Influence of near‐surface volcanic structure on long‐period seismic signals and on moment tensor inversions: Simulated examples from Mount Etna

Abstract

[1] Long-period (LP) seismicity on volcanoes is thought to be associated with moving fluids or resonating fluid-filled conduits, hence LP moment tensor (MT) source inversions might have a direct bearing on our understanding of the plumbing system. Using 3-D full wavefield simulations and 2-D sensitivity kernels in a digital elevation model of Mount Etna, we investigate the influence of near-surface volcanic structure on LP signals and on moment tensor inversions. Contrary to common wisdom in crustal seismology we find that, despite their relatively long wavelengths, LPs are severely distorted by near-surface structures including layering and topographic features. In particular near-surface low-velocity structures which are commonly observed on volcanoes play a critical role in controlling the nature of LP signals. If not accounted for, these path effects leak into the source solution, leading to the emergence of erroneous source geometries, spurious forces and incorrect source time functions. This is particularly problematic if one adopts an ''unconstrained'' solution space for the source, with many free parameters. Hence there is a fine balance in the trade-off between the velocity model and the source. In the absence of high-resolution near-surface velocity control we demonstrate the importance of employing a priori source information from other fields (e.g., structural geology), for shallow LPs, constraining the number of free parameters in the inversion. A probabilistic approach should then be taken, as the model with the ''best fit'' is not necessarily the ''true'' solution.Citation: Bean, C., I. Lokmer, and G. O'Brien (2008), Influence of near-surface volcanic structure on long-period seismic signals and on moment tensor inversions: Simulated examples from Mount Etna,

Long-period seismicity in the shallow volcanic edifice formed from slow-rupture earthquakes

“…The wavefield distribution for LP events from that experiment (Fig. 1a) shows extreme spatial variability, with pulse-like low frequency signals at the summit stations that appear as classical resonating LP signals at off-summit stations, in excellent agreement with numerical predictions 11 . This unequivocally demonstrates that the apparent resonance in these low-frequency seismic events is caused by wavefield distortions as a consequence of wave propagation effects and is not source-related.…”

Moment tensor inversion for the source location and mechanism of long period (LP) seismic events from 2009 at Turrialba volcano, Costa Rica

“…The network was designed to have a high density distribution of stations across the summit and flanks of the volcano, which has been suggested by Bean et al (2008) andDe Barros et al (2011) to be necessary for accurate source inversion: this was difficult to achieve in some areas on the northern flank of the volcano which is inaccessible.…”

“…This study is therefore important as a better comprehension of LP events at Turrialba volcano may improve the understanding of the present stage of activity within a potentially dangerous volcano that appears to be "reawakening". Turrialba volcano is also an ideal candidate for moment tensor inversion of LP events due to the relative ease of access to the summit of the volcano in comparison to many other active volcanoes; the summit region is large and relatively flat, allowing for the dense deployment of seismometers close to the active craters, which reduces the likelihood of erroneous solutions (Bean et al, 2008;Kumagai et al, 2010).…”

Distinguishing high surf from volcanic long‐period earthquakes

“…Periodic strain changes induced by the oceanic microseism have also been cited as a trigger for hydrothermal LP events [Stich et al, 2011]. Locating and interpreting LP seismicity at volcanoes is complicated by the lack of sharp first arrivals and the potential for waveform modification by complex subsurface structures and topography [Bean et al, 2008]. A sudden increase in either the number or amplitude of LP events is often a sign of volcanic unrest, and LP signals are frequently seen as swarms or groups of repeating events concentrated in time.…”