Marco Pilz

Combining Seismic Noise Techniques for Landslide Characterization

A strong topographic relief and the presence of weakly consolidated sediments create favorable conditions for the development of landslides around the eastern rim of the Fergana Basin (Central Asia). In summer 2012, a field experiment employing small aperture seismic arrays was carried out on an unstable slope, using ambient vibration recordings. The aim of the study was to constrain the seismic response of a potential future landslide and to map lateral and vertical changes in the shear-wave velocity of the surficial soil layers. Strong variations of horizontal-to-vertical spectral ratios in terms of amplitude and directionality indicated clear differences in local site effects, probably reflecting the stability of different sections of the slope. Results further showed resonant frequencies of both the entire unstable block, as well as for smaller, individual parts. The use of an ad hoc, passive seismic tomography approach based on noise correlograms allowed for the mapping of the shear-wave velocities of the sliding material, even in cases of significant topography relief. Based on the recording of seismic noise only, we clearly identified a low-velocity body of weakly consolidated claystone and limestone material, which can be interpreted as the landslide body, with laterally varying thickness.

Improving the spatial resolution of ground motion variability using earthquake and seismic noise data: the example of Bishkek (Kyrgyzstan)

Site response analysis plays an important role in seismic hazard and risk assessment, and in defining the optimal engineering design for civil structures. However, due to increasing urbanization, target areas are often too vast to be covered by standard approaches, resulting in large uncertainties in the spatial variability of the expected ground motion. Here, we propose a method to improve the spatial resolution of ground motion variability in terms of Standard Spectral Ratios (SSRs), using earthquakes recorded at a few selected sites for a relatively short amount of time, and seismic noise data collected over a denser grid, taking advantage of clustering and correlation analysis. The method is applied to Bishkek, Kyrgyzstan. Using the K-means clustering algorithm, three clusters of site response types have been identified, based on their similarity of SSRs. The cluster’s site responses were adopted for sites where only single station noise measurements were carried out, based on the results of correlation analysis. The spatial variability of the site response correlates well with the main geological features in the area. In particular, variability is noted from south to north, consistent with both the changes in the thickness of the sedimentary cover over the basin and in the Quaternary material outcropping at the surface. This method has therefore the potential to improve the estimation of site effects at the local scale in the future.

k0: The role of Intrinsic and Scattering Attenuation

Abstract Knowledge of the acceleration spectral shape is important for the prediction of ground motion. At high frequencies, the rapid decrease of the spectral amplitude, which controls the peak values, has been modeled by the spectral decay factor k , allowing an estimate of the apparent attenuation and which currently constitutes a basic input parameter for the generation of stochastic ground motion and the calibration of ground‐motion prediction equations. Based on numerical simulations of ground motion, we investigate the role of intrinsic and scattering attenuation in determining the high‐frequency decay of earthquake‐induced ground motion. We show that the attenuation term related to scattering depends nonlinearly on the intrinsic term, meaning that the commonly used explanation for the high‐frequency decay spectrum parameter might not be appropriate when analyzing signal windows of several seconds’ width.

The contribution of scattering to near-surface attenuation

The rapid decrease of the acceleration spectral amplitude at high frequencies has widely been modeled by the spectral decay factor kappa (κ). Usually, the path-corrected component of κ, often called κ0, is believed to be a local and frequency-independent site characteristic, in turn representing attenuation related to waves propagating vertically through the very shallow layers beneath the study site. Despite the known relevance of κ0 in a wide range of seismological applications, most methods for its calculation do not fully consider the influence of the scattering component. To account for the scattering component, we present a summary of statistical observations of the seismic wavefield at sites of the Swiss seismic networks. The intrinsic properties of the wavefield show a clear dependency on the local shallow subsoil conditions with differences in the structural heterogeneity of the shallow subsoil layers producing different scattering regimes. Such deviations from the ballistic behavior (i.e., direct waves that sample only distinct directions) are indicative for local structural heterogeneities and the associated level of scatter. Albeit the attenuation term related to scattering depends nonlinearly on the intrinsic term, the results indicate that the commonly used explanation for the high-frequency decay spectrum might not be appropriate but involving the amount of scattering might allow better constrained estimates of κ0.

The Multi-Parameter Wireless Sensing System (MPwise): Its Description and Application to Earthquake Risk Mitigation

The Multi-Parameter Wireless Sensing (MPwise) system is an innovative instrumental design that allows different sensor types to be combined with relatively high-performance computing and communications components. These units, which incorporate off-the-shelf components, can undertake complex information integration and processing tasks at the individual unit or node level (when used in a network), allowing the establishment of networks that are linked by advanced, robust and rapid communications routing and network topologies. The system (and its predecessors) was originally designed for earthquake risk mitigation, including earthquake early warning (EEW), rapid response actions, structural health monitoring, and site-effect characterization. For EEW, MPwise units are capable of on-site, decentralized, independent analysis of the recorded ground motion and based on this, may issue an appropriate warning, either by the unit itself or transmitted throughout a network by dedicated alarming procedures. The multi-sensor capabilities of the system allow it to be instrumented with standard strong- and weak-motion sensors, broadband sensors, MEMS (namely accelerometers), cameras, temperature and humidity sensors, and GNSS receivers. In this work, the MPwise hardware, software and communications schema are described, as well as an overview of its possible applications. While focusing on earthquake risk mitigation actions, the aim in the future is to expand its capabilities towards a more multi-hazard and risk mitigation role. Overall, MPwise offers considerable flexibility and has great potential in contributing to natural hazard risk mitigation.

Assessing Earthquake Early Warning Using Sparse Networks in Developing Countries: Case Study of the Kyrgyz Republic

The first real-time digital strong-motion network in Central Asia has been installed in the Kyrgyz Republic since 2014. Although this network consists of only 19 strong-motion stations, they are located in near-optimal locations for earthquake early warning and rapid response purposes. In fact, it is expected that this network, which utilizes the GFZ-Sentry software, allowing decentralized event assessment calculations, not only will provide useful strong motion data useful for improving future seismic hazard and risk assessment, but will serve as the backbone for regional and on-site earthquake early warning operations. Based on the location of these stations, and travel-time estimates for P- and S-waves, we have determined potential lead times for several major urban areas in Kyrgyzstan (i.e., Bishkek, Osh, and Karakol) and Kazakhstan (Almaty), where we find the implementation of an efficient earthquake early warning system would provide lead times outside the blind zone ranging from several seconds up to several tens of seconds. This was confirmed by the simulation of the possible shaking (and intensity) that would arise considering a series of scenarios based on historical and expected events, and how they affect the major urban centres. Such lead times would allow the instigation of automatic mitigation procedures, while the system as a whole would support prompt and efficient actions to be undertaken over large areas.

Ground‐Motion Forecasting Using a Reference Station and Complex Site‐Response Functions Accounting for the Shallow Geology

Abstract The distribution of damage due to recent earthquakes has shown that the effects of shallow geological structures on the level of ground shaking represent an important factor in engineering seismology. Whereas many previous studies have estimated site amplification factors in the frequency domain, their application to the real‐time modeling of ground motion is not yet fully established. In this article, a method for the real‐time correction of frequency‐dependent site‐response factors is proposed, which accounts not only for the modulus, but also for the changes in the signal phase related to local site conditions. The transformation of the complex standard spectral ratios to a causal recursive filter in the time domain allows for the forecasting of the waveforms for soft‐soil sites almost in real time when the signal is recorded earlier at a reference site. When considering travel‐time differences of the various seismic phases between the hypocenter and the studied sites, the level of ground motion at soft‐soil sites with respect to arrival time, energy, duration, and frequency content can be well constrained, even in cases of a high spatial variability of the amplification patterns.

MIIC: Monitoring and Imaging Based on Interferometric Concepts

The capability of seismic interferometry to create virtual sources at receiver sites from records of ambient seismic noise is used for seismic monitoring and tomography of different targets. We present hardware developed specifically for the needs of seismic data acquisition in the context of monitoring and ambient noise tomography. Digitizers are capable of continuous recording and real time wireless data transmission in self organizing meshes to allow for robust telemetry in difficult circumstances such as cities or landslides that may cause the loss of stations. A software tool is described that implements required processing and analysis procedures for the interferometric processing. We have applied the novel 3D ambient noise surface wave tomography approach to the Issyk-Ata fault in Kyrgyzstan. It shows that seismic interferometry can successfully be used for structural investigations on length scales of only 100 m. The method uses 3D sensitivity kernels for a single-step inversion of phase velocity dispersion curves for subsurface S-wave velocity structure and incorporates topography. We recover lateral differences in sediment velocities and an offset of the bedrock depth across the fault. Applications of interferometric monitoring to the geological \(\mathrm{CO}_2\) storage test site in Ketzin (Germany) and to the Piton de la Fournaise volcano (La Reunion island) emphasize the value of this approach. At Ketzin site we identify variations of the subsurface velocities that are correlated with changes in the ground water level and mask potential signals from the reservoir depth. At Piton de la Fournaise volcano, seismic velocity changes are linked to volcanic processes as shown by comparison with surface displacement and seismicity that are typically used to characterize volcanic activity. We observe a clear distinction between phases of inflation prior to eruptions and deflation during periods of quiescence.

Earthquake Risks: Hazard Assessment of the City of Santiago de Chile

Santiago de Chile is located at the top of a deep sedimentary basin close to the active tectonic San Ramon Fault. In the case of rupture, this fault at the eastern edge of the city can generate earthquakes with a magnitude of up to 7.1. As seen in past earthquakes, the soil characteristics within the Santiago basin change the level of seismic hazard, since they heavily modify the level of ground shaking over short distances. This chapter takes a closer look at the relationship between these parameters and their influence on local site conditions. The methodology it presents to provide a rough estimate of the seismic hazard combines a high resolution map of the fundamental resonance frequency of the soil and a 3D shear wave velocity model for the northern part of the city. By comparing the results with mapped intensities of recent events, the chapter estimates the areas of the city that are more endangered and recommends a more thorough investigation for these parts of the basin. Although the findings cannot be generalized for all possible earthquakes affecting the city, it concludes with some practical recommendations such as retrofitting the existing building stock in areas particularly under threat of seismic hazard.