Residual tide gauge records for
Gibraltar and Newlyn
by Alexander B. Rabinovich, D.Sc. -
Institute of Ocean Sciences, Canada (30 Jan 2005)
Philip Woodworth, Permanent Service on Mean Sea Level (POL,
Liverpool, UK) kindly sent me the residual tide gauge records for
Gibraltar and Newlyn; suggesting that I try to examine these
records more carefully (from the preliminary analysis "nothing
obvious can be seen"). Unfortunately, both records have
15-minute sampling interval, which is not for tsunami analysis.
Nevertheless, I decided to try and the results were quite interesting.
The main problem causing difficulties for the tsunami analysis of these
records were significant low-frequency oscillations. In particular,
it was a surge in Newlyn with maximum height (~30 cm) at approximately
00:00 UTC, December 28. Besides, both records had quite evident
seiche oscillations. Therefore, to suppress these
oscillations I used a high-pass (6-hour) Kaiser-Bessel filter and
examine the filtered data.
The record in Gibraltar indicated a train of oscillations with maximum
troug-to-crest height of about 3.5 cm beginning at about 07:00 UTC,
December 27, i.e. about 30 hrs after the main Sumatra shock.
However, the visible period of these oscillations (~1.5 hrs) is by my
opinion too large for tsunami waves.
The Newlyn record (above) is quite different from this point of vie.
It has very clear tsunami-like oscillations. Certainly, we cannot exclude
the possibility that these oscillations were associated with the cyclone
passage (that induced the observed storm surge). However, the
general character of oscillations, the observed periods (40-60 minutes),
relatively abrupt beginning, arrival time (that is in good agreement
with the Bermuda/Atlantic City/Halifax timing), and long ringing
are important arguments that the recorded oscillations are associated
with the Sumatra tsunami. Newlyn is located near the southwestern end
of Cornwall Peninsula and this is the most natural site to record the
tsunami waves arriving to the northeastern Atlantic (Newlyn is open
for such waves, while Gibraltar is in a shadow). The Sumatra
tsunami waves have been clearly recorded in the Aleutian Islands,
so we should not be surprized that they reached England!
The parameters of the Newlyn are the following:
1st train: Arrival ~07:45 UTC, Dec. 27 (about 30 hrs 45 min after
the main shock);
Max height 5 cm;
Periods ~ 60 min (note that 15-min sampling causing problems
in detecting exact periods)
2nd (main) train: Arrival ~14:30 UTC, Dec 27 (37 hrs 30 min)
Max height 16 cm
Periods 45-60 min.
Alexander B. Rabinovich, D.Sc.
Institute of Ocean Sciences
9860 West Saanich Rd.
Sidney, BC, V8L 4B2 Canada