**3.1 Spatial distribution of seismic activity**

The seismicity in this region is mainly concentrated at the western side of "tectonic bottle neck zoning " of Wudu, Wen County - Pingwu, Qingchuan, in the area of 103° ~ 105°E, which is a nearly S-N banded zone. The main seismogenic structures were the boundary faults in the north and south parts and the faults within the Minshan block. However, the earthquake frequency and maximum earthquake magnitude inside the fault block are higher than those along the Maqin-Lueyang fracture and the Longmen Mountain fault zone (Fig.10). Compared with the southern boundary zone, the seismic intensity in Maqin-Lueyang fracture is far larger (Table.3). The main causative faults inside the block triggered by a nearly E-W regional tectonic stress-strain field are:


### **3.2 Migration pattern of the large magnitude earthquake sources**

According to the historical seismic data (Table.1), the general pattern in this region is that the large magnitude seismicity migrates from the boundary faults to the triangularshaped fault block in northwestern Sichuan in nearly N-S direction, then migrates to the boundary faults. There are some differences between the migration patterns before 1900 and after 1900.


The seismicity in this region is mainly concentrated at the western side of "tectonic bottle neck zoning " of Wudu, Wen County - Pingwu, Qingchuan, in the area of 103° ~ 105°E, which is a nearly S-N banded zone. The main seismogenic structures were the boundary faults in the north and south parts and the faults within the Minshan block. However, the earthquake frequency and maximum earthquake magnitude inside the fault block are higher than those along the Maqin-Lueyang fracture and the Longmen Mountain fault zone (Fig.10). Compared with the southern boundary zone, the seismic intensity in Maqin-Lueyang fracture is far larger (Table.3). The main causative faults inside the block triggered

1. the Huya fault, striking nearly N-S, it is the source of reverse fault earthquakes;

**3.2 Migration pattern of the large magnitude earthquake sources** 

2. the Songpinggou fault, in NW-SE, it is the source of left-lateral strike-slip earthquakes ; 3. the Dongmengou fault in NEE-SWW, it is the source of right-lateral strike-slip

According to the historical seismic data (Table.1), the general pattern in this region is that the large magnitude seismicity migrates from the boundary faults to the triangularshaped fault block in northwestern Sichuan in nearly N-S direction, then migrates to the boundary faults. There are some differences between the migration patterns before 1900

1. The migration laws of strong seismic activities before 1900 is: earthquake in north boundary fault (186 BC, February 22, Wudu, Wen County magnitude 7.0 earthquake)→(to the south) earthquake inside the fault block (January 16, 1630, Songpan and Pingwu magnitude 6.7 earthquake)→(to the south) earthquake in the Longmen Mountain fault in the southeast boundary (April 21,1657, Wenchuan magnitude 6.5 earthquake) →(to north) earthquake inside the fault block (September 4,1713, Diexi magnitude 7.0 earthquake) →(to the north) earthquakes in the north boundary fault (June 19,1879, Wudu magnitude 6.0 earthquake, and July 1,1879, Wudu, Wen County magnitude 7.5 earthquake, and Zhouqu, Wudu magnitude 6.5 earthquake in 1881). Generally speaking, the seismic activity began in the north boundary fault, finally returned to north boundary fault again after the north-south round-trip before 1900. Seismic activities mainly occurred inside the fault block or along the north

2. The migration laws of strong seismic activities after 1900 is: earthquake inside the fault block(August 25, 1933, Diexi magnitude 7.5 earthquake) →(internal adjustment) earthquakes inside the fault block(two adjustable strong seismic activities: March 14, 1938, Zhenjiangguan magnitude 6.0 earthquake, and October 8,1941, Heishui magnitude 6.0 earthquake) →(to south)earthquake of Longmen Mountain in southeast fault block (February, 1958, Maoxian magnitude 6.2 earthquake)→(to north)earthquake inside fault block(November 9, 1960, Zhangla magnitude 6.7 earthquake)→(to south) earthquakes in southeast boundary fault block(February 24, 1970 Dayi magnitude 6.25 earthquake)→(to north)earthquakes inside fault block (August 11, 1973, Songpan, Huanglong magnitude 6.2 earthquake, and 1976 August 16~August 23 Songpan, Pingwu magnitude 7.2 swarm earthquake)→(to south)(internal adjustment)earthquake

**3.1 Spatial distribution of seismic activity** 

earthquakes.

and after 1900.

boundary fault.

by a nearly E-W regional tectonic stress-strain field are:

inside fault block(in the southwest area of Lianghekou in Xiao Jin), and this belongs to the adjustable phase of seismicity. Obviously, strong seismicity has presented the trend of migration to internal of fault block, and southward. Seismic activities mainly occurred inside the fault block or along the southeast boundary fault.

In summary, these migration patterns suggest that the triangular-shaped Minshan fault block is transferred along the deep fault zones on both sides of the boundary faults in an irregular process. As the northern boundary fault zone is the main impedance boundary for eastward movement, every cycle of eastward fault-seismic activity started at the northern zone, and then migrated into the fault block and towards the southeastern block boundary fault.

#### **3.3 Periodicity of large magnitude seismicity**

Analysis of the historical seismic data in this region shows that their seismicity has been alternating active and quiet periods since 1920 (Fig.11).

1924-1941 was the first active period in the 20th century, which lasted about 18 years; largest earthquake during this period with magnitude 7.5 occurred in Diexi on August 25, 1933. This high activity period was followed, by the first quiet period in the region, which lasted about 10 years(1941-1952).

From 1952-1978, the second active period occurred in the region, which lasted about 26 years; its strongest earthquakes are the 2 Songpan-Pingwu magnitude 7.2 earthquakes on August 16 and 23, 1976. This period was followed by the second quiet period in the region, which lasted about 10 years (1978-1989). From 1989 to the present, we experience the third active period, which may last until 2012(±8). Wenchuan earthquake magnitude 8.0 occurred during this period, whose epicenter is south boundary of the Minshan block (Table 3).

Fig. 11. M-t diagram of the strong earthquakes in northwestern Sichuan


Table 3. Location and period of occurrence of strong earthquakes in northwestern Sichuan

#### **3.4 Recurrence interval of strong earthquakes (Ms**≥ **7.0)**

Table.4 shows that the time interval between strong earthquakes in the region ( the recurrence interval) is shortening in a slightly exponential way.


Table 4. The recurrence interval between strong earthquakes (Ms≥7.0)

From this time series, it may be assumed that the time interval until the occurrence of the next over magnitude 7.0 earthquake is about 40 years. In fact, it only took 32 years from Pingwu earthquake to Wenchuan earthquake. The accelerating tendency of strong seismicity in the region is obviously closely related to the triangular tectonic system controlling strong earthquakes.

#### **3.5 Fractal characteristics of the seismic magnitude/frequency pattern**

Aki(1981)shows that the G-R relation(logN=a-bM, Gutenberg-Richter, 1954) is equivalent to the definition of fractal distribution( Chen Chunzai, 1997). D. L. Turcotte(1989) suggested that that there is a quantitative relationship between the value b and the fractal dimension, Df, from the point of seismic wave energy and seismic distance, that is Df=2b, and then revealed the fractal characteristics of the G-R relation, i.e. the Df stands for the frequency of the earthquake occurring. The equation of logN=a-bM show that, if the value of Df is smaller, the region is more prone to large magnitude earthquakes.

We conducted a special fractal study of the southern and middle part of the Longmen Mountain (I), the Minshan Block (II2), and the tectonic seismic belt of west Qinling (III). The results show that the seismicity fractal dimension (Df) of the zone (I) is the highest: 1.2105;

The Minshan block 7 4 Before 1900, twice

Table 3. Location and period of occurrence of strong earthquakes in northwestern Sichuan

Table.4 shows that the time interval between strong earthquakes in the region ( the

Diexi earthquake 7.0 September 4, 1713 1898 years

earthquake 7.5 July 1, 1879 166 years Diexi earthquake 7.5 August 25, 1933 54 years

earthquake 7.2 August 16~23, 1976 43 years

From this time series, it may be assumed that the time interval until the occurrence of the next over magnitude 7.0 earthquake is about 40 years. In fact, it only took 32 years from Pingwu earthquake to Wenchuan earthquake. The accelerating tendency of strong seismicity in the region is obviously closely related to the triangular tectonic system

Aki(1981)shows that the G-R relation(logN=a-bM, Gutenberg-Richter, 1954) is equivalent to the definition of fractal distribution( Chen Chunzai, 1997). D. L. Turcotte(1989) suggested that that there is a quantitative relationship between the value b and the fractal dimension, Df, from the point of seismic wave energy and seismic distance, that is Df=2b, and then revealed the fractal characteristics of the G-R relation, i.e. the Df stands for the frequency of the earthquake occurring. The equation of logN=a-bM show that, if the value of Df is

We conducted a special fractal study of the southern and middle part of the Longmen Mountain (I), the Minshan Block (II2), and the tectonic seismic belt of west Qinling (III). The results show that the seismicity fractal dimension (Df) of the zone (I) is the highest: 1.2105;

magnitude The time of occurrence

**3.4 Recurrence interval of strong earthquakes (Ms**≥ **7.0)** 

The names of earthquakes Earthquake

recurrence interval) is shortening in a slightly exponential way.

earthquake 7.0 February 22, 186 BC

Table 4. The recurrence interval between strong earthquakes (Ms≥7.0)

**3.5 Fractal characteristics of the seismic magnitude/frequency pattern** 

smaller, the region is more prone to large magnitude earthquakes.

6.0 and 6.9 Magnitude over 7.0

The times of earthquakes Occurring interval,

2 2 Before 1900, 4 times

3 0 Before 1900, once

After 1900,9 times

After 1900, twice

The time interval between two strong earthquakes

and times Magnitude between

The names of fault zones

The north boundary fault

The south boundary fault

Wudu, Wen County

Wudu, Wen County

Songpan, Pingwu

controlling strong earthquakes.

zone

zone

that (Df) value of the zone (II2) is the lowest: 0.913; and that the (Df) value of the zone (III) has an intermediate value: 0.9788. This suggest that, the magnitude level of seismicity of the Minshan block is the largest; and the south and intermediate section of Longmen Mountain has the lowest number of large magnitude earthquakes.

For the region as a whole, the fractal value (Df) is 1.0054, close to 1, meaning that the seismic activities of all magnitudes in the region were controlled by a scale-independent earthquake mechanism. This proves that an triangular block tectonic system controls the earthquakes in the region(Table 5).


Table 5. The results of fractal analysis of seismic activity in northwestern Sichuan
