The Community Seismic Network and Quake-Catcher Network: Monitoring building response to earthquakes through community instrumentation

The Community Seismic Network (CSN) and Quake-Catcher Network (QCN) are dense networks of low-cost ($50) accelerometers that are deployed by community volunteers in their homes in California. In addition, many accelerometers are installed in public spaces associated with civic services, publicly-operated utilities, university campuses, and high-rise buildings. Both CSN and QCN consist of observation-based structural monitoring which is carried out using records from one to tens of stations in a single building. We have deployed about 150 accelerometers in a number of buildings ranging between five and 23 stories in the Los Angeles region. In addition to a USB-connected device which connects to the host's computer, we have developed a stand-alone sensor-plug-computer device that directly connects to the internet via Ethernet or WiFi. In the case of CSN, the sensors report data to the Google App Engine cloud computing service consisting of data centers geographically distributed across the continent. This robust infrastructure provides parallelism and redundancy during times of disaster that could affect hardware. The QCN sensors, however, are connected to netbooks with continuous data streaming in real-time via the distributed computing Berkeley Open Infrastructure for Network Computing software program to a server at Stanford University. In both networks, continuous and triggered data streams use a STA/LTA scheme to determine the occurrence of significant ground accelerations. Waveform data, as well as derived parameters such as peak ground acceleration, are then sent to the associated archives. Visualization models of the instrumented buildings' dynamic linear response have been constructed using Google SketchUp and MATLAB. When data are available from a limited number of accelerometers installed in high rises, the buildings are represented as simple 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. The approach has been applied to a CSN-instrumented 12-story reinforced concrete building near downtown Los Angeles. The frequencies were identified directly from spectra of the 8 August 2012 M4.5 Yorba Linda, California earthquake acceleration time series. When the basic dimensions and the first two frequencies are input into a prismatic Timoshenko beam model of the building, the model yields mode shapes that have been shown to match well with densely recorded data. For the instrumented 12-story building, comparisons of the predictions of responses on other floors using only the record from the 9th floor with actual data from the other floors shows this method to approximate the true response remarkably well.