Prediction of High-Frequency Ground Motion Parameters Based on Weak Motion Data

Large earthquakes that have occurred in recent years in densely populated areas of the world (e.g. Izmit, Turkey, 17 August 1999; Duzce, Turkey, 12 November 1999; Chi-Chi, Taiwan 20 September 1999, Bhuj, India, 26 January 2001; Sumatra 26 December 2004; Wenchuan, China, May 12, 2008; L’Aquila, Italy, April 6, 2009; Haiti, January 2010 Turkey 2011) have dramatically highlighted the inadequacy of a massive portion of the buildings erected in and around the epicentral areas. For example, the Izmit event was particularly destructive because a large number of buildings were unable to withstand even moderate levels of ground shaking, demonstrating poor construction criteria and, more generally, the inadequacy of the application of building codes for the region. During the L’Aquila earthquake (April, 06, 2009; Mw=6.3) about 300 persons were killed and over 65,000 were left homeless (Akinci and Malagnini, 2009). It was the deadliest Italian earthquake since the 1980, Irpinia earthquake, and initial estimates place the total economic loss at over several billion Euros. Many studies have already been carried out describing the rupture process and the characteristics of local site effects for this earthquake (e.g. D’Amico et al., 2010a; Akinci et al., 2010). It has been observed that many houses were unable to withstand the ground shaking. Building earthquake-resistant structures and retrofitting old buildings on a national scale may be extremely costly and may represent an economic challenge even for developed western countries, but it is still a very important issue (Rapolla et al., 2008). Planning and design should be based on available national hazard maps, which, in turn, must be produced after a careful calibration of ground motion predictive relationships (Kramer, 1996) for the region. Consequently, the assessment of seismic hazard is probably the most important contribution of seismology to society. The prediction of the earthquake ground motion has always been of primary interest for seismologists and structural engineers. For engineering purposes it is necessary to describe the ground motion according to certain number of ground motion parameters such as: amplitude, frequency content and duration of the motion. However it is necessary to use more than one of these parameters to adequately characterize a particular ground motion. Updating existing hazard maps represents one of the highest priorities for seismologists, who contribute by recomputing the ground motion and reducing the related uncertainties. The quantitative estimate of the ground motion is usually obtained through the use of the so-called predictive relationships (Kramer, 1996), which allow the computation of specific ground-motion parameter as a function of magnitude, distance from the source, and frequency and they should be calibrated in the region of interest. However this is only possible if seismic records of large earthquakes are available for the specific region in order to derive a valid attenuation relationship regressing a large number of strong-motion data (e.g. Campbell and Bozorgnia, 1994; Boore et al., 1993; Ambraseys et al., 1996, Ambraseys and Simpson, 1996; Sabetta and Pugliese, 1987, 1996; Akkar and Bommer 2010). For the Italian region the most used attenuation relationships are those obtained by Sabetta and Pugliese (1987, 1996) regressing a few data recorded for earthquakes in different tectonic and geological environments. It has been shown in several cases that it is often not adequate to reproduce the ground motion in each region of the country using a single model. Furthermore the different crustal properties from region to region play a key role in this kind of studies. However, the attenuation properties of the crust can be evaluated using the background seismicity as suggested by Chouet et al. (1978) and later demonstrated by Raoff et al. (1999) and Malagnini et al (2000a, 2007). In other words, it becomes possible to develop regionallycalibrated attenuation relationships even where strong-motion data are not available. One of the purposes of this work is to describe quantitatively the regional attenuation and source characteristics for constraining the amplitude of strong motion expected from future earthquakes in the area. In this work we describe how to use the background seismicity to perform the analysis (details in Malagnini et. 2000a, 2007). In particular, this chapter describes the procedures and techniques to study the ground motion and will focus on describing both strong motion attenuation relationships and the techniques used to derive the ground motion parameters even when strong ground motion data are not available. We will present the results obtained for different regions of the Italian peninsula, showing that the attenuation property of the crust and of the source can significantly influence the ground motion. In addition, we will show that stochastic finite-fault modeling based on a dynamic frequency approach, coupled with field investigations, confirms to be a reliable and practical method to simulate ground motion records of moderate and large earthquakes especially in regions prone to widespread structural damage

The 2012 Ferrara seismic sequence: from a 1D reliable crustal structure for moment tensor solutions to strong implications for seismic hazard

On May 20 2012, an event of Ml 5.9 (Mw 5.6) stuck the southem edge of the Po river plain (Pianura Padana). The earthquake was preceded by a foreshock of Ml 4.1 (Mw 3.8), less than 3 hours before the Mw 5.6 main. Hypocentral depths were 6.3 km for both events. Centroid depths were 5 and 6 km, respectively. The activated fault was a reverse one, dipping to the south. Then a complex seismic sequence started, in which more than six earthquakes with Ml greater than 5 stuck the area, the last one on June 3, 2012. Aftershocks delineated a 50 km long and 10-15 km wide zone, approximately elongated in the WE direction. More than 2100 events were located between May 19 and June 25 2012 by the INGV National Seismic Network, 80 of them with Ml greater than 3.5. The damage due to the Ml 5+ earthquakes was widespread, as they severely hit historical towns and industrial infrastructures. However, a striking inconsistency exists between the relatively small moment magnitudes and the corrisponding high level of damage. In order to define a velocity structure for the crust beneath the Pianura Padana, to be used for waveform inversion of moment tensors, we gathered all the geophysical and geological information available for the area. The model is characterized by very thick and shallow Quaternary sediments, to be used for the inversion of broadband waveforms for moment tensor (MT) solutions, in the frequency band between 0.02-0.1 Hz. We calculated moment tensors for 20 events down to Mw~3.2. We demonstrate how surface waves dominate the seismograms in the region, which may have played a major role in enhancing the damage to industrial structures observed in the epicentral area. Synthetic seismograms computed using the developed model well reproduced the anomalous durations of the ground motion observed in Pianura Padana, also highlighting important implications for the seismic hazard in the entire area. The present seismic hazard assessment as well as the size of the historical earthquakes in the region (and so their recurrence times), may need to be re-evaluated in the light of this new results

Uncertainty analysis for seismic hazard in Northern and Central Italy

In this study we examine uncertainty and parametric sensitivity of Peak Ground Acceleration (PGA) and 1-Hz Spectral Acceleration (1-Hz SA) in probabilistic seismic hazard maps (10% probability of exceedance in 50 years) of Northern and Central Italy. The uncertainty in hazard is estimated using a Monte Carlo approach to randomly sample a logic tree that has three input-variables branch points representing alternative values for bvalue, maximum magnitude (Mmax) and attenuation relationships. Uncertainty is expressed in terms of 95% confidence band and Coefficient Of Variation (COV). The overall variability of ground motions and their sensitivity to each parameter of the logic tree are investigated. The largest values of the overall 95% confidence band are around 0.15 g for PGA in the Friuli and Northern Apennines regions and around 0.35 g for 1-Hz SA in the Central Apennines. The sensitivity analysis shows that the largest contributor to seismic hazard variability is uncertainty in the choice of ground-motion attenuation relationships, especially in the Friuli Region (?0.10 g) for PGA and in the Friuli and Central Apennines regions (?0.15 g) for 1-Hz SA. This is followed by the variability of the b-value: its main contribution is evident in the Friuli and Central Apennines regions for both 1-Hz SA (?0.15 g) and PGA (?0.10 g). We observe that the contribution of Mmax to seismic hazard variability is negligible, at least for 10% exceedance in 50-years hazard. The overall COV map for PGA shows that the uncertainty in the hazard is larger in the Friuli and Northern Apennine regions, around 20-30%, than the Central Apennines and Northwestern Italy, around 10-20%. The overall uncertainty is larger for the 1-Hz SA map and reaches 50- 60% in the Central Apennines and Western Alps

Crustal permeability changes inferred from seismic attenuation: Impacts on multi-mainshock sequences

We use amplitude ratios from narrowband-filtered earthquake seismograms to measure variations of seismic attenuation over time, providing unique insights into the dynamic state of stress in the Earth’s crust at depth. Our dataset from earthquakes of the 2016–2017 Central Apennines sequence allows us to obtain high-resolution time histories of seismic attenuation (frequency band: 0.5–30 Hz) characterized by strong earthquake dilatation-induced fluctuations at seismogenic depths, caused by the cumulative elastic stress drop after the sequence, as well as damage-induced ones at shallow depths caused by energetic surface waves. Cumulative stress drop causes negative dilatation, reduced permeability, and seismic attenuation, whereas strong-motion surface waves produce an increase in crack density, and so in permeability and seismic attenuation. In the aftermath of the main shocks of the sequence, we show that the M ≥ 3.5 earthquake occurrence vs. time and distance is consistent with fluid diffusion: diffusion signatures are associated with changes in seismic attenuation during the first days of the Amatrice, Visso-Norcia, and Capitignano sub-sequences. We hypothesize that coseismic permeability changes create fluid diffusion pathways that are at least partly responsible for triggering multi-mainshock seismic sequences. Here we show that anelastic seismic attenuation fluctuates coherently with our hypothesis

SAFE Project: An improved integrated system of earthquake physics study from ground and satellite observations

Trabajo presentado en 35th General Assembly of the European Seismological Commission, celebrado en Trieste (Italia), del 4 al 10 de septiembre de 2016The Swarm satellite mission by ESA has the primary goal to measure the magnetic signals from the Earth to get new insights of the geomagnetic field and its sources. The SAFE (“Swarm for Earthquake study”) project (funded by ESA in the framework "STSE Swarm+lnnovation", 2014) aims at applying the new approach of geosystemics to the analysis of Swarm satellite electromagnetic data for investigating the preparatory phase of large earthquakes. The main objective of the project is to explore the possible link between large earthquakes and precursory electromagnetic anomalies detected by Swarm and ground based data (seismic, magnetic, GNSS, etc.). This work will show some recent case studies analysed in the framework of the project.Peer reviewe

Nucleation speed limit on remote fluid-induced earthquakes

Earthquakes triggered by other remote seismic events are explained as a response to long-traveling seismic waves that temporarily stress the crust. However, delays of hours or days after seismic waves pass through are reported by several studies, which are difficult to reconcile with the transient stresses imparted by seismic waves. We show that these delays are proportional to magnitude and that nucleation times are best fit to a fluid diffusion process if the governing rupture process involves unlocking a magnitude-dependent critical nucleation zone. It is well established that distant earthquakes can strongly affect the pressure and distribution of crustal pore fluids. Earth's crust contains hydraulically isolated, pressurized compartments in which fluids are contained within low-permeability walls. We know that strong shaking induced by seismic waves from large earthquakes can change the permeability of rocks. Thus, the boundary of a pressurized compartment may see its permeability rise. Previously confined, overpressurized pore fluids may then diffuse away, infiltrate faults, decrease their strength, and induce earthquakes. Magnitude-dependent delays and critical nucleation zone conclusions can also be applied to human-induced earthquakes.Publishede1700665T. Modelli di pericolosità sismica e da maremotoJCR Journa

On the Relationship betweenMwandMLfor Small Earthquakes

Estimating the moment magnitudes (Mw) of a small earthquake is a challenging task. One viable option to measure its size is to calculate its local magnitude (ML) and convert it to the physically basedMw. Unfortunately, to correctly perform such a conversion is not easy; moreover, even though many studies demonstrate that the equivalence betweenML andMw is incorrect for small events, these two parameters are sometimes thought to be strictly equivalent, regardless of the earthquake’s size. Using random vibration theory, we show that, below Mw ∼ 4, the ML of a small earthquake is proportional to the logarithm of its seismic moment, and the following relationship holds: EQ-TARGET;temp:intralink-;;55;536 MwSMALL 2 3 MLSMALL C′: We test our findings on a high-quality data set in the Upper Tiber Valley (northern Apennines, Italy), composed of events in the range of 0 ≤ ML ≤ 3:8, for which we compute accurate estimates of ML and Mw. Online Material: Details of the processing procedure, figures of the empirical regional attenuation functional, and source terms of 1191 events from the Alto Tiberina fault (ATF) data set and earthquake catalog.Published2402-24082T. Sorgente SismicaJCR Journa

Moment magnitude and local magnitude of small earthquakes nucleating along a low angle normal fault in the Upper Tiber Valley (Italy)

The computation of the moment magnitude of small earthquakes (MW < 3) allows the investigation of key aspects of the physics of the seismic source, like the scaling properties of earthquakes. In order to do that, we analyse the crustal propagation of seismic waves in the Upper Tiber Valley (Northern Apennines, Italy) using 38,000 high-resolution broadband seismograms from 1192 well-located micro-earthquakes that occurred between 2010 and 2014, in the local magnitude range -1.0 ≤ ML ≤ 3.8. Because we use weak-motion data, we maximize the signal-to-noise ratios by applying a complex technique based on Random Vibration Theory (RVT). Our analysis of the data produced two main results: i) we are able to calculate the seismic moment (and moment magnitude) for very small events, down to at least MW = -1.5. ii) we determined a relationship between MW and ML , and use RVT to show that ML ~ log10 (M0) for small earthquakes.PublishedSan Francisco2T. Sorgente Sismic

Characteristics of the strong ground motion from the 24th August 2016 Amatrice earthquake

The 2016 August 24 Amatrice earthquake occurred at 03:36 local time in central Apennines Italy with an epicentre at 43.36°E, 38.76°N, Istituto Nazionale di Geofisica e Vulcanologia (INGV), few kilometers north of the city of Amatrice. The earthquake ruptured a North-West (NW)–South-East (SE) oriented normal fault dipping toward the South-West (SW) (Scognamiglio et al., 2016). High values of peak ground acceleration (~0.45 g) were observed close to Amatrice (3 stations being few kilometer distances from the fault). The present study presents an overview of the main features of the seismic ground shaking during the Amatrice earthquake. We analyze the ground motion characteristics of the main shock in terms of peak ground acceleration (PGA), peak ground velocity (PGV) and spectral accelerations (SA, 5 per cent of critical damping). In order to understand the characteristics of the ground motions induced by Amatrice earthquake, we also study the source-related effects relative to the fault rupture directivity