This Grant for Rapid Response Research (RAPID) investigates low-cost, strong-motion instrumentation for the earth sciences and structural engineering research communities to provide a new interdisciplinary data type: real-time, full spectrum displacements based on an optimal combination of high-rate global positioning system (GPS) and very high-rate accelerometer data. These new data both fully capture dynamic and static near-field strong motion and enable its analysis and characterization in real time. The ability to obtain full spectrum waveforms in three dimensions with millimeter precision is also a breakthrough for rapidly and fully estimating the response of large engineered structures (for example, bridges, buildings, and dams) at the full range of periods. This approach improves the timeliness of earthquake characterization by an order of magnitude and improves its fullness by spanning the full spectrum of ground deformation from the very high frequencies all the way through to direct current (DC) offsets. The project will gather perishable structural response data from five prototype sensor packages placed on the five-story building test specimen, constructed under NSF award CMMI 0936505, that will undergo strong seismic motion on the NEES outdoor shake table at the University of California, San Diego in early 2012.
The sensors enable a new technological paradigm for studying the processes of large earthquakes and the hazards they pose by taking fuller advantage of seismic and geodetic instrumentation through integration, thereby providing a new data observation type. The sensor devices have applications in the fields of seismology, tsunamis, and structural engineering, where early detection, warning or damage assessment is needed. Information gathered and processed with the device could be used by authorities and first responders in the very immediate aftermath of a strong earthquake. This project also promotes collaborative and interdisciplinary research, education of technologically sophisticated graduate and undergraduate students, strengthening of diversity in support of workforce development, and enhancement of curricula through engagement with integrated data sets that have direct societal impact.
The last few years have seen significant natural disasters worldwide and severe loss of life and property due to earthquakes, tsunamis, volcanic activity, landslides, severe storms, and flooding. The latest example is the April 4, 2011 magnitude 9.0 Tohoku-oki earthquake and ensuing tsunami and nuclear meltdown in Japan, whose devastating humanitarian and socio-economic effects still ripple throughout Japan and the world. The Western U.S. is thought to be primed for a series of destructive earthquakes on the southern section of the San Andreas fault system in southern California, the Cascadia subduction zone in the Pacific Northwest including Northern California, and the northern Hayward fault in the San Francisco Bay Area. Nevertheless, there is still not an effective earthquake early warning system in place in the Western U.S., mainly because seismic instruments, although abundant in the Western U.S., are limited in their ability to observe large earthquakes at close range. We at UC San Diego’s Scripps Institution of Oceanography have developed technology that fills this need by leveraging the more than 500 geodetic-quality Global Positioning System (GPS) real-time monitoring stations in the Western U.S. (Figure 1). By upgrading these stations with low-cost sensors of our design, we can accurately measure ground motions in real time and better forecast, assess, and mitigate natural hazards, including earthquakes, tsunamis, and extreme storms and flooding for use by scientists, decision makers, first responders, and the public. Our technology, which is based on an integration of traditional seismic instruments (accelerometers) and GPS, can also be used to monitor, assess damage, and tag large engineered structures such as dams, tall buildings and bridges during significant seismic events and over the long-term to monitor structural degradation. To these ends we participated in April-May 2012 in a five-story shake table experiment (Figure 2) at the NEES Large High Performance Outdoor Shake Table (LHPOST) at UCSD (http://nees.ucsd.edu/). A shake table is a platform on which engineering structures are placed and then subjected to ground motions (accelerations) recorded during large earthquakes in the past. In this project, the recordings were from 4 large earthquakes (1994 Northridge, 2002 Denali, Alaska, 2007 Peru and 2010 Chile). The experiments were carried out in two stages. In the first the building rested on a rubber base that isolated the building from the shake table platform (base-isolated condition); the building was then raised, the isolation was removed and the building lowered to sit directly on the shake table platform (fixed-base condition). We collected data from GPS receivers deployed on the building (three on the roof and two near its foundation) at a sample rate of 20 times a second (Figure 3). These data were combined with co-located accelerometer instruments to record the motions of the building during shaking. Combining the GPS and accelerometer data allowed us to measure permanent deformation at the roof of the building during the last two events (this is not possible with seismic instruments only), as well as the motions of the building for all experiments at a rate of 200 samples per second. We achieved excellent results overall. The final results were distributed to structural engineers for further analysis. The results of our analysis will be of direct use to engineering seismologists who design buildings to withstand large earthquakes, and to geophysicists who develop earthquake early warning systems. Both of these avenues should help reduce casualties and property damage during large earthquakes such as the great earthquake in 2010 in Japan.
