Physical Modeling of Long Period Events in a Controlled-Source Condition

Michigan Technological University, Houghton MI

Investigators

Abstract

Pressure changes in rock cavities filled with fluids produce specific seismic signals. These signals are abundant in volcano "plumbing systems". They are called long-period (LP) events because of their duration. LP seismicity is an important precursor for volcanic eruptions because it is triggered by magma transport preceding the eruptions. Yet, the source mechanism of LP events is still poorly understood. A theoretical model developed in the 1980s, The Chouet’s model, has been widely used to describe this mechanism. The model has been thoroughly explored with numerical simulations but not tested experimentally. This is because producing LP signals in the laboratory is challenging. Here, the researchers are developing a new apparatus to artificially generate LP signals. The apparatus consists of a crack filled with fluid and embedded within a large concrete slab. The slab is 6-meter long (~20 ft) and instrumented. Pressure in the fluid is varied while sensors record the produced signals. Analysis of the signal allow unveiling the source mechanism of LP events. The project outcomes have strong implications for volcanic eruption hazard assessment. They also find applications in other disciplines, such as glaciology where LP events are triggered by water-filled cracks or the geothermal energy sector. The project provides support for an early career scientist and a graduate student at Michigan Technological University. It also provides training for undergraduate students, notably from groups underrepresented in Science, as well as outreach to K-12 students. It is funded by both the Geophysics and the Petrology and Geochemistry programs. The goal of the researcher is to quantitively characterize LP signals with respect to the fundamental parameters that influence LP seismicity. These critical parameters include crack stiffness (rheology contrasts of the materials), fluid viscosity, triggering location, source frequency, crack orientation, and mass transport. The new apparatus is 6m x 6m wide and 22-cm thick. The concrete slab is equipped with controlled piezoelectric sources which generate pressure pulses from different locations/orientations with respect to the crack. It is also equipped with transducer and strain sensors to analyze the slab response and the produced signals. The cracks can be filled with water or ethanol. In addition to providing constraints for computational modeling, this research explores regions of the parameter space which cannot be easily probed numerically. It, thus, bridges a critical gap between theoretical models and the LP signals observed at active volcanoes. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.