Lateral variation in slab orientation beneath Toba Caldera, northern Sumatra

Abstract

The Investigator Fracture Zone (IFZ) subducts beneath Toba Caldera, the Earth's largest Quaternary caldera, in northern Sumatra, suggesting a possible relationship between them. Locations of sub‐crustal earthquakes based on arrival times of P and S waves at a seismograph network surrounding Toba reveal the geometry of the subducted slab and the IFZ beneath Toba. A vertical tear of less than 20 km in the slab across the IFZ, as previously suggested, cannot be ruled out but the large‐scale geometry of the slab is dominated by a broad bend of slab contours parallel to the concaveseaward indentation of the trench. The slab shape is probably a response to the trench curvature, can explain the change in trend of the volcanic arc near Toba, and may cause shallowing of the forearc basin near Nias Island. The decrease in radius of curvature of the slab contours is not accompanied by an observable decrease in dip angle, possibly resulting in lateral compressive stress in the slab. The high rate of seismicity along the subducted Investigator Fracture Zone, that intersects the slab obliquely to its plunge direction, is uncommon at subducted fracture zones and is likely caused by such lateral stress in the slab.

Submergence and uplift associated with the giant 1833 Sumatran subduction earthquake: Evidence from coral microatolls

“…Convergence is nearly orthogonal to the trench axis near Java but is highly oblique near Sumatra, where strain is strongly partitioned between dip slip on the subduction zone interface and right-lateral slip on the Great Sumatran fault [Fitch, 1972;McCaffrey, 1991]. Earthquake focal mechanisms and hypocentral distributions indicate that the subducting plate dips less than 15 ø beneath the outerarc ridge and steepens to about 50 ø below the volcanic arc [Newcomb and McCann, 1987;Fauzi et al, 1996]. Newcomb and McCann [1987] conclude that the source of the 1833 earthquake was a 550-km-long segment of the subduction interface that extended from the Batu Islands to Enggano Island and that the rupture extended from the trench to a point below the outer-arc islands.…”

Weak Tectono-Magmatic Relationships along an Obliquely Convergent Plate Boundary: Sumatra, Indonesia

“…Even though the volumes of Toba are DRE, assuming a first-order but likely correction of 0.5 for the non-DRE equivalent (e.g., Rose and Chesner, 1987), the obtained value is still 1-2 orders of magnitude larger than that of the other Sumatran volcanoes. This exceptional productivity can be explained by the presence of a tear in the slab below Toba (Fauzi et al, 1996;Hall and Spakman, 2015). The volcanoes with an evident or possible link to the GSF do not usually show the highest erupted volumes, suggesting that the structural setting may enhance the location of a volcano, but not its longer-term production.…”

“…The largest and most representative volcanic complex is Toba Caldera, located along a major bend in the subducting plate below, probably characterized by a tear reactivating a pre-existing fracture zone (Figure 1; Fauzi et al, 1996;Pesicek et al, 2008;Hall and Spakman, 2015;Koulakov et al, 2016). Toba is the site of Earth's largest Quaternary eruption, ejecting ∼3000 km 3 of magma, after at least 150,000 years of storage and evolution (Vazquez and Reid, 2004;De Silva et al, 2015).…”

Structure of the Central Sumatran Subduction Zone Revealed by Local Earthquake Travel Time Tomography Using Amphibious Data

“…Seismicity occurring in the prolongation of the IFZ down to depths of 200 km presumably reflects the subducting trace of the IFZ (Fauzi et al, 1996. At shallower depths beneath the Batu Islands both the forearc crust and the plate interface are characterised by enhanced seismicity levels with a number of persistent clusters.…”

“…9E). IFZ is reflected by seismicity down to ~200 km depths (Fauzi et al, 1996;Lange et al 2010); in Fig. 9F it is visible as a band of seismicity striking north (i.e.…”