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{{#Wiki_filter:- _ _ _ _ E a. ~ l' 7' ^ i ' J, ' 1). .e h m1, t!. ;y p -pS bG a i FRANK NEUMANN 'i g pE ~ Brisuotoaier, azotoor ospaarunwr "f.t.~~ .i UNevERStry QF WAeMiN5tDN B EATTLC 6, WAS H'4 N GTO N 1 May 22, 1963. Dr. Robert H. Bryan, Division of Licensin$ and Regulation, Atomic Energy Commission, Washington 26, D. C.
Dear Dr. Bryan:
There are enclosed, in duplicate, answere to the three questions raised at the Urbana conference. I had previously prepared a statement on the maximtun accelerations to be expected and am includ-ing this in the discussion. The material is therefore not too system-atically arranged. A further study has been made of Housner's spectra and it appears that even when his " average" spectrum values are multiplied by 2.7 they fall quite short of the spectra derived direct from the El Centro seismograph record. Reference is made especially to the well known Caltech report " Spectrum Analysis of Strong-Motion Earth-quakes", by Alford, Housner and Martel published in August,1951. Within the next few days I hope to answer the questions raised in Mr. Price's letter of May 3,1963. As you suggested discuss-ing any subjects we might wish to this would seem a good way to review the entire problem again and perhaps reveal something of significance. Sincerely yours (G-WN t.-<.- c<w - Frank Neumann. 1 Encis Report in duplicate on questions raised at the Urbana conference. P. S. The requested bibliographical material will be furnished, shortly. 'p l go gs q . i:. y lll %f h MA - % y>I7 l963 x g--- .C. sjj~Mseg z-TOlik' s i s m, 3738 8709170445 051217 ES -660 PDR [} Yb
~ ~ - 3 t,, (. 4.3 @ d $c nllU2h :... g 2 o s d -m 7.Sl #w &m: - [, t,' r Q_uestion: Estbate the magnitude at Bodega Head of the "maxhum credible"7j earthquake. 1 The best that can be deduced from less than 200 years of earthquake-history in California is that violent disturbances shook the San Francisc o Bay area in 1838 and 1906. The 1906 shock vas definitely magnitude 8.2 and the 1838 may be presumed to have been of comparable magnitude. -l l This is a 68-year interval. Sixty-nine years prior to the 1838 earthquake explorers reported violent shocks in southern California. 1 (TheBay areawasvirtuallyuninhabited). If the terrain on either side of the San Andreas Fault is shifting at a rate of 0.2 feet per year , according to triangulation re-surveys, and if the fault slips 15 feet (as in 1906) 1 l and assumes at least temporarily a state of no strain, then another 75 i years vould be required to build up another 15-foot deformation along th } e fault system. This, along with the fact that in the Seattle, Washington area a statistical analysis revealed an approximately 70 year cycle fo r its strongest shocks, leads to the feeling that on the vest coast 70 year might be considered a periodic cycle for strong shocks. The nearness of Bodega Head to both the 1838 and 1906 epicenters (roughly 20 and I ) K miles away) point to the head as an area of extremely high seismicity. It is quite possible that stronger shocks in an early geologic era may have cauced secondary faulting in the rocks of Bodega Head. A 2000-year history of earthquakes in China seems to indicate that a given area may be active for a period of several hundred years and then become t relatively quiescent. There is no vay of knowing whether the seismicity ' 3738 .I ',.. y.p n ~ 2,, .,_. ~ -- ~~*~F~'~~ ~ - - - -
. o. < o ( (. 2 i, of Bodego Head is increasing or decreas!..ng; it seems most " credible" to assume that its seismic history over the next 100 or 200 years will not be materially different from that indicated by the historical record. This refers to both the frequency and maximum magnitude of future shocks. 1 If this cannot be justified then either the power plant should not be constructed on Bodega Head or it should be designed to withstand accelem-tions of 1.0g or over, that is, intensity XII. In any event it would seem vise to design a power plant possessing mm1=m resistance to earth-l quake forces, that is, design it for. accelerations beyond which it vould no longer be econcenically feasible to build such a plant. (See attached chart showing the Gutenberg relationrhfu between magnitude and maximum intensity.) e 0 e 6 l lt s
L 1 l ~ o. ( ( w i 1 i, ~tr, Question: Estimate the credible ground acceleration (peak and avera;e), i both vertical and horizontal that are likely to occur. -{ This has been done in large measure in the attached previously j prepared statement titled "Determimttion of the probably maximum intensity and acceleration at the epicenter of the California earthquake of 1906". It suggests 0 5g as an absolute min 4== in the epicentral area on basement j rock. This vould correcpend to a low value for intensity 9 5 Any attempt -to provide a factor of safety would require increasing this at least several tenths of gravi y. The unknown factor here is that intensity G.3 or 8 5 f is the greatest intensity that has been registered on this country's strong motion seismographs and extrapolations are necessarily uncertain. i j l (About.35g was registered at El Centro in 19 0 and it seems quite possible 1 i j from descriptive evidence alonk that this could have beeil nearly doubled inthe1906 shock.) All accelerations generally denote peak or m"4=n accelerations i unless otherwise specified. However, on the attached chart titled i j " Illustrating vide ranges of earthquake motions registered on strong-l motion seismographs" four types of earthquakes are illustrated having a 1 rather vide variety of periods and amplitudes. In virtually all of the writer's papers reference is made to type B shocks described on the chart just mentioned. (Note' that the deviations on the chart all refer to \\ intensity VI earthquakes). Type B might be considered " average" type I shocks for engineering purposes. It should be noted, however, that type i A shocks, a shock-vave type, can register considerably higher accelerations. This is one reason why the acceleration associated with the El Centro recording is not necessarily a m ei= = for this intensity of shock; also i 1 'O \\ b' -E ' +
~ A' c 1 4 s;. i a reason why estimated accelerations of future destructive shocks should be overestimated rather than u'derestimated. Shock type earthquakes may be considered as associated with " peak" accelerations.E l The original l question might vell be considered in connection with earthqualm spectra which represent the muitmm (equivalent static) accelerations that will 1 be impressed on structures by complex ground accelerations. On page 7 ' I of the writers attached paper titled "A Lateral-Force Formula Based on Seismological Concepts", in Figure 3 there is a " spectrum" of the El Centro ground motion and above it four oscillator response spectra computed at l l Cal-Tech. It vill be seen how the responses of low damped oscillators i deviate greatly from the average. It is also seen how the aversge of each spectrum more or less outlines the shape of the ground spectrum so that one might say that each oscillator spectrum is roughly some multiple of the ground spectrum. It is now admitted that there may 'w sg be considerable variation in oscillator spectral values depend on the I technique used to compute them. It should also be noted that all oscillator spectral values are greater than those in the ground spectrum, l_ in other words (x-y) is always greater than y for the damping values used. Since Housner plans to use the El Centro ground motion as a measure of the probably motion at the proposed power plant but uses " Average Acceleration Spectra'. and " Average Ve'tocity Spectra" to compute earth-l l quake forces instead of,, the~ El Centro spectra shown in the " Lateral Force" i paper (See Housner's average spectra Figs. 9 and 10. Appendix V of the j' F.G,.&E. Hazard's report) one is interested to know how the ts sets of ~ spectra compare. Comparable velocity scales have been placed on the right i o ' = 3N y aw n e e sr 'me',*-+i+we4,,sw isw.wwwe.,ysy, g ge-w s w e +w*+=o.,, p*y w .re.ypy+,=-,.= q eu.., --a..+--ew=+w+ a( e-. e:=-=s-mwe-e=*w i< mw w
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l (, ( l j j -s- ) (Gu. % kk @ ) ? side of each illustration,so that such comparison can be made. The ] l Housner " average response" curves (presumably the average of many spectra computed from different earthquake motions) are consistently lower than shown by the El Centro spectra. Thus, for example, an oscillator with ] l h =.02 damping, the El Centro average =v4=n velocity is about 2 7 ) l ft/sec. at about 1.0 sec. period. The Housner awrage curve shows less than 2.0 ft./sec. Note that deviations from the average (see Fig. 3) { a may raise this to 3 5 or 4 ft/sec. instead of the 2.0 or 2 3 ft/sec. shown ) l on the Housner average curves. It should also be noted that in the j critical 0 5 period area the average spectral velocities range consid-1 erably below the ground velocities for highly damped oscillators, a thing l 1 that never happens on. the El Centro oscillator response curves. A vord of explanation from Dr. Housner as to why he chooses " average" spectral response values instead of the actual El Centro values would be enlightening, especially since he sets up the El Centro ground motion as I a guide for est bating seismic forces at Bodega Head. With reference to horizontal and vertical motion the max hum motions in general refer to horizontal motions. In general vertical motions may be assumed to be about two-thirds the horizontal motion although this might vary considerably. Short period waves generally dominate in the vertical motions as compared with horizontal motions. 1 ' i ~ .r e ur-.ew e _ e-w.; _ 4, pr,w.m. ywee.,, m. y m.y y.p,wmn,,,y,g_,Ig ,p .mwr y,py-,. ,,, s. n y w,,o,,. mm. . m v 7,,. m.,pp.mw_,,
c f l Question: Estimate the amount of permanent (faulting) displacement of the head for such an earthquake. Any answer to this question must necessarily be pure conjecture;.a geologist would be better qualified to answer it than a seismologist I because he is better able to judge the geologic history of the head. I The one significant fact that stands out is that the 1906 shock, center-ing about 22 miles to the south,.did not rupture the rock. In 22 miles the intensity in basement rock can drop three grades of intensity, to perhaps one/ quarter its original vibrational amplitudes. Whether Bodega Head could withstand the hammer blev shocks of a 1906 type shock origi-nating virtually beneath it is a matter of conjecture. Intensity 9, l 10 and 11 shocks could conceivably weaken it to the point where one more chock might trigger a rupture. With San Andreas Fault absorbing most of the strain it vould seem that any rupturing of the Bodego Head basement rocks vould be relatively minor, if it did occur, involving perhaps a few inches of displacement. l The Bodego Head rocks, like all other rock masses within roughly 5 miles of the San Andraas Fault, must be continun11y experiencing stresses and strains as a result of crustal movements along the San Andreas. Even though.' eological evidence points to a condition of stabil-g ity for more than a thousand years it does not guarantee immunity. The likelihood of such a rupture occurring in the forseeable future, however, 4 is so remote that, if it did occur, it could b'e classed as an " unforeseeable" type of nstural phenomena. (One does not refuse to ride in an automobile i d_______ -,,-..,-.~..-.-.-4
- ~ n ~ (.. ( 4 4 I -7 ', ' i 1 l because 3hooo persons are killed every year on the public highways.) on i the other hand the applicant should be bound to employ every device possible ] 1 to mitigate the results of such a break if it did occur (analogous to ) using a seat belt.) This vould especially involve flexibility in all 1 types of pipe lines connecting the v' rious units of the plant. a s i 4 i l I t 4 I e a 6 6 -l 1 I l l i. ~3 i
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+I I raoM Et, CENTRO EAaTHQUAxe RsconD (MAY 18, 1940) ~ log 10ht.862 + (log 10t 12 ha other destruct,ve earthquake motions. i There may be some inaccuracy in the long-period Fio. 4. MAcNITUDE Curves rom S. H. M. portion of this spectrum because the displacement was computed by double integrating the acceleration record, and in this process it was assumed that no Steady State Forced Vibrations permanent displacement of the ground had occurred. In all forced-vibration Fc A subsequent triangulation resurvey of the area re-6-- g---- 8 i studies interest is centered f vealed that a permanent shift had occurred. The i primarily on the relative writer believes, however, that any differences in the j motion between oscillator l period-amplitude readings that might result from i and ground, as this meas-j reprocessing the original acceleration record would I ures the deformation of the j have but little engineering significance. A discussion l oscillator at any instant. In of the problem follows an article by B. K. hieade en-Y x-y .the adjoining illustration i titled " Report on Triangulation for Earthquake In- ///////// / / / / / / this is indicated by (x-y), ) vestigation in the Vicinity of El Centro, California," where x is the absolute mo-L in the IM7 issue of the Transactions of the American tion of the oscillator and y the motion of the ground. I Crophysical Union. Engineering interest is limited chiefly to the maxi-In any formula designed to include all the ex-mum deformation (x-y) experienced during a forted pectable periods and amplitudes in an earthquake of vibration and the acceleration (V- ) and velocity ar.y intensity, the writer prefers to use the El Centro (x-y) associated with it. The engineer wants to spectrum as a representative ground spectrum. Spec. know in particular just how (x-y) is related to y. l, tra for earthquakes of lower intensity (than the For steady-state forced vibrations of simple har-h1M-8.3 adopted as the El Centro intensity) can be monic character, the relatively simple solution is approximated by simply droppingdhe El Centro found in all textbooks on vibration analysis. The so-called magnification curves in Fig. 4 are typical ) Curve in Fig. I to any other intensity level. Empha-e q sis is laid on the fact that a particular earthquake of the standard mathematical solution. Such curves motion may or may not contain wave periods that. show the maximum relative motion (x-y) between j i can set up resonant and perhaps damaging building the moving oscillator and the moving ground y. The l vibrations, but if such periods are present the pro-much-desired rat o (x-y)/y can be read from the l i posed formula is designed to take them into account graph if the period of the ground motion, T,; the and thus determine the maximum lateral force under period T,; and the damping, h, of the osciliator are the worst possible conditions. known. The curves readily show the responses of a JULY,1960 7 l .,s y, .--.r.n.w,... - 3,3 m..,
p. t .r n a A t' single damped oscillator to steady-state ground vi- ~ ' Norsperiodie Forced Vibrations brations of constant amplitude but variable period, ' This section concerns the theoretical -(x-y) re-T,, or the responses of a series of oscillators to a ground motion, y, of fixed period and amplitude. In o, sponse of an oscillator of given period, T., and - Fig. 4 these are designated cases a and b respectively.- { damping, h, to a given earthquake motion such When the well-known simple harmonic motion equa-that registered on a strong-motion seismograph. One tions are used to obtain the maximum velocity (i-j) may disregard the wave-train concept just advanced and maximum a' acceleration (2-f) associated with as a feasible: approach to the oscillator-response l ~ any maximum relative displacement, (x-y), the period problem because a very direct solution was ~ proposed T. must be used since the oscillator is forced to by M. A. Biot
- in the early 1930's. His mathematical vibrate in unison with the ground regardless of its solution for obtaining maximum (x-y) from a com-own period, T..
plex earthquake ground motion is now widely The structural engineer's interest, however, is lim-known; but for some years application of his equa. ited primarily to the maximum deformations associ-tion to practical problems was delayed because of the ated with resonance, or near resonance, and low labor involved in solving the integral portion of the equation. The. writer independently suggested the values of damping. This implies that he is generally. not interested in the small deformations associated use of a torsion pendulum for computing oscillator with ground periods that are much greater or much responses of this kind.' In 1943 Biot reported the results of comprehensive torsion-pendulum studies less than the oscillator period / For this special case made at the California Institute of Technology.8' it may be assumed that, since T. and . are about equal, one may compute the acceleration and veloei-Subsequently Alford, Housner, and Martel found ties associated with the maximum resonant displace-the electrical analog computer better adapted to this ment by simply substituting T. for T,. This makes a purpose and, after adding a damping factor to Biot's calculation based on this assumption independent of equation,' proceeded to calculate a large number of-the ground period. The point is given emphasis be-so-called " earthquake spectra."" These curves show cause, when T. is substituted for T., the expressions the maximum (x-y) responses of groups of damped for maximum (x-y), (i j), and (#-y) then take on oscillators to many strong earthquake motions regis-the same forms (to be shown later) as are used in tered on Coast and Geodetic Survey strong-motion expressing similar maximum oscillator responses to seismographs. The following form of equation was complex nonperiodic earthquake vibrations. used in these final analyses: The writer believes that virtually all earthquake' rh T vibrations can be treated basically as steady state (x-y)= 2r f(r)s T sin (t-r)dt= 2r 5, T vibrations for the following reason: the basic ele-ment of an earthquake vibration is a relatively smooth wave train (Reference 4, pp.19-20). If an (f-7)"S' oscillator were to be subjected to such a wave mo-and tion, its response in terms of (x-y)/y, could be esti. mated quite readily from stan,dard magnification 2r curves such as those illustrated in Fig. 4. Among (*-7) " Y' seismologists, application of this principle is stand-i j ard practice when computing the ground motion. y, where t is the t.ime m seconds, e is the time parameter from " smooth" types of ordinary seismograph rec-in integration, and h is the damping. The oscillator ords that provide the (x-y) data. The difficulty in spectra for the El Centro record of May 18, 1940, strong-motion records is that two, three, or more together with the corresponding ground spectrum wave trains may be impressing several types of wave determined by the writer, are shown in Fig. 3. trains on the seismograph pendulum simultaneously. The problem confronting engineers is to utilize if the (x-y) response of a particular oscillator could these spectra to the best possible advanta he computed for each wave train present and if all previously stated, though maximum (x-y)ge. As was values are. of the individual responses wgre then combined, the important, they must be associated with the corre-resulting motion would approximate that obtained sponding maximum values of y if a solution similar when the (x-y) values are computed directly from to that for steady-state vibrations is to be developed. strong-motion seismograph recortis' (see Fig. 5). This solution appears to be feasible. This last procedure will be discussed further in the next section. The wave-train concept is important: Oscillator Spectra vs Ground Spectra it will later be shown to play an important part in the As previously. stated, so-called " earthquake spec-development of the proposed lateral-force formula. tra," or oscillator spectra, have been made available' 8 THE TREND IN ENGINEERING t - se em, w - . i;l --- we - cm "'T"* ' ~ " ~ " ~ ~
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+ - e [ >@[f / - g3 41.3 cop f DETERMINATION OF THE PROBABLE MAXIMUM INTENSITY AND ACCEL J AT THE EPICENTER OF THE CALIFORNIA EARTHQUAKE OF 1906 l' (By Frank Neumann) This determination presumes the validity of graphs showing the relationship between earthquake intensity and epicentral distance on basement rock. Each graph shows, for a particular earthquake, the minimum epicentral distance at which. each grade of intensity is reported. It is presumed that such minimum intensities represent intensity on basement rock or a condition equivalent to a base: cent rock response to earthquake forces. Such graphs arc derived entirely from intensity distribution maps (isoscismal map data). For further details see the writer's booklet entitled " Earthquake Intensity and Related Ground Motion," University of Washington Press, 1940. On the accompanying chart are shown the distance-intensity graphs for the Imperial Valley earthquake of May 18, 1940 and the Kern County (California) earth-quake of.fuly 21, 1952. In addition to a so-called granitic basement, the seismo-graphic data reveal (what tectonic maps also show) a massive layer of sedimentary rock which serves as a secondary basement. On this sedimentary basement minimum earthquake intensities (with respect to epicentral distance) are always about one grade of intensity higher than in the surrounding areas,where the granitic type basement dominates. In both the 1940 and 1952 earthquakes the plotted data show that one must go to an epicentral distance of about 100 miles before the lower. granitic basement intensities are noted. In the Puget Sound area the lower gran - itic basement intensities are found all over a shaken area; there is no sedimentary "1 basement. Without attempting to repeat here how instrumental (strong-motion) data can { help to determine granitic basement rock intensities at the shorter epicentral distances, it will be assumed that on the accompanying chart we have both a granitic
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y ( 3 (' a sedimentary basement graph for the 1952 shock. The data for the 1952 shock, not shown here, are quite similar to t!\\ose for the 1940 shock ex I cept that at the chorter epicentral dist.ances the reported intensities are not too cons s ent be-cause of lack of structures in the epicentral area needed to eval and weaknceses in the intensity scale itself. uate the intensity, No attempt has been made to draw { an equivalent granitic basement graph for the 1952 shock although thi i s is quite possible. { Gutenberg and Richter developed an earthquake magnitude scal e and, for Cali-fornia earthquakes, developed a relationship between earthquak e taagnitude and i corresponding " maximum" intensity as shown on the accompanying chart It is im-portant to show these intensities, or magnitude equivalents { , on the distance vs intensity graph to see what they actually signify: At a one-mile _ epicentral distance the magnitude equivalent defines the posi tion (in the 1940 shock) of the granitic basement gra h p. At a _two-mile epicentral distance it defines the positi on of the sedimentary basement graph for both the 1940 and 1952 shocks At three miles _ the magnitude equivalents indicate the i t n ensities _ observed in the irmeediate epicentral areas of both shocks In the 1940 shock it was de-finitely MM-96 in the 1952 shock,only MM-8's and 10's were ] reported from the isolated epicentral area so that MM-9.5, used here \\ , seems a satisfactory and reasonable compromise. l Because of the consistency of the relationships found in th ese two shocks, and in all others studied, it is seen how through entirely empi i r cal relationships
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magnitude can be used to es .blish a basement rock distance-intensity graph for any earthquake for which the magnitude is known. li The accompanying magnitude-intensity cbArt shows that the \\ ,1 very well determined magnitude of the 1906 shock was 8.2; the corresponding " maxim t um intensity" accord-ing to Gutenberg and Richter was MM-11.5. This intensity plotted on the distance \\\\ .g .e . e s* v*~ n - - - ~
] [ intensity graph at one-mile epicentral distarice, as previously stated, defines the position of the 1906 granitic basement rock distance-intensity graph assuming i j the slope or attenuation is the same as for other California shocks. The descrip-tive data published for the 1906 shock is not adequate for reliable intensity j evaluations at all epicentral distances so that a graph cannot be based on such data. In San Francisco, however, 20 miles from the 1906 epicenter, H. O. Wood made an excellent evaluation of intensity distribution and found that on the ser-j pentine hills of the city the intensities were a low M -7 or 6.5. If these values l. are plotted on the accompanying graph, it helps to establish the validity of the f granitic basement intensities for 1906, which virtually overlaps the distance-1 intensity graph for sedimentary basement rock obtained for the 1952 shock. Using the previously stated criterion that the intercept at three miles epicentral dis-tance yicids the best observed value of epicentral intensity, a value of M -9.5 is obtained for the granitic basement rock intensity in the epicentral area of the 1906 shock. This is a legitimate correc' tion to the magnitude equivalent of M-11.5 which erroneously assumes that great variations in intensity exist all over a shaken area. Current studies show there is very little deviation within an epicentral distance of five or ten miles. The 1906 intensity thus exceeds the El Centro intensity by about 1.3 inten-sity grades. If one assumes that the ground motions are doubled for each grade increase in intensity (as from M-1 to M-8), the.33g registered at El Centro would be increased 2.5 times to.85g. (this is a geometric progression, not arith-metic). If the common ratio is reduced to 1.5 (which seems more probable for all intensities above M-8)f,the~ acceleration would be increased 1.6 times to.538 Within three miles of the epicenter * .s would be virtually the ground accelera-tion on all types of rocks and soils according to all the distance intensity data thus far studied. Perhaps a compromise between.85g and.53g would not be too far fetched. 0 !m w,m, e se. p es er ve.se p, -3 es-m yweese,emmypewsm, a ,^7s w es-pe.e, e p ser -=sem me.m y sm pe-g i -.g, e t - ep a-e p g g-e ,y we e. e -,-e +---w+ -+
(.. ( ~ . s v i This estimate of maximum ground accelera' tion presumes a " normal" type of record,- resembling in substance, other records ebtained on strong motion seismograph records. 5 Exhibit..,however, shows that for the same intensity such as HM-6 it is possible 3 that a single pulse, or shock wave type of recording is possible, having perhaps.50 l per cent greater acceleration but of shorter duration. Ue do not know too much ~ about such shocks, but it is well to know that the above estimate of maximum accel- ~ eration does not necessarily represent.the maximum that can reasonably be' postulated. 1 1 1 1 q q j 2 .M ~ l 1 i
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.w.-.. ( COMPARISON OF OBSERVED MAXIMUM INTENSITIES IN THE IMPERIAL VALLEY EARTHQUAKE 'i OF 1940 AND THE CALIF 0ItNIA EARTHQUAKE OF 1906 It is important to compare the observed intensity at El Centro with other om violent earthquake intensity because it is the ground motion registered here on a strong motion seismograph that Dr. Housner assumes is representative of the greatest ground motion experiencal in the California earthquake of 1906. Imperial Valley earthquake: At El Centro about all the official Coast and Geodetic Survey reports say is the following: "In El Centro a number of old brick buildings were so much damaged that they were condemned. Damage was chiefly to old brick construction, to walls that were not reinforced or tied into the struc-l ture, and to balconies projecting over sidewalks. There were a few breaks in the l water mains, but practically no damage to the water plant or power lines." Inten-l sity rated VIII to IX by U.S.C.G.S.. l In Imperial 80 per cent of the buildings were damaged to some degree. In l the business district all buildings were damaged, many so seriously that they were condemned. Intensity rated X by U.S.C.G.S. In Brawley 50 per cent of buildings in the business district were condemned. Intensity rated X by USCGS. The USCGS abstracts of earthquake reports devotes 25 lines to describing earthquake effects in El Centro, and 90 lines to Imperial. It would appear from a study of the original material that the maximum intensity experienced in the Imperial Valley was at least a grade higher than that experienced at El Centro where the well known sei'emograph record was obtained. California earthquake of 1906. Pages 192 and 193 of Vol. I " Report of the State Earthquake Investigation Commission" furnishes all of what little is known about the intensity of ground motion in the epicentral area of the 1906 shock. e..+.me-:.+eg.,-ve ev+eh v gade ameip
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.... a ( It is clear that many dairy farmers and, cows were knocked over by the force of -} the ground motion; some frame structures were totally demolished, others were not (It is not recorded that bither people or animals were thrown over in the Imperial Valley earthquake). From a. reading of the available literature on both the 1940 and 1906 shocks the writer would conclude that the intensity in the epicentral area of the 1906 shock could easny have been a grade or two higher than at El Centro or a grade higher than that et Imperial. This. is independent of all considerations based on scientific and technical data. From these considerations alone the writer would at least double the ground motions measured at El Centro to obtain an estimate of the probable ground motion in the epicentral area of the 1906 earthquake. (Dr. Housner concludes that they are substantially the same for reasons in which the writst does not concur. The writer does not believe that the ground shaking in the center of a magnitude 8.2 shock is no greater than near the center of a mag-nitude 7 shock. This view is not supported by any of the writer's intensity inves tiga tions). I a' e - y,m.. ,~. u..e y..-.. .~.r _+. y-.v~... ..-,. - +. -s - ~ - - ~ ~ <,, -. - - -.. -. - - - - - -,M-- .}}