The 2021 Mw 5.6 Earthquake in the Guyana Shield, South America: An Intraplate Event with No Reason to Occur

Abstract

In 31 January 2021, an Mw 5.6 earthquake occurred in the Guyana shield, causing damage to adobe houses and ground cracks. A four-station network, installed in March, showed a shallow (1-3 km) southeast-northwest-oriented reverse fault, consistent with global and regional Centroid Moment Tensor (CMT) solutions. The rupture size (5 km long x 3 km wide) and 1 m slip (from Interferometric Synthetic Aperture Radar data) indicate a stress drop of 5-12 MPa, larger than the average in mid-plate South America. The large non-double-couple (DC) component of all CMT solutions is probably due to a complex rupture geometry, suggested by the aftershock distribution. None of the usual hypotheses to explain intraplate seismicity (weakness zones, stress concentration, and flexural stresses) is applicable to this event. There are no signs of crustal weakness zones: the major Paleoproterozoic faults trend southwest-northeast, orthogonal to the southeast-northwest rupture; the Mesozoic Takutu graben is 65 km away. None of the surface-wave tomographies indicate lithospheric thinning. Free-air anomalies near the epicenter have average values and do not indicate crustal loads increasing stresses locally. In the isolated epicentral area there is no mining activity nor hydroelectric reservoir to induce seismicity. No prior seismogenic feature was found to explain this activity.