Micro-earthquakes could reveal Earth's water sources

Aerial view of Long Beach Harbor and San Pedro, California. Image: FreeImages

Discovery of new water sources could be made possible by a new method of recording planet Earth's natural vibrations, say an international team of scientists from the US and China.

The technique lets researchers visualise rocks containing fluids better, leading to potential advancements in the discovery of water and oil resources. There is also potential for applications in identifying urban geologic hazards and developing early warning systems for tsunamis, along with better understanding of the water cycle.

The method uses portable seismometers to record the Earth's natural vibrations, making it a cost-effective and easy way to study locations in cities and oceans.

"This innovative method has the potential to revolutionise our approach to discovering and utilising water and oil resources, enhancing urban safety measures, and deepening our understanding of the environment."

Professor Xin Liu, University of Hong Kong

In the study, 42 seismometers were placed along a line across the Los Angeles basin, from Long Beach to Whittier Narrows. Researchers found that rocks about 1-2 km below the surface near the Newport-Inglewood Fault, which causes earthquakes, contain significant volumes of fluid. in tiny holes, which may explain the occurrence of small earthquakes in Long Beach, California.

The abundance of fluid within these tiny holes reduces friction along the fault plane, allowing the two rock blocks on either side to slide past each other more easily and generate small earthquakes.

The paper published in Nature Communications journal suggests that the technique - known as ambient noise differential adjoint tomography - can be used to find water and oil resources without the need for expensive drilling, by generating images of the ground covered by seismometers, revealing how fast seismic waves travel in soils and rocks. In some locations, the seismic wave travels much slower compared to other regions at the same depth, indicating the presence of fluid.

As water and oil are fluids in rocks, this method can identify rocks containing such fluids.

Map of Los Angeles Basin, where a live seismometer at Long Beach is recording earth’s background vibrations along with 10-minute noise logging display. Image: Raspberry Shake.

The research team is led by Professor Xin Liu, from the department of earth sciences at the University of Hong Kong (HKU), who says, "Previously, groundwater aquifers or deep fluid reservoirs were difficult to find without drilling multiple expensive wells or costly seismic surveys with loud artificial sound that are not environmentally friendly on land or ocean. Using just weak seismic noise recordings by two dozen seismometers on land or seafloor, our new technique can create images containing fluid information within rocks, and pinpoint the location anddepth of fluid-rich rocks."

The scientists say that this innovative method can be used to create detailed images of the ground in urban areas and the deep ocean, serving various purposes such as assessing urban geologic hazards, implementing early-warning systems for tsunamis and enhancing understanding of the water cycle under the seafloor. In urban settings, a series of land seismometers can be deployed over the area of interest.

At sea, a line of ocean bottom seismometers (OBS) can be installed to record background vibrations. In both cases, a detailed image is created right underneath the line of seismometers, providing information about the locations of loose soil, sediments and fluid-bearing rocks that directly relate to regions with slow seismic wave velocity.

An ongoing project is imaging fluids under the seafloor in the Nankai Trough, southeast Japan. Image: Dr Lina Gao

"In conclusion," says Professor Liu, "this innovative method has the potential to revolutionise our approach to discovering and utilising water and oil resources, enhancing urban safety measures, and deepening our understanding of the environment.

"Its direct impact on our daily lives spans from efficient resource exploration to effective disaster preparedness and promoting sustainable environmental management practices. This scientific breakthrough holds excellent promise in shaping a better future for us all."