The Study On The Method Of Seismic Hazard Assessment Based On Historical Intensity Observations

The development of society and the seismic hazard characteristic in recent years show that it is necessary to further study the method of analyzing seismic hazard. Two approaches, probabilistic seismic hazard analysis (PSHA) and deterministic seismic hazard analysis (DSHA), are commonly used for seismic hazard assessment. The methods used in Seismic hazard mapping of China and United States, as well as the method used in Japan, were compared and discussed.In this dissertation, an alternative method, seismic hazard assessment based on historical intensity observations and statistics, was used. The key element of this method is to establish the frequency-intensity curve that describes the relationship between intensity and annual frequency (or return period) at a site. The main content and steps are showed as follows. Firstly, the historical intensity observations with higher precision were collected after testing the completeness of earthquake catalog and deleting the aftershocks. Second, the intensity database was established in geographic information system (GIS). Third, the statistical cells were divided also in GIS, and the size of cells was decided on the characteristic of seismicity, the coverage, geological characteristic and population density. Fourth, analyses on the digitized intensity records were performed to determine frequency-intensity relationship for each cell. The characteristic of seismic hazard was quantified with the intensity and return period in a site or an area. And finally, the seismic hazard was estimated by using the Poisson model.Considering the completeness of earthquake records, return periods of strong earthquakes and deleting the aftershocks, the database of intensity for North China was established. And the temporal and spatial characteristic of seismicity were analyzed.Two methods were used to adjust the parameters of frequency-intensity curves for the cells of scarce intensity observations. One was that the uniform slope of intensity attenuation was used in one seismic zone, and the other is to use the slope of intensity attenuation in the closest cell. By comparing the two kinds of results, the author thought that the first method was more reasonable and reliable.Moreover, The seismic hazard for North China were estimated from 246-year (1484～1730) intensity observations again. Obviously, the numbers of intensity observations of 246-year period are much less than those of 500-year period. Based on analyses of the completeness of intensity records, the reliability of results and the seismicity trend in the next 100 years, I found that the seismic hazard assessment from 500-year intensity observations are more reliable. So, I calculated the return periods of I≥Ⅶ,Ⅷ,Ⅶand seismic hazard in terms of the exceedance probability of I≥Ⅶ,Ⅷ,Ⅶin 50 years.The high intensity value in the cells that have rare observations does not significantly affect the reliability of cells. In addition, the method can be applied well to the areas that have ample intensity observations, and also fit to the geological tectonic area. For the areas that does not have so much strong intensity observations, the seismic hazard also can be assessed using intensity records, because the distribution of low intensities contributes to projection of the expected frequency of high intensities. I also estimated the corresponding intensity with 10 percent probability of exceedance in 50 years. The results showed that North China has high seismic hazard, which can be compared with the current design requirement for this area.The seismic seismic assessed using intensity observations is described by probability of exceedance, which shows the probability of a specified ground motion occurring at least once in a period. The intensity records reflect the influence from multiple factors, so in a certain extent, the usage of intensity records can reflect the seismic hazard in a site or an area. In summary, seismic hazard assessment based on intensity can be an alternative method, at least in the area that is sufficient of intensity records, and also can be used in engineering design.