Concentration of seismic energy within the two active domains beneath individual volcanoes and groups of volcanoes of Java, Indonesia

Abstract

It is shown that beneath all individual active volcanoes or volcanic groups of Java there are two seismically active domains ( Ac 1 and Ac 2), separated by an aseismic space ( As 2). The concentration of released seismic energy is calculated for all active domains, in the cases of altogether eighteen individual volcanoes or groups, respectively. The medium value of energy-concentration ( E∗) in the case of the shallower active domains ( Ac 1)is: E ̄ ∗ (Ac 1) = 5.51 c erg. cm −3 ( c erg = 0.01 erg), while the medium value for the deeper active domains ( Ac 2) is: E ̄ ∗ (Ac 2) = 3.38 c erg. cm −3 that is, they are in the same order of magnitude, thus expressing a remarkably uniform process of strain-accumulation and -release inside well-separated domains under active volcanoes if a longer period of time was considered. The variation of the E∗-values, especially that of E∗ (Ac 2) along the arc of the volcanoes of Java, roughly speaking from west to east, shows a general tendency towards decrease. The western part is characterized by high, the eastern one by low values. By and large, a similar graph can be drawn concerning the reciprocal number of tectonic shocks within domains Ac 2. That is, the western part of the belt is characterized by relatively few shocks of higher magnitude while their number is essentially greater in the eastern part. Finally, the variation of the probable minimum depth of the primary magma chambers (sources of andesitic magma), calculated by one of the present authors corresponding to the Toksöz-model and on the basis of the analysis of Benioff-zones underneath Java, also shows a similar tendency, because the primary chambers at the western part of the belt are to be found at a relatively shallower level than at the eastern part.