Towards the Understanding of Deep Crustal Faulting and Fluid Movement through the Analysis of Long Period Earthquakes at Clearlake, CA

Volcanos pose serious threats to human societies yet can be sources of geothermal energy. Clear Lake Volcanic Field is a volcanic steam field located in California. It hosts numerous geothermal power plants.

Although it has not erupted for 11,000-years, it exhibits volcanic-type seismic activity such as tremors and long-period (LP) events. LP events are produced by fluids surging through cracks and faults in the Earth’s crust. Studying these events allows better understanding fluid movement and fault dynamics.

This is important economically and to better assess seismic and volcanic hazards. However, seismic signals from LP events are small and challenging to detect and analyze. Here the researchers investigate these events at Clear Lake Volcanic Field.

They use seismology which is the study of seismic waves generated by earthquakes and travelling through the Earth. The waves are detected at the surface and analyzed. The researchers leverage on existing data.

They combine state-of-the-art analytical techniques to better identify and locate the LP events. They examine the dynamic properties of the events (stress drops), their scaling with magnitude, and when possible the focal mechanisms of the earthquakes that produced them. The improved catalog of LP events, high-resolution locations, and focal mechanisms is used to investigate the fluid transport system in the region.

The project outcomes – which include refined methods and development of hybrid approaches to improve LP event analysis – could be applied at other volcanic systems. The one-year project also supports a collaboration between academia and the industry, and educational outreach to the public.

The study is based on existing catalog of seismic waveform data that have shown to include LP events. These events are compared to non-volcanic tremor and low-frequency earthquakes at other places along the San Andreas fault system. The researchers search the continuous waveform data set using adaptive waveform template matching to identify LP events related to events already in the catalog.

They develop new templates - based on waveform similarity - to further interrogate the data and detect and catalog new LP events. They use the double-difference location method to develop a high-resolution catalog. The detected events are analyzed using spectral modeling methods to estimate the scalar seismic moment and corner frequency; the goal is to assess possible scaling properties of the LP earthquakes.

The team also utilizes three-component waveform data to determine focal parameters from moment tensor inversion. LP events are small and challenging to model and therefore require efforts toward the refinement of methods and the development of hybrid approaches to extract more reliable results. Because of their small magnitude, the researchers need to explore combined inversion scheme that utilizes a source-type inversion of both waveforms and first-motion polarities, possibly expanding the method to utilize body wave amplitude ratios.

The results of the multipronged analysis will be used to investigate potential fault structure and the fluid transport system of the Clear Lake Volcanic Field. It will also allow evaluating whether continued study is warranted with a possible portable array deployment to improve coverage to better investigate LP earthquake seismicity.

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.