Study on spatio-temporal characteristics of earthquakes in southwest China based on z-value

1 Introduction

In recent years, people have experienced many disasters [1–4] which are partly caused by human activities [5–8], while others have occurred naturally [9,10]. From these disasters, the high frequency of seismic activity has attracted extensive attention of researchers, which has set off a wave of research on seismic activity phenomena [11,12]. According to the relevant statistical data of the United States Geological Survey (USGS) [13] and China Seismic Network (CENC) [14], the harm brought by earthquakes to human beings cannot be underestimated. Compared with other natural disasters, the disaster caused by earthquakes is the most enormous direct threat to human life and property safety. Hence, the study of the law of seismicity is both important and urgent.

Seismicity is intense because China is located geologically on the collision and suture zone between the Indian and Pacific plates (on the Eurasian plate). In particular, southwest China is one of the most significant seismically active areas in mainland China. It is located in a number of seismic zones and is seismically active.

The study of seismicity focuses on the distribution characteristics of seismic activity in time, space, and intensity [15]. It is an important aspect for seismologists to study the temporal and spatial transmission characteristics of earthquakes based on certain statistical characteristics of seismic activity, such as spatial concentration, temporal compliance, and intensity proportionality, and it also plays a vital role in earthquake prevention [16]. As some scholars have pointed out, when we study the classification of seismic activities, we should first consider the intensity of seismic activity, study the main period of seismic activity, and the distribution relationship between local and global seismic activities. The Mediterranean–Himalayan seismic zone was seismically active in 1897–1916, 1934–1951, 1972–1979, and 1999–2002 [17]. Simultaneously, the segmented seismic activity of the Pacific seismic zone was also unusually significant [18], and a good, complementary relationship was formed with the active and inactive intervals of the Mediterranean–Himalayan zone, i.e., the seismically active periods of this seismic zone are 1980–1998, 1952–1971, and 1917–1933, which roughly correspond to the periods of relative calm of the Mediterranean–Himalayan zone [19].

Singh et al. [20] researched and discovered by the use of Anderson’s theory that for various tectonic regimes, the global seismicity trend for crustal earthquakes of less than 20 km depth suggests that the source fault inclination also plays a critical part in the propagation of the aftershocks in the subsurface rupture with respect to the mainshock earthquake. It has also been observed that majority of the compression earthquakes that display upward seismicity migration typically occur along steep faults. However, in the context of the Himalayan seismicity, the opposite migratory behaviour of the aftershock sequence is observed even along the gentle dipping source faulting. In order to study the ongoing tectonic process, Sharma et al. analyze crustal deformation at a selected transect in a part of western Himalayas as well as of the Indian plate through a GAMIT/GLOBK platform. Insignificant movement across the Himalayan Frontal Thrust indicates greater strain accumulation and the possibility of earthquakes in near future. Yongshuang et al. [22], based on previous studies, carried out field geological survey and remote sensing interpretation in the fault zone. In addition, geophysical surveys, trenching, and age-dating were conducted in the key parts to better understand the geometry, spatial distribution, and activity of the fault zone. Combining the spatial and temporal distribution of the historical earthquakes, the seismic hazard of the Xianshuihe fault zone has been predicted by using the relationship between the magnitude and the frequency of earthquakes caused by different fault segments. Console et al. [23] established a simulation algorithm to the Corinth Gulf Fault System, which has shown realistic features in time, space, and magnitude behaviour of the seismicity. These features include long-term periodicity of strong earthquakes, short-term clustering of both strong and smaller events, and a realistic earthquake magnitude distribution departing from the Gutenberg-Richter distribution in the higher magnitude range. According to the historical earthquake list (from 1973 to 2017), Li et al. [24] divided the data into five periods. Compared with the commonality and diversity of the spatiotemporal characteristics of seismic activity in different periods, it shows that there is a positive spatial autocorrelation in each period, which proves the spatial aggregation of the seismic activity in the Alpide Himalayan seismic belt. Nevertheless, over time, the spatial aggregation distribution gradually diminished from 1973 to 2017; from the Local Spatial Autocorrelation analysis, the spatial aggregation characteristics of seismic activity change with time and have a significant differentiation among the regions. According to the comparison of Local Spatial Autocorrelation characteristics in different periods, seismic activities in the Alpide Himalayan seismic belt were consistent with the periodic characteristics. Wang et al. [25] used three different methods to study China’s western Sichuan-Yunnan region of seismic risk and the results show that the multiple faults of the Sichuan-Yunnan region, including fracture, kerry fracture, Jiang nu river north of fracture, South river, Red river – pu ‘er breaking and Fresh water river fracture and connection of Ganzi-Yushu fault rupture, can produce all accumulated M > 7.5 earthquake seismic moment.

As the analysis of the time–space propagation characteristics of historical earthquakes plays a crucial role in earthquake prediction and z-value can be used to quantitatively study the characteristics of seismic activity as it accurately represents seismic activity visually, this study applies z-value of statistical geology to study the variation characteristics of seismic activity. In this context, this study mainly used earthquake statistics, based on the historical earthquake catalog, investigating the seismic activity from two aspects of time and space. Based on the target area, this paper analysis and researches this area's space-time distribution of seismic activity, making some beneficial discussion that works for protecting against and mitigating earthquake disasters.

2 Methods

The main theoretical founder of geostatistics is legal statistician Matheron G. Atheron. It is a subject developed for its geographical distribution and has been widely applied and developed in many fields such as geology, geophysics, soil, and ecology [26]. Using the spatial distribution characteristics of regional variation as the foundation, the variation function cannot only study the characteristics of spatial data and dependency, stochastic properties and structural characteristics, the pattern of the distribution and variation but also simulates the volatility and discreteness of various spatial data for the optimal unbiased interpolation estimates. [27]. Therefore, it is very effective to apply geostatistics to study the spatial and temporal characteristics of earthquakes. Based on this, we discuss the seismic rate with the help of geostatistics tool ArcGIS and seismic analysis tool ZMAP, and use z-value to study the spatial and temporal characteristics of earthquakes. The basic principle of z-value is as follows:

The average magnitude variation model can help understand the variation of the average in different periods of time in the study area by analysing the changes [28]:

where M ¯ is the average value of m ¯ j of all the average magnitude samples in the whole time period, which is a relatively stable amount, indicating the background characteristics of the area of study. m ¯ is the mean value of m ¯ j over the study period: σ M and σ m are the variances of the two samples.

Because z is the difference of the average value separated from the same parent, it is approximately subject to normal distribution, so it has a significant feature of normal distribution, i.e., when z = 1.64, the significance level was 90%; when z = 1.96, the significance level was 95%; and when z = 2.57, the significance level was 99%. From this, we can analyse the significant level of the calm characteristic reflected by the change of the average magnitude.

This method is based on the seismic activity change rate in a long period of time in a region, so as to analyse the seismic activity change rate in a certain period of time.

Assuming that the time series of the earthquake is { T 1 , T 2 , … , T n } , the seismic rate sequence is

R 1 is the average earthquake generating rate of the whole time period in the area of study, which is also the number of earthquakes occurring in a unit time period. R 2 is the average occurrence rate of earthquakes in a certain period of time (suppose W is the prediction time or window length, with T i − W ≤ R 2 < T i ); σ 1 and σ 2 are the standard deviation of seismic initiation rate sequence in two time periods. n 1 and n 2 are the number of samples in two time periods.

Then, the z-value parameter is defined as [29]

Evidently, when z = 0 , the background earthquake incidence is the same as the earthquake incidence; when z > 0 , the earthquake incidence in this period decreases; when z < 0 , the earthquake incidence rises, and the calm before a major earthquake can be measured by this.

The significance of z-value is the seismic rate. According to z-value, the variation of seismic rate in two different periods ( T 1 − T 2 and T 3 − T 4 ) in the selected area can be obtained. Where a negative value T 1 − T 2 < T 3 − T 4 indicates that the earthquake incidence is increasing, while a positive value T 1 − T 2 > T 3 − T 4 indicates that the earthquake incidence is gradually decreasing [30].

In the research on crustal tectonic activity, stress variation characteristics, and subsequent earthquake hazard prediction within a certain space and time range, it is usually necessary to gather statistics on the seismic energy released in the space–time domain of the area of study [31]. The most direct manifestation of seismic activity is the release of seismic energy. In other words, the energy released by the earthquake can directly indicate the strength of seismic activity in the region, but also implies the transmissibility of seismic energy, i.e., the subsequently large earthquake may be in the process of formation. In addition, the variation characteristics of energy release from seismic activities are used to explore earthquake prediction, which is an important application of study the law of seismic activity. Therefore, seismic energy is a very important factor for the study of seismic activity law.

The estimation formula of energy released by earthquakes [32] is

where M is the magnitude of a certain earthquake and c is a constant. Its value is obtained according to the previous studies and it is taken as 4.8 as an empirical value in this paper.

3 Results

Southwest China (including Sichuan, Yunnan, and Tibet) is located in the collision zone between the Indian plate and the Eurasian plate. This region has a complicated geological structure and is located at the intersection of the edges of several plates. It is often regarded as a special “extrusion structure” formed by the collision and extrusion between India and the Qinghai-Tibet Plateau. It borders Sichuan Basin, Yangtze block, and South China fold belt in the east and its interior is mainly composed of Songpan-Ganzi fold belt, Qiangtang plate, and the southeast branch of Lhasa block, and borders the India and Bangladesh plates in the southwest, and the Indochina plate in the south. There are several faults, or suture zones, developed around and within it. Due to its special geographical location and complex geological structure, earthquakes of magnitude 7.0 and above have occurred many times in history, such as, on May 12, 2008, the 8.0 magnitude earthquake occurred in Wenchuan, Sichuan, resulting in more than 70,000 deaths, millions of people displaced; on April 14, 2010, a 7.1-magnitude earthquake struck Yushu, Qinghai province, mercilessly killing hundreds of thousands of people. The earthquakes caused great loss to the safety of human life and property. However, earthquake predictions have not achieved satisfactory results until now and the pace of seismic activity has not slowed down. Owing to the frequent occurrence of strong and large earthquakes in southwest China, its seismicity has been paid much attention in seismological circles. Therefore, this paper focuses on the southwest region of China as the area of research and studies the temporal and spatial characteristics of recurring earthquakes.

Using CENC and USGS cite, this paper checks for China’s earthquake catalog, deletes duplicates, supplements the missing item, and reduces the size to the southwest China (21 degrees longitude and latitude range: latitude 37 degrees, longitude 78–110 degrees). From 1900 to 2013, all epicenters of an earthquake with a magnitude greater than 3.0 in southwest China are shown in Figure 1. This paper uses the K–K method to remove the handle of aftershocks and delete before and after the aftershock frequency from the USGS data shown in Figure 2.

Figure 1

Epicenter distribution of Ms greater than or equal to 3.0 in southwest China.

Figure 2

Histogram of earthquake frequency in southwest China from 1900 to 2013. (a) Before deletion of aftershock (b) after deletion of aftershock.

As the seismic activity is the result of the dynamic change of the lithospheric stress field, the earthquake is usually considered to be caused by the sudden fracture and dislocation of the rock mass in the earth due to tectonic stress. Therefore, to study the temporal and spatial distribution characteristics of the earthquake, it is necessary to use certain basic geographical background and geological structure background. Hence, this study collects auxiliary information for research [33]. Among them, the basic geographical background includes national boundaries, provincial boundaries, administrative divisions, and other layers. The background layer of geological structure related to seismic activity includes geological layer, fault, and other map layers. The spatial data required in this article are shown in Table 1.

During map projection, this study chooses the World Geodetic System 1984 (WGS 84) coordinate system as the unified coordinate system, the reference ellipsoid as WGS 84 geosphere ellipsoid, and the latitude and longitude as the unit, so as to facilitate data space superposition analysis and data management.

The software ArcGIS 10.0 provided by Esri was used in this paper to study the relationship between the distribution of seismic fault zones and seismic activity. In ArcGIS, fault zones are represented by lines, earthquake catalogs by points, and earthquake depth, size, energy, intensity, longitude, latitude, and other attributes are represented by the attribute values of points. Provincial and national boundaries are expressed as polygons. Other more complex geological phenomena are represented in the form of points and lines. According to many scholarly studies, the occurrence of earthquakes is not poison distribution but has a certain regularity. So, this article uses ArcGIS 10.0 statistics module and spatial analysis and studies the spread characteristics of the earthquakes in space and time, and the overlay and earthquake fault zone, the buffer analysis, and point density analysis. According to the results of the analysis, research division provides eight major seismic activity area, with the aid of ZMAP seismic analysis tools and the analysis of seismic migration of space and time characteristics. Therefore, the relationship between fault zone and seismic activity is analysed quantitatively and qualitatively, and the regularity between active fault zone and seismic activity is grasped.

Through observation, it is found that seismic activity is distributed in a zonal pattern. Those powerful earthquake with a magnitude greater than 7 in southwest China are all from new active faults. In order to facilitate further analysis related to the study of regional seismic activity, this study will use the following method, namely by combining the seismic activity with the main geological fault zone and according to the distribution of seismic point density, we will conduct regional research in the southwest region to study the characteristics of seismic activity. In the specific operation of regional division, we consider the following two aspects:

1. Generate administrative boundary data containing seismic data and the data provided by 1:400,000 basic geographic databases according to the southwest earthquake directory, and process and preserve the fault map data provided by 1:200,000 geological maps of China. It should be noted that the WGS 84 coordinate system and Mercator projection were used for all the data [34]. After the completion of projection, the spatial analysis module was used for density analysis in ArcMap and finally, the density distribution of seismic points was obtained.

2. Based on the distribution of seismic faults given by geological maps in China, ArcGIS was applied to buffer the eight fault zones of Kangding-Ganzi, Chengdu-Majian, Himalayas, Central Xizang, Tarim, Nujiang-Lancang, Zhendong and Jinshajiang-Yuanjiang, respectively, for 30–50 km, and point density analysis was performed for earthquakes with magnitudes greater than or equal to 3.0 in southwest China.

In this study, the density distribution of historical seismic points and the distribution of fault zones in southwest China are divided into eight concentrated earthquake zones, as shown in Figure 3. Among them, 1 is the Wudu-Mabian earthquake zone; 2 is the Himalayan seismic zone; 3 is the central seismic zone of Xizang; 4 is the Tarim seismic zone; 5 is the Nujiang-Lancang River seismic zone; 6 is the East seismic zone; 7 Kangding-Ganzi seismic zone; and 8 for Jinsha River–Yuanjiang seismic zone.

Figure 3

Division of major seismic belts in southwest China.

Using the data from 1900 to 2013, the eight divided seismic activity areas were analysed and buffer analysis was carried out on each fault zone. For all earthquakes greater than or equal to a magnitude of 3.0 to overlay, and all the earthquake as input elements, earthquake buffer for cutting elements, cut out fault zone of the buffer earthquake, a magnitude – sequence diagrams and frequency–time chart, the law of seismic activity is analysed. Through the open source Matlab seismic activity analysis toolbox ZMAP [31], the seismic activity of two periods in the study area was analysed, namely the z-value of the earthquake generating rate.

3.1 Wudu-Mabian

The Wudu-Mabian fault zone, also known as the Longmenshan fault zone, is a very dangerous fault zone. The Longmenshan tectonic zone is mainly composed of three major fault zones, namely the Wenchuan-Maowen fault zone, the Beichuan-Yingxiu fault zone, and the Anxian-Guanxian fault zone.

As shown in Figure 4, from 1900 to the end of 2013, earthquakes with a magnitude of over 7.0 occurred in 1933, 1955, 1970, 2008, and 2013, respectively. From the 6.8 magnitude earthquake in 1976 to the 8.0-magnitude Wenchuan earthquake in 2008, there were no earthquakes with a magnitude of more than 7.0 or even more than 5.0. During this period, the region must belong to the stage of stress accumulation. The stress accumulation reached a certain degree, but it was not released all the time, which eventually led to the 8.0-magnitude Wenchuan earthquake. As shown in Figure 5, the frequency statistical chart of Wudu-Mabian illustrates that the 8.0 magnitude Wenchuan earthquake in 2008 caused the highest frequency of earthquakes in this area since 1900.

Figure 4

M–T of Wudu-Mabian region.

Figure 5

N–T of Wudu-Mabian region.

The seismic directory of Wudu-Mabian area was introduced into the ZMAP toolbox, and the time of the 8.0 magnitude Wenchuan earthquake, which occurred on May 12, 2008, was taken as the dividing line while the seismic rates of Wudu-Mabian during the two periods from 2008 to 2013 and 1901 to 2008 were analysed, as shown in Figure 6. It can be found that the seismic rates along the Maoxian-Wenchuan fault zone and the Beichuan-Yingxiu fault zone increased after 2008, indicating that the seismic activity in this area has also increased. Before the 2008 earthquake, the seismic activity was mainly distributed in the northwest direction, on the fault zone, and the earthquake occurrence rate gradually decreased after 2008 towards the southwest and northeast migration, which entered the active seismic period.

Figure 6

Longmen mountain area (a) z-value; (b) incidence of earthquakes.

Within the buffer of the fault zone (with 50 km radius), this paper analyzes all the earthquakes with a magnitude greater than or equal to 5.5, calculates the strain energy change over time, makes the strain energy of time sequence and the M–T figure of the buffer zone (earthquake magnitude–time figure), and analyzes each partition’s seismic activity of small and strong earthquakes clusters pattern.

As shown in Figure 7, the period from 1970 to 2008 should belong to the stress accumulation stage, but it has not been released, so it finally leads to the occurrence of 8.0 magnitude Wenchuan earthquake, which is consistent with the conclusion drawn from the magnitude–time diagram. After the occurrence of a large earthquake, the crustal balance was destroyed, leading to the active period of seismic activity. The periods from 1955 to 1970 and 2008 to 2013 were the active periods of large earthquakes in Wudu-Mabian earthquake area, and the rest periods were the quiet period of large earthquakes.

Figure 7

Time series diagram of strain energy in Wudu-Mabian earthquake zone.

3.2 Jinsha River–Yuanjiang

The study area consists of two main fault zones, namely Delai-Dingqu fault zone and Jinshajiang fault zone. Jinshajiang fault zone belongs to the measured reverse fault with the North–Northwest/West trend and belongs to the right-aligned, strike-slip fault zone, which is a joint fault zone between the Qiangbei-Simao-Qamdo micro continental block and Songpan-Ganzi active fault zone. The Delai-Dingqu fault zone belongs to the torsional fault zone, which is dominated by basalt rocks and protrudes slightly like the east arc line. It belongs to the boundary fault zone between Yidun Belt and Zhongza block.

Figure 8 shows that from 1990 to 2013, there are only two earthquakes with a magnitude greater than 7.0 Richter scale (1908 and 1988). From 1989 to 2007, there is no earthquake with a magnitude greater than 6.5, which suggests that this period is the stress accumulation stage. In Figure 9, the frequency–time chart of the area is shown. The earthquake in 2007 marked the maximum number of earthquakes in the area. Taking the occurrence time of the 7.0-magnitude earthquake in 2007 as the limit, the results are shown in Figure 10. It can be seen from this figure that the seismic activities from 1900 to 2007 were concentrated in the central part, and the earthquakes from 2007 to 2013 shifted to the middle part of Delai-Dingqu and the northernmost end of the Jinshajiang fault zone.

Figure 8

M–T of Jinsha River–Yuanjiang.

Figure 9

N–T of Jinsha River–Yuanjiang.

Figure 10

Jinsha River and Delai-Dingqu area (a) z-value; (b) incidence of earthquakes.

The buffer zone (with a buffer radius of 50 km) was analysed to make the time series of strain energy in various seismic regions, as shown in Figure 11. The periods from 1908 to 1976, 1988 to 1993, and 2001 to 2013 were seismically active periods in the Jinshajiang and Delai-Dingqu seismic regions, and the rest periods were the calm period of major earthquake.

Figure 11

Strain energy time series of Jinsha River and Delai-Yuanjiang earthquake zone.

3.3 Kangding-Ganzi

Kangding-Ganzi seismic zone, also known as Xianshuihe seismic zone, is located in the transition zone between the Qinghai-Tibet Plateau and the mountainous western margin of Sichuan. Historically, the seismic activity of this fault zone is quite frequent; the damage intensity is very large; and the focal depth is basically within 20 km. According to the magnitude–time (Figure 12) of this area, since 1900, there have been a total of seven times of earthquakes with a magnitude greater than 7.0, which occurred in 1908, 1923, 1925, 1947, 1955, 1973, and 2013. As can be seen from Figure 13, the number of earthquakes in the area of study increased significantly from 1900 to 2013, and it was estimated that the area was in a period of in-situ stress activity.

Figure 12

M–T of the Kangding-Ganzi.

Figure 13

N–T of the Kangding-Ganzi.

With 1997 as the boundary, the z-value of earthquake recurrence rate from 1997–2013 and 1970–1997 are compared in Figure 14a and the change rate of seismic activity is shown spatially in Figure 14b. It can be seen that the fault zone in 1997–2013 shows a raise in the northwest and southeast branch and in 1970–1997 shows a reduction of the earthquake seismogenic rate in the middle section of the seismic belt.

Figure 14

Kangding-Ganzi Area. (a) z-value; (b) incidence of earthquakes.

A buffer zone (with a buffer radius of 50 km) was analysed for this fault zone, as shown in Figure 15. The regularities of the major earthquake activity period and the calm period in the Kangding-Ganzi earthquake area were not obvious.

Figure 15

Strain energy time series of Kangding-Ganzi earthquake zone.

3.4 The Nujiang-Lancang

The Nujiang-Lancang fault zone has small transverse and longitudinal faults on both sides of the whole territory. The topography is very complicated, and it is the suture zone between the Nianqing Tanggula plate and the Gangdis-Qinghai-Tibet Plateau. It belongs to the thrust fault zone, nappe tectonic zone, and faulted basin tectonic zone.

The M–T diagram of the Nujiang-Lancang River earthquake area is made, as shown in Figure 16. Since 1970, there has been one earthquake with a magnitude greater than 7.0, the 1988 Yunnan Lancang River earthquake with a magnitude of 7.4. As can be seen from the frequency–time in Figure 17, the years 1993, 2004, 2008, and 2013 are the earthquake frequent periods. The earthquake with a magnitude of 7.4 in 1988 was selected as the characteristic earthquake. As shown in Figure 18, the seismic activity before 1988 was in the northwest along the southeast direction, and then in the northwest and southeast direction.

Figure 16

M–T of Nujiang-Lancangjiang.

Figure 17

N–T of Nujiang-Lancangjiang.

Figure 18

Nujiang area (a) z-value; (b) incidence of earthquakes.

The buffer zone (with a buffer radius of 50 km) was analysed for the fault zone, as shown in Figure 19. The periods from 1908 to 1976, 1986 to 2001, and 2007 to 2013 were the activate period of major earthquake in the Nujiang seismic zone, and the rest periods were the calm period of major earthquake.

Figure 19

Strain energy time series of Nujiang area.

3.5 Tarim region

Tarim seismic zone is a complete, ancient, and solid block in Mainland China.

The M–T diagram of the Tarim Region is shown in Figure 20. There were three earthquakes with a magnitude of more than 6.5 from 1970 to 2013, namely two earthquakes with a magnitude of 6.7 in 1976 and one earthquake of a magnitude of 6.8 in 2008. No earthquake with a magnitude of more than 6.5 has occurred since 2008, and it is still in the state of stress accumulation. As shown in Figure 21 for frequency–time figure, the years 1990, 1997, 2001, 2004, and 2008 are taken as the seismically active period, where the 2008 earthquake recurrence rate is highest. Taking the 2008 earthquake that has a 6.8 magnitude as the characteristic earthquake, the seismogenic rate diagram is made, as shown in Figure 22. As can be gleaned from the figure, the seismicity before 2008 was in the southern middle section and it decreased in a small part of the earthquake. After 2008, the seismicity increased in the southwest direction.

Figure 20

M–T of Tarim.

Figure 21

N–T of Tarim.

Figure 22

Tarim area (a) z-value; (b) incidence of earthquakes.

A buffer zone (with a buffer radius of 50 km) was analysed for this fault zone, as shown in Figure 23. The seismic area of the Tarim earthquake was in an active period of major earthquakes most of the time.

Figure 23

Strain energy time series of Tarim earthquake zone.

3.6 Central Tibet

The activity and earthquake of Tibet are very strong and obvious. There are mainly Gondise fault block upwarping zone, Himalayan boundary upwarping zone, and Tibet active tectonic system type.

As shown in Figure 24, M–T in the central Tibet Earthquake region, five earthquakes with a magnitude of more than 6.5 have occurred since 1970: in 1975 (6.8), 2008 (6.9), 2005 (6.6), 2008 (6.7), and 2004 (6.6). No earthquake with a magnitude greater than 6.5 had occurred so far in 2008, which explained that this region was at the stress accumulation stage. As shown in Figure 25, the year 2008 was the time when earthquake seismogenesis rate reached the highest since 1970, so the M6.9 earthquake in 2008 was selected as the characteristic earthquake, then the z-value was calculated, and the seismogenesis rate chart was worked out, as shown in Figure 26. It can be seen that most areas were in the seismogenesis period after 2008.

Figure 24

M–T of central Tibet.

Figure 25

N–T of central Tibet.

Figure 26

Central Tibet area. (a) z-value; (b) incidence of earthquakes.

The seismic activity in the central earthquake region of Tibet is mainly controlled by the Tibetan active tectonic zone, followed by the Himalayan boundary upwarping zone, and the Gangdise fault block upwarping zone. The tectonic activity and earthquake are very strong and obvious, and the seismic activity in the central earthquake region of Tibet is in the active period of major earthquakes most of the time, as shown in Figure 27.

Figure 27

Strain energy time series of central Tibet earthquake zone.

3.7 The Himalayas

The Mediterranean–Himalayan seismic zone is also known as the Eurasian seismic zone, which is mainly distributed in Eurasia and has very active seismic activities. As shown in Figure 28, the eight 7.0 magnitude earthquakes were basically grouped and distributed symmetrically. In the study area, a fan shape is formed with the Nyalam 8.0-magnitude earthquake as the handle and the main compressive stress direction near the north and south as the axis. Earthquakes of magnitude 7.0 or less occur in the middle part of the fan, and earthquakes greater than magnitude 7.0 occur at the edges of the fan. Each time group is relatively concentrated and the previous period is mainly concentrated in the middle of the seismic zone.

Figure 28

M–T of the Himalayas.

The frequency–time diagram of this area is shown in Figure 29. The earthquake occurrence rate from 1970 to 2013 reached the highest in 2008. Taking 2008 as the characteristic time, as shown in Figure 30, only a small area in the middle of the country has seen a decrease in the earthquake occurrence rate after 2008 while most of the other areas are in an active period.

Figure 29

N–T of the Himalayas.

Figure 30

The Himalayas area. (a) z-value; (b) incidence of earthquakes.

The buffer zone (with a buffer radius of 50 km) was analysed, as shown in Figure 31. As the Indian ocean plate collides with the Eurasian plate from south to north, arching the Tibetan plateau, and the Indian plate advances northward at a rate of about two inches a year, the Himalayan region remains largely seismically active.

Figure 31

Strain energy time series of The Himalayas earthquake zone.

3.8 The Eastern Yunnan fault zone

As shown in Figure 32, there were three earthquakes with a magnitude greater than 7.0: in 1970 (magnitude 7.2), 1995 (magnitude 7.0), and 2013 (magnitude 7.0). The major earthquake in 1995 was caused by the sinistral dip-slip dislocation caused by a fault close to the north-south fault under NW principal stress. The seismogenic structure belongs to the near-south-north-trending Yimen fault.

Figure 32

M–T of the eastern Yunnan.

According to Figure 33, 2013 and 1995 are periods of frequent earthquakes. The year 2008 is shown as the characteristic time to make the seismic occurrence rate chart in Figure 34. It can be seen that the earthquakes in the period after 2008 mainly concentrated in the south-central and northernmost section of the Kang-Dian axis, namely the Purdue fault zone.

Figure 33

N–T of the eastern Yunnan.

Figure 34

The eastern Yunnan area (a) z-value; (b) incidence of earthquakes.

The buffer zone (with a buffer radius of 50 km) was analysed for this fault zone, as shown in Figure 35. The periods from 1936 to 1970 and 2008 to 2013 were active periods of large earthquakes in the Eastern Yunnan earthquake area, and the rest periods were quiet periods.

Figure 35

Strain energy time series of the eastern Yunnan earthquake zone.

The seismic activity characteristics of the southwest region can be obtained from the seismic generating rates of the above-mentioned eight major seismically active regions, as shown in Figure 36.

Figure 36

The characteristics of sub-area seismic activity in southwest China.

Based on the analysis of the buffer zones of the eight fault zones (with a buffer radius of 50 km), it can be seen that the increase in seismic frequency of the Jinshajiang-Yuanjiang seismic Nujiang seismic zones is consistent with that of the Eastern Yunnan seismic zone. The seismic frequencies of Kangding-Ganzi and Wudu-Mabian seismic zones are also in good agreement. The sudden increase of seismic frequency in central Tibet, the Himalayas, and the Tarim seismic belt also showed good coincidence. The reason may be that these regions are closely related in structure. When a major earthquake occurs in a region, the seismic waves of the region will radiate to other related or similar earthquake regions, so that these regions are also accompanied by the occurrence of earthquakes and the increase of earthquake frequency.

4 Discussion

In this chapter, ArcGIS spatial analysis module [35] is applied to divide the southwest region into eight major frequent seismic occurrence regions. The z-value of earthquake occurrence rate, earthquake time, and earthquake frequency in each sub-region were studied, and the propagation characteristics of earthquakes in time and space were analysed. The main conclusions were as follows:

1. The results show that the Indian ocean plate is bumping into the Eurasian plate from south to north, arching the Tibetan Plateau, and the Indian plate has been pushing north at a rate of about two inches per year [36]. The seismic activity in the central seismic region of Tibet is mainly controlled by the Tibetan active tectonic belt, followed by the Himalayan boundary upwarping zone, and the Gangdise fault block upwarping zone. The tectonic activity and earthquake are very strong and obvious. The Tarim–Alxa continental block subducted and collided to the north of the Qinghai-Tibet Plateau, and the mountains around the Tarim basin rose and moved strongly, which led to the active period in the Himalayan mountain earthquake zone, central Tibet, and most areas of the Tarim earthquake zone. Although the Wudu-Mabian area appears to have no strong tectonic activity, it may be in the process of stress accumulation which, unreleased to a certain extent, will cause the rupture of the earth’s crust, resulting in an earthquake. It can be seen that the Wenchuan earthquake stimulated the seismic zone, the seismic activity to the southwest direction and the northeast direction movement is more violent; Kangding-Ganzi is located in the transition zone between the western mountain range of Sichuan Basin and the Qinghai–Tibet Plateau. Seismic activity also increases from the middle to the northwest direction and southeast direction; the Nujiang–Lancang seismic belt is strengthened from the middle to the southwest and northeast. The Jinshajiang fault zone belongs to the measured reverse fault, and the Jinshajiang-Honghe fault zone belongs to the right-aligned, strike-slip fault zone. The seismic activity of the Jinshajiang-Yuanjiang earthquake zone shifted to the middle part of Delai-Dingqu and the northernmost part of the Jinshajiang fault zone. The eastern Yunnan earthquake zone is close to the south-north fault, which results in the sinistral dip-slip dislocation. The seismogenic structure belongs to the near-south-north-south Yimen fault. The seismic activity in this area is mainly concentrated in the south-central and northernmost section of the Kang-Dian axis, namely the Purdue fault zone. Simultaneously, it can be seen that due to the impact of the Wenchuan earthquake, the Wudu-Mabian earthquake area affects the surrounding earthquake area, so the seismic activity of the surrounding earthquake area increases towards the Wudu-Mabian earthquake area.

2. It is found that there is no significant characteristic of seismicity in the time of seismicity in each fault. However, further study shows that the seismicity of each fault has its own relative quiet period and relative active period [37].

3. Study the regional seismicity, analyse its spatial correlation (N–T map), and analyse the change of seismic activity and seismic fault zone over time. The geological tectonic background of the fault is related or the same, and the change of seismic activity over time will be consistent. The Jinshajiang-Yuanjiang and Nujiang earthquake areas are consistent with the seismic frequency surge in the Eastern Yunnan earthquake area. The Kangding-Ganzi seismic area is consistent with the Wudu-Mabian seismic area, and the central Tibet, the Himalayas and the Tarim seismic areas also have a good stability.

Due to the limitation of geophysics and geology, this article mainly uses the historical data, the application of the theory of nonlinear statistical method, and the macroeconomic research area for seismic analysis of the propagation characteristics of time and space to get preliminary results [38]. Further research direction should be combined with the characteristics of the structure and the media composition, strength, hardness, flexibility, electromagnetic properties and phase behaviour, along with the sun’s magnetic field and magnetic field interaction model [39] to analyse the seismic time–space propagation on clearer physical properties.

5 Conclusion

Based on historical earthquake catalog, this study mainly used earthquake statistics to investigate seismic activity from time and space aspects and analyse and research the southwest China area. Based on the study of earthquake activity, we make some beneficial discussion and contribute to protecting and mitigating earthquake disasters. From the results, we can find that the seismic rates along the Maoxian-Wenchuan fault zone and the Beichuan-Yingxiu fault zone increased after 2008, indicating that the seismic activity in this area increased. The periods from 1908 to 1976, 1988 to 1993, and 2001 to 2013 were the seismically active period in the Jinshajiang and Delai–Dinqou seismic regions, and the rest periods were the calm period of major earthquake. The regularities of the major earthquake activity period and the calm period in the Kangding-Ganzi earthquake area were not obvious. The periods from 1908 to 1976, 1986 to 2001, and 2007 to 2013 were the activate periods of major earthquake in the Nujiang seismic zone, and the rest periods were the calm period of major earthquake. The seismic area of the Tarim earthquake was in an active period of major earthquakes most of the time. The tectonic activity and earthquakes are very strong and obvious. The seismic activity in the central earthquake region of Tibet is in the active period of major earthquakes most of the time. As the Indian Ocean plate collides with the Eurasian plate from south to north, arching the Tibetan plateau, and the Indian plate advances northward at a rate of about two inches a year, the Himalayan region remains largely seismically active. The periods from 1936 to 1970 and 2008 to 2013 were active periods of large earthquakes in the eastern Eastern Yunnan earthquake area, and the rest periods were quiet periods.