Localization of seismic coda at Merapi volcano (Indonesia)

Abstract

The energy of the seismic coda generated by multiple scattering at small scale heterogeneities in the Earth is generally uniformly distributed in space at some late lapse time. 3‐component seismogram envelopes of volcano tectonic B‐type earthquakes recorded at Merapi volcano, on the contrary, show a systematic decrease of the coda amplitude with distance from the summit of the volcano. We present two different models to explain this observation. In the first model seismic energy transport is described as a diffusion process and the observed coda localization is explained by an inhomogeneous distribution of the diffusion coefficient (D = D(r)). As a simple analytical model we use a strongly scattering cylinder (representing the volcano) embedded in a homogeneous half‐space (representing the surrounding crust). In the second model we assume the disorder within the strato volcano to be so large that diffusion theory breaks down and a transition to the so called ‘Anderson localization regime’ occurs. This model predicts a concentration of seismic energy near the source region and can thus also explain the observed coda localization.

Anomalous character of the coda envelopes on Mt Vesuvius explained in terms of depth dependentQ

“…The coda amplitude decreases with increasing source–receiver distance, contrary to what is experimentally observed worldwide for most tectonically active areas. This hypothesis is consistent with the interpretation of coda localization under volcanoes by Friedrich & Wegler (2005). These authors analysed seismic data collected at the volcano Merapi finding a systematic decrease of the coda amplitude as a function of the distance from the volcano summit and argued that this is the result of the trapping of the scattered waves inside the volcano area.…”

“…Aki and Ferrazzini explained this phenomenon as being caused by the generation of slow waves trapped in the fluid-solid two-phase system of the magma body. Similar observations were made by Friedrich & Wegler (2005) and Wegler & Luhr (2001) at Merapi volcano, Indonesia. Mayeda et al (1992) analysed the local earthquakes recorded at Mauna Loa and Kilauea volcanoes, Hawaii, and discovered that coda waves non-uniformly sample the volume below the Kilauea Caldera.…”

“…These authors analysed seismic data collected at the volcano Merapi finding a systematic decrease of the coda amplitude as a function of the distance from the volcano summit and argued that this is the result of the trapping of the scattered waves inside the volcano area. More recently, Parsiegla & Wegler (2008) demonstrate that a heterogenous medium with the shape of the volcano topography over a homogeneous half‐space can give the dependence of the coda properties with distance that was observed by Friedrich & Wegler (2005). In the heterogeneous layer, strong multiple scattering phenomena occur, and the coda amplitude can be larger than the amplitude of the direct wave, as found for moonquakes by Nakamura (1970).…”

“…Yoshimoto et al (2006) underlined that it is expected, in synthesis, that a non-uniform distribution of coda wave energy should in general be observed between regions with differing geological structures. Aki & Ferrazzini (2000) and Friedrich & Wegler (2005) named the phenomenon of coda energy concentration 'coda localization'. The term localization comes from the concept of 'Anderson localization' phenomenon introduced by Anderson (1958) for electrons in a semiconductor.…”

Imaging the dynamics of magma propagation using radiated seismic intensity

“…In future studies a more complete inverse problem should be done (with inversion of n and Q as well as location) including scattering effect that could be dominant in active volcanos [ Wegler , 2003; Friedrich and Wegler , 2005; Parsiegla and Wegler , 2008].…”