The seminar is part of the Berkeley Seismology Lab Seminar Series.
The Laguna del Maule volcanic field, straddling the Chile-Argentina border at 36° S, is currently the subject of a multi-disciplinary collaborative investigation supported primarily by the U.S. National Science Foundation Integrated Earth Systems program and the Observatorio Volcanológico de Los Andes del Sur (OVDAS) of SERNAGEOMIN. At least 50 post-glacial (younger than 20 ka) eruptions from more than two dozen vents encircling the 25 x 17 km lake basin have produced rhyodacitic to rhyolitic lava flows and ash deposits totaling > 30 km3, suggesting that a large, active, silicic magma reservoir fuels this system. Since 2007, GPS and InSAR geodesy reveal that Laguna del Maule has been experiencing rapid uplift at 20 to 25 cm/year centered within the ring of silicic vents. Moreover, a deformed paleo-shoreline that has been 36Cl-dated implies magma-driven surface uplift of > 60 m and that growth of this large shallow reservoir has occurred over at least the past 9,400 years. UW-Madison, Cornell, and OVDAS deployed a seismic array covering ~450 km2 that surrounded the lake basin. The array consisted of 18 broadband stations in 2015, and was enlarged to 47 stations in 2016 (37 broadband, 10 short-period). The full array remained in place until late March 2018. A variety of seismic studies have been initiated for the seismic array data: body-wave tomography, seismic interferometry, surface-wave tomography, and focal mechanism determination. Other studies are planned, including attenuation tomography, teleseismic tomography, receiver function analysis, and moment tensor determination. The main goals are to detect the magma chamber underlying Laguna del Maule, characterize its dimensions and properties, and assess the state of stress of the system.
I will report on results from surface-wave tomography, and compare them to the results from other geophysical techniques. Due to the small array aperture (~30 km) and the limited frequency range of usable ambient noise data, we combined three types of data for the surface-wave tomography: standard noise cross-correlation analysis using pairs of stations within the array, correlation of earthquake coda at pairs of array stations, and differential dispersion of ambient noise for pairs of array stations cross-correlated with remote stations. Somewhat unexpectedly, the Vs image shows evidence of a strong upper crustal low velocity anomaly mainly along the southwest side of the lake, which does not extend under the entire lake basin. Depending on how the boundaries of the low-Vs anomaly are defined, we obtain estimates of ~250 to ~400 km3, and average melt percentages of 5% to 8%. The position of the anomalous body is very close to the estimated source area for the uplift measured by InSAR and GPS. A Bouguer gravity low, interpreted to reflect the magma reservoir, is observed in the same area, but appears to be offset from the seismic anomaly. In contrast, magnetotelluric results image a large low-resistivity zone near the north side of the lake and much smaller anomalies elsewhere.
Our current hypothesis is that during the Holocene, rhyolitic magma batches have been repeatedly extracted from different parts of a magma reservoir system beneath the lake basin. The magma batches that have fed the individual eruptions were likely of relatively modest volume, comparable to the geophysical results. These inferences are consistent with a “cold storage” model for the silicic system.
About the Speaker: Dr. Clifford H. Thurber (Department of Geoscience, University of Wisconsin-Madison)
Dr. Clifford H. Thurber, Department of Geoscience, University of Wisconsin-Madison. Prof. Thurber is an international leader in research on three-dimensional seismic imaging (seismic tomography) using microearthquakes. His primary research interests are in the application of seismic tomography to fault zones, subduction zones, and volcanoes, with major areas of emphasis on the San Andreas fault and volcanoes in Hawaii and Alaska. Other areas of Thurber’s expertise include earthquake location and geophysical inverse theory.
Invited by: Evan Um