Mentawai Fault

Mentawai Fault
Name	Mentawai Fault
Named for	Mentawai Islands
Country	Indonesia
Region	West Sumatra
Length	~300 km
Displacement	variable
Plate type	Transform/oblique-slip

Mentawai Fault is an active offshore fault system located along the outer arc near the Mentawai Islands off the western coast of Sumatra in Indonesia. It forms a major structural element accommodating oblique convergence between the Indo-Australian Plate and the Eurasian Plate and is associated with significant earthquake and tsunami hazard for the Indian Ocean rim, notably affecting coastal provinces such as West Sumatra and island chains including Siberut Island. The fault has been the subject of multidisciplinary studies involving organizations such as the Geological Agency of Indonesia, the US Geological Survey, and the International Seismological Centre.

Overview

The Mentawai Fault system comprises a series of strike-slip fault strands and splay faults running roughly northwest–southeast along the outer-arc high adjacent to the Sunda Trench, extending roughly parallel to the Sumatra Fault. It lies south of the megathrust rupture zones that produced the 2004 Indian Ocean earthquake and tsunami and the 2005 Nias–Simeulue earthquake, and has been inferred from bathymetric mapping, seismic reflection profiles, and offshore drilling undertaken by institutions including the British Geological Survey and the Lamont–Doherty Earth Observatory. The fault interacts with named tectonic features such as the Mentawai Basin, the Nicobar Basin, and the outer-rise seafloor.

Geology and Structure

Geological mapping, multichannel seismic reflection, and sub-bottom profiling reveal that the Mentawai Fault occupies an outer-arc ridge composed of uplifted accretionary complex sediments and imbricated thrust slices derived from the Sunda Trench accretionary prism. Structural studies show a complex partitioning of oblique convergence into right-lateral strike-slip motion and reverse faulting on adjacent thrusts, comparable in mechanics to the Great Sumatran Fault system. Key lithologies include accreted turbidites, trench fill, and metamorphosed mélange correlated to formations described in regional stratigraphic syntheses by the Indonesian Petroleum Association and the Netherlands Geotechnical Institute.

Tectonic Setting and Plate Interactions

The Mentawai Fault lies within the highly oblique convergent margin where the Indo-Australian Plate subducts beneath the Sunda Plate at the Sunda Trench with convergence rates of tens of millimeters per year estimated by GPS networks and geodetic studies coordinated by the Asian Seismic Network. Oblique plate motion is partitioned landward into the right-lateral outer-arc strike-slip activity of the Mentawai system and the trench-parallel megathrust slip that produces great earthquakes such as the 1833 Sumatra earthquake. Regional kinematic models reference global plate reconstructions from the Plate Tectonics paradigm and utilize datasets from the International GNSS Service and seismic catalogs maintained by the Global Centroid Moment Tensor Project.

Seismicity and Historical Earthquakes

Instrumental seismicity catalogs record numerous moderate earthquakes along and near the Mentawai Fault, with centroid moment tensors showing predominantly strike-slip and oblique mechanisms. Paleoseismological evidence from uplifted coral terraces and submerged peat deposits indicates episodic seismic rupture and coseismic uplift events similar to those documented for the 2004 Indian Ocean earthquake and tsunami and the 2007 Bengkulu earthquake. Historic archives and tsunami testimonies preserved in colonial records from the Dutch East Indies era complement radiocarbon-dated stratigraphic sequences used to infer recurrence intervals and seismic slip per event.

Tsunami Generation and Hazard Assessment

Because the Mentawai Fault lies adjacent to shallow continental shelves and the outer-arc high, certain types of ruptures—particularly those with a substantial vertical component on splay thrusts—can generate local tsunamis that threaten coastal communities of Padang, Bengkulu, and smaller islands such as Sipura Island. Hazard assessments combine numerical tsunami modeling produced by groups like the National Oceanic and Atmospheric Administration tsunami group and regional modeling centers with scenario-based planning developed by the Indonesian Agency for Meteorology, Climatology and Geophysics (BMKG). Risk analyses integrate population data from national censuses, port infrastructure inventories, and evacuation-route mapping conducted with support from United Nations Office for Disaster Risk Reduction programs.

Monitoring, Research, and Risk Mitigation

Ongoing monitoring employs broadband and short-period seismic networks, offshore ocean-bottom seismometers, GPS campaigns, and seabed geodetic instruments deployed in collaborations between universities such as Institut Teknologi Bandung, international research centers like the Woods Hole Oceanographic Institution, and government agencies. Research priorities include high-resolution seismic profiling, paleotsunami trenching, and coupled seismic–tsunami early-warning algorithm development leveraging real-time telemetry systems used by regional warning centers. Mitigation efforts encompass community-based preparedness programs, land-use planning guided by hazard zonation, engineering retrofits of critical facilities including hospitals and ports, and integration of traditional knowledge with formal evacuation drills supported by NGOs such as the Red Cross.

Category:Geology of Indonesia Category:Seismic faults