In the Virgin Islands, a network of 10 devices called “strong motion sensors” record constantly.
In the event of an earthquake, they capture information about it.
Otherwise, the data they record loops back over on itself every 60 seconds to save memory.
The VI sensors are among 66 placed around the Caribbean to measure and triangulate seismic activity as part of the Puerto Rico Strong Motion Program, which works to inform safer construction regulations by providing academics with hard data.
Last week, programme Director Juan Carlos Martínez Cruzado, a professor at the University of Puerto Rico, visited the VI to conduct his annual check of the sensors set up here.
As the shadows lengthened on June 4, he and two colleagues made their last stop for the day at a sensor in East End after checking sensors on Virgin Gorda.
Types of sensor
At the site, Mr. Martinez Cruzado explained the difference between a strong motion sensor and the better-known seismometer. Seismometers, he said, are very sensitive instruments designed to pick up the smallest measurable seismic activity in a given area.
“They want to study the earth; they want to learn where the seismic faults are, the movement of the tectonic plates, and in which direction they’re moving,” he said.
To illustrate a possible use of a seismometer, he provided an off-hand example.
“[Seismometers] are studying the earth,” Mr. Martinez Cruzado said. “So they get an earthquake in China, they can record it here [in the VI], because they’re very sensitive.”
Infrastructure
Strong motion sensors, on the other hand, are less-sensitive devices that can be placed in urban areas and used to measure the forces that act on infrastructure, he explained.
“We are more interested in the infrastructure that can be damaged with a big earthquake,” he said. “You see, our instruments are not as sensitive as the seismograph in the seismic network. But when you get a big earthquake, then ours will record, while [seismometers] will be out of range. They won’t be able to record [the seismic event] properly.”
To measure the effects on infrastructure, he added, the sensors must be placed near the infrastructure in question.
“We are interested in the big earthquake, the high-intensity earthquake that can destroy buildings, bridges or dams,” he said. “So we need to put instrumentation where the buildings are.”
Quake in May
On May 4, sensors around the region were tripped by an earthquake that was felt by many VI residents.
The tectonic activity lasted roughly 40 seconds, according to preliminary data captured by one of the territory’s sensors.
The East End sensor is one of 10 currently operating around the VI: Two are on Beef Island and one each is at Cappoons Bay, the Ralph T. O’Neal Administration Complex, McNamara, North Sound on Virgin Gorda, Guana Island, Norman Island and Jost Van Dyke.
More resources
The Puerto Rico Strong Motion Program began in the 1970s with seven strong motion sensors and one “instrumented building,” according to its website.
Today, the programme manages a network of 66 sensors and two “instrumented buildings with digital accelerographs.”
Despite the progress over the decades, Mr. Martinez Cruzado would like to see the PRSMP run more efficiently.
“We need money, and we need people to work,” he said. “This is a hobby for me, you know? I am a professor at university: I have to be teaching my courses; I have to be doing research and these kinds of things. And then also we are also taking care of this strong motion programme.”
Collaboration
To ease the burden of yearly data collection, he suggested that host countries and territories could assist in performing upgrades to the strong motion sensors so that they could wirelessly transmit the data back to Puerto Rico.
For more information on the Puerto Rico Strong Motion Program, go to its website at prsmp.uprm.edu/.
