Tomographic imaging of deep-velocity structure beneath the Eastern and Southern Carpathians, Romania: Implications for continental collision

The Vrancea seismic zone, located beneath the "elbow" of the Eastern and Southern Carpathians, is characterized by intermediate-depth seismicity associated with subduction and plate collision. Among many strong earthquakes that have occurred in the region, the 1977 Vrancea earthquake (March 4, 1977, M-s = 7.1, depth of 86 km) caused extensive damage and fatalities. The Vrancea seismic zone is defined by a tabular region of seismicity that extends from the surface to a depth of 200 km. The small size (about 80 km in length and 40 km in width) and geometry of the seismic zone have made it difficult to interpret the kinematics of subduction and continental collision in the Romanian Carpathians. To better understand the collision/subduction environment, body wave seismic tomography was used to image the deep structure beneath the Eastern and Southern Carpathians in Romania. A total of 7837 P and 2292 S arrival times from 433 shallow and intermediate-depth earthquakes recorded by local and regional seismic stations were inverted for the three-dimensional (3-D) seismic velocity structure. The travel times and ray paths were calculated by an efficient 3-D ray-tracing technique. By utilizing the LSQR algorithm P and S wave velocity perturbations were determined from the surface down to a depth of 200 km. The high-resolution P wave tomographic images reveal a pattern of broad heterogeneity in the velocity structure of the region. One of the most important features is a high-velocity body existing beneath the Carpathian are at depths between 100 and 170 km. This high-velocity body outlines a slab dipping near vertically where most of the intermediate-depth earthquakes occur. Beneath the Transylvanian Basin, a high-velocity zone is dipping toward the SE and may represent the base of lithosphere, which is in convergence with the subducted slab. Low-velocity bodies are found along the forearc of the Carpathian foredeep down to depths of 70 to 80 km. These low-velocity bodies may be correlated with the shallow sedimentary layer in the foreland and may represent crustal materials wedged and underthrust along with the subducted lithosphere.