Southern Puerto Rico was hit by a series of massive, devastating earthquakes that began late December in 2019. The quakes, which included a magnitude 6.4 in early January, shocked seismologists. After thorough research, they may have found out why. 

According to USGS seismologists, earthquakes were known to occur on the northern side of the island. Puerto Rico rests between the Caribbean and North American plates. The northeast corner of the Caribbean plate moves less than an inch towards the east annually. Historical records detailed earthquakes and tsunamis since the 1500s. However, the seismic activity has always been limited in northern Puerto Rico. 

Read: Earthquake Shakes Puerto Rico Causing Damage to Buildings in Ponce

The USGS uncovered previously unknown seafloor faults that may have triggered the series of earthquakes between 2019 and 2020. The scientists are working to identify and chart the faults. Mapping active fault can help them estimate the location and the magnitude of potential earthquakes. Conducting seismic research can also help experts inform local and state governments better building codes, safer zoning, and educating the public about earthquake hazards. 

Exploring the Seabed

USGS geophysicist Uri ten Brink launched a seismic research cruise at the island's south coast following the quakes. Uri ten Brink heads a marine geohazards project that documents undersea tectonic processes. The team is also responsible for assessing the potential hazards caused by landslides, tsunamis, and earthquakes. According to Uri ten Brink, the earthquake sequence would have likely involved a previously unknown fault system off Puerto Rico's south coast. If proven true, his theory would be very valuable to science as well as the well being of the people living in southern Puerto Rico. 

The USGS team collaborated with researchers from the University of Puerto Rico Mayaguez to run a series of one-day research cruises in La Parguera from March 7th to 13th. The Puerto Rican educational institution provided logistical support including a 43-foot R/V Sultana. 

Deep-Sea Faults

Preliminary results from the research cruises confirmed Brink's theory. The team found evidence of at one deep-sea fault near Guayanilla Bay. The undersea fault could possibly be the extension of a fault mapped on land. Ten Brink and the researchers also identified several more faults seven and fifteen kilometers from the shore, which as near areas identified as the epicenters of the recent quakes. 

The team also identified several scars in shore-facing cliffs which may have been caused by rockfalls due to tremendous shaking. Ten Brink expects to publish more data in the coming months. The researchers would conduct more thorough data processing to uncover additional faults and is expected to measure ruptures along the newly discovered faults. 

The team aboard the R/V Sultana used a device called the "sparker." The device uses electrical charges to produce air bubbles in the water. The expanding bubbles create a sound wave that travels through the water and the seabed. The research team uses hydrophones to measure the time the signals take to bounce back from the ocean floor. The differences in the signal's return suggest differences in the elevation of the rock beneath the ocean floor and are often considered to be signs of faults.

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