Fault zone structures of northern and southern portions of the main central fault generated by the 2008 Wenchuan earthquake using fault zone trapped waves

The rupture process of the May 12, 2008 Ms8.0 Wenchuan earthquake was very complex. To study the rupture zones generated by this earthquake, four dense temporary seismic arrays across the two surface breaking traces of the main-shock were deployed in July and recorded a great amount of aftershocks. This paper focuses on the data interpretation of two arrays across the central main fault, the northern array line 1 and southern array line 3. The fault zone trapped waves recorded by the two arrays were used to study the structure of the central main fault and the difference between the northern and southern portions. The results show that the widths of the rupture zone are about 170-200 m and 200-230 m for northern and southern portions respectively. And the corresponding dip angles are 80° and 70°. The seismic velocity inside the fracture zone is about one half of the host rock. By comparison, the northern portion of the rupture zone is slightly narrower and steeper than the southern portion. Besides these differences, one more interesting and important difference is the positions of the rupture zone with respect to surface breaking traces. At the northern portion, the rupture zone is centered at the surface breaking trace, while at the southern portion it is not but is shifted to the northwest. This difference reflects the difference of rupture behaviors between two portions of the central main fault. The width of the rupture zone is smaller than that of MS.1 Kunlun earthquake though these two earthquakes have almost the same magnitudes. Multiple ruptures may be one factor to cause the narrower rupture zone.

Fault zone trapped waves at Longmenshan fault belt

Trapped waves in different sections of Longmenshan fault belt were observed, and the results show the difference between the northern and southern portions of this fault belt. Guanzhuang and Leigu surveying lines are located at the northern portion of the fault belt, and the result indicates that the width of the rupture zone underground in this area is about 160 - 180 m. The center position of rupture zone underground corresponds to the surface breaking trace, and is equally distributed at the edges of the two fault walls. However, Hongkou surveying line is located at the southern portion of the fault belt, and the result indicates that the width of the rupture zone underground in this area is about 180 -200 m. The rupture zone underground is mainly distributed below fault scarp. The Wenchuan MsS. 0 earthquake and Lushan Ms7.0 earthquake both occurred at the Longmenshan fault belt. The results will provide information for the structure background of the two violent earthquakes.

SEM Numerical Simulation of Vertical and Inclined Fault Zone Trapped Waves and Comparison of Their Wave Fields

Fault zone trapped waves （FZTWs） mainly travel along the fractured fault zone （FZ） which is of low velocity and high attenuation. FZTWs often carry significant information about a fault＇s internal structure, so it is important to understand their wave field characteristics for FZ structure inversion. Most previous simulations are based on vertical faults, while in this paper we implement the FZTW simulations on vertical or inclined faults and compare their wave fields in both time and frequency domains. The results show that the existence of fault zone and inclined angle of fault can significantly influence the features of waves near faults. In amplitude, a fault zone can generate a larger amplitude of waves. The velocity contrast between two wails of fault may lead to amplification of amplitudes in the low velocity fault wall. In frequency, a fault zone tends to influence the waves in the low frequency range. In a pattern of particle polarization of FZTWs, it tends to be single direction for vertical faults but fork to multiple directions for inclined faults, which might provide a new way to study the fault zone with FZTWs. These conclusions may be valuable for FZ structure inversion, and will enhance the knowledge on near-fault strong ground motions.