A major earthquake with a magnitude of 7.7 ruptured parts of the seismic gap along the plate boundary system, the Orient Fault (Figure 1). We used the slip inversion method of Kikuchi and Kanamori (1993) to derive the slip distribution of the event.
1. Slip Inversion of the Event
We used 66 P and 51 SH waveforms recorded at 71 seismic stations in distance ranges of 38° to 95°. Our preferred slip solution infers a seismic moment of 4.53E+20 Nm, equivalent to a moment magnitude of 7.7 (Figure 2).
The rupture initiated from the eastern end of the failed fault segment and unilaterally propagated westward. We used the strike, 257°, and dip, 87°, angles that were reported by the Global CMT catalog (Ekströem et al., 2012). The computed average rake angle of 5° matches acceptably with the angle reported by the Global CMT, 3°. The average rake angle of 5° suggests an almost pure left-lateral strike-slip mechanism.
Figure 3 shows two distinct slip distributions about 60 km apart and each consists of sub-patches. The eastern slip distribution is about 115 km long and its width reaches 15 km. The main asperity, with a maximum slip of more than 16 m, extends for about 30 km, between 45 and 75 km along the strike in Figure 3. The western slip distribution with a length of about 65 km and a maximum slip of about 6 meters, consists of two sub-patches.
2. Interpretation of the Slip Inversion
Figure 4 shows the slip distribution of the event with the real dip angle. To elaborate the consistency of the slip distribution and the bathymetry, in Figure 5 the ruptured fault segment is hinged about its shallow edge such that it becomes a horizontal plane. Figures 4 and 5 show one-week aftershock activity, while the known seismicity before the earthquake is shown in Figure 6.
The following points can be deduced from the slip distribution of the event:
- The absence of aftershocks along the major part of the fault indicates the total rupture of the brittle crust.
- The substantial slip-reduction along the shallow part, between 75 and 110 km in Figure 3, corresponds to bathymetry depths more than 7500 m, between 79.4°W to 80°W in Figures 4 to 6.
- The 60-km gap in the slip distribution corresponds with the bathymetry deeper than 7000 m. This may indicate a thinner brittle crust, which fails with smaller stress compared to the region of the main asperity.
- The above point can be verified by comparing the slip distribution of the main asperity, from 45 to 75 km in Figure 3, and the slip for the fault segment located between 75 and 110 km; the maximum slip values for the segments are 16 and 9 m in turn.
- The rupture propagated unilaterally from east to the west.
- The low-slip area, located about 215 km from the initiation point in Figure 3, is the location of the large aftershock of 2020-01-28 21:55:16 (Mw 6.1) and a cluster of smaller aftershocks. This can be observed in Figure 5 at the longitude of about 80.75°W.
Ekströem, G., M. Nettles, and A. M. Dziewonski (2012), The global CMT project 2004-2010: Centroid-moment tensors for 13,017 earthquakes, Phys. Earth Planet. In., 200, 1–9, doi: 10.1016/j.pepi.2012.04.002.
Kikuchi, M., H. Kanamori, and K. Satake (1993), Source complexity of the 1988 Armenian earthquake: Evidence for a slow after-slip event, J. Geophys. Res., 98, 15,797–15,808.Last Updated on September 1, 2021