T H E NORTH SEA EARTHQUAKE O F 1931 JUNE 7.
H . C . Versey, D S c .
(Communicated by R. Stoneley)
(Received 1938 October 17)
At 0.25 a.m. G.M.T. on 1931 June 7 a large area of the British Isles
was shaken by an earthquake. The disturbed area extended from Norway
in the north to Le Mans in the south, and from Waterford in the west to
Brunswick in the east. This region of nearly 400,000 sq. miles is larger
than that shaken by any British earthquake previously recorded.
Instrumental records were obtained at all British stations. At Kew,
from a comparison between the N.-S. and E.-W. components of the primary
waves and from the time of the receipt of primary and secondary waves,
the epicentre was calculated at 53’ 8’ N., I’ 2‘ E. By comparing the
times of receipt of primary waves at various stations, Father Rowland
calculated the epicentre at 53’ 57’ N., I’ 25’ E., a position which agrees
closely with Stoneley’s determination * from the velocity of the primary
waves. This epicentre is situated near the Dogger Bank, sixty miles from
Yorkshire and Norfolk.
This paper attempts to summarise the macroseismic effects as recorded
by some four hundred observers scattered over the disturbed area. These
have been plotted on the accompanying map (fig. I ) using the MercalliCancani-Sieberg scale. This scale has been adopted to bring the records
into line with a number of Continental records.
Nature and Effect of Shock.-Epicentral area : The navigator of a motorboat some 25 miles south-east of Flamborough reported a sound like a
distant gun, followed by a confused series of noises like the sound of underwater explosions at some distance. After about fifteen to twenty minutes
a heavy swell developed and appeared to roll from all directions in a sea
that was previously absolutely calm. Throughout there was no wind.
A similar report was made by a vessel 70-80 miles off Scarborough. The
heavy swell must have spent itself very quickly for there was no indication of
any tsunami on the neighbouring coast. Mr. R. M. Robson of Filey has
recorded that there was no sign of any exceptional “tidal wave” on the coast
near Filey.
Outside the epicentral area : It is impossible and unnecessary to give
all the details from which the isoseismal map has been constructed. Brief
reference will be made to the general distribution of the effect and emphasis
placed on those regions where the intensity is apparently abnormal. Damage
to buildings was confined to the area between Scarborough, Hull and
*
R. Stoneley, Brit. Assoc. Report, p. 256, 1931.
1939 Jan.
The North Sea Earthquake of 1931 June 7
4'7
Grimsby. Stones in the steeple of a Filey church were displaced in a
manner suggesting a torsional couple. Boulders were displaced from the
Castle Hill, Scarborough, and fell into a gully below. I n a parallel zone
about 45 miles wide, extending from the mouth of the Tees to Yarmouth, the
effects correspond to intensity 6. Cracks in Lincoln Cathedral were widened
inch, and at York, Nonvich and Redca; the cracking of ceilings, the
4'8
Dr. H . G. Vrsey,
4, 6
disturbance of movable objects and the stopping of clocks are typical of
this zone.
Beyond this line the intensity diminishes gradually, but apparently falls
away more quickly towards the south, a condition which may be correlated,
by analogy with other shocks, with the proximity to the surface of more rigid
rocks, in this case the London Palaeozoic Platform. There is, however, to
the south of this a roughly elliptical area with an E.-W. elongation where
the intensity increases again. It is admitted that the variation in intensity is
FIG.2.-Recorded
directions of movement.
small, but the area in question is the one from which come the greatest
number of personal observations. An attempt is made on the map to
represent this variation by the dotted isoseismal in the London region.
Inside this closed curve the intensity seems to be less thap 4.
The Hereford Area-Observers in an area between Ludlow in the north
and Gloucester in the south, and between Malvern in the east and Dorstone
in the west, report effects which are referable to intensity 5 on the scale. The
region thus delimited on the map is surrounded by the extensive and wide
belt of intensity 4.
Such local increases in the intensity can sometimes be explained by a
reference to the geological structure. A large area of thick alluvial strata
might react to produce more marked superficial effects. The region in
question has a core of Precambrian rocks and should, by analogy with such
shocks as the Charleston earthquake of 1886, diminish the effects of the
The North Sea Earthquake of 1931 June 7
1939 Jan.
419
shock. The long axis of the isoseismal 5 agrees very closely with that of the
curves of previous earthquakes in the Hereford area. It may be suggested,
therefore, that the major disturbance in the North Sea stimulated a movement
of the Hereford centre and caused the higher local intensity.
The Channel Islands.-A perfectly comparable case may be deduced from
observations in the Channel Islands. I am indebted to Dr. A. E. Mourant
for many valuable data from the area. In the island of Jersey the intensity
was generally 5 , but locally disturbance of movable objects points to
intensity 6 . In Guernsey the intensity was reduced to 5 , while on the
neighbouring coasts of the Cotentin and Brittany nothing more violent than
intensity 4 is reported. The Channel Islands, as shown by Dr. Mourant,
very frequently contain the epicentres of severe shocks, and probably the
explanation put forward above for Hereford is applicable to the Jersey centre.
The Continent.-Kolderup * has described the effects in Norway. The
disturbed region and accompanying sound was confined to a narrow strip in
the extreme south-west between Fitjar and Mandal and a small detached
area near Larvik. The Larvik phenomena are explained by him as due to
an existing strain having come to its release by the main shock. At Ogna
an intensity of 5 seemed indicated, but the consensus of observation suggests
the position of isoseismal4 as shown on the map.
Records from Germany collected by Professor Tams have been forwarded
to me by the courtesy of Kew Observatory. The intensity is estimated as
between 3 and 4 on the Mercalli scale. It appears, however, that the disturbed area extended farther to the east over the low ground of the Weser
Valley than on the highlands to the south.
Twin-shocks.-Davison has argued that certain British earthquakes are
twin-earthquakes in which the shock consists of two parts, separated by a
brief interval of quiet. There is, in the present earthquake, nothing in the
form of the isoseismals or in the distribution of sounds to suggest that it is
a twin-shock, but the fact that the most disturbed region was submarine
makes it improbable that such evidence would be found. Records all over
the country show that a large number of observers noticed two independent
shocks, while three observers noticed three shocks. The existence of a
definite isokinetic area (comparable with that mapped by Davison for certain
earthquakes) cannot be proved. ' In Germany a double shock is recorded
from the Hamburg and Aachen areas. Most observers report the second
shock as the more severe.
The interval between the two shocks shows wide variation, but there is no
direct geographical relation between the recorded interval and distance from
the epicentre. In southern England from Kent to Devonshire, and also in
Gloucestershire and the Welsh Border, an interval from three to five seconds
is frequently mentioned, but many other observers in these areas felt only one
shock. In Norfolk, South Lincolnshire and Derbyshire an interval of ten
seconds is mentioned, while on the west coast from Westmorland to North
Wales an interval of one minute was noted, the same interval being given by
a Dorset observer.
*
C . F. Kolderup, Bergens Museum Arsbok, No. 9 , 1931.
G
28
Dr. H. C. Versey,
420
4, 6
It would appear that this evidence is not in accordance with a twinearthquake hypothesis, nor does it seem possible to relate it to the varying
time of receipt of the well-known pulses. The small interval, for southern
England, allowing for the difficulty of estimating short periods of time,
probably refers to successive jerks of the main shock. The longer intervals
are not incompatible with the probable times of receipt of some of the earlier
pulses.
Foreshocks.-Only two pieces of evidence can possibly be considered as
pointing to premonitory shocks. At Linlithgow tremors lasting ten seconds
are reported, occurring on June 6 at about 9 p.m. In the Larvik region of
Norway, as mentioned by Kolderup, an earthquake took place twelve days
previously, which is noteworthy in view of the detachment of this region from
the main disturbed area in Norway.
Aftershocks.-Evidence comes from many areas as widely separated as the
Midlands and N.W. Germany of disturbances following the main shock. At
Stoke-on-Trent an interval of 45 minutes is mentioned, at Sheffield the
interval was longer, and in Germany two days, the latter of intensity 5. The
water-gauge at Hunstanton, mentioned by Kendall and Sheppard * as having
been affected by the main shock, was again disturbed at 3.10 on the same
morning.
Other E#ects.-Kendall
and Sheppard have recorded the result of
enquiries sent out by them, to determine the effect of the shock on waterlevels in wells, etc. Of about thirty such enquiries only four gave a positive
result, two of which were within isoseismal 7 and two within isoseismal 6.
The most remarkable is that at Filey, where the water-level was permanently
raised by 8 feet and where the well in question is situated within a few yards
of the northern fault of the Vale of Pickering. Other people near this line
complained of dirty water being yielded by their wells, but as this took place
some considerable time (3 months) after the earthquake, it is doubtful
whether a causal connection can be traced. The bore at Boston is reported
as having moved zg inches out of alignment at 400 feet.
Distribution of Sounds. -Observers in Norfolk, Lincolnshire and the
East Riding of Yorkshire are unanimous in recording an accompanying sound,
and thus an isoacoustic line of IOO per cent. seems approximately coincident
with isoseismal 6. Our records are not sufficiently numerous to draw
isoacoustic lines for the rest of the country, but isoseismal4 may be regarded
as the approximate boundary of the sound area. The sound is variously
described as resembling the noise of heavy traffic, of thunder, of rushing
water, of wind in trees, of sieving, of groaning, or ils metallic.
The majority of observers who were able relatively to time tremors and
sounds report that the sound was experienced first, and this is especially so
within the area enclosed by isoseismal 5. In the outer zones the noise was
noticed continuing after tremors, while in many cases where the shock was
double the noise was confined to the second shock.
Causes of the Earthquake.-According to Davison no previous record
t
*
P. F. Kendall and T. Sheppard, Naturalist, p. 3 0 1 , 1931.
t C.Davison, History of
British Earthquakes, 1924.
1939 Jan.
The North Sea Earthquake of 1931 June 7
421
of an earthquake with epicentre near this one is known, although several
centres in the northern North Sea are recorded by Kolderup.* The effect
in Scotland and northern England of one of these has been fully described
by Tyrrel1.t
The epicentre is sufficiently near to the English coast to warrant some
reference to known tectonic lines in Yorkshire and Lincolnshire, especially
to those where a repetition of movement can be demonstrated (fig. 3). The
0
10
20
30
40
Scale o f Miles
FIG.3.-Map showing position of epicentre in relation to tectonic lines on mainland.
most striking of these lines is that of the Market Weighton disturbance,f
but there is no evidence of movement on this line since Middle Tertiary
times. On the supposed continuation of this line in Europe,§ the records of
Montessus de Ballore 1) or the more recent ones of Dijks 7 show little seismic
activity, and most of the area is regarded by Ballore as aseismic. Furthermore, the epicentre, as seen in fig. 3, is situated to the north of the supposed
Saxonian line.
* C . F. Kolderup, Bergens Museums Arsbok, No. 6, 1930.
-t G. W. Tyrrell, Trans. Geol. SOC.Glasgow, 19,I , 1932.
1 H. C. Versey, Proc.
Yorks. Geol. SOC.,21, 197, 1929.
§ W. A. J. M. V a n der Gracht,Jaarsverslag der Rijksopsporing van Delfstoffen over
1913.
11 F. Montessus de Ballore, L a Geographie Sismologique, 1906.
7 G . Dijks,Jaarsverslag 1929-1932, Geol. Bur. voor het Nederlandsche Mijngebied
te Heerlen, 1930-33.
Dr. H . C. Versey,
422
4, 6
The epicentre is almost in alignment with the marked belt of faulting
which extends from the Magnesian Limestone near Ripon through the
Howardian Hills to the coast between Filey and Bridlington. Repetition of
faulting on this line has taken place,* with its latest movement probably later
than the formation of the principal peneplane of Cleveland. As mentioned
before, severe effects were felt at Filey, but these were no more marked than
those of Hull and elsewhere in areas not associated with this faulting.
The continuation of Cleveland structures into the North Sea area is more
problematical. As at present understood, this region consists of a series of
elongated domes arranged en &echelon,as shown by Lees and Cox,t and for
a more extended area in unpublished work by the present writer. Crossing
obliquely between these domes is the line of the Cleveland dyke, which, by
reason of its great length and by analogy with dykes of the same swarm in
Scotland, may be regarded as a tensional line operating during the Tertiary
uplift of Cleveland. This line if continued passes almost exactly through
the epicentre of the earthquake. The present eastern coastal boundary is
marked by a series of faults which have an echelon arrangement from Whitby
to Red Cliff. The Peak fault is well known as one of repeated movement,
while the last disturbance in this eastern fault zone is represented by the
Tertiary movement of the Red Cliff-Hunmanby fault.
Although any of these tectonic line% either singly or collectively, may
have been instrumental in determining the actual epicentre, it is unlikely that
the cause of the shock is to be found in any of the types of movement connected with them. As is seen from the map (fig. I), the isoseismal lines
are broad ellipses with their long axis in a S.W.-N.E. direction, similar to
those of the North Sea shock of 1927 given by Tyrrel1.f That author has
hinted at the possibility of a concealed fault of Caledonoid trend being
involved in the 1927 earthquake, but in the southern part of the North Sea
off Yorkshire all the flanking structures are in directions other than Caledonoid. Some Caledonoid structures are known in the older rocks, and the
Doncaster earthquake of 1905 and a previous one in 1902 were associated
with movement along such a line. The possibility of the existence of a
concealed fault with S.W.-N.E. trend cannot therefore be dismissed.
The irregular submarine topography of the North Sea, the details of
which have recently been discussed by R. G. Lewis,§ may provide evidence
which is relevant. The epicentre was very near to the notable depression
known as the Outer Silver Pit, which is regarded by Lewis as part of an
old course of the Rhine and produced by overtlow, through the ridge of the
Dogger Bank, after glacial damming. It is difficult on this hypothesis to
understand why the Outer Silver Pit should be so markedly superdeepened
unless much of the old channel has been silted up. Furthermore, the
channel is parallel and not transverse to his alleged rises. As it appears on
the contoured map its size, shape and alignment precludes its being merely
*
t
3
5
H. C . Versey, Proc. Yorks. Geol. SOC.,
21, 197,1929.
G.M.Lees and P. T. Cox, Quart. Journ. Geol. SOC.,
93,170,1937.
G.W.Tyrrell, Trans. Geol. SOC.
Glasgow, 19, I, 1932.
R. G.Lewis, Geog.Journ., 86, 334, 1935.
1939 Jan.
The North Sea Earthquake of 1931 June 7
423
an irregularity in glacial or post-glacial accumulation and strongly suggest
a tectonic origin. The scale of the map is, however, misleading, for the
actual submarine slopes are quite gentle. The Dogger Bank itself has a
distinct south-west-north-east trend, but all the available evidence seems to
show that it is made up of Pleistocene deposits and that its northern edge
represents a temporary coast line.
It is not improbable that an earthquake as strong as the one under discussion would disturb the loose deposits on some of the steeper slopes with
a consequent disturbance of the water as recorded in the epicentral region.
Earthquakes which have occurred in the northern North Sea have all
been attributed by Kolderup * to the isostatic recovery of the region following
removal of ice-load. Similarly in north-eastern North America, a region
which is generally aseismic, earthquakes have been attributed by Hobbs t
to the same cause. It is concluded, therefore, that the earthquake of 1931
June 7 must be explained as due to isostatic recovery, the release of strain
being localised on the submarine continuation of one of the tectonic lines of
North-East Yorkshire.
Acknowledgments.-The author wishes to express his thanks to all those
who, by contributing details of the earthquake, have helped in this work.
Special thanks are due to Professor A. Gilligan, Dr. A. E. Mourant, Dr. C. F.
Kolderup, Messrs. S. M. Bower and J. J. Shaw, and the authorities of Kew
Observatory for further details collected by them; to Dr. J. N. Carruthers
and Rear-Admiral Edge11 for particulars of North Sea bathymetry, and to
Dr. R. Stoneley for continued help and criticism.
*
C . F. Kolderup, Bergens Museums Arsbok, No. 6, 1930.
+ W. H. Hobbs, Smithsoniun Report for 1926, p. 257, 1927.