Rotaphone, a Self-Calibrated Six-Degree-of-Freedom Seismic Sensor and Its Strong-Motion Records

Abstract

Rotational seismology is attracting attention in the seismological community including seismic engineers interested in possible damages to man‐made constructions due to strong rotational ground motions. Mathematically, seismic rotation represents the curl of the seismic displacement u , ![Graphic][1] , with Cartesian components: ![Graphic][2] (1) Seismic rotation, due to S waves, is nonzero even in a homogeneous isotropic medium. The inhomogeneity and anisotropy of seismic structures generate more complex wavefield patterns and hence more significant seismic rotations. Moreover, a complex seismic source can contribute considerably to the rotation field, namely in a near‐source zone. In this paper we present a new six‐degree‐of‐freedom (6DOF) seismic sensor called Rotaphone and its strong‐motion records both from an anthropogenic source (blast) and from a tectonic event. By strong rotational motion we mean rotation rate of the order 10−4 rad/s and higher (according to a common convention accepted in the seismological community; see, e.g., Graizer, 2009). The sensor is portable, and easy to install and operate. It has a wide dynamic range of 120 dB; it is able to detect weak ground motions (from the order of 10−7 in SI units) as well as stronger motions (up to the order of 10−1 in SI units). The sensor seems to meet requirements of operative deployment for the needs of strong rotational ground‐motion studies. Rotaphone is a mechanical sensor system for recording ground velocity and determining the rotation rate by measuring the spatial velocity gradients at a point. It is based on differencing records from proximal geophones attached to a rigid frame. The first prototypes based on this principle were presented by Brokesova and Malek (2010) and Brokesova, Malek, and Kolinský (2012). Those instruments measured only the vertical‐axis rotation (torsion) and two perpendicular horizontal velocity components. The 6DOF instrument presented here represents an enhanced design of those … [1]: /embed/inline-graphic-1.gif [2]: /embed/inline-graphic-2.gif