Utilizing urban noise to monitor the changing site response of the Mexico City basin

Abstract:

Ciudad de Mexico (CDMX) is exposed to strong seismic hazard due to its particular site conditions. A large area of the city is built on quaternary lake sediments which amplify and prolong seismic shaking. In addition, the fundamental period of basin resonance, an important hazard factor, has been observed to have noticeably shifted in response to groundwater extraction during the past decades. Therefore, we turn to CDMX as a natural laboratory to study the interplay of environmental processes, human activity and seismic site response by applying correlation-based monitoring methods.

We utilize data from temporary and permanent broadband seismic stations, as well as a continuously recording accelerometer, to create a long-term baseline of observed changes in seismic properties. To measure velocity changes, we rely on urban noise correlations. Due to the variability and inhomogeneous spatial distribution of the urban seismic noise sources, we extract repetitive signals using a clustering technique, and base the measurement on the correlation of selected signal windows. Measurements allows us to observe both seasonal and long-term trends in ground response.

Results from continuous recordings will then be used to evaluate the possibility of using auto-correlations of triggered accelerometer recordings for supplementing the monitoring. Due to their instrumental properties, such observations need to be based on the coda of regional seismic events. We perform a comparison to the results from urban noise in order to investigate whether coda correlations of regional earthquakes permit the monitoring of shallow seismic properties.

Finally, we intend to expand our dataset to explore the potential of new types of instrumentation, such as raspberry shake citizen science stations and low-cost MEMS accelerometers, in monitoring seismic site effects and their changes. Both are increasingly deployed in urban areas and therefore may provide higher spatial coverage in the future than traditional networks can offer.