Cornell seismic scientist explains how separate fault events can appear as 'back-to-back' quakes across multiple regions

Earthquake reports coming in close together on Wednesday, including shocks in northern Venezuela as well as in Japan and northern California, prompted questions about whether the events were connected. NPR spoke with Julia Hubbard, an earthquake scientist at Cornell University, about how seismologists interpret clusters of earthquakes that occur near-simultaneously but may originate from different faults and tectonic settings.

Hubbard said that when earthquakes happen within a short time window, viewers often assume the events are physically linked. In practice, she said, “back-to-back” timing can be misleading because earthquakes are widespread across active plate boundaries and fault systems. Even when two events are separated by thousands of miles, they can still be detected and reported around the same time due to the scale and reach of modern monitoring and communication.

On the question of whether one earthquake can trigger another far away, Hubbard explained that aftershock sequences are generally tied to the fault that ruptured. Aftershocks occur as the crust adjusts following the initial break, including stress changes in the surrounding region. Those local adjustments can extend over time, but they are not the same as a guaranteed, direct cause-and-effect link to distant earthquakes in other countries.

For the specific pattern of Wednesday’s reports, Hubbard emphasized that northern Venezuela, parts of Japan, and northern California each sit in distinct seismic environments. She described how seismologists look first at the earthquake source regions, including where the rupture likely occurred and what kind of tectonic structure is involved. When those source characteristics differ, she said, the most likely explanation is that each earthquake reflects separate activity on different faults rather than a single, planet-wide cascade.

Hubbard also discussed how earthquake waves travel through the Earth. While seismic energy can be recorded over large distances, the presence of shaking on seismometers far from the epicenter does not automatically mean that distant faults will rupture. The decision points for whether additional earthquakes occur involve local stress conditions on faults near the receiving location, not just the timing of when shaking is recorded.

Separately, seismologists use magnitude, depth, and aftershock behavior to classify what a sequence may mean for communities. Hubbard said that the safest public response is to treat each significant earthquake as its own event, follow local emergency guidance, and remain alert for aftershocks in the affected area where the shaking was strongest.

As more data are compiled after an earthquake sequence, Hubbard noted that scientists can refine estimates of faulting and timing, which helps clarify whether two events are independent or related within the same regional system. She said the process underscores the importance of keeping public messaging anchored to what is known about each event’s source and hazard footprint rather than to the headline similarity of “back-to-back” reports.

In the immediate aftermath, officials generally rely on operational guidance that prioritizes life safety: checking injuries, monitoring infrastructure, and following official instructions for sheltering or evacuation where applicable. Hubbard’s explanation of why near-simultaneous events do not automatically imply a single trigger is aimed at helping the public interpret what seismic reports mean for their specific location.