Estimation Of Seismic Risk Research Articles

Visibility of fault strands in exploratory trenches and timing of rupture events

Many estimates of seismic risk depend crucially on how well the date of faulting events can be determined from the stratigraphic position of fault strands exposed in exploratory trenches. However, fault strands cut by trenches may seem to die out where they are in fact only poorly expressed, or they may actually die out. Such ambiguity can lead to misinterpretation of the time of the most recent displacement and also of the recurrence interval between faulting events. To investigate the frequency of occurrence and other characteristics of strands that falsely appear to die out or that actually do die out, more than 1200 fault strands were analyzed. For strands associated with individual faulting events, 45% could not be visibly traced in the trench walls to the ground surface that existed at the time of faulting; for reverse and strike-slip faults, the number exceeds 70%. Thus, any apparent upward termination of a fault strand that is thought to indicate the date of a faulting event requires critical examination and corroboration from other evidence. Nonvisibility (i.e., dieout upward, obscure segments, and dieout downward) is more common on strike-slip and reverse faults than on normal faults and is more common in soil horizonsmore » than in clay and gravel. Nonvisibility occurs on strands that have had large (>1m) as well as small displacements.« less

Tectonic synthesis and seismic risk along the Rio Grande de Santiago fault, in Jalisco, Mexico

Various kinds of geological evidence has recently been used to show the presence of a master fault along the course of the Rio Grande de Santiago (RGS). Triangulation data of small motions of survey monuments located in the neighbourhood of the Santa Rosa Dam for the period 1964–1981, show a pattern that closely matches the shape and orientation of the strain ellipsoid deduced previously from the regional structural framework. These and other observations on the mode of fracturing of both margins, and the instability of the wall rocks, indicate that the fault is active and therefore potentially dangerous to civil constructions. Petrologic parameters, sustained by major element chemical analysis and Strontium isotope data support the hypothesis of emplacement of nepheline bearing mildly alkaline basalts through fractures controlled by a complicated pattern of synthetic and antithetic faults, and a pull-apart basin development along the right lateral fault of the RGS. The area does not have a seismic history like the southern continental margin of Mexico (i.e. southern Jalisco, Guerrero, Michoacan, Oaxaca and Chiapas), yet in February 11, 1875, an earthquake of estimated magnitude 7.5, had its epicentre located in Zapopan, Jalisco, a village immediately adjacent to Guadalajara City, and only a few kilometres to the southeast of the study area. After that the region has remained relatively free of seismic activity. An important conclusion is that if the motion of two large blocks sliding past each other along the surface of the fault has been aseismic, it implies the possible accumulation of stress at depth. Seismic risk estimation should therefore be considered at a yellow alert, based on the following features: It is recommended to establish a close survey of microseismic activity, a high precision triangulation and studies of paleoseismicity. A civil protection plan should therefore be carefully carried out to prevent a major disaster.

Gross design and construction errors, and seismic risk studies

The seismic risk estimates and seismic margin assessment for nuclear power plants should consider the potential effects of design and construction errors. These errors generally modify the component responses and fragilities. The concern with design and construction errors is that they may simultaneously affect a large number of systems and structures and negate the possible benefits of redundancy. A methodology for treating the effects of gross errors in seismic PRA and seismic margin studies consist of enumeration and classification of errors, sensitivity studies to identify significant errors, collection of data on errors and incorporation into the margin and risk estimation.

A Historical Perspective on Seismic Risk Assessment and Bayesian Updating

Drawing on an evaluation of past and current risks to light water power reactors arising from seismic design and construction errors, as well as the results of recent probabilistic risk assessments, a historical comparison is made between expert estimates of seismic risk and the implications of experience with past seismic deficiencies. A few methods of annual Bayesian updating of expert opinion on seismic risk as a function of new information are compared. The implications of the results include the suggestions that care be exercised in using a self-estimate of uncertainty in performing weighting and that the use of common information may lead consciously or unconsciously to a disadvantageous excessive weight placed on prevalent rather than innovative opinion.

Seismic Risk for the Largest Cities of the World; Intensity VIII or More

General methodology of probabilistic estimation of seismic risk, as a base for decision making, is described by an analysis of seismic risk for the cities with population ≥ 1 mln in active seismic regions of the world. The estimations are based on seismotectonic regionalization of the world and on the three models: of earthquake occurrence, strong motion (isoseists) and dynamics of population. The estimations are tested by comparison with actual earthquake history of the cities considered. The test is successful, in spite of the fact, that all these models are grossly averaged due to the usual incompleteness of the data. This shows, that available data may be sufficient to estimate the seismic risk for a large set of objects, while not for each separate object. During the 30 years, 1971-2000, 8 ± 3.5 cities and 40.1 ± 21.6 mlns of people in these cities will experience strong motions with macroseismic intensity ≥ VIII MM (Modified Mercalli Scale). Counted are only the events, when such shaking covers ≥ 100 km2, so that each event does spell disaster. The second estimation is large due to the explosion of urban population. More conservative data on the population growth reduce this estimation not more, than by 30 %. It indicates, that global seismic risk is rapidly increasing, presenting new unexplored problems.

Engineering applications of new probabilistic seismic ground-motion maps of Canada

New peak horizontal acceleration and velocity zoning maps with a probability of exceedance of 10% in 50 years and seven seismic zones are developed from new probabilistic strong seismic ground-motion estimates for replacement of the 1970 seismic zoning map in the National Building Code of Canada. The adoption of a probability of exceedance of 10% in 50 years produces reference seismic ground motion appropriate to the level of protection afforded by provisions of the current code; the use of two ground-motion parameters, the relative levels of which vary considerably throughout the country, provides independent reference levels for structures having short and long fundamental periods.For calculating seismic base shear, a new seismic response factor is derived in which seismic forces for long-period structures are directly proportional to zonal velocities, and for short-period structures proportional to zonal accelerations, with an upper limit on the acceleration/velocity ratio applicable for any location. To maintain the same design standard as provided by the current code, the base shear is calibrated to remain the same, on average, in large population centres in regions of moderate to high seismic risk. The resulting changes in the base shear applicable at various locations reflect the improved estimates of seismic risk, in particular the introduction of additional zones in the higher risk regions of the country and the higher levels of short-period ground motion estimated for some regions of eastern Canada.These and associated changes in seismic design provisions have been recommended for adoption in the 1985 edition of the National Building Code of Canada.

Relationships among the source parameters of earthquakes in Europe

There are many relationships describing the seismic regime, but not yet sufficient for the prediction of earthquakes and for the estimation of seismic risk. To obtain further information we must find other parameters describing the source properties of individual shocks, in addition to the geographical coordinates, the focal depth, the time of origin and the magnitude. Earthquake source parameters such as the seismic moment, the source dimension, the stress drop, the average of the relative displacement of the two sides of a fault, the seismic energy, etc. could serve as an important data base for describing the source and the seismic regime of the individual focal regions.

What are the main uncertainties in estimating earthquake risk?

The general heading of seismic risk covers a variety of problems, of which the following at least may be distinguished:
 
 risk analyses for a particular structure on a particular site; 
 the determination of large-scale zoning schemes; 
 microzoning; 
 risk analyses for earthquake insurance ; 
 risk analyses for civil defence and hazard reduction programmes. 
 
 This paper is concerned chiefly with the first of these, although a few remarks concerning the others appear in the final section. Its intention is to identify the major uncertainties which enter into the estimation of seismic risk (in the sense of 1. above) and to place an order of magnitude estimate on the errors each is likely to cause. Some earlier papers with a similar theme are by Cornell and Vanmarke (1969), Donovan and Bernstein (1978), McGuire and Shedlock (1981), McGuire and Barnhard (1981) among others. Evidently there is a considerable element of personal judgement in such an attempt; the principal aim is to draw attention to the effects of aspects which are difficult to bring into explicit consideration and as a consequence are in danger of being ignored or otherwise brushed under the carpet.

Seismic risk to pipelines with application to Northern Canada

Proposed northern pipeline projects necessitate the estimation of seismic risk in the Arctic. Such estimates are difficult to obtain due to the lack of a good instrumental or historical data base, and the sparsity of tectonic information.Seismic risk estimates are commonly based on the procedures proposed by Milne and Davenport or those described by Cornell. The methods are inherently similar and the choice between the two is largely a matter of convenience. A modified Milne and Davenport approach (based on the amplitude recurrence distribution) appears to offer advantages in Northern Canada since it avoids the seismological problems of defining tectonic zones and maximum magnitudes.Uncertainties in the model assumptions are described by a factor having a log-normal distribution that modifies the predicted acceleration. The variability of the distribution can be adjusted regionally to reflect the quality and extent of seismic information.Seismically induced hazards to pipelines include overstressing due to travelling waves, liquefaction of silt-laden soils (possibly resulting in landslides), and abrupt displacement due to fault crossings. The potential for seismically induced liquefaction and for overstressing can also be evaluated probabilistically using modified forms of the amplitude recurrence distribution.

Consequence of an earthquake prediction on statistical estimates of seismic risk

abstractThis paper formulates a procedure for estimating the consequence of an earthquake prediction on statistical estimates of seismic risk. Several examples, using this procedure, indicate the following general conclusions: if a prediction with a confidence level of 1.0 significantly affects the risk at a site, then the same prediction at smaller confidence levels will also affect the risk. If an earthquake is predicted in a high risk zone, it can be important for an entire adjacent low risk zone; a prediction for the adjacent low risk zone may not affect the high risk zone at all. Finally, a prediction for a region which may significantly affect the amplitudes of shaking that can be expected in the next year may have no importance for structures which are designed (or planned) with a 50 or more year lifetime, depending on the size of the predicted earthquakes.These conclusions are drawn from a hypothetical, yet plausible, case study in the Los Angeles, California, region, where hypothetical predictions are considered for events on the San Andreas fault, the Sierra Madre fault, and a region of about 4300 km2 covering the most populated region of the Los Angeles basin. For the background seismicity model used in this study, the threshold of a significant (to the seismic risk) prediction for 1-yr risk is M = 5 on the San Andreas fault, M = 412 on the Sierra Madre fault, and M = 4 in the Los Angeles zone. For the 50-yr risk, the thresholds of significance are M = 7, M = 612, and M = 6, respectively, for the same three zones.

Seismic gaps and recurrence periods of large earthquakes along the Mexican subduction zone: A reexamination

abstractSeismic gaps and recurrence periods of large, shallow interplate earthquakes along the Mexican subduction zone are reexamined after combining information from a catalog of nineteenth century's earthquakes, some relocated epicenters of the early part of this century, source parameters of recent large earthquakes, and redetermined magnitudes of great, shallow earthquakes of this century. Tehuantepec and Michoacan gaps have not experienced a large shock in this century and perhaps none in the past century; they are either aseismic or have anomalously large repeat times. Guerrero, Jalisco, and Ometepec regions presently appear to have a high seismic potential. Observed average repeat times of large earthquakes (Ms ≳ 7.4) in six regions (east, central, and west Oaxaca, San Marcos, Petatlan, and Colima) are between 32 to 56 yr. Data of this century indicate that the strain is released mostly in large events (Ms ≳ 7.4). A simple dislocation model with parameters obtained from the studies of recent earthquakes explains the observed recurrence periods quite well. The b value for this zone is not meaningful, an observation which is of significance for seismic risk estimation. Most of seismic moment (or, equivalently, seismic energy) release since 1800 appears to occur for 15 yr followed by relative quiescence in the next 15 yr.

A reply to “Comments on ‘Canadian methodologies of probabilistic seismic risk estimation’”

A reply to “Comments on ‘Canadian methodologies of probabilistic seismic risk estimation’”

Comments on “Canadian methodologies of probabilistic seismic risk estimation” by D. H. Weichert and W. G. Milne

Comments on “Canadian methodologies of probabilistic seismic risk estimation” by D. H. Weichert and W. G. Milne

A Comparison of Estimates of Seismic Risk in the Central United States

Abstract Using the maximum likelihood method and a simple least-squares fit relating magnitude and the number of earthquakes per year in the central Mississippi Valley, Nuttli obtained equations which predict that the average recurrence period for a New Madrid-size earthquake is 810 and 980 years, respectively. Using different variants of Gumbel’s theory of extreme events, the average recurrence period is found to be 1100 years (Type I) to 215,000 years (Type III based on recent data only). Conventional methods, including Gumbel’s Type I theory, which make no allowance for an upper limit on the size of expectable earthquakes may overpredict the hazard from earthquakes which are larger than any which have occurred in the historic past.

Regional assessment of seismic risk in eastern Canada

Earthquake recurrence data for eastern Canada are used to derive estimates of probabilistic seismic risk at the low levels desirable for critical structures. The historical earthquake data (1661 to 1975) are employed to produce a seismicity model consisting of six zones of earthquake occurrence in the continental region and an isolated zone on the Grand Banks. Magnitude recurrence relations of the form N ( ≧ M ) = N 5 e − β M are derived for each of the zones. N 5( N ≧ M 5) ranges from 0.0003 earthquakes per annum per 104 km2 in the general background zone to 0.08 in the confined Charlevoix Zone in the St. Lawrence Valley; β values range from 1.5 to 2.4. Seismic risk estimates (probability of exceedence of peak ground acceleration) are derived using the Cornell method of integrating risk contributions from the zones of earthquake occurrence. For sites within the larger zones of relatively low seismicity, e.g., Northern Appalachian Zone, with little influence from adjacent zones, the risk estimates near 10−3 per annum are relatively insensitive to the uncertainties in the magnitude recurrence parameters; the maximum risk contribution comes from the lower magnitude earthquakes at the near distances. For sites within the range of influence of highly active areas such as the Charlevoix Zone, peak ground motions with risk of exceedence near 10−3 per annum can vary by a factor of two depending on assumptions of the magnitude recurrence extrapolation and the maximum magnitudes. Constraints on the seismicity based on tentative geological correlations increase ground motion for sites near some active zones by factors of up to 1.5 at the same risk level compared to that derived from the model based only on historical earthquake distributions. With the present knowledge of tectonic processes in eastern Canada, the uncertainties in magnitude recurrence extrapolations and maximum magnitudes, the assumptions about the temporal and spatial stationarity of earthquake zones, and the uncertainties on near-field effects and attenuation, seismic risk estimation at probabilities below 10−3 per annum becomes increasingly deterministic.

On Canadian methodologies of probabilistic seismic risk estimation

abstract Three probabilistic methods for the estimation of seismic risk have been used in Canada. A reevaluation of the extreme value method shows no advantages over the average value method of Milne and Davenport. Conceptual improvements in the underlying assumptions of the latter method are a constrained release of historical earthquakes from their presumed epicenters and the averaging of earthquake rates over variable periods. Risk estimation can then proceed as suggested by Cornell. Comparison of the results of this modification of the average number method shows similar results as the Milne and Davenport average value method. The stability of risk estimates against new earthquakes is improved, but sensitivities at typical sites toward unavoidable deterministic elements in the model are similar to the older method. For certain site-source-seismicity combinations probabilistic estimates of ground motion could become almost quasi-deterministic.

Recurrence relations of Koyna and Bhadrachalam Aftershock activity

Estimates of seismic risk require a recurrence relation between the expected number of earthquakes per year and a measure of earthquake size. The recurrence relations of Koyna and Bhadrachalam after schocks are discussed in the light of low seismicity and poor geographic distribution of seismographic stations in south India. As the definition of an ‘active fault’ may vary according to the type of land use contemplated, it is emphasized that multi instrument micro-earthquake investigations should be carried out in order to take into account even the smallest earthquakes and know not only which faults may move but how they may move.

Earthquake risk prediction as a stochastic process

The extrapolation in time of an earthquake sequence considered as a multidimensional stochastic point process is discussed. Estimates of seismic risk for both long- and short-term predictions are considered and an algorithm for the calculations is proposed. Several examples of short-term extrapolations are carried out by means of Monte Carlo simulations of the process. An assessment of the predictability of the seismic process shows that the catalog of strong earthquakes ( M ⩾ 7.0) contains about 0.4 bits of information per earthquake for the particular model of the process applied here.

Attenuation of Intensities Based on Isoseismals of Earthquakes in Central U.S.

The spatial attenuation characteristics of seismic waves can be significantly different in different geographic regions. Attenuation of ground motion in regions other than the western United States has to be determined principally using the intensity observations because of the paucity of instrumental strong-motion data. Isoseismals of ten earthquakes of epicentral intensity, Io equal to or greater than VI on the Modified Mercalli (MM) scale, all occurring in the central United states have been investigated. By measuring the areas occupied by isoseismals of various intensities and applying the least squares method, the following attenuation relationship is obtained I ( R ) = Io + 2.35 - 0.0316 R - 1.79 log R ( for R > 20 km ) where I(R) is the site intensity at an epicentral distance, R. This relationship can be useful in providing estimates of maximum probable site intensity and seismic risk due to a large earthquake at a known distance from the site.