·

Series of earthquakes hit Sumatra prompting fears of possible tsunamis, Indonesia

series-of-earthquakes-hit-sumatra-prompting-fears-of-possible-tsunamis-indonesia

A series of earthquakes has been recorded along the Indonesian island of Sumatra on November 8, 2015 prompting fears of possible tsunamis in the region. The strongest so far measured M6.4 on the Richter scale as registered by the USGS.

The first earthquake, measuring M4.5, was registered at 01:38 UTC at a depth of 25.3 km (15.7 miles). It was located about 238 km (148 miles) WNW of Sikabaluan, Indonesia.

The second, measuring M6.1 (USGS), hit at 09:34 UTC about 82 km (51 miles) SSW of Padangsidempuan at a depth of 75.1 km (46.6 miles). Based on earthquake information and historic tsunami records it was not sufficient to generate a tsunami, PTWC said.

Singapore’s National Environment Agency (NEA) has put the quake's magnitude at 6.5, warning about possible tsunamis in the region.

USGS issued green alert for shaking-related fatalities and economic losses, estimating a low likelihood of casualties and damage. Overall, the population in this region (about 863 562 people within 100 km (62 miles) radius) resides in structures that are vulnerable to earthquake shaking, though some resistant structures exist.

Over the next couple of hours, 11 more moderately strong earthquakes were recorded off the northern coast of Sumatra, the source of December 26, 2004 ~M9.2 earthquake and subsequent devastating tsunami which claimed lives of between 230 000 and 280 000 people. 

With a magnitude of Mw 9.1 – 9.3, this was the third-largest earthquake ever recorded on a seismograph. It had the longest duration of faulting ever observed, between 8.3 and 10 minutes and caused the entire planet to vibrate as much as 1 centimetre (0.4 inches).

Earthquakes off the coast of Sumatra, Indonesia on November 8, 2015. Image credit: USGS

  1. 4.7110km NNW of Sabang, Indonesia2015-11-08 17:46:51 UTC10.0 km
  2. 5.2126km NNW of Sabang, Indonesia2015-11-08 16:59:20 UTC10.0 km
  3. 6.4123km NNW of Sabang, Indonesia2015-11-08 16:47:01 UTC7.7 km
  4. 4.9143km NW of Sabang, Indonesia2015-11-08 14:47:01 UTC10.0 km
  5. 5.1130km NW of Sabang, Indonesia2015-11-08 14:34:05 UTC10.0 km
  6. 4.7149km NNW of Sabang, Indonesia2015-11-08 13:38:38 UTC7.4 km
  7. 5.2135km NW of Sabang, Indonesia2015-11-08 13:23:48 UTC13.5 km
  8. 5.1125km NW of Sabang, Indonesia2015-11-08 11:54:42 UTC32.2 km
  9. 4.9137km NNW of Sabang, Indonesia2015-11-08 10:53:35 UTC53.8 km
  10. 5.2138km NW of Sabang, Indonesia2015-11-08 10:42:04 UTC27.1 km
  11. 4.8147km NW of Sabang, Indonesia2015-11-08 10:04:36 UTC18.0 km
  12. 6.181km SSW of Padangsidempuan, Indonesia2015-11-08 09:34:57 UTC75.1 km
  13. 4.5238km WNW of Sikabaluan, Indonesia2015-11-08 01:38:59 UTC25.3 km
Data source: USGS (Updated at 18:25 UTC on November 8, 2015)

 

  • For an updated USGS map with earthquakes recorded near Sumatra, please see the following page.

Seismotectonics of the Sumatra Region

The plate boundary southwest of Sumatra is part of a long tectonic collision zone that extends over 8000 km from Papua in the east to the Himalayan front in the west. The Sumatra-Andaman portion of the collision zone forms a subduction zone megathrust plate boundary, the Sunda-Java trench, which accommodates convergence between the Indo-Australia and Sunda plates. This convergence is responsible for the intense seismicity and volcanism in Sumatra. The Sumatra Fault, a major transform structure that bisects Sumatra, accommodates the northwest-increasing lateral component of relative plate motion.

Relative plate motion between the Indo-Australia and Sunda plates is rapid, decreasing from roughly 63 mm/year near the southern tip of Sumatra (Australia relative to Sunda) to 44 mm/year north of Andaman Islands (India relative to Sunda) and rotating counterclockwise to the northwest, so that relative motion near Jakarta is nearly trench-normal but becomes nearly trench-parallel near Myanmar. As a result of the rotation in relative motion along the strike of the arc and the interaction of multiple tectonic plates, several interrelated tectonic elements compose the Sumatra-Andaman plate boundary. Most strain accumulation and release occurs along the Sunda megathrust of the main subduction zone, where lithosphere of the subducting Indo-Australia plate is in contact with the overlying Sunda plate down to a depth of 60 km. Strain release associated with deformation within the subducting slab is evidenced by deeper earthquakes that extend to depths of less than 300 km on Sumatra and 150 km or less along the Andaman Islands. The increasingly oblique convergence between these two plates moving northwest along the arc is accommodated by crustal seismicity along a series of transform and normal faults. East of the Andaman Islands, back- arc spreading in the Andaman Sea produces a zone of distributed normal and strike-slip faulting. Similar to the Sumatran Fault, the Sagaing Fault near Myanmar also accommodates the strike-slip component of oblique plate motion. Plate-boundary related deformation is also not restricted to the subduction zone and overriding plate: the subducting Indo-Australian plate actually comprises two somewhat independent plates (India and Australia), with small amounts of motion relative to one another, that are joined along a broad, actively-deforming region producing seismicity up to several hundred kilometers west of the trench. This deformation is exemplified by the recent April 2012 earthquake sequence, which includes the April 11 M 8.6 and M 8.2 strike-slip events and their subsequent aftershocks.

Paleoseismic studies using coral reefs as a proxy for relative land level changes associated with earthquake displacement suggest that the Sunda arc has repeatedly ruptured during relatively large events in the past, with records extending as far back as the 10th century. In northern Simeulue Island, the southern terminus of the 2004 megathrust earthquake rupture area, a cluster of megathrust earthquakes occurred over a 56 year period between A.D. 1390 and 1455, resulting in uplift substantially greater than that caused by the 2004 event. Studies that look at large sheeted deposits of sand on land interpreted as the transport of debris from a tsunami wave also indicate that this region has experienced significant tsunamis in the past centuries, albeit infrequently.

Prior to 2004, the most recent megathrust earthquakes along the Sumatran-Andaman plate boundary were in 1797 (M 8.7-8.9), 1833 (M 8.9-9.1) and 1861 (M8.5). Since 2004, much of the Sunda megathrust between the northern Andaman Islands and Enggano Island, a distance of more than 2,000 km, has ruptured in a series of large subduction zone earthquakes – most rupturing the plate boundary south of Banda Aceh. The great M 9.1 earthquake of December 26, 2004, which produced a devastating tsunami, ruptured much of the boundary between Myanmar and Simeulue Island offshore Banda Aceh. Immediately to the south of the great 2004 earthquake, the M 8.6 Nias Island earthquake of March 28, 2005 ruptured a 400-km section between Simeulue and the Batu Islands. Farther south in the Mentawai islands, two earthquakes on September 12, 2007 of M 8.5 and M 7.9 occurred in the southern portion of the estimated 1797 and 1833 ruptures zone, which extends from approximately Enggano Island to the northern portion of Siberut Island. Smaller earthquakes have also been locally important: a M 7.6 rupture within the subducting plate caused considerable damage in Padang in 2009, and a M 7.8 rupture on October 25, 2010 occurred on the shallow portion of the megathrust to the west of the Mentawai Islands, and caused a substantial tsunami on the west coast of those islands.

In addition to the current seismic hazards along this portion of the Sunda arc, this region is also recognized as having one of the highest volcanic hazards in the world. One of the most dramatic eruptions in human history was the Krakatau eruption on August 26-27, 1883, a volcano just to the southeast of the island of Sumatra, which resulted in over 35, 000 casualties.

Subduction and seismicity along the plate boundary adjacent to Java is fundamentally different from that of the Sumatran-Andaman section. Relative motion along the Java arc is trench-normal (approximately 65-70 mm/year) and does not exhibit the same strain partitioning and back-arc strike- slip faulting that are observed along the Sumatra margin. Neither has the Java subduction zone hosted similar large magnitude megathrust events to those of its neighbor, at least in documented history. Although this region is not as seismically active as the Sumatra region, the Java arc has hosted low to intermediate-magnitude extensional earthquakes and deep-focus (300-700 km) events and exhibits a similar if not higher volcanic hazard. This arc has also hosted two large, shallow tsunami earthquakes in the recent past which resulted in high tsunami run-ups along the southern Java coast. (USGS)

Featured image credit: USGS

If you value what we do here, create your ad-free account and support our journalism.

Share:

Related articles



Your support makes a difference

Dear valued reader,

We hope that our website has been a valuable resource for you.

The reality is that it takes a lot of time, effort, and resources to maintain and grow this website. We rely on the support of readers like you to keep providing high-quality content.

If you have found our website to be helpful, please consider making a contribution to help us continue to bring you the information you need. Your support means the world to us and helps us to keep doing what we love.

Support us by choosing your support level – Silver, Gold or Platinum. Other support options include Patreon pledges and sending us a one-off payment using PayPal.

Thank you for your consideration. Your support is greatly appreciated.

Sincerely,
Teo Blašković

$5 /month

  • Ad-free account
  • Clean user interface and fast browsing
  • Direct communication with us via chat and email
  • Suggest new features, content and applications
  • Early access to new apps and features

$50 /year

$10 /month

  • Ad-free account
  • Clean user interface and fast browsing
  • Direct communication with us via chat and email
  • Suggest new features, content and applications
  • Early access to new apps and features

$100 /year

$25 /month

  • Ad-free account
  • Clean user interface and fast browsing
  • Direct communication with us via chat and email
  • Suggest new features, content and applications
  • Early access to new apps and features

$200 /year

You can also support us on Patreon

support us on patreon

or by sending us a one-off payment using PayPal:


Commenting rules and guidelines

We value the thoughts and opinions of our readers and welcome healthy discussions on our website. In order to maintain a respectful and positive community, we ask that all commenters follow these rules:

  • Treat others with kindness and respect.
  • Stay on topic and contribute to the conversation in a meaningful way.
  • Do not use abusive or hateful language.
  • Do not spam or promote unrelated products or services.
  • Do not post any personal information or content that is illegal, obscene, or otherwise inappropriate.

We reserve the right to remove any comments that violate these rules. By commenting on our website, you agree to abide by these guidelines. Thank you for helping to create a positive and welcoming environment for all.

Leave a reply

Your email address will not be published. Required fields are marked *