Understanding the Outer Rise Zone from the West Nias Earthquake, Not to Be Neglected Page all

KOMPAS.com – Friday (14/5/2021) afternoon at 13.33 WIB, the Meteorology, Climatology and Geophysics Agency (BMKG) recorded a strong earthquake M 7.2, rocking the West Nias region, North Sumatra.

The results of the BMKG analysis show initial information about this earthquake with a magnitude of 7.2, which was then updated to M 6.7.

“Friday, May 14, 2021 at 13.33.09 WIB in the Indian Ocean, West Coast of Sumatra was rocked by a tectonic earthquake. Magnitude strength of 7.2 which was subsequently updated to 6.7,” said Head of the Meteorology, Climatology and Geophysics Agency Dwikorita Karnawati at a press conference in Jakarta. , Friday.

BMKG monitoring results showed that the earthquake did not have the potential to cause a tsunami, but the shock was felt by the surrounding community with varying intensities.

Also read: No Tsunami Potential, West Nias Earthquake Shakes Feel Up to Aceh

According to the Head of the BMKG Earthquake and Tsunami Mitigation Division, Daryono, this earthquake did not have the potential for a tsunami because the M 6.7 was not strong enough to cause sea floor deformation to disrupt the sea water column.

The epicenter is located at coordinates 0.2 degrees North latitude and 96.69 degrees East longitude or is at sea at a distance of 125 kilometers southwest of Lahomi City, West Nias Regency, North Sumatra, with a depth of the epicenter (hypocenter) 10 kilometers.

By paying attention to the location of the epicenter and the depth of the hypocenter, the earthquake that occurred was a shallow earthquake that was located in the outer-rise zone or outside the plate subduction zone.

So, what is meant by the outer rise zone?

The devastating earthquake that triggers tsunamis is generally caused by rock deformation in a shallow subduction system, precisely in the contact area between the plates, which is called the megathrust zone.

In this zone, maximum elastic energy can be accumulated due to a meeting or collision between the plates which is then suddenly released during an earthquake.

So, the earthquake epicenter southwest Nias on the map appears to be outside the subduction zone. This is what characterizes the source of the outer rise earthquake.

“The tectonic force that works in this zone is not compressional or pressing, but extensional or tensile force because it is a bending zone (strain) on the slab plate that bends before plunging into another plate,” said Daryono, Saturday (5/15/2021) .

According to the BMKG analysis, this earthquake has a fault-down mechanism (normal fault). This reinforces the fact that the earthquake originated in the deformation zone due to the build-up of the maximum tension or strain that has been accumulating so far.

In Indonesia, the source of the most active outer rise earthquakes is outside the Sunda subduction system, which is spread across the Indian Ocean west of Sumatra, South Java, to southern Bali to NTB.

The following is a list of significant earthquake records originating from earthquake sources outside the subduction zone (outer rise), namely:

1. The southern Java earthquake measuring M7.5 triggered a tsunami in southern Java on September 11, 1921.
2. The southern Sumbawa earthquake had a magnitude of M8.3 which triggered a destructive tsunami south of Sumbawa on August 19, 1977.
3. The southern Bali earthquake measuring M6.0 on 9 June 2016 rocked the island of Bali.
4. The southern earthquake of West Java M5.1 on July 15 2016 rocked the southern coast of West Java.
5. A magnitude 5.4 earthquake of Bengkulu and Lampung shook the coast of Bengkulu to Lampung on 23 July 2016.
6. The southern Bali earthquake with a magnitude of M5.3 rocked South Bali on March 17, 2017.
7. The southern Bali earthquake with a magnitude of M5.1 rocked South Bali on June 9, 2019.
8. The southern Bali earthquake with a magnitude of M6.5 on March 19, 2020, was felt in almost all of Bali and Lombok with an intensity scale of up to MMI IV.
9. Nias earthquake with a magnitude of M 6.7 on May 14, 2021, felt in Nias Island, North Sumatra, Aceh and West Sumatra.

The Nias earthquake yesterday was the 3rd of the previous significant earthquakes, namely on April 19, 2021, with a magnitude of M6.0 and on April 14, 2021, with a magnitude of M5.6.

Daryono emphasized that earthquakes in the outer rise zone such as the one that rocked West Nias should not be ignored.

In the historical record of seismicity in Indonesia, there are two tsunamis caused by earthquakes originating in the outer rise zone, namely:

• Destructive tsunami in Sumbawa 1977
• 1921 Java Tsunami

“The source of this outer rise earthquake triggered the Lunyuk Tsunami, Sumbawa, on August 19 1977. At that time, the massive earthquake M 8.3 was known by seismologists as The Great Sumba, “said Daryono.

A devastating earthquake at that time triggered the formation of a fault in the seabed by means of a descending mechanism.

The fracture of the seabed with the descending mechanism triggered a tsunami about 8 meters high and killed more than 300 people.

Abroad, the earthquake source zone outer rise has also triggered a deadly tsunami.

The Sanriku Tsunami in Japan in 1933 was triggered by an earthquake measuring 8.6 which originated in the outer rise zone. The Sanriku tsunami killed more than 3,000 people.

Then an M 8.1 magnitude earthquake in the outer rise zone near the Tonga subduction on September 29, 2009 also triggered the Samoan tsunami in the Pacific, killing 189 people.

Also read: Earthquake Today: M 7.2 Shakes West Nias, No Tsunami Potential

“The outer rise is an earthquake zone that has been neglected, because the megathrust earthquake source zone is more popular,” said Daryono.

Even though it has been neglected, keep in mind that earthquakes in the outer rise zone are no less dangerous and can trigger a tsunami.

“The records of the tsunami originating outside the subduction zone above are sufficient to teach us all that the outer rise zone, including in Indonesia, is a tsunami trigger earthquake zone that should be watched out for and should not be ignored,” concluded Daryono.

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