Aseismic mid-crustal magma reservoir at Cleveland Volcano imaged through novel receiver function analyses

Abstract

processes related to eruptions at arc volcanoes are linked by structures that transect the entire crust. Imaging the mid-to lower-crustal portions (here, ~5-15 km and >15 km respectively) of these magmatic systems where intermediate storage may occur has been a longstanding challenge. Tomography, local seismic source studies, geodetic, and geochemical constraints, are typically most sensitive to shallow (<5 km) storage and/or have insufficient resolution at these depths. Geophysical methods are even further limited at frequently-erupting volcanoes where well-developed trans-crustal magmatic systems are likely to exist, due to a lack of deep seismicity. Here we show direct evidence for mid-crustal magma storage beneath the frequently erupting Cleveland volcano, Alaska, using a novel application of seismic receiver functions. We use p-s scattered waves from the Moho as virtual sources to investigate S-wave velocities between the Moho and the surface. our forward modeling approach allows us to provide direct constraints on the geometry of low velocity regions beneath volcanoes despite having a comparatively sparse seismic network. our results show clear evidence of mid-crustal magma storage beneath the depths of located volcanic seismicity. future work using similar approaches will enable an unprecedented comparative examination of magmatic systems beneath sparsely instrumented volcanoes globally. Recent research on trans-crustal magmatic systems (TCMS) demonstrates the importance of relationships between temporally longer-lived zones of crystals, melt, and volatiles ("mush") that extend throughout the crust, and comparatively ephemeral and depth-restricted magma reservoirs comprised primarily of eruptible melt 1 in driving volcanic eruptions. A longstanding barrier to understanding these systems is the challenge of imaging the mid-to lower-crustal portions of TCMS 1,2. Commonly-used approaches for detecting crustal magma reservoirs include analyses of volcanic seismicity, analyses of geodetic observations, and geochemical and petrological analyses of eruption products. Volcanic seismicity is linked to differential strain and pressure transfers related to magma and volatile movement in the crust and can either be similar to tectonic earthquakes (volcano-tectonic or VT events) or have primarily long period energy (LP events) 3-7. Similarly, geodetic methods constrain locations of magma storage by measuring deformation associated with magma emplacement and movement within the crust, and are typically most sensitive to shallow magma storage regions 8-10. Both methods are most sensitive to the results of magma movement; therefore, the absence of seismicity or geodetic deformation at volcanoes cannot be interpreted as the absence of stable magmatic (or mush) storage regions, limiting the ability of these techniques to provide information on comparatively stable volcanic systems. Geochemical and petrological approaches for assessing magma storage depths are most sensitive to the region of last storage in the crust prior t...

Seismo-Acoustic Characterization of Mount Cleveland Volcano Explosions

“…This suggests a vertically extensive magma storage region with a lack of sharp horizontal boundaries at the top and bottom of this region (Janiszewski et al, 2020). Janiszewski et al (2020) note that their results are consistent with a well-developed open volcanic conduit system, which may help explain the general lack of precursory seismicity at Cleveland. A recent study by Werner et al (2020) used a combination of volcanic gas emission rates and melt inclusion compositions from 2016 and found evidence that magma may be residing and degassing in a vertically extensive conduit region ranging in depth between 0.5 and 3.0 km below the summit.…”

“…Previous studies on Mount Cleveland include those using satellite observations (Simpson et al, 2002;Dean et al, 2004;Gu et al, 2005;Worden et al, 2014;Wang et al, 2015;Werner et al, 2017), sparse summit gas flights (Werner et al, 2017;Werner et al, 2020), temporary seismic deployments (Janiszewski et al, 2020;Power et al, in review;Haney et al, 2019), and long range infrasound recordings (De Angelis et al, 2012;Iezzi et al, 2019b). Janiszewski et al (2020) used receiver functions to find a low seismic velocity zone below Cleveland with a minimum vertical extent of 10-17.5 km below sea level that is <5 km in diameter. This suggests a vertically extensive magma storage region with a lack of sharp horizontal boundaries at the top and bottom of this region (Janiszewski et al, 2020).…”

“…Janiszewski et al (2020) used receiver functions to find a low seismic velocity zone below Cleveland with a minimum vertical extent of 10-17.5 km below sea level that is <5 km in diameter. This suggests a vertically extensive magma storage region with a lack of sharp horizontal boundaries at the top and bottom of this region (Janiszewski et al, 2020). Janiszewski et al (2020) note that their results are consistent with a well-developed open volcanic conduit system, which may help explain the general lack of precursory seismicity at Cleveland.…”

“…Cleveland is also monitored by AVO using satellite imagery, both in the visible and infrared bands, but the region is regularly cloudy which often obscures smaller explosion plumes below the cloud deck. Temporary seismic deployments, such as a year-long deployment from 2015 to 2016 (Janiszewski et al, 2020;Haney et al, 2019;Power et al, in review), have also been used to gain information about Mount Cleveland, though they were not telemetered in real time.…”

Ps‐P Tomography of a Midcrustal Magma Reservoir Beneath Cleveland Volcano, Alaska

“…Similarly to prior studies of the upper mantle (Si et al, 2017(Si et al, , 2019, we invert the virtual source travel times, but at the crustal scale. Importantly, our technique mitigates the limitations of the forward modeling approach of Janiszewski et al (2020) by solving for the three-dimensional (3-D) crustal velocity structure surrounding Cleveland Volcano and iteratively re-tracing rays through the 3-D heterogeneous structure. This new technique, which is distinct from receiver function tomography studies that fit receiver function waveforms for averaged layer velocity profiles, confirms the existence of a middle-to-lower crustal magma reservoir and reveals the surrounding heterogeneous structure.…”