Tomographic Imaging of the Agung-Batur Volcano Complex, Bali, Indonesia, From the Ambient Seismic Noise Field

Abstract

The Agung-Batur Volcanic Complex (ABVC), part of the Sunda volcanic arc, is the source of some of the most hazardous volcanic activity in Indonesia. The ABVC has undergone many small (VEI 1-2) eruptions since historical records began in the early 19th century, but Mt. Agung has experienced much larger (VEI 5) eruptions, both in the modern (1963) and historical (1843) eras, as well as several times during the past 2000-3000 years. The 1963 eruption caused more than 1000 deaths, and a more recent eruption in 2017 caused the evacuation of 140,000 people. Delineating the magma structure beneath ABVC is an important first step in understanding the physics of these eruptions. This paper presents the first local-scale study of Rayleigh wave group velocity structure and the seismic velocity structure beneath the ABVC using ambient seismic noise tomography. Seismic data were collected using 25 seismometers deployed across the ABVC during early January to March 2019. The local seismic network provides good resolution beneath both Mt. Agung and Mt. Batur. We obtained 158 Rayleigh Green's Functions, extracted by cross correlating noise simultaneously recorded at available station pairs. We used sub-space inversion to calculate group velocity at different periods and to estimate the lateral variations in group velocity for given periods. 2-D tomographic maps obtained from the inversion of the group velocity of Rayleigh waves clearly showed some pronounced velocity anomalies beneath the ABVC. We applied the Neighbourhood Algorithm (NA) technique to invert the Rayleigh wave dispersion curves to obtain shear wave velocity (Vs) vs. depth profiles. These profiles indicate a low Vs of about 1 km/s underlying the volcanic complex between Mt. Agung and Mt. Batur at depths up to 2 km, which we suggest is due to a combination of low-Vs volcanic deposits as well as a shallow hydrothermal fluids system associated with magma fluids and/or gases produced by magma intrusion at depths >7 km.

Use of Local Seismic Network in Analysis of Volcano-Tectonic (VT) Events Preceding the 2017 Agung Volcano Eruption (Bali, Indonesia)

“…The VT seismic events during the seismic crisis presented in Figures 6C, 10A indicate that most of the seismic events were located midway between Agung and Batur, along a ∼N65 • E seismic trend. Interestingly this trend is also in agreement with the S-wave velocity boundary obtained by Zulfakriza et al (2020). This trend also acted as a boundary between the positive and negative station corrections obtained in this study.…”

“…The Kou09 model was selected because no velocity model of Agung region was available. Recently, Zulfakriza et al (2020) performed an S-wave velocity inversion using ambient noise tomography called the Zul20 model. However, in their S-wave data inversion, the Vp/Vs ratio for each layer remained fixed, whereas the density was estimated from the P-wave velocity.…”

Delineation of Sedimentary Basin Structure beneath the Banyumas Basin, Central Java, Indonesia, Using Ambient Seismic Noise Tomography

“…The subspace inversion method is also an iterative method, part of a class of highly-effective algorithms that minimizes objective/mis t functions using a quadratic approximation of the parameter space model in a relatively small dimension (Kennett et al1988). This method has been successfully applied to produce velocity variation maps of the Rayleigh group (Saygin and Kennet 2010;Martha et al 2017;Zulfakriza et al 2020).…”

“…Wathelet et al (2008) successfully applied NA to produce new computer code for creating inversions of Rayleigh surface wave dispersion curves. The NA inversion algorithm was also used by Zulfakriza et al (2020) to produce variations in velocity Vs to describe tomography beneath the Agung-Batur volcanic complex on Bali Island.…”

“…ANT has been used extensively to image crustal structures in countries worldwide: Australia (Saygin and Kennet 2010), the Netherlands (Yudistira et al 2017), China (Liu et al 2016;Liu et al 2017), Scotland (Nicolson et al 2012), Tibet (Jiang et al 2014;Zheng et al 2017;Yao et al 2009), and the United States (Ritzwoller et al 2011). ANT has also been applied in Indonesia to determine S-wave velocity structures in West Java (Rosalia et al 2020), Central Java (Zulfakriza et al 2014), East Java (Martha et al 2017), the Bandung Basin (Wuryani et al 2019;Pranata et al 2020), as well as on the islands of Bali (Zulfakriza et al 2020), and Lombok (Sarjan et al 2020), and the Banda Arc (Porritt et al 2016).…”