A collapsed building following the 22nd April 1997 Tobago Earthquake. Image: University of the West Indies Seismic Research Centre.
On April 22nd, 1997, the largest earthquake to strike near Tobago in recorded history occurred at 5:31 AM. It struck south of Tobago, registering a moment magnitude of (MW) 6.7 and a duration magnitude of (Mt or md) 6.1 by the University of the West Indies Seismic Research Centre (SRC).
The Earthquake Sequence
The April 1997 Tobago seismic sequence consisted of two subsequences. On 2nd April the first subsequence began with a magnitude Mt 5.6 (SRC) mainshock west of Tobago. This earthquake also produced damage across Southwestern Tobago. That sequence ran its course over a 2 day period, with most of the earthquakes in the area west of Tobago occurring during that time.
On April 4th, 1997 foreshock activity began in the area south of Tobago that would build to the magnitude (MW) 6.7 mainshock on April 22nd, 1997. This was part of a seismic sequence of over three hundred (>300) located events and 500 overall events.
The highest level of seismic activity occurred during April 1997, with over 200 earthquakes. Aftershocks continued through the end of 1997 when seismic activity returned to background levels.
The first sub-sequence could be identified by its clear difference in depth, geometry, and kinematics of the second sub-sequence which produced the magnitude (MW) 6.7 quake. Events that occurred north and west of Tobago generally occurred between 35-50 kilometers, while events that occurred south of Tobago were notably shallower, between 5-15 kilometers. These depths were recorded by both the SRC and the United States Geological Survey National Earthquake Information Center (NEIC).
Based on research, it is posited that the magnitude (Mt) 5.6 triggered the (MW) 6.7 quake, even though these events occurred on different faults and fault planes.
The difference in magnitudes
Across the globe, different seismic monitoring agencies use different methods, or several methods, for processing earthquake parameters. Each method has its limitations and will likely produce different results within the ranges of the uncertainty of that data. This is generally accepted within the scientific community.
In addition, differences between the above determinations represent to some extent the uncertainty involved in locating earthquakes in this region due to local and global differences in velocity models and seismic phases used in calculations.
Based on research and GPS observations published in the American Geophysical Union Tectonics Journal, GPS sites in Tobago had measurable movement following the mainshock.
In Southern Tobago, closer to the epicenter of the Mt 6.1 earthquake, researchers estimated a northward displacement of 123 (±5) mm, an eastward displacement of 57 (±7) mm, and a vertical displacement of -7 (±9) mm. This means that overall, the site moved towards the northeast and sank.
In Northeastern Tobago, further away from the epicenter of the quake, researchers estimated a northward displacement of 58 (±5) mm and eastward displacement of 36 (±6) mm. However, the vertical displacement was highly uncertain at 4 (±18) mm, based on their modeling.
The Ground Shaking
Based on reports from the USGS, moderate shaking was reported from the first large quake at magnitude (md) 5.6, internationally (MW) 6.1, with a Modified Mercalli Intensity (MMI) of V, which indicates light damage is possible.
Based on reports from the University of the West Indies Seismic Research Centre, felt reports from this first quake indicated the highest MMI of VII, which indicates very strong shaking and moderate damage in southwestern Tobago, with light to moderate shaking elsewhere across the island.
With the April 2nd, 1997 quake that registered a duration magnitude of (Mt or md) 5.6 struck at 2:45 AM, one home collapsed with a number of buildings damaged.
Following the magnitude (MW) 6.7 quake on April 22nd, 1997, however, six homes collapsed with more than thirty (30) houses and buildings damaged according to media reports. These buildings included the NIB mall in Tobago, Tobago’s PTSC Central Bus Terminal, the Scarborough library, and the post office.
Over 15 people were left homeless. Two persons were injured, one of which was woman who was hurt in Canaan-Bon Accord Village in Western Tobago after her home suffered unspecified damage.
This quake left millions in damage across the island, estimated at 18 million dollars.
Why was this quake more damaging than the earlier magnitude (md) 5.6? Based on research, the ground shaking from both quakes caused a significant increase in groundwater discharge, which in some cases threatened building foundations. This increased groundwater discharge was restricted to south-west Tobago between Shirvan Road in the west and Hillsborough West River in the east.
In addition, there was an increase in mortality in trees in the area following the mt 6.1 quake, which has been linked to the change in groundwater.
Earthquakes continue to occur in the region, albeit infrequently compared to other areas in the Trinidad and Tobago Region. A seismic swarm did occur beginning on December 7th, 2016 with a magnitude 6.1 (md) earthquake South of Tobago along the Southern Tobago Fault System. After over 450 recorded seismic events, earthquake activity returned to background levels by the end of March 2017.
According to the University of the West Indies Seismic Research Centre, “Research into the 1982 Tobago earthquakes revealed the potential for
earthquakes in the magnitude range 6.0-6.9 and the occurrence of the
magnitude 6.1 earthquake supported that conclusion, Further research has
strengthened the case for even larger magnitude earthquakes near Tobago.”
Hence, it is imperative to not only imperative that we practice “Drop. Cover. Hold.” during an earthquake, but how best to prepare for our “Big One.”