Tonga Islands 7.5 Mag / Time (UTC) 2026-03-24 04:37:50 / Depth: 229 km
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The March 23, 2026, M 7.5 Tonga earthquake occurred as the result of normal faulting at intermediate depth. Focal mechanism solutions indicate rupture occurred on either a shallowly dipping fault striking southwest, or a steeply dipping fault striking northeast. In this region, the Pacific Plate subducts westward beneath the Australia Plate. The location and depth of the earthquake are consistent with faulting within the subducting Pacific Plate.
While commonly plotted as points on maps, earthquakes of this size are more appropriately described as slip over a larger fault area. Normal faulting events of the size of the March 23, 2026, earthquake are typically about 70 km x 30 km in size (length x width).
Earthquakes like this event, with focal depths between 70 and 300 km, are commonly termed “intermediate-depth” earthquakes. Intermediate-depth earthquakes represent deformation within subducted slabs rather than at the shallow plate interface between subducting and overriding tectonic plates. They typically cause less damage on the ground surface above their foci than is the case with similar-magnitude shallow-focus earthquakes, but large intermediate-depth earthquakes may be felt at great distance from their epicenters.
Since 1950, six previous earthquakes of magnitude 7 or larger have occurred within 250 km of this earthquake. The most recent was a magnitude 7 in March 2025. The largest of these was a magnitude 8 earthquake on May 3, 2006, that caused minor damage in the region.
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This record shows several teleseismic wave phases from the 24 March 2026 Tonga earthquake as observed at Raspberry Shake R01A0 in Horrenberg. Because Tonga and Horrenberg are separated by about 150°, the waves did not all travel along simple near-surface paths. Instead, they sampled deep parts of the Earth, including the core.
The first packet, PKIKP–PKP–PKiKP, is a group of compressional (P-type) core phases. In this notation, P means a compressional wave in the mantle, K a compressional wave in the liquid outer core, and I a compressional wave in the solid inner core. These phases arrive first because P-waves are the fastest body waves.
The second packet, p/s-PKP depth-phase complex, is linked to the earthquake’s source depth. The lower-case p and s indicate phases that first travel upward from the source as P or S, reflect near the surface above the hypocenter, and then continue as PKP-type core phases. That is why this part is a broader, more complex wave train.
The third packet, PP (+PKS/PKIKS), contains a strong PP phase, a P-wave reflected once at the Earth’s surface, mixed with other nearby arrivals such as PKS and PKIKS.
The last packet, SKIKS / PPP, is a later-arriving complex. It includes waves that either cross the core as S-related phases or reflect multiple times at the surface as PPP. Together, these arrivals illustrate how one large earthquake can send energy through very different parts of the Earth.
Graphic adapted from: SEWilco - http://pubs.usgs.gov/
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