Modern Digital Seismology: Instrumentation, and Small Amplitude Studies in the Engineering World

Abstract

The recording of ground motions is a fundamental part of both seismology and earthquake engineering. The current state-of-the-art 24-bit continuously recording seismic station is described, with particular attention to the frequency range and dynamic range of the seismic sensors typically installed. An alternative method of recording the strong-motions would be to deploy a velocity sensor rather than an accelerometer. This instrument has the required ability to measure the strongest earth motions, with enhanced long period sensitivity. An existing strong motion velocity sensor from Japan was tested for potential use in US seismic networks. It was found to be incapable of recording strong motions typically observed in the near field of even moderate earthquakes. The instrument was widely deployed near the M8.3 Sept 2003 Tokachi-Oki earthquake. The dataset corroborated our laboratory observations of low velocity saturations. The dataset also served to show all inertial sensors are equally sensitive to tilting, which is widespread in large earthquakes. High-rate GPS data is also recorded during the event. Co-locating high-rate GPS with strong motion sensors is suggested to be currently the optimal method by which the complete and unambiguous deformation field at a station can be recorded. A new application of the modern seismic station is to locate them inside structures. A test station on the 9th floor of Millikan Library is analysed. The continuous data-stream facilitates analysis of the building response to ambient weather, forced vibration tests, and small earthquakes that have occurred during its lifetime. The structure's natural frequencies are shown to be sensitive not only to earthquake excitation, but rainfall, temperature and wind. This has important implications on structural health monitoring, which assumes the natural frequencies of a structure do not vary significantly unless there is structural damage. Moderate to small earthquakes are now regularly recorded by dense, high dynamic range networks. This enhanced recording of the earthquake and its aftershock sequences makes possible the development of a Green's Function deconvolution approach for determining rupture parameters.