THE UNIVERSITY OF THE WEST INDIES
ST. AUGUSTINE, TRINIDAD AND TOBAGO, WEST INDIES
THE SEISMIC RESEARCH CENTRE
Telephone: (868) 662-4659 Fax: (868) 663-9293 E-mail: [email protected]
Seismic Activity Advisory 1
2011 September 09,
The Antigua-St. Kitts area is one of the more seismically active zones in the Eastern
Caribbean and has hosted some of the largest magnitude earthquakes to have occurred in the
Eastern Caribbean since the 1600’s. The average number of background earthquakes, i.e. those
that recur on a daily/weekly basis, manifest minor fluctuations. The output level sometimes
increases in association with the occurrence of a significant magnitude earthquake. This can take
the form of foreshocks and aftershocks or only aftershocks. One such period in 20002001exhibited elevated earthquake activity associated with a magnitude 5.6 event located east of
Barbuda on 30th October, 2000.
In recent months, we have seen an increase in earthquake activity in the area. Fig. 1 is an
epicentral plot of earthquake in the area 16.5°N – 18.5°N and 60.0°W – 64.0°W from the start of
2011 up to 4th September.
Fig. 1:
There are 165 earthquakes displayed in the plot.
cumulative count of those events.
Fig. 2 shows the daily occurrence and
Fig. 2
Fig. 3 shows the magnitude distribution of those events. It should be noted that, due to technical
constraints, some of events do not have associated magnitudes, but estimates based on
experience assess some of them in the range 3.1-3.5.
Fig. 3
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Seismic Activity Advisory 16.5°N - 18.5°N & 60°W - 64°W
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The largest earthquake experienced, so far, in the area during 2011 was a magnitude 5.1
located west of Saba. This occurred in an area where the output level is relatively sparse in
contrast to the Antigua-Barbuda area, which is an area that has high output level. This is the
region where earthquakes are concentrated. Activity in the area during the instrumental era,
which extends from 1955 to the present time, based on the catalogue of the SRC is shown in Fig.
4
Fig. 4: Seismic activity in the region 1955 -2011/09/04. Subplots show
earthquakes in the depth ranges: (a) 0-15 km (b) 15-35 km (c) 35-100 km
(d) 100–300 km
(a)
(c)
(b)
(d)
At this time, it is not possible to project on the likely progression of the activity that is
being observed. It should be noted that the 8th October, 1974 magnitude 7.4 earthquake was
located in this area, north-east of Antigua/south-west of Barbuda, and the region’s great
earthquake, at magnitude estimated to be in the range 8.0-8.5, was also located in the vicinity,
south-east of Antigua. It may also be worthy of note that on the 6th of September there was an
earthquake of magnitude 5.6 located at 21.66°N and 60.20°W, which is an unusual area for
earthquakes to occur and significantly extends the area of increased activity. A major earthquake
is always possible and, therefore, hazard awareness should be sustained; given that present
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Seismic Activity Advisory 16.5°N - 18.5°N & 60°W - 64°W
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activity is somewhat elevated, some risk-related preparations for a 1974 type earthquake or even
greater magnitude might be prudent at this time.
These earthquakes are occurring now when instrumental coverage is downgraded due to
delinquency in servicing contributions to the Centre. We, therefore, cannot take the best
advantage of the strong motion information inherent in the data and this is most unfortunate.
Such data would feed into informing earthquake resistant building design and development of
better preparedness plans. Table 1 shows the network as it stood 10 years ago and the ideal
configuration, given current technology.
Table 1: Current and minimum instrument density in the region
Network
Existing Seismic
Stations
Minimum
Recommended
2
1/1
7
4/3
5
3/2
7
4/3
11
10
Antigua/Barbuda
St. Kitts/Nevis
Montserrat
Existing Strong
Motion Stations
Minimum
Recommended
1
5
1
5
0
5
Within the constraints of the existing networks, we will continue to monitor the activity
and provide periodic updates.
Sincerely yours,
Joan L. Latchman (Dr.)
Seismologist
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