Thursday, October 3, 2013
Mechanical and Civil Engineering Seminar
The Community Seismic Network and Quake-Catcher Network: monitoring building response to earthquakes through community instrumentation
Monica Kohler, Senior Research Fellow , Mechanical and Civil Engineering, California Institute of Technology
The Community Seismic Network (CSN) and the Quake-Catcher Network (QCN), dense networks of low-cost accelerometers that are deployed by community volunteers in homes, high-rises, schools, civic service buildings, and utilities in California. The CSN and QCN projects are developing advanced computational and data management resources that make it possible to take ground and building monitoring from sparse or non-existent to city block-size sampling scales in southern California. These projects are increasing the number of seismic observations in urban regions from 100s to 10,000s by exploiting recent advances in MEMS sensing technologies and cyberinfrastructure, e.g., archiving and processing data in the Google App Engine cloud computing service. In addition to homes, we have deployed hundreds of accelerometers in a number of buildings ranging between five and 23 stories in the Los Angeles region. Key goals are to record earthquake ground shaking at high density in buildings and to implement new tools that compute and visualize the vibrational properties of these buildings through simple but realistic physical models. We represent the buildings, wherever possible, as shear beam or prismatic Timoshenko beam models with soil-structure interaction. Small-magnitude earthquake records are used to identify the first two pairs of horizontal vibrational frequencies which are then used to compute the response on every floor of the building, constrained by the observed data. When the basic dimensions and the first two frequencies of instrumented buildings are input into the shear or Timoshenko beam model of the building, the model yields mode shapes that match well with the densely recorded data. Predictions of responses on other floors using only the calculated mode shapes and a limited duration traveling wave coupled with the low-frequency modal response are then used to approximate the true response. Application to several instrumented buildings and verification with a finite-element model of one of the buildings show that the method approximates the true response well. It is anticipated that expansions of these networks will benefit building model validation and calibration associated with regional response to scenario and actual excitation. In the long term, advances in sensor/network development could lead to imaging the full wavefield and monitoring significant changes in the properties of the coupled ground-civil structure systems over a city-wide area.