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Puerto Rico’s Winter 2019-2020 Seismic Sequence Leaves the Island On Edge

Tiegan Hobbs, PhD

Alberto M. López, K. Stephen Hughes, and Elizabeth Vanacore
Department of Geology, University of Puerto Rico – Mayagüez
Puerto Rico Seismic Network, University of Puerto Rico – Mayagüez

Citation: López, A.M., Hughes, K.S., & Vanacore, E. (2020), Puerto Rico’s Winter 2019-2020 Seismic Sequence Leaves the Island On Edge, Temblor, http://doi.org/10.32858/temblor.064.

The largest shock in Puerto Rico in 100 years struck on a fault discovered only several years ago. The swarm-like earthquake sequence, with a M 5.8 foreshock and two large aftershocks so far, caused extensive damage and, unfortunately, may not be over.

The largest quake in a century

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 M 4.7 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 M 7.2 in 1918 (Doser et al., 2005). 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 the December 28, more than 14 earthquakes of M 4.5 or greater and hundreds of smaller events have been felt and recorded. The strongest of these events have been the M 5.0 (29 Dec), the M 5.8 (6 Jan), and the M 6.4 and 6.0 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 M 6.4 event occurred, causing locally catastrophic damages to structures in the south coast region of the island and widespread anxiety among the population.

Thematic map of the current seismic state of Puerto Rico. Refer to the legend at left for details of the illustration. Circles represent seismic events located by the Puerto Rico Seismic Network since December 28, 2019. The different colors indicate when a particular event occurred since activity commenced in late December, while the size of the circle represents the magnitude of the event. Red lines show the estimated location of known predominant faults. The different color tones in Puerto Rico corresponds to estimated intensities most probable the population were exposed to. Most significant events are shown with a white star with the magnitude and date plotted next to it. The largest event observed to date is the M6.4 7-Jan, similarly, the main precursor (M5.7 on 6-Jan) and the main aftershock (M5.8 on 7-Jan) are also shown.

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. The reason for this increase is the end of year sequence in southwestern Puerto Rico, and the seismic sequence associated to the M 6.0 Mona Canyon event of Sept 23, which so far includes 1,227 located aftershocks. Although the M 6.0 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.

Topography of the island of Puerto Rico, located along the seismically active tectonic interface between the North American and Caribbean plates. Events larger than M 2.5 detected by the Puerto Rico Seismic Network between December 8, 2019 to January 7, 2020 are shown. Red are events on or before December 28, 2019 and greens are January 7, 2020. The size of each symbol corresponds to the event’s magnitude, with all events plotted between M 2.5 and M 6.4. 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. The red box near the cluster of seismic events corresponds to the extent of the figures below. Topographic basemap from USGS and bathymetric map from ESRI.

Extensive damage

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 M 6.4 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 M 6.4 quake has been reported, a result of a collapsed wall that fell on a man sleeping in his house 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 6-Jan.

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. A submarine landslide is exactly what it sounds like: a slide of sediment and rock on the ocean floor which is capable of producing localized tsunami.

A house in southern Puerto Rico in the municipality of Guánica that was built upon stilts. The columns buckled during the shaking caused by the M 5.8 earthquake just offshore on January 6, 2020. This is a classic ‘soft first story’’ building. Photo credit: Fernando Martínez Torres.
Collapse of most of one of the walls of the old Guánica lighthouse. The lighthouse was built in 1892 by the Spanish. Photo credit: Fernando Martínez Torres on 6-January, 2020.

Before and after views of the Punta Ventana in Guayanilla. Shaking during the M 5.8 event on January 6, 2020 caused its collapse. Photo credit: twitter.com/UPRMetLab/

A newly recognized fault

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, Prentice and Mann (2005) 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. 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.

Aerial (top) and shaded relief (bottom) views of the on-land expression of the Punta Montalva fault (PMf). A pronounced topographic lineament and deflected drainage highlight the sinistral (left-lateral) fault trace. The cluster of hundreds of seismic events lies just to the south east of the map view along the same trend as the PMf. LiDAR DEM from USGS data.
Events larger than M 2.5 detected by the Puerto Rico Seismic Network recorded on or after the first significant event 4.7 ml that occurred on December 28, 2019. The events are color coded by the time after the 4.7 ml event and the size of the symbol represents the magnitude recorded in the PRSN catalog. The yellow line indicates the position of the Punta Montalva Fault.

What next?

Local media has reported that there are approximately 100,000 housing structures built on stilts 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 7 Jan M 6.4 event, and classes are due to resume in the coming days. 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.

Histograms showing the quantity of events occurring since the beginning of the seismic activity on 28-Dec. Bold, vertical lines indicate the moment in which a particular event occurred in the time window, shown on above the line. Events from 6 and 7 January do not show significant activity due to completeness. PRSN analysts were locating these events still by the time this article had to be published.

Unfortunately, we still don’t know whether the M 6.4 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 certain, that after feeling aftershocks all day, we are on edge and uncertain about what will happen next.

References

Adames, A., 2017, Geomorphic and geophysical characterization of the north Boquerón Bay-Punta Montalva fault zone: A capable fault system in southwestern Puerto Rico. M.Sc. Thesis, Department of Geology, University of Puerto Rico, Mayagüez

Addarich-Martinez, L. 2009, The geologic mapping and history of the Guánica Quadrangle, southwestern Puerto Rico, M.Sc. Thesis, Department of Geology, University of Puerto Rico, Mayagüez.

Doser, D.I., Rodriguez, C. M., and Flores, C., 2005, Historical earthquakes of the Puerto Rico – Virgin Islands region, in Mann, P., ed., Active tectonics and seismic hazards of Puerto Rico, the Virgin Islands, and offshore areas: Geological Society of America Special Paper 385, p. 103-114.

Prentice, C., S., and Mann, P., 2005, Paleosismic study of the South Lajas fault: First documentation of an onshore Holocene fault in Puerto Rico, in Mann, P., ed., Active tectonics and seismic hazards of Puerto Rico, the Virgin Islands, and offshore areas: Geological Society of America Special Paper 385, p. 215-222.

Roig-Silva, C. M., Asencio, E., and Joyce, J., 2013, The Northwest Trending North Boquerón Bay-Punta Montalva Fault Zone; A Through Going Active Fault System in Southwestern Puerto Rico.