Fluid ascent and magma storage beneath Gunung Merapi revealed by multi-scale seismic imaging

Abstract

Magma is fed to a volcano through a complex "plumbing" system that involves not only shallow structures beneath the volcano edifice, but also deep structures and processes within the underlying crust and upper mantle. This paper summarizes seismic experiments carried out over many years at Gunung Merapi in Central Java. These have resolved the 3D seismic velocity structure of the Merapi edifice, and provided a 3D structural image of the lithosphere and subduction zone beneath Central Java. Earthquake locations reveal that with distance from the trench, the dip of the subducting slab steepens from nearly horizontal (0-150 km), through 45 degrees (150-250 km), to 70 degrees (>250 km). The slab appears as a 30 km thick double layer of seismicity in a depth range of 80 km to 150 km, and it can be identified seismically to a depth of more than 600 km. The active volcanoes of Merapi, Sumbing, and Lawu are located at the edge of a large low velocity body that extends from the upper crust to the upper mantle beneath Central Java. Shear wave signals recorded above this anomaly are strongly attenuated compared to neighboring areas. The anomalous body 2 has a detected volume of >50,000 km 3 and a decrease in P and S velocities relative to adjacent regions of up to 30%. The resulting Vp/Vs ratio of up to 1.9 is unusually high for lower crust. Additionally, the anomaly extends along a 45 degree-slope downward from beneath the volcanic arc and meets the slab at 100 km depth. We interpret this sloping anomaly as a pathway for fluids and partial melts. Increased seismicity is observed at depths of ~100 km, possibly as a result of dehydration of the subducting slab with related fluid releases causing partial melting of overlying mantle material. The large velocity reduction and high Vp/Vs ratio in the region are consistent with an increase in temperature, a reduction of shear strength, and the presence of fluids or melts of 13 to 25 vol. %. The detected strong anomaly beneath Central Java is unique in size and amplitude compared to other subduction zones. The geophysical evidence suggests that this segment of the arc has a high magma flux and is thus capable of developing even larger shallow crustal reservoirs and more voluminous explosive eruptions in the future.

Volcano-tectonic control of Merapi's lava dome splitting: The November 2013 fracture observed from high resolution TerraSAR-X data

“…As the amphitheater and breaching of the Gendol valley have been initiated in 2006 already, this trend appears to be a more sustained structural trend that may affect further developments and hazards at Merapi. It is interesting to note that this general trend is also parallel to the border of a deep crustal velocity anomaly observed by means of seismic tomography (Luehr et al, 2013).…”

The feeder system of the Toba supervolcano from the slab to the shallow reservoir

“…Figure 3c shows P-velocity anomalies in the upper mantle derived as a result of regional tomography studies. An earlier version of this model has previously been presented 14 ; however, we subsequently modified it slightly by tuning the parameters and incorporating new data. This tomography model was constructed using seismic travel time data from the global catalogue of the International Seismological Centre (ISC) and the algorithm developed by Koulakov and Sobolev 15 .…”

“…3c , we present a horizontal section of the P-velocity anomalies at 220 km depth. This result is a detailed section from a regional tomography model for the entire Sunda Arc, which is an updated version of a previously published model 14 . The P- and S-wave velocity heterogeneities down to 1,000 km depth were computed based on the tomographic inversion of P- and S-body wave travel time data from the ISC 32 for the time period from 1964 to 2012.…”

Simulation of block‐and‐ash flows and ash‐cloud surges of the 2010 eruption of Merapi volcano with a two‐layer model

“…Pallister et al [] give a detailed description of the eruption chronology and estimate the extrusion rates. Budi‐Santoso et al [], Luehr et al [], and Jousset et al [] present the 2010 eruption from several geophysical points of view and Drignon et al [] based on textural analyses and glass water content. Cronin et al [], Charbonnier and Gertisser [], Komorowski et al [], and Jenkins et al [] provide a chronology of the eruption and map and describe the deposits.…”