Giuliano Panza

Why are the Standard Probabilistic Methods of Estimating Seismic Hazard and Risks Too Often Wrong

Abstract According to the probabilistic seismic hazard analysis (PSHA) approach, the deterministically evaluated or historically defined largest credible earthquakes (often referred to as Maximum Credible Earthquakes, MCEs) are “an unconvincing possibility” and are treated as “likely impossibilities” within individual seismic zones. However, globally over the last decade such events keep occurring where PSHA predicted seismic hazard to be low. Systematic comparison of the observed ground shaking with the expected one reported by the Global Seismic Hazard Assessment Program (GSHAP) maps discloses gross underestimation worldwide. Several inconsistencies with available observation are found also for national scale PSHA maps (including Italy), developed using updated data sets. As a result, the expected numbers of fatalities in recent disastrous earthquakes have been underestimated by these maps by approximately two to three orders of magnitude. The total death toll in 2000–2011 (which exceeds 700,000 people, including tsunami victims) calls for a critical reappraisal of GSHAP results, as well as of the underlying methods. In this chapter, we discuss the limits in the formulation and use of PSHA, addressing some theoretical and practical issues of seismic hazard assessment, which range from the overly simplified assumption that one could reduce the tensor problem of seismic-wave generation and propagation into a scalar problem (as implied by ground motion prediction equations), to the insufficient size and quality of earthquake catalogs for a reliable probability modeling at the local scale. Specific case studies are discussed, which may help to better understand the practical relevance of the mentioned issues. The aim is to present a critical overview of different approaches, analyses, and observations in order to provide the readers with some general considerations and constructive ideas toward improved seismic hazard and effective risk assessment. Specifically, we show that seismic hazard analysis based on credible scenarios for real earthquakes, defined as neo-deterministic seismic hazard analysis, provides a robust alternative approach for seismic hazard and risk assessment. Therefore, it should be extensively tested as a suitable method for formulating scientifically sound and realistic public policy and building code practices.

Improving earthquake hazard assessments in Italy: An alternative to “Texas sharpshooting”

The 20 May 2012 M = 6.1 earthquake that struck the Emilia region of northern Italy illustrates a common problem afflicting earthquake hazard assessment. It occurred in an area classified as “low seismic hazard” based on the current national seismic hazard map (Gruppo di Lavoro, Redazione della mappa di pericolosita sismica, rapporto conclusivo, 2004, http://zonesismiche.mi.ingv.it/mappa_ps_apr04/italia.html) adopted in 2006. That revision of the seismic code was motivated by the 2002 M = 5.7 earthquake that struck S. Giuliano di Puglia in central Italy, also a previously classified low-hazard area, resulting in damage and casualties. Previous code was updated in 1981–1984 after earlier maps missed the 1980 M = 6.5 Irpinia earthquake.

Broadband NDSHA computations and earthquake ground motion observations for the Italian territory

The aim of this work is two-fold: 1) to compare the results of national scale NDSHA modelling for the Italian region at 10 Hz cut-off, based on the relevant available knowledge, with observations (e.g., peak ground motion values) and existing empirical attenuation relations; 2) to update the scaling law for source spectra (SLSS) to be used for the selected area. The new set of source spectra, defined along the lines suggested by the comparison with empirical attenuation relations, produces acceptable results in terms of PGV and spectral acceleration at long periods. Synthetic PGA and SA at short periods show a faster attenuation with respect to the observed ones and, therefore, the effect of complex attenuation factors should be explored in future ad hoc studies.

Neo-deterministic seismic hazard scenarios for India???a preventive tool for disaster mitigation

Current computational resources and physical knowledge of the seismic wave generation and propagation processes allow for reliable numerical and analytical models of waveform generation and propagation. From the simulation of ground motion, it is easy to extract the desired earthquake hazard parameters. Accordingly, a scenario-based approach to seismic hazard assessment has been developed, namely the neo-deterministic seismic hazard assessment (NDSHA), which allows for a wide range of possible seismic sources to be used in the definition of reliable scenarios by means of realistic waveforms modelling. Such reliable and comprehensive characterization of expected earthquake ground motion is essential to improve building codes, particularly for the protection of critical infrastructures and for land use planning. Parvez et al. (Geophys J Int 155:489–508, 2003) published the first ever neo-deterministic seismic hazard map of India by computing synthetic seismograms with input data set consisting of structural models, seismogenic zones, focal mechanisms and earthquake catalogues. As described in Panza et al. (Adv Geophys 53:93–165, 2012), the NDSHA methodology evolved with respect to the original formulation used by Parvez et al. (Geophys J Int 155:489–508, 2003): the computer codes were improved to better fit the need of producing realistic ground shaking maps and ground shaking scenarios, at different scale levels, exploiting the most significant pertinent progresses in data acquisition and modelling. Accordingly, the present study supplies a revised NDSHA map for India. The seismic hazard, expressed in terms of maximum displacement (Dmax), maximum velocity (Vmax) and design ground acceleration (DGA), has been extracted from the synthetic signals and mapped on a regular grid over the studied territory.

How geodesy can contribute to the understanding and prediction of earthquakes

Earthquakes cannot be predicted with precision, but algorithms exist for intermediate-term middle-range prediction of main shocks above a pre-assigned threshold, based on seismicity patterns. Few years ago, a first attempt was made in the framework of project SISMA, funded by Italian Space Agency, to jointly use seismological tools, like CN algorithm and scenario earthquakes, and geodetic methods and techniques, like GPS and SAR monitoring, to effectively constrain priority areas where to concentrate prevention and seismic risk mitigation. We present a further development of integration of seismological and geodetic information, clearly showing the contribution of geodesy to the understanding and prediction of earthquakes. As a relevant application, the seismic crisis that started in Central Italy in August 2016 with the Amatrice earthquake and still going on is considered in a retrospective analysis of both GPS and SAR data. Differently from the much more common approach, here, GPS data are not used to estimate the standard 2D velocity and strain field in the area, but to reconstruct the velocity and strain pattern along transects, which are properly oriented according to the a priori information about the known tectonic setting. SAR data related to the Amatrice earthquake coseismic displacements are here used as independent check of the GPS results. Overall, the analysis of the available geodetic data indicates that it is possible to highlight the velocity variation and the related strain accumulation in the area of Amatrice event, within the area alarmed by CN since November 1st, 2012. The considered counter examples, across CN alarmed and not-alarmed areas, do not show any spatial acceleration localized trend, comparable to the one well defined along the Amatrice transect. Therefore, we show that the combined analysis of the results of intermediate-term middle-range earthquake prediction algorithms, like CN, with those from the processing of adequately dense and permanent GNSS network data, possibly complemented by a continuous InSAR tracking, may allow the routine highlight in advance of the strain accumulation. Thus, it is possible to significantly reduce the size of the CN alarmed areas.

Evaluation of liquefaction potential for building code

The standard approach for the evaluation of the liquefaction susceptibility is based on the estimation of a safety factor between the cyclic shear resistance to liquefaction and the earthquake induced shear stress. Recently, an updated procedure based on shear‐wave velocities (Vs) has been proposed which could be more easily applied.These methods have been applied at La Plaja beach of Catania, that experienced liquefaction because of the 1693 earthquake. The detailed geotechnical and Vs information and the realistic ground motion computed for the 1693 event let us compare the two approaches. The successful application of the Vs procedure, slightly modified to fit historical and safety factor information, even if additional field performances are needed, encourages the development of a guide for liquefaction potential analysis, based on well defined Vs profiles to be included in the italian seismic code.

Uncertainties in the estimate of strong ground motion in the surroundings of a large earthquake

The engineering problem of understanding the collapse of a building during a major earthquake is greatly eased if the actual seismic motion on the site is known. Whenever a sufficient number of strong motion records is available, as in the case of the november 23, 1980 Irpina (Italy) earthquake, we can realistically simulate it for frequencies up to 1 Hz using “complete” synthetic seismograms. In our case we apply a normal mode summation technique. Through a simple minimization process we obtain a source model that reproduces the observed records. Then we use it to estimate the seismic ground motion at other sites. We also investigate the stability and uncertainties in our solution.