Vanuatu Islands M7.3 / Time (UTC) 2026-03-30 08:44:13
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Tectonic Summary
The March 30, 2026, M 7.3 earthquake near the island of Espiritu Santo in the Vanuatu island chain of the southwest Pacific Ocean occurred as a result of oblique-thrust faulting near the boundary between the Australia and Pacific plates. Moment tensors of the event indicate that rupture occurred on a north-northwest striking moderately dipping fault or a south-southwest striking steeply dipping fault. The earthquake is located about 90 km to the east of the New Hebrides Trench, the bathymetric expression of the plate boundary between the Australia and Pacific plates, where the lithosphere of the Australia plate subducts into the mantle beneath the North Fiji Basin. At the location of this earthquake, the Australia plate moves east-northeast with respect to the Pacific plate at a velocity of approximately 80 mm/yr.
Earthquakes like this one, with focal depths between 70 and 300 km, are commonly termed “intermediate-depth” earthquakes. Intermediate-depth earthquakes often represent deformation within subducted slabs rather than at the shallow plate interfaces 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 distances from their epicenters.
The Vanuatu region frequently experiences large earthquakes; 19 events of M 7 or larger have occurred within 150 km of this March 30, 2026, event over the preceding century. The largest was a M 7.7 earthquake in May 1965, 75 km to the north-northwest of the March 30, 2026, event.
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For my station in Dielheim-Horrenberg, the Vanuatu M7.3 earthquake of 30 March 2026 was recorded at a teleseismic distance where the first obvious vertical-component arrivals are not expected to be simple direct P waves, but rather core-traversing P phases. That is why I interpret the first highlighted packet as a PKP-family cluster, most likely PKIKP–PKP–PKiKP: PKP travels as a compressional wave through the mantle and outer core, PKIKP also crosses the inner core, and PKiKP is reflected from the inner-core boundary. At distances near the P-wave shadow zone, these phases commonly dominate the earliest clear signal.
The second highlighted packet is broader and later, which makes a phase group more likely than a single arrival. Because the earthquake had an intermediate depth of about 121 km, depth phases such as pPKP and sPKP are plausible; these are PKP-related waves that begin with an upward leg from the source before continuing along a core-crossing path. A contribution from the PP complex is also likely.
The third packet, much later in time, fits best with delayed teleseismic energy such as PPP and possibly SKIKS. Since this interpretation is based on a single filtered Raspberry Shake EHZ trace, the assignments should be regarded as physically well-motivated but should also be taken with care.
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