Mid January 2020

Introduction

The southwestern coast of Puerto Rico has been experiencing an earthquake swarm. It started in December and the U. S. Geological Survey and other agencies have recorded over 1,495 earthquakes of magnitude 1.5 or greater in the last 30 days. Click here for the most recent statistics.

Most earthquakes are small, recorded by seismographs but not felt. Here is a brief description of magnitudes and potential for damage.

• Magnitude less then 2.5: Not usually felt
• Magnitudes between 2.5 and 5.4: Usually felt, minor damage.
• Magnitudes between 5.5 and 6.0: Felt, slight damage to buildings and other structures
• Magnitudes between 6.1 and 6.9: May cause widespread damage in populated areas.

So far, the largest magnitude earthquake was a 6.4 which occurred about 4: 20 AM on January 7. It caused widespread damage in communities along the southwest coast: Ponce, Yauco, Guanica. We felt it in San Juan, about 70 miles from the epicenter. We also felt two aftershocks that same morning. Fortunately, San Juan is too far from the epicenter to experience any damage.

Earthquakes: The Big Picture

I remember, as a bored second or third grader, staring at the world map in our classroom. I was sure Africa and South America fit together. Of course, I wasn’t the only one to notice this. The German atmospheric scientist Alfred Wegener had noticed the same thing, in the late 1800’s and early 1900’s. Wegener mustered geologic, fossil, and other data to theorize that the continents were once connected, and had somehow moved to their current position. Click here for a short lecture on Wegener’s evidence.

The map below shows the current configuration of the 17 major tectonic plates. Note that the Caribbean plate adjoins the North American, South American and Cocos plates.

Pangaea – The Supercontinent

So what was the position of the plates in times past? It is now accepted that the super continent Pangaea began to break up about 250 million years ago. The map below shows Pangaea with the modern continents labelled.

Massimo Pietrobon wondered how modern political boundaries related to Pangaea. Using a good bit of cartographic license, he created the map below. The map shows a representation of Pangaea with current countries (which of course did not exist 250 million years ago) overlain. Visit the artist’s website here.

The Break Up of Pangaea

By about 200 million years ago, the pates had separated into two super continents – Laurasia and Gondwana.

Plate tectonics explains a lot of disparate data. As shown above, Madagascar had separated from Africa and India as early as 200 million years ago. Lemurs evolved on that isolated island, and nowhere else.

Note also that the Indian subcontinent was separated by 200 million years ago. It moved rapidly to the north north east before crashing into the Asian plate, and causing the uplift that created the Himalaya mountains and the Tibetsn plateau. During its travels the Indian subcontinent sometimes moved at a speed of 5 to 6 inches per year.

For a short video of the plate movements click here. For an animation of the movement of the Indian subcontinent, complete with music from India, click here.

Plate Boundaries

It stands to reason that if tectonic plates are moving, they are separating (diverging), colliding (converging) or moving parallel to each other. It makes sense that earthquakes and volcanoes occur at the borders of plates. In fact, that is the case – for the most part. The schematic below depicts these situations.

Diverging Zones

If two plates separate and the area of divergence is under the ocean, an ocean ridge forms. If the divergence is on land, rift valleys form.

A nineteenth centruy British expedition used cable soundings to map some of the bottom of the Atlantic Ocean. They were the first to find the mid ocean ridge system. After World War II, sonar and other exploration techniques provided greater detail and led to the idea that new crust was being formed as the plates moved away from each other.

As noted above, if land masses are spreading, the result is a rift valley system.

Converging Zones

Plates moving into each other create converging zones. One plate sinks under the other in a process called subduction. If the convergence occurs under the ocean, a trench is created along the leading edge of the plate being subducted. Again, these zones are seismic and volcano active regions. In addition, earthquakes sometimes cause tsunamis.

Transform Plate Boundary

Two plates sliding against each other form a transform plate boundary. The crust is deformed and sometimes pulverized but, unlike the other two boundaries, no new crust is formed.

The San Andreas fault in California is a famous example of this type of boundary. The North American plate to the east is moving approximately southeast; the adjoining Pacific plate to the northwest.

The Situation in Puerto Rico

The Caribbean plate, as noted above, is surrounded by four other plates. Each one is moving relative to the others. That creates a complex environment. In addition, Puerto Rico might be on its own microplate, itself moving with respect to both the North American and Caribbean plates. (This is further proof to what I have said many times: Puerto Rico is its own unique self).

As shown in the map above, the Caribbean plate, moving eastward, is converging with and overriding the North American plate which is moving westward. This explains the volcaones in the Lesser Antilles volcanoc arc. To the north, nearer to Puerto Rico, the boundary between these two plates transitions from a converging zone to a transform boundary. Note the faults that cross the Dominican Republic and Haiti. It was one of these faults that ruptured and caused the devastating Haitian earthquake (magnitude 7.0) of 2010. Puerto Rico is in the area where the boundaries transition from converging to transform. In addition, it is near the edge of the Caribbean plate. The fact that Puerto Rico is its own microplate makes for a very complex earthquake environment.

The recent earthquake swarm has been centered off the southwest coast of Puerto Rico. (Google earthquakes pr – that will take you to a USGS site with information on recent earthquakes). The Puerto Rico microplate is being squeezed from both the north and the south.. To the north, the North American plate is sliding under, in an oblique direction, the Puerto Rico microplate. The North American plate in effect lifting and moving the Puerto Rico microplate onto the much larger Caribbean plate.This creates the possibility of seismic activity to both the north and south of Puerto Rico. It is the southern fault that has generated the current sequence of earthquakes.

And the earthquakes have occurred in a so-called swarm – a series of events of about the same size, as opposed to one large event followed by smaller aftershocks.

So will there be more earthquakes here? Count on it. Just like it is a sure bet there will be earthquakes in California, or Japan. It is also possible that the volcanic activity will return to the Lesser Antilles. But anyone who tells you they can predict these things with great certainty is a charlatan. One has to assume the Caribbean attitude: Be happy, don’t worry.

Whatever the cause, the earthquakes have been disconcerting. Here in San Juan, we can barely feel them – they feel, if you were in a flimsy building, like the vibrations caused by a large truck passing nearby. Never the less, the earthquakes will not stop the SanSe20 street festival in Old San Juan this weekend. More on that later.

Notes and Sources

The schematics of the plates and actions along the plant boundaries were taken from the geology.com website. Visit that site here.

The first Pangaea map is from here.

The second Pangaea map is from here.

The Gondwana and Laurasia schematic is from here.

The map of the mid-ocean ridges comes from here.

The representation of the East African rift valleys is from here.

The schematic of the converging zone under Japan is from here.

Look here for the map of the Caribbean plate.

In addition, I gleaned information from various Wikipedia articles.