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Combining Land and Lake Bottom Magnetotelluric Measurements to Study Volcanic Systems in Mono Basin, California


In volcanic settings, magnetotelluric methods can be a useful tool for geophysical investigation because the host rock is typically resistive while in contrast the features of interest, such as fluids and heat sources, will be electrically conductive. The work presented in this thesis are findings from magnetotelluric data taken in Mono Basin, California to study volcanic systems in the area and give suggestions for future eruption potential. The Long Valley volcanic region and Mammoth Mountain area have experienced heightened geologic unrest in the recent decades with ground deformation, earthquake swarms and CO2 emissions, which are an indication of an ongoing volcanic threat.

Since Long Valley Caldera's formation 0.76 Ma, there has been a north-trending cycle of eruptions creating the Inyo-Mono Crater chain likely due to extensional tectonics. The most recent event occurred less than 350 years ago in Mono Lake, creating Paoha Island. Previous geophysical investigations of this area have identified conductive anomalies in Mono Basin that suggest fluid flow and heat sources. Mono Lake previously served as an equivocal gap in the data set, motivating a field campaign using modified seafloor EM receivers. Following lake deployments, several land stations were deployed north of the lake for further investigation. 2D inversions of the data show a large conductive body that underlies a smaller anomaly 2-5 km under Mono Lake. Guided by existing geologic observations, this system is interpreted to be a magma chamber or hydrothermal reservoir that underlies hot fluids with a local fault facilitating fluid and heat flow between the two bodies. The interpretation of the shallow section is supported with observations of hot springs at the surface. The melt fraction of the larger, deep conductor is estimated in a range of 5 - 40%. The anomaly extends to the north of Mono Lake past the limits of the data set, suggesting that another eruption is likely following the northward trend.

Additionally, a process for using lake-bottom MT measurements in 3D inversion is presented in this work. Bathymetry cannot be included in 3D models using the ModEM code but the measurements collected on the lake bottom can be upward continued to the surface using the 1D MT recursion relation. Using these modified data, the 3D inversion is in good agreement with the 2D inversion and shows a deep conductive feature that funnels upwards to a shallower conductor directly beneath Mono Lake.

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