4
THE HARWICH EARTHQUAKE
On September 1994 at 7.36 am Essex experienced its largest earthquake for more than
100 years. The magnitude 3.2 earthquake had an epicentre in the North Sea, 40 Km
south-east of Harwich, and was reported felt by coast guards at Walton-on-the-Naze. The
focus of the earthquake was at a depth of 7.3 Km and was in an area where no previous
seismicity has been detected in the last 20 years.
On average about 360 earthquakes occur in Britain every year, of which about 30 are
strong enough to be felt. Very few are damaging although eleven people are known to
have died as a result of British earthquakes, mostly from falling masonry.
The most damaging U.K. earthquake occurred at Colchester in 1884 and the Essex Field
Club played an important part in researching this event. It most seriously affected the
villages to the south of Colchester and it is thought that the shallow depth of the focus (2
Km) was a major contiibutory factor to the damage. The club published the results of this
research in an illustrated Special Memoir in 1885, which makes fascinating reading.
The most devastating earthquakes in the world occur at the boundaries of the earth's
tectonic plates; Britain is located near the centre of one of these plates and so is
fortunately not affected by large scale plate motion. Nevertheless, it is under great
pressure as it is squeezed from the north-west by the mid-Atlantic spreading ridge and
fron the south-east by the effects of the Alpine mountain building phase. Deep
underground the rocks adjust to this stress by the occasional sudden and violent release of
energy which we experience as an earthquake.
The British Geological Survey has anational network of seismic monitoring stations
which detect and record not only British earthquakes, but also large earthquakes
elsewhere in the world. The devastating earthquake at Kobe, Japan on 16 January 1995
was detected by monitoring stations throughout Britain. A typical station is solar-
powered and entirely automatic, relaying information on an event direct to the BGS in
Edinburgh. Two of these stations are situated in Essex, at Brentwood and Colchester.
I am grateful to Frances Wright of the BGS Global Seismology Group in Edinburgh for
the above information, and to James Berry of the Geologists' Association (Essex Group)
for bringing the Harwich earthquake to my attention.
Gerald Lucy
EFC NATIVE BLACK POPLAR SURVEY
Although male treesare said to dominate the Native Black Poplar population nationally,
and certainly seem to do so in N.E. Essex, our survey has so far this year been turning up
more female trees than males.
On the EFC Black Poplar Hunt on the 9th April we checked out the three huge
magnificent trees on the east bank of the Chelmer, just south of the footbridge and the
ruins of Lt. Waltham watermill, at TL 710122. These trees (notified to us by Mrs Ruth
Phillips), were all female. The northernmost tree was smothered in ivy, which perhaps
ought to be removed, and the southernmost had obviously regrown from the rootstock
after the original trunk had fallen.