Seismic scattering anomalies at Taal volcano, Philippines, inferred from inversion of envelope widths of volcano-seismic events.
Abstract
Estimating seismic heterogeneous structures beneath volcanoes, described by the mean free path (l0) and the quality factor for medium attenuation (Qi) for S waves, is important to understand spatio-temporal variations of magma and hydrothermal systems. A previous study used envelope widths (p) of volcano-tectonic (VT) earthquakes in a high-frequency band (5-10 Hz) and estimated depth-dependent 1D l0 and Qistructures at Taal volcano, Philippines, by a grid-search approach using Monte Carlo waveform simulations. However, this approach is computationally expensive, and is not applicable to estimate a more complex model. To overcome this problem, we developed an inversion approach to estimate 3D l0 and Qistructures, and applied it to observed p values at Taal. In this method, we estimated deviations of l0 and Qi in assumed anomalies from those of the 1D model by performing inversion of envelope widths with the damped least squares method. We analyzed 58 VT earthquakes that occurred between November 2011 and May 2013 at Taal. We set two anomaly regions in our inversion based on previous studies at Taal. One (Anomaly 1) is beneath the eastern flank of Volcano Island, and the other (Anomaly 2) is beneath Main Crater. As a result of our inversion using p values of all the events, the estimated l0 and Qivalues in Anomaly 2 were smaller than l0 and close to Qiin the first layer of the 1D model, respectively, indicating the existence of stronger heterogeneity in Anomaly 2. The estimated l0 and Qivalues in Anomaly 1 were close to those in the first layer. However, several events, which occurred in similar hypocenters with similar mechanisms, showed variations in observed p values at a station located above Anomaly 1. We performed the inversions of the observed p values for these events individually, and found that the estimated l0 values in Anomaly 1 temporally changed. The strong scattering in Anomaly 2 may be caused by cracks or fractures in solidified magma conduits developed in previous eruptions. The temporal variations in l0 in Anomaly 1 can be explained by ascends of mushy magma with various initial water contents, which varied scattering characteristics in magma due to heterogeneous distributions of degassed bubbles.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMV016.0026H
- Keywords:
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- 8499 General or miscellaneous;
- VOLCANOLOGY