Surface Rupture and Geotechnical Features of the July 2, 2013 Tanah Gayo Earthquake

DOI:10.17014/ijog.3.2.95-105An assessment of surface rupture and collateral ground failures can help to evaluate the impact of future earthquakes. This paper presents the results of a field survey conducted to map the surface rupture and geotechnical phenomena associated with the ground shaking during the July 2, 2013 earthquakes in Tanah Gayo Highland. The objectives of this survey are to document and to characterize the surface ruptures as well as to identify types of earthquake-induced ground failures. Results of the survey identified four best sites of possible surface rupture. Two locations are obvious surface ruptures that can be traced on primary topographic feature of the active fault segment from the north to the south, crossing Pantan Terong Hill. The fault segment has a total mapped length of 19 km, with WNW trending zone and a dextral rupture offset. The ground shaking also resulted in landslides and liquefaction in areas underlain by very fine-grained tuffaceous sands. Based on the field survey, it can be concluded that the newly defined active fault segment, the Pantan Terong segment, is likely the segment that ruptured at the July 2, 2013 Tanah Gayo earthquake. Due to the soil types and unstable rocky slopes in the hilly Central Aceh region, large-scale landslides are primary risks during an earthquake event in this region

Distribusi Vektor Aliran Air Tanah Dua Dimensi Dalam Media Rekahan Di Big Gossan, Tembagapura, Papua

. PT. Freeport Indonesia (PTFI) is planning to open a new underground mine at Big Gossan by using stop mine method which requires dry mining area. In some pilot areas, groundwater flowed from some stope holes significantly, while in other areas, some holes were dry. Previous hydrogeologic modeling, assuming that aquifers were intergranular media, cannot answer the variation of the groundwater occurrence. The aquifers in the study area are composed of fractures. In this research, hydrogeology of the area is remodeled, stressing that the aquifers are fractured media and recharge takes place at the intersections of faults and rivers. In this modeling, fracture geometry and orientation are analysed first. The results are used to calculate groundwater table distribution by using Gale Method and hydraulic conductivity by using Oda et.al. Method. Both parameters are used to construct two dimensional hydrogeological modeling, to know distribution of groundwater flow vector in the research area. This research reveals that hydraulic conductivity in this area is heterogeneus, not homogeneous, as was assumed in the previous modeling. The hydrogeologic model shows that groundwater flow is concentrated to the northwest of Stope # 6

Rupture and variable coupling behavior of the Mentawai segment of the Sunda megathrust during the supercycle culmination of 1797 to 1833

We refer to periods of subduction strain accumulation beneath the Mentawai Islands, Sumatra, as “supercycles,” because each culminates in a series of partial ruptures of the megathrust in its final decades. The finale of the previous supercycle comprised two giant earthquakes in 1797 and 1833 and whatever happened in between. This behavior between two great ruptures has implications for how the megathrust will behave between its more recent partial failure, during the M_w 8.4 earthquake of 2007, and subsequent large ruptures. We synthesize previously published coral microatoll records and a large new coral data set to constrain not only these two giant ruptures but also the intervening interseismic megathrust behavior. We present detailed maps of coseismic uplift during the two earthquakes and of interseismic deformation during the periods 1755–1833 and 1950–2000, as well as models of the corresponding slip and coupling on the underlying megathrust. The large magnitudes we derive (M_w 8.6–8.8 for 1797 and M_w 8.8–8.9 for 1833) confirm that the 2007 earthquakes released only a fraction of the moment released during the previous rupture sequence. Whereas megathrust behavior leading up to the 1797 and 2007 earthquakes was similar and comparatively simple, behavior between 1797 and 1833 was markedly different and more complex: several patches of the megathrust became weakly coupled following the 1797 earthquake. We conclude that while major earthquakes generally do not involve rupture of the entire Mentawai segment, they may significantly change the state of coupling on the megathrust for decades to follow, influencing the progression of subsequent ruptures

Twin‐surface ruptures of the March 2007 M>6 earthquake doublet on the Sumatran fault

Although the 2000‐km‐long Sumatran fault is one of Earth’s longest and historically most active strike‐slip faults, previous large historical ruptures have gone unmapped, although some have been inferred from felt reports or from modeling of sparse geodetic data. Through field observation, we documented twin‐surface ruptures associated with two M>6 earthquakes in March 2007. The first rupture broke a 22 km part of the 60‐km long Sumani segment southeast from a releasing stepover; the second rupture occurred on a 22.5 km part of the 90‐km‐long Sianok segment northwest from the stepover. Dextral displacements on the two segments averaged 51 and 36 cm. The earlier earthquake doublet in 1926 also straddled this stepover but also did not seem to have ruptured the entire fault segments. Thus, using fault lengths between large stepovers to estimate earthquake magnitude will overestimate what is likely to occur

Earthquake Geology of the Lembang Fault, West Java, Indonesia

The Lembang Fault is a major fault in western Java that skirts the northern edge of Bandung, one of Indonesia's largest cities, just south of the active Tangkuban Perahu volcano. Although it has no recorded or historical large earthquakes, the Lembang Fault shows obvious geomorphic evidence of recent activity and has long been thought to be active. In this study, we use geomorphic analysis to unequivocally establish that the fault has a dominantly sinistral sense of movement with a slip rate of 1.95–3.45 mm/yr. This proves that the fault is accommodating trench parallel slip resulted from a slight obliquity in plate convergence at the JavaTrench. With a length of 29 km, this suggests that the Lembang Fault could produce a Mw 6.5–7.0 earthquake with a recurrence time of 170–670 years. We also conducted paleoseismological trenching of the Lembang Fault and found evidence for at least 3 earthquakes in the 15th century, 2300–60 BCE and 19,620–19,140 BP. The 2300–60 BCE earthquake had a measurable vertical displacement of 40 cm, which is consistence with a Mw 6.5 earthquake. This is the first mapping of a source of crustal earthquakes in Java, Indonesia, the world's most densely populated island in one of its most tectonically active areas. The Lembang Fault and other faults in Java are likely to pose substantial risk to not only Bandung but many of Java's major urban agglomerations

Stratigraphic record of Holocene coseismic subsidence, Padang, West Sumatra

Stratigraphic evidence is found for two coseismic subsidence events that underlie a floodplain 20 km south of Padang, West Sumatra along the Mentawai segment (0.5°S–0.3°S) of the Sunda subduction zone. Each earthquake is marked by a sharp soil-mud contact that represents a sudden change from mangrove to tidal flat. The earthquakes occurred about 4000 and 3000 cal years B.P. based on radiocarbon ages of detrital plant fragments and seeds. The absence of younger paleoseismic evidence suggests that late Holocene relative sea level fall left the floodplain too high for an earthquake to lower it into the intertidal zone. Our results point to a brief, few thousand year window of preservation of subsidence events in tidal-wetland stratigraphic sequences, a result that is generally applicable to other emergent coastlines of West Sumatra

Distribusi Vektor Aliran Air Tanah Dua Dimensi dalam Media Rekahan di Big Gossan, Tembagapura, Papua

Abstrak. PT. Freeport Indonesia (PTFI) berencana membuka tambang bawah tanah baru di Big Gossan dengan metode stope mine, yang menuntut daerah yang akan ditambang berada dalam kondisi kering. Di beberapa lokasi uji, air tanah keluar dari beberapa lubang stope dengan debit yang cukup mengganggu, tetapi di beberapa lokasi lainnya lubang berada dalam kondisi kering. Pemodelan hidrogeologi terdahulu, yang mengasumsikan bahwa akifer adalah media antar butir, tidak dapat menjawab variasi keberadaan air tanah tersebut. Akifer di daerah ini disusun oleh rekahan-rekahan. Dalam penelitian ini, dilakukan kembali pemodelan hidrogeologi dengan penekanan bahwa akifer adalah media rekahan dan pada titik-titik perpotongan antara sesar dan sungai terjadi peresapan. Dalam pemodelan ini, pertama-tama dilakukan analisis geometri dan orientasi rekahan. Hasil analisis digunakan untuk menghitung distribusi muka air tanah dengan Metode Gale dan konduktivitas hidrolik akifer dengan Metode Oda dkk. Kedua parameter tersebut digunakan untuk melakukan pemodelan hidrogeologi dalam dua dimensi, untuk mengetahui distribusi vektor aliran air tanah. Dari penelitian ini diperoleh bahwa konduktivitas hidrolik di daerah ini adalah heterogen, bukan homogen, seperti asumsi dalam pemodelan terdahulu. Hasil pemodelan hidrogeologi menunjukkan bahwa aliran air tanah terkonsentrasi disebelah barat laut dari Stope # 6.Abstract. PT. Freeport Indonesia (PTFI) is planning to open a new underground mine at Big Gossan by using stop mine method which requires dry mining area. In some pilot areas, groundwater flowed from some stope holes significantly, while in other areas, some holes were dry. Previous hydrogeologic modeling, assuming that aquifers were intergranular media, cannot answer the variation of the groundwater occurrence. The aquifers in the study area are composed of fractures. In this research, hydrogeology of the area is remodeled, stressing that the aquifers are fractured media and recharge takes place at the intersections of faults and rivers. In this modeling, fracture geometry and orientation are analysed first. The results are used to calculate groundwater table distribution by using Gale Method and hydraulic conductivity by using Oda et.al. Method. Both parameters are used to construct two dimensional hydrogeological modeling, to know distribution of groundwater flow vector in the research area. This research reveals that hydraulic conductivity in this area is heterogeneus, not homogeneous, as was assumed in the previous modeling. The hydrogeologic model shows that groundwater flow is concentrated to the northwest of Stope # 6

Late Quaternary eruption of the Ranau Caldera and new geological slip rates of the Sumatran Fault Zone in Southern Sumatra, Indonesia

Abstract Over the last decade, studies of natural hazards in Sumatra have focused primarily on great earthquakes and associated tsunamis produced by rupture of the Sunda megathrust. However, the Sumatran Fault and the active volcanic arc present proximal hazards to populations on mainland Sumatra. At present, there is little reliable information on the maximum magnitudes and recurrence intervals of Sumatran Fault earthquakes, or the frequency of paroxysmal caldera-forming (VEI 7–8) eruptions. Here, we present new radiocarbon dates of paleosols buried under the voluminous Ranau Tuff that constrain the large caldera-forming eruption to around 33,830–33,450 calender year BP (95% probability). We use the lateral displacement of river channels incised into the Ranau Tuff to constrain the long-term slip rate of two segments of the Sumatran Fault. South of Ranau Lake, the Kumering segment preserves isochronous right-lateral channel offsets of approximately 350 ± 50 m, yielding a minimum slip rate of 10.4 ± 1.5 mm/year for the primary active fault trace. South of Suoh pull-apart depression, the West Semangko segment offsets the Semangko River by 230 ± 60 m, yielding an inferred slip rate of 6.8 ± 1.8 mm/year. Compared with previous studies, these results indicate more recent high-volume volcanism in South Sumatra and increased seismic potency of the southernmost segments of the Sumatran Fault Zone

Geomorphic expression of the suspected active back-arc thrust fault around Jakarta, Indonesia

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