TERRESTRIAL HYDROLOGIC RESPONSE TO THE 2020 SOUTHWEST PUERTO RICO SEISMIC SEQUENCE

After the M6.4 mainshock of the 2020 Southwest Puerto Rico Seismic Sequence on January 7, changes in the temperature and discharge were reported at the Baños de Coamo hot springs site in the south-central region of the island.

Temperature and pH sensors were placed in the main well of the Baños de Coamo to measure possible changes in response to the ongoing seismic activity. Daily water samples were collected to track any changes in hydrogen and oxygen isotope ratios in the thermal water. In addition, USGS Water Data was used to obtain and analyze rivers’ streamflow conditions to determine the impact of the earthquakes in this area.

In the initial months after the mainshock, no significant increase in thermal water temperature was observed. The temperature remained very consistent always within 0.5 ̊C of 43 ̊C. The pH monitoring showed no correlation to seismicity either. In the same period, δ18O remained consistent while δD displayed some fluctuation. The isotopic data are consistent with the local meteoric water line. Changes in discharge at the thermal springs was also measured during the seismic sequence but a relationship with seismicity is clouded by a regional drought and later a tropical storm. Ongoing data acquisition and synthesis will be presented.

In addition, increases in base flow discharge were recorded in at least 11 Puerto Rican rivers after the largest event in the seismic sequence. Increase in base flow conditions was recorded in river systems whose drainage basins include the Great Southern Puerto Rico fault zone, which trends WNW from the south coast of the island to the western coast. The Baños de Coamo thermal spring site is also located along this crustal scale structure and highlights its importance in channeling the terrestrial hydrologic response to the seismic sequence.

This investigation funded by NSF-RAPID also aims to take advantage of the interest in the seismic events to educate the general public at the popular Baños de Coamo thermal spring site. Bilingual outreach materials are in development and will be distributed by our partners in the municipality of Coamo. This initiative will help to clarify myths related to the origin of the thermal waters and give general geologic introduction to visitors at the hot springs site.