SEISMIC RISK STUDY （English version） .doc

report onseismic risk study and definition of design spectrum for thermal power plant in coronel (punta puchoco and port sites)(rev. 1)table of contents1introduction32seismic threat study32.1micro zoning and seismicity42.1.1identification of seismic sources52.1.2micro zoning or discretization62.2probabilistic analysis model92.2.1occurrence modeling92.2.2frequency/magnitude ratio102.2.3spread and attenuation features122.2.4integrated probabilistic model132.2.5design philosoply132.3probabilistic analysis outcomes142.4historical seismicity / deterministic study163design spectrum definition203.1soil mechanics data203.2seismic design spectrum214bibliography25annex a seismic catalog, seismic catalog, geophysics department, partamento de geofsicauniversidad de chile.27annex b:seismic catalog, usgs31incostas chile s.a.rev.134 de 3327-03-06 1.introductioncolbn s.a. is currently studying the development of an investment to install a future conventional thermal power plant (ctco) that use the coal as the primary fuel for a generating capacity of approx. 350mw gross to supply energy to the chilean electric power grid (sic). colbn has established the plant location at baha de coronel, coast of the viii region.consequently, as part of the basic local condition studies necessary for the development of the engineering, colbn has commissioned incostas the preparation of the seismic risk study and the definition of a design spectrum.this report provides a seismic risk study valid for two sites in the area of baha de coronel, located some 30km to the south from concepcin. the first site is punta puchoco, located to a side of the current port “jureles”, at coordinates 37.07s and 73.17w. the other site is located in the same bay, at a distance of around 4-5km, in the sector of port coronel, at coordinates 37.03s and 73.14w.the study consists of two main parts. the first one covers the seismic threat study based on the maximum horizontal soil acceleration expectable in the site for a certain level of design defined in principle (probability of exceedance at a given time interval). the second part includes the design spectrum proposed on the basis of the soil properties. the study also provides the ways to modify the spectrum in accordance with features of energy dispersion (response modification coefficient, r) and absorption (x ) of the structures and equipment of the project.2.seismic threat studythe outcomes herein shown are based on data included in the catalog of the chilean seismic events occurred between 1570 and 2006, between latitudes 35 and 40s and meridians 70 and 7 5w.the earthquake catalog used in this work was obtained from the seismologic service of universidad de chile, supplemented with data from the united state geological survey (usgs), which are equivalent and supplementary. the catalog comprises all the events (earthquakes) occurred in the period and area shown, of magnitude = 5.it is interesting to note that the amount of earthquakes of a magnitude 5 recorded in the catalogs (dgf and usgs) is not significant. this fact makes the models appear with a relatively significant uncertainty.as early information, it should be noted that any seismic threat analysis must comply with the following stages: discover, based on the seismic event catalog, the types of earthquake- generating sources, specify a space distribution of potential areas for the generation of earthquakes, and evaluate the parameters characterizing the seismicity in each of them. define the probabilistic models considered to model the occurrence and relate frequency and magnitude of the earthquakes occurred in the region for each of the seismic sources identified. establish the way how the energy released by the earthquake is spread and attenuated (attenuation laws). estimate the parameters of interest (horizontal acceleration) for the study site, according to the exceedance probability defined in the design criteria.at the same time, it is always advisable to conduct a deterministic analysis based on the history seismicity of the region and an estimate of the expectable seismic magnitude levels along a period time comparable to the useful life of the project being considered. this would help estimate the design accelerations expectable at the project site.the previous points are developed below, providing the outcomes obtained for our study case.2.1micro zoning and seismicity:from the chilean seismicity study, we know in advance that the most important earthquakes take place around the seismogenic zone (benioff zone), both at the ocean plate subduction zone (nazca plate) under the south american plate (thrust or intraplate earthquakes), and within the subduction plate at an intermediate depth (intraplate earthquakes.)given the form of the subduction zone and the extent area in which take place the earthquakes considered for this study, it is necessary to develop the discretization for such area, so as to take into account the different distances of each subzone to the study site (any), and the different seismicity (earthquake occurrence) in each subzone.2.1.1 identification of seismic sources:from the observation of the seismic event catalog data used in this study, provided in annexes a and b, it was determined that the earthquakes in the region may be divided as originated in two different sources within the subduction mechanism: “thrust”-type earthquake source or “intermediate depth intraplate”-type earthquake source.a) location in depth and longitude (west) of all the catalog earthquakessismos en zona de intraplacasismos en zona thrust-73,00-72,50-72,00-71,50-71,00-70,50-70,000-20profundidad (km)-40-60-80-100-120profundidad (km)-140-160-180longitud oeste-75,5-75-74,5-74-73,5-73-72,50-10-20-30-40-50-60-70-80longitud oesteb) intermediate depth intraplate zone and its estimated slopec) “thrust” zone and its estimated slopefigure 2.1: determination of seismogenic sourcesfigure 2.1 a) shows all the catalog earthquakes depicted in depth as per their location in longitude (e w). figures 2.1 b) and c) show the same data, separated by source type, and for each of which, a slope has been estimated by lineal regression providing the estimated tilt of the fault zone in each. this estimate excludes some events that would not correspond to the already identified main seismogenic mechanisms and which, because of their reduced number, would have an insignificant influence on the outcomes.it should be noted that the catalog of events for this region does not show any representative events of the “cortical” type source, which are effectively noted in the central zone; consequently, this type of source has not been considered in the model.figure 2.1.a) shows the limit among the events generated by both sources considered as located in the longitude 72.75w. in addition, it will be considered that both source types roughly correspond to a zone extending between 35 to 40 of latitude south, horizontal between 75 and 72.75of longitude west (25-km mean depth) and then tilted in an approx. 24 angle in the “intermediate depth intraplate” zone.2.1.2 micro zoning or discretizationsince a wide area has been considered for the seismogenic sources, it is necessary to complete a discretization of each of the sources in subzones of about 25 x 25-km. figure 2.2 illustrates the scheme of the micro zoning being completed.figure 2.2: scheme of subduction fault zone (benioff) and its discretization in specific areas.for each discretization subzone, it is determined, from the history seismicity information, the annual occurrence rate (ni) for events of magnitude or = 5:n i =number of eventsoccurrence period (years) (2.1)this n value is assigned to the whole zone, proportionally to the discretization area.the distance to the site of interest (ri of figure 2.2) is estimated by considering the central point of each discretization subzone and that the average depth of each subzone is established from the already described source geometry (see trends in figure 2.1). finally, it is considered that each latitude and/or longitude degree is equivalent to 111km.figure 2.3 shows in plan the events considered with the micro-zoning completed, in addition to two cross-sections that better illustrate the shape and tilt of the fault zone or source.figure 2.3 : seismic events considered. location in plan and depth.to evaluate the seismicity as per the equation 2.1, it is first required to uniform the magnitude units of the earthquakes included in the catalog, as the latter lists the magnitudes for some events based on surface waves or richter magnitude (ms ) and for others, based on body waves (mb). this study will show the event magnitude in terms of the ms magnitude, being the remaining values converted in accordance with the ratio (2.2).m s = 1.1127 * mb - 0.789(2.2)the ratio (2.2) was obtained from a lineal regression on all the data for the catalog earthquakes that provided both magnitudes.the seismicity (ni) for each subzone is obtained from a seismicity value calculated for a larger area associated to individual seismogenic sources; in other words, for the area associated to the thrust-type fault zone and the area associated to the intraplate zone, a representative average earthquake occurrence rate (seismicity) is determined and then this value is used, for each source type, to calculate the seismicity associated to each microzone or grid used in the detailed calculation of probabilities.2.2probabilistic analysis modelbelow, you will find the probabilistic analysis models supporting the procedure used in this work.2.2.1 ocurrence modelingit is usually considered that the earthquake occurrence in a given region during a “t” time interval can be modeled by using the poisson distribution, as shown by the equation 2.3:p( n t(v * t) n= n) =*n!e ( -v *t )(2.3)where p(nt = n) is the probability of occurrence of “n” earthquakes during the time period of “t” years for the zone with an annual average occurrence rate “n”.in a poisson model, the occurrences are “statistically independent,” meaning that the earthquake occurrence probability does not depend on the previous occurrence of other earthquakes. this is not consistent with certain aspects of the seismological phenomenon, since there is in fact certain relationship among successive events resulting from the released energy and the plate accommodation. in spite of this model unaccuracy, it is usually accepted that the model provides outcomes adequate for the risk studies, as it properly represents the occurrence of significant magnitude events.the parameter characterizing the event occurrence frequency for each of the sources (ni) is determined from the catalog data, as explained in the final paragraphs of chapter 2.1.2. nevertheless, given the reduced catalog data amount, it has been deemed necessary to consider two modeling alternatives: the total events, without distinguishing the two source types, model #1 the events for each source considered on a separate basis, model #2the outcomes are shown in table (t2.1):table t2.1: annual average earthquake occurrence rate values (n)modeln1, no distinction of sources0.25372.1 thrust-type source0.3032.2 intraplate-type source0.26142.2.2 frequency/magnitude ratioto establish the relationship between the earthquake magnitude and the amount of events associated to each magnitude, the gutenberg/richter ratio has been used, with the following equation:log10 n (m ) = p - q * m ,(2.4)or:n (m ) = ea - b *m(2.5)where,m = richter magnituden(m)= number of events of magnitude m or higher for a time period.a = 2.3*pb = 2.3*qthe previous expressions require the determination of coefficients “p” and “q”, which are computed by adjusting the minimum squares over the universe of points log10n(m) vs. m, which are directly obtained from the seismic catalog (see figure 2.4).as already mentioned, within the whole region considered for the study, a distinction must be made between the “thrust”-type and the “intraplate”-type sources, each of which has a particular gutenberg/richter ratio (different “p” and “q”.)as in the case of the occurrence frequency parameter, the g/r ratio parameters are obtained for both models considered, i.e., without distinction among different sources and distinguishing between the thrust-type source and the intermediate depth intraplate-type source. this is the way how the calculation of “p” and “q” is completed for magnitudes m 5.0. one of the outcomes obtained is shown in figure 2.4:p = 4.72q = 0.4835figure 2.4: example of estimate. parameters defining the gutemberg/richter ratio (model #1)from the expression (2.5), we have the function describing the probability that the magnitude of a certain event be lower than a certain magnitude m, given by the equation (2 .6):f(m m ) = 1- e - b ( m - m 0 )(2.6)where,p(m = m) = probability that the earthquake magnitude will be = to a given m.m0 =minimum richter magnitude on which the statistical values are considered. = already explained in equation (2.5).2.2.3 spread and attenuation featuressome relevant seismic threat studies conducted in our country (see references 1 and 2) show that there are several empirical ratios to model the way of attenuating the energy released by the earthquakes based on the distance to the focus (reflected through the variation of any intensity parameter like, for example, the maximum horizontal acceleration value.) this work initially considers two attenuation ratios for the maximum horizontal acceleration of the soil (a), which are the following: martin attenuation ratio (1990):a(cm / s 2 ) = 71.3 * e0.83m * (r + 60)-1 .03(2.7) medina attenuation ratio (1998):a(cm / s2 ) = 1917 * e0.39 m * (r + 60) -1.12(2.8)where,r = distance between the site and the focus or hypocenter in (km)m = richter earthquake magnitudea = horizontal acceleration at a soil level, at the site in question in (cm/s2)these expressions are the maximum accelerations measured in sites characterized as “rock”.while the literature provides some attenuation expressions applicable to the softer soil case, particularly for the type of soil at the sites under study, as per their authors, the expression involves great uncertainty, as it is based in a very small amount of experiment data and must, therefore, be carefully used and only as a reference. in view of the above, it will not be considered to generate the risk analysis outcomes.the specialized literature (campos et al. 2002 6, campos and kausel 1990 7, kausel 1991 8, etc.) extensively discusses the applicability of the attenuation laws to the different types of seismic sources. it is accepted that the available attenuation laws are adequate for the events from the “thrust”-type sources; nevertheless, for the intermediate-depth intraplate-type events, we actually see maximum accelerations that are, in the average, 50% higher than the ones provided by the attenuation laws, so it is necessary to correct them.2.2.4 integrated probabilistic modelto date, we have explained the way of modeling the earthquake occurrence, the way the energy is spread and attenuated based on the soil acceleration, and the way to relate the richter magnitude with the occurrence density. all the above can now help combine the equations 2.2-2.8 so as to obtain an expression to calculate the probability that, for a given site and a certain time period, a certain horizontal soil acceleration value (a) may be exceeded. for example, the following equation shows the resulting expression considering the martin attenuation ratio (1990):k- b i 1* ln a( r + 60)1.03 - mi0 () 0.83 71.3 f amax a= 1 -i =1exp -v i * t * e(2.9)where,k = number of grids of the micro zoning = 25ni = average frequency of occurrence of events in the source corresponding to grid ib i = parameter of the recurrence ratio (gutenberg/richter) of the source corresponding to the grid i,t = time period for which you want to determine the probability2.2.5 design philosophy:it is common that the seismic design regulations specify the levels of earthquake “severity” that must be considered by the designer engineer, based on the exceedance probability shown in a give time period. the following earthquake intensity levels are typically established as follows: service earthquake:p(a maxa / 50 years) = 0.5an earthquake with an intensity such that it has a 50% exceedance probability in 50 years. design earthquake:p(a maxa / 50 years) = 0.1an earthquake wth an intensity such