Puerto Rico’s 2019-2020 Seismic Swarm Continues and Leaves the Island on Edge

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The largest earthquake in Puerto Rico in 100 years struck on a fault discovered only several years ago.

The swarm-like earthquake sequence, with a M5.8 foreshock and two large aftershocks so far, caused extensive damage and, unfortunately, is not over.

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Current seismic state of Puerto Rico. Earthquake map by Temblor

On December 28, 2019, the southwestern region of the island of Puerto Rico — located in the northeastern corner of the Caribbean plate — began to experience an increase in seismicity.

It started with an M4.7 event that rattled a population that has not experienced a significant damaging earthquake in their lifetime.

That is because the last major event to affect the island was an M7.2 in 1918.

Although there are two main fault systems that cross the island in a roughly east-to-west fashion, it is the southwest tip of the island where most seismicity occurs as evidenced by more than four decades of seismic monitoring by the Puerto Rico Seismic Network.

Since December 28, more than 14 earthquakes of M4.5 or greater and thousands of smaller events have been felt and recorded. The strongest of these events have been the M5.0 (29 Dec), the M5.8 (6 Jan), and the M6.4 and M6.0 both on 7 Jan.

The January 6th event during the early morning surprised many children opening their gifts as part of the traditional Three Kings Day (Epiphany), and placed the entire population of Puerto Rico on alert.

Less than 24 hours later, the larger M6.4 event occurred, causing locally catastrophic damages to structures in the south coast region of the island and widespread anxiety among the population.

Meanwhile, the earthquake swarm continues with at least 34 earthquakes larger than M2.5 on January 17, 2020.

2019: Highest seismic activity on the island

Interestingly, the year 2019 proved to be the year with the highest seismic activity recorded by the Puerto Rico Seismic Network.

With 6,517 events (171 with M>3.5), the year 2019 saw almost twice the amount of events located to that of an average year since 2014.

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Topographic map of Puerto Rica and its main earthquake fault lines. The larger Great Northern Puerto Rico fault zone (GNPRfz) and Great Southern Puerto Rico fault zone (GSPRfz) are shown in addition to the South Lajas Valley Fault (SLVF), and the North Boquerón Bay – Punta Montalva fault zone (NBBPMfz) in the southwest of the island. Map by Temblor

The reason for this increase is the end of year sequence in southwestern Puerto Rico, and the seismic sequence associated to the M6.0 Mona Canyon event of Sept 23, which so far includes 1,227 located aftershocks.

Although the M6.0 sequence might indicate a change in the tectonic motions in the region, it was farther away from Puerto Rico and as such was not felt as strongly as the current activity in the southwest.

Earthquakes create extensive damage on Puerto Rico

The majority of the seismicity has been offshore by a few kilometers, but residents along the southern coast closest to the epicentral area have felt moderate to strong shaking up to a Mercalli Intensity of VII to VIII (very strong to severe shaking) during the M6.4 on 7 Jan.

The very shallow quake depths have made the shaking more evident for the population. The strongest shaking events have resulted in the collapse of several houses built on stilts, as well as the partial collapse of both the historic church in Guayanilla and the abandoned Spanish lighthouse in Guánica.

At least one death attributed to the M6.4 quake has been reported, a result of a collapsed wall that fell on a man sleeping in Ponce.

People that have evacuated destroyed or severely damaged homes have been provided temporary shelter, although many of them do not trust the structural integrity of the shelter centers and prefer to stay outdoors.

Psychological counseling and services have been arranged for those who need them.

Additionally, the ground movement has also provoked rock falls in the south coast limestone bedrock. The blocks of limestone scattered across roadways and also impacted houses.

Sadly, the scenic natural rock formation known as the Punta Ventana (‘Window point’) in Guayanilla collapsed during the M 5.8 event of Jan 6th.

Sea level rise reported off Puerto Rico

Although no major tsunami has been generated, a local rise of around an inch in sea level was reported just west of the epicentral area in La Parguera by the Caribbean Tsunami Warning Program.

The possibility of submarine landsliding has been a popular talking point given its proximity to land, its shallow rupture and higher magnitude potential.

New earthquake fault discovered on Puerto Rico

The first decade of the millennium sparked an increased interest in the past sliding of faults on the island of Puerto Rico — a field of earthquake science known as ‘paleoseismology’.

Inspired by recently acquired LIDAR (Laser radar topographic) data, scientists dug up trenches on a fault across the Lajas Valley in southwestern Puerto Rico. These trenches allow scientists to identify offsets and other clues that the fault has slipped in the past. Their findings suggested activity on the South Lajas Valley fault (SLVF), at its western terminus near the Bay of Boquerón, within the last 12,000 years.

The current seismicity appears to represent activation of a further southeasterly seaward extension of this relatively newly recognized fault.
The current seismicity appears to represent activation of a further southeasterly seaward extension of this relatively newly recognized fault. Map via Temblor

However, the fault’s eastward extension was difficult to follow and identify, primarily due to the thick young sediment cover of the Lajas Valley. Shortly after, a renewed effort was put forward by professors of the Geology Department of the University of Puerto Rico to understand better the seismic potential of this region. A new fault was identified using a plethora of geophysical (Roig et al., 2013 ; Adames, 2017) and geological (Addarich-Martinez, 2009) analyses.

The discovery of the North Boquerón Bay – Punta Montalva fault was important because it was not in the USGS fault database for Puerto Rico, and even then scientists knew it posed a serious threat to the nearby population.

The current seismicity appears to represent activation of a further southeasterly seaward extension of this relatively newly recognized fault.

So what’s next for Puerto Rico?

Local media has reported that there are approximately 100,000 housing structures built on silts in Puerto Rico, and approximately 95% of public school buildings have been built prior to the existing building code currently in use.

Both of these categories of buildings are especially vulnerable to shaking. Such a statement is particularly worrisome because a school has already collapsed after the Jan 7th M6.4 event, and classes are due to resume in the coming days.

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Quantity of quakes that have occurred since the beginning of the seismic activity on 28-Dec. Via Temblor

Fear of what has just been lived and anxiety about what may happen is evidently worrisome for the population, who have preferred to sleep outdoors.

Unfortunately, we still don’t know whether the M6.4 event was the mainshock event or if it will become another precursor in the sequence.

The swarm-like character of the sequence also suggests that it could continue. One thing is sure… After feeling aftershocks all day, they are on edge and uncertain about what will happen next. Similar headlines on Strange Sounds and Steve Quayle. [Temblor]

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