Combined wave propagation analysis of earthquake recordings from borehole and building sensors

Abstract:

In regions highly exposed to natural hazards, Early Warning Systems can play a central role in risk management and mitigation procedures. To improve at a relatively low cost the spatial resolution of regional earthquake early warning (EEW) systems, decentralized onsite EEW and building monitoring, a wireless sensing unit, the Self-Organizing Seismic Early Warning Information Network (SOSEWIN) was developed and further improved to include the multi-parameter acquisition. SOSEWINs working in continuous real time mode are currently tested on various sites. In Bishkek and Istanbul, an instrumented building is located close to a borehole equipped with downhole sensors. The joint data analysis of building and borehole earthquake recordings allows the study of the behavior of the building, characteristics of the soil, and soil-structure interactions.

The interferometric approach applied to recordings of the building response is particularly suitable to characterize the wave propagation inside a building, including the propagation velocity of shear waves and attenuation. Applied to borehole sensors, it gives insights into velocity changes in different layers, reflections and mode conversion, and allows the estimation of the quality factor Qs.

We used combined building and borehole data from the two test sites: 1) to estimate the characteristics of wave propagation through the building to the soil and back, and 2) to obtain an empirical insight into soil-structure interactions. The two test sites represent two different building and soil types, and soil structure impedance contrasts. The wave propagation through the soil to the building and back is investigated by the joint interferometric approach. The propagation of up and down-going waves through the building and soil is clearly imaged and the reflection of P and S waves from the earth surface and the top of the building identified. An estimate of the reflected and transmitted energy amounts is given, too.