Puerto Rico was rocked by another large earthquake early Saturday, the latest and largest of dozens of aftershocks that followed a larger quake off the island’s southern coast Tuesday.

The latest seismic activity had its epicenter off the coast of Guanica, a city on the island’s southern coast. The 5.9-magnitude quake — initially believed to be a 6.0-magnitude quake — shook buildings and left concrete strewn about many of the islands’ streets. Puerto Rico has been racked with nearly 45 aftershocks of at least a 3.0-magnitude since Tuesday’s 6.4-magnitude quake, which killed one person and injured nine others, according to the Puerto Rico Seismic Network (PRSN).

Ahead of the latest earthquake, nearly one million Puerto Ricans were without power, hundreds of thousands lacked access to clean water, and more than 2,000 people were living in shelters. The Puerto Rico Electric Power Authority said electricity was nearly fully restored following Saturday’s quake, claiming 93 percent of the island now has power.

Nevertheless, Saturday’s shakes are expected to exacerbate an already critical situation. Thousands of homes and schools have simply collapsed, adding to already extensive damage, including the destruction of the noted natural landmark Punta Ventana.

President Donald Trump declared a state of emergency on the island late Tuesday, the first since Puerto Rico was hit by Hurricane Maria in 2017. The declaration will free up federal disaster funds to assist with recovery, but local officials worried that the assistance will be too slow to make a difference.

“FEMA is a very bureaucratic agency and it moves very slowly. So slowly that we’re still waiting for federal funds from Maria,” Daniel Hernández, director of generation for Puerto Rico’s Electric Power Authority, told The Associated Press.

For now, the island’s residents find themselves once again trying to survive a natural disaster that has devastated the island’s infrastructure. And unfortunately, more aftershocks could hit the island.

Víctor Huérfano, the head of PRSN, said Saturday’s seismic activity is likely to trigger aftershocks.

“It’s going to re-energize an unstable situation,” Huérfano told The Associated Press. “It’s a complex zone.”

Earthquakes are difficult to predict — but Puerto Rico may face more in the days to come

The quakes plaguing Puerto Rico come almost exactly ten years to the day a massive 7.0-magnitude hit nearby Haiti in 2010. Experts, however, have noted the timing is just a coincidence, and that while historical and real-time seismometer data can lay out expected earthquake trends, predicting exact seismic activity in order to save lives is generally a much more difficult proposition.

Puerto Rico has felt about 139 seismic activities above a 3.0-magnitude since early December, according to the US Geological Survey (USGS). Seismologist Lucy Jones suggested on Twitter that they were all part of one continuous seismic event. The frequency of these events, and Puerto Rico’s susceptibility to earthquakes stems from the manner in which the island is situated along the meeting points of the South American, Caribbean, and North American plates:

As of yesterday, the USGS seismic forecast for the region predicted a decreasing number of aftershocks after the larger quake Tuesday.

Ahead of Saturday’s quake, the USGS had given about a 3 percent chance of Puerto Rico experiencing another 6.4-magnitude quake, which Saturday’s quake nearly eclipsed. The organization did warn, however, that there was a “high likelihood” aftershocks of magnitude-3.0 or greater could come in the the next week.

As Vox’s Umair Irfan has explained it is difficult to give those in earthquake zones more detailed predictions than these:

It’s difficult to figure out when an earthquake will occur, since the forces that cause them happen slowly over a vast area but are disbursed rapidly over a narrow region. What’s amazing is that forces built up across continents over millions of years can hammer cities in minutes.

Forecasting earthquakes would require high-resolution measurements deep underground over the course of decades, if not longer, coupled with sophisticated simulations. And even then, it’s unlikely to yield an hour’s worth of lead time. So there are ultimately too many variables at play and too few tools to analyze them in a meaningful way.

Some research shows that foreshocks can precede a larger earthquake, but it’s difficult to distinguish them from the hundreds of smaller earthquakes that occur on a regular basis.

Despite the difficulty involved, USGS and PRSN officials have been busy late this week trying to install temporary seismometers along the southern coast of Puerto Rico in order to monitor seismic activity and assist in predicting aftershocks on the already reeling island. In the meantime, thousands will remain in shelters, awaiting concrete assurances it is safe to return and begin rebuilding.

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