25 December 2016 Magnitude 7.7 Chile shock might be a late aftershock of the world’s largest recorded earthquake

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Ross S. Stein, Temblor

Almost anywhere else in the world, a M=7.7 earthquake would be considered large and rare, but today’s shock is exceptional: It struck in the aftershock zone of the great M=9.5 Concepción earthquake, which ruptured 1000 km (600 mi) of the Chile trench, and slipped the megathrust by up to 45 m (150 ft). As a result, the 1960 quake was a stunning 800 times larger than today’s shock.

Fortunately, today’s quake stuck beneath a national park, Parque Tantauco, on the southern tip of Isla Grande de Chiloé, a remote and lightly populated area of southern Chile. As of this writing, there are no deaths but some damage reported.

At the level of M=3.0 and larger, there were no foreshocks in the preceding month to today’s quake. The first 12 hours of aftershocks all lie 30-60 km (20-40 mi) to the west of the mainshock, which suggests the rupture propagated upward and westward toward the offshore trench.

Temblor screen with ‘Earthquake Forecast’ layer clicked, showing that earthquakes of M=7.7 are rare at this location; they occur at a rate of about 0.2% per year. In contrast, 500 km to the north, near Los Angeles, Chile, today’s quake would be much more common.
Temblor screen with ‘Earthquake Forecast’ layer clicked, showing that earthquakes of M=7.7 are rare at this location; they occur at a rate of about 0.2% per year. In contrast, 500 km to the north, near Los Angeles, Chile, today’s quake would be much more common.
Coulomb stress imparted by the M=9.5 Chile earthquake, calculated by Lin and Stein (2004) based on the seismic slip model of Barrientos and Ward (1990). The large black stars are the main 1960 sub-events. Today’s quake struck on a part of the fault that was likely promoted by the 1960 shock. But if so, why did it wait for 56 years?
Coulomb stress imparted by the M=9.5 Chile earthquake, calculated by Lin and Stein (2004) based on the seismic slip model of Barrientos and Ward (1990). The large black stars are the main 1960 sub-events. Today’s quake struck on a part of the fault that was likely promoted by the 1960 shock. But if so, why did it wait for 56 years?
Seismic slip in the 22 May 1960 M=9.5 Concepción earthquake, from Barrientos and Ward (1990), compared to today’s quake, which lands in a ‘saddle’ with lower than average slip.
Seismic slip in the 22 May 1960 M=9.5 Concepción earthquake, from Barrientos and Ward (1990), compared to today’s quake, which lands in a ‘saddle’ with lower than average slip.

Whether the M=7.7 earthquake is an aftershock, or unrelated, is difficult to establish, in part because in the 1960’s seismic monitoring in the southern hemisphere was quite poor. In 1960, the fault slipped about 10 m at the site of the M=7.7 event, and the longterm fault slip rate is 73 mm/yr, and so about 135 years would need to elapse until the effect of the 1960 quake would have been erased. We are only halfway there, suggesting today’s event might indeed be a far flung aftershock of a leviathan.

Sources

Sergio E. Barrientos and Steven N. Ward (1990), The 1960 Chile earthquake: inversion for slip distribution from surface deformation, Geophys. J. Int., 103, 589-598, doi: 10.1111/j.1365-246X.1990.tb05673.x

Ines L. Cifuentes (1989), The 1960 Chilean earthquakes, J. Geophys. Res., 94, 665 – 680, doi: 10.1029/JB094iB01p00665

Jian Lin and Ross S. Stein (2004), Stress triggering in thrust and subduction earthquakes, and stress interaction between the southern San Andreas and nearby thrust and strike-slip faults, J. Geophys. Res., 109, B02303, doi:10.1029/2003JB002607.

USGS

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