1990 South Sudan earthquakes

In 1990, present day South Sudan was rocked by a series of violent earthquakes. It started with the largest event, a Mw  7.2, and continued with multiple very large aftershocks for the next couple of months. It contains some of the largest recorded earthquakes anywhere in Africa.

1990 South Sudan earthquakes
1990 South Sudan earthquakes is located in South Sudan
Juba
Juba
1990 South Sudan earthquakes
1990 South Sudan earthquakes is located in Africa
Juba
Juba
1990 South Sudan earthquakes
UTC time1990-05-20 02:22:01
 1990-05-24 19:34:44
 1990-05-24 20:00:08
 1990-07-09 15:11:20
ISC event369792
 370208
 370216
 362203
USGS-ANSSComCat
 ComCat
 ComCat
 ComCat
Local dateMay 20, 1990 to July 9, 1990
Local time04:22:01
 21:34:44
 22:00:08
 17:11:20
Magnitude7.2 Mw[1]
 6.5 Mw[2]
 7.1 Mw[3][4][1]
 6.6 Mw[1]
Epicenter5.121°N 32.145°E / 5.121; 32.145
TypeStrike-slip & Normal
Areas affectedSouth Sudan
Total damageYes
Max. intensityVIII (Severe)[5]
Casualties31 dead[6]

Tectonic setting

The East African Rift System (EARS) is a system of rifts and associated rift lakes within the eastern portion of the African continent. It accommodates the internal breakup of Africa. It is a boundary between the Somali and African Plate. The Victoria and Rovuma microplates help accommodate additional more local stresses. The eastern portion of the rift runs from the Afar rift to Tanzania, while the western portion runs from Lake Albert (Africa) in Uganda all the way down to Mozambique.[3]

The Aswa Rift Zone (ARZ) is an important geologic structure in the area near the earthquakes. The ARZ may act as a broad zone of faulting that links together the eastern and western portions of the EARS.[7] The mainshock occurred along a fault where the ARZ and EARS meet.[4]

Earthquake sequence

May 20 mainshock

The first and largest earthquake in the sequence struck near Juba at 02:22:01 on May 20, 1990. The event had a mb  of 6.6, with a Ms  and Mw  of 7.2, and struck at a depth of 7–15 km (4.3–9.3 mi).[4][3][8][1] Its focal mechanism shows left-lateral (sinistral) strike-slip faulting, which is consistent with the shear forces in the area of the ARZ.[3] The event killed 31 people and damaged some buildings in Juba, and some in Moyo, Uganda. This event is believed to be the largest earthquake ever recorded in South Sudan, and is one of the largest events known in all of Africa.[5][3][7][8][1][6]

May 24 foreshock

At 19:34:44 on May 24, 1990, another large earthquake struck southern South Sudan. It occurred at a depth of 10–14 km (6.2–8.7 mi) with a mb  of 6.1, Ms  of 6.5-6.8 and (Mw ) of 6.5-6.6.[2][3][4][1] The focal mechanism solution varies, with some showing the earthquake being the result of normal dip-slip faulting, while others suggesting it was the result of reverse dip-slip faulting instead.[9][3]

May 24 mainshock

At 20:00:08, only a little over 25 minutes after the foreshock, an even stronger earthquake struck the region: the second strongest of the whole sequence. This event was a mb  6.6, Ms  7.0, Mw  7.1 quake that occurred at a depth of 10–40 km (6.2–24.9 mi).[3][4][1] The focal mechanisms by various sources again disagree, with some showing normal faulting, with others showing strike slip faulting.[3][10][11] It caused additional structural damage to buildings previously affected by the first and strongest shock, however, no casualties occurred.[6] The worst damage was observed in uninhabited areas.[6]

July 9 mainshock

After the sequence had largely finished, another large shock struck the area for a final time. The last big earthquake in the sequence was a mb  6.0, Ms  6.4-6.5 and Mw  6.6 event. It occurred at a depth of 10–11 km (6.2–6.8 mi).[1] Focal mechanism solutions are split, with a roughly equal proportion of agencies and studies preferring a strike-slip event, with another equal proportion believing it to be a normal faulting event.[12][3][13]

Damage

The mainshock killed 31 people and damaged buildings in Juba and Moyo, Uganda. Shaking was felt as far as Nakuru, Kenya.[6] The mainshock of May 24 did further damage. Government buildings and private institutions were damaged in Terekeka, South Sudan.[14] Older buildings in Juba reported cracked walls.[14] Between 8,000 and 10,000 of the displaced required extra relief and shelter.[14] 300,000 people were made homeless across the region as a result of the earthquakes.[15]

See also

References
  1. Ambraseys & Adams 1991.
  2. "M 6.5 - 54 km NNE of Juba, South Sudan". USGS. USGS. Retrieved 28 August 2022.
  3. Gaulon et al. 1992.
  4. Mulwa & Kimata 2014.
  5. "M 7.2 - 69 km ENE of Juba, South Sudan". USGS. USGS. Retrieved 28 August 2022.
  6. National Geophysical Data Center / World Data Service (NGDC/WDS) (1990), Significant Earthquake Database, National Geophysical Data Center, NOAA, doi:10.7289/V5TD9V7K
  7. Giardini & Beranzoli 1992.
  8. Moussa 2008.
  9. "ISC". ISC-GEM. ISC-GEM. Retrieved 28 August 2022.
  10. "M 7.1 - 63 km NNE of Juba, South Sudan". USGS. USGS. Retrieved 28 August 2022.
  11. "ISC". ISC-GEM. ISC-GEM. Retrieved 28 August 2022.
  12. "M 6.6 - 60 km N of Juba, South Sudan". USGS. USGS. Retrieved 28 August 2022.
  13. "ISC". ISC-GEM. ISC-GEM. Retrieved 28 August 2022.
  14. "Sudan Earthquake May 1990 UNDRO Information Reports 1-2". reliefweb. UN DHA. Retrieved 28 August 2022.
  15. "More earthquakes hit Sudan, 300,000 said homeless". UPI. UPI. Retrieved 29 August 2022.

Sources

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