The Influence Of The Wenchuan Ms8.0 Earthquake On The Occurrence Probability Of Future Earthquakes On Neighboring Faults

New evidence from recent studies supports the hypothesis that sudden change of Coulomb stress can influence the distribution and occurrence probability of future earthquakes. I utilize the rich geilogical data and earthquake catalogs to determine all parameters of the fault model and numerical simulation. They include the distribution of b-values, magnitude of completeness, the background seismicity rate, a value of Aσn (here A is a rate/state constitutive parameter andσn is the total normal stress) and the duration for the transient effect (ta). Then, I forecast the earthquake occurrence probabilities on neighboring faults after the Wenchuan earthquake.1) Based on the geometry and movement characteristics of the Wenchuan earthquake, the Wenchuan seismic source fault is devided into six sections: the Hongkou section in the southwest end of the Beichuan-yingxiu fault (BY1); Longmenshan town-Gaochuan section (BY2); Gaochuan-Leigu section (BY3); Leigu-nanba section (BY4) ; Qingchuan section to the south of Nanba (BY5), and the Pengguan fault. On the basis of field investigation, we determine the geometric parameters and co-seismic displacements of each fault section for the model. I then use field data of reference points to calculate the average co-seismic vertical and horizontal displacements of each fault section.2) I assume the Longquan Shan fault and the Huya fault are the receiving faults, and calculate the Coulomb stress changes at depth 7.5 km for the two faults, which corresponds to the medium depth of peak co-seismic displacements, using software Coulomb 3.1. The results indicate that the Longquan Shan fault is closer to failure, but the Huya fault is far away from the failure. The Coulomb stress is increased by 0.46-0.58 bar in the northern portion of the Longquan Shan fault due to the Wenchuan earthquake, and such a change declines gradually from north to south along the fault, and is nearly zero at the south end of the fault. In contrast, the Huya fault is in an inhibited state, on which the Coulomb stress is reduced by about 0.1 bar in the north, and along the fault the change of the inhibited Coulomb stress increases gradually from north to south, and is nearly 0.5 bar far away from the Coulomb failure in the south end of the fault.3) I utilize the epicenter distribution of M≥1.0 small earthquakes on the Longquan Shan and the Huya faults after the Wenchuan earthquake to study the correlation between the seismicity and the Coulomb stress change. The frequency of seismic activity one year after the Wenchuan earthquake is consistent with the distribution of the Coulomb stress change: the frequency of small quakes is largely increased in zone A, slightly increased in zone B, and remains almost unchanged in zone C. Also, such frequency appears to decline gradually from north to south along the fault. Furthermore, the greater Coulomb stress change, the more the change in the frequency of seismicity along the fault. The epicenter distribution of Ms≥1.0 small earthquakes on the Longquan Shan fault and the Huya fault after the Wenchuan earthquake is consistent with the distribution of the Coulomb stress change. This implies that the Wenchuan earthquake may have triggered small quakes on the Longquan Shan fault, but inhibited those on the Huya fault.4) I use the rate/state friction law to calculate the forecast 10a seismicity rate. The rusult is that the seismicity rate is growing in Longquan Shan fault, but falling in the Huya fault. I analyze the ratio of the expected seismicity rate R to the background rate r with time. The result indicates that it will take 400 years for the seismicity rate in the northern section of the Longquan Shan fault (zone A) to return to the background level, 300 years for the middle section (zone B), and almost 100 years for the southern section (zone C). Similarly, it will take 400 years for the seismicity rate in the northern section of the Huya fault (zone A) to return to the background lever, 600 years for the middle section (zone B), and almost 700 years for the southern section (zone C). This means that the larger the change of Coulomb stress on the neighboring faults, the more time needed for their seismicity to return to the background level. 5) I then calculate the occurrence probability of future earthquakes in the Longquan Shan and Huya study region for the next decade. The result is that the earthquake occurrence probability of the Longquan Shan region in Zone A is estimated to be 16-31% for 5.0≤Ms<6.0 shocks during the next decade, and 3-8% before the Wenchuan earthquake; the earthquake probability in Zone B of Longquan Shan region is 9-14% for 5.0≤Ms<6.0 shocks during the next 10 years, and 3-8% before the Wenchuan earthquake; the earthquake probability of Longquan Shan region in Zone C is estimated to be 4-8% for 5.0≤Ms<6.0 shocks during the next decade, and is the same as it before the Wenchuan earthquake. The earthquake occurrence probability in Zone A of the Huya region is estimated to be 4-7% for 5.0≤Ms<6.0 shocks during the next decade, and is 7-13% before the Wenchuan earthquake; the earthquake probability in Zone B of the Huya region is 4-9% for 5.0≤Ms<6.0 shocks during the next 10 years, and is 10-17% before the Wenchuan earthquake; the earthquake probability in Zone C of the Huya region is 2-6% for 5.0≤Ms<6.0 shocks during the next 10 years, and it is 9-20% before the Wenchuan earthquake. Above all, the occurrence probability of future earthquakes in the Longquan Shan grows from lower-lever probability events to middle-lever probability events, but the earthquake occurrence probability in the Huya region reduce from middle-lever probability events to lower-lever probability events.6) Combined with the aftershocks of the Wenchuan earthquake, I speculate the NW-directed Xiaoyudong fault as a independent rupture. Calculated the Coulomb stress change with different initial ruptures. One is that the southwestern section of the NE directed Beichuan-Yingxiu fault serves as the initial rupture, and the other is that the NW trending Xiaoyudong fault ruptures first. Under the first condition, most of the Xiaoyudong fault is inhibited by 1.5 bar Coulomb stress from failure, though its northwestern section is in the stress trigger region. If such a process was true, then the Xiaoyudong fault should had been stable during the main shock of the Wenchuan Mw7.9 event and no surface ruptures and small aftershocks would had occurred along this NW trending fault. Evidently this is in contrary to the fact. Under the second condition, the Xiaoyudong fault ruptures first, and then other faults fail successively. In response to the initial rupture on the Xiaoyudong fault, most of the southwestern section of the Beichuan-Yingxiu fault is in the stress trigger region of 1.0-1.5 bar, and the parallel Pengguan fault is also brought closer to failure, implying the whole source fault system starts to rupture toward northeast. This study suggests that the NW trending Xiaoyudong fault should be the initial rupturing member that triggered the NE directed Beichuan-Yingxiu fault and Pengguan fault to fail leading to the main shock.