Mw 7.8, Southwest of Sumatra, Indonesia Wed, 2 March 2016 at 12:49:48 UTC M /03/03

Transcription

1 Earthquake overview AFGHANISTA N PAKISTA N INDIA A moment magnitude (Mw) 7.8 earthquake struck in South West, Indonesia. The epicentre was centered about 800 km West South West of Padang, Sumatra province, Indonesia. M /03/03 Pakistan The depth has been estimated to be about 24 km (~15 miles). No fatalities or damaged to population centres have been reported. Image courtesy of Google Earth.

2 Earthquake hazard map Moderate (IV) shaking was felt in some of the Sumatran coastal regions closest to the epicenter. Otherwise Slight (III) shaking was felt in the rest of the surrounding region. Shaking Intensity Modified Mercalli Intensity Scale (MMI) I. Instrument al Not felt by many people unless in favourable conditions. II. Weak Felt only by a few people at best, especially on the upper floors of buildings. Delicately suspended objects may swing. III. Slight Felt quite noticeably by people indoors, especially on the upper floors of buildings. Many to do not recognise it as an earthquake. Standing motor cars may rock slightly. Vibration similar to the passing of a truck. Duration estimated. IV. Moderate Felt indoors by many people, outdoors by a few people during the day. At night, some awakened. V. Rather Strong Felt outside by most, may not be felt by some people in non-favourable conditions. Dishes and windows may break and large bells will ring. Vibrations like train passing close to house. VI. Strong Felt by all; many frightened and run outdoors, walk unsteadily. Windows, dishes, glassware broken; books fall off shelves; some heavy furniture moved or overturned; a few instances of fallen plaster. Damage slight. VII. Very Strong Difficult to stand; furniture broken; damage negligible in building of good design and construction; slight to moderate in well-built ordinary structures; considerable damage in poorly built or badly designed structures; some chimneys broken. Noticed by people driving motor cars. VIII. Destructive Damage slight in specially designed structures; considerable in ordinary substantial buildings with partial collapse. Damage great in poorly built structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture moved. IX. Violent General panic; damage considerable in poorly designed structures, well designed frame structures thrown out of plumb. Damage great in substantial buildings, with partial collapse. Buildings shifted off foundations. X. Intense Some well build wooden structures destroyed; most masonry and frame structures destroyed with foundation. Rails bent. XI. Extreme Few, if any masonry structures remain standing. Bridges destroyed. Rails bent greatly. XII. Cataclysmi c Total destruction everything is destroyed. Lines of sight and level distorted. Objects thrown into the air. The ground moves in waves or ripples. Large amounts of rock move position. Landscape altered, or leveled by several meters. In some cases, even the routes of rivers are changed. Image courtesy of US Geological Survey

3 Earthquake hazard map Green alert for shaking-related fatalities and economic losses There is a low likelihood of casualties and damage USGS PAGER Map with MMI contour overlay Images courtesy of the US Geological Survey

4 Earthquake slip model N/S transform faults Left: Graph displaying the total energy release over time for the rupture. The most energy release occurred approximately 15 seconds after the start of the earthquake. Right: Cross section of the preliminary slip model generated by the USGS from seismic data Images courtesy of US Geological Survey

5 Historical seismicity M /04/11 Large strike-slip earthquakes are not uncommon in the boundary region between the India and Australia plates. M /04/11 One such event occurred in 2000; a M7.9 earthquake was generated as a strike slip ruptured to the south east of the March 3, 2016 earthquake. In 2012, two earthquakes M8+ occurred in the same day due to a series of ruptures to the north. M /06/18 M /03/03 Image courtesy of Google Earth Despite their marine location, the strike slip nature of these earthquakes tends to NOT generate large tsunamis.

6 Earthquake focal mechanism The depth of this earthquake is 24 km, which is known as a crustal earthquake. The focal mechanism shows that this earthquake has a strike slip mechanism. ra at u bd Su n io ct n Zo e The rupture fault plane is orientated N-S, showing left lateral strike slip motion m Su The earthquake occurred in the Wharton basin transform faults which can be easily seen from the bathymetry. N/S transform faults Above: Corresponding focal mechanism for Indonesian earthquake (obtained from USGS) Left: Rupture of a strike slip fault showing left lateral displacement indicated by arrows, fault plane is indicated by thin black arrows Below: Location of the earthquake (yellow star). Transform faults trending North-South give information on ruptured fault plane

7 Tectonic setting The 2016 Mw 7.8 earthquake occurred nearly 600 km to the southwest of the Sumatra subduction zone, where the India and Australia plates subduct to the north-northeast beneath the Sunda plate. In the diffuse plate boundary region separating the India and Australia plates, southwest of the major subduction zone, there are also other strike slip earthquakes occurred before. All these events are caused by north striking fault or right lateral slip on an approximately west striking fault. Map showing the location of the plate boundary, subduction zone and the The relative plate motion rate between the Indo-Australia and Sunda plates is rapid. At southern tip of Sumatra is roughly 63 mm/yr northeast and then decreasing and counterclockwise rotating to 44 mm/yr northwest at Andaman Islands.

8 Seismic waves recorded in the UK P-waves S-waves Even though the earthquake occurred more that 10,900 km (~6700 miles) from the UK, the seismic waves emitted by this earthquake can still be measured. The plot above shows the vertical component of the seismic waves measured at seismic stations across the UK. The colour of the line corresponds to the station at which it was recorded, shown on the map on the right hand side of the slide.

9 Find more BGS (British Geological Survey) seismology and earthquakes frequently asked questions GO! IRIS (Incorporated Research Institutions for Seismology) learning about earthquakes GO! UK School Seismology Project classroom activities, videos and support documents GO! USGS (United States Geological Survey) FAQs, glossary, posters, animations GO! USGS summary of the earthquake GO! 9

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