1990 South Sudan earthquakes

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.{{Sfn|Gaulon|Chorowicz|Vidal|Romanowicz|1992}}

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.{{Sfn|Giardini|Beranzoli|1992}} The mainshock occurred along a fault where the ARZ and EARS meet.{{Sfn|Mulwa|Kimata|2014}}

The first and largest earthquake in the sequence struck near Juba at 02:22:01 on May 20, 1990. The event had a {{M|b}} of 6.5-6.7, {{M|s}} of 7.1-7.4, and {{M|w}} of 7.2, and struck at a depth of {{convert|7-15|km|abbr=on}}.{{efn-lr|{{M|b|6.5}}{{sfn|Girdler|McConnell|1994|p=68}}-{{M|b|6.7}}{{Sfn|Moussa|2008|p=90}}
{{M|s|7.1-7.4}}{{Sfn|Giardini|Beranzoli|1992|p=106}}
{{M|w|7.2}}{{Sfn|Gaulon|Chorowicz|Vidal|Romanowicz|1992|p=96}}
7 km depth{{Sfn|Mulwa|Kimata|2014|p=493}}-15 km{{Sfn|Ambraseys|Adams|1991|p=399}}}} Its focal mechanism shows left-lateral (sinistral) strike-slip faulting, which is consistent with the shear forces in the area of the ARZ.{{Sfn|Gaulon|Chorowicz|Vidal|Romanowicz|1992}} The earthquake ruptured over an area of {{cvt|26-60|km|abbr=on}} by {{cvt|40|km|abbr=on}}. The average slip in the rupture was {{cvt|0.56-1.1|m|abbr=on}} and it had a stress drop of 1.63-1.65 MPa.{{sfn|Moussa|2008|p=94}}{{sfn|Mulwa|Kimata|2014|p=493}} The earthquake had an aftershock length of {{cvt|120|km|abbr=on}}.{{sfn|Pegler|Das|1996|p=906}} The earthquake was preceded by a low-frequency slow slip signal immediately before the high-frequency main event, which could mean the event was an amalgamation of a slow and normal earthquake.{{sfn|Ihmle|Jordan|1994|p=1547}} The earthquake killed 31 people and damaged some buildings in Juba, and some in Moyo, Uganda. The Nile's banks had fissures, and a bridge near Torit was destroyed. The shock was felt for up to {{cvt|800|km|abbr=on}}.{{sfn|Ambraseys|Adams|1991|p=398}} 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.{{Sfn|Gaulon|Chorowicz|Vidal|Romanowicz|1992|p=87}}

At 19:34:44 on May 24, 1990, another large earthquake struck southern South Sudan. It occurred at a depth of {{convert|10-14|km|abbr=on}} with a {{M|b}} of 6.1, {{M|s}} of 6.5-6.8 and {{M|w}} of 6.5-6.6.{{Sfn|Gaulon|Chorowicz|Vidal|Romanowicz|1992}}{{Sfn|Mulwa|Kimata|2014}}{{Sfn|Ambraseys|Adams|1991}} By analyzing body waves, the earthquake was found to have slipped in a normal sense.{{Sfn|Gaulon|Chorowicz|Vidal|Romanowicz|1992|p=98}}

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 {{M|b}} 6.6, {{M|s}} 7.0, {{M|w}} 7.1 earthquake that occurred at a depth of {{convert|10-40|km|abbr=on}}.{{Sfn|Gaulon|Chorowicz|Vidal|Romanowicz|1992}}{{Sfn|Mulwa|Kimata|2014}}{{Sfn|Ambraseys|Adams|1991}} The focal mechanism of the earthquake was normal faulting.{{Sfn|Girdler|McConnell|1994|p=67}} It caused additional structural damage to buildings previously affected by the first and strongest shock, however, no casualties occurred. The worst damage was observed in uninhabited areas.

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 {{M|b}} 6.0, {{M|s}} 6.4-6.5 and {{M|w}} 6.6 event. It occurred at a depth of {{convert|10-11|km|abbr=on}}.{{Sfn|Ambraseys|Adams|1991}} 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.{{Sfn|Gaulon|Chorowicz|Vidal|Romanowicz|1992}}

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

• List of earthquakes in 1990
• 2006 Mozambique earthquake

{{notelist-lr}}

{{reflist|refs={{cite web |title=M 7.2 – 69 km ENE of Juba, South Sudan |url=https://earthquake.usgs.gov/earthquakes/eventpage/usp00049hf/executive |publisher=United States Geological Survey |access-date=28 August 2022}}{{cite web |title=M 6.5 – 54 km NNE of Juba, South Sudan |url=https://earthquake.usgs.gov/earthquakes/eventpage/usp00049q9/executive |publisher=United States Geological Survey |access-date=28 August 2022}}{{cite web |title=M 7.1 – 63 km NNE of Juba, South Sudan |url=https://earthquake.usgs.gov/earthquakes/eventpage/usp00049qe/executive |publisher=United States Geological Survey |access-date=28 August 2022}}{{cite web |title=M 6.6 – 60 km N of Juba, South Sudan |url=https://earthquake.usgs.gov/earthquakes/eventpage/usp0004bkr/executive |publisher=United States Geological Survey |access-date=28 August 2022}}{{cite web |title=ISC |url=http://www.isc.ac.uk/cgi-bin/web-db-run?event_id=362203&out_format=ISF2&request=COMPREHENSIVE |website=ISC-GEM |access-date=28 August 2022}}{{citation|title=Significant Earthquake Database|url=https://www.ngdc.noaa.gov/hazel/view/hazards/earthquake/event-more-info/5237|author=National Geophysical Data Center / World Data Service (NGDC/WDS)|year = 1990|publisher=National Geophysical Data Center, NOAA|doi=10.7289/V5TD9V7K}}{{cite web |title=Sudan Earthquake May 1990 UNDRO Information Reports 1-2 |date=29 May 1990 |url=https://reliefweb.int/report/sudan/sudan-earthquake-may-1990-undro-information-reports-1-2 |publisher=ReliefWeb |access-date=28 August 2022}}{{cite web |title=More earthquakes hit Sudan, 300,000 said homeless |url=https://www.upi.com/Archives/1990/05/26/More-earthquakes-hit-Sudan-300000-said-homeless/4401643694400/ |work=UPI |access-date=29 August 2022}}

}}

Sources

{{refbegin}}

• {{cite journal |last1=Ambraseys |first1=N. N. |last2=Adams |first2=R. D. |title=Reappraisal of major African earthquakes, south of 20°N, 1900–1930 |journal=Natural Hazards |date=December 1991 |volume=4 |issue=4 |pages=389–419 |doi=10.1007/BF00126646 |s2cid=129238922 |url=https://doi.org/10.1007/BF00126646 |access-date=28 August 2022|url-access=subscription }}

{{refend}}

• {{cite journal |last1=Gaulon |first1=R. |last2=Chorowicz |first2=Jean |last3=Vidal |first3=Gérard |last4=Romanowicz |first4=Barbara |title=Regional geodynamic implications of the May–July 1990 earthquake sequence in southern Sudan |journal=Tectonophysics |date=August 1992 |volume=209 |issue=1 |pages=87–103 |doi=10.1016/0040-1951(92)90012-U |bibcode=1992Tectp.209...87G |url=https://doi.org/10.1016/0040-1951(92)90012-U |access-date=28 August 2022|url-access=subscription }}
• {{cite journal |last1=Giardini |first1=Domenico |last2=Beranzoli |first2=Laura |title=Waveform modelling of the May 20, 1990 Sudan earthquake |journal=Tectonophysics |date=20 August 1992 |volume=209 |issue=1–4 |pages=105–114 |doi=10.1016/0040-1951(92)90013-V |bibcode=1992Tectp.209..105G |url=https://doi.org/10.1016/0040-1951(92)90013-V |access-date=28 August 2022|url-access=subscription }}
• {{cite journal |last1=Girdler |first1=R. W. |last2=McConnell |first2=D. A. |title=The 1990 to 1991 Sudan Earthquake Sequence and the Extent of the East African Rift System |journal=Science |date=1 Apr 1994 |volume=264 |issue=5155 |pages=67–70 |doi=10.1126/science.264.5155.67 |pmid=17778134 |url=https://www.science.org/doi/abs/10.1126/science.264.5155.67 |access-date=13 July 2024|url-access=subscription }}
• {{cite journal |last1=Ihmle |first1=Pierre F. |last2=Jordan |first2=Thomas H. |title=Teleseismic Search for Slow Precursors to Large Earthquakes |journal=Science |date=2 Dec 1994 |volume=266 |issue=5190 |pages=1547–1551 |doi=10.1126/science.266.5190.1547 |pmid=17841716 |url=https://www.science.org/doi/10.1126/science.266.5190.1547 |access-date=13 July 2024|url-access=subscription }}
• {{cite journal |last1=Moussa |first1=Hesham Hussein Mohamed |title=Spectral P-wave magnitudes, magnitude spectra and other source parameters for the 1990 southern Sudan and the 2005 Lake Tanganyika earthquakes |journal=Journal of African Earth Sciences |date=2008 |volume=52 |issue=3 |pages=89–96 |doi=10.1016/j.jafrearsci.2008.05.004 |bibcode=2008JAfES..52...89M |url=https://doi.org/10.1016/j.jafrearsci.2008.05.004 |access-date=28 August 2022 |issn=1464-343X|url-access=subscription }}
• {{cite journal |last1=Mulwa |first1=Josphat K. |last2=Kimata |first2=Fumiaki |title=Tectonic structures across the East African Rift based on the source parameters of the 20 May 1990 M7.2 Sudan earthquake |journal=Natural Hazards |date=21 February 2014 |volume=73 |issue=2 |pages=493–506 |doi=10.1007/s11069-014-1082-y |bibcode=2014NatHa..73..493M |s2cid=140581505 |url=https://doi.org/10.1007/s11069-014-1082-y |access-date=28 August 2022|url-access=subscription }}
• {{cite journal |last1=Pegler |first1=G. |last2=Das |first2=S. |title=Analysis of the relationship between seismic moment and fault length for large crustal strike-slip earthquakes between 1977–92 |journal=Geophysical Research Letters |date=1 May 1996 |volume=23 |issue=9 |pages=905–908 |doi=10.1029/96GL00963 |url=https://agupubs.onlinelibrary.wiley.com/doi/10.1029/96GL00963 |access-date=13 July 2024}}

• [https://earthquake.usgs.gov/earthquakes/eventpage/usp00049hf/executive M 7.2 – 69 km ENE of Juba, South Sudan] – United States Geological Survey
• [https://earthquake.usgs.gov/earthquakes/eventpage/usp00049q9/executive M 6.5 – 54 km NNE of Juba, South Sudan] – United States Geological Survey
• [https://earthquake.usgs.gov/earthquakes/eventpage/usp00049qe/executive M 7.1 – 63 km NNE of Juba, South Sudan] – United States Geological Survey
• [https://earthquake.usgs.gov/earthquakes/eventpage/usp0004bkr/executive M 6.6 – 60 km N of Juba, South Sudan] – United States Geological Survey
• {{EQ-isc-link|369792}}
• {{EQ-isc-link|370208}}
• {{EQ-isc-link|370216}}
• {{EQ-isc-link|362203}}

{{Earthquakes in 1990}}

{{Earthquakes in Africa}}

South Sudan earthquakes

South Sudan earthquakes

South Sudan earthquakes

South Sudan earthquakes

Category:Earthquakes in Africa

South Sudan earthquakes