Seismic Hazard Maps Research Articles

A probabilistic seismic hazard map of India and adjoining regions

This paper presents the results of an exercise carried out under GSHAP, over India and adjoining regions bound by 0°N-40°N and 65°E-100°E. A working catalogue of main shocks was prepared by merging the local catalogues with the NOAA catalogue, and removing duplicates, aftershocks and earthquakes without any magnitude. Eighty six potential seismic source zones were delineated based on the major tectonic features and seismicity trends. Using the probabilistic hazard assessment approach of McGuire, adopted by GSHAP, the Peak Ground Accelerations (PGA) were computed for 10% probability of exceedance in 50 years, at locations defined by a grid of 0.5° x 0.5°. Since no reliable estimates of attenuation values are available for the Indian region, the attenuation relation of Joyner and Boore (1981) was used. The PGA values over the grid points were contoured to obtain a seismic hazard map. The hazard map depicts that a majority of the Northern Indian plate boundary region and the Tibetan plateau region have hazard level of the order of 0.25 g with prominent highs of the order of 0.35-0.4 g in the seismically more active zones like the Burmese arc, Northeastern India and Hindukush region. In the Indian shield, the regional seismic hazard, covering a major area, is of the order of 0.05-0.1 g whereas some areas like Koyna depict hazard to the level of 0.2 g. The present map can be converted into a conventional seismic zoning map having four zones with zone factors of 0.1 g, 0.2 g, 0.3 g and 0.4 g respectively.

A Monte Carlo approach to seismic hazard analysis

AbstractWe have developed a Monte Carlo methodology for the estimation of seismic hazard at a site or across an area. This method uses a multitudinous resampling of an earthquake catalog, perhaps supplemented by parametric models, to construct synthetic earthquake catalogs and then to find earthquake ground motions from which the hazard values are found. Large earthquakes extrapolated from a Gutenberg-Richter recurrence relation and characteristic earthquakes can be included in the analysis. For the ground motion attenuation with distance, the method can use either a set of observed ground motion observations from which estimates are randomly selected, a table of ground motion values as a function of epicentral distance and magnitude, or a parametric ground motion attenuation relation. The method has been tested for sites in New England using an earthquake catalog for the northeastern United States and southeastern Canada, and it yields reasonable ground motions at standard seismic hazard values. This is true both when published ground motion attenuation relations and when a dataset of observed peak acceleration observations are used to compute the ground motion attenuation with distance. The hazard values depend to some extent on the duration of the synthetic catalog and the specific ground motion attenuation used, and the uncertainty in the ground motions increases with decreasing hazard probability. The program gives peak accelerations that are comparable to those of the 1996 U.S. national seismic hazard maps. The method can be adapted to compute seismic hazard for cases where there are temporal or spatial variations in earthquake occurrence rates or source parameters.

A simplified approach to the global seismic hazard assessment

Some earthquakes in recent years (such as Tangshan earthquake (1976, China), North Ridge earthquake (1994, USA), Kobe earthquake (1995, Japan)) did not occur in known faults; moreover, the detailed data of geology, earthquake, crust deformation, etc. needed by the fault model are very difficult to obtain in many regions of the world. A simplified method of global seismic hazard analysis is developed, based on an area source model and by using seismicity data, and the global seismic hazard map is compiled. To evaluate the effectiveness and credibility of the new method, comparison study of this map with the existing national maps has been performed, which implies that it is considerably valuable to apply this method to practical use.

Correlation of seismic intensity with Fourier acceleration spectra

We present a method for estimating the seismic intensity in terms of MMI or MSK scale using Fourier amplitude spectra of ground acceleration. The method implies that severity of earthquake ground motion is determined by spectral amplitudes in relatively narrow frequency band: so-called “representative frequencies”, at decreasing frequencies (from 7–8 Hz for small intensities to 0.7 – 1.0 Hz for MMI(MSK) = VIII–IX) with increasing intensity level. It is examined through estimation of probable intensity at a site using recordings of recent earthquakes in several seismic regions and prediction of intensity distribution patterns for some earthquakes. Seismic hazard maps, in terms of intensity levels based upon the proposed approach, should describe regional features of seismic waves excitation and propagation, as well as local ground conditions.

Seismic hazard map of the western hemisphere

Vulnerability to natural disasters increases with urbanization and development of associated support systems (reservoirs, power plants, etc.). Catastrophic earthquakes account for 60% of worldwide casualties associated with natural disasters. Economic damage from earthquakes is increasing, even in technologically advanced countries with some level of seismic zonation, as shown by the 1989 Loma Prieta, CA ($ 6 billion), 1994 Northridge, CA ($ 25 billion), and 1995 Kobe, Japan (> $ 100 billion) earthquakes. The growth of megacities in seismically active regions around the world often includes the construction of seismically unsafe buildings and infrastructures, due to an insufficient knowledge of existing seismic hazard. Minimization of the loss of life, property damage, and social and economic disruption due to earthquakes depends on reliable estimates of seismic hazard. National, state, and local governments, decision makers, engineers, planners, emergency response organizations, builders, universities, and the general public require seismic hazard estimates for land use planning, improved building design and construction (including adoption of building construction codes), emergency response preparedness plans, economic forecasts, housing and employment decisions, and many more types of risk mitigation. The seismic hazard map of the Americas is the concatenation of various national and regional maps, involving a suite of approaches. The combined maps and documentation provide a useful global seismic hazard framework and serve as a resource for any national or regional agency for further detailed studies applicable to their needs. This seismic hazard map depicts Peak Ground Acceleration (PGA) with a 10% chance of exceedance in 50 years for the western hemisphere. PGA, a short-period ground motion parameter that is proportional to force, is the most commonly mapped ground motion parameter because current building codes that include seismic provisions specify the horizontal force a building should be able to withstand during an earthquake. This seismic hazard map of the Americas depicts the likely level of short-period ground motion from earthquakes in a fifty-year window. Short-period ground motions effect short-period structures (e.g., one-to-two story buildings). The largest seismic hazard values in the western hemisphere generally occur in areas that have been, or are likely to be, the sites of the largest plate boundary earthquakes. Although the largest earthquakes ever recorded are the 1960 Chile and 1964 Alaska subduction zone earthquakes, the largest seismic hazard (PGA) value in the Americas is in Southern California (U.S.), along the San Andreas fault.

Seismic hazard map of North and Central America and the Caribbean

Minimization of the loss of life, property damage, and social and economic disruption due to earthquakes depends on reliable estimates of seismic hazard. National, state, and local governments, decision makers, engineers, planners, emergency response organizations, builders, universities, and the general public require seismic hazard estimates for land use planning, improved building design and construction (including adoption of building construction codes), emergency response preparedness plans, economic forecasts, housing and employment decisions, and many more types of risk mitigation. The seismic hazard map of North and Central America and the Caribbean is the concatenation of various national and regional maps, involving a suite of approaches. The combined maps and documentation provide a useful regional seismic hazard framework and serve as a resource for any national or regional agency for further detailed studies applicable to their needs. This seismic hazard map depicts Peak Ground Acceleration (PGA) with a 10% chance of exceedance in 50 years. PGA, a short-period ground motion parameter that is proportional to force, is the most commonly mapped ground motion parameter because current building codes that include seismic provisions specify the horizontal force a building should be able to withstand during an earthquake. This seismic hazard map of North and Central America and the Caribbean depicts the likely level of short-period ground motion from earthquakes in a fifty-year window. Short-period ground motions effect short-period structures (e.g., one-to-two story buildings). The highest seismic hazard values in the region generally occur in areas that have been, or are likely to be, the sites of the largest plate boundary earthquakes.

Seismic Hazard Maps of Japan and Computational Differences in Models and Parameters

To construct a seismic hazard map for intraplate earthquakes in Japan, historical records, paleoseismology data and a time-dependent conditional earthquake recurrence model were combined to crtate two types of contour maps: a probability map of peak ground acceleration (PGA) of 0.2 g or higher between 2001 AD and 2050 AD, and a PGA map of 10% probability during the same period. The resulting maps demonstrate the effectiveness of conditional seismic hazard analysis, although there are several uncertainties in the estimation of the slip rate, the elapsed time, and the segmentation of seismogenic fault systems. To create these maps, the historical seismicity rate for the last 400 years, and synthetic earthquake frequency from active fault data are first compared to examine the effect of uncertainties in fault segmentation and slip rate estimation. Then hazard maps based on time-dependent and time-independent models are derived. The results suggest that the conditional hazard map shows better agreement with current understanding of the recurrence behavior of active faults. For example, (1) low probabilities are obtained for faults that are considered to have ruptured within the historical period, and (2) higher probabilities are calculated for faults with long elapsed times or high slip rates. In addition, some seismogenic active fault systems are indicated as precautious faults based on the time-dependent earthquake recurrence model.

On the Correlation of Seismic Intensity with Fourier Amplitude Spectra

This paper presents a method for estimating the seismic intensity (MMI or MSK scale) using Fourier amplitude spectra of ground acceleration. The method implies that the severity of earthquake ground motion is determined by spectral amplitudes in a relatively narrow frequency band: so-called “representative frequencies”, at decreasing frequencies (from 7-8 Hz for small intensities to 0.7-1.0 Hz for MMI=VIII-IX) with increasing intensity level. It is examined through estimation of probable intensity at a site using recordings of recent earthquakes in several seismic regions, and prediction of intensity distribution patterns for the Coalinga, California earthquake of May 2, 1983, and the Spitak, Armenia earthquake of December 7, 1988. Seismic hazard maps, in terms of intensity levels based upon the proposed approach, should describe regional features of seismic waves excitation and propagation, as well as local ground conditions.

How Well Does the Spatial Distribution of Smaller Earthquakes Forecast the Locations of Larger Earthquakes in the Northeastern United States?

Since the processes that cause earthquakes in the eastern United States are poorly understood, it is not yet possible to predict where and when future earthquakes are likely to occur in this region. One approach to forecasting future locations of large earthquakes in this region is that used as part of the methodology for developing the latest generation of National Seismic Hazard Maps. This approach involves using the spatial distribution of small earthquakes ( m ≥ 3) to forecast the locations of large ( m ≥ 5) earthquakes. The hypothesis that underlies this approach is that the smaller earthquakes are delineating locations that are capable of generating larger earthquakes. We were curious to see how well such an approach would work for a scaled-down simulation in which the spatial distribution of even smaller earthquakes ( m ≥ 2) recorded by seismic networks in the northeastern United States and adjacent areas is used to “forecast” the locations of the larger earthquakes ( m ≥ 4) that have already occurred. We use such a scaled-down simulation to investigate the extent to which, in this situation, the spatial distribution of small earthquakes forecasts the locations of larger earthquakes in an intraplate environment. The essence of our procedure is to analyze systematically how often previously occurring small earthquakes occurred in the vicinity of large earthquakes. We devised three methods of summarizing and reporting whether small earthquakes tended to occur in the epicentral regions of future large earthquakes. We found that, on average, large earthquakes occurred in the vicinity of small earthquakes more frequently than would be expected for a random distribution of large earthquakes. There are, however, a significant number of large earthquakes that would not have been forecast based on the distribution of small earthquakes. This suggests that, if we are going to use this approach, we will have to be satisfied with a significant number of “misses” in our ability to forecast locations of large earthquakes. Perhaps this level of ability to forecast locations of large earthquakes is a realistic estimate of the level of success we can expect to achieve given the current state of knowledge of the mechanisms that cause earthquakes in this region.

Shear-Wave Velocity Characteristics of Geologic Units in California

Site conditions can be classified by the average shear-wave velocity to 30 meters (Vs30) and used for estimating site effects in seismic hazard calculations. Large scale seismic hazard maps, which include site effects, may be produced, providing Vs30 can be well correlated with geologic units. Vs30 values for several geologic units can be easily classified into soil profile types of the UBC (ICBO 1997). Most geologic units have wide variations in Vs30 and some extensive geologic units, such as older alluvium, the Franciscan Complex or the Puente Formation cannot be easily classified.

Seismic hazard maps of Italy

The Italian "Gruppo Nazionale per la Difesa dai Terremoti" has conducted a project in recent years for assessing seismic hazard in the national territory to be used as a basis for the revision of the current seismic zonation. In this project the data on the major earthquakes were reassessed and a new earthquake data file prepared. Definition of a seismotectonic model for the whole territory, based on a structural-kinematic analysis of Italy and the surrounding regions, led to the definition of 80 seismogenic zones, for which the geological and seismic characteristics were determined. Horizontal PGA and macroseismic intensity were used as seismicity parameters in the application of the Cornell probabilistic approach. The main aspects of the seismic hazard assessment are here described and the results obtained are presented and discussed. The maps prepared show the various aspects of seismic hazard which need to be considered for a global view of the problem. In particular, those with a 475-year return period, in agreement with the specifications of the new seismic Eurocode EC8, can be considered basic products for a revision of the present national seismic zonation.

Compatible ground-motion time histories for new national seismic hazard maps

Ground-motion time histories which are compatible with the uniform hazard spectra (UHS) provided by the new national seismic hazard maps of the Geological Survey of Canada (GSC) are simulated. Time histories are simulated for the following cities: Halifax, La Malbaie, Québec, Montreal, Ottawa, Toronto, Prince George, Tofino, Vancouver, and Victoria. The target UHS for the time history simulations are the GSC 5% damped horizontal-component spectra for "firm ground" (Class B) sites for an annual probability of 1/500. The Canadian Council on Earthquake Engineering is currently considering the adoption of these maps as the seismological basis for the earthquake design requirements for future editions of the National Building Code of Canada. It is therefore useful to have compatible time histories for these spectra, in order that dynamic analysis methods requiring the use of time histories can be employed. The simulated records provide a realistic representation of ground motion for the earthquake magnitudes and distances that contribute most strongly to hazard at the selected cities and probability level. For each selected city, two horizontal components are generated for a moderate earthquake nearby, and two horizontal components are generated for a larger earthquake farther away. These records match the short- and long-period ends of the target UHS, respectively. These simulations for local and regional crustal earthquakes are based on a point-source stochastic simulation procedure. For cities in British Columbia, records are also simulated for a scenario M8.5 earthquake on the Cascadia subduction zone, using a stochastic finite-fault simulation model. Four different rupture scenarios are considered. The ground motions for this scenario event are not associated with a specific probability level, but current information suggests that their probability of occurrence is comparable to that of the 1/500 UHS (the probabilistic analyses performed for the national hazard maps do not explicitly include the Cascadia subduction event). Thus it would be reasonable to conduct engineering analyses for cities in British Columbia using both the simulated crustal-event motions and the simulated Cascadia-event motions for the Cascadia event. The time histories simulated for this study are available free of charge to all interested parties.Key words: compatible time-histories, seismic hazard, ground motions.

The Spectral Source Model: A Tool for Deterministic and Probabilistic Seismic Hazard Assessment

Seismic-hazard assessment at small scales requires the computation of realistic broad-band accelerograms near the sources. Methods that use only natural records are often limited because of the limited data set. Among recently developed methods for source modelling, the spectral-source model, based on the k−2 dislocation distribution seems particularly promising as it enables computation of synthetics for any source-receiver distance and directivity configuration. The spectral model takes into account important source characteristics, such as stress drop and directivity effects predicting acceptable high-frequency levels, whatever the station location (Bernard et al., 1996b). Firstly, some tests to explain the directivity properties of the model are presented, followed by the main results of a deterministic study carried out for the 1992 Erzincan earthquake, Turkey. Finally, results of a probabilistic study in the Moyenne-Durance region (France) are used to demonstrate the ability of this method to produce seismic-hazard maps and spectra linked to a specific return period.

Seismic hazard assessment in Armenia

An evaluation of seismic hazard assessment in the territory of Armenia is presented in this work. The catastrophic M = 7.0 Spitak earthquake in 1988 revealed the drawbacks of the acting seismic zoning map. Two seismic hazard maps have been compiled in NSSP RA during 1991–1996. The deterministic approach was applied in the first one and the probabilistic in the second one. Both maps were compiled on the basis of the same database. Comparison between the maps shows good correlation.

Compatible ground-motion time histories for new national seismic hazard maps

Ground-motion time histories which are compatible with the uniform hazard spectra (UHS) provided by the new national seismic hazard maps of the Geological Survey of Canada (GSC) are simulated. Time histories are simulated for the following cities: Halifax, La Malbaie, Québec, Montreal, Ottawa, Toronto, Prince George, Tofino, Vancouver, and Victoria. The target UHS for the time history simulations are the GSC 5% damped horizontal-component spectra for "firm ground" (Class B) sites for an annual probability of 1/500. The Canadian Council on Earthquake Engineering is currently considering the adoption of these maps as the seismological basis for the earthquake design requirements for future editions of the National Building Code of Canada. It is therefore useful to have compatible time histories for these spectra, in order that dynamic analysis methods requiring the use of time histories can be employed. The simulated records provide a realistic representation of ground motion for the earthquake magnitudes and distances that contribute most strongly to hazard at the selected cities and probability level. For each selected city, two horizontal components are generated for a moderate earthquake nearby, and two horizontal components are generated for a larger earthquake farther away. These records match the short- and long-period ends of the target UHS, respectively. These simulations for local and regional crustal earthquakes are based on a point-source stochastic simulation procedure. For cities in British Columbia, records are also simulated for a scenario M8.5 earthquake on the Cascadia subduction zone, using a stochastic finite-fault simulation model. Four different rupture scenarios are considered. The ground motions for this scenario event are not associated with a specific probability level, but current information suggests that their probability of occurrence is comparable to that of the 1/500 UHS (the probabilistic analyses performed for the national hazard maps do not explicitly include the Cascadia subduction event). Thus it would be reasonable to conduct engineering analyses for cities in British Columbia using both the simulated crustal-event motions and the simulated Cascadia-event motions for the Cascadia event. The time histories simulated for this study are available free of charge to all interested parties.Key words: compatible time-histories, seismic hazard, ground motions.

Dating precariously balanced rocks in seismically active parts of California and Nevada

Precariously balanced boulders that could be knocked down by strong earthquake ground motion are found in some seismically active areas of southern California and Nevada. In this study we used two independent surface-exposure dating techniques—rock-varnish microlamination and cosmogenic 36 Cl dating methodologies—to estimate minimum- and maximum-limiting ages, respectively, of the precarious boulders and by inference the elapsed time since the sites were shaken down. The results of the exposure dating indicate that all of the precarious rocks are >10.5 ka and that some may be significantly older. At Victorville and Jacumba, California, these results show that the precarious rocks have not been knocked down for at least 10.5 k.y., a conclusion in apparent conflict with some commonly used probabilistic seismic hazard maps. At Yucca Mountain, Nevada, the ages of the precarious rocks are >10.5 to >27.0 ka, providing an independent measure of the minimum time elapsed since faulting occurred on the Solitario Canyon fault.

The Role of Earthquake Hazard Maps in Loss Estimation: A Study of the Northridge Earthquake

Local-scale seismic hazard maps are an important component of loss estimation because they provide information on possible site effects. This paper evaluates how well seismic hazard maps predicted damage in the Northridge earthquake. Normalizing for residential density, the research found that structures built on several geologic map units shown on pre-1994 hazard maps had significantly more damage than the area-wide average. Specifically, buildings on fine-grained Holocene alluvium, hillside bedrock, and “moderate” and “very high” liquefaction susceptibility zones were 1.5 to 2.5 times more likely to be damaged than the area average. Pipes were four times more likely to break in “very high” liquefaction susceptibility zones than in the area as a whole. The use of hazard maps in loss estimation can help state and local governments to set priorities in managing land use, enforcing building codes, conducting seismic strengthening programs for existing structures, and planning for emergency response and longterm recovery.

Benefits of scenario ground motion maps

Earthquake hazards can be communicated with both probabilistic seismic hazard maps and “deterministic” maps showing estimated ground motions expected from a subset of the possible earthquakes. This paper describes the latter class of maps as “scenario” ground motion maps. The principle behind the scenario maps is to portray a median estimate of ground motions from events that are reasonable, and regionally the most significant. The intent is that the scenario map answers the common question: “If the fault ruptures, what do you expect?” This paper proposes rules for construction of scenario maps, argues that a parallel map (a scenario probability map) should be constructed giving estimates of the annual recurrence rates of the scenario ground motions, and compares probabilistic and scenario ground motions for areas with high and moderate seismic hazards. Uses for scenario maps include educating the public about the earthquake hazards, some engineering applications and hazard response planning.

Probability-based seismic safety evaluation of existing buildings

A probabilistic second-moment seismic safety measure is proposed and the seismic safety of eleven existing buildings is evaluated. These buildings are analyzed using lumped-mass models and the seismic hazard at each building site is estimated by utilizing an available probability-based seismic hazard map. Results for the distribution of the safety measure with story level suggest that significant damage may be concentrated only in a few stories in some buildings. Second-moment seismic margin indices for some buildings are relatively high, while lower seismic margins result for others, perhaps due to underestimation of the ultimate limit state resistance. The feasibility of the procedure for evaluating seismic safety of existing buildings is demonstrated.

Seismic hazard in central southern Africa

SUMMARY Seismic hazard maps of central-southern Africa where hazard has been expressed in terms of peak ground acceleration for an annual probability in excess of show relatively high values that distinguish the seismic hazard potential of the Deka fault zone, the mid-Zambezi basin-Luangwa rift and western central Mozambique. In areas such as central-southern Africa where little is known about the geology of the region and the fault systems have not been fully mapped, seismic hazard potential may be estimated from seismicity and broad-scale fault features. For this region, such potential is based on earthquake magnitude M, 2 6. Events of such magnitude have recently occurred in the mid-Zambezi basin, southern Zimbabwe and western-central Mozambique. This paper follows the conventional probabilistic hazard analysis procedure, defining seismic source zones from seismicity based on instrumental records from a cataloque that spans a period of 83 years. Geological and geomorphological features in the region are described on the mesoscale and are correlated with the seismicity as broad fault zones. The scarcity of strong-motion accelerogram data necessitated the formulation of attenuation values based on random vibration theory