New constraints on the intraplate stress field of the Amurian plate deduced from light earthquake focal mechanisms

The Amurian plate plays an important role for the understanding of eastern Asian plate dynamics. Different models of rigid-rotation and relative velocities with respect to adjacent plates have been proposed. As few GPS data and sparse stress observations have been available until today—particularly for the intraplate region—these models have proven hard to confirm or deny. To address this we determine new focal mechanisms for this region and perform a stress inversion for selected parts of the plate in order to provide better constraints on the plate dynamics.Most strong earthquakes occur on active plate margins and for these events focal mechanisms are routinely calculated by moment tensor inversion using teleseismic data on a global scale. Away from the plate margins, earthquake magnitudes are smaller and the efficiency of such automated procedures strongly depends on the number of seismic records available. Using the Frequency Sensitive Moment Tensor Inversion allows us to determine source mechanisms of light events with a smaller number of data. For the Amurian plate interior, we calculate 41 focal mechanisms for crustal earthquakes from 1993 to 2008 (3.9 ≤ MW ≤ 5.4). We combine these data with additional solutions from the literature to perform formal stress inversions for three separate regions. The reliability of the resulting stress orientations is evaluated by bootstrap analysis to show the variability of the principle stress axes.For all regions the resulting stress field shows a dominant strike-slip regime with an ENE–WSW orientation of the maximum horizontal compressional stress SH and is consistent with borehole stress data from the World Stress Map at the margins and in the central part of the Amurian plate. Moreover, our analysis reveals differences in both the stress ratio and the SH orientation within the plate interior. While the northern part of the Amurian plate might be influenced by the Baikal rift in terms of a small anti-clockwise rotation of SH, the eastern part of the plate clearly responds to the far field plate-boundary forces from the subducting Pacific plate.