Ground Motion Hazard Research Articles

A procedure to develop a backbone ground-motion model: A case study for its implementation

The backbone modeling in ground-motion characterization (GMC) is a useful methodology to describe the epistemic uncertainty in median ground-motion predictions. The approach uses a backbone ground-motion model (GMM) and populates the GMC logic tree with the scaled and/or adjusted versions of the backbone GMM to capture the epistemic uncertainty in median ground motions. The scaling and/or adjustment should represent the specific features and uncertainties involved in source, path, and site effects at the target site. The identification of the backbone model requires different considerations specific to the nature of the ground-motion hazard problem. In this article, we present a scaled backbone modeling approach that considers the magnitude- and distance-scaling predictors as well as their correlation to address the epistemic uncertainty in median ground-motion predictions. This approach results in a trivariate normal distribution to fully define a range of epistemic uncertainty in a model sample space. The simultaneous consideration of magnitude and distance scaling while defining the epistemic uncertainty and the methodology followed for the simplified representation of trivariate normal distribution in ground-motion logic tree are the two important features in our procedure. We first present the proposed approach that is followed by a case study for Central and Eastern North America (CENA) stable continental region. The case study discusses the underlying assumptions and limitations of the proposed approach.

Site-Response Analysis Using the Shear-Wave Velocity Profile Correction Approach

ABSTRACT The traditional approach used to incorporate site response into the ground-motion hazard analysis is to compute a design spectrum for a rock-site condition and then propagate the rock motion from the base of the soil model to the surface. The main limitation with this approach is that it can be inconsistent with the ground-motion models (GMMs) used to develop the input rock motion. The VS profile implicit in the GMMs is unlikely to match the site-specific VS profile (value and gradient), because the GMMs were developed for ground motions from different VS profiles over large regions and are unlikely to match the profile of any one site well. This article presents the VS profile correction method for developing surface ground motions as an alternative to the soil-over-rock approach routinely used in earthquake engineering practice. This approach is similar to the standard soil-over-rock analysis, but uses different input motions and involves performing two site response analyses—one for the generic profile associated with the GMM(s) and one for the site-specific profile—then applying the ratio of the two site response analysis results to correct the design spectrum for the reference site condition developed using the GMMs. Two example applications are included to illustrate the VS profile correction methodology as well as some of the challenges that may arise when doing so.

Analysis of the risk-targeting approach to defining ground motion for seismic design: a case study of Iran

Modern seismic codes employed the uniform hazard basis for seismic design, which defines design ground motion with a fixed return period for different sites. Seismic design for uniform hazard ground motion does not lead to the goal of uniform structural safety. As a potential solution to address this problem, the risk-targeting approach has been considered in recent years. This study aims to investigate the changes applied by this approach to the current uniform hazard ground motions. For this purpose, hazard curves for Iran from the Earthquake Model of Middle East (EMME14) have been used. The risk-targeting approach has been performed in two cases, once considering GMs with a 475 year return period and then considering GMs with a 2475 year return period. For each case, a generic fragility function for buildings has been defined. A 1% probability of collapse in 50 years was selected as the target risk in both cases. For each case, the map of the distribution of the theoretical collapse risk is presented. It was discussed that by employing a generic fragility function, the risk-targeting could not guarantee to harmonize risk amongst the sites with different hazard levels, but it could have such an impact for the sites with the same design GM but the different slope of hazard curves. Finally, it was found that basing the seismic design on the 2% in 50 years GMs level leads to a more uniform collapse risk across the country.

Towards an Iranian national risk-targeted model for seismic hazard mapping

Ideally, hazard assessment for seismic design should quantitatively control the risk of exceeding different limit states (LS) over the structure's lifetime. In fact, the geographical variation of the ground motion hazard curves has an impact on the total seismic risk of code-conforming buildings. This paper examines the path towards a ground motion intensity measure (IM) reflecting a consistent seismic risk using research results referring to Iran, which is one of the most seismically active regions on Earth and has a high level of vulnerability and loss. Recently, risk-targeted (RT) seismic hazard maps have been developed in regions such as the United States and Europe as a whole. This paper prepares a new seismic hazard map for peak ground acceleration (PGA) and spectral acceleration, Sa(T), at periods of 0.2 s and 1.0 s, at the national level, based on the most recent and well-founded hazard curves. Result stability (validation) is emphasized through an extensive analysis of the risk-targeting approach applied to Iran. Subsequently, as the performance of the buildings designed according to the Iranian national design code (Standard No. 2800), is currently unknown, the RT hazard adjustment for the previous hazard maps with the uniform return period (RP) are highlighted innovatively providing a basis for choosing appropriate seismic hazard levels within the country. Also, the results of this study recommend that the mapped design ground motions be changed even within the same seismic zones of Standard No. 2800. It is emphasized the necessity of the development of an updated and more accurate hazard model for Iran (the work is in progress). Finally, the presence of aleatory uncertainty in the risk-targeted IM (IMR) is first illustrated in this paper. Examples equip the study with how the quantified uncertainty may be relevant to geographic locations.

Displacement hazard curves derived from slope-specific predictive models of earthquake-induced displacement

Probabilistic hazard curves for earthquake-induced permanent slope displacements incorporate the important variabilities associated with predictions of slope movements, such that a rational assessment of slope performance can be obtained. To date, displacements estimated from sliding block analyses predominantly have been used to compute displacement hazard curves, despite the fact that nonlinear finite element analysis is becoming the preferred method to evaluate the performance of slopes. This paper extends the probabilistic approach for use with displacements from finite element analyses. We develop slope-specific displacement prediction models using displacements computed by nonlinear finite element analyses. Various models are developed that utilize different ground motion intensity measures and each model is used to compute a displacement hazard curve for a site in Northern California. The computed hazard curves reveal that the different models generate a large range of epistemic uncertainty, and the model with the smallest standard deviation does not necessarily predict the smallest displacement hazard due to the influence of the shape of the ground motion hazard curve and of the displacement model. As a result, multiple displacement models that incorporate different intensity measures should be considered in any assessment of the seismic performance of slopes, in the same way that multiple ground motion models are incorporated into ground motion hazard analyses.

A new ground motion prediction model for Northeast India (NEI) shallow crustal

In this paper, a new ground-motion prediction model (GMPE) for North-eastern India (NEI) is developed based on the available recorded events. NEI has already faced several high magnitude earthquakes (moment magnitude [Mw ]>7.0) during the period 1200 to 2018. It is difficult to develop GMPE and estimating the seismic hazard of NEI, due to sparse recorded strong motion database. To date, seismic hazard maps for NEI have not been prepared. All developed countries have developed seismic hazard maps for their cities using region-specific GMPE. So, to prepare the seismic hazard maps of NEI, a ground motion prediction model will be required. The success of earthquake hazard mitigation depends to a large extent on how accurately the ground motion hazard can be estimated at a vulnerable site. The GMPE developed in this study has been compared with other ground-motion models as well as with the available limited recorded strong motion database. Based on the comparison, it is observed that the new model is reasonably correct as the model is unbiased with respect to both magnitude and hypocentral distance. This model needs to be upgraded as and when more recorded strong ground motion database is available for this region.

Seismic reliability of axially loaded vertical piles

The reliability of single vertical pile foundations subjected to seismic loads is assessed and compared with the minimum acceptable reliability level for static load conditions mandated by the Canadian codes. The analysis is executed for a site with a mean shear-wave velocity of the top 30 m of the ground equal to 250 m/s subjected to the ground motion hazard of five Canadian cities. Using both a full probabilistic analysis and simplified probabilistic model, the results seem to indicate that the current design practice is unable to achieve the reliability target of the codes. The shortfall is particularly significant when the limiting pile settlement is relatively small. The calculated reliability level of small limiting settlements is impacted by the geotechnical variability, whereas the seismic hazard variability affects large pile limiting settlements. Finally, the simplified probabilistic model produces the same results as the full probabilistic model for large pile settlement and is a convenient tool to execute code calibration.

Incorporating results from seismic microzonation into probabilistic seismic hazard analysis: An example in western Liguria (Italy)

This study presents a first attempt to couple two of the most important components of seismic risk mitigation strategies: probabilistic seismic hazard analysis (PSHA) and seismic microzonation. We show an application in the westernmost sector of the Liguria region in northwestern Italy, an area where strong earthquakes (up to about magnitude 6.5) occurred in the past producing hundreds of casualties. The final goal is to produce probabilistic seismic hazard maps for peak ground acceleration and spectral acceleration that incorporate site amplification (by amending existing ground motion attenuation models for rock conditions with an additional amplification term provided by microzonation studies) on a municipal scale. To this end, a partially non-ergodic approach is applied according to modern practice. Compared to conventional, ergodic hazard assessments for rock conditions and flat topography, such maps provide at a glance a finer picture of the actual hazard of the area under study, highlighting those sites where the ground motion hazard is dominated by local amplification effects. At these sites, the expected ground motions are up to twice the values resulting from the traditional, ergodic PSHA on rock. Besides hazard maps, results are also presented in terms of uniform hazard spectra for a number of target sites. Comparison with the isoseismal maps of past earthquakes have revealed a good agreement between the spatial distribution of the assessed hazard and that of felt intensities, with greater damage observed in areas presenting significant amplification effects and, consequently, a higher seismic hazard.

Simple GMPE for underground mines

A simple ground motion prediction equation (GMPE) is developed for peak ground velocity, PGV, and for cumulative absolute displacement, CAD, for underground mines. Assuming the ground velocity at source, $$\text{PGV}_{0}=0.63 v_{S} \Delta \epsilon$$ where $$v_{S}$$ is S-wave velocity and $$\Delta \epsilon$$ is the average strain change at seismic sources (Brune in J Geophys Res 75(26):4997–5009, 1970; Kanamori in Phys Earth Planet Inter 5:426–434, 1972), is independent of seismic potency, P, then $$\overline{\text{PGV}}\left( P,R\right) = \text{PGV}_{0} \cdot B$$ , where $$B = \left[ c_{L}P^{1/3}/\left( R+c_{L}P^{1/3}\right) \right] ^{c_{R}}$$ and R is distance. Assuming after Eshelby that at source $$\text{CAD}_{0} = q_{0}$$ $$\Delta \epsilon ^{2/3} P^{1/3}$$ , then $$\overline{\text{CAD}}\left( P,R\right) = \text{CAD}_{0} \cdot B$$ , where $$q_{0}=0.828494$$ . The S-wave velocity and the strain drop are strongly constrained by the type of rock and can be assumed, therefore both GMPE have only two parameters to be inverted from ground motion data: $$c_{L}$$ and $$c_{R}$$ . There is no provision made for site effect since in mines almost all sensors are placed in boreholes away from excavations. The basic outcome of ground motion hazard analysis for a given site is a seismic hazard curve that shows the annual rate, or probability, at which a specific ground motion level will be exceeded. It is expected that CAD that includes both the peak and the duration of ground motion may be a better indicator of damage potential than PGV alone, being a single measurement over the whole waveform. Two simple applications are presented. (1) A graphical trigger for damage inspection when the PGV predicted for an event at selected sites exceeds a predetermined level. (2) The cumulative CAD plot that may be a useful tool to monitor the consumption of the deformation capacity of the support due to seismicity.

Increases in Life-Safety Risks to Building Occupants from Induced Earthquakes in the Central United States

Earthquake occurrence rates in some parts of the Central United States have been elevated for a number of years; this increase has been widely attributed to deep wastewater injection associated with oil and gas activities. This induced seismicity has caused damage to buildings and infrastructure and substantial public concern. In March 2016, the U.S. Geological Survey (USGS) published its first earthquake ground motion hazard model that accounts for the elevated seismicity, producing a one-year forecast encompassing both induced and natural earthquakes. To assess the potential impact of the elevated seismicity on buildings and the public, this paper quantifies forecasted risks of (1) building collapse and (2) the falling of nonstructural building components by combining the 2016 USGS hazard model with fragility curves for generic modern code-compliant buildings. The assessment shows significant increases in both types of risk compared to that caused by noninduced earthquakes alone; the magnitude of the increases varies from a few times to more than 100 times, depending on location, building period (which is correlated to building height), alternatives for the hazard model, and type of risk of interest. For exploratory purposes only, we also estimate revised values of the risk-targeted ground motion that are currently used for designing buildings.

Seismic risk analysis for reinforced concrete structures with both random and parallelepiped convex variables

This article develops an improved seismic risk assessment formulation exhibiting both random and bounded uncertainties using a probability and parallelepiped convex set mixed model. Limit thresholds for different types of components are described via a probabilistic model. The distribution parameters of limit thresholds are originally treated by employing a multidimensional parallelepiped convex model, in which marginal intervals are utilised to represent scattering levels for the distribution parameters, while relevant angle are employed to express the correlation between uncertain distribution parameters. The structural responses, i.e., engineering demand parameters (EDPs), are considered as correlated random variables and are assumed to follow a multidimensional lognormal distribution. A performance limit state function, which allows considering the relationship between the EDPs and the corresponding limit thresholds, is employed to reflect the coexistence of both random and parallelepiped convex variables. The limit state function is mapped into the standard parameter space via a transformation technique. Then, the improved seismic risk formulation, characterised through a probability and parallelepiped convex mixed variables, can be derived with the combination of the seismic fragility function and the ground motion hazard curve. The main purpose is to illustrate that the performance limit states should be properly modeled as random and parallelepiped convex mixed variables rather than only random or deterministic quantities. A six-story reinforced concrete building designed according to Chinese codes are used to illustrate the proposed approach for constructing hazard curves. The interstory drift and the peak floor acceleration are the selected EDPs, calculated through incremental dynamic analysis. The results demonstrate that the calculated failure probabilities for different limit states in 50 years are found capable of meeting the requirements of Chinese seismic norms after the proposed seismic risk formulation is adopted.

Probabilistic seismic hazard assessment of Peshawar District, Pakistan

The seismic provisions for the Building Code of Pakistan were revised after the 2005 Kashmir earthquake and these have resulted in the introduction of a macrozonation ground motion hazard map in the seismic provisions. The macrozonation map proposes a peak ground acceleration (PGA) for the return period of 475 yr for Pakistan for flat rock sites. After the macrozonation, the next step is to develop the surface ground motion assessment studies for the cities, districts and tehsils of Pakistan. In this study, the probabilistic seismic hazard analysis (PSHA) approach is used for the Peshawar District. The PSHA, consistent with the classical Cornell approach, is carried out to obtain the seismic hazard curves and uniform hazard spectra of PGA values for the return periods of 150, 475, 975 and 2475 yr at a grid spacing of $$0.1{^{\circ }}\times 0.1{^{\circ }}$$ . The PGA for Peshawar at 150, 475, 975 and 2475 yr return period is estimated as 0.23, 0.34, 0.39 and 0.45g, respectively, for rock flat outcrop site conditions. The surface ground motion maps proposed in this study incorporate the local soil effects using amplification factors based on shear wave velocity obtained as a proxy to the topographic slope. The resultant ground surface hazard assessment proposes the PGA value of 0.63g for the return period of 475 yr and 0.89g for the return period of 2475 yr. The maps developed in the current study are important inputs for the structural designing, risk assessment and land use planning of the Peshawar District.

A New Formulation on Seismic Risk Assessment for Reinforced Concrete Structures with Both Random and Bounded Uncertainties

A new formulation on seismic risk assessment for structures with both random and uncertain-but-bounded variables is investigated in this paper. Limit thresholds are regarded as random variables. The median of random variables is described through an improved multidimensional parallelepiped (IMP) convex model, in which the uncertain domain of the dependent bounded variables can be explicitly expressed. The corresponding Engineering Demand Parameters are taken to be dependent and follow a multidimensional lognormal distribution. Through matrix transformation, a given performance function is transformed into the regularized one. An effective method based on active learning Kriging model (ALK) is introduced to approximate the performance function in the region of interest rather than in the overall uncertain space. Based on ALK model, the failure probabilities for different limit states are calculated by using Monte Carlo Simulation (MCS). Further, the failure probabilities for different limit states in 50 years can be obtained through coupling the seismic failure probability with the ground motion hazard curve. A six-story reinforced concrete building subjected to ground motions is investigated to the efficiency and accuracy of the proposed method. The interstory drift and the acceleration as two responses of the case study are, respectively, obtained by utilizing Incremental Dynamic Analysis and nonlinear history analysis.

Probabilistic seismic performance evaluation of SMA-braced steel frames considering SMA brace failure

This study explores seismic performance of steel frame buildings with SMA-based self-centering bracing systems using a probabilistic approach. The self-centering bracing system described in this study relies on superelastic response of large-diameter cables. The bracing systems is designed such that the SMA cables are always stressed in tension. A four-story steel frame building characterized until collapse in previous research is selected as a case-study building. The selected steel frame building is designed with SMA bracing systems considering various design parameters for SMA braces. Numerical models of these buildings are developed by taking into account the ultimate state of structural components and SMA braces as well as the effect of gravity frames on lateral load resistance. Nonlinear static analyses are conducted to assess the seismic characteristics of each frame and to examine the effect of SMA brace failure on the seismic load carrying capacity of SMA-braced frames. Incremental dynamic analyses (IDA) are performed to compute seismic response of the designed frames at various seismic intensity levels. The results of IDA are used to develop probabilistic seismic demand models for peak inter-story and residual inter-story drifts. Seismic demand hazard curves of peak and residual inter-story drifts are generated by convolving the ground motion hazard with the probabilistic seismic demand models. Results show that steel frames designed with SMA bracing systems provide considerably lower probability of reaching at a damage state level associated with residual drifts compared to a similarly designed steel moment resisting frame, especially for seismic events with high return periods. This indicates reduced risks for the demolition and collapse due to excessive residual drifts for SMA braced steel frames.

Incorporating the local faults effects in development of seismic ground-motion hazard mapping for the Peninsular Malaysia region

The probabilistic seismic ground-motion hazard maps for the Peninsular Malaysia region have been developed by the present study. Despite the earlier seismic hazard maps for this region, which were proposed only based on far-field Sumatran seismic sources, this study has attempted to present the new maps using the combination of the local faults within the region, Sumatran fault, and Sumatran subduction source zones. The composite earthquake catalogue of the study region has been extended to cover an area limited by 10°S–10°N Latitude and 90°–110°E Longitude with the period from 1900 to 2014. The seismic source zones were categorized into subduction and fault sources. Line, area, and background source models were used to model the seismic sources with variable characteristics along them. In order to consider the epistemic uncertainty, logic-tree framework was used to incorporate basic quantities such as different source modelling, maximum magnitudes, and ground-motion prediction equations (GMPEs). The hazard maps are presented over a 12.5 km grid in terms of peak ground acceleration (PGA) for 10 and 2% probabilities of exceedance in 50 years corresponding to 475 and 2475 years return periods (RP), respectively. The proposed new hazard maps give the expected ground motions based on the extended earthquake catalogue, upgraded seismic source parameters, and more compatible GMPEs than applied in the development of previous hazard maps. The estimated PGAs on rock site condition across the Peninsular Malaysia region for RP475 and 2475 years are in the range of 1.0–10.0%g and 2.0–20.0%g, respectively. The horizontal elastic acceleration response spectra on different soil site conditions have been also presented for the region of interest following the principles of Eurocode 8 through the calculated uniform hazard spectra.

Effects of Epistemic Uncertainty in Seismic Hazard Estimates on Building Portfolio Losses

In catastrophe risk modeling, a defensible estimation of impact severity and its likelihood of occurrence to a portfolio of assets can only be made through a rigorous treatment of uncertainty and the consideration of multiple alternative models. This approach, however, requires repeating lengthy calculations multiple times. To limit the demand on computational time and resources, a frequent practice in the industry is to estimate the distribution of earthquake-induced portfolio losses using a simulated catalog of events from a single representative mean ground motion hazard model for the region. This simplified approach is faster but may provide biased estimates of the likelihood of occurrence of the large and infrequent losses that drive many risk mitigation decisions. Investigation through case studies of different portfolios of assets located in the San Francisco Bay Region shows the potential for both a bias in the mean loss estimates and an underestimation of their central 70% interpercentile. We propose a simplified and computationally practical approach that reduces the bias in the mean portfolio loss estimates. This approach does not improve the estimate of the inter-percentile range, however, a quantity of no direct practical use.

Variabilities in probabilistic seismic hazard maps for natural and induced seismicity in the central and eastern United States

Probabilistic seismic hazard analysis (PSHA) characterizes ground-motion hazard from earthquakes. Typically, the time horizon of a PSHA forecast is long, but in response to induced seismicity related to hydrocarbon development, the USGS developed one-year PSHA models. In this paper, we present a display of the variability in USGS hazard curves due to epistemic uncertainty in its informed submodel using a simple bootstrapping approach. We find that variability is highest in low-seismicity areas. On the other hand, areas of high seismic hazard, such as the New Madrid seismic zone or Oklahoma, exhibit relatively lower variability simply because of more available data and a better understanding of the seismicity. Comparing areas of high hazard, New Madrid, which has a history of large naturally occurring earthquakes, has lower forecast variability than Oklahoma, where the hazard is driven mainly by suspected induced earthquakes since 2009. Overall, the mean hazard obtained from bootstrapping is close to the published model, and variability increased in the 2017 one-year model relative to the 2016 model. Comparing the relative variations caused by individual logic-tree branches, we find that the highest hazard variation (as measured by the 95% confidence interval of bootstrapping samples) in the final model is associated with different ground-motion models and maximum magnitudes used in the logic tree, while the variability due to the smoothing distance is minimal. It should be pointed out that this study is not looking at the uncertainty in the hazard in general, but only as it is represented in the USGS one-year models.

Seismic hazard assessment of northern Pakistan

This article presents probabilistic seismic hazard analyses of northern Pakistan region carried out to produce macro-seismic hazard maps for the region that define new regional ground motion design parameters for 95-, 475-, 975- and 2475-year return period earthquakes as regional contour maps and horizontal uniform hazard at important cities. The Cornell–McGuire approach (Cornell in Bull Seismol Soc Am 58(05):1583–1606, 1968; McGuire in FORTRAN computer program for seismic risk analysis. US Geological Survey, Open file Report, 76-6768, 1976) is used to carry out the analyses at 0.1° rectangular grid. The seismotectonic model of the region used in analysis consists of shallow and deep area zones differentiated based on the focal depths of the earthquakes. Earthquake catalogue compiled and used in the analysis is a composite catalogue composed of 19,373 events. Ground motion prediction equations (GMPEs) used are calibrated using goodness-of-fitness measures and visual inspection with local strong motion data. Epistemic uncertainty in the GMPEs is taken into account through the logic tree approach. Comparison of ground motions due to deep earthquakes is made for the first time for the region. The comparison between ground motion due to shallow and deep earthquakes indicates that the seismic hazard would be underestimated if the deep earthquakes are excluded. Ground motion values obtained in this study considering all the earthquakes suggest ground motions are dominant towards the north east of the region. The proposed study indicates that the ground motion hazard values suggested by the current Building Code of Pakistan underestimate the seismic hazard. Final results of this study are in close agreement with the recent studies on the region.

An operational-oriented approach to the assessment of low probability seismic ground motions for critical infrastructures

Extreme natural hazard events have the potential to cause significant disruption to critical infrastructure (CI) networks. Among them, earthquakes represent a major threat as sudden-onset events with limited, if any, capability of forecast, and high damage potential. In recent years, the increased exposure of interdependent systems has heightened concern, motivating the need for a framework for the management of these increased hazards. The seismic performance level and resilience of existing non-nuclear CIs can be analyzed by identifying the ground motion input values leading to failure of selected key elements. Main interest focuses on the ground motions exceeding the original design values, which should correspond to low probability occurrence. A seismic hazard methodology has been specifically developed to consider low-probability ground motions affecting elongated CI networks. The approach is based on Monte Carlo simulation, which allows for building long-duration synthetic earthquake catalogs to derive low-probability amplitudes. This approach does not affect the mean hazard values and allows obtaining a representation of maximum amplitudes that follow a general extreme-value distribution. This facilitates the analysis of the occurrence of extremes, i.e., very low probability of exceedance from unlikely combinations, for the development of, e.g., stress tests, among other applications. Following this methodology, extreme ground-motion scenarios have been developed for selected combinations of modeling inputs including seismic activity models (source model and magnitude-recurrence relationship), ground motion prediction equations (GMPE), hazard levels, and fractiles of extreme ground motion. The different results provide an overview of the effects of different hazard modeling inputs on the generated extreme motion hazard scenarios. This approach to seismic hazard is at the core of the risk analysis procedure developed and applied to European CI transport networks within the framework of the European-funded INFRARISK project. Such an operational seismic hazard framework can be used to provide insight in a timely manner to make informed risk management or regulating further decisions on the required level of detail or on the adoption of measures, the cost of which can be balanced against the benefits of the measures in question.

Assessing Potential Ground-Motion Hazards from Induced Earthquakes

AbstractEarthquakes induced by the hydraulic fracturing and fluid injections pose hazards to communities in the United States, Canada, as well as other places in the world. Although several types o...