Yesterday at 6 pm local time (3 pm in Arizona) pair of large earthquakes occurred in rapid succession along the northern coast of Venezuela. The first, an M7.2, ruptured at a depth of about 20 km and about 180 km west of Caracas. 40 seconds later (while shaking from the first was still propagating), the second, an M7.5, initiated at a depth of about 10 km and ruptured from the eastern end of the M7.2 towards the east (towards Caracas). This pair of events, a foreshock followed rapidly by an adjacent mainshock, is being called a "doublet". The first ruptured into the second like one domino following into the next.
This is a devastating pair of earthquakes for Venezuela; they are shallow and occurred near to populated regions. The US Geological Survey anticipates significant loss of life and economic impacts.
The foreshock occurred along the large east-west Yaracuy valley that connects to the northern Venezuelan coast. Cities with hundreds of thousands of residents (San Felipe, Puerto Cabello, Los Guayos) experienced very strong to severe shaking (a total of ~1M people).
The second event with its ground motions propagating along with those of the first, ruptured eastward along the northern Venezuelan coast nearly to Caracas (a distance of ~150 km). This "directivity"--rupture moving in the same direction as the waves it is producing--can significantly enhance ground motions and that is likely what happened for places like Caracas. For this event, more than 2.5M people experienced very strong to severe shaking for at least tens of seconds. Eyewitness reports show many buildings collapsed. Many slopes may have failed in the nearby mountains, enhancing damage via cascading hazard.
The earthquakes occurred along a well known active fault zone variably named the Boconó-San Sebastián-El Pilar Fault zone. It represents the plate boundary between northern South America and the Carribean plates. The Carribean plate (north side of the fault) moves eastward about 20 mm/yr relative to the South American Plate. This dextral motion was thus accommodated in part by these two mostly right lateral earthquakes. This side to side rather than up-down sense of motion--even though the rupture was along the coast--is consistent with the lack of observation of any tsunamis.
These are the largest earthquakes to occur in the region in at least the past century. M6 events have occurred within 250 km in the last century, but nothing this large. An ~M7.4 event occurred in 1812 in the same area as yesterday's M7.2, while an ~M7.6 occurred possibly along these faults just offshore Caracas in 1900.
Aftershocks are occurring and will continue to make rescues more challenging. I expect that today the response will be vigorous and we will know more about this catastrophe.
Authoritative Links:
Additional useful links:
- https://earthquakeinsights.substack.com/p/catastrophic-m75-earthquake-strikes
- ASU recordings: M7.5 and M7.2
Some papers I found useful:
- Audemard, F. A., 2007, Revised seismic history of the El Pilar fault, Northeastern Venezuela, from the Cariaco 1997 earthquake and recent preliminary paleoseismic results, J Seismol, 11, no. 3, 311–326, doi: 10.1007/s10950-007-9054-2.
- Pousse‐Beltran, L., R. Vassallo, F. Audemard, F. Jouanne, J. Carcaillet, E. Pathier, and M. Volat, 2017, Pleistocene slip rates on the Boconó fault along the North Andean Block plate boundary, Venezuela, Tectonics, 36, no. 7, 1207–1231, doi: 10.1002/2016TC004305.
- Higgins, M., P. C. La Femina, J. C. Weber, H. Geirsson, G. A. Ryan, and C. Wauthier, 2021, Strain Partitioning and Interseismic Fault Behavior Along the Caribbean‐South American Transform Plate Boundary, Tectonics, 40, no. 8, e2021TC006740, doi: 10.1029/2021TC006740.
