Shaking Intensity Level Research Articles

What 25+ Years of “Did You Feel It” Intensities Tell Us About Shaking in California

Abstract “When will the Big One happen?” is a question that people often have for earthquake scientists. But while waiting for the “Big One” to occur, people will usually experience frightening or damaging shaking from multiple relatively smaller-magnitude earthquakes. Given this context, it raises the question: “Where does most of the damage come from?” Could smaller, yet more frequent, earthquakes account for the majority of reported impactful shaking? To explore this question, we consider reports of earthquake damage and felt shaking experiences from a catalog of community-collected intensity values from the U.S. Geological Survey’s “Did You Feel It?” system. Comparing these intensities to expectations from a ground-motion model, we find that earthquakes of magnitudes smaller than expected are responsible for most reported intensities of community decimal intensities (CDI) 4.5 and above (moderate and higher shaking intensity levels). (Here “expected value” is meant in its mathematical sense of the mean or equivalently median shaking intensity.) We also present a regional analysis of observed earthquake shaking for specific areas, detailing the maximum intensity experienced within a predetermined area. We identify several instances of M < 4.5 events that generated maximum intensities of CDI > 5 in regions in California surrounding the cities of Eureka, San Francisco, Los Angeles, and San Diego. Our results motivate the need to include smaller-magnitude earthquakes in communications about earthquake hazard and risk reduction.

Direct loss-based seismic retrofit of reinforced concrete frames

In earthquake-prone areas, structures not compliant with modern design codes significantly contribute to seismic risk. Therefore, risk mitigation strategies (e.g., seismic retrofit) should be employed to reduce the expected economic and human losses. This paper introduces a procedure for the design of retrofit solutions for reinforced concrete (RC) frame buildings to achieve - rather than be bounded by - a desired target level of earthquake-induced loss for a given site-specific seismic hazard profile. The presented methodology is “direct” because the designer and/or client can set a loss target in the first step of the procedure and no design iterations are virtually required. Direct loss-based seismic retrofit (DLBR) relies on a simplified loss assessment methodology enabled by a surrogate probabilistic seismic demand model. This defines the probability distribution of seismic deformation demand of single degree of freedom (SDoF) systems conditioned on different shaking intensity levels. The proposed design methodology enables designers to account for risk/loss-based considerations from the conceptual/preliminary design phase, thus facilitating the choice among different retrofit solutions. Starting from two under-designed case-study buildings, four illustrative applications of the procedure are provided. They involve considering different economic expected annual loss targets and different retrofit solutions involving the addition of RC walls and RC column jacketing. Benchmark loss estimates are calculated using non-linear time-history analyses of refined, multi-degree-of-freedom models showing satisfactory results: the simplified loss estimate introduces an overestimation maximum equal to 15.4% among the four illustrative applications.

Seismic fragility evaluation of embankments on liquefiable soils and remedial countermeasures

This paper focuses on the evaluation of seismic fragility curves for embankments on liquefiable soils with the aim of improving the reliability of the risk assessment of transport infrastructure systems. Nonlinear dynamic analyses of embankments on liquefiable soils are carried out using the finite difference program FLAC2D with the PM4sand model. Validations of numerical modelling are performed in comparison to the dynamic centrifuge model tests reported in prior investigations. For the establishment of seismic fragility curves, ten input motions are selected and scaled to evaluate the embankment response under increasing shaking intensity levels. The seismic risk of embankments on liquefiable soil is evaluated through fragility curves with the damage states (DSs) described in terms of embankment settlement. The criteria, including correlation, efficiency, practicality, and proficiency, are assessed based on the relationship between the intensity measures (IMs) and the engineering demand parameters (EDPs) for the studied embankments. The embankment responses are compared with the benchmark scenario results to distinguish the relative contribution of the considered liquefaction mitigation strategies (i.e., soil densification, gravel berm, and sheet–pile enclosure). The seismic fragility curve serves as an efficient seismic risk analysis in the selection of liquefaction remedial measures in engineering design.

Scenario-based earthquake risk assessment for central-southern Malawi: The case of the Bilila-Mtakataka Fault

A regional seismic risk analysis for central-southern Malawi is conducted by focusing on the Bilila-Mtakataka Fault within the East African Rift System and by incorporating local information on population exposure and building vulnerability. The scenario-based earthquake risk assessments account for uncertainty in geometry, position, and rupture pattern of the Bilila-Mtakataka Fault as well as ground-motion variability and are based on the latest 2018 national census data. In addition, Malawi-specific seismic fragility functions, which were developed based on building surveys and laboratory tests of local construction materials, are implemented to reflect realistic seismic vulnerability of unreinforced masonry constructions in Malawi. The results from the earthquake risk assessments and sensitivity analyses based on alternative data and models highlight the importance of incorporating local information on seismic hazard characterisation, population data, and seismic vulnerability of buildings, in comparison with global data and models. For the considered case study region, individual effects of the above-mentioned model components tend to result in 20–30 % or greater differences in regional seismic risk metrics, such as the affected population experiencing a certain ground shaking intensity level or the number of collapsed housing units. The improved seismic hazard-risk assessments are more effective in informing future seismic risk mitigation policies and actions.

Vulnerability modellers toolkit, an open-source platform for vulnerability analysis

Vulnerability functions describe the expected loss for a given ground shaking intensity level and are an essential component in probabilistic seismic risk assessment. This manuscript presents a novel open-source platform for the derivation of analytical fragility and vulnerability models, covering state-of-the-art methodologies, and addressing critical issues in vulnerability modelling such as uncertainty propagation, validation/verification of results and sufficiency/efficiency of intensity measure types. This framework is divided into seven modules designed to guide users through the different stages of analytical vulnerability modelling from the selection of ground motion records to the validation and verification of the models. The platform was implemented in the Python programming language and it is freely accessible through a public GitHub repository. A graphical user interface is included with the toolkit and is intended to be a general-purpose method for modellers to interact with the vulnerability modellers toolkit (VMTK). Experienced users are encouraged to use Python’s scripting capabilities to explore all the features of the VMTK source code and to contribute to future releases of the toolkit.

A simplified procedure for predicting the soil amplification factor based on the strain compatible period

This paper proposed a simplified procedure for predicting the amplification factor based on a new empirical model for estimating the site period considering nonlinear strain range of soil. The analysis results suggested the depth to bedrock of the soil profile affects the site period at certain shaking intensity levels. Considering this finding, we first proposed a new empirical model to estimate the site period by propagating 78 input motions through 119 soil profiles. A statistical method confirmed the accuracy of the proposed empirical method, which represents the effect of both dynamic properties of soil and input motion features on site response. We then presented a simplified procedure for predicting the amplification factor by taking to account the site period shift and soil nonlinear behavior due to the interaction of damping increase and the resonance controlling effect. Finally, the accuracy of the simplified procedure results in predicting the amplification factor was evaluated by considering the equivalent linear and nonlinear approaches. The results revealed that the simplified model captures the trends of equivalent linear site response analyze in predicting the amplification factor with acceptable accuracy.

Torsional irregularity indices for the seismic demand assessment of RC moment resisting frame buildings

Design standards across the world use different parameters to quantify degree of torsional irregularity of buildings. Popular among such descriptors are the normalized static eccentricity ratio, torsional radius to mass radius of gyration ratio, maximum to minimum (or average) elastic floor displacement ratio, etc. However prior research, particularly on idealized single storey systems have indicated that these parameters do not correlate well with the seismic demands at all stages of structural behaviour (i.e. elastic and inelastic). The present study aims to identify a robust torsional irregularity index that captures the susceptibility of a structure to torsional effects at all ground motion intensity levels. To this end, the correlation between the maximum interstorey drift demands and popular torsional irregularity descriptors is first examined under different intensities of ground shaking using an approach that integrates multiple stripe analysis and statistical regression analysis. The numerical study is based on the nonlinear seismic assessment of thirteen three-storey gravity designed RC moment frame buildings subjected to bidirectional seismic excitation. The results indicate that none of the scalar indices considered correlate well with the drift demands at all intensities of ground shaking. A vector comprising of the normalized static eccentricity ratio and the torsional radius to mass radius of gyration ratio is found to perform well at low shaking intensity levels, where the structure responds in an elastic manner. However, at high intensity levels where the structure undergoes significant excursions into the inelastic range of behaviour, the strength eccentricity is observed to be a better descriptor of torsional irregularity. A three-element vector comprising of the above indices is found to capture the maximum interstorey drifts accurately at all ground motion intensity levels. A seismic demand model for maximum interstorey drift, sensitive to the above torsional irregularity descriptors, is also defined.

From Earthquake Source Parameters to Ground‐Motion Warnings near You: The ShakeAlert Earthquake Information to Ground‐Motion (eqInfo2GM) Method

We present a new near‐real‐time method for converting earthquake source parameters into ground motion (GM) at locations across the west coast of the United States. This method, called earthquake information to ground motion (eqInfo2GM), has been implemented as part of the ShakeAlert earthquake early warning (EEW) system and makes estimated GM accessible to users on the EEW timeframe of seconds, as the U.S. Geological Survey ShakeMap does at higher resolution and accuracy in the minutes following a seismic event. Whereas the higher fidelity ShakeMap comes at the cost of longer processing times, eqInfo2GM effectively provides a predicted ShakeMap before shaking arrives at many locations. We describe key design details, including ground‐motion prediction equations (GMPEs) implemented, modifications made to optimize for speed, and formats created for conveying GM severity. The GM output format determines added latency and reflects a trade‐off between speed and accuracy; for our test earthquake data set, added latency is in the 0.01–1.5 s range after earthquake source parameters have been generated. GMPE implementations are validated against predicted ShakeMaps (without observations), with almost all events showing minimal mean shaking intensity level differences, reflecting variations only in treatment of source distances and VS30 data. Comparison against ShakeMaps computed with observations (a proxy for true GM) show larger differences, demonstrating the challenges of working in the EEW timeframe, when full source characterization and peak ground motion observations are both unavailable. Although specific configurations and features of the method will evolve as the needs of the EEW user community become evident, eqInfo2GM is expected to improve the overall utility of EEW alerts by providing end users with estimates of predicted local GM hazard. Such near‐real‐time estimates will enable users to decide more accurately what action to take to reduce the impact of imminent, potentially damaging shaking.

Modified Mercalli intensities for the M7.8 Kaikōura (New Zealand) 14 November 2016 earthquake derived from ‘felt detailed’ and ‘felt rapid’ online questionnaires

This paper describes the shaking intensity levels caused by the M7.8 Kaikōura earthquake of 14/11/2016 according to the information from the two current GeoNet online questionnaires, ‘Felt Detailed’ and ‘Felt RAPID’. A recently developed method to extract intensity levels at a community scale using ‘Felt Detailed’ data is used. These are compared with individual intensities from ‘Felt RAPID’ survey, instrumental intensities from two recent ground motion to intensity conversion equations, and traditional intensity assignments. While maximum Modified Mercalli instrumental, traditional, ‘Felt RAPID’ and individual ‘Felt Detailed’ intensities go up to 8, community intensities using ‘Felt Detailed’ mostly only go up to 5, with only four communities with MM 6-7. Reasons for this discrepancy include a) lack of data around the epicentre; b) few reports from this event compared to other smaller recent earthquakes; and c) lack of public awareness of ‘Felt Detailed” surveys, released shortly after the earthquake. In addition, only 47% of reports were used to calculate community intensities, based on a minimum requirement for robust calculation of 5 reports. Although ‘Felt RAPID’ provided a much larger number of reports (more than 15,000) for this earthquake compared to ‘Felt Detailed’ (3500), the reliability of the former may be compromised by their lack of detail. Results from this paper suggest that, when enough reports are submitted, ‘Felt Detailed’ can provide good quality data that can be used in tools such as the near-real time shaking intensity maps provided in ShakeMapNZ.

Ground Motion Selection for Seismic Slope Displacement Evaluation Analysis of Earthen Levees

Guidelines for selecting ground motions for evaluation of seismic slope displacements for earthen levees are proposed. These guidelines were developed based on results from two-dimensional equivalent-linear dynamic analyses of characteristic earthen levee cross sections using a wide range of ground motions (∼1 ;500). The effect of a number of ground motion parameters on the dynamic response of the levees in terms of seismic slope stability was studied, and the parameters that correlated best to the response were identified. For the seismic slope stability evaluation, the peak ground velocity ( PGV) best correlates to the seismic slope displacements ( u). Regression relationships between u and PGV are proposed for a series of levee types and shaking intensity levels, which are used in the proposed methodology for selecting ground motions for site specific analysis of levees.

Values for peak ground acceleration and peak ground velocity using in seismic hazard assessment for Song Tranh 2 hydropower region

Values for peak ground acceleration and peak ground velocity using in seismic hazard assessment for Song Tranh 2 hydropower region

Experimental study for seismic demands on in-cabinet equipment in NPPs

This study aims to establish ordinary seismic demands imposed on safety-related electrical instruments mounted in electrical cabinets (referred to as in-cabinet equipment) in nuclear power plants and to understand the basic characteristics of the dynamic responses for electrical cabinets. An existing cabinet used in the Maanshan nuclear power plant was adopted as a specimen for a shaking table test. The vibration responses and spectral amplification factors of the tested cabinet under different shaking intensity levels, as well as the variation of total mass and anchorage locations of additional weights attached to the cabinet, are studied and summarized in this study. The results of this preliminary study can contribute to developing a complete test plan for motor control centers in the Lungmen nuclear power plant in future studies.

The Mw 5.8 Mineral, Virginia, Earthquake of August 2011 and Aftershock Sequence: Constraints on Earthquake Source Parameters and Fault Geometry

The Mw 5.8 earthquake of 23 August 2011 (17:51:04 UTC) (moment, M0 5:7 × 10 17 N·m) occurred near Mineral, Virginia, within the central Virginia seis- mic zone and was felt by more people than any other earthquake in United States history. The U.S. Geological Survey (USGS) received 148,638 felt reports from 31 states and 4 Canadian provinces. The USGS PAGER system estimates as many as 120,000 people were exposed to shaking intensity levels of IV and greater, with approximately 10,000 exposed to shaking as high as intensity VIII. Both regional and teleseismic moment tensor solutions characterize the earthquake as a northeast- striking reverse fault that nucleated at a depth of approximately 7 2 km. The distri- bution of reported macroseismic intensities is roughly ten times the area of a similarly sized earthquake in the western United States (Horton and Williams, 2012). Near- source and far-field damage reports, which extend as far away as Washington, D.C., (135 km away) and Baltimore, Maryland, (200 km away) are consistent with an earthquake of this size and depth in the eastern United States (EUS). Within the first few days following the earthquake, several government and aca- demic institutions installed 36 portable seismograph stations in the epicentral region, making this among the best-recorded aftershock sequences in the EUS. Based on modeling of these data, we provide a detailed description of the source parameters of the mainshock and analysis of the subsequent aftershock sequence for defining the fault geometry, area of rupture, and observations of the aftershock sequence mag- nitude-frequency and temporal distribution. The observed slope of the magnitude- frequency curve or b-value for the aftershock sequence is consistent with previous EUS studies (b 0:75), suggesting that most of the accumulated strain was released by the mainshock. The aftershocks define a rupture that extends between approxi- mately 2-8 km in depth and 8-10 km along the strike of the fault plane. Best-fit modeling of the geometry of the aftershock sequence defines a rupture plane that strikes N36°E and dips to the east-southeast at 49.5°. Moment tensor solutions of the mainshock and larger aftershocks are consistent with the distribution of aftershock locations, both indicating reverse slip along a northeast-southwest striking southeast- dipping fault plane.

Developing Empirical Collapse Fragility Functions for Global Building Types

Building collapse is the dominant cause of casualties during earthquakes. In order to better predict human fatalities, the U.S. Geological Survey's Prompt Assessment of Global Earthquakes for Response (PAGER) program requires collapse fragility functions for global building types. The collapse fragility is expressed as the probability of collapse at discrete levels of the input hazard defined in terms of macroseismic intensity. This article provides a simple procedure for quantifying collapse fragility using vulnerability criteria based on the European Macroseismic Scale (1998) for selected European building types. In addition, the collapse fragility functions are developed for global building types by fitting the beta distribution to the multiple experts’ estimates for the same building type (obtained from EERI's World Housing Encyclopedia (WHE)-PAGER survey). Finally, using the collapse probability distributions at each shaking intensity level as a prior and field-based collapse-rate observations as likelihood, it is possible to update the collapse fragility functions for global building types using the Bayesian procedure.

An Empirical Model for Global Earthquake Fatality Estimation

We analyzed mortality rates of earthquakes worldwide and developed a country/region-specific empirical model for earthquake fatality estimation within the U.S. Geological Survey's Prompt Assessment of Global Earthquakes for Response (PAGER) system. The earthquake fatality rate is defined as total killed divided by total population exposed at specific shaking intensity level. The total fatalities for a given earthquake are estimated by multiplying the number of people exposed at each shaking intensity level by the fatality rates for that level and then summing them at all relevant shaking intensities. The fatality rate is expressed in terms of a two-parameter lognormal cumulative distribution function of shaking intensity. The parameters are obtained for each country or a region by minimizing the residual error in hindcasting the total shaking-related deaths from earthquakes recorded between 1973 and 2007. A new global regionalization scheme is used to combine the fatality data across different countries with similar vulnerability traits.

Simulating soil stiffness degradation in transient site response predictions

A series of nonlinear seismic ground response analyses are conducted to investigate the effect of soil degradation on the seismic response of typical sedimentary deposits in Southern California. For this purpose, three downhole array station sites at the Los Angeles basin are subjected to broadband synthetic ground motion excitations. A characteristic strain measure, defined as the averaged maximum shear strain in the top 30 m of the soil profile ( γ 30 ), is proposed to enable the correlation between the ground shaking intensity level (here measured by means of the peak ground acceleration on the surface of rock-outcrop, PGA RO ) and the average deviation in ground motion response spectral predictions (denoted as e SA ) attributed to the effect of modulus degradation due to cyclic loading. It is shown that strong linear correlations exist among log ( PGA RO ) , log ( γ 30 ) and log ( e SA ) for the sites investigated. On the other hand, the number of loading cycles typically employed to indicate the importance of soil degradation is shown to play a much less important role. This becomes more apparent for near-field large amplitude ground motions characterized by a dominant pulse and overall low number of cycles, which nonetheless impose strains large enough to trigger modulus degradation effects. Based on this observation, it is suggested that the soil degradation effect in seismic ground motion is primarily controlled by the ground shaking intensity. The level of PGA RO beyond which soil degradation should be accounted for and incorporated in seismic ground response analysis is also depicted for the sites investigated.

The effect of joint ductility on the seismic fragility of a regular moment resisting steel frame designed to EC8 provisions

In order to evaluate the seismic reliability of moment resisting frames at performance levels associated with highly nonlinear structural responses, structural modelling of the rotational capacity of the joints needs to address potential failure modes. This paper begins by reviewing European experimental studies on the cyclic behaviour of steel joints. A linear regression equation, which relates the joint plastic rotation capacity to the beam depth, is proposed based on an analysis of the test data. Thus, the effect of joint ductility and failure on structural response is quantified, within a probabilistic context, through seismic fragility analysis of a mid-rise steel frame designed to Eurocode 8. The variability in structural demand is estimated at increasing ground shaking intensity levels, and fragility curves, conditional on a given ground motion record, are derived for two different performance levels. The inherent randomness in ground motion is taken into account by using an ensemble of recorded accelerograms. The effect of joint rotation capacity is noticeable in mean fragility curves corresponding to high seismic demand and response levels.