4 big quakes in 3 different places within half an hour of each other. Are they related?

On Thursday, June 25 (Manila time), a few minutes after 6 am, two powerful earthquakes, each with a magnitude above 7.0 and focal depths shallower than 20 kilometers, struck the South American nation of Venezuela just 40 seconds apart.

The epicenters were on land near the town of Yumare, about 200 kilometers west of the capital, Caracas.

Less than 30 minutes later, a magnitude 6.9 earthquake struck Japan on the opposite side of the Pacific, with its epicenter offshore near the coast of northern Honshu.

Both the Venezuelan and Japanese earthquakes were preceded about seven hours earlier by a magnitude 5.6 earthquake in northern California in the United States.

Are these earthquakes, which occurred within hours of one another but thousands of kilometers apart, related?

Yes and no

Yes, these earthquakes are related in the grand scheme of Plate Tectonics, the theory that says that the earth’s crust is divided into several broken pieces called plates which are continuously moving, allowing them to interact with one another.

But sensu stricto, they are not directly related. The magnitude 5.6 earthquake in California was generated by a fault related to the San Andreas Fault, which defines the boundary between the Pacific Plate and the North American Plate.

These two plates are grinding horizontally against each other as the highly mobile Pacific Plate presses eastward against the more stable North American Plate.

The San Andreas Fault, which moves in a dextral strike-slip sense — meaning that when a person stands on the Pacific Plate and looks across the moving fault, that person would perceive the North American Plate to the right — runs for more than 1,200 kilometers traversing the entire length of the state of California from San Francisco to San Diego.

It has generated several of the most devastating earthquakes in American history and is feared by many for its threat of being able to generate the Big One.

The two magnitude 7-plus earthquakes in Venezuela were generated by a fault system that moves in a similar manner to the San Andreas Fault.

In Venezuela, it is the Caribbean Plate that moves rightwards with respect to the South American Plate along an intricate system of dextral slip faults belonging to the Boconó-Morón-El Pilar Fault System.

The magnitude 7.2 and 7.5 earthquakes, which struck 39 seconds apart, have been interpreted by certain earthquake scientists as a “doublet,” a phenomenon that involves two earthquakes generated by two distinct faults operating independently of each other, but occurring closely together in space and time.

A similar seismic event struck eastern Turkey in 2023 when two earthquakes with magnitudes 7.1 and 7.5 struck the country nine hours apart within a 100-kilometer radius.

The magnitude 6.9 earthquake that struck northern Honshu in Japan around 30 minutes after the Venezuelan earthquakes was caused by the subduction of the Pacific Plate beneath the Japan Arc which can be considered part of the Okhotsk microplate of the North American Plate (western Honshu is part of the Eurasian Plate).

Unlike the Venezuelan and Californian earthquakes, the Japan earthquake was generated by a thrust fault, which involves the movement of plates along an inclined plane. In Honshu, the Pacific Plate dives underneath the Okhotsk microplate along a fault plane inclined to the west manifested offshore as the Japan Trench.

This same region was struck by the powerful magnitude 9.0 earthquake of Tohoku on March 11, 2011. That earthquake released an energy which is almost a hundred times stronger than the magnitude 6.9 earthquake of June 25, 2026.

Back on the home front

On Friday, June 26, at 7:34 PM (Manila Time), Sarangani was once again struck by a magnitude 6.5 earthquake which is likely an aftershock of the magnitude 7.8 tremor that hit the region on June 8.

After an earthquake, the crust needs to adjust by continuing to deform. These crustal deformations cause the aftershocks (seismic events that happen “after” the main shock), albeit with diminishing magnitudes. – Rappler.com

Mario A. Aurelio, PhD is a professor at the National Institute of Geological Sciences – University of the Philippines (UP NIGS). Among his fields of interest include earthquake studies.