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RELATIONSHIP BETWEEN FAULT AND EARTHQUAKE IN SUMATERA, INDONESIA 1. INTRODUCTION

Indonesia is located at the triple junction of the Australian Plate, Eurasian Plate, and Pacific Plate. The Australian Plate is converging with the southeastern segment of the Eurasian Plate, called the Sunda Plate, at a relative plate motion rate of approximately 60 mm/a. This plate motion is oblique to the Sumatran Subduction Zone. The boundary-parallel shear motion of the convergence transpires mainly along the Sumatran Fault, while the boundary-perpendicular convergent motion is accommodated by underthrusting along the Sunda Megathrust at a rate of approximately 45 mm/a. Because of these complex tectonics, Indonesia is an earthquake-prone country. Figure 1 shows that the tectonic and geographic location of Sumatran Fault.

The great Sumatra – Andaman earthquake and Tsunami of 2004 was dramatic reminder of the importance of understanding the seismic and tsunami hazards of subduction zones. There’s several earthquakes happen in Sumatran which are in March 2005 Sunda Megathrust ruptured producing moment magnitude amount 8.6; in 1797 producing moment magnitude amount of 8.8 and in 1833 producing moment magnitude amount of 9.0 at the same area. There is several ways to understand between the relationship between fault and earthquake. The commonly method is using GPS measurements, join inversion of teleseismic body wave data and strong ground motion data, InSAR, etc.

Here, we shows the relationship between those topics using two paper and what’s the historical earthquake occured to understand the patterns. Those earthquakes in Sumatran are correspond to each other and connected by the geographical of Sumatran Island.

Figure 1. Tectonic and Geographic Location of the Sumatran Fault (GSF). The red triangles mark the volcanoes.

2. METHODS AND DATA From different cases (Mentawai Earthquake and Sumatran – Andaman), this paper using joint inversion of

teleseismic and strong ground motion data to estimate 2010 of the Mentawai tsunami earthquake rupture process. The data from IRIS – DMC and BMKG station near the mainshock occurred. InSAR and GPS measurement to understand partial rupture of a locked patch of the Sumatra megathrust during the 2007 earthquake sequence. The data from calculation of GPS measurement in the field and also interpretation of InSAR image. All of these datas are use for understanding the relationship between fault and earthquake. For all details it will explain at results and discussion section.

3. RESULTS AND DISCUSSION

From combining two papers we know that the relationship between earthquake and fault is Earthquake occur on Faults. Strike – slip earthquakes occur on strikes – slip faults, normal earthquakes occur on normal faults and thrust earthquake occur on thrust or reverse faults. When an earthquake occurs on one of these faults, the rock on one side of the fault slips with respect to other. The fault surface can be vertical, horizontal, or at some angle to surface of the earth. The slip direction can also be at any angle. From this relationship, we take two papers as an example. It can reinforce our statement about the relationship between earthquake and faults. In Paper 1 (Estimation of the 2010 Mentawai tsunami earthquake rupture process from joint inversion of teleseismic and strong ground motion data) shows The 2010 Mentawai earthquake generated a locally devastating tsunami much larger than expected based on the seismic magnitude. Source process inversion results indicate a shallow dip, consistent with an origin on the Sunda Megathrust. The rupture nucleated around the hypocenter and propagated to the southwest and broke the first asperity centering at 14 km from the epicenter with maximum slip amounting to 3.9 m, then propagated along the strike direction to the northwest where the second asperity was broken, which was centered about 78 km from the epicenter. this paper identify this earthquake as a tsunami earthquake because of its excessively long rupture duration and its generation of a greater than expected tsunami. However, there are large differences in slip distributions from the different modeling methods and datasets. Although this paper cannot reconcile these complexities and large uncertainties on the amount of slip still exist, this paper found the conclusion that the majority of slip occurred far from the islands at very shallow depths to be robust. As mentioned above, many large earthquakes have occurred in this region. After comprehensive analysis, the 2010 Mentawai Mw7.8 earthquake ruptured immediately updip of and was probably triggered by stress changes following the September 2007 Mw8.5 Sumatran earthquake. This area may have last ruptured as part of the 1797 Mw8.6 and 1833 Mw8.9 events, described by Natawidjaja et al. as having about 18 m of megathrust slip to explain the co-seismic uplift. Further north, the 2005 Mw8.6 Nias earthquake ruptured the same approximate area. Available high resolution

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bathymetry along the trench suggests that significant faulting in the region may be due to rupture through the prism toe during the 2004 Sumatran Giant earthquake and previous earthquakes. The large slip estimated in the shallow trench and the considerable faulting near the trench toe further north support the hypothesis that the subduction zone off western Indonesia is capable of supporting shallow megathrust slip. It challenges the conventional wisdom that the shallow tips of subduction megathrusts are aseismic and, therefore, raises important questions both about the mechanical properties of the shallow fault zone and the potential seismic and tsunami hazards of this shallow region

Figure 2. Tectonic Map of the 2010

Mentawai Earthquake Region. An Expanded View

of The Study Region is Shown in upright

Corner.

Figure 3. Joint Inversion Result of

the Mentawai Earthquake

Figure 4. Slip Distribution Map

View for The Week After the Mainshock

From other paper (Partial Rupture of Locked Patch of the Sumatra Megathrust during the 2007 Earthquake

Secquence) shows that the rupture area of the 2007 Mentawai earthquakes was confined to a subset of a locked portion that is surrounded by creep during the interseismic period. Such permanent barriers, which are found to influence the down-dip extent as well as the lateral extent of megathrust ruptures, can be imaged from the modelling of interseismic strain except when they lie in stress shadows, inparticular along the up-dip portion of the plate interface. The complex spatio-temporal pattern of the 2007 rupture is probably related to the fact that it produced much less slip than did historical earthquakes in the area. The 2007 ruptures released only 25% of the deficit of moment that had accumulated since the last rupture. The sequence essentially ruptured a set of asperities, which triggered each other through static and dynamic interactions but did not cooperate efficiently, because of the intervening barriers. Some of these barriers are most likely not permanent and are related to the slip in past earthquakes. Whereas permanently creeping barriers should tend to favour some regularity and similarity of earthquakes, the presence of non-permanent barriers due to the stress distribution left over from previous ruptures is probably the major factor introducing irregularity, as observed in dynamic fault models. This is probably the main reason that neither the slip-predictable nor the time-predictable models apply, and why the 2007 earthquakes did not grow as big as the 1833 earthquake. This supports the view that seismic asperities are probably not permanent features 28 but rather move from one rupture to another within the area that is locked in the interseismic period.

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Figure 5. Patches with Strong Inteseismic Coupling on the Sunda Megathrust Coincide

With Large Seismic Ruptures.

Figure 6. Models of Megathrust Slip During the Mw = 8.4 and Mw = 7.9 Earthquakes Show

Principal Slip on Widely Separated Patches

Figure 7. Comparison of The Moment Release Deficit

Accumulated Since the 1797 and 1833 Earthquakes.

The conclusion of the relationship between faults and earthquakes in Sumatra according two paper is first, these

Earthquakes are correspond each other. Besides, earthquake occurred because the fracture between two blocks of rock allows the blocks to move relative to each other and the movement may occur rapidly so earthquake happened, The geographic of Sumatran shows the variety of topography such as volcanoes, seamount and etc triggered the tectonic plate activity around Sumatra.

Second is, from two paper explains the historical of earthquake happened in Sumatran. From historical of earthquake in Sumatra, we can understand about the estimation of rupture processed. It indicates that every earthquakes in Sumatra are related each other along subduction zone or coupling zone. It can influence one to another faults to move rapidly or slowly (creep) around Sumatra area.

4. REFERENCE Konca, A Ozgun, Jean Phillipe Avovac, Anthony Sladen, Aron J Meltzner, Kerry Sieh, Peng Fang, Zhenhong Li, et al. 2008.

"Partial Rupture of a Locked Patch of the Sumatra Megathrust During the 2007 Earthquake Sequence." Nature ,

December 4: 631-635.

Lifen, Zhang, Liao Wulin, Li Jinggang, and Wang Qioliang. 2015. "Estimation of the 2010 Mentawai Tsunami Earthquake

Rupture Process From Joint Inversion of Teleseismic and Strong Ground Motion Data." Science Direct, February

20: 180-186.