1、.,Magnitude 7.7 NORTHERN SUMATRA, INDONESIATuesday, April 6, 2010 at 22:15:02 UTC,A major 7.7 earthquake occurred Tuesday 205km (125miles) west-northwest of Sibolga, Sumatra, Indonesia. There was widespread panic as residents rushed to higher ground. Local media report that hundreds of houses have b

2、een damaged, and there are electricity blackouts across the province. Following the earthquake, people remained out on the streets, wary of aftershocks.,USGS,Residents sit on a truck as they prepare to flee after an earthquake in Banda Aceh on Sumatra island, Indonesia, Tuesday, April 6, 2010. A 7.7

3、 earthquake shook Indonesias northwest island of Sumatra early Tuesday, prompting a brief tsunami warning and sending residents rushing for higher ground. (AP Photo/Heri Juanda),.,The red star shows the epicenter of the earthquake while the arrows show the direction of motion of the Australia-Indian

4、 Plate. At the location of this earthquake, the Australia-Indian Plate moves north-northeast with respect to the Sunda microplate at a velocity of approximately 60-65 mm/yr. The Sunda trench axis strikes approximately N 37W. The Indian Ocean crust is moving in an azimuth of N 23E relative to Southea

5、st Asia, giving an angle of obliquity of 60.,UNAVCO,Magnitude 7.7 NORTHERN SUMATRA, INDONESIATuesday, April 6, 2010 at 22:15:02 UTC,.,Magnitude 7.7 NORTHERN SUMATRA, INDONESIATuesday, April 6, 2010 at 22:15:02 UTC,The subduction zone surrounding the immediate region of this event last slipped during

6、 the M8.6 earthquake of March 2005, and todays event (red star) appears to have occurred within the rupture zone of that earthquake. Within a few years, almost the entire length of the Sumatra megathrust has ruptured: two M7.3 earthquakes 125 km to the north in 2002 and 2008; a M9.1 earthquake that

7、ruptured to within 125 km north of this earthquake in 2004; a M7.9 earthquake 250 km to the south in 2007; a M8.4 earthquake 375 km to the south in 2007; and a M7.5 earthquake 260 km to the south near in 2009.,NASA,Earthquake,.,The Indonesian region is one of the most seismically active zones of the

8、 earth. The map on the right shows historic earthquake activity near the epicenter (star) from 1990 to present. As shown on the cross section, earthquakes are shallow (orange dots) at the SundaTrench (purple line) and increase to 300 km depth (blue dots) towards the northeast as the Australia-Indian

9、 Plate dives beneath the Sunda microplate. The earthquake of April 6, 2010 nicely fits the pattern of depths for earthquakes that occur on the subsurface interface between the Australia Indian Plate and the Sunda microplate.,A,A,Magnitude 7.7 NORTHERN SUMATRA, INDONESIATuesday, April 6, 2010 at 22:1

10、5:02 UTC,USGS,.,USGS,Magnitude 7.7 NORTHERN SUMATRA, INDONESIATuesday, April 6, 2010 at 22:15:02 UTC,In most subduction zones, motion of the subducted plate is nearly perpendicular to the trench axis (purple line). In some cases, for example Sumatra, the convergent motion is oblique to the trench an

11、d a strike-slip fault is observed running near or along the volcanic chain (green line).,The geography of Indonesia is dominated by volcanoes that are formed due to subduction of the Australia-Indian Plate beneath the Sunda microplate.,.,This Sumatra earthquake occurred as a result of thrust faultin

12、g on or near the subduction interface plate boundary between the Australia-India and Sunda plates. On the basis of the,Magnitude 7.7 NORTHERN SUMATRA, INDONESIATuesday, April 6, 2010 at 22:15:02 UTC,USGS Centroid Moment Tensor Solution,The tension axis (T) reflects the minimum compressive stress dir

13、ection. The pressure axis (P) reflects the maximum compressive stress direction.,Idealized Focal Mechanism for Thrust Fault,currently available fault mechanism information and earthquake depth, it is likely that this earthquake occurred along the plate interface.,USGS,.,Magnitude 7.7 NORTHERN SUMATR

14、A, INDONESIATuesday, April 6, 2010 at 22:15:02 UTC,NOAA,Thailand Meteorological Department (TMD) STB - SAIC Tsunami Buoy (DART II payload),This map shows projected tsunami travel times from this earthquake. This map indicates forecasted times only, not that a wave traveling those distances has actua

15、lly been generated. Tsunami monitoring systems have been strategically deployed near regions with a history of tsunami generation, to ensure measurement of the waves as they propagate towards coastal communities and to acquire data critical to real-time forecasts.,A nearby tsunami buoy detected a sm

16、all elevation in wave height from this earthquake (indicated in red on the graph below). This caused a local tsunami watch to be issued for Indonesia but that watch has now been cancelled.,.,The record of the M7.7 Northern Sumatra, Indonesia earthquake on the University of Portland seismometer (UPOR

17、) is illustrated below.,Magnitude 7.7 NORTHERN SUMATRA, INDONESIATuesday, April 6, 2010 at 22:15:02 UTC,.,Portland is about 13,408 km (8331 miles, 120.79 degrees) from the location of this earthquake.,Direct P and S waves cannot travel to stations more than epicentral distance 103 because of the lar

18、ge decrease in wave velocities across the boundary between the mantle and the liquid outer core. There is a shadow zone for direct P waves in the range 103 103 because the liquid outer core blocks S waves that cannot travel through liquids.,PP is a compressive wave that traveled through Earths mantl

19、e and bounced midway between the epicenter and Portland (arrived 20 minutes 15 seconds); SS is a shear wave that also bounced midway between the epicenter and Portland (arrived 36 minutes 44 seconds).,Surface wave energy required approximately 51 minutes 6 seconds to travel around the perimeter of the Earth from Sumatra, Indonesia to Portland, Oregon.,Magnitude 7.7 NORTHERN SUMATRA, INDONESIATuesday, April 6, 2010 at 22:15:02 UTC,.,Magnitude 7.7 NORTHERN SUMATRA, INDONESIATuesday, April 6, 2010 at 22:15:02 UTC,Quick