Fanis Moschas

Dynamic Deflections of a Stiff Footbridge Using 100-Hz GNSS and Accelerometer Data

AbstractThe authors investigated the possibility of expanding the application of global navigation satellite systems (GNSS) into the monitoring of stiff civil-engineering structures from the current limit of dominant frequencies of 3–4 Hz to a new limit of 6–7 Hz. On the basis of experience from previous supervised learning experiments, the authors analyzed 100-Hz GNSS data collected with an optimal phase-locked loop and collocated accelerometer data, both describing the attenuation of forced excitations of a timber pedestrian bridge. Computed vertical apparent deflections were masked by noise, but it was still possible to accurately identify the natural frequency of the bridge, equal to 6.5 Hz, and weak fusion with accelerometer data led to accurate oscillation waveforms. The quality of the latter was assessed by structural constraints and by comparison of waveforms of acceleration and of damping ratios derived from GNSS and from accelerometers. These results made it possible to (1) assess the quality of...

Time-space modeling of the dynamics of Santorini volcano (Greece) during the 2011–2012 unrest

The 2011–2012 unrest of Santorini (Thera) volcano (Aegean Sea, Greece) was associated with microseismicity confined to the Kameni Line (KL), a major tectonovolcanic lineament, and has been regarded as a single magmatic episode, produced by a spherical source derived from inversion of GPS data. However, such a source is a few kilometers away from the KL and cannot explain observed microseismicity. For this reason, we divided the unrest episode into five periods based on the fluctuations of seismicity and deformation rates and investigated the connection between seismicity and two spherical magmatic point sources for each period. Based on a new inversion algorithm and consistent GPS data, we recognized during the volcano unrest episode an unstable pattern of intrusions correlating with both the KL and Columbo Line (CL), a second major tectonovolcanic lineament. Intrusions correlating with CL appear relatively persistent, aseismic, small, and shallow, which is consistent with marine geophysical evidence for arrested shallow dykes and geodetic evidence from a previous inflation episode. During the two periods of intense seismicity, sources close to the KL, explaining seismicity, were obtained. This unstable pattern of intrusions explains both the well-observed location and timing of seismicity as well as ground deformation and is consistent with results of an Okada-type inversion for a sill and a dyke. The stress interactions between the two sources agree with Coulomb failure stress models. Santorini appears to be affected by concurrent offset magma pulses, and only recent activity from a magma pulse below the KL produced microseismic swarms.

Statistical Estimation of Changes in the Dominant Frequencies of Structures in Long Noisy Series of Monitoring Data

Damage in structures is reflected in permanent changes of their natural frequencies and theoretically can be derived through measurements. Still, measurement-derived frequencies of structures usually reflect a superimposition of various effects, fluctuations due to environmental and loading conditions, noise, and possible permanent changes (damage or repair). The amplitude of the latter is usually of the same order of magnitude with the other effects; hence permanent shifts are masked by noise and cannot be identified, especially in long monitoring records. In order to overcome this problem, essential for the assessment of the structural health of various key structures, we adopt a statistical approach developed for the identification of shifts (inhomogeneities) in normally distributed climatological data, in particular the SNHT test. The efficiency of the SNHT was first tested on synthetic data and then on sets of estimates of dominant frequencies of a decaying pedestrian bridge. It was found that under certain conditions the SNHT can identify the location of shifts in dominant frequencies of structures; the amplitude of the shifts can then be easily computed. Since the efficiency of the test increases with the length of the time series, this test seems especially suitable for the analysis of long monitoring records.