Massive earthquake M7.6 struck off the coast of Solomon Islands - strong aftershocks continue

Massive earthquake M7.6 struck off the coast of Solomon Islands - strong aftershocks continue

A massive earthquake measuring M7.6 (USGS) struck Solomon Islands on April 12, 2014 at 20:14 UTC. EMSC initially reported M8.2 but later downgraded to M7.8 at depth of 20 km. USGS is reporting depth of 29.3 km (18.2 miles).

Epicenter was located 100 km (62 miles) SSE of Kirakira and 323 km (201 miles) SE of Honiara, Solomon Islands.

GDACS estimates there are about 17 954 people living within 100 km radius.

USGS issued green alert for shaking-related fatalities and economic losses. 

Overall, the population in this region resides in structures that are vulnerable to earthquake shaking, though some resistant structures exist. The predominant vulnerable building types are mud wall and informal (metal, timber, GI etc.) construction.

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

Geoscience Australia estimated this earthquake could have been felt by people up to 2512 km away.

JRC tsunami wave height calculation model said maximum tsunami wave height is 0.8 m in Mwaniwowo, Solomon Islands. This height is estimated for April 12, 2014 at 20:39 UTC.

Tsunami maximum wave height. Image credit: JRC. See animation.

Updates

By 13:29 UTC on Sunday, April 13, 2014 USGS registered a total of 29 strong earthquakes in the area. The strongest, after M7.6 yesterday evening, registered as M7.5 at 12:36 UTC on April 13. USGS reported depth of 35 km. EMSC reported this one as M7.6 at depth of 10 km.

Earthquake swarm on April 13, 2014 - Solomon Islands. Image credit: USGS

 

  1. 7.6100km SSE of Kirakira, Solomon Islands2014-04-12 20:14:39 UTC29.3 km
  2. 7.5111km S of Kirakira, Solomon Islands2014-04-13 12:36:18 UTC35.0 km
  3. 5.996km SSE of Kirakira, Solomon Islands2014-04-12 20:24:46 UTC26.8 km
  4. 5.7119km SSE of Kirakira, Solomon Islands2014-04-13 12:46:43 UTC31.4 km
  5. 5.269km SSE of Kirakira, Solomon Islands2014-04-13 09:29:20 UTC11.7 km
  6. 5.195km S of Kirakira, Solomon Islands2014-04-13 10:09:49 UTC8.1 km
  7. 5.185km SSE of Kirakira, Solomon Islands2014-04-12 21:39:05 UTC43.1 km
  8. 5.1127km SSE of Kirakira, Solomon Islands2014-04-12 23:52:14 UTC7.0 km
  9. 5.0149km SE of Kirakira, Solomon Islands2014-04-12 21:17:05 UTC35.0 km
  10. 5.087km S of Kirakira, Solomon Islands2014-04-13 10:05:39 UTC21.1 km
  11. 4.965km S of Kirakira, Solomon Islands2014-04-13 11:12:48 UTC10.0 km
  12. 4.9122km SE of Kirakira, Solomon Islands2014-04-13 08:55:12 UTC19.1 km
  13. 4.9123km SSE of Kirakira, Solomon Islands2014-04-12 20:49:56 UTC40.3 km
  14. 4.9129km SSE of Kirakira, Solomon Islands2014-04-12 22:40:33 UTC34.9 km
  15. 4.9124km S of Kirakira, Solomon Islands2014-04-13 03:17:56 UTC24.1 km
  16. 4.9124km SSE of Kirakira, Solomon Islands2014-04-13 05:59:24 UTC14.4 km
  17. 4.8131km S of Kirakira, Solomon Islands2014-04-13 03:41:01 UTC18.1 km
  18. 4.882km S of Kirakira, Solomon Islands2014-04-13 04:36:26 UTC23.8 km
  19. 4.8146km SE of Kirakira, Solomon Islands2014-04-13 07:02:20 UTC10.0 km
  20. 4.8102km SSE of Kirakira, Solomon Islands2014-04-13 10:54:28 UTC10.0 km
  21. 4.7124km SE of Kirakira, Solomon Islands2014-04-13 12:24:35 UTC20.0 km
  22. 4.772km S of Kirakira, Solomon Islands2014-04-12 21:11:02 UTC35.3 km
  23. 4.789km SW of Kirakira, Solomon Islands2014-04-13 05:36:01 UTC20.0 km
  24. 4.782km S of Kirakira, Solomon Islands2014-04-13 10:26:24 UTC34.9 km
  25. 4.687km S of Kirakira, Solomon Islands2014-04-13 05:25:24 UTC19.1 km
  26. 4.6142km SSE of Kirakira, Solomon Islands2014-04-12 22:18:51 UTC35.0 km
  27. 4.6126km S of Kirakira, Solomon Islands2014-04-13 06:19:25 UTC20.0 km
  28. 4.6124km SE of Kirakira, Solomon Islands2014-04-12 21:34:39 UTC38.1 km
  29. 4.695km SSW of Kirakira, Solomon Islands2014-04-13 04:31:10 UTC24.0 km
Data source: USGS (Updated: 13:29 UTC on April 13, 2014)

Tectonic summary

The April 12, 2014 M 7.6 Solomon Islands earthquake occurred as the result of nearly pure transform faulting, likely on a NW-SE oriented left-lateral fault, on or near the plate boundary between the Australia and Pacific plates. At the location of the earthquake, the Australia Plate converges with and slips past the Pacific plate at a rate of 95 mm/yr. The April 12 earthquake occurred along a portion of this plate boundary that transitions from thrust to transform tectonics between the New Britain Trench to the northwest and the New Hebrides Trench farther east. The earthquake occurred about 100 km SSE of Kirakira, Solomon Islands. The region of the April 12 earthquake is very seismically active, with 27 earthquakes of M6+ occurring within 100 km and 62 events of M7+ within 500 km since 1900. The majority of these earthquakes are grouped to the northwest around the Solomon Islands and to the east near Vanuatu and the Santa Cruz Islands. Notable earthquake within 100 km include a doublet of M7.0 events in November 1978, events of M7.1 in 1931 and 1937, and an M7.2 in 1910. The most recent local event prior to the April 12 earthquake was an M6.0 on April 4, 2014 94 km to the NW. The April 12 earthquake also occurred about 950 km to the SE of an earthquake sequence south of Bougainville Island, Papua New Guinea that began with an M6.1 on April 11, 2014. On February 6, 2013, an M8.0 struck offshore of the Santa Cruz Islands about 340 km east of the April 12 event. The 2013 earthquake triggered a regional tsunami of about 1.5 m and was followed by aftershocks of M7.0 and two of M7.1.

Seismotectonics of the Eastern Margin of the Australia Plate

The eastern margin of the Australia plate is one of the most sesimically active areas of the world due to high rates of convergence between the Australia and Pacific plates. In the region of New Zealand, the 3000 km long Australia-Pacific plate boundary extends from south of Macquarie Island to the southern Kermadec Island chain. It includes an oceanic transform (the Macquarie Ridge), two oppositely verging subduction zones (Puysegur and Hikurangi), and a transpressive continental transform, the Alpine Fault through South Island, New Zealand.

Since 1900 there have been 15 M7.5+ earthquakes recorded near New Zealand. Nine of these, and the four largest, occurred along or near the Macquarie Ridge, including the 1989 M8.2 event on the ridge itself, and the 2004 M8.1 event 200 km to the west of the plate boundary, reflecting intraplate deformation. The largest recorded earthquake in New Zealand itself was the 1931 M7.8 Hawke's Bay earthquake, which killed 256 people. The last M7.5+ earthquake along the Alpine Fault was 170 years ago; studies of the faults' strain accumulation suggest that similar events are likely to occur again.

North of New Zealand, the Australia-Pacific boundary stretches east of Tonga and Fiji to 250 km south of Samoa. For 2,200 km the trench is approximately linear, and includes two segments where old (>120 Myr) Pacific oceanic lithosphere rapidly subducts westward (Kermadec and Tonga). At the northern end of the Tonga trench, the boundary curves sharply westward and changes along a 700 km-long segment from trench-normal subduction, to oblique subduction, to a left lateral transform-like structure.

Australia-Pacific convergence rates increase northward from 60 mm/yr at the southern Kermadec trench to 90 mm/yr at the northern Tonga trench; however, significant back arc extension (or equivalently, slab rollback) causes the consumption rate of subducting Pacific lithosphere to be much faster. The spreading rate in the Havre trough, west of the Kermadec trench, increases northward from 8 to 20 mm/yr. The southern tip of this spreading center is propagating into the North Island of New Zealand, rifting it apart. In the southern Lau Basin, west of the Tonga trench, the spreading rate increases northward from 60 to 90 mm/yr, and in the northern Lau Basin, multiple spreading centers result in an extension rate as high as 160 mm/yr. The overall subduction velocity of the Pacific plate is the vector sum of Australia-Pacific velocity and back arc spreading velocity: thus it increases northward along the Kermadec trench from 70 to 100 mm/yr, and along the Tonga trench from 150 to 240 mm/yr.

The Kermadec-Tonga subduction zone generates many large earthquakes on the interface between the descending Pacific and overriding Australia plates, within the two plates themselves and, less frequently, near the outer rise of the Pacific plate east of the trench. Since 1900, 40 M7.5+ earthquakes have been recorded, mostly north of 30°S. However, it is unclear whether any of the few historic M8+ events that have occurred close to the plate boundary were underthrusting events on the plate interface, or were intraplate earthquakes. On September 29, 2009, one of the largest normal fault (outer rise) earthquakes ever recorded (M8.1) occurred south of Samoa, 40 km east of the Tonga trench, generating a tsunami that killed at least 180 people.

Across the North Fiji Basin and to the west of the Vanuatu Islands, the Australia plate again subducts eastwards beneath the Pacific, at the North New Hebrides trench. At the southern end of this trench, east of the Loyalty Islands, the plate boundary curves east into an oceanic transform-like structure analogous to the one north of Tonga.

Australia-Pacific convergence rates increase northward from 80 to 90 mm/yr along the North New Hebrides trench, but the Australia plate consumption rate is increased by extension in the back arc and in the North Fiji Basin. Back arc spreading occurs at a rate of 50 mm/yr along most of the subduction zone, except near ~15°S, where the D'Entrecasteaux ridge intersects the trench and causes localized compression of 50 mm/yr in the back arc. Therefore, the Australia plate subduction velocity ranges from 120 mm/yr at the southern end of the North New Hebrides trench, to 40 mm/yr at the D'Entrecasteaux ridge-trench intersection, to 170 mm/yr at the northern end of the trench.

Large earthquakes are common along the North New Hebrides trench and have mechanisms associated with subduction tectonics, though occasional strike slip earthquakes occur near the subduction of the D'Entrecasteaux ridge. Within the subduction zone 34 M7.5+ earthquakes have been recorded since 1900. On October 7, 2009, a large interplate thrust fault earthquake (M7.6) in the northern North New Hebrides subduction zone was followed 15 minutes later by an even larger interplate event (M7.8) 60 km to the north. It is likely that the first event triggered the second of the so-called earthquake "doublet". (USGS)

More information on regional seismicity and tectonics

Featured image: USGS

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