3D attenuation model of Long Valley Caldera (CA)

Unrest at Long Valley caldera, California has been attributed to magma
recharge or the ascent of hydrothermal fluids. The difference is critical
for assessing volcanic hazard. To better constrain subsurface structures
in the upper crust and to help distinguish between these two competing
hypotheses for the origin of unrest, we model the 3D seismic attenuation
structure by analyzing more than 47000 vertical waveforms with the coda
normalization method. We identify high attenuation anomalies in the
center-south of the caldera. Low attenuation anomalies are imaged in the
fluid-rich western and eastern areas of the caldera, one of which
corresponds to the location of an earthquake swarm in 2014. From a
comparison with other geophysical images (magnetotellurics, seismic
tomography) we attribute high attenuation anomalies to hydrothermal
systems. Average to high attenuation values are also observed at Mammoth
Mountain (south-west of the caldera), and may also have a hydrothermal
origin.  The attenuation images, however, are not able to image
the deeper crust where the magma bodies might exist.