On some geological aspects of the east anglian earth-quake of april 22nd, 1884

20

11.

HICKS ON BONE-CAVES IN WALES.

that the borders of the Bristol Channel, where the caverns mainly occur were but little affected at the time in comparison with such an area as the Vale of Clwyd, in the immediate neighbourhood of a mountainous district with great glaciers, opening also to the north and therefore subject during the period of submersion to the influence of floating ice from more northern areas.

ORDINARY MEETING. FRIDAY, DECEMBER 5TH, 1884. HENRY HICKS, ESQ., M.D., F.G.S., President, in the chair. The list of donations to the library since the last meeting was read, and the thanks of the Association were returned to the donors. The following were elected members of the Association :-J. Alston; R. B. Grantham; T. D. Palmer Jones j H. Lawrence j A. G. Leeson j W. Mawer, F.G.S.; H. L. Millar. The following paper was then read: _ I f Preliminary Notice of the East Anglian Earthquake of April 22nd, 1884." By Raphael Meldola, Esq., F.R.A.S., F.C.S. ON SOllE GEOLOGICAL ASPECTS OF THE EAST ANGLIAN EARTHQUAKE OF APRIJ, 22ND, 1884. By RAPHAEL MELDOLA, F.R.A.S., F.C.S., &c. Rarely in the history of British earthquakes has a shock been experienced of such destructiveness as that which I now propose to bring under the notice of the Members of the Geologists' Association. Among the most noteworthy of these visitations during the period extending from the 12th century down to the present time, we have but about half-a-dozen records which as regards structural danger can be compared with the present disturbance. In 1185 Lincoln Cathedral and many other buildings in the county were seriously damaged. In 1248 a violent shock in the west of England caused the partial destruction of the Cathedrals of Wells and St. David's and many other churches in Somersetshire were damaged. In 1275 again many churches are said to have been damaged in the western counties, and St. Michael's, Glastonbury, thrown down. In 1382, the sixth year of Richard II., many

OEOLoolcAL ASP~CTS OF THE EAST ANOLIAN EARTHQUAKE.

21

churches in the south-east of Eugland are reported to have been thrown down. In 1480, buildings were thrown down by a violent earthquake having its focus in the eastern counties. In 1580 a strong shock in Kent and Middlesex damaged old St. Paul's and the Temple Church and threw down many chimneys in London. In 1750 structural damage was again caused in London, and later in the year another shock threw down chimneys in Northampton. In 1786 some destruction was caused by an earthquake which visited the north and north-western counties and extended into Scotland, the Isle of Man, and Ireland. The present disturbance appears to have originated about 9.18 a.m. on April 22nd, beneath the earth in the neighbourhood of the little villages of Abberton and Peldon in Essex, between Colchester and Mersea Island. Immediately after the occurrence I commenced to collect information, being fully alive to the necessity of not allowing such a visitation to pass without bcing, in the interests of science, as far as possible exhaustively reported upon. The establishment in the county comprising the seismic centre of a wellknown and energetic local society, having members scattered throughout the various parts of Essex, promised to afford unusual facilities for the preparation of such a report, and the Essex Field Club, as the nearest organized scientific body, appeared to me to be the proper central authority for undertaking the investigation. On behalf of this society I took upon myself the labour of drawing up the report, although not without certain misgivings, as I was aware that my own pursuits in other fields had not hitherto led me to the serious consideration of seismological problems, and above all I felt that the demands upon my time would leave me with but little leisure to pursue my task uninterruptedly. Since April, however, I have been steadily carrying on the work, 'and the report is now in a state approaching completion. The necessary slowness of the work, due in part to the delay ill receiving the large amount of scattered evidence from the various parts of England, has prevented any statement of results up to the present time, and although some of these results have in consequence been to a certain extent forestalled by other observers, I have thought it advisable to reserve publication until the history of the East Anglian earthquake can be presented to the scientific and general public as a connected whole. Taking the earliest opportunity of visiting the area of destruction, accompanied by my friends Mr. T. V. Holmes, F.G.S., (late

22

n.

MELDOLA ON SOME GEOLOGICAL ARPECTS OF

of H.M. Geological Survey), and Mr. William Cole, lIon. Sec. of the Essex Field Club, I left for Colchester on May 3rd, and made [\ detailed inspection of all those places where the damage had bcen greatest. This visit was much facilitated by the attentions we received at Colchester from Mr. Henry Laver, F.L.S., and Mr. J. C. Shenstolle, F.R.:VLS. On May 10th, at the invitation of the Mayor of Maldon, I visited that town in order to address a mecting on behalf of thc Mansion Honse Relicf Fund, and through the kindness of my friend Mr. E. A. Fitch, F .1,.8., I was enabled to collect further information. On August 4th, a meeting of the Essex Field Club was held in the earthquake district, and although most of the damaged buildings had by that time been repaired, a few additional observations were made. Before our first visit to the scene of damage, I learnt that my friend Mr. G. J. Symons, F.R.S., had already been over the ground, and an interview with that well-known meteorologist fortunately secured his hearty co-operation. Mr. Symons had written to the Times tbe day after the shock, asking for information, and during his inspection had marked all the cases of damage that came under his observation on the 6-incb Ordnance map, and had also made many notes in tbe field. After publishing an abstract account of the earthquake in the May number of the 'Meteorological Magazine,' Mr. Symons was good enough to hand over to me the whole of his correspondence, notes and maps, and whilst expressing my thanks for this courtesy I will take the present opportunity of stating that tho completeness of the records is largely due to the assistance which I have from the first received from him. Want of space forbids me here from thanking in detail my own numerous correspondents, and I will only state that by means of direct application and a set of questions most kindly circulated through the local papers I have received many hundred letters from Essex and other parts of England. It is impossible also to mention here individually the names of all those from whom I have received assistance of various kinds, but I mnst state that my labours have been considerably facilitated by the co-operation of my friend, Mr. William White, who kindly undertook for me much of the correspondence, and who is now assisting we in drawing up a complete preliminary catalogue of British Earthquakes. I must not omit to add that Mr. J. C. Shenstone, of Colchester,

TilE EAST

A~GI.L\N

EARTHQUAKE.

23

has afforded me much valuable local-aid, and I have great pleasure also in acknowledging my indebtedness to the local press, the excellent, and, on the whole, rcmarkably accurate accounts having been of the greatest service in drawing up the report. The intensity of the present disturbance will be best learned by examining the area over which the shock was felt. Taking Abberton as the centre, the extreme distances are Brigg (Lincolnshire), 135 miles north-north-west; Altrincham (Cheshire), 180 miles north-west; Street (Somersetshire), 170 miles west-southwest; Freshwater (Isle of Wight), 135 miles south-west; Bonlogne, 90 miles south-south-east; and Ostend, 100 miles south-east. A parallelogram drawn through these stations would be 242 miles east and west, and 200 miles north and south, giving an area of 48,400 square miles. In round numbers the shock may he said to have entended over nearly 50,000 square miles. An approximate comparison of the intensity of the present shock with that of other well-known earthquakes is given by the ratio between the squares of the radii of the disturbed areas. This follows according to Prof. Milne." from the circumstance that the quantity of material shaken being as the square of the radius of thc disturbed area, and taking the quantity of material moved as proportional to the moving force, the intensity of tho shock varies as the square of the radius, assuming that the shock originates at the same depth beneath the two areas being compared. Thus the mean radius of the present earthquake is about 135 miles, and that of the great Lisbon earthquake of 1755 was 600 miles. The relative intensities are therefore : 600 2

:

135 2 = 360,000 : 18,225.

The East Anglian disturbance, according to this estimate, was a little over one-twentieth the iutensity of the Lisbon earthquake, but this result is possibly too high, owing to the exaggeration of the radius by the conduction of the shock along the older rocks, as will be explained subsequently. The area of structural damage comprised 50 or 60 square miles, the main axis of disturbance extending for about five miles in a northeast and south-west direction between Peldon and Wyvenhoe on the river Colne. Along the main axis a very large percentage of the buildings were damaged, chimneys being dislodged or fractured

* 'Trans. Scism. Soc. Japan,' Vol. i, Part 2, p. 53.

24

n,

)IEI.DOLA ON SOME GEOLOGICAL ASPECTS OF

and twisted, gables thrown down, roofs untiled, and in the worst cases substantial brickwork was cracked and rent in many directions. It is noteworthy that the main axis is divided by an area of lesser destruction between Wyvenhoe and Abberton, and I am inclined to attribute this to the fact that the shock actually originated beneath the Abberton-Peldon rcgion, but was totally reflected at Wyvenhoe, causing excessive damage at this last village. The general facts upon which this conclusion is based are given in the report, the chief argument being that the tract of country to the north-east of Wyvenhoe lay in "seismic shadow." The detailed description of the nature of the damage belongs more especially to the local section of the report, and need not be entered into on the present occasion. The towns and villages where damage was sustained, after making due allowance for local conditions, have been classified as follows in the estimated order of the intensity of the shock :_t<

1. The main axis of disturbance. a, Peldon, Abberton, Wyvenhoe and Rowhedge. (Maximum destruction.) {:J. Langenhoe, Fingringhoe, East Donyland. (In the main axis, but damage not so great). 2. Colchester and Greenstead, West Merses, 3. The Wigboroughs, Layer De La Haye. 4. East Mersea, Bradwell. 5. Alresford, Brightlingsea. 6, Layer Breton, Layer Marney, Tillingham, Lexden. 7. Birch, St. Lawrence, Tolleshunt D'Arcy, Tollesbury, SalcottYirley. (Damage slight.) The remaining stations in Essex where the shock was fclt have been recorded with as much detail as appeared advisable, as also those in the counties of l:'uffolk, Norfolk, Cambridgeshire, Hertfordshire, Buckinghamshire, Oxfordshire, Gloucestershire, 'Vorcestershire, Warwickshire, Nort.hamptonshire, Rutland, Leicestershire, Lincolnshire, Derbyshire, Cheshire, and, in the south, Middlesex, Kent, Surrey, Sussex, Hampshire, Berkshire, and Somersetshire, and across the Channel in France and Belgium. The present earthquake, baving been quite unrecorded by instru• The reader is referred to the County Map for the situation of these places.

THE EAST ANGLIA.N EARTIlQUAKE.

25

ments, has not been susceptible of exact dynamical treatment, but the ensemble of evidence goes to show that in general character it resembled those disturbances which occur so frequently in the Plain of Yedo in Japan, and which have been made the subject of much excellent scientific investigation by Profs. J. A. Ewing and John .Milne, and the Japanese seismologists. It is true that previous earthquakes, and more especially the great Calabrisu earthquake of 1857, have been subjected to elaborate mathematical treatment by Mallet and others, but for my own part I feel bound to coincide with Prof. Ewing,· and those who recognise that the data upon which these calculations arc based are, for the most part, faulty and open to so much error as to render the results practically of but little value. As in the case of those earthquakes which have been made in Japan to write t1e story of their movement by means of seismographs, there is good evidence to show that the present disturbance commenced with a series of small vibrations, which increased in amplitude till the "shock" was felt, and then died away as gradually as they commenced. Any attempts to find the velocity of displacement from the projection of bodies, based on the supposition that the movement was of the nature of a Rudden jerk, would thus have been open to the same objections as those already alluded to. It is of interest, and may hereafter be of some importance, to note that the present disturbance occurred during a period of general seismic activity throughout the world. On March 4th, 1881, and July 28th, 1883, there occurred the well-known and destructive earthquakes in the Island of Ischia. In May, 1883, there commenced the eruptions in the Island of Krakatoa, which, on the night of August 26th, culminated in one of the most stupendous volcanic cataclysms of modern times . Numerous shocks, some of them of destructive intensity, have been recorded in various parts of Europe and America during the last two or three years, and the present earthquake, as has so frequently been observed in the case of similar disturbances, had its premonitory shocks. Of these many were only remembered after the occurrence, and must be received with incredulity in the absence of independent confirmation; but there seems to be trustworthy evidence that the present area of disturbance was shaken by a sharp shock, accom• • Earthquake Measurement,' No. !J of the Memoirs of the Science Dspa.rtment of the University of Tokio, 1883, p.75.

26

R. MELDOLA ON SOME

GEOLOGICAL ASPEC':tS 01"

panied by a loud report, on February 18th, 1884, at about 1.10 or 1.20 a.m., and Mr. Henry Laver, of Colchester, informs me that he and his household were all awakened by another slight shock about 5 a.m, on June 24th. Since April 22nd America has been visited (August 10th) by a strong shock, which was felt in New York and all along the Atlantic coast from Philadelphia to Portland, and which is reported to have been the most serious that has occurred during the present century. A strong vibration, accompanied by the usual rumbling sound, was experienced in Jersey on August 26th. Only last month (November 14th) a sharp shock, accompanied by a loud report, was feIt at Clitheroe, in Lancashire, and on tile 27th several strong shocks were reported from Marseilles, Geneva, and generally throughout the Department of the Alpes Maritimes. Whether these different manifestations of seismic energy are, with our own earthquake, to be referred to the action of some common cosmic cause, it does not enter into my province to consider on the present occasion, but the general impression which the facts appear to have left upon my mind seemed worthy of record. '" The aspects of the earthquake, which will perhaps be of more special interest to the members of this Association, are those connecting the disturbance with the geology of our island. The central area is entirely occupied by the London Clay, covered here and there with patches of Drift sands and gravels, and intersected along the river valleys by stretches of alluvium. There seemed to be a general impression at Colchester, on our arrival at that town, that the chief damage was experienced by buildings on the London Clay, and that less destruction was sustained by those on the gravel. On priori dynamical considerations this seemed a very reasonable supposition, inasmuch as the clay, regarded as a vibrating mass, is more coherent, and would be more likely to vibrate as a whole, than the looser gravel among which the destructive energy of the vibrations would be absorbed by numerous internal reflexions

a

* While preparing this paper for the press, a violent earthquake is reported from Spain on December 25th, shortly before 21h. In the southern provinces, where the shocks were tho most severe, great destruction of buildings has taken place, and many hundred lives have been lost. A great landslip was caused in a mountain ill the province of Malaga, which destroyed many houses in the town of Periana, The shocks were renewed, and further damage and 1088 of life caused, on the five following days. On December 28th a severe shock was felt at 'I'arvis, in Carinthia (Austria), and the walls of buildings were fractured. Mr. Alfred Batson states (' Nature,' Jan. Ist, 1885) that the disturbance extended to England, as he felt a shock at Ramsbnry, Wilts, about 22h. 20m. on December 25th. The first shock of this dato in Spain occurred, however, about 21h. 3m. 40s. Greenwich time. [Note added Jan. Ist, 1885.J

TlIE EAST ANGLIAN EARTHQUAltE.

21

and refractions. This phase of the subject has, I believe, been treated of by Mr. G. H. Kinahan, F.G.S., of the Irish Geological Survey, who went over the area of destruction soon after tbe shock j but the evidence is necessarily of a very complex character, and my own investigations into the matter being as yet uncompleted, I prefer for the present reserving any expression of opinion on this point. Westward of the village of West Mersea there is a low hill of London Clay covered with Drift gravel, the southern side of which slopes somewhat steeply towards the sea shore. A road runs along the southern edge of this hill, and the shock opened a long and sinuous cast and west crack for a distance of about 200 or 300 yards, the crack running more or less parallel with the road, and terminating at the mouth of St. Peter's Well, which is a shallow surface well, fed by drainage from the clay underlying the gravel, and situated on the shore, above high water, a little to the southwest of West Mersea "Church . This crack, when first opened, was about two yards in depth, and, in parts, 4-5 inches wide j but it had almost closed up and was nearly obliterated at the time of our visit. Although described as a "fissure" in the newspapers, I came to the conclusion that it might be more correctly regarded as a small incipient landslip, whicb, if the vibrations had been more severe, might have resulted in the precipitation of a few hundred feet of clay from the southern slope of the hill on to the shore, The water of St. Peter's "W ell usually runs in a clear stream from the square wooden cistern forming the mouth, and the first effect of the shock was to project the water forcibly from the mouth, after which the water ran turbid for about two hours, and then became clear again. This projection of water has been frequently observed as an effect of earthquake shocks, and in the present case, where a shallow surface well is concerned, calls for no other explanation than that of motion imparted from below to a mass of fluid having freedom to move in an outward direction, only. The turbidity of the water, which was observed also in the well at the coastguard station near by, was probably caused by the fine particles of clay, &c., detached by the temporary grinding motion of the strata through which the water percolates, caused by the passage of the seismic wave. At a place known as Cross Farm, about a milo east-north-east of West Merses, situated on London Clay, about 54 feet above sealevel, the shock opened another small crack, having an east and

28

R. MELDOLA ON SOME

GEOLOGICAL ASPECTS OF

west direction, from which two little streamlets of fresh water burst forth, bringing with them a reddish-coloured sand j these springs trickled downwards for eight or nine hours, and then ceased to flow. Some of the sand was kindly forwarded to me by Dr. Alexander Wallace, of Colchester, with the following remarks: "Plenty of water just beneath soil. Sand like this not found about tkese parts whcn digging foundations." In the absence of precise knowledge of the underlying beds, it is of course impossible to offer any satisfactory suggestions as to the origin of this sand, as neither microscopical nor chemical analysis could be expected to throw any- light upon its history. This was substantially confirmed by Prof. T. G. Bonney, F.R.S., who was good enough to submit a specimen to microscopical examination for me. The nearest well-boring, which is at East Mersea, passed through the following beds:-· Red Clay... ... { Black silt ... ." Stone, with brackish water ... 1 Gravel { Pebbles, I. supposed old beach" Red Clay ... London Clay {Green sand, very hard Reading Beds, 60 feet {Black Clay... ... ...

Alluvium, 37 feet

To Chalk Chalk ...

Feet. 15 22

1; 1; 15 80 80 ... 115 •.. 225

340

If I may venture to express any opinion respecting the origin of the sand thus brought up, I should like to point out that there is no occasion for supposing that it came from any great depth. The farm is situated close to the junction of the London Clay with a patch of Drift gravel, and the soil contains much water just beneath the surface, as stated by Dr. Wallace. Now sand is but the finer particles from gravel, and it is just possible that the sand in question was simply washed out of the Drift, leaving the larger and coarser particles behind by a process of natural filtration. The sand thus separated from the larger pebbles would 1I0t be recog-

* I am indebted to Mr. W. Whitaker, F.G.S., of H.M. Geological Survey, for the use of some of the proof.sheets of the Survey Memoir, 48 N., containing the account of the above and other well-sections supplementary to those published in Mr. Dalton's Memoir, 48 S.W.

THE EAST A};C.LIAN EAUTHQUAKE.

29

nized by those familiar with the subsoils of the neighbourhood, and the impression would arise that it had been brought up from some unknown depth. The temporary squeeze to which the watersaturated beds were subjected by the passage of the wave of com· pression, was doubtless the cause of the water bursting forth at the surface, the crack opened by the same disturbance affording the means of escape in the direction of least resistance. I hope to be able to procure a specimen of the Drift gravel from the neighbourhood of this farm, in order to see whether, by washing, a similar sand can be separated from it. The effects of the shock upon the wells at Colchester and Bocking have already been brought under the notice of the Association on a former occasion by Mr. C. E. De Rance, but additional facts have since come to light, and the whole subject is of sufficient interest to geologists to justify its consideration in some detail. At the Colchester Waterworks there are two wells sunk into the Chalk, the depth being about 420 feet, and the Chalk about 143 feet from the surface. Previous to the earthquake the water-level had been gradually sinking, and the Committee had decided that a deepening of the wells would be necessary, when the shock came and caused a rise of seven to seven-and-a-half feet, this increased level being maintained for about six months, after which Mr. Charles Clegg, C.E., the borough engineer, reported to me that the level had fallen four feet niue inches. Whether this fall indicates a permanent relapse to the pre-seismic or some lower level cannot at present be decided, as the long drought had caused a general falling off in the water-supply throughout the country. All the other wells in the neighbourhood of Colchester were affected in a similar manner, but as no systematic measurements are made, not much is known beyond the fact that the shock caused a rise in the level of the water. At the Castle Brewery well the water is said to have returned to its former level in November. At Booking a well sunk to a depth of 244 feet (reaching the Chalk), belonging to the factory of Messrs. S. Courtauld and Co., has been systematically measured for some years past, the height of the water being taken on Monday mornings (holidays excepted). The readings indicate the number of inches above the surface to which the water rises. At the request of Mr. Sydney Courtauld, the following table was kindly supplied by Mr. D. Radford Sharpe,

30

R.

&IELDOLA ON SOME GEOLOGICAL .ASPECTS OF

of Braintree, who takes these weekly measurements on behalf of the Underground Water Committee of the British Association:Height of Water about time of Height of Water for about the Earthquake, 1884, same period in 1883. March 3rd 13 inches. Mareh 5th ll~ inches. ,,10th 16l" ,,12th 16" ,,17th 12" 19th 15" ,,24th 1I~" 26th 19" ,,31st 14~ " Ap~il 2nd 13 ,. 15" sth 12" April 7th ,,14th 12~" 16th 13~" " 21st 12" ,,23rd 141" 28th 31t" ,,30th 16" M;y 5th 42 May 7th 15" ,,12th 44" 15th 14" 19th 49" 21st 13" 26th 49" ,,28th 13t" J;~e 3rd 57" June 4th 13" ,,9th 56~" 11th 12~ ,,16th 52" ,,18th 18" These results show that the earthquake caused a rise of 19i inches, the rise increasing up to June 3rd, after which the level was falling. Some later measurements forwarded to me by Mr. Sharpe indicate that this increased level has, with certain fluctuations, been maintained since April 22nd, a reading taken on December 15th giving a height of 42 inches. Of the various conjectures offered to account for this rise of the water in wells as an effect of the earthquake, there is no doubt that the true explanation, as pointed out by Mr. C. E. De Rance at a previous meeting of the As sociation, is that of the widening of the fissures in the Ohalk by the shock, so that an increased supply percolates into the ' wat er-bearing beds, thus raising the general level of the water.-

Effects of Free Margins and Boundaries, When an earthquake wave passes from one formation into another of different elasticity 11 portion of the wave is reflected at the surface bounding the two formations, and the remainder becomes refracted in accordance with the general laws of wave motion. The shock thus tends to become exaggerated along such

* Mr. G. J. Symons has forwarded the following note from the Danbury (Connecticut) News referring to the earthquake whieh occurred there

on August 10th, 1884:-" At Rocky Glen, where all attempts to obtain water by digging wells had always failed, there has been an abundanceof water since the occurrence of the earthquake. Previous to that all water used in the houses and greenhouses WRS brought from a long distance, neces8itatin~ much labour and expense. The stream has continued unabated in quantity, and is of excellentquality."-Monthly Weather Review, October, 1884. C.S. Signal Office.

THE EAST ANGLIAN EARTHQUAKE.

31

boundaries owing to the interference of th e direct and reflected waves, and as such reflection is most likely to occur along the lines of jun ction of different g eological formations , I proposc, for brevity, to speak of the se effects as the result of "junctional vibrations." This portion of the subject has not yet been exhaustively examined with respect to the present earthquake, and I will therefore content myself with the gcneral statement that there appears to be a certain amount of evidence revealing such connexion between the perception of the shock and the situation of the station at or near a line of junction. Again, when an earthquake shakes a large tract of country the vibrations t end to be exaggerated along free margins, such as coast lines, riv er valleys, lines of outcrop , &c., because in these cases there is no resistance offered in one direction to the vibrating particl es beyond the cohesive force binding them to the material of their respective beds. Th e particles along such margins are, in fact, somewhat similarly situated to the last of a row of marbles placed in contact with one another, a blow delivered at one end of the series causing the last marbl e in the row to move away from the direction of the motion. The perception of the shock at stations thus situated may be ascrib ed to "marginal vibrations." Although this phase of earthquake motion has not been dwelt upon with much st ress by previous seismologists, th ere is good evidence that in the pr esent earthquake such results were actually experienced. Referring to the River Blackwat er, for instance, one of my correspondents g raphically describes the effects" as though th e shock followed th e water." In and about London also the shock appears to have been especially distinct along the river margin, whilst round the south and east coasts the vibrations were felt at many towns, viz., P ortsmouth, several places on the coast of the Isle of Wight, Brighton, Eastbourne, Hastings, Dover, Westgate-on-Sea, Herne Bay, Sheern ess, Shoeburyness, Bradwellon-Sea, Clacton-on-Sea, W alton-on-Naze, Harwich, Aldborough, Lowestoft, Y armouth, Lynn, Skegness, and other stations on the Lincolnshire coast. Propaqation of the Shock along the Older Rocks. The velocity of propagation of an earthquake throngh any stratum depends upon the elasticity of th e rock material, and the attention of geologists is naturally directed on the occasion of such disturbances to the older and harder, and therefore more clastic, rocks which underlie the newer furmations of this country. After

32

R. MELDOLA ON SOME GEOLOGICAL ASPECTS OF

due consideration of the various opinions which have from time to time been expressed with reference to this aspect of earthquake transmission, I am induced to offer a few suggestions which, if not adding anything new by way of observation, may possibly help to put the problem in a clearer light for future discussion. It is frequently said that on the occurrence of an earthquake in this country the geologist instinctively connects the disturbance in some way with the older rocks, because their stratigraphical history shows that OT. the whole there appears to have been a greater amount of igneous activity during Palreozoic times than dnring the more recent periods. Without pausing to refute this belief,. it seems to IDe desirable, in order to prevent any misconception, to bring into greater prominence the question as to the natnre of the connection between the older rocks and modern earthquakes. If it is supposed that the presence of the Paleeozoic rocks immediately beneath any area gives a greater probability of that area being visited by earthquake disturbances, at the present time, this supposition is certainly not borne out by the facts of earthquake distribution in Britain. It is true that in some of our seismic arrears, such as in Wales and Scotland, the older rocks come to the surface, but, on the other hand, an inspection of l\, complete catalogue of British earthquakes, which, with the assistance of my friend :Mr. William White, I have been preparing for the present report, will show that many of our earthquake districts arc on the more recent formations, and separated by beds of softer materials of enormous thickness from the harder ridge beneath. As far as the evidence goes, and as the present earthquake indicates with special emphasis, it appears that a seismic disturbance may take place anywhere beneath the surface quite irrespectively of the geology of the district, so that in the present state of knowledge the origination of an earthquake cannot be associated with the presence of the older rocks in the relationship of cause and effect. In regions remote from volcanic centres the proximate causes of earthquakes are as yet only conjectural, nor is there at present any evidence-to put the case perhaps somewhat strongly-that any

* "I have shown that ordinary volcanic rocks have been ejected in Silurian, Devonian, Carboniferous, Jurassio, Cretaceo-oolitic, Cretaceous, Eocene, Miocene, and Pliocene times, and from all that I have seen or read of those ancient volcanoes I have no reason to believe that volcanic forces played a more important part in any period of geological time than they do in this our modern epoch."-Prof. Sir A. C. Ramsay's Presidential Address to the British Association, Swansea, 1880. See also Mallet's fourth Report on Earthquake Phenomena, • Brit. Assoc. Reports,' Leeds. 1858, p. 47 et seq.

33

THE EA ST AKGL IAN E AR T HQ UAK E .

two earthquakes are due to th e same immediate cause. But although the origination of an earthquake shock maya priori be considered apart from the pre sence or absence of the harder rocks, th ere is no doubt that in a large number of cases, both here and abroad, such distur bances do actually originate eith er within or beneath the P alseozoic rocks, and in our present ignoranc e of the immediate causes of t hese disturbances we 'can only regard such occurr ences as accidents cont ingent upon the necessary circumstan ce that th e harder and more elast ic strata are lowermost, and th erefore more nearly over the focus of dist urbance. It is for future observation and mathemat ical reasoning to det ermine whether th e production of an earthquake must be placed in One of the casual connection with the older rock-formations. fundamental problems at pres ent before seismologists is, in fact, the determination of the depth of the origin of the disturbance, and alt hough much has been done in this dir ection by th e labours of the late Robert Mallet (who may be considered as th e founder of modern seismology) and other observers, th e result s arri ved at are confessedly but rough appr oximations to th e truth deduced iu most cases from very uncertai n data obtained without the assis tance of instrum ents. E ven where seismographic records have been available th e results vary with in considerable limits, or, in other words, are not precise enough for the pure geologist at present to make use of them. Thus, t o take a specific instance from th at excellent observer Prof. John Milne," the angl es of emergence of t he Japanese earthquake of F ebruary, 22nd, 1880, as given by various obser vations at Tokio and Yokohama, were 1'5°, 6'25°, 10'5°, II 0 , 18°, 31 0, or 45°, corres ponding to a depth of 0' 3, 1'6, 2'8, 3-2, 5-3, 9-7, or 16 miles. The 1st, 6th , and 7th of thes e angl es are rej ected as improbable, and th e depth thus brought within the limits of one-and-a-half to five miles. Prof. Milne adds :- " This calculation for th e depth of th e shock, it will be observed, depends on Ilo certain vertical movement which was registered. If this vertic al motion is due to transv erse vibrations, th e calculation is valueless." This result, obtained by a skilful observer, aided by instruments, well serves to illu strate the uncertainty which still surrounds th e subject . In the light of th e preceding considerat ions we have , th erefore, t o regard the older rocks beneath t he E ast Anglian earthqu ake distri ct, not as necessarily being concerned in any way with the

* VOL.

'Tran s. Seism, Soc. J apan, ' Vol. i, part 2, p. 104.

IX., No. 1.

8

34

R. MELDOLA O~ sOME GEOLOGICAL ASPECTS OF

origination of the disturbance, but simply from the mechanical point of view of their being better conductors of the elastic wave, or series of waves, than the softer formations overlying them. In passing through a material such as clay the energy of the vibrations would be to a greater extent spent in overcoming internal viscosity than when travelling through a more elastic crystalline rock; in other words, more heat would be developed in a soft material, and the mechanical vibrations would die out more rapidly than in a hard rock-stratum. The localization of the effects of the recent earthquake is thus, doubtless, the result of the shock having originated beneath a clay area. Soon after the earthquake of April 22nd Mr. W. Topley, F.G.S., of H.l\I. Geological Survey, and other geologists," directed attention to the possible connection of the disturbance with the Palseozoic rocks, and, after carefully considering all the facts which have been available, I think it may fairly be concluded that the westward extension of the vibrations was, so to speak, exaggerated by tbe spreading of the older rocks in this direction. As might be expected, however, the connection in question is not very intimate nor particularly well defined; nor is this surprising when we consider how slight were the effects towards the extreme limits of the disturbance, and how unobservant are the majority of people with respect to such unexpected movements. In presenting the facts respecting the distribution of' the earthquake movement in relation to the older rocks, it will, perhaps, be most instructive, in the first place, to consider broadly the extreme western limits of tbe shock, because it is in this direction that the older rocks come nearer to or actually crop out at the surface. Thus there are records from : Cheshire on the Trias. Derbyshire on the Trias and Carboniferous. Leieestershire on tbe Lias. Staffordshire near tbe junction of the Carboniferous and Permian. Warwickshire on the Trias and Lias. W orcestersbire on the Trias. Northamptonshire on the Lias and Oolite. Gloucestershire on the Trias and Lias. Somersetshire on the Lias.

* •Nature,' May 1st, 1884, p. 17, and map, ibid... May 15th, p. 61. See also a Jetter by .. Seisrnos " in the' Standard' of April 26th, and J. E. Taylor, F.G.S., in Hardwicke's 'Science G088ip,' June, 1884, p. 122.

'IHE EAST ANGLIAN EARTHQUAKE.

35

Although most of these localities are on Lower Secondary formations, it is known that the older rocks, in most cases, lie at no very great depth beneath the surface, and in some instances there is evidence that the most ancient of the Palroozoic series underlie the Carboniferous rocks. Thus there are Silurian outcrops in Staffordshire, W orcestershire, and Gloucestershire, whilst in Leicestershire Cambrian, or probably pre-Cambrian rocks, rise above the surface. The connection of the older rocks of the west of England with those of Belgium and the north of France has appeared with increasing probability ever since Godwin-Austen, in 1856," promulgated the view of the possible extension of the Coal-Measures beneath the south-eastern parts of this country. The presence of the older rocks beneath the Tertiaries of the London Basin has been proved by certain well-borings now so familiar to geologists through the abstracts given in Mr. Whitaker's 'Guide to the Geology of London.' Thus at Meux's Brewery, Tottenham Court Road, the Devonian rocks were reached at a depth of about 1,066 feet; the Tumford (Cheshunt) boring reached the same formation at a depth of 980 feet; at Ware a boring, 831'5 feet in depth, reached the Silurian rocks, and at Harwich Carboniferous rocks were reached at a depth of 1,025'5 feet.] Although the evidence is somewhat scanty, and the position of the Palroozoic rocks under those parts of the London Basin where no deep borings have been made, is thus left doubtful, it may safely be admitted that the Secondary and Tertiary formations of East Anglia rest on beds of harder material, which are in connection with (if not actual extensions of) those of the west of England and Belgium, and this is all that we have to consider for the present purpose. As it appeared probable that the seismic vibrations were transmitted more freely along the deep-seated harder rocks, and consequently travelled further westward than they would have done had these beds been of the same imperfect elasticity as clay, it occurred to me that the evidence might be dealt with statistically, and that towards the extreme limits of the disturbance we should have more numerous records from those localities where the

* 'Quart. Journ. Geol, Soo.,' Vol. xii, p. 38. t The Kentish Town boring reached the Gault at a depth of 130l feet, and then passed through l88~ feet of doubtful beds between the New Red series and the Carboniferous. The recent boring at Richmond. at a depth of about 1,200 feet, reached beds of doubtful age, which may be New Red or Old Red sandstone.

86

R. HELDOLA ON

SOME GEOLOGICAL ASPECTS OF'

older rocks come nearer to the surface than from districts where great thicknesses of softer formations overlie these rocks. The facts of the case certainly favour this supposition, although their fair estimation has been a matter of considerable difficulty owing to the numerous disturbing influences tending to complicate the results, such, for example, as the exaggeration of the shock by marginal and junctional vibrations in the manner previously described. In such cases, of course, the shock tends to make itself felt quite irrespective of the harder rocks beneath. On account of these difficulties all attempts to make a detailed comparative analysis of the records from the newer and older geological districts has only served to convince me that it is impossible to present the evidence in a manner calculated to carry conviction except by the very broadest treatment. The geological structure of that portion of England over which the earthquake shock extended favours, in a perfectly legitimate manner, such a comparison as that required for the present investigation. If a line be drawn diagonally from the eastern corner of the Wash in Lincolnshire to the Bill of Portland this line marks the Cretaceous boundary, and roughly divides the seismic area into two regions, that to the south-east comprising the newer and softer formations down to the base of the Upper Secondaries, and that to the north-west including the series from the Palseozoic rocks up to the highest beds of the Lower Secondaries (see Map). The south-eastern area, however, includes the focus of the disturbance, and in order to eliminate the effects of the direct shock, as propagated through the newer formations under which it originated, it is necessary to select some radial circle so as to compare only districts which may fairly be considered to be well out of the range of the direct movement. Plotting on a map all the places where the shock was felt, the distribution of these places round the focus leads me to conclude that the westernmost limits of the direct shock may be taken as somewhere about Reading', 86 miles in a line from the focus. Taking a point ncar Abberton as the centre, and describing a circle with a radius representing this distance, we may compare the north-western with the south-western parts of England outside this radius, because the Tertiary formations of the London Basin and the underlying Upper Secondaries stretch away into and cover a large portion of this south-western area, whilst in the corresponding north-western area no formations uiore recent than the Lower

f·:·:::-' : .. :.. : ,/ ::.",

MAP OF THE AREA AFFECTED BY THE EARTHQUAKE OF APRIL 22ND,

1884.

~

,'.

TilE EAST ANGLIAN EAIlTIiQUAKF:.

37

Secondaries come to the surfac e.- Reduced to its most general terms the case now stands thus:-W e have for comparison two areas , in one of which the older rocks come to the surface or lie at no very great depth below it, whilst in the other (south-western) area the older rocks arc in great part covered by softer materials, and come near the surface only at the western margin. It will be convenient, in the first place, to take as a boundary between the two areas a [inc drawn due west from the focus and passing a little to the south of Cheltenham. North of this boundary, and outside the 86-mile radius, the shock was felt at the following places : -

1. Altrincbam, Cheshire. New Red Series. Carboniferous a few miles to N. and N .E. 2. Brassington, Derbyshire. Near junction of Lower and Upper Carboniferous. 3. Derby. Trias, near outcrop of Upper Palseoaoic. 4. Leicester. Near junction of Triassic and Jurassic. Cambrian and Carboniferous a few miles to N.W. 5. Market Harborongh, Leicestershire, Lias. 6. Husband's Bosworth, Leicestershire. : Lias. 7. Ashby Parva, Leicestcrshire, Lias. Old rocks II few miles to N.W. 8. W olverhampton. Near junction of Carboniferous and Permian. 9. Birmingham. Trias. Csrbouiferous and Permian a few miles to W. 10. Leamington. Trias. Mr. Topley informs me that the Palreozoic rocks are probably rather deep here. 11. Rugby. Lias. Mr. Topley states :-" Deep boring in Trias. Paleeozoic rocks probably deep." 12. Weedon, Northamptonshire. Lias. Palseozoio rocks proved by boring at Northampton. 13. Worcester. Trias. Upper Silurian (Malvern ridge) a few miles to W. 14. Cheltenham, Gloucestershire, Li.as. 15. Banbury, Oxfordshire. Lias, near outcrop of a patch of Lower Oolite. 16. Bicester. Oxfordshire. Lower Oolite. South of the boundary line there are records from the following places : 1. Stroud, Gloucestershirc. At junction of Lias and Lower Oolite.

* Superficial deposits lIa ve been ignored in this discussion, which dealR only with the solid geology.

38

R.

MELDOLA ON BOME GEOLOGICAl. ARPECTS OF

2. Oxford. Middle Oolite. 3. Sulhampstead, Berkshire. Lower Eocene, near outcrop of Cretaceous. 4. Bristol. Trias. Carboniferous rocks in neighbourhood. 5. Street, Bomersetshire. Lias. Carboniferous rocks in neighbourhood. 6. Portsmouth. Eocene. 7. East Cowes. Eocene. 8. Ryde. Eocene. 9. Freshwater. Eocene, near outcrop of Cretaceous. The evidence even as it stands thus shows ] 6 records from the north-western area against 9 from the south-western area, and the facts may now be submitted to a closer analysis. Portsmouth and the Isle of Wight stations, being all on the coast, may fairly be rejected, as the shock was probably the result of marginal vibrations. The records from Lincolnshire have been omitted, as many of the places are also on the coast, and the inclusion of this county would tend to make the comparison too unfair by giving too large an area north of the boundary line. The latter being removed and replaced by the natural boundary separating the Cretaceous from the older formations, Oxford, Stroud, Bristol, and Street may be added to the north-western area, as they are all on the formations older than the Cretaceous. We thus have 20 records from stations on rocks older than the Cretaceous against one (Sulhampstead) on the Tertiaries. The vibrations felt at this last place were probably propagated directly along the Tertiaries, as were those experienced at Kew, London, Woolwich, and other stations in the London Basin." Outside the radius, with the exception of Snlhampstead and the marginal stations, there is not a single record from any station on formations more recent than the Lower Secondaries. Paleeozoic rocks may safely be assumed to lie under Bristol and Street, and most probably under Stroud and Oxford. At Burford between these two towns, Palaiczoic rocks
TlIE EAST ANGLlAN EARTHQUAKE.

39

perfect fairness j i.e., all the extra-radial stations where thc shock was felt have been entered in the map. It may fairly be objected that the radius chosen is purely arbitrary, and that some other circle, snch, for instance, as a circle touching the extreme western limit of the London Basin Tertiaries, might have been equally well taken as a boundary. In so far as it is impossible to determine the true limits of the disturbance for each formation this objection is perfectly valid j it is, in fact, impossible to say where the vibrations died out in the Clay and were continued through the Chalk, or where they died out in the latter and were propagated along the harder rocks. Could the actual limits of the disturbance be mapped out for each of the superposed formations over which its effects extended, we should certainly not have a set of concentric circles, but a series of curves of much greater complexity. To the foregoing objection it may be replied that a boundary of SOUle kind was absolutely necessary for the comparison undertaken, inasmuch as two sorts of vibrations had to be distinguished from each other, viz., pl'imal'!J vibrations transmitted directly from the seat of the disturbance through the strata surrounding this centre, and secotularst vibrations due to the agitation of strata overlying the harder rocks by the passage of the direct shock along the latter. It is obvious that these two effects could only be separated by giving a sufficiently wide area round the focus to allow the primary vibrations to die out, and the radius taken appeared to fulfil this condition. One other consideration will completely dispose of the above obj ection, however weighty it may appear at first sight. A further examination of the facts will show that the radius may be brought in or pushed out to the extent of many miles without materially affecting the argument. Without burdening the discussion with too many details I may state that I have compared the number of records from stations on Cretaceous and post-Cretaceous areas with those on older formations with circles of various radii, and the general result, as will be seen on reference to the map accompanying this paper, is that there is invariably a preponderance of records from the older districts.·

* [Since reading this paper I have received No. 19 of the' Journal of the Nortbsmptonsbire Natural History Society' containing a paper on the earthquake, by Mr. C. A. Markham, who recordssome stations in this county which I had not included in my report. Some twoor three stations willthus be added to the north-western area, all being on Lower Secondary formations.]

40

n, MELDO L A o x

a OME GE OLOGI CAL AS PE CT:; OF

Th e view that t ho disturbanc« was spread to the north -west , west and south-w est along the older rocks thus seems t o be fairly borne out on examination, and from thi s it follows thnt the transmission of the shock to the east and south-east at Ostend and Boulogne ma.y ha ve been du e t o the sam e cause, as th e olde r rocks ar e kn own to unde rlie th ese pla ces. It will be remarked tha t th e actual records from Palreozoic districts are few compared with tho se from Lower Secondary ar eas, such as th e T rias and Lias. 'I'his fact may be ascribed to th e g eologi cal accident that th e Pelseozoic rocks come to th e surface only at th e ex treme west and north -west of th e area affected, so that the vibrati ons may have died out to ordina ry perception before reaching th ese Palreoz oic outcr ops at extreme distances from the focus. I by no mea ns wish to assert that at every stat ion in the extraradial area the shock was felt on account of th e presence of underlying hard rocks. Our knowledg e of th e deep geo logy of thi s country is as yet too incompl et e to ven tur e upon such an assertion j indeed some of tb e st atio ns mark ed are known to he on Lower Secondary beds of gr eat thickness, and the exact position of tho Palreozoic rocks is quite unknown j but al th ough SOUle of th e Lower Secondary strata consist of clays or other soft mat erial s, it may fairly be admitted that these beds are , as a whole, har der than the Upper Secondaries and Tertiaries, and thi s is all that we have to take into consideration. From the seismological point of view we may" lump" the Palreozoics and Lower Secondaries, as we arc concerned with th ese formations only from n mechanical standpoint, and it is impossible to discriminat e by a purely dynam ical meth od betw een formations which the geologist separates with such scrupulous and necessary exactness by mineral ogical and palreontological charact ers. After thus consider ing t he records from the Palreoz oie and Upper Secondary districts en bloc, it will be seen tbat a very great maj ority of the ext reme stations li e on or neal' known Paleeozoic outcrops, and as the latter formations arc on the whole harder than the Upp er Secondaries it Tllay be inferred that the shock mad e itself felt at th ese extreme stations by virtue of the superior elasticity of their rocky beds. A question of some interest which bus sometim es been put to me by geolog ist s during th e preparatio n of th e report is th e following : -Admi tting th at the vibrat ions travelled more freely along the Pal eozoic rocks, can the argum ent. be inverted, and t.he presence of the se rocks beneath allY locali ty be inferred because

THE EAST ANGLIAN EARTHQUAKE.

41

the shock happened to be felt there? In reply to this, it may be pointed out that supposing we have a record from a station which is fairly beyond the radius where the energy of the primary vibrations may be supposed to have been spent in the softer rocks, the seismological evidence is by itself valueless unless combined with geological and topographical evidence. Thus the southeastern extension of the shock into France and Belgium has been ascribed to the extension of the Palreozoic rocks in this direction, because these rocks come to the surface near Boulogne and have been proved by boring at Calais." Again, with respect to the Wealden Area, although the Palreozoic locks may, by faults, folds, anticlinal ridges, or the thinning away of superincumbent formations be brought nearer the surface in some districts and thus cause the shock to be felt, it would be very unsafe, in the absence of geological knowledge, to predict the presence of theso rocks under such stations from the seismological evidence only. Tbus the shock was felt, among other places in this area, at Tunbridge and Crowborough ; but it would be rash to assume on these grounds the existence of a Sub-Wealden Palreozoic ridge, inasmuch as the shock was felt all over the Chalk in Norfolk at a distance extending even further north of the focus than these Wealden stations are south of this point, so that if the vibrations travelled northwards through the Chalk as far as, let us say, Fakenham, thero is no reason why they should not have been transmitted southwards through the Wealden beds for an equal or nearly equal distance.j-

Summary and Conclusion. Summing up the results of the present discussion the following conclusions may, I venture to think, be safely admitted : (1.) The seismic disturbance originating beneath the newer formations of Essex was spread to the N.W., W., S.W., and probably to the E. and S.E. along the harder Bub-Cretaceous rocks. '" The situations of these towns will probably suggest that marginal effects were experienced. This may be the case, but if so it is by no means inoompatible with the above explanation, as a little oonsideration will show that the effect of a free margin would be the same, whether the distriot were shaken by primary or secondary vibrations. t Althongh a certain number of experiments have been made with the objeot of determining the velocity of earthquake vibrations in various rocks by Mallet, there is yet much to be done in this field of investigation. Important observations on the velocity of the normal and transverae vibrations in granite, marble, tuft', clay-rook, and slate in artifioially produoed earthquakes have been made by Messrs. Milne and Gray, in Japan. - ' Phil. Mag.,' Nov., 1881, p. 365.

42

GEOLOGICAL ASPECTS OF THE EAST ANGLIAN EARTHQUAKE.

(2.) At the extreme limits of the disturbance tile shock was propagated along the Palreozcic rocks, and thus shook the overlying formations. (3.) The extreme stations are thus shown to be in seismic continuity (in the sense of conductivity only) with the rocks surrounding the centrum i that this continuity is also geological appears probable from the stratigraphical evidence. (4.) The theory of the extension of the Paleeozoic rocks beneath the south-east of England receives seismological support from the fact that the shock was felt near Palreozoic outcrops in Somersetshire and France at the opposite margins of this area. It will be instructive, in concluding, to present the general results in another way. If we imagine the newer formations of East Anglia to be extended eastwards across the German Ocean to the same distance as Street or Bristol in the west, the older rocks remaining at their present depth below the Chalk at Harwich, the disturbance would probably not have had an eastIn the ward extension equal to its present westward extension. present case, for instance, no records have been received from Holland. If, again, we imagine the seat of the disturbance to be transferred to the west, say near Bristol, it is improbable that the effects would have been felt as far eastwards as those of the present shock extended towards the west. In fact the majority of the earthquakes which in this country are reported to have been spread over a very wide area, appear to have originated towards or In the year 1248, to take an at the eastern parts of England. instance, there occurred in the west of England the earthquake which wrecked many churches and damaged the Cathedrals of Wells and St. Davids. This shock, which appears to have been even more serious than the present one, did not extend to any great distance eastwards. On tho other hand the great eastern earthquake of 1185, by which Lincoln Cathedral was" rent from top to bottom" was, according to Holinshed, " a sore earthquake," and felt" through all the parts of this land." In the light of the evidence furnished by the present earthquake, these and similar facts which I hope to have an opportunity of dealing with more fully on another occasion, may perhaps be explained by the free conduction of the vibrations along the harder rocks which dip under the newer formations towards the eastern parts of England.