Earthquake Reaearch in China  2017, Vol. 31 Issue (3): 414-421
Intensity-frequency Estimation Based on Historical Seismic Intensity in the Yunnan Area
Feng Wei1,2, Zhen Meng2, Liu Qin2     
1. Institute of Earthquake Science, CEA, Beijing 100036, China;
2. China Earthquake Networks Center, Beijing 100045, China
Abstract: For earthquake disaster mitigation, we use historical records and more complete intensity investigation data from 1500 to 2015 to analyze and estimate the seismic intensity and frequency of the earthquake-prone areas in Yunnan. We digitized intensity observations and divided the Yunnan region into cell size of 0.2°×0.2° to calculate the seismic intensity-frequency relationship for each cell. Combined with a repeated cycle of intensity of one hundred years and population economics data in Yunnan, we analyze future areas of concern. The results can provide a reference for earthquake hazardous area zoning. This method is based on historical earthquake data, reducing as much as possible the various hypotheses for the assessment, and thus can concisely reflect the different intensity-frequency distributions of the region.
Key words: Intensity-frequency     Seismic intensity     Yunnan area    

INTRODUCTION

China is an earthquake-prone country, where historical strong earthquakes have caused huge economic losses and casualties, and there are written records of seismic hazards in historical materials such as inscriptions and local chronicles in Chinese dynasties. Since the founding of the Peoples Republic of China, the earthquake administrations have attached great importance to the work of data accumulation. The Earthquake Catalogue incorporates not only three elements of earthquakes, but also earthquake disaster loss description and seismic intensity maps. Based on the information provided, Chen Kun et al. (2008) realized the implementation of GIS platform and calculated seismic intensity probability based on MapInfo.According to the location of earthquake epicenters, Liu Kai et al. (2008), estimated the average intensity distribution map of earthquakes in China and the maximum earthquake intensity distribution map by long-short axis attenuation relationship, and analyzed the spatial results. Using 500 years worth of historical earthquake intensity data in the Beijing-Tianjin-Tangshan region, Liu Jingwei et al. (2010) made a tentative analysis of seismic hazard and risk, provided the probability of exceedance in 50 years for specific intensity and compared the results with "Seismic Ground Motion Parameters Zonation Map of China".

In this article, based on historical seismic intensity-frequency relationships, historical documents of earthquake disasters and modern scientific survey data (the years of 1500-2000) in the Yunnan region (21°-29°N, 98°-106°E) are collected, ArcGIS is applied to the digitalization of intensity data, and data from two time spans, the years of 1500-2015 and 1900-2015 respectively, is used to calculate different intensity-frequency relations in 0.2°×0.2°latitude-longitude grids. In addition, in combination with repeated cycle intensity of one hundred years and population economics data in Yunnan, we analyze future areas concerned. The results of regional intensity-frequency estimation reflect the influence of distribution of geological structures and historical earthquakes, which can provide reference for seismic fortification and emergency rescue.

1 SEISMIC INTENSITY DATA PROCESSING AND DIGITALIZATION

Yunnan Province lies to the east of the collision zone of the Indian and Eurasian plates, with complex geological structure and intense tectonic movements, characterized by high-frequency, disastrous, widespread seismicity. From 1900-2015, approximately 340 MS≥5.0 destructive earthquakes occurred in the Yunnan region, including 259 earthquakes with magnitude of 5.0-5.9, an average of 2-4 earthquakes every year, 67 earthquakes with magnitude of 6.0-6.9 and 13 earthquakes with magnitude of 7.0-7.9, with an average of 1 earthquake every year. Since the 1970s, Yunnan Province has been hit with a total of 8 earthquakes with MS≥7.0, which caused 18, 317 fatalities, more than 50, 000 injuries and direct economic loss of more than 20 billion Yuan(RMB), It is one of the provinces that has suffered the most severe earthquake disaster from regular destructive earthquakes. Relevant seismic data obtained is abundant, and predecessors have carried out estimation of partial intensity distribution (Department of Earthquake Disaster Emergency Management, CEA, 2010). The earliest estimation of intensity distribution is based on the Jianzhou Earthquake in Yunnan Province on July 15, 1481. According to "Stories Witten in Shuyuan"Ⅶ: In the period from 07:00 p.m. to 09:00 p.m., an earthquake was felt in the Dali Government Office. People and objects were shaken, which stopped after the second quake. In the period from 09:00 p.m. to 11:00 p.m. at the same day, according to the Heqing military and civilian government office, the whole area was shaken by earthquakes until dawn, and more than 100 quakes were recorded, which stopped at noon the next day. Government office buildings and walls collapsed, killing more than 20 soldiers, civilians, prisoners and Yamen runners, and injuring many. Half of the village houses collapsed, and countless men and women were crushed to death. Previous studies have given an approximate intensity range. By reference to documentary records and contemporary real scientific observations, the seismic intensity data during 1500-2015 and 1900-2015 (the gray part in Fig. 1) is selected for further calculation.

Fig. 1 Statistics of seismic intensity data

ArcGIS is firstly used to digitalize historical earthquake data and resources in the Yunnan region (21°-29°N, 98°-106°E) to obtain the spatial distribution data of the earthquake affecting field. For an incomplete seismic intensity circle, we use the long-short axis relational expression (Li Shicheng et al., 2003) for seismic intensity in the Yunnan area to close the incomplete line elements, and using ArcGIS, the line elements are transformed into surface elements, which can be calculated with grid superposition.

The long axis:

$ {{I}_{b}}=4.1805+1.3543M-(1.6139+0.0173M)\rm{ lg }({{\mathit{{R}_a}}}+13), ~\sigma =0.5269 $ (1)

The short axis:

$ {{I}_{b}}=12.9240+1.2225M-(3.6095-0.1627M)\rm{ lg }({{\mathit{{R}_b}}}+36), ~\sigma =0.5927 $ (2)

The above research area is divided into 40×40 grids with a unit of 0.2°, and we number the grids from the left to right, from bottom to top, and count the number of seismic intensity scales in each grid. For earthquakes in recorded history, we count the number of quakes with seismic intensity of degree Ⅳ or above in each grid (Fig. 2). It can be seen that the number of quakes with seismic intensity over degree Ⅳ is as many as 45 in some areas in Yunnan (the year of 1500-2015). Areas with more abundant data are mainly concentrated in central Yunnan Province and along the Xiaojiang fault in the southeast of Kunming.

Fig. 2 Distribution of the number of seismic intensity data in the study area
2 SEISMIC INTENSITY-FREQUENCY ANALYSIS

We calculate the seismic intensity-frequency relationship of each grid based on a latitude-longitude grid, reflecting the probability of repeated intensity scales in each grid in a given period of time. Ross S. Stein et al. (Elliot et al., 2006; Ross et al., 2006; Kuei-Pao Chen et al., 2010; Bozkurt et al., 2007) used an algorithm similar to the Gutenberg Richter recurrence relationship to calculate the seismic intensity-frequency relationship in the Taiwan-Japan region, as part of a regional probabilistic seismic risk assessment. The linear relationship of seismic intensity-frequency logarithmic form is as follows:

$ 1g~f=a-b \times I $ (3)

Where, f denotes yearly probability of occurrence for seismic intensity greater than or equal to I, I seismic intensity, a, b the coefficients of linear relation, which is fitted with the least square method.

$ \prod = \frac{{\sum {{{({\rm{lg}}({\mathit{f}_{{\rm{observed}}}}) - {\rm{log}}({\mathit{f}_{{\rm{curve\;\; fit}}}}))}^2}} }}{N} $ (4)
$ {R^2} = 1.0 - \left[ {\frac{{\sum {{{({\rm{lg}}({\mathit{f}_{{\rm{observed}}}}) - {\rm{lg}}({\mathit{f}_{{\rm{curve \;\; fit}}}}))}^2}} }}{{\sum {{{({\rm{lg}}({\mathit{f}_{{\rm{observed}}}}) - {\rm{lg}}({\mathit{f}_{{\rm{curve\;\; fit}}}}))}^2}} }}} \right] $ (5)

Where, N denotes the number of seismic intensity data, R2 regression coefficient, ∏ the minimum variance, and f is the yearly frequency of a certain intensity scale. We get ∏ < 0.1 and R2>0.81, indicating higher intensity-frequency logarithmic correlation of each grid. The annual occurrence probability f of each grid is calculated using equation (3), and by reciprocal, we can estimate the probability for an earthquake occurrence in a given period of time and corresponding intensity of the average repetition cycle. We calculate the average repetition cycle intensity of one hundred years using seismic intensity data for the years 1500-2015 and 1900-2015 respectively, and find that high intensity distributions calculated by both are roughly matched. However, there are differences in intensity values in some regions.More precisely, results calculated from seismic intensity data from 1900-2015 exhibit higher intensity values in sub-regions such as Longling, Gengma in southwestern Yunnan, Zhaotong in northeastern Yunnan and Lijiang, Eryuan in the central west of Yunnan, when compared with that from seismic intensity data during 1500-2015. Considering that the lack of intensity records of small and moderate earthquakes in some areas during 1500-1900 would cause the change of parameters, thus affecting the calculation results, the intensity data is relatively complete during 1900-2015, thus the calculation results are more reliable, which can provide reference for earthquake hazard area zoning.

On the basis of intensity data from 1900-2015, we get repetition cycles (year) for seismic intensity of Ⅵ, Ⅶ and Ⅷ degree respectively in Kunming, Qujing, Yuxi and Dali (Table 1). At the same time, the distribution map of the average repetition cycle intensity of one hundred years (Fig. 4) and the distribution map of economy and population in Yunnan Province are drawn (Fig. 5). By comparing Fig. 4 and Fig. 5, it can be seen that the average repeated intensity of one hundred years is higher than degree Ⅷ for 9 grid units along the Zhaotong and Lianfeng fault zones in northeast Yunnan, which belong to a high intensity area. Kunming and Yuxi, to the south of Kunming, are economically developed, densely populated, with convenient transportation, and are important tourism and trade areas. Kunming, as the capital of Yunnan Province, is a state-list historical and cultural city and the core of the metropolitan region in central Yunnan. The above areas are located near the Xiaojiang fault zone and the Chuxiong-Tonghai fault zone, with a relatively higher intensity-frequency value, and the average repeated intensity of one hundred years is higher than degree Ⅶ for 21 grid units, showing a higher seismic risk, which are key areas deserving deep regard in future years. The Lijiang-Ninglang, Zhongdian, Chenghai and Honghe faults crisscross near Dali and Lijiang in west Yunnan, with average repeated intensity of one hundred years higher than degree Ⅶ for 12 grid units, and these grid areas cover many domestic tourist resorts, where there is a large floating migrant population, which also belongs to areas in need of attention.

Table 1 Repetition cycles for different seismic intensity in major cities

Fig. 3 The average repeated cycle intensity of one hundred years (the period of 1500-2015)

Fig. 4 The average repeated cycle intensity of one hundred years (the period of 1900-2015)

Fig. 5 Economy and population distribution in the study area
3 ANALYSIS AND DISCUSSION

In this article, based on historical seismic intensity-frequency relationships, we calculate various intensity-frequency relationships for 0.2°×0.2°grids in the Yunnan region (21°-29°N, 98°-106°E) using digitalized seismic hazard data. In combination with contrastive analysis of repetition cycle intensity for one hundred years and demographic and economic statistics in the Yunnan region, high intensity areas that need further attention are proposed. The importance of the research in this article is as follows:

(1) Seismic hazard data used in this article is of varied content and long duration time. Seismic hazard dataspanning 115 years is collected and adopted, incorporating most of the disastrous earthquakes recorded in the Yunnan region since 1900. Related data used in previous research is mostly up to the year of 2000, and we incorporate new earthquakes in the Yunnan region during 2000-2015 for calculation (for example: the Dayao MS6.2 earthquake in 2003, the Ninger MS6.4 earthquake in 2007, the Ludian MS6.5 earthquake in 2014 and the Jinggu MS6.6 earthquake in 2014).

(2) The superiority of the method used in this article lies in minimizing the assumptions of the assessment, incorporating elements such as site effect, building response and regional geographic information based on historical documents and actual investigation materials, and the regional intensity-frequency estimation results reflect the influence of distribution of geological structures and historical earthquakes. By comparison, the distribution of high intensity obtained from intensity data in different time spans varies, and the results obtained from the use of modern information with better completeness are more valuable.

(3) By combining repetition cycle intensity for one hundred years with populated and economic areas in the Yunnan region, we can carry out the study of disaster prediction for urban or regional engineering units, groups and systems that might suffer earthquake damage, and thus provide decision-making support information for earthquake fortification and post-earthquake emergency rescue force scheduling and relief distribution in areas concerned.

ACKNOWLEDGEMENT

Many thanks to associate research professor Yang Guiling from China Earthquake Networks Center, who helped significantly with digitization of seismic intensity data. The authors express their sincere gratitude to Zheng Chuan and Li Min from the Earthquake Administration of Yunnan Province, who provided basic geographic information in the Yunnan region. In addition, we are grateful to the peer reviewers for detailed reviews and valuable suggestions.

This paper has been published in Chinese in the Journal of Earthquake, Volume 36, Number 3, 2016.

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