The location of El Ejido town over a deep sedimentary basin in a zone of high seismicity in the Spanish context has motivated research on its seismic response characterization. To this aim, S-wave velocity models have been obtained from joint inversion of Rayleigh wave dispersion curves and full-wavefield modelling of the horizontal-to-vertical spectral ratio of ambient noise (HVSR) under the diffuse field assumption (DFA). Combination of spatial autocorrelation surveys (SPAC) with array apertures of several hundred metres and HVSRs displaying low-frequency peaks allowed to characterize deep ground features down to the Triassic bedrock. Predominant periods in the town ranged from 0.8 to 2.3 s, growing towards the SE, with few secondary peaks at higher frequencies.The shallow structure has been explored by means of geotechnical surveys, Multichannel Analysis of Surface Waves (MASW) and SPAC analysis in small-aperture arrays. Resulting models support a general classification of the ground as stiff soil.

South and SouthEast of Spain are the regions with a higher seismic hazard in Spain. Therefore, a regional normative, focused on the importance of developing seismic emergency planning in many of the municipalities of the Valencian community, was established in 2011. consequently, all the municipalities in Alicante province have to develop a seismic emergency planning. however, only Elche and Alicante have completed the seismic risk analysis and they have started to prepare their emergency plans which will be finished before the end of 2019. This paper shows the main results of the seismic risk analysis carried out in both municipalities. The seismic hazard update in the region has shown that the main earthquake scenarios, which may hit both cities, correspond to the crevillente and the Bajo Segura faults (also responsible for damaging historical earthquakes). In both cases, the urban areas are on a sedimentary deposit, which can reach hundreds of meters so site effects and possible site-building resonance can be important. Additionally, most of the building stock belongs to periods without seismic normative, increasing, therefore, their vulnerability and the obtained damage. The mean damage ratio for a magnitude of 5.5 increases from a 0.8% to a 10.3% at Alicante and Elche, respectively. Besides, if the magnitude increases to a 6.5 then the mean damage ratio increases from a 16.6% to a 60.3%. In conclusion, we recommend that the emergency planning developed for both municipalities has to take into consideration that even the occurrence of a probable earthquake (475 yrs return period) corresponding to a mw 5.5 will affect both cities so the procedures and protocols should be written in close cooperation.

Summary The stability of the low-frequency peaks (< 1 Hz) obtained in the passive seismic survey of Campo de Dalías basin (CDB) by applying the horizontal-to-vertical spectral ratio (HVSR) method was investigated. Three temporary seismic stations were installed in remote sites that enabled studying the stationarity of their characteristic microtremor HVSR (MHVSR) shapes. All stations began to operate in mid-2016 and recorded at least one year of continuous seismic ambient noise data, having up to two years in some. Each seismic station counted with a monitored borehole in their vicinity, registering the groundwater level every 30 minutes. The MHVSR curves were calculated for time windows of 150 s and averaged hourly. Four parameters have been defined to characterize the shape of the MHVSR around the main peak and to compare them with several environmental variables. Correlations between MHVSR characteristics and the groundwater level showed to be the most persistent. The robustness of MHVSR method for applications to seismic engineering was not found to be compromised since the observed variations were within the margins of acceptable deviations. Our results widen the possibilities of the MHVSR method from being a reliable predictor for seismic resonance to also being an autonomous monitoring tool, especially sensitive to the S-wave modifications.

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The seasonal soil losses and frequent shallow landslides in the Cerrado region of Brazil have high destructive potential with social, economic, and climatic implications. As fluvial systems substantially drive such environmental threats; therefore, it is essential to conduct geological site characterization and continuously monitor the seasonal erosive potentials of the rivers and streams. However, in such unstable and sensitive conditions, traditional intrusive investigation approaches may not be safe; therefore, the geophysical investigation might offer a good alternative. For the present study, a geophysical approach (particularly the seismic method) was adopted to examine the seismic footprints and GPR site characterization of a seasonal stream in the Rua do Matto, Brasilia, Brazil. The monitoring was conducted (at a safe distance) on the intermittent stream over several durations of dry (no rain) and rainy (flood) conditions. After pre-processing the raw data, the power spectral density (PSDs) was computed as a function of several variables (wind speed), time-frequency spectrograms, ambient noise displacement root mean square (RMS), the single station horizontal-to-vertical spectral ratio (HVSR) curves. In addition, change-point analysis was used for comparing the ambient noise with wind speed (both were well correlated). The GPR amplitude and waveform variation features were attributed to the subsurface material and the presence of boulders in the floodplain as well as regions (low coherence value) susceptible to erosion (weak spots). The river flows were evident on the mean probabilistic PSD values, spectrograms, HVSR curves and different patterns of RMS displacements (at selective ranges of frequency). The multi-peaks emerged on the HVSR curve are further analyzed for changes in amplitude, width and troughs possibly related to river activities and soil moisture due to rain. The approach provides the basis for non-destructive monitoring tools enabling the detection of 'seismic signatures' and weak spots of the fluvial channels for improving their environmental management.

Identifying ambient noise-based (ANb) signatures of streams can help in the estimation of their erosive potential (EP) that promotes reverie landslides and soil losses in the fluvial valleys. This is particularly imperative on flooding or rainy days, leading to stronger erosion-prone conditions (colluvium and boulders) of the valley beds inferred from georadar attribute analysis. Developing such research direction can benefit the local communities, as is the case with the Cerrado region of Brazil, where these phenomena have high destructive potential with social, economic, and climatic implications. For the present study, a seasonal stream in the Federal District of Brazil was investigated by ANb monitoring supported by Ground Penetration Radar (GPR) for site characterization. The ANb monitoring was conducted (at a safe distance) with a seismometer over several durations of dry and rainy conditions. The power spectral density (PSDs) was computed as a function of several variables, including weather conditions (rainfall, wind speed, and pressure), time-frequency spectrograms, and ambient noise displacement root mean square (RMS). This analysis also considered the single station horizontal-to-vertical spectral ratio (HVSR), where rain, wind, pressure, river flow and anthropogenic signatures were evident (at selective frequency ranges). Multi-peaks that emerged on the HVSR curve were further analyzed to identify amplitude and frequency changes, and the three peaks shift on average to a lower position during the rainy period. The GPR amplitude and waveform variation features were attributed to the stratigraphy (i.e., the boundary between valid and invalid regions and coherence value) of the floodplain and regions susceptible to erosion (erosion-prone lithological spots). This approach provides the basis for non-destructive monitoring tools enabling the detection of 'seismic signatures' and weak spots of the fluvial channels for improving environmental management.

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<p>Preceding the seismic modelling of Campo de Dalías sedimentary basin by single-station ambient noise measurements, research focused on the reliability of the methodology employed, namely the microtremor horizontal-to-vertical spectral ratio (MHVSR), is conducted. It is known that MHVSR may present some dependence on weather and site-specific conditions as topographic effects, anthropogenic activities or the variability in the microtremor source distribution. In this context, the stationarity of MHVSR curves with their fundamental peaks below 1 Hz is studied after the installation of three long-term stations in rural sites and another one for a week in the urban area of El Ejido town.</p><p>The robustness of the MHVSR methodology is often assured by looking into the stationarity of mainly two peak parameters: frequency and amplitude. In this study, two new parameters are tested: the peak-width and the trough frequency. We have up to two years of microtremor and weather data that helped to track the variability of wind speed, atmospheric pressure, and temperature, as well as sea tide and aquifer levels compared to the peak shape of MHVSR curves.</p><p>Most weather variables only show short or punctual correlations with MHVSR parameters, which is the case of wind gusts above 10 m/s that totally blurred the MHVSR peak-shape for periods of a few days in the more poorly isolated station. The wind-speed time series collected in Campo de Dalías show high correlations with the total microtremor energy in the frequency band of secondary microseisms (0.3 - 1 Hz) with a clear seasonal behaviour. However, the MHVSR peaks studied in that band are uncorrelated with them. Our results show that the piezometric level maintains a moderate to high correlation with MHVSR peak-variability during a time span of 9 months. Campo de Dalías hosts a system of karst aquifers, which constitutes the main water supply for this semi-arid region. Modelling the groundwater flow in that kind of aquifers is a challenging field of research and monitoring it by means of investigation wells has a high cost. The results observed in this study widen the possibilities of MHVSR for being an aquifer-monitoring tool on time scales as short as a few days.</p>