Three‐dimensional attenuation model of Sierra Negra Volcano, Galápagos Archipelago

Abstract

The shallow magma system beneath Sierra Negra was imaged using attenuation tomographic methods. The t∗ spectral decay method for P wave phases was used to highlight regions of high Qp−1 which suggest the presence of magma melt. High‐ Qp−1 anomalies ranging from 0.005 to 0.04 are concentrated below the caldera from 0.5 to 10.5 km depths. Attenuation is sensitive to temperature and fluid presence; thus, this high attenuation is interpreted as possible zones of magma accumulation. An imaged shallow body is consistent with geodetic studies on caldera deformation that modeled a magma sill or flattopped diapir of unknown thickness at ∼1 km depth below sea level.

Using Machine Learning to Automatically Detect Volcanic Unrest in a Time Series of Interferograms

“…Sierra Negra (Rodd et al, 2016). This theory is in broad agreement with studies of other Galapagos volcanoes, such as Bagnardi and Amelung (2012), which identified at least two areas of magma storage below Fernandina Volcano, and Stock et al (2018), which identified two magma reservoirs beneath Wolf Volcano.…”

“…Between the 2005 and 2018 eruptions, the deployment of a temporary broadband seismic network termed the SIGNET array (Tepp et al., 2014) has provided additional insights into the structure of the crust beneath Sierra Negra. Body wave tomography has been used to infer the existence of large low velocity zone at depths of 8–15.5 km below sea level (BSL) coupled with smaller areas of high and low velocities at depths of 3–15.5 km BSL (Tepp et al., 2014), while attenuation tomography has been used to infer the existence of separate low velocity zones from 0.5–3 and 3–10.5 km BSL (Rodd et al., 2016). Combining these measurements with the geodetic studies of deformation before the 2005 eruption has led to the conclusion that both a shallow magma chamber and a deeper magma chamber embedded in a larger mush zone exist below Sierra Negra (Rodd et al., 2016).…”

“…Body wave tomography has been used to infer the existence of large low velocity zone at depths of 8–15.5 km below sea level (BSL) coupled with smaller areas of high and low velocities at depths of 3–15.5 km BSL (Tepp et al., 2014), while attenuation tomography has been used to infer the existence of separate low velocity zones from 0.5–3 and 3–10.5 km BSL (Rodd et al., 2016). Combining these measurements with the geodetic studies of deformation before the 2005 eruption has led to the conclusion that both a shallow magma chamber and a deeper magma chamber embedded in a larger mush zone exist below Sierra Negra (Rodd et al., 2016). This theory is in broad agreement with studies of other Galapagos volcanoes, such as Bagnardi and Amelung (2012), which identified at least two areas of magma storage below Fernandina Volcano, and Stock et al.…”

Seismicity patterns during a period of inflation at Sierra Negra volcano, Galápagos Ocean Island Chain

“…Earthquake magnitudes are estimated using a local magnitude scale and distance correction terms for incipient seafloor spreading zones where local attenuation is high [Keir et al, 2006], in the absence of a local magnitude scaling relation. This scaling is consistent with the high P-wave attenuation at depths of 0.5-8.5 km below sea level estimated by the attenuation tomography model of Rodd et al [2016]. A site-specific magnitude scale awaits a time series that spans an eruption cycle.…”

“…The geometry of the bottom of the sill(s) is poorly constrained by surface deformation patterns [Yun et al, 2007]. The 3D attenuation model of Rodd et al [2016] reveals patchy, highly attenuating zones between 0.5 km -3 km below sea level, interpreted as shallow magma bodies. The zone between the shallow sill and the deeper crystal mush zone remained poorly determined.…”