Multiplets, i.e. events with similar waveforms, are selected from shallow earthquakes recorded on Merapi volcano (Indonesia) before the eruption of February 2nd, 1992. Two multiplet families are found with their sources close to the summit. Their seismograms are analyzed using the Moving Window Cross Spectrum technique which measures the precise time delay between seismic phases in the entire seismogram. For both families of multiplets, a gradual decrease in the arrival times of coda waves is observed as a function of the date prior to the eruption: coda waves are becoming progressively faster (up to 1.2 per cent) as the time interval to the eruption shortens. This observation is interpreted as the consequence of an increase in the seismic velocity inside the volcano. The increase in velocity started in May 1991 and was observed until September 1991, 4 months before the eruption. This velocity increase may be related to an increase in pressure in the magma chamber or in the conduits and to the resulting closure of the surrounding cracks.

[1] Here we present the results of local source tomographic inversion beneath central Java. The data set was collected by a temporary seismic network. More than 100 stations were operated for almost half a year. About 13,000 P and S arrival times from 292 events were used to obtain three-dimensional (3-D) Vp, Vs, and Vp/Vs models of the crust and the mantle wedge beneath central Java. Source location and determination of the 3-D velocity models were performed simultaneously based on a new iterative tomographic algorithm, LOTOS-06. Final event locations clearly image the shape of the subduction zone beneath central Java. The dipping angle of the slab increases gradually from almost horizontal to about 70°. A double seismic zone is observed in the slab between 80 and 150 km depth. The most striking feature of the resulting P and S models is a pronounced low-velocity anomaly in the crust, just north of the volcanic arc (Merapi-Lawu anomaly (MLA)). An algorithm for estimation of the amplitude value, which is presented in the paper, shows that the difference between the fore arc and MLA velocities at a depth of 10 km reaches 30% and 36% in P and S models, respectively. The value of the Vp/Vs ratio inside the MLA is more than 1.9. This shows a probable high content of fluids and partial melts within the crust. In the upper mantle we observe an inclined low-velocity anomaly which links the cluster of seismicity at 100 km depth with MLA. This anomaly might reflect ascending paths of fluids released from the slab. The reliability of all these patterns was tested thoroughly.Citation: Koulakov, I., et al. (2007), P and S velocity structure of the crust and the upper mantle beneath central Java from local tomography inversion,

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Mt. Merapi is one of the most dangerous volcanoes in Indonesia, located within the tectonically active region of south‐central Java. This study investigates how Mt. Merapi affected ‐ and was affected by ‐ nearby tectonic earthquakes. In 2001, a Mw6.3 earthquake occurred in conjunction with an increase in fumarole temperature at Mt. Merapi. In 2006, another Mw6.3 earthquake took place, concomitant with an increase of magma extrusion and pyroclastic flows. Here, we develop theoretical models to study the amount of stress transfer between the earthquakes and the volcano, showing that dynamic, rather than static, stress changes are likely responsible for the temporal and spatial proximity of these events. Our examination of the 2001 and 2006 events implies that volcanic activity at Mt. Merapi is influenced by stress changes related to remote tectonic earthquakes, a finding that is important for volcano hazard assessment in this densely inhabited area.