Rotational Motions Extracted From Delaney Park Downhole Array in Anchorage, Alaska

Date: 4/25/2019

Time: 05:00 PM

Room: Pine

Strong-ground motion data recorded at the Delaney Park Geotechnical Array during a number of earthquakes including the 2018 M7.0 Anchorage Alaska event were used to extract rotational motions at different depths and different levels of shaking. The downhole array is located in downtown Anchorage, about a city block from the Atwood building, also instrumented. The ground motions are measured at the surface and at six levels below the surface (up to 61 m depth) using three-component borehole accelerometers that record motions in the glacial outwash sediments near the surface, the sensitive clays of the bootlegger cove formation and at its deepest into the glacial till. Only few direct reliable measurements of rotational component of strong-ground motions from earthquakes are obtained so far. In the meantime, high quality earthquake data recorded at downhole arrays provide an opportunity to estimate deformations based on the differences in recordings at various depths. Deformation or simple shear strain with the rate γ is the combination of pure shear strain with the rate γ/2 and rotation with the rate α=γ/2. High level rotational motions, especially tilting may have adverse effects on tall buildings and bridges. High dynamic range, well synchronized and properly oriented instrumentation is necessary for reliable calculation of rotations from downhole array data. So far, the largest rotation of 0.60E-03 rad was observed at the Eureka downhole array corresponding to ground velocity of 35 cm/s during the 2010 M6.5 Ferndale earthquake in Northern California. The largest rotation rate of 0.55E-02 rad/s associated with the S-wave was observed at an epicentral distance of 4.3 km during the M_L 4.2 event in Southern California at the La Cienega downhole array (Graizer, 2017).

Presenting Author: Vladimir Graizer

Authors