M6.1 aftershock hits southeast of Loyalty Islands, New Caledonia

A strong aftershock registered by the USGS as M6.1 hit southeast of Loyalty Islands, New Caledonia at 06:41 UTC on May 23, 2023, at a depth of 12.1 km (7.5 miles).

This is the fourth M6+ earthquake to hit the region since May 19, including the M7.7 mainshock at 02:57 UTC on May 19. Other significant aftershocks include M7.1 on May 20 and M6.5 on May 20.

Since the start of the sequence, USGS registered a total of 72 earthquakes with magnitudes above M4.

The Loyalty Islands region is very active seismically, and the region within 250 km (155 miles) of May 19, 2023, earthquake has hosted 13 other M 7+ earthquakes over the preceding 50 years. The largest prior events were an M7.7 on February 10, 2021, and an M7.7 earthquake on May 16, 1995. Because of their remote locations, earthquakes in this region as less likely to create strong shaking in populated areas.

Tsunami wave observations from coastal and/or deep-ocean sea level gauges reveal the following measurements of the maximum tsunami height following May 19 M7.7 (the mainshock), measured with respect to the normal tide level:

Lenakel, Vanuatu recorded a height of 0.61 m (2 feet) at 05:11 UTC with a wave period of 4 minutes; East Cape, New Zealand observed a height of 0.11 m (0.4 feet) at 05:44 UTC with a wave period of 8 minutes; Fishing Rock Raoul located at Raoul Island, New Zealand measured a height of 0.15 m (0.5 feet) at 05:35 UTC with a wave period of 4 minutes; Raoul Is Boat Cove, Raoul Island, New Zealand documented a height of 0.07 m (0.2 feet) at 05:23 UTC with a wave period of 6 minutes; North Cape, New Zealand registered a height of 0.21 m (0.7 feet) at 05:32 UTC with a wave period of 6 minutes; Hienghene, New Caledonia observed a height of 0.03 m (0.1 feet) at 04:54 UTC with a wave period of 20 minutes; Thio, New Caledonia recorded a height of 0.07 m (0.2 feet) at 04:09 UTC with a wave period of 20 minutes; Mare, New Caledonia measured a height of 0.19 m (0.6 feet) at 04:13 UTC with a wave period of 6 minutes; Ouinne, New Caledonia documented a height of 0.20 m (0.7 feet) at 04:06 UTC with a wave period of 8 minutes; and Lifou, New Caledonia registered a height of 0.12 m (0.4 feet) at 04:00 UTC with a wave period of 6 minutes.

For all earthquakes in this series, the USGS issued a Green alert for shaking-related fatalities and economic losses, stating there is a low likelihood of casualties and damage.

Overall, the population in this region resides in structures that are highly vulnerable to earthquake shaking, though some resistant structures exist. The predominant vulnerable building types are unknown/miscellaneous types and wood construction.

Recent earthquakes in this area have caused secondary hazards such as tsunamis that might have contributed to losses.

The Australia-Pacific margin is a highly dynamic plate boundary known for generating exceptionally strong earthquakes, tsunamis, and volcanic eruptions of global significance.

While the majority of this boundary is characterized by direct convergence between the Australian and Pacific plates, there are also transitional zones where oblique convergence occurs, as well as areas experiencing extension. Notably, the direction of subduction undergoes changes at multiple locations along the plate boundary, making it a distinctive attribute of this particular region.

Featured image credit: TW/SAM, Google