At 11:10 a.m. ET on Wednesday, a magnitude 5.6 earthquake struck Northern California, near the small city of Ukiah. Then, at 6:04 p.m. ET, an M7.2 foreshock rocked the northeast coast of Venezuela, immediately followed by an M7.5 mainshock. Minutes later, an M6.9 temblor hit offshore of Kuji, Japan.
It was an exceptional day of seismic activity. The quakes in California and Japan caused no major damage but did result in some injuries. Unfortunately, the situation in Venezuela is far worse. The M7.2 and M7.5 doublet—a pair of similar-magnitude earthquakes that are causally linked but seismologically distinct—destroyed buildings across the northern coastal state of La Guaira and the capital city of Caracas. As of 9 a.m. ET on Thursday, at least 164 deaths and nearly 1,000 injuries have been reported, according to the Associated Press.
“Dozens of buildings have collapsed there… and we are currently carrying out intensive rescue operations to save lives,” Acting President Delcy Rodríguez reportedly said Thursday morning.
When four large earthquakes strike within less than eight hours of each other, it’s natural to wonder if they were connected. Let’s dive into the seismic mechanics of these events.
Coincidence or connection?
First, a brief primer on what causes earthquakes. Most quakes occur when there is a sudden release of built-up pressure along fault lines, or fractures, in Earth’s crust. These fractures are the boundaries between independently moving slabs of crust. As those slabs grind against each other, friction can cause them to become locked in place. Eventually, so much pressure accumulates along the fault that the slabs suddenly slip, triggering an earthquake.
While California, Venezuela, and Japan are all earthquake-vulnerable regions, their seismic activity stems from completely separate fault systems.
The quake in Northern California occurred on the Maacama fault, a right-lateral strike-slip fault (meaning pieces of Earth’s crust are moving past each other horizontally to the right) that is part of the broader San Andreas system. It’s situated between the San Andreas Fault itself to the west and the Bartlett Springs Fault zone to the east. The Maacama fault is capable of producing large earthquakes, but Wednesday’s event was the largest ever recorded there, according to earthquake scientists Judith Hubbard and Kyle Bradley, who co-author the blog Earthquake Insights.
The epicenters of the Venezuelan earthquakes were within just a few miles of each other, but seismic wave information from the U.S. Geological Survey (USGS) suggests they likely originated from different faults with different rupture styles, ssociate professor of earthquake science at the University of Melbourne, explains in an article for The Conversation. It’s likely that the first quake triggered the second one, according to Quigley, but there is no evidence to suggest these events were physically related to the earthquake in California.
The offshore quake in Japan also appears disconnected from these earlier events. Seismicity off the coast of northeastern Honshu, where this event occurred, primarily stems from the Japan Trench subduction zone. This is where the Pacific Plate slides beneath northern Japan, making it one of the most seismically active regions in the world.
Large earthquakes can trigger other quakes in distant locations through a process known as dynamic stress transfer, according to the USGS. This is when energy from the seismic wave passing through other vulnerable locations triggers a rupture. However, that’s probably not what happened on Wednesday. While the two quakes in Venezuela were related to each other, the quakes in California and Japan were most likely “coincidental,” William Barnhart, a USGS geodesist, told the New York Times.
As seismologists gather more data on these quakes, the mechanisms underlying them will become clearer, but it’s unlikely that they will uncover any connection between them. Wednesday’s events serve as a reminder that seismically vulnerable regions can suffer a major quake at any time, underscoring the critical importance of preparedness.
