locations and event size estimation, and a better understanding of regional seismic wave phases. The model will also serve as a basis for estimating and calibrating travel times of regional phases in this region, and it will complement, with a better resolution, global crustal models such as CRUST5.1. Here we focus on the crustal structure in the study region and present seismic velocity structures in terms of preliminary regional contour maps, and seismic velocity transects. N-S trending transects in the western Barents Sea show Moho depths between 10 and 45 km, with average values around 35 km. Thicknesses of sedimentary layers vary considerably and reach locally more than 10 km. Strongest variations are observed in the west between northern Norway and Svalbard, whereas transects further east exhibit a more simple, layered crustal structure. The crustal velocity structure causes variations of first arrival travel times of more than 1 s in distances more than about 300 km from the sources. The detailed crustal velocity model is therefore also important for the monitoring of events at regional distances, where corresponding ray paths run mainly in the upper mantle. Furthermore, delay times of a few seconds for events at Novaya Zemlya, recorded at the NORSAR array in Norway, indicate significant velocity variations in the mantle also for our study region.
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Development of a 3D velocity model for the crust and upper mantle Scandinavia Nowaya Z. Svalbard KFJL 20˚E 30˚E 40˚E 50˚E 64˚N 66˚N 66˚N 68˚N 68˚N 70˚N 70˚N 72˚N 72˚N 74˚N 74˚N 76˚N 76˚N 78˚N 0 200 400 600 800 1000 area of location error ellipse (km2) 0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 0 100 200 300 400 500 600 700 800 0 100 200 300 400 500 600 700 800 900 1000 1100 0 100 200 300 400 500 600 700 800 900 1000 20˚E 30˚E 40˚E 50˚E 64˚N 66˚N 66˚N 68˚N 68˚N 70˚N 70˚N 72˚N 72˚N 74˚N 74˚N 76˚N 76˚N 78˚N 20˚E 30˚E 40˚E 50˚E 64˚N 66˚N 68˚N 70˚N 72˚N 74˚N 76˚N 50 km Depth 2 4 6 8 VP [km/s] 20˚E 30˚E 40˚E 50˚E 64˚N 66˚N 68˚N 70˚N 72˚N 74˚N 76˚N 150 km Depth 2 4 6 8 VP [km/s] 20˚E 30˚E 40˚E 50˚E 64˚N 66˚N 68˚N 70˚N 72˚N 74˚N 76˚N 250 km Depth 2 4 6 8 VP [km/s] 20˚E 30˚E 40˚E 50˚E 64˚N 66˚N 68˚N 70˚N 72˚N 74˚N 76˚N 350 km Depth 2 4 6 8 VP [km/s] 0 20 40 60 depth [km] 020040060080010001200140016001800 distance [km] 0 20 40 60 depth [km] 020040060080010001200140016001800 distance [km] 0 20 40 60 depth [km] 020040060080010001200140016001800 distance [km] 0 20 40 60 depth [km] 020040060080010001200140016001800 distance [km] a EW 0 20 40 60 80 100 depth [km] 0 20 40 60 80 100 depth [km] 0 20 40 60 80 100 depth [km] 0 20 40 60 80 100 depth [km] 23456789VP [km/s] b A 0 20 40 60 depth [km] 0 200 400 600 800 1000 distance [km] 0 20 40 60 depth [km] 0 200 400 600 800 1000 distance [km] 0 20 40 60 depth [km] 0 200 400 600 800 1000 distance [km] 0 20 40 60 depth [km] 0 200 400 600 800 1000 distance [km] S N 0 20 40 60 80 100 depth [km] 0 20 40 60 80 100 depth [km] 0 20 40 60 80 100 depth [km] 0 20 40 60 80 100 depth [km] 23456789VP [km/s] C 0 20 40 60 depth [km] 0 200 400 600 800 distance [km] 0 20 40 60 depth [km] 0 200 400 600 800 distance [km] 0 20 40 60 depth [km] 0 200 400 600 800 distance [km] 0 20 40 60 depth [km] 0 200 400 600 800 distance [km] S N 0 20 40 60 80 100 depth [km] 0 20 40 60 80 100 depth [km] 0 20 40 60 80 100 depth [km] 0 20 40 60 80 100 depth [km] 2 3 4 5 6 7 8 9VP [km/s] B 0 20 40 60 depth [km] 02004006008001000 distance [km] 0 20 40 60 depth [km] 02004006008001000 distance [km] 0 20 40 60 depth [km] 02004006008001000 distance [km] 0 20 40 60 depth [km] 02004006008001000 distance [km] a EW 0 20 40 60 80 100 depth [km] 0 20 40 60 80 100 depth [km] 0 20 40 60 80 100 depth [km] 0 20 40 60 80 100 depth [km] 23456789VP [km/s] b D A 0 100 200 300 400 depth [km] 23456789VP [km/s] 0 2 4 6 8 10 t - x/8 [s] 02505007501000125015001750 distance [km] EW A B 0 100 200 300 400 depth [km] 23456789VP [km/s] 0 2 4 6 8 10 t - x/8 [s] 0 250 500 750 distance [km] S N B C 0 100 200 300 400 depth [km] 23456789VP [km/s] 0 2 4 6 8 10 t - x/8 [s] 0 250 500 750 1000 distance [km] S N C D 0 100 200 300 400 depth [km] 23456789VP [km/s] 0 2 4 6 8 10 t - x/8 [s] 02505007501000 distance [km] EW D 20˚E 30˚E 40˚E 50˚E 64˚N 66˚N 66˚N 68˚N 68˚N 70˚N 70˚N 72˚N 72˚N 74˚N 74˚N 76˚N 76˚N 78˚N 20˚E 30˚E 40˚E 50˚E 64˚N 66˚N 66˚N 68˚N 68˚N 70˚N 70˚N 72˚N 72˚N 74˚N 74˚N 74˚N 76˚N 76˚N 78˚N 0 5 10 15 20 Depth to Basement [km] 20˚E 30˚E 40˚E 50˚E 64˚N 66˚N 66˚N 68˚N 68˚N 70˚N 70˚N 72˚N 72˚N 74˚N 74˚N 74˚N 76˚N 76˚N 78˚N 0 5 10 15 20 Depth to Basement [km] - 1 km-contours IASP91 23456789VP [km/s] IASP91 BAREY -2 0 2 4 6 8 10 t - x/8 [s] 0 250 500 750 1000 1250 1500 1750 2000 distance [km] 20˚E 30˚E 40˚E 50˚E 64˚N 66˚N 66˚N 68˚N 68˚N 70˚N 70˚N 72˚N 72˚N 74˚N 74˚N 74˚N 76˚N 76˚N 78˚N 0 10 20 30 40 50 Depth to Moho [km] 20˚E 30˚E 40˚E 50˚E 72˚N 76˚N b 6 8 10 12 14 16 18 20 Sediment thickness [km] 0 10 20 30 40 50 Igneous crustal thickness [km] 0 10 20 30 Sediment thickness [km] 0 10 20 30 40 50 Igneous crustal thickness [km] 0 10 20 30 Sediment thickness [km] 0 10 20 30 40 50 Igneous crustal thickness [km] 0 10 20 30 Sediment thickness [km] 0 10 20 30 40 50 Igneous crustal thickness [km] 0 10 20 30 Sediment thickness [km] 0 10 20 30 40 50 Igneous crustal thickness [km] 0 10 20 30 Sediment thickness [km] 0 10 20 30 40 50 Igneous crustal thickness [km] 0 10 20 30 Sediment thickness [km] 0 10 20 30 40 50 Igneous crustal thickness [km] 0 10 20 30 Sediment thickness [km] outlier outlier outlier 0 10 20 30 40 50 Igneous crustal thickness [km] 0 10 20 30 Sediment thickness [km] a of the Barents Sea and adjacent regions Nils Maercklin1, Oliver Ritzmann2, Jan-Inge Faleide2,1, Hilmar Bungum1,2, Johannes Schweitzer1, Walter D. Mooney3, Shane T. Detweiler3 & William S. Leith3 1NORSAR, POB 53, N-2027 Kjeller, Norway, e-mail: [email protected] 2Dept. of Geosciences, University of Oslo, POB 1072 Blindern, N-0316 Oslo, Norway 3U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, California, USA Data and geological provinces Relation between sediment and igneous crustal thickness Mantle P velocities Data versus final model: example transects and first arrival travel times Establishment of a “Ground Truth” database input velocity model node-point 3D model Introduction The observational basis for our velocity model are velocity-depth functions from previous seismic studies in the region (red triangles). The 50x50 km grid tiles for our model are defined in an optimum way such that the tiles form a fully equidistant grid. The final model will consist of seven crustal layers (ice, water, soft sediments, hard sediments, and upper, middle, and lower igneous crust). The upper mantle is described by continous velocity-depth curves, courtesy of A. Levshin (see Shapiro & Ritzwoller, 2002). The inset map shows defined geological provinces used to construct regional mean velocity models. The provinces are chosen on the basis of distinct tectono-sedimentary history, and within each province a representative relation between sediment-thickness and the thickness of the igneous crust is given. Sediment thickness data are taken mainly from the Magnetic Mapping of the Barents Sea (MMBS) project (Myklebust, 1994), based on potential field, seismic reflection/ refraction, and borehole data (left). Gridding of these and some additional data provides a depth-to-basement map for the target region (right). The sedimentary layers reach more than 20 km thickness in some basins, e.g. in the eastern Barents Sea. Within individual (non-oceanic) geological provinces we use the sediment thickness to derive the total crustal thickness, where model tiles are not constrained by velocity-depth functions. The method is outlined in the following section. Our main objective is to develop a 3D seismic velocity model of the crust and upper mantle in the study region. To compile our 3D model with a nominal resolution of 50x50 km, we use results from previous, crustal-scale 2D seismic wide-angle and normal incidence profiles. Passive seismological recordings, previous receiver function studies, and potential field data provide additional constraints for our model. The upper mantle of our model is based on published regional models. In order to verify our initial model, we model observed arrival times of several seismic phases with known origins. The final model aims to improve seismic monitoring in this region, which includes improved event locations and event size estimation, and a better understanding of regional seismic wave phases. The model will also serve as a basis for estimating and calibrating travel times of regional phases in this region. The defined geological provinces bear a similar geological history, hence a similar basin architecture can be assumed to be present throughout the entire province. It is reasonable to assume that this is due to a common setting in a broader tectonic regime and a common physical behavior of the deeper igneous crust and its isostatic response. An extensional regime would therefore result in increasing sediment thickness with decreasing igneous crustal thickness, while strong compression might lead to crustal thickening and subsequent erosion of sedimentary strata. This assumption gains substantial support from a strong correlation between the sediment thickness and the remaining igneous crustal thickness. Three examples of this are shown in the figure on the right (a), where data are taken from the sampled velocity functions that appear as inverted triangles in the map (b). On this basis, area-wide sediment thickness data (see above), can be used to calculate the igneous crustal thickness for those provinces that hold sufficient sampled profiles to manifest a relationship. Model tiles that are constrained by sampled profiles are equipped with the measured depth-to-basement and depth-to-Moho, whereas unconstrained tiles are assigned a sediment thickness taken from areawide data and an igneous crustal thickness inferred from manifested relationships. Mean seismic velocities for crustal layers are calculated from the sampled velocity-depth functions and are used for the entire province. Compared to simple linear interand extrapolation, this technique provides a geologically more reasonable subsurface structure. The Moho depth map (right) illustrates the derived crustal model. Moho depths of more than 50 km occur below northern Fennoscandia and Novaya Zemlya. Regions with oceanic crust in the western part of the model region are characterised by Moho depths of around 10 km. Crustal thicknesses in the central Barents Sea are around 35 km. The parameterization of all crustal layers (depth-to-top of layer, layer thickness, and mean velocity) have been visualised by similar maps (see Bungum et al., 2004). Transects and upper mantle velocity maps are shown below. We constructed four example transect through our velocity model for the crust and upper mantle (see map). The panels on the right show (from top to bottom): Initial 1D velocity-depth functions (data; red triangles in map), 1D velocity-depth functions from our model (hexagons in map), first arrival travel time curves, and at the bottom the gridded velocity transects together with wavefronts and ray paths for first arrivals. Note that the colour scale is devided into a section for crustal velocities up to 7.8 km/ s and another section for mantle velocities between 7.8 and 9 km/s. Wave fronts and travel times for each transect are calculated using a finite-difference (FD) scheme after Podvin & Lecomte (1991) with a grid node spacing of 0.5 km in both directions. Since the FD scheme assumes a Cartesian grid, a transformation has been applied to account for the spherical Earth. Wave fronts are shown every 5 s (dashed) and 10 s (solid). Travel time curves for our model are shown as blue lines, and curves for two 1D velocity models are shown for comparison: the regional model BAREY (Schweitzer & Kennett, 2002; green dashed) and the global model IASP91 (Kennett & Engdahl, 1991). Mantle P-wave velocities at four depths, based on the model of Shapiro & Ritzwoller (2002). This model is used initially in conjunction with the crustal and Pn velocity model established in this project. Courtesy of A. Levshin, Univ. of Colorado, Boulder. A database of around 50 potential “Ground Truth” (GT) events has been compiled for this project. The events are all relocated based on the available travel time data. The map shows travel paths for these GT events (Pg black, Pn gray, and longer-distance P gray dashed). Seismic stations are indicated by blue triangles while the events are plotted as open circles, color coded based on formal location uncertainties. For all of these travel paths, seismic data are archived and available at NORSAR, constituting an excellent basis for subsequent model validation and refinement. For a given source-receiver distance travel times of first arrival P phases (blue dots) show a variation of a few seconds, reflecting the lateral velocity variation in the model region. Travel times for BAREY (green) and IASP91 (red) are given for comparison. For long distances, travel times from IASP91 are generally too high in our model region. References and further reading H. Bungum, O. Ritzmann, J.-I. Faleide, N. Maercklin, J. Schweitzer, W. Mooney, S. Detweiler & W. S. Leith (2004). Development of a Three-dimensional Velocity Model for the Crust and Upper Mantle in the Barents Sea, Novaya Zemlya, Kara Sea and Kola-Karelia Regions. 26th Seismic Research Review, Orlando, Florida, paper 204. https://www.nemre.nnsa.doe.gov/prod/srr/2004/PAPERS/02-04.PDF B. L. N. Kennett & E. R. Engdahl (1991). Travel times for global earthquake location and phase identification. Geophysical Journal International,105, 429-465. R. Myklebust (1994), Magnetic mapping of the Barents Sea (MMBS). Interpretation report, Amarok A.S., Oslo, Norway. P. Podvin & I. Lecomte (1991). Finite difference computation of traveltimes in very contrasted velocity models: a massively parallel approach and its associated tools. Geophysical Journal International,105, 271-284. J. Schweitzer & B. L. N Kennett (2002). Comparison of location procedures - the Kara Sea event 16 August 1997. In: Scientific Report 2-2001/2002, NORSAR, Kjeller, Norway. N. M. Shapiro & M. H. Ritzwoller (2002). Monte-Carlo inversion for a global shear-velocity model of the crust and upper mantle. Geophysical Journal International,151, 88-105. Depth to Moho Depth to basement Model DataFirst arrivals Model A B C D East - West N. Norway - SPITS Kursk accident - SPITS Novaya Zemlya - ARCES IASP91 BAREY ESC General Assembly, September 13-17, 2004, Potsdam, Germany. SCD-1, P-187.