PACIFIC-ANTARCTIC RIDGE, July 20 — A magnitude 5.5 earthquake struck the Pacific-Antarctic Ridge at 05:38 UTC on July 20, at a depth of 10 kilometers, according to the U.S. Geological Survey. No tsunami warning was issued. The record shows the Pacific-Antarctic Ridge is a seismically active region.
Over the past two decades, at least 139 earthquakes of magnitude 5.0 or greater have been recorded within about 300 kilometers of the epicenter. The strongest of those events was a magnitude 6.4 earthquake in 2010.
At 10 kilometers deep, this was a shallow earthquake. Seismologists note that shallow events — those less than 70 kilometers below the surface — typically produce stronger shaking at the surface than deeper earthquakes of the same magnitude. Earthquakes in the magnitude 5 range are classified as moderate, capable of causing minor to moderate damage near the epicenter, depending on local building conditions and population density.
The U.S. Geological Survey detected the event through its real-time global seismic monitoring network. Aftershocks are common following an earthquake of this size, and may continue for days or weeks.
The general advisory in such cases is for people in the affected area to be prepared for further shaking. On the timeline,
The Pacific-Antarctic Ridge is a divergent tectonic plate boundary located in the southern Pacific Ocean, separating the Pacific Plate from the Antarctic Plate. It is part of the global mid-ocean ridge system, where new oceanic crust is formed as the plates move apart.
Seismic activity along such ridges is common, driven by the ongoing process of seafloor spreading and associated faulting. Shallow earthquakes like this one typically occur along these spreading centers, where the brittle upper crust undergoes stress from tectonic forces. Because the ridge lies in a remote, sparsely populated part of the ocean, moderate earthquakes in this region rarely pose a direct threat to human life or infrastructure.
The primary hazard from such events is the potential for underwater landslides or localized changes in seafloor topography, though the magnitude of this earthquake is below the threshold generally associated with significant tsunami generation. The absence of a tsunami warning issued by monitoring agencies is consistent with standard protocols for events of this size and depth in deep-ocean settings.
Seismic monitoring of remote oceanic ridges relies on a global network of seismometers, which allows scientists to detect and locate earthquakes even in areas without local instruments. Data from these events contribute to the understanding of plate tectonics, fault mechanics, and the seismic hazard profile of oceanic regions. Continued monitoring of aftershocks can provide further insight into the stress state of the crust along this ridge segment.
