Micro-earthquakes “Just-underneath” Seismic Stations as Ground Truth Events——Application to the 2008 Wenchuan Aftershock Sequence

Analyzing the aftershock sequence of the 2008 Wenchuan earthquake,we considered 26 micro-earthquakes "just underneath" seismic stations.Making use of such special station-event configurations to determine the depth of these micro-earthquakes provided accurate relocation of aftershocks with a reference set of "ground truth(GT)events".

Characteristics of the May 12,2008 Wenchuan M_S8.0 Earthquake Sequence and Discussion on Relevant Problems

The aftershock activity of the May 12, 2008 Wenchuan Ms8.0 Earthquake Sequence shows an obvious segmented feature. Most of the large aftershocks were distributed in the north and south parts of the aftershock zone. Thrusting was dominant with a small amount of strike-slip component in the south part. The aftershock activity decayed gradually, presenting the sequence features of a mainshock-aftershock pattern. The north part was the ending area of the malnshock fracture where strike-slipping was dominant, showing an obvious swarm feature. Therefore it became the major area for large aftershocks. The modulation of the earth tide on aftershock activity is remarkable; most large aftershocks occur during the period of flood and neap tide. The time period around 16：00 was the dominant occurring time for large aftershocks. The p-value, a parameter of modified Omori formula, increases gradually with time, and reaches about 1 at the end. Based on previous study, the sequence patterns, magnitude of maximum aftershock, as well as the duration of aftershock activity has been discussed. The primary results also show that the magnitude difference between the maiushock and the maximum aftershock is proportional to the rupture size of the maiushock for huge earthquakes of about Ms8.0. This means that when the magnitudes of the earthquakes are nearly the same, large rupture size corresponds to sufficient energy release.

The Focal Mechanism Solutions of the M_S 8.0 Wenchuan Earthquake in Sichuan on May 12,2008 and Some of Its Aftershocks

The focal mechanism solutions of the Wenchuan earthquake （Ms8.0） of May 12, 2008 and some of its aftershocks occurring up to December I0, 2008 are determined with lower semisphere of equal-projection and first motion sign data of P waves from regional and distant stations. The focal mechanism solutions of the Ms8.0 Wenchuan earthquake are： Nodal plane I：strike 5°, dip angle 48°, slip angle 39°; Nodal plane II： strike 247°, dip angle 62°, slip angle 131°; P axis azimuth 309°, plunge 8°, T axis azimuth 208°, plunge 54°, B axis azimuth 44°, plunge 35% Combining geological tectonics and spatial distribution of aftershocks, nodal plane II can be identified as a seismogenic fault. According to focal mechanism solutions, the fault activity that triggered the huge earthquake is reverse thrusting. The main rupture surface is S67°W, basically identical to the fault strike on which the earthquake occurred. The main compression stress P axis is N51°W, which is basically the same as the direction of the regional tectonic stress field. According to the results of focal mechanism solutions of aftershocks, the aftershocks occurring in the southern and northern sections of the Longmenshan fault zone have predominant orientations and are obviously different. For the main shock and the early aftershocks occurring on the southern section of the Longmenshan fault, the rupturing is mainly characterized by reverse-dip slip with some strike-slip, and over time, the aftershocks migrated towards the northern section. The rupturing in the source is mainly characterized by strike-slip with some reverse-dip slips. The stress field is controlled by the main shock stress field in the southern section of the Longmenshan tectonic zone, while it is controlled by the main shock stress field and regional stress field in the northern section of the Longmenshan tectonic zone.

Slope Seismic Response Monitoring on the Aftershocks of the Wenchuan Earthquake in the Mianzhu Section

Previous investigations have shown that the seismic response of slopes during the Wenchuan earthquake was highly variable. The present study tries to give an answer to the question： Which are the main factors affecting the seismic response degree of slopes？ With the support of the China Geological Survey Bureau, we set 3 monitoring sections in Jiulong slope, Mianzhu city, China with the aim to record the site response of the slope during the affershoeks of the Wenehuan earthquake. After the Wenchuan earthquake, which happened on 12 May 2008, 30 aftershocks have been recorded in these monitoring points. We analyzed 11 records, with magnitudes ranging from ML = 4.6 to ML = 3.1. The amplification factors of the horizontal compound PGA and 3D compound PGA have been determined for the 3 points at different elevations on the slope. Results showed that the dynamic response of the slope on the earthquake was controlled by factors such as topography and the thickness of the Quaternary overburden.

Corrections for the Location Errors of Wenchuan Aftershocks Recorded by the Nagqu and Hotan Arrays

We use the slowness-azimuth station correction( SASC) method to improve the location accuracy of the Wenchuan aftershocks recorded by the Nagqu and Hotan seismic arrays.The results show that the standard deviations of back-azimuth and slowness errors of Wenchuan aftershocks recorded by the Nagqu array decreased by 32% and 58%respectively after correction. The decrease is 38% and 71% for the Hotan array. After the correction,the location accuracy of all Wenchuan aftershocks recorded by the Nagqu array is improved. For the Hotan array,the accuracy is improved in the slowness estimation for 78% of aftershocks and in back-azimuth estimation for all aftershocks.

A Statistical Analysis on the Wenchuan Aftershock Activity Triggered by Earth Tide

A statistical analysis on the Wenchuan aftershock activity triggered by tidal forces is systematically studied based on Schusters test, including earthquakes triggered by tidal force, tidal stress and tidal coulomb failure stress. The results show that a group of strong aftershocks which occurred at the end of July to early August in 2008 at the north of Wenchuan were obviously triggered by earth tide, the same conclusion is drawn by Schusters smooth test of the tidal force, tidal stress and tidal coulomb failure stress. In addition, the Wenchuan aftershock activity is obviously triggered by fortnight tide. In the north, the aftershocks happened more frequently in the first and last quarters of the moon, and in the south, the aftershocks happened more frequently in the first and last quarters of the moon and during the full moon.

A Study on Tidal Force Triggering of the Wenchuan M_S8.0 Earthquake and Its Strong Aftershocks

By calculating the azimuth, phase angle and change rate of tidal force, and contrasting their rupture types, we find that in 6 of 11 reverse faulting earthquakes, the angles between the direction of the horizontal tidal force and compressive principal stress are within 33°, the vertical phase angles of all 8 strike-slip earthquakes are in the upward peak section and the vertical phase angles of two extensional normal faulting earthquakes are in the downward peak section. According to the above statistics, the mechanism of tidal force triggering of the Wenchnan Ms8. 0 Earthquake and its strong aftershocks is discussed.

Stress changes on major faults caused by 2013 Lushan earthquake and its relationship with 2008 Wenchuan earthquake

On April 20, 2013, an Ms7.0 earthquake occurred in Ya'an-Lushan region, Sichuan Province, China, killing and injuring morethan one thousand people. Therefore, it is critical to outline the areas with potential aftershocks before reconstruction andre-settlement as to avoid future disasters. Based on the elastic dislocation theory and multi-layered lithospheric model, we calculate the co-and post-seismic stress changes caused by the Wenchuan and Lushan earthquakes to discuss the relationshipbetween Mw7.9 Wenchuan earthquake and Ms7.0 Lushan earthquake, the influences on the distribution of aftershock caused bythe Lushan earthquake, and the stress changes on major faults in this region. It is shown that the Coulomb failure stress increment on the hypocenter of Lushan earthquake caused by the Wenchuan earthquake is about 0.0037-0.0113 MPa. And the possible maximum value (0.0113 MPa) is larger than the threshold of stress triggering. Therefore, the occurrence of Lushanearthquake is probably effectively promoted by the Wenchuan earthquake. The aftershock distribution is well explained by theco-seismic stress changes of Lushan earthquake. By the two ends of the rupture of Lushan earthquake with increased Coulombfailure stress, a lack of aftershock recordings indicates the high seismic hazard. The stress accumulation and correspondingseismic hazard on the Kangding-Dafu segment of the Xinshuihe fault, the Beichuan-Yingxiu fault, the Pengxian-Guanxianfault, and the Ya'an fault are further increased by the Lushan earthquake and post-seismic process of Wenchuan earthquake.