A sanity check for earthquake recurrence models used in PSHA of slow deforming regions: the case of SW Iberia

<p>Probabilistic Seismic Hazard Assessment (PSHA) is the most common tool used to decide on the acceptable seismic risk and corresponding mitigation measures. One key component of these studies is the earthquake generation model comprising the definition of source zones and recurrence relationships. Slow deforming regions are particularly challenging for PSHA since the inferred return period for large earthquakes is longer than the instrumental and historical seismicity records, and the relationship between known or probable active faults and seismicity is uncertain. Therefore, in these areas PSHA results show a large variability that impairs its acceptance by the political decision-makers and the public in general. We propose two consistency tests to address the variability of earthquake generation models found in PSHA studies: i) one rule-of-thumb test where the seismic moment release from the model is converted to an average slip on a typical fault and compared with known plate kinematics or GNSS deformation field; ii) using a neotectonic model, the computed deformation is converted into seismic moment release and to a synthetic earthquake catalogue. We apply these tests to the W and SW Iberia slow deforming region, where two earthquake source areas are investigated: 1) the Lower Tagus Valley, one of the largest seismic risk zones of Portugal; and 2) the offshore SW Iberia area, considered to be the source for the 1<sup>st</sup> November 1755 event (M~8.7). Our results show that some of the earthquake source models should be regarded as suspicious, given their high/low moment release when compared to the expected values from GNSS observations or neotectonic modelling. In conclusion, PSHA studies in slow deforming regions should include a similar sanity check on their models&#8217; evaluation, downgrading the weight of poorly compliant models.</p>

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A sanity check for earthquake recurrence models used in PSHA of slow deforming regions: the case of SW Iberia

10.5194/nhess-2020-300 ◽

2020 ◽

Author(s):

Margarida Ramalho ◽

Luis Matias ◽

Marta Neres ◽

Michele M. C. Carafa ◽

Alexandra Carvalho ◽

...

Keyword(s):

Seismic Moment ◽

Seismic Risk ◽

Seismic Hazard Assessment ◽

Earthquake Source ◽

Mitigation Measures ◽

Probabilistic Seismic Hazard Assessment ◽

Lower Tagus Valley ◽

Source Models ◽

Seismic Moment Release ◽

Tagus Valley

Abstract. Probabilistic Seismic Hazard Assessment (PSHA) is the most common tool used to decide on the acceptable seismic risk and corresponding mitigation measures. We propose two consistency tests to address the variability of earthquake generation models found in PSHA studies: i) one rule-of-thumb test where the seismic moment release from the model is converted to an average slip on a typical fault and compared with known plate kinematics or GNSS deformation field; ii) using a neotectonic model, the computed deformation is converted into seismic moment release and to a synthetic earthquake catalogue. We apply these tests to the W and SW Iberia slow deforming region, where two earthquake source areas are investigated: 1) the Lower Tagus Valley, one of the largest seismic risk zones of Portugal; and 2) the offshore SW Iberia area, considered to be the source for the 1st November 1755 event (M~8.7). Results show that some of the earthquake source models should be considered as suspicious, given their high/low moment release when compared to the expected values from GNSS observations or neotectonic modelling. In conclusion, PSHA studies in slow deforming regions should include a similar sanity check on their models' evaluation, downgrading the weight of poorly compliant models.

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Style of faulting of expected earthquakes in Italy as an input for seismic hazard modeling

Natural Hazards and Earth System Science ◽

10.5194/nhess-20-3577-2020 ◽

2020 ◽

Vol 20 (12) ◽

pp. 3577-3592

Author(s):

Silvia Pondrelli ◽

Francesco Visini ◽

Andrea Rovida ◽

Vera D'Amico ◽

Bruno Pace ◽

...

Keyword(s):

Seismic Hazard ◽

Seismic Moment ◽

Outer Part ◽

Seismic Hazard Assessment ◽

Source Model ◽

Probabilistic Seismic Hazard Assessment ◽

Seismic Zone ◽

Moment Tensors ◽

Area Source ◽

Seismic Moment Release

Abstract. The style of faulting and distributions of nodal planes are essential input for probabilistic seismic hazard assessment. As part of a recent elaboration of a new seismic hazard model for Italy, we defined criteria to parameterize the styles of faulting of expected earthquake ruptures and to evaluate their representativeness in an area-based seismicity model. Using available seismic moment tensors for relevant seismic events (Mw≥4.5), first arrival focal mechanisms for less recent earthquakes, and also geological data on past activated faults, we collected a database for the last ∼100 years by gathering a thousand data points for the Italian peninsula and regions around it. In this dataset, we adopted a procedure that consists, in each seismic zone, of separating the available seismic moment tensors into the three main tectonic styles, making a summation within each group, identifying possible nodal plane(s), taking into account the different percentages of styles of faulting, and including where necessary total or partial (even in terms of tectonic style) random source contributions. Referring to the area source model used, for several seismic zones we obtained robust results; e.g., along the central and southern Apennines we expect future earthquakes to be mostly extensional, although in the outer part of the chain reverse and strike-slip events are possible. In the northern part of the Apennines we expect different styles of faulting for different hypocentral depths. In zones characterized by a low seismic moment release, the possible style of faulting of future earthquakes is less clear and it has been represented using different combinations of random sources. The robustness of our results is confirmed when compared with recent relevant earthquakes occurring in Italy.

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Earthquake source parameters that display the first digit phenomenon

Nonlinear Processes in Geophysics ◽

10.5194/npg-22-625-2015 ◽

2015 ◽

Vol 22 (5) ◽

pp. 625-632

Author(s):

P. A. Toledo ◽

S. R. Riquelme ◽

J. A. Campos

Keyword(s):

Seismic Moment ◽

Waiting Times ◽

Source Parameters ◽

Tohoku Earthquake ◽

Earthquake Source ◽

Coseismic Slip ◽

Self Organized ◽

Earthquake Source Parameters ◽

Self Organized Criticality

Abstract. We study the main parameters of earthquakes from the perspective of the first digit phenomenon: the nonuniform probability of the lower first digit different from 0 compared to the higher ones. We found that source parameters like coseismic slip distributions at the fault and coseismic inland displacements show first digit anomaly. We also found the tsunami runups measured after the earthquake to display the phenomenon. Other parameters found to obey first digit anomaly are related to the aftershocks: we show that seismic moment liberation and seismic waiting times also display an anomaly. We explain this finding by invoking a self-organized criticality framework. We demonstrate that critically organized automata show the first digit signature and we interpret this as a possible explanation of the behavior of the studied parameters of the Tohoku earthquake.

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Methodology for Earthquake Rupture Rate estimates of fault networks: example for the Western Corinth Rift, Greece

10.5194/nhess-2017-124 ◽

2017 ◽

Cited By ~ 1

Author(s):

Thomas Chartier ◽

Oona Scotti ◽

Hélène Lyon-Caen ◽

Aurélien Boiselet

Keyword(s):

Active Faults ◽

Slip Rate ◽

Seismic Hazard Assessment ◽

System Level ◽

Fault System ◽

Normal Faults ◽

Accurate Estimation ◽

Probabilistic Seismic Hazard Assessment ◽

Earthquake Rupture ◽

Multiple Faults

Abstract. Modelling the seismic potential of active faults is a fundamental step of probabilistic seismic hazard assessment (PSHA). An accurate estimation of the rate of earthquakes on the faults is necessary in order to obtain the probability of exceedance of a given ground motion. Most PSHA studies consider faults as independent structures and neglect the possibility of multiple faults or fault segments rupturing simultaneously (Fault to Fault -FtF- ruptures). The latest Californian model (UCERF-3) takes into account this possibility by considering a system level approach rather than an individual fault level approach using the geological , seismological and geodetical information to invert the earthquake rates. In many places of the world seismological and geodetical information long fault networks are often not well constrained. There is therefore a need to propose a methodology relying only on geological information to compute earthquake rate of the faults in the network. In this methodology, similarly to UCERF-3, a simple distance criteria is used to define FtF ruptures and consider single faults or FtF ruptures as an aleatory uncertainty. Rates of earthquakes on faults are then computed following two constraints: the magnitude frequency distribution (MFD) of earthquakes in the fault system as a whole must follow an imposed shape and the rate of earthquakes on each fault is determined by the specific slip-rate of each segment depending on the possible FtF ruptures. The modelled earthquake rates are then confronted to the available independent data (geodetical, seismological and paleoseismological data) in order to weigh different hypothesis explored in a logic tree. The methodology is tested on the Western Corinth Rift, Greece (WCR) where recent advancements have been made in the understanding of the geological slip rates of the complex network of normal faults which are accommodating the ~15 mm/yr North-South extension. Modelling results show that geological, seismological extension rates and paleoseismological rates of earthquakes cannot be reconciled with only single fault rupture scenarios and require hypothesising a large spectrum of possible FtF rupture sets. Furthermore, in order to fit the imposed regional Gutenberg-Richter MFD target, some of the slip along certain faults needs to be accommodated either with interseismic creep or as post-seismic processes. Furthermore, individual fault’s MFDs differ depending on the position of each fault in the system and the possible FtF ruptures associated with the fault. Finally, a comparison of modelled earthquake rupture rates with those deduced from the regional and local earthquake catalogue statistics and local paleosismological data indicates a better fit with the FtF rupture set constructed with a distance criteria based on a 5 km rather than 3 km, suggesting, a high connectivity of faults in the WCR fault system.

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Hydrodynamics of pedestrians' instability in floodwaters

Hydrology and Earth System Sciences ◽

10.5194/hess-21-515-2017 ◽

2017 ◽

Vol 21 (1) ◽

pp. 515-531 ◽

Cited By ~ 25

Author(s):

Chiara Arrighi ◽

Hocine Oumeraci ◽

Fabio Castelli

Keyword(s):

Experimental Data ◽

Froude Number ◽

Flood Risk ◽

Risk Mitigation ◽

Three Dimensional ◽

Numerical Results ◽

Mitigation Measures ◽

Dimensionless Numbers ◽

Large Variability ◽

Mobility Parameter

Abstract. People's safety is the first objective to be fulfilled by flood risk mitigation measures, and according to existing reports on the causes of casualties, most of the fatalities are due to inappropriate behaviour such as walking or driving in floodwaters. Currently available experimental data on people instability in floodwaters suffer from a large dispersion primarily depending on the large variability of the physical characteristics of the subjects. This paper introduces a dimensionless mobility parameter θP for people partly immersed in flood flows, which accounts for both flood and subject characteristics. The parameter θP is capable of identifying a unique threshold of instability depending on a Froude number, thus reducing the scatter of existing experimental data. Moreover, a three-dimensional (3-D) numerical model describing the detailed geometry of a human body and reproducing a selection of critical pairs of water depth and velocity is presented. The numerical results in terms of hydrodynamic forces and force coefficients are analysed and discussed. Both the mobility parameter θP and the numerical results hint at the crucial role of the Froude number and relative submergence as the most relevant dimensionless numbers to interpret the loss of stability. Finally, the mobility parameter θP is compared with an analogous dimensionless parameter for vehicles' instability in floodwaters, providing a new contribution to support flood risk management and educating people.

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Contrasting seismic risk for Santiago, Chile, from near-field and distant earthquake sources

10.5194/nhess-2019-30 ◽

2019 ◽

Author(s):

Ekbal Hussain ◽

John R. Elliott ◽

Vitor Silva ◽

Mabé Vilar-Vega ◽

Deborah Kane

Keyword(s):

Low Income ◽

Seismic Risk ◽

Near Field ◽

Active Faults ◽

Rapid Expansion ◽

Seismic Risk Analysis ◽

Earthquake Sources ◽

Maule Earthquake ◽

The City ◽

Distant Earthquake

Abstract. More than half of all the people in the world now live in dense urban centres. The rapid expansion of cities, particularly in low-income nations, has enabled the economic and social development of millions of people. However, many of these cities are located near active tectonic faults that have not produced an earthquake in recent memory, raising the risk of losing the hard-earned progress through a devastating earthquake. In this paper we explore the possible impact that earthquakes can pose to the city of Santiago in Chile from various potential near-field and distant earthquake sources. We use high resolution stereo satellite imagery and derived digital elevation models to accurately map the trace of the San Ramón Fault, a recently recognised active fault located along the eastern margins of the city. We use scenario based seismic risk analysis to compare and contrast the estimated damage and losses to the city from several potential earthquake sources and one past event, comprising (i) rupture of the San Ramón Fault, (ii) a hypothesised buried shallow fault beneath the centre of the city, (iii) a deep intra-slab fault, and (iv) the 2010 Mw 8.8 Maule earthquake. We find that there is a strong magnitude-distance trade-off in terms of damage and losses to the city, with smaller magnitude earthquakes on more local faults, in the magnitude range 6–7.5, producing 9 to 17 times more damage to the city and estimated fatalities compared to the great magnitude 8+ earthquakes located offshore on the subduction zone. Our calculations for this part of Chile show that unreinforced masonry structures are the most vulnerable to these types of earthquake shaking. We identify particularly vulnerable districts, such as Ñuñoa, Santiago and Macul, where targeted retrofitting campaigns would be most effective at reducing potential economic and human losses. Due to the potency of near-field earthquake sources demonstrated here, our work highlights the importance of also identifying and considering proximal minor active faults for cities in seismic zones globally, in addition to the more major distant large fault zones that are typically focused on in the assessment of hazard.

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