SIKLUS MEGA-TSUNAMI DI WILAYAH ACEH-ANDAMAN DALAM KONTEKS SEJARAH

Abstract Mega‐tsunami Aceh‐Andaman 2004 revolutionary changed people awareness of earthquakes and tsunami threats. The event also caused major changes in politics and social infrastructures, from a period of terror to a new government of NAD.  Paleoseismological studies indicate two penultimate tsunami events prior to 2004 around 1390 AD and 1440 AD. These are confirmed by the GPS study suggesting the 2004-like event (Mw9.15) can be repeated every six hundred years. In 1236 AD, the well known Islamic state, Samudra Pasai, was arise, marking a new era in Aceh. After 1450 AD, Samudra Pasai seems to be slowly dissapeared.  Later in 1496 AD, a new Islamic Kingdom, Aceh Darussalam, appeared and dominated the Aceh region. It is strongly suspected that the changes of power from Samudra Pasai to Aceh Darussalam was linked to the mega‐tsunami events in 1390 and 1440 AD. Understanding ancient natural catastrophic and the affected society is crucial in developing awareness and in natural‐dissaster mitigations, including to rejuvinate a true local wisdomAbstrak Mega tsunami di wilayah Aceh-Andaman pada tahun 2004 merubah masyarakat menjadi melek terhadap ancaman bencana gempa dan tsunami .  Bencana 2004 merubah pemerintahan dan tatanan masyarakat di Aceh, dari masa teror ke pemerintahan NAD yang baru.  Penelitian paleoseismologi menguak peristiwa bencana gempa-tsunami tahun  sebelumnya, sekitar tahun 1390 M dan 1450 Masehi.  Fakta ini ditunjang oleh data tektonik geodesi (GPS) bahwa siklus perulangan gempa 2004 (Mw9.15) dapat terjadi sekitar 600 tahunan sekali.   Pada tahun 1236, berdirinya Kerajaan islam Samudra Pasai yang cukup dikenal menandai era baru di Aceh.   Setelah tahun 1450 Masehi, Kerajaan Samudra Pasai ini seperti meredup dan menghilang.   Kemudian  pada tahun 1496 Masehi berdiri Kerajaan Baru Islam, Aceh Darussalam yang tidak ada hubungannya dengan Samudra Pasai. Diduga peralihan masa Samudra Pasai  ke masa Aceh Darussalam berkaitan erat dengan kejadian tsunami tahun 1390 dan 1440 Masehi tersebut.   Memahami kejadian bencana katastropik purba dan masyarakat yang terkena dampaknya adalah aspek yang sangat penting dalam pendidikan kebencanaan, khususnya dalam mengembangkan kesiapsiagaan dan kearifan lokal. 

Major Bifurcations, Slip Rates, and A Creeping Segment of Sumatran Fault Zone in Tarutung-Sarulla-Sipirok-Padangsidempuan, Central Sumatra, Indonesia

DOI: 10.17014/ijog.5.2.137-160A detailed active fault study in Tarutung-Sarulla-Sipirok-Padangsidempuan was conducted based on their tectonic-morphological features using SRTM-30, 3D-visualization, and LIDAR data, combined with field and shallow geophysical surveys using the GPR method. Sumatran Fault Zone is bifurcated from the single major Sianok fault segment into two major branches: Angkola and Barumun-Toru Faults that run (sub) parallel to each other. In the studied area, they are merged gradually to become the Renun Fault. The total slip rates from Sianok to Renun segments are constant at about ~ 14 mm/year (13.8 ± 0.3 mm/yr on Renun and 13.7 ± 1.6 mm/yr on Sianok segments). In the bifurcation zone, it is partitioned into 9.3 ± 1.8 mm/yr slip on Toru, and about 4 - 5 mm/yr on Angkola segments. Based on field evidence supported by the seismicity and historical record, the Toru Fault appears to move continuously (creeping). This is crucial for understanding tectonics and its significance to hazard mitigations. Further investigations on Angkola and Toru Faults are crucial for mega installations of Sarulla Geothermal Power Plant, which is located in between Angkola and Toru Fault zones.</p

Source Processes of the March 2007 Singkarak Earthquakes Inferred from Teleseismic Data

The rupture processes of two sequentialearthquakes have been inverted from teleseismic data. The first event released a total seismic moment of 7.9×1018 Nm (Mw 6.5) and the slip distribution shows three asperities, 1.5 m at the shallowside, 0.7 m at the rightsouth-east deep side and 0.5 m atthe north-west deep side. The second event had one asperity with 1.7 m slip and released a seismic moment of 7.5×1018 Nm (Mw 6.5). In both cases, maximum slip occurred above the hypocenter which was responsible for the surface displacement pattern

Interseismic deformation above the Sunda Megathrust recorded in coral microatolls of the Mentawai islands, West Sumatra

The geomorphology and internal stratigraphy of modern coral microatolls show that all the outer arc Mentawai islands of West Sumatra have been subsiding over the past several decades. These same islands rose as much as 3 m during the giant megathrust earthquakes of 1797 and 1833, and the current subsidence probably reflects strain accumulation that will lead to future large earthquakes. Average subsidence rates over the past half century vary from 2 to 14 mm yr^(−1) and increase southwestward, toward the subduction trench. The pattern is consistent with rates of subsidence measured by a sparse network of continuously recording Global Positioning System (cGPS) stations and with locking of a 400-km-long section of the underlying subduction megathrust, between about 1°S and 4°S. This record of subsidence and tilting, extending nearly a century into the past, implies that the region is advancing toward the occurrence of another giant earthquake. However, evidence of episodic rather than steady subsidence reflects a behavior that is more complex than simple elastic strain accumulation and relief. Most prominent of these episodes is an extensive emergence/subsidence couplet in about 1962, which may be the result of rapid, aseismic slip on the megathrust, between the islands and the trench. Lower subsidence rates recorded by the corals since about 1985 may reflect failure on many small patches within the locked section of the megathrust

Source parameters of the great Sumatran megathrust earthquakes of 1797 and 1833 inferred from coral microatolls

Large uplifts and tilts occurred on the Sumatran outer arc islands between 0.5° and 3.3°S during great historical earthquakes in 1797 and 1833, as judged from relative sea level changes recorded by annually banded coral heads. Coral data for these two earthquakes are most complete along a 160-km length of the Mentawai islands between 3.2° and 2°S. Uplift there was as great as 0.8 m in 1797 and 2.8 m in 1833. Uplift in 1797 extended 370 km, between 3.2° and 0.5°S. The pattern and magnitude of uplift imply megathrust ruptures corresponding to moment magnitudes (M_w) in the range 8.5 to 8.7. The region of uplift in 1833 ranges from 2° to at least 3.2°S and, judging from historical reports of shaking and tsunamis, perhaps as far as 5°S. The patterns and magnitude of uplift and tilt in 1833 are similar to those experienced farther north, between 0.5° and 3°N, during the giant Nias-Simeulue megathrust earthquake of 2005; the outer arc islands rose as much as 3 m and tilted toward the mainland. Elastic dislocation forward modeling of the coral data yields megathrust ruptures with moment magnitudes ranging from 8.6 to 8.9. Sparse accounts at Padang, along the mainland west coast at latitude 1°S, imply tsunami runups of at least 5 m in 1797 and 3–4 m in 1833. Tsunamis simulated from the pattern of coral uplift are roughly consistent with these reports. The tsunami modeling further indicates that the Indian Ocean tsunamis of both 1797 and 1833, unlike that of 2004, were directed mainly south of the Indian subcontinent. Between about 0.7° and 2.1°S, the lack of vintage 1797 and 1833 coral heads in the intertidal zone demonstrates that interseismic submergence has now nearly equals coseismic emergence that accompanied those earthquakes. The interseismic strains accumulated along this reach of the megathrust have thus approached or exceeded the levels relieved in 1797 and 1833

Partial rupture of a locked patch of the Sumatra megathrust during the 2007 earthquake sequence

The great Sumatra–Andaman earthquake and tsunami of 2004 was a dramatic reminder of the importance of understanding the seismic and tsunami hazards of subduction zones [1,2,3,4]. In March 2005, the Sunda megathrust ruptured again, producing an event [5] of moment magnitude (Mw) 8.6 south of the 2004 rupture area, which was the site of a similar event in 1861 (ref. 6). Concern was then focused on the Mentawai area, where large earthquakes had occurred in 1797 (Mw = 8.8) and 1833 (Mw = 9.0) [6,7]. Two earthquakes, one of Mw = 8.4 and, twelve hours later, one of Mw = 7.9, indeed occurred there on 12 September 2007. Here we show that these earthquakes ruptured only a fraction of the area ruptured in 1833 and consist of distinct asperities within a patch of the megathrust that had remained locked in the interseismic period. This indicates that the same portion of a megathrust can rupture in different patterns depending on whether asperities break as isolated seismic events or cooperate to produce a larger rupture. This variability probably arises from the influence of non-permanent barriers, zones with locally lower pre-stress due to the past earthquakes. The stress state of the portion of the Sunda megathrust that had ruptured in 1833 and 1797 was probably not adequate for the development of a single large rupture in 2007. The moment released in 2007 amounts to only a fraction both of that released in 1833 and of the deficit of moment that had accumulated as a result of interseismic strain since 1833. The potential for a large megathrust event in the Mentawai area thus remains large

Paleogeodetic records of seismic and aseismic subduction from central Sumatran microatolls, Indonesia

We utilize coral microatolls in western Sumatra to document vertical deformation associated with subduction. Microatolls are very sensitive to fluctuations in sea level and thus act as natural tide gauges. They record not only the magnitude of vertical deformation associated with earthquakes (paleoseismic data), but also continuously track the long-term aseismic deformation that occurs during the intervals between earthquakes (paleogeodetic data). This paper focuses on the twentieth century paleogeodetic history of the equatorial region. Our coral paleogeodetic record of the 1935 event reveals a classical example of deformations produced by seismic rupture of a shallow subduction interface. The site closest to the trench rose 90 cm, whereas sites further east sank by as much as 35 cm. Our model reproduces these paleogeodetic data with a 2.3 m slip event on the interface 88 to 125 km from the trench axis. Our coral paleogeodetic data reveal slow submergence during the decades before and after the event in the areas of coseismic emergence. Likewise, interseismic emergence occurred before and after the 1935 event in areas of coseismic submergence. Among the interesting phenomenon we have discovered in the coral record is evidence of a large aseismic slip or “silent event” in 1962, 27 years after the 1935 event. Paleogeodetic deformation rates in the decades before, after, and between the 1935 and 1962 events have varied both temporally and spatially. During the 25 years following the 1935 event, submergence rates were dramatically greater than in prior decades. During the past four decades, however, rates have been lower than in the preceding decades, but are still higher than they were prior to 1935. These paleogeodetic records enable us to model the kinematics of the subduction interface throughout the twentieth century

Neotectonics of the Sumatran Fault and Paleogeodesy of the Sumatran Subduction Zone

Under the Sumatran plate boundary the Australian-Indian plate is subducting at about 60 mm/yr in the direction N11°E. The oblique convergence is partitioned into trench-parallel slip - accommodated largely by the Sumatran fault zone and trench-perpendicular slip - accommodated by the subduction zone. Our detailed map of the Sumatran fault zone (SFZ) shows that the Sumatran fault is highly segmented. The influence of these fault segmentations on historical seismic source dimensions suggests that the dimensions of future events will also be influenced by fault geometry. The largest geomorphic offsets along the Sumatran fault zone are about 20 km, and may represent the total offset across the fault. The shape and location of the Sumatran fault and the active volcanic arc are highly correlated with the shape and character of the underlying subducting oceanic lithosphere. We utilize coral microatolls in west Sumatra to document evidence for deformation of the underlying subduction interface. Microatolls are very sensitive to fluctuations in sea level, and thus act as natural tide gauges. They record not only the magnitude of vertical deformation associated with earthquakes (paleoseismic data), but also continuously track long-term aseismic deformation that occurs during intervals between earthquakes (paleogeodetic data). Numerous microatolls from the region around the equator record a simple pattern of tilt away from the trench axis in 1935 related to an Mw7.7 earthquake. About 115 km from the trench axis, uplift was nil. Nearer to the trench, uplift progressively increased trench-ward to at least 90 cm. Farther than 115 km from the trench, submergence of up to 35 cm occurred. We model these paleogeodetic data by a 2.3 m slip event on the interface between 88 and 125 km from the trench axis. A large aseismic event or ?silent earthquake? in 1962 is among the most interesting phenomena discovered in the coral record, and is the second largest short-lived event recorded throughout the equatorial region. Furthermore, paleogeodetic data reveal that the interseismic deformation rates have varied both temporally and spatially.</p

The Sumatran Fault System, Indonesia: Supplement 1 from "Neotectonics of the Sumatran fault and paleogeodesy of the Sumatran subduction zone" (Thesis)

Under the Sumatran plate boundary the Australian-Indian plate is subducting at about 60 mm/yr in the direction N110E. The oblique convergence is partitioned into trench-parallel slip?accommodated largely by the Sumatran fault zone and trench-perpendicular slip?accommodated by the subduction zone. Our detailed map of the Sumatran fault zone (SFZ) shows that the Sumatran fault is highly segmented. The influence of these fault segmentations on historical seismic source dimensions suggests that the dimensions of future events will also be influenced by fault geometry. The largest geomorphic offsets along the Sumatran fault zone are about 20 km, and may represent the total offset across the fault. The shape and location of the Sumatran fault and the active volcanic arc are highly correlated with the shape and character of the underlying subducting oceanic lithosphere. We utilize coral microatolls in west Sumatra to document evidence for deformation of the underlying subduction interface. Microatolls are very sensitive to fluctuations in sea level, and thus act as natural tide gauges. They record not only the magnitude of vertical deformation associated with earthquakes (paleoseismic data), but also continuously track long-term aseismic deformation that occurs during intervals between earthquakes (paleogeodetic data). Numerous microatolls from the region around the equator record a simple pattern of tilt away from the trench axis in 1935 related to an Mw7.7 earthquake. About 115 km from the trench axis, uplift was nil. Nearer to the trench, uplift progressively increased trench-ward to at least 90 cm. Farther than 115 km from the trench, submergence of up to 35 cm occurred. We model these paleogeodetic data by a 2.3 m slip event on the interface between 88 and 125 km from the trench axis. A large aseismic event or ?silent earthquake? in 1962 is among the most interesting phenomena discovered in the coral record, and is the second largest short-lived event recorded throughout the equatorial region. Furthermore, paleogeodetic data reveal that the interseismic deformation rates have varied both temporally and spatially

Tectonic Strain in Sumatera Based on Continuous Sumatran GPS Array (SuGAR) Observation 2007-2008

Abstract. Sumatra is located near the place where the collision between Indo-Australian Plate and Eurasian Plate heppened. When Indo-Australian Plate moves below Eurasian Plate, the friction that occur between both plates causes the strain is being accumulated. The strain that exceeds the elastic limit will be released as an earthquake. GPS observation in Sumatra was conducted to analyse the velocity of vector displacement and the heterogenous of tectonic strain on the surface as one of the tectonic indication to earthquake mitigation in the future. The result from data processing shows vector displacement in Sumatra has northeast direction that indicate inter-seismic and southwest direction that indicate post-seismic. The strain distribution is extension that indicate post-seismic equally scatteres dominantly in zone where happened Aceh earthquake on 2004, Nias earthquake on 2005, Bengkulu on Sptember 12th 2007, and earthquake on Mentawai Island on September 13th 2007. While strain as compression shows Sumatera still has inter-seismic effect.Keywords: compression, earthquake, extension, GPS