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We analyze 25 episodes of volcanic tremor recorded from 22 November until31 December 2009 at Mt Semeru  volcano in order toinvestigate their spectral and dynamical properties. The overtone frequenciesfor most of the tremor events indicate a pattern of period-doubling, which isone possible route that can lead a system to chaotic behavior. Exponentialdivergence of the phase space orbits is a strong indicator of chaos and wasquantified by estimating the maximal Lyapunov exponent (MLE) for all tremorevents. MLEs were found to vary linearly with the number of frequencyovertones present in the tremor signals. This implies that the tremor sourceat Semeru fluctuates between a quasi-periodic state with few overtonefrequencies (2–3) and small MLEs (~0.013), and a chaotic onewith more overtones (up to 8) and larger MLEs (up to 0.039). These resultsagree well with the tremor generation model suggested previously by Julian(1994), which describes wall oscillations of a crack excited by unsteadyfluid flow. In this model, as fluid pressure increases, a period-doublingcascade leads to numerous new frequencies and a chaotic tremor signal. Thetemporal variation of MLEs exhibited significant fluctuations from 23 until31 December when the eruptive activity shifted from explosive to effusive.Such a situation may reflect variable fluid pressure conditions inside theconduit, where at first magma is accumulated and subsequently is erupted,releasing the buildup of pressure. Our results give further evidence for therole of nonlinear deterministic processes in generating volcanic tremor andcall for similar investigations to be conducted in other volcanoes

Maximal Lyapunov exponent variations of volcanic tremor recorded during explosive and effusive activity at Mt Semeru volcano, Indonesia