Dr. Margaret D. Wiggins-Grandison, Contributor
WIGGINS-GRANDISON
THE MISSION OF the Earthquake Unit (EQU) is to try to understand earthquake processes and forecast seismic hazard for the benefit of Jamaica and its citizens. The small staff of six (and one is currently on extended study leave) operates instruments to detect and record earthquakes, process on average 270 events per year which amount to over 3,000 data records, conduct research on various aspects of Jamaican seismology, and engage in data exchange and outreach activities such as visits by schools, scientific exhibitions and talks. The Unit is unmanned outside of normal working hours which are from 8:30 am to 4:30 pm weekdays, although an analyst is always available during weekends and public holidays.
The Earthquake Unit operates three types of earthquake monitoring equipment. Twelve high gain seismographs and eight accelerographs constitute the Jamaica Seismograph Network (JSN) and the Jamaica Strong Motion Network (JSMN), respectively. The seismograph stations are sensitive to very small earthquakes, ambient cultural and earth noises, such as traffic, high winds that rock trees and cause the ground to move (imperceptibly to humans) and sea activity (called microseisms), whereas the accelerographs detect and record strong ground motion that is perceptible or felt by humans, and only in the proximity of the instrument. The instruments operate unmanned around the clock automatically detecting and recording events based on various pre-set thresholds. There are no automatic processing or alarm systems in place though we are seeking to have these implemented in the near future. Despite the latter, staff invariably respond promptly to felt earthquakes irrespective of the time of day.
The Earthquake Unit also operates three Global Positioning Systems (GPS) which do not measure earthquakes directly but measure the slow drift of tectonic plates and rates of strain accumulation along faults. This helps to determine which faults are active and how big an earthquake it can generate. In collaboration with the University of Wisconsin, 24 measuring points have been established on Jamaican bedrock, which is the solid part of the crust since 1999, including one on the Pedro Banks last year. Another four sites are to be established this year, including on the Morant Cays with the help of the JDF Coast Guard. The rates of movement are of the order of millimetres per year hence a long time is required to adequately measure and characterise the motions. One GPS is moved from site to site and two stations are permanently monitored.
WHEN A FELT EARTHQUAKE OCCURS
Recall the magnitude 5.1 earthquake of June 12, 2005 that took place on Sunday night 10:58 pm near Aenon Town in central Jamaica. Within 15 minutes, three staff members were in the Earthquake Unit processing the event and responding to phone calls. By 11:30 pm the preliminary location (still unchanged) was available. It was reviewed and shortly before midnight the information was being faxed to over 20 recipients comprising the ODPEM, the relevant Minister of Government, local electronic and print media and the Associated Press. When the phones stopped ringing at about 1:20 am the staff went home again. At that time there were no reports of damage but we felt sure there had been some consequences and therefore made plans for a team of persons to be dispatched to the epicentral area first thing in the morning. By 6:00 am I was back in office just in time to do the first radio interview, even as some stations were reporting having no news on the event. It appeared that the radio stations were largely unmanned that night as none had called even after the faxes were transmitted to their offices and that was highly unusual. However, by 8:30 am four persons (including two co-opted summer workers) set off for Aenon Town and called back before noon with reports of damage. Nine out of 12 seismograph stations recorded that earthquake - the other three were down at the time. Three accelerographs recorded the main event only. Two others that ought to have captured the event were found to be inoperable.
Dr. Margaret D. Wiggins-Grandison is a seismologist and head of the Earthquake Unit, University of the West Indies.
