The April 20th, 2013 Lushan M7.0 earthquake sequence occurred on the southern segment of the Longmen Shan fault zone, southwestern China. Utilizing the waveform data of M >=-3.0 earthquakes between 20 Apr. 2013 and 31 Dec. 2015 from Sichuan Regional Seismic Network, our present study determined the focal mechanism solutions, centroid depths and moment magnitudes for the Lushan mainshock and 87 M >= 3.0 aftershocks by CAP waveform inversion method. Along with analysis on the strain rosette and areal strain (As), we aimed to discuss the focal mechanism of the sequence and its implications to the tectonic deformation in the seismogenic area. The major findings are as follows: (1) The parameters of the ruptured plane are of strike 219 degrees, dip 43 degrees and slip 101 degrees for the Lushan M7.0 mainshock with a moment magnitude of M-w 6.55 and centroid depth of 15 km. The 87 M >= 3.0 aftershocks are distributed asymmetrically near the mainshock in a region similar to 37 km long along the strike of the Longmen Shan fault zone, and similar to 16 km wide perpendicular to the fault zone. All the aftershocks are located within a depth range of 7 similar to 22 km. Most of the aftershocks are above the mainshock, and the average depth of the sequence is about 13 km. No aftershocks were found shallower than 7 km, indicating the seismogenic source was relatively deep. The estimated seismogenic source of the Lushan earthquake is 37 km X 16 km X 16 km. (2) The values of areal strain (As) show that reverse faulting is dominant for the sequence. The plots of the hypocenters of the sequence on the vertical cross-sections indicate that a NE-SW-striking thrust fault dipping about 45 degrees to NW is the main seismogenic fault of the Lushan earthquake sequence, a portion of aftershocks occurred on the SE-dipping back thrust fault nearly perpendicular to the NW-dipping main fault, and the range-front fault of Longmen Shan fault zone may be responsible for some aftershocks. P-axis is nearly horizontal and orientated in NW-SE direction, coinciding with the regional tectonic stress field. This finding indicates that the seismogenic area is controlled by the stress field, and the Lushan earthquake sequence was resulted from the reverse faulting of the NE-SW-trending faults under a nearly horizontal principle stress with NW-SE orientation. Existence of non-thrusting type earthquakes and obvious variation of the plunge angle of P-axis surrounding the mainshock together indicate the stress disturbance in the local area was influenced by the Lushan mainshock. (3) The strain rosettes for the entire sequence and different classes of magnitudes all display NW-trending compressional white lobe, therefore, we infer that the geological structures for the entire seismogenic area are of thrust faulting under NW-oriented compressional deformation. The strain rosettes exhibit self-similarity in terms of orientation and shape for all classes of magnitudes, reflecting that the deformation pattern of the seismogenic faults is independent of magnitude. (4) The shape of the strain rosette at each depth is dominated by compressional white lobe in NW-NWW direction, indicating all the geological structures for the entire seismogenic area or individual segments within different depths are under NW-NWW-oriented compressional deformation. However, variation of both the shapes and orientations of the strain rosettes with depth is observed, indicating existence of segmentation of tectonic deformation in the vertical direction. (5) The dimension of the seismogenic source of the Lushan earthquake is relatively small, and the mainshock was not on the main faults along the southern segment of the Longmen Shan fault zone, and further we suggest that the cumulated strain energy was not released thoroughly, posing strong earthquake risk on the southern segment.
