A review study of the source parameters of the 23 August 2011 M (sub w) 5.7 Virginia earthquake

The source parameters of the 23 August 2011 M (sub w) 5.7 Virginia earthquake have been further studied. To perform our source parameter review study, we first retrieved waveform records from Incorporated Research Institutions for Seismology. We then carefully selected 17 stations that had good Rayleigh-wave records. We used these 17 vertical records to retrieve a crustal shear-wave velocity model surrounding the epicenter. Using the regional depth phase modeling method, we retrieved focal depths for the mainshock and seven larger aftershocks, which ranged from about 3 to 10 km. With the retrieved focal depths, the arrival times of the P and S phases at four close stations and a revised hypoDD program package, we relocated the mainshock and seven larger aftershocks. On the vertical profile with an azimuth of 119 degrees , the hypocenter trend was at 52 degrees down from horizontal. The moment tensor of the mainshock was inverted by modeling long-period Rayleigh-wave records at 11 stations selected with consideration of a balanced distribution around the epicenter. Combined with the hypocenter trend, a plane solution with a strike of 29 degrees , a dip of 55 degrees , and a slip of 113 degrees was identified to be close to the real rupture plane of the mainshock. The rupture process of the mainshock was preliminarily analyzed using regional waveform records. At least four subevent ruptures were found. The initiate depth of the first subevent rupture was at about 7.8 km, and the rupture speed was at about 1.3 km/s. The time gap between subevent ruptures 1 and 2 was about 0.34 s.