Active Tectonics of Northeast Asia: Using GPS Velocities and Block Modeling to Test Okhotsk Plate Motion Independent from North America

Edwin (Trey) Apel and Roland Bürgmann

Introduction

Horizontal surface velocities of 96 GPS sites (41 from the Eurasian, North American and Pacific plate and 55 from NE Asia) constrain the plate kinematics of NE Asia and allow for a rigorous test of the possibility of Okhotsk plate motion independent of the North American plate. A block modeling approach is used to incorporate both rigid block rotation and near-boundary elastic strain accumulation effects in a formal inversion of the GPS velocities. Considered models include scenarios with and without independent Okhotsk plate motion and a number of different plate boundary locations and locking depths. We are also considering the possible influence of an independent Amurian plate that may also affect the determination of Okhotsk plate existence and motion.

Background

The current plate kinematics of Northeast Asia are somewhat enigmatic due to subduction dominated deformation in the east and little to no differential plate motion in the west which results in diffuse and sparse seismicity. An Okhotsk plate that rotates independently of North America is not a particularly new idea (Seno et al., 1996). However, it is important for defining the plate boundary geometry and constraining the relative motion of the major and minor plate in Northeast Asia and provides a rigorous framework for interpreting seismicity and the surface deformation observed by geodesy. Because the increasingly dense GPS networks in this region are in such close proximity to plate boundaries a sophisticated plate motion model that includes both rigid block rotations and elastic plate boundary strain effects is required to discriminate independent Okhotsk plate motion.

GPS Velocities

The GPS velocities used in our inversion are an updated subset of the 151 global stations processed for the final solution by Steblov et al. (2003). The data were processed using GAMIT/GLOBK by Bob King at MIT and Misha Kogan at Columbia. Processing details can be found in Steblov et al. (2003). Before our inversion the GPS sites were weighted according to site stability, distance from plate boundaries, and a simple declustering routine.

Blocks

In order to test for independent Okhotsk plate motion we tested two block configurations. In our 3-plate system we assumed that the Okhotsk region was part of the North American plate (Figure 20.1). Our 4-plate model (Figure 20.2) allows the Okhotsk plate to rotate independently. We then compared the misfit of the inversion to the data to test for significance. The segments that bound the blocks represent uniformly slipping elastic dislocations locked to some specified depth. Because our inversion combined rigid block rotation with elastic strain accumulation effects, the parameterization of the block boundary geometry is critical. Geometry of the plate boundaries was based heavily on seismicity. However, topography, geodetic gradients, and other modeling (such as single distributed slip inversions) were used to constrain the location and strike and dip of some of our elastic dislocations. Diffuse boundaries surrounding the Okhotsk region like the northern and western edges are not manifested as discrete structures in any geological or geophysical data set. These plate boundary deformation zones are represented in our model by vertical dislocations locked to optimal depths of approximately 75 km that are allowed both strike-slip and tensile motions. These deeply locked elastic dislocations generate diffuse surface deformation consistent with observed patterns of surface deformation. The kinematics of subduction zones are represented by dipping dislocations, locked to 40 km depth and allowed to accommodate both strike slip and dip slip motion.

Inversion

The GPS data and pre-defined block boundary geometry were used as inputs in our block modeling inversion code (Meade and Hager, 2004). We used all of these data (96 sites) to invert for predicted horizontal surface velocities at each GPS station while simultaneously considering rigid block rotation and elastic strain effects for both block configurations. Our 3-plate model shows a clear, systematic pattern of residual velocities that suggests independent Okhotsk plate motion (Figure 20.1). Euler vectors, calculated from our optimized 4-plate inversion, suggest the Okhotsk plate rotates 0.206 deg/Myr clockwise, with respect to North America, about a pole located north of Sakhalin (Figure 20.2 inset).

Discussion and Conclusions

Our inversion favors a scenario with independent Okhotsk plate motion but does not require it, based on the application of F-test statistics, which indicate that the improved fit is significant only at 87% confidence. The plate-motion parameters of the Okhotsk plate are consistent with right-lateral motion in northern Sakhalin and contraction in southern Sakhalin, inferred from focal mechanism solutions. However, subtle changes in block and segment geometry can cause significant changes in the estimated pole of rotation of the Okhotsk plate. This is due, in large part, to the close proximity of most GPS stations in northeast Asia to plate boundaries, such as the Kamchatka-Kurile subduction zone and the Sakhalin Island contraction and strike-slip shear zone. GPS velocities on the Kamchatka peninsula capture a complex pattern associated with the locked subduction zone. This locked subduction zone may require a more complex model than a simple elastic dislocation for the rotational signal to be resolvable. Additional blocks may also affect the determination of an independently rotating Okhotsk plate. Our continued work includes formally examining the potential role of adjacent blocks such as the Amurian, northern Hokkaido, and Magadan blocks.

Figure 20.1: Observed, predicted,and residual velocities for the 3-plate model. The inset map shows the position of the Eurasian pole of rotation (2-sigma error ellipses) with respect to North America and is labeled in degrees per million years.
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Figure 20.2: Observed, predicted,and residual velocities for the 4-plate model. The inset map shows the position of the Eurasian and Okhotsk poles of rotation (2-sigma error ellipses) with respect to North America and are labeled in degrees per million years.
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References

Meade, B.J. and B.H. Hager, Block Models of Crustal Motion in Southern California Constrained by GPS Measurements, J. Geophys. Res. (IN PRESS), 2004.

Seno, T., T. Sakurai, and S. Stein, Can the Okhotsk plate be discriminated from the North American plate?, J. Geophys. Res., 101, 11305-11315, 1996.

Steblov, G.M., M.G. Kogan, R.W. King, C.H. Scholz, R. Bürgmann, and D.I. Frolov, Imprint of the North American plate in Siberia revealed by GPS, Geo. Res. Let., 30(18), 2003.

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