Study of seismic hazard mitigation at different
time scales: applications and technological
[Tesi di dottorato]
Earthquakes, weak or strong, represent always a psychological and emotional stress for people, but also a strong socio-economic impact for the affected area. The earthquake generation, the propagation of seismic waves, the seismic waves modification due to the propagation media and the interaction between seismic wave and human structures, are the main topics of different research disciplines. In the last years a wide interdisciplinary research program (physics, seismology, mathematics, geology, engineering, etc.) was world wide experienced aiming at improving the earthquake knowledge and mitigating the ground-shaking effects.
The seismic risk, namely the probability of occurrence of losses , can be described as the convolution of three variables:
Seismic Risk = Seismic Hazard * Vulnerability * Exposition
The Seismic Hazard represents the probability for a selected strong motion parameter (Peak Ground Acceleration, Velocity, Spectral Acceleration…) to be exceeded in a given time interval at a target site. The Seismic Hazard term mainly depends on the earthquake characteristics, target site-epicenter distance and geomorphologic conditions. More generally the Seismic Hazard describes the potential for a dangerous, earthquake-related natural phenomena such as ground shaking, fault rupture, or soil liquefaction. These phenomena could result in adverse consequences to society such as the destruction of buildings or the loss of life. From the seismological point of view all new information about the incoming ground shaking or the possible future ground motion amplitude represent a contribution to the “seismic hazard mitigation”. In fact the knowledge of the expected ground acceleration is the first step for a better build design, while the knowledge of the incoming acceleration amplitude, during an earthquake occurrence, can define an alert action rather than others.
The Vulnerability represents the probability that people, infrastructure and activity suffer the consequence of an earthquake. The damages can be direct as the collapse of structures or collateral as the fall of productivity due to power station injuries.
The Exposition represents a qualitative and quantitative evaluation of the elements exposed to seismic hazard also in terms of their geographical distribution.
Seismic risk increases as earthquake-prone regions become more densely populated and urbanized. Although local planning and zoning activities can help to shape regional growth over time, additional development is generally (and understandably) promoted as a means of strengthening of local economies.
The seismic hazard mitigation can be subdivided in two main time intervals:
The “pre-event” period is characterized by different methodology and actions to prevent the casualties and damages produced by an earthquake. The main goal of the pre-event action is the estimation of casualties, structural collapse or damages as consequence of an earthquake occurrence that can affect the area of interest. As it will be reported in the following chapters the seismological results of the pre-event actions like seismic hazard analysis strongly depend on the knowledge about the seismicity characteristics of the target area, seismic wave generation and propagation.
The main pre-event approaches to the seismic hazard mitigation are the Probabilistic Seismic Hazard Analysis (PSHA) (Chapter 1) and the Hybrid Seismic Hazard Analysis (Chapter 2).
A PSHA study for Molise region, Italy, was conducted to refining the seismic hazard characterization for identifying a set of relevant earthquakes for the engineering analysis of structures. The results of this study, in combination with an experimental structural health monitoring system, represent a real innovation for the pre-event action to minimize the casualties and the structural damages. In fact the possibility to tracking at different time scales the structural response and the evolution of damage can provide important information to support rescue operations. The methodology and the results for the PSHA analysis at Molise region are reported in the Chapter 1.
The hybrid approach to seismic hazard analysis represents a combination of the main characteristic of the Deterministic Seismic Hazard Analysis (DSHA) and the probabilistic methodology. The probabilistic/deterministic approach is able to overcome the limitations of PSHA when a single causative fault and an associated maximum (credible) earthquake is considered as the threat for the site of interest and to considered the earthquake return period using the DSHA. This methodology was applied to high densely populated volcanic areas in Campania region, Italy, where the occurrence of a moderate seismic event represents a threat for the inhabitants and for the civil and/or industrial infrastructures. The hybrid approach in these high dangerous zones is considered as the first step for the hazard mitigation. The results of the analysis are reported in Chapter 2.
The “real-time” phase represents the modern seismological challenge. In fact only at the present the technology allows to perform analysis on seismic signal during the earthquake occurrence. However the technological development is not enough advanced to assure to undertake always the right alert actions. The shake maps play a relevant role to reduce the earthquake damages. It was reported by Wald et al. that: “For rapid response, ShakeMap ground motion values are used for emergency response and loss estimation, assessment of damage to the lifeline and utility networks, and for providing information to the general public” . Shake maps are representations of the space distribution of the more representative parameters useful for description of the possible injuries to the infrastructures, which are the Peak Ground Acceleration (PGA) and the Peak Ground Velocity (PGV) and Spectral accelerations at different periods (Sa). The shake maps are a possible reference instrument for the civil protection operating units, for the army, for the Red Cross, etc. in order to assure timely and effective service on the territory. However at the present the shake maps calculated in real-time represent an approximation of the real ground motion parameters space distribution. The lack of reliability of the maps is strictly related to the lack of knowledge about the fault extension and the not well-defined source to target site distance. As discussed in the Chapter 3, a new methodology has been developed to define the dimension of the surface projection of fault plane during the earthquake to calculate a better spatial distribution of acceleration and velocity using the appropriate source to target site distance.
Instruments able to recognize an earthquake, to estimate the incoming ground motion and subsequently produce an early warning signal represent a valid real-time approach to mitigate the seismic hazard. This instruments combines seismological know-out with technological aspect about for instance seismic sensors, data-loggers, computer elaboration, and so on, and usually are defined as Earthquake Early Warning System (EEWS). The EEWS represent a challenge from different point of view because different knowledge is need to perform a signal analysis in real time to assure the large and reliable possible warning time interval. At the present the time interval necessary to perform very fast-automated action such as to stop the elevators, to shut-off gas, to stop the trains, to start-up the generators, is of order of few seconds. In high densely populated or industrialized areas few seconds could be sufficient to minimized the casualties and damages, to maximize the efficiency of rescue operation and the faster return to a normal and safe condition. Chapter 4 reports a preliminary study for “EEWS-BOX” that represents a new approach for a stand-alone earthquake early warning system.
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