Relocation of the 5 December 2004 Waldkirch, Germany, earthquake with regional 1D and 3D seismic velocity models in the presence of upper-mantle anisotropy

摘要

The exact hypocenter location, that is, the depth of the 5 December 2004 Waldkirch earthquake in the southern Rhinegraben area, has been at the center of debate for some time. After various relocation efforts by several geophysical institutions, a depth range of 9-12 km was eventually reported for this event. Here we examine whether the Waldkirch event can be located with improved accuracy by a regional network with newly derived regional 1D and 3D seismic velocity models for the crust and upper mantle that also take into account the existing Pn upper-mantle anisotropy beneath Germany. The preliminary arrival-time analysis of the phase data hints at possible misidentifications of several phases (Pg versus Pn), in addition to that of errors due to anisotropy that lead to unrealistic shallow depth locations in the range of 0-5 km. After removal of these adverse effects on the relocation, the more realistic depth range of 10-15 km is obtained. The effects of an anisotropic upper mantle, the influence of the V_P/V_S ratio, and the influence of a varying Moho depth on the event relocation are then investigated. With the 1D model, the Waldkirch depth is located between 14 and 15 km, when P waves only or both P and S waves are used, whereas with the 3D P-wave velocity model, the depth again becomes 14 km. The effect of the Pn anisotropy is stronger for the epicenter than for the depth. The statistical location precision is determined by calculation of covariances and confidence ellipses as well as by Monte Carlo simulations with both randomly perturbed arrival times and initial hypocenters. In addition, the effects of uncertainties in the velocity model on the relocation are investigated. Based on this comprehensive analysis, the optimal Waldkirch event depth is determined to be 14 km, with a 95% confidence interval of ±2.5 km.