The reason for the abnormal rise of groundwater after the major earthquake

After the Kumamoto earthquake (Japan), an abnormal rise in the groundwater level was observed in the recharge area of ​​the large-scale groundwater flow system in the Kumamoto region. Credit: Associate Professor Takahiro Hosono

Increases in groundwater levels and volumes after observing the large earthquakes around the world, but the details of this process remained unclear due to the lack of groundwater data directly before and after the earthquake. Fortunately, researchers from Kumamoto and Kwansi Gakuen Universities (Japan) and the University of California Berkeley (USA) realized that they had a unique research opportunity to analyze changes in the level of groundwater around Kumamoto City after large earthquakes struck the region in 2016.

Changes in the hydrological environment were recorded after the earthquake, such as the drying of ponds or wells, the sudden onset of running water, or the rise in water levels since the Roman era. Various theories have been proposed to cause such changes, such as fluctuations in pore water pressure (groundwater pressure in pores or gaps in rocks and soil), increased water permeability, and water movement through new cracks.

To determine the actual cause, data should be collected from observation sites in wells, water sources and rivers. However, especially in the case of internal earthquakes, it is generally rare for these sites to be spatially arranged in an area where a major earthquake occurred. Additionally, it is rare for you to have sufficient comparison data before and after a disaster. These difficulties were a barrier to obtaining a clear picture of how hydrological environments changed after the earthquakes.

Kumamoto, on the island of Kyushu, in southern Japan, is famous for its waters. Almost 100% of the city’s drinking water is obtained from the groundwater in the area, so there are many monitoring wells in the area that constantly record the water level and quality data. Early in the morning (Japan time) on April 16, 2016, a 7.0 magnitude earthquake struck the city resulting in a wealth of groundwater data before and after the earthquake. Kumamoto University researchers realized this unique opportunity to assess how earthquakes can change hydrological environments in more detail than ever before, so they established international cooperation to study the event.

Prior to the earthquake, the groundwater of the Kumamoto City area had wide-ranging structural features that included low-altitude mountain springs, soil water in the feeding area, and the water of the Shirakawa River (Black Box B, C). Credit: Associate Professor Takahiro Hosono

An abnormal rise in the groundwater level occurred after the main shock and was particularly noticeable in the feeding area of ​​the groundwater flow system. The water levels reached their peak within a year after the main shock at about 10 meters, and despite their calm after that, the water levels were still high after more than three years. This was thought to be due to the outflow of water from a place not part of the hydrological cycle prior to the earthquake, so researchers attempted to identify sources using stable isotopic ratios of water.

The proportions of stable isotopes of water on the Earth’s surface change slightly with different processes (evaporation, condensation, etc.) so that the mark values ​​become unique depending on the location. These signs make it possible to identify the processes that affected the water sample as well as its source.

A comparison of stable isotope groups before and after revealed that, before the earthquake, groundwater in the Kumamoto City area came mainly from low-rise aquifers, soil water in feeding areas, and leakage from the central Shirakawa River. Area. After the earthquake, researchers believe seismic fractures are on the western side of a mountain. Aso also increased the permeability of the mountain water, which released groundwater towards the feeding area in the flow system and increased water levels. Moreover, groundwater levels in the flow area that decreased almost immediately after the major shock were restored within only one year.

“Our research is the first to capture the hydro-environmental changes caused by a major earthquake in detail,” said Assistant Professor Takahiro Hosono. “The phenomenon we have discovered can occur anywhere on Earth in regions with climatic and geological conditions similar to Kumamoto. We hope our research will be useful for academics and guidelines for regional use of water in disasters.”

Ground level of helium in groundwater could indicate a potential risk of an earthquake. More information: Takahiro Hosono et al, Stable isotopes show that earthquakes enhance permeability and release water from mountains, Nature Communications (2020). DOI: 10.1038 / s41467-020-16604-y Provided by Kumamoto University

Citation: The reason for the abnormal groundwater rise after the Great Earthquake (2020, July 2) retrieved on July 2, 2020 from https://phys.org/news/2020-07-abnormal-groundwater-large-earthquake.html

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