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Situation in SS Sumatra after earthquakes September 30, М7.3 and October 1, М6.6, 2009 on the west of Sumatra, Indonesia. /October 02, 2009/

MONITORING AND FORECAST

SUMATRA, subsystems

NORTH and SOUTH

Мs≥7.3

MONITORING AND FORECAST

SUMATRA

Мs≥7.8

In a communication dated 16.08.2009 were given forecasts for strong earthquakes Ms≥7.8 in the SS Sumatra. It was reported that in zone A (see Fig.1) would be dangerous since March-April to August 2010. At the relative seismic gap here will be prepared a catastrophic earthquake with M=8.5-8.4. The earthquake of September 30, 2009 ☼ happened in the predicted zone A, but it is not expected earthquake; by the geometry of faulting we attribute it to the specific, not a typical earthquake for the Sunda trench. This earthquake is similar to the recent earthquake of September 2, 2009, M7.0 in the south-west of the Java (Indonesia), described in detail in a communication dated 06.09.2009.

SS Sumatra

IV level (badly determined).
Length of 1400 km.
Depth 80 km.
Threshold magnitude 7.8.
Monitoring since 2004.
Predicted quakes 1.

Major cities: Medan, Padang, Bengkulu, Palembang, Dzhanbi, Islands of Nias, Batu, Mentavay.

Seismicity caused by subduction of the Australian plate beneath the Sunda plate at the Sunda trench (to the west of Sumatra). The Australian plate is moving at a speed of 65 mm per year to the north-northeast with respect to the Sunda plate.

Last devastating earthquake was in September 12, 2007, M=8.5 (with the source depth of 30 km). 25 people died.

SPECIAL FEATURE OF THE MECHANISM OF EARTHQUAKE on September 30, 2009, SUMATRA, INDONESIA

Tectonics within the framework SS Sumatra is caused by the northeastern motion of Australian plate and by its sinking under the plate of the Sunda. The sources of strong typical earthquakes are timed to the plane of the contact of these plates in subduction zone and have thrust mechanism. Earthquake on September 30 occurred at the depth of 80 km; it also had the thrust mechanism (see Fig.1); however, at this earthquake occurred faulting within the subducting Australian plate, oriented perpendicularly to the Sunda trench (see Fig.3). Because of the special feature of such mechanism this earthquake did not excite the sensitive waves of tsunami, it did not have noticeable aftershock activity and did not cause such damage, which could cause the same force an earthquake with a typical mechanism for subduction zones. As it was earlier noted, until, unfortunately, we do not have an experience and have available small statistical material for the forecast of these earthquakes by the method of seismic entropy. But in both cases (in Java, on September 2, M7.0 and Sumatra, on September 30, M7.3) it is evident that the untypical earthquakes occurred near the zones A of the expectation of strong earthquakes, i.e., zones of maximum stress concentration at this period of time in the Sounda Trench. It is possible that between them there is some connection, then the Sunda trench could prepare here new surprises. Now the problem is - how to continue monitoring after these earthquakes in the SS Sumatra and Java. While in each case will decide, based on the seismic situation in the system.

Fig.2. Dynamics of an increase the probability of strong earthquake with М≥7.8 in SS Sumatra after quake in October 1, 2009, М6.6.

FORECAST EARTHQUAKES Мs≥7.8 IN THE SS SUMATRA. Earthquake on September 30, 2009, because of depth and geometry of rupturing, we add neither strong nor indicator-earthquakes. We consider that it did not discharge system and in the depth of source it was concluded it beyond the framework SS Sumatra (H<80 km). Thus, all forecasts, given here in a communication from 16.08.2009 remain valid. One ought not in no case to be calmed, in the zone A (near the islands Mentavay - Siberut and Sipura and coastal from Padang) occurs preparation and is expected catastrophic earthquake with M=8.4. Undoubtedly, earthquake dated September 30, 2009 will contribute and can accelerate the preparation for this catastrophe. Now, on October 2, 2009 (after indicator-earthquake dated October 1, 2009, M6.6) the probability of strong earthquake with M≥7.8 in SS Sumatra P=9% (on entropy P_W=18%, and on cumulative energy P_K=49%) (see Fig.2).

Fig.1. Sumatra, Indonesia. Controlled area of the forecast of earthquakes with M≥7.8. Danger remain in zone А. At August 2010 here it can be prepared catastrophic earthquake with М=8.4. Are shown epicentres and focal mechanisms of earthquakes on September 30 and October 1, 2009.

☼ Earthquake on the west of Sumatra, Indonesia

2009.09.30 07:38:26 1.40S 99.47E H=80 km Мs=7.3 CCD GS RAS

Fig.3. Intensity, the mechanism of seismic source 30.09.2009. Schematic 3D image of Australian plate and its sinking under the Sunda plate in the region of Sumatra (along the profile AB, view form the north). Are shown typical mechanisms and special feature of the mechanism of earthquake on September 30.

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SUMATRA, FORECAST earthquakes with Мs≥7.3. Subsystem NORTH. It will be dangerously by May 2010. Probability of the earthquake 29% (on entropy 38%, on cumulative energy 78%) (see Fig.4). Subsystem SOUTH. It will be dangerously by 2015. Probability of the earthquake 5% (on entropy 9%, on cumulative energy 51%). Let us recall that the earthquakes from M6.6-7.2 are not forecast. /October 02, 2009/

Fig.4. Dynamics of an increase the probability of strong earthquake with М≥7.3 in subsystems North and South of Sumatra after the earthquake October 1, 2009, М6.6.

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