Content uploaded by Max Semenovich Barash
Author content
All content in this area was uploaded by Max Semenovich Barash on Mar 19, 2014
Content may be subject to copyright.
Oceanology, Vot. 42, No. 4, 2002, pp. 547-556. Trans lated fr om Okeanologiya , Vol. 42, No. 4, 2002, pp. 572-5 81.
Original Russian Text Copyright © 2002 b y Kuzin, Barash.
Engl ish Translation Copyright © 2002 by M AIK "Nauka /Interperiod ica” (R ussia).
____________________________________
MARINE
______
GEOLOGY
On the Influence of the Rayleigh Waves from the Strongest
(M > 7.5) Central American Earthquakes on the Sedimentary
Formations within the Clarion-Clipperton Province
I. P. Kuzin and M. S. Barash
Shirshov Institute o f Oceanology, Russian A cademy of Sciences, Moscow, Russia
Received M ay 28, 2001
Abstract— During the expeditionary studies of the sedimentary cover at two test sites within the Clarion-Clip
perton Province, an erosional cut of the Tertiary sediments was observed and paradoxical bedding of massive
ancient manganese nodules over the sediment surface of different ages right up to the recent ones was recog
nized. The explanations available for the observed pattern are essentially qualitative and are not supported by
sufficient argumentation. We proposed a new approach considering the mechanical impact of the Rayleigh
waves generated by the strongest (M > 7.5) earthquakes within the nearest Central American seismic active
region on the sediments. This approach is based on the data on the ground oscillations on land that were excited
by the Rayleigh waves of selected catastrophic earthquakes (Lisbon, 1775; Assam, 1950; and Alaskan, 1964).
These oscillations were observed at epicentral distances from 2000-4000 to 8000 km. The mechanism consid
ered is realistic and allows one to use quantitative characteristics of the Rayleigh wave oscillations for the expla
nation of their mechanical impact on the sediments. The study of the Rayleigh wave amplitudes was carried out
using about 200 records of earthquakes with M = 6.0-8.2 at distances of 560-9200 km obtained at the Petro-
pavlovsk-Kamchatskii and Severo-Kuril’sk seismic stations. For the analysis, we used the waves arriving from
various seismically active regions of the Pacific Ocean. The close seismotectonic and seismic analogy between
the Kuril-Kamchatka and Central American segments of the Pacific belt allows us to apply the data recorded
in the former region for studies of the same phenomena in the latter area. Then, at distances between the test
sites and the seismically active Central American region of 3000-5400 km, the amplitudes of the Rayleigh
waves reach about 0.5 mm. These values are large enough to explain the “seismological erosion” of the sedi
ments; however, they are insufficient for displacement of manganese nodules because of their large size (from
3-5 to 10 cm). For explanation of the nodule displacement, another mechanism is required.
INTRODUCTION
The results of numerous studies in the tropical zones
of the Pacific Ocean north and south o f the equaton
allowed the establishment of the presence of vast fields
of ferrom anganese nodules located on the floor surface
(Fig. 1). In selected regions, the nodule concentrations
reach values feasible for industrial mining. One such
region is located in the eastern equatorial part of the
Pacific Ocean between the Clarion and Clipperton frac
ture zones. This region was surveyed by various geo
logical and geophysical m ethods including deep-sea
drilling (see [3]). It was found that the structure of the
sedimentary formations is characterized by a series of
special features. The entire sedimentary cross section
with a thickness from a few tens to 500 m includes
deposits from the Late Eocene to recent times. The sed
iment bedding is distorted by folds and normal faults.
Over a vast area, the Tertiary deposits were subjected to
a regional erosion and redeposition. The Tertiary sedi
ments of different ages are located on the floor surface
or beneath the Quaternary deposits. Their erosion is
supposed to be related to the enhancement of convec
tive near-bottom currents from climatic changes that
took place at continental glaciations in the Northern
and Southern hemispheres; according to different esti
mates, the beginning of this kind o f erosion ranges from
15 to 1 My BP. Direct measurements showed that the
velocities of the present-day near-bottom currents are
not sufficient for erosion of deposits.
The nodules are mostly spread over the surface of
the floor. At some places in the C larion-Clipperton
Province, the nodules occupy more than 50% of the
floor surface [5]; however, they are also encountered in
the thickness of the Quaternary and Tertiary deposits.
The rate of the nodule growth is orders of magnitude
sm aller than the rates of the Quaternary sedimentation;
this proportion should lead to their burial within the
sedimentary sequence. Moreover, direct micropaleon
tological exam inations showed that often the nodules
are actually older than the sediments over which they
rest. Hence, there should be a certain m echanism of
their “floating” to the surface of the floor, or, more pre
cisely, of their maintenance at the surface.
The paradox of the old massive heavy nodules rest
ing over younger unconsolidated sedim ents was
observed by many researchers. Several hypotheses
were posed: long-term gaps in sedimentation, erosion
of the sedim ents by the near-bottom currents, accumu
548 KUZIN, BARASH
Fig. 1. Distribution of ferrom anganese nodules over the floor o f the Pacific and Indian (southeastern part) oceans (from [8]) and
seismicity of the Pacific belt over the period 1900-1985. / —Areas free of ferromanganese nodules; 2— areas with low nodule con
centrations; 3— areas with high nodule concentrations; 4— axes of mid-ocean ridges. Magnitude ranges of the earthquakes M: 5— 7.0-
7.4; 6—1.5-1 9', 7—8.0-8.4; 8—> 8,5. The sizes of the foci are shown according to the recommendations from [15]: log L, km = 0.6 M - 2.5.
For better presentation, these sizes in Fig. I are magnified twofold.
lation of the nodules as residual elements at the regional
transformation of the Tertiary sediments of different
ages, bottom stirring by seism ic shocks and the screen
separation effect (movement of coarse-grained parti
cles to the surface in a medium inhomogeneous with
respect to grain-size), microfluxes of “geogas,” biotur-
bation and pushing the nodules up to the surface by the
bottom animals, rheological properties of the sedi
ments, and hydrodynamical properties of the near-bot
tom water stratified with respect to density (see [3]).
Detailed studies of the stratigraphy of the sediments
and microfossils in ferromanganese nodules and crusts
collected in tw o test areas in cruise 41 of R/V Dmitrii
Mendeleev in 1988 have been made [1-3]. The areas
were centered at points with the coordinates 10° N,
140° W (test areaM -1 ) and 13.5° N, 133.5° W (test area
M-2). It was found out that, at the surface of the floor,
beneath a thin layer of the Quaternary sedim ents, there
are deposits of the Late Eocene to Early Miocene out
crop comprising a continuous sequence in the interval
from about 40 to 17.5 My BP. These deposits are partly
eroded since they outcrop at the surface of the floor.
Here, sediments with an age from 17 to 1 My BP are
absolutely absent. If one assum es the mean sedimenta
tion rate in this region to be about 2 mm/ky, this means
that from 80 to 34 m o f the Tertiary deposits were
removed (Fig. 2).
Detailed micropaleontological examinations of the
ferromanganese nodules showed that they are mostly of
the Oligocene age. The studies of the m icrofossils from
a block of dense ancient clay covered with a ferroman
ganese crust suggested that the crust was formed over
an eroded surface o f the Late Oligocene clay probably
at the Quaternary time, i.e., 0.9-0.7 My BP, at the
beginning of the “glacial” Pleistocene. In papers [1-3]
a hypothesis is suggested that the erosion in the test
OCEAN OLO GY Vol. 42 No. 4 2002
ON THE INFLUENCE OF THE RAYLEIGH WAVES 549
areas might be induced by the disintegration and stir-
ring-up of the surface sediments due to seismic excite
ment from strong earthquakes in the subduction zone of
Central America. Fine-grained fractions of the stirred
sediments might be removed w ith the near-bottom cur
rents, while the coarse-grained fractions, in particular
nodules, rem ained on the surface of the floor. Thus, this
hypothesis proposed a common reason and a single
interval of the formation of the features o f the sedim ent
distribution over the region (the existence of a regional
stratigraphic gap, formation of the nodule fields, and
position of the ancient nodules over the surface of the
Quaternary sediments).
This paper is devoted to a special study of the poten
tiality of a seismic impact as a reason for the deposit
erosion and formation of ferromanganese nodule fields
over the floor surface. This hypothesis implies a dou
bled m echanism of the seismic impact on the sedim ents
from strong earthquakes. On the one hand, this is the
action o f the near-bottom “storms,” which generate cur
rents even stronger than those measured to date do. The
action o f these “storms” consists of disintegration and
stirring-up of the bottom sedim ents with their subse
quent removal with the near-bottom current. On the
other hand, a stirring-up of heavier nodules under the
action of seismic oscillations was suggested with their
final floating due to vibration (the screen separation
effect known in the dressing operations for placer min
erals).
At first sight, in addition to the seismological factor,
in order to solve the problem discussed, one can assess
the m echanism of the lu nar-solar tides. It is known that,
due to the gravitation of the moon, the earth’s hydro
sphere suffers deform ations with an amplitude of a few
tens of centimeters (up to 30 -50 cm ) [14, 17]. The tides
are of diurnal and semidiurnal types. Their maximums
of the diurnal and semidiurnal tides are observed near
the earth’s poles and in the equatorial zone, respec
tively. According to the data available, the dissipation
of the tidal energy in the deep ocean is three times as
small (1.1 x 1011 J/s) as in the upper mantle
(3.6 x 10u J/s) and 20-fold smaller than in the shallow-
water seas (2.3 x 1012 J/s). Nevertheless, the am ount of
energy dissipating in the ocean over a semidiurnal tidal
cycle is 2.38 x 1015 J, which is equivalent to the seismic
energy released from an earthquake with a m agnitude
of M = 6.6.
However, the tidal deformations cannot affect the
currents and, therefore, the redistribution of the bottom
sediments in the ocean. This is defined by the static
character of the tides [17]. Tidal deform ations induce
slow vertical pulsations of the ocean floor due to the
radial deformations of the earth. Therefore, no tangen
tial component of the motion exists that could induce
tidal currents in the ocean. Due to this reason, the tide
My (a)
(c)
Fig. 2. Inferred schem atic o f the development o f erosion in
the Tertiary sediments (from [3]): (a) 1.5 My BP, (b) 0.9 -
0.7 My BP, and (c) present-day situation. The dots mark the
ferromanganese nodules, the arrows show the near-bottom
currents, and S marks the present-day erosional surface.
as a possible source for the displacement o f the oceanic
bottom sediments is removed from assessment.
ANALYSIS O F THE SEISM OLOG ICAL DATA
The above discussion leads to the conclusion that
the most probable source providing the “floating” of the
nodules to the surface of the floor (more precisely, their
maintenance at the floor surface) in the equatorial part
of the Pacific Ocean is the high seismic activity of the
margins of its central part, as well as of its other mar
gins. On the whole, the mechanism of the impact of
strong earthquakes is com patible with the ideas posed
in [1-3 ]; nevertheless, it requires a certain refinement.
One can agree with the generation of near-bottom
О ГРатм ш n r .v Vr,i л-> мл л о п т
550 KUZIN, BARASH
“storms” and disintegration of the sediments; however,
it seems that the action of the screen separation effect
should be rejected. This effect may reveal itself only
under the influence of high-frequency oscillations of
great amplitudes excited by the body waves (p-waves,
converted waves o f the SP type, and the vertically
polarized component of an S- or SV-wave). This is pos
sible only in the imm ediate vicinity of the earthquake
focus (a few tens of kilometers), since body waves
attenuate rather rapidly— inversely proportional to the
distance. At greater distances, sufficiently intensive
oscillations of the ocean floor may be excited by the
Love and Rayleigh surface waves, which attenuate
inversely proportional to the square root from the dis
tance (see, for example, [7]) and begin to dominate
already from distances greater than 300 km. This sugges
tion is confirmed by the effects observed from such cat
astrophic earthquakes as the Lisbon earthquake of 1755
(M ~ 8.5), the Assam earthquake of 1950 (M = 8.7), and
the Alaskan earthquake of 1964 (M = 8.6).
In the Lisbon earthquake, oscillations of the water
surface in channels and lakes were observed up to Scot
land and Sweden (distances from 2100 to
2600-4 000 km) [6]. Sim ilar phenomena were observed
in England and on the Scandinavian Peninsula in the
Assam earthquake at distances of approxim ately
7500-8000 km [19]. Finally, in the Alaskan earth
quake, water oscillations in wells were noted up to the
coasts of the Gulf of Mexico; on the Mississippi River
(distance about 4500 km), fastenings for a barge moor
ing moved apart [6].
Taking into account the above data, one can expect
a significant seism ic effect from strong earthquakes
over the ocean floor as well. However, in order to
resolve the problem set, one should be based on the
quantitative data rather than on the descriptive inform a
tion. To do this, one should assess the changes in the
am plitudes o f the Rayleigh surface waves with dis
tance. The vertical com ponent o f these oscillations con
tains no less than two-thirds of the total amplitude [7],
which is o f greatest importance in our case.
Unfortunately, no data has been published on the
am plitude changes of the Rayleigh waves with dis
tance. Therefore, we im plemented a special study of the
am plitudes of these waves from earthquakes with dif
ferent magnitudes at different distances virtually for all
of the seismically active areas of the Pacific Ocean. In
Fig. 1 one can see the epicenters of the earthquakes of
the Pacific active belt with focal depths up to 60 km and
magnitudes of 7 .0-8.5 during the years 1900-1985
obtained from the catalogue of the World Data Center
(Moscow).
As one can see in Fig. 1, the active zone closest to
the test areas in the region of high concentrations of the
ferromanganese nodules in the C larion-Clipperton
Province is the epicentral zone of Central America
(2800-5400 km), followed by the zones o f the Solomon
Islands (5800-6200km ), South A merica and the Aleu
tian Islands (from 6000-6300 to 7400-7500 km), New
Guinea (6800-8400 km), the K uril-K amchatka and
Japan arcs (from 8400 to 8700 km), and, finally, the
Philippine Islands (9300-9600 km).
While studying the amplitudes of the Rayleigh
waves, w e used the data of the displacement determ ina
tions from the vertical component records at the Petro-
pavlovsk-Kam chatskii and Severo-Kuril’sk seismic
stations taken from the seismological bulletins of
1980-1994 for the earthquakes of alm ost all the seism i
cally active regions o f the Pacific O cean (see Fig. 1). In
the Southern Ocean, the data for 91 earthquakes with
M = 6.0-7.8 at epicentral distances o f 5.24-26.65°
(582-2958 km) were conventionally related to the
“nearby” zone and the data for 98 earthquakes with M =
6.3-8.2 at distances of 26.69-89.55° (2963-9940 km)
were related to the “intermediate” zone.
To a first approximation, the amplitudes of the Ray
leigh waves at the registration points should correspond
to the possible ocean floor oscillations at similar epi
central distances.
Since the data on the Rayleigh wave amplitudes
were taken regardless of the focal m echanism , we
assessed the mean values calculated as the weighed
averages over specified intervals o f epicentral dis
tances. The earthquakes were subdivided into four
groups with respect to their magnitudes: M = 6.0-6.4;
6.5-6.9; 7.0-7.4, and 1.5-1.9. In addition, for M = 8.2,
there were two values for relatively small (36.89° and
47.32°) and two more for relatively great (80.23° and
82.49°) distances.
It should be noted that the data on the am plitudes of
the former two groups (M = 6.0-6.4 and 6.5-6 .9) have
no practical significance due to the smallness of their
values (0.03-0.06 mm at A < 20° for the first group and
0.1 5-0.20 mm for the second group). O nly for the two
latter groups (M = 7.0-7.4 and 7.5-7.9) do the oscilla
tion amplitudes reach a few tenths of a m illimeter (from
0.13-0.19 to 0.33-0.52 mm). Sim ilar amplitude values
are observed for the earthquakes with M = 8.2 (0.30-
0.50 mm for Д = 36.89° and 47.32° and 0 .30-0.40 mm
for A = 80.2° and 82.5°).
Figure 3 shows the amplitude distribution of the
Rayleigh waves with distances from 5.2° to 82.5° for
the earthquakes with m agnitudes 6.0-8.2. The wide
magnitude range was accepted in order to study the pro
portionality of the changes in the oscillation amplitudes
between the magnitude groups of the earthquakes. In
Fig. 3, one can observe a clearly expressed decrease in
the oscillation amplitude in the Rayleigh waves with
distance for the first three magnitude groups of earth
quakes— M = 6.0-6 .4, 6 .5-6.9, and 7.0-7.4 , mani
fested against the background of their significant dis
persion. For magnitudes M = 7.5-7 .9 and 8.2, this ten
dency to a decrease in the oscillation am plitude with
distance is more poorly manifested; in addition, due to
the sm aller amount of data for this magnitude range, the
results are less representative in the statistical respect
OCEANOLOGY Vol. 42 No. 4 2002
ON THE INFLUENCE OF THE RAYLEIGH WAVES 551
logAs , (Am
Fig. 3. Changes in the amplitudes of the vertical oscillations in the Rayleigh waves with distance for earthquakes with different
magnitudes. 7 -5— Magnitude groups; of earthquakes, M: 1—6.0-6.4; 2— 6.5-6.9; 3— 7.0-7 .4; 4— 7.5-7.9; 5— 8.2; 6— mean
amplitude values at M = 6.0-7.9; 7—same for M = 8.2.
than for the first three magnitude groups of earth
quakes. The data shown in Fig. 3 point to a nonpropor
tional change in the R ayleigh wave amplitudes with the
transition from one m agnitude group to another with a
0.5 magnitude increment. This is confirmed by the dif
ference in the coefficients o f the fitting equations pre
sented in Table 1. From Fig. 3 and Table 1 it is seen that,
with the magnitude growth, the sloping o f the approxi
mating linear relationships becom es smaller. This sug
gests the weakening of the distance dependence of the
Rayleigh wave amplitude with growth in the earth
quake m agnitude.
In Table 2, the values of the am plitudes o f the verti
cal displacements in a Rayleigh wave are listed for dif
ferent magnitude groups and distances calculated from
the fitting equations.
It is interesting to compare these amplitude changes
with the changes o f the earthquake magnitude obtained
from the correlation of the magnitudes with the earth
quake energy, on the one hand, and calculated from the
observed data, on the other. Using the known relations
betw een the earthquake m agnitude and its energy
derived by B. Gutenberg [20] (logE = 1.5 M + 4.8) and
T. Rautian [12]: (logE = 1.8 M + 4), we find that, on
average, at each m agnitude change o f 0.5 units, the
energy changes 6.7-fold. A ssuming that the oscillation
amplitude is approximately equal to the square root of
the energy, its change at the same magnitude increment
should be approximately 2.6 tim es. The amplitude
increm ent observed with the transition from m agni
tudes o f 6.0-6.4 to magnitudes of 6.5-6 .9 in the epicen-
tral distance range 10°-50° is, on average, 4.67 times or
1.8 times as great. With the transition from magnitudes
of 6.5-6.9 to magnitudes of 7.0 -7.4, the amplitude
increment is, on average, 4.0 times or 1.5 times as great
as the value obtained from the magnitude-energy rela
tionship. These data point to a more rapid amplitude
growth at a m agnitude increase of 0.5 units.
The amplitude relations between the m agnitude
ranges M = 7.5-7 .9 and 7.0-7.4 at distances 10°-75°
and between the magnitude M = 8.2 and the interval
M = 7.5-7.9 at distances 40°-80° feature opposite
trends. In the former case, the amplitude increment is
only 1.5 times, while in the latter case it is 1.8 times.
This suggests a slower amplitude growth than that pre
dicted by calculations; the differences per 0.5 magni
tude unit are 1.7 in the former case and 1.4 times in the
latter.
Thus, for magnitudes 6.0-7.4, the amplitude incre
ment with the magnitude growth by 0.5 units proceeds
1.5-1.8 times faster as com pared to the expected theo
retical value of 2.6 times derived from the correlation
betw een the magnitude and the energy of an earth
quake. For the magnitudes 7.4-8.2 , the amplitude
increase at the same magnitude step is 1.4-1.7 times as
slow. From here, it follow s that the 2.6-fold amplitude
growth at a m agnitude increase of 0.5 times expected
from the magnitude-energy relative actually does not
Table 1. Changes in the attenuation coefficient of the Ray
leigh wave amplitude with distance at different magnitudes
of earthquakes
Magnitude
groups, M Epicentral
distances, Д0 Angular coefficient.
values
6.0-6.4 9.63-47.98 -0.0190 ± 0.0070
6.5-6.9 7.34-76.22 -0 .01 52 ± 0.0007
7.0-7.4 5.24-74.68 -0 .0122 ±0.0045
7.5-7.9 6.69-83.70 -0.01 04 ±0.0036
8.2 38.54-81.36 -0.0032 ± 0.0003
T ab le 2. A mpli tudes of th e v ertica l d ispla cem en ts in a R ay leigh w ave for d iff erent m ag nitu de s o f eart hq ua ke s and epic en tra l
distances
552 KUZIN, BARASH
Magnitudes
A° 6.0-6.4 6.5-6.9 7.0-7.4 7.5--7.9 8.2
АЛД,(1 ДЛЯ, ц AAr, Ц. Ar, ЦAAr> ДAr, ЦAAr, Ц
10 53 ±10 208 ±4 650 ±103 845 + 166 - -
20 34 "146 tt 490 ft 655 tt --
30 22 t/ 103 // 370 tt 525 tt --
40 14 "73 tt 280 tt 410 tt 505 ±18
50 9tt 51 ft 210 tr 325 tt 470 ft
60 --36 tt 160 rt 255 tt 435 tt
70 --25 tt 120 tt 200 tt 405 tf
75 --21 rt 105 "180 tt 390 tt
80 ------160 ft 375 tt
exist. Therefore, the suggestion that the amplitude
value at a given distance is close to the square root of
the energy is not valid.
Along with the oscillation amplitudes of the Ray
leigh waves, we also studied their frequency composi
tion (periods). In contrast to the amplitudes, the distri
bution of the periods of the Rayleigh waves for differ
ent m agnitudes and epicentral distance ranges is
irregular and chaotic. Thus, no relationship betw een the
oscillation period and earthquake magnitude is
observed. The range of the period variations is from 12
:o 30 s, while the bulk of the data imply a period value
of about 18 ± 2 s (140 values from the total of 177 or
ibout 80%).
It is necessary to note that, fundamentally, an alter
native to the labor-consuming study of the amplitudes
ind periods of the Rayleigh waves lies in the applica-
:ion of calculations using well-known relations. For
:xam ple, paper [9] presents a form ula that may be writ-
;en in the following form:
A i = A0R-°-5e ^ RievT\ (1)
ivhere Л0 is the amplitude of the displacement in the
;arthquake focus, R is the distance in km, Г is the oscil-
ation period, v is the group velocity of the Rayleigh
A'aves, and Q is the mechanical reactance factor of the
nedium . The disadvantage of this form ula is caused by
he use of the very uncertain parameters A0 and Q.
An approximate value of Л0 may be obtained from
m additional relation [13]:
logD , cm ± 0.4 = 0.757M - 3.235. (2)
This relation is derived from the correlation of the
seismic mom ent with the magnitude for strong earth
quakes. For example, for an earthquake with a magni
tude of M = 8.2, the value of the displacement in the
focus Л0 or D should equal 3.7 -23.6 m, or 9.5 m on
average.
According to [10], the mean value o f the mechanical
reactance factor Q for a lithosphere 80 km thick is equal
to 300.
In order to determine the calculated value of the
Rayleigh wave am plitude from an earthquake with
M = 8.2, we assume that the distance value is 40° or
4444 km (the minimum distance for which the
observed amplitude is known, see Table 2) and the
period is 18 s (see above). A ccording to the dispersion
curve for oceanic paths [22], this period corresponds to
a group velocity of 3.6 km/s. With a focal displacem ent
value of 9.5 m, the remaining parameters being the
same, at a distance of 4444 km from the source, one
obtains a calculated Rayleigh wave amplitude of about
70 |im . Taking into account that the amplitude of the
vertical m otion in the Rayleigh wave is two-thirds of
the total am plitude, the vertical amplitude value should
be about 45 (im.
According to Table 2, the amplitude of the vertical
motion actually observed at the same distance equals
500 |Jm; that is, it is greater by a factor of more than 11.
In addition, one should have in mind the numerous
assum ptions on the param eter values in equation (1).
The com parison performed gives evidence in favor of
the experimental studies of the amplitudes of the Ray-
OC EANO LOG Y Vol. 42 No. 4 2002
Earthquake
ON THE INFLUENCE OF THE RAYLEIGH WAVES 553
g. 4. Central American seismically active zone and test areas 1 and 2, where the studies of the relationships between the bottom
diments and ferromanganese nodules were performed in cruise 41 of R/V Dmitrii Mendeleev. Earthquake magnitudes, M:
—7.0—7.4; 2—7.5-7.9; 3— 8.0-8.4; 4—> 8.5.
waves, when one has no need to preset any
own quantity in equations of type (1).
THE RAYLEIGH WAVE IMPACT
ON TH E SEDIMENTARY FORMATIONS
THE CLA RION-C LIPPER TON PROVINCE
n analysis of the observation data presented in the
ding section allowed us to determine the values of
assible vertical oscillations in the Rayleigh waves
Terent magnitudes o f strong earthquakes and at
ent distances from the test areas located in the
on-Clipperton Province. In order to resolve the
em of the interaction of the sediments with the fer-
nganese nodules in these test areas, one should
s tw o principal issues:
) Explanation of the erosional cut of the Tertiary
lents in the test areas considered in papers [1-3];
) Explanation of the old Oligocene ferromanga-
nodules resting over the recent sediments.
> assess the first issue, let us analyze the data on the
is of the interrelations between the sedimentary
■ and ferrom anganese nodules in test areas 1 and 2
: Clarion-Clipperton Province acquired in cruise 41
V Dm itrii Mendeleev, Fig. 4 [1-3]. In the figure,
an see that the seismically active zone closest to
st areas is the Central Am erican link o f the Pacific
lie belt. This zone is about 2600 km long extend-
om the northwest to the southeast. The mean dis-
s from test area 2 to its northwestern and south-
m segments are 30° (more than 3300 km ) and 39°
; than 4300 km), respectively. The respective dis-
s from test area 1 are 41° (more than 4500 km) and
more than 5400 km).
e have already noted that, in the region of the test
, erosion of the Eocene to Early Miocene
17 My BP) sediments was observed; meanwhile,
younger sediments up to the Q uaternary are absent. At
the sites where the gap in sedimentation lasted from the
Early Miocene to the Quaternary (17-1 M y BP), the
erosion amplitude is estim ated at 34 m. I f older sedi
ments, down to the Eocene, were subjected to erosion,
the erosional cut increases up to 80 m (Fig. 2). A ccord
ing to the latest evaluation from [3], the erosion noted
might start at 0.9-0.7 My BP at the beginning of the
“glacial Pleistocene.”
Assum ing the constancy of the tectonic regime over
this period, one can also postulate the invariance o f the
param eters of seismicity, which represent derivatives
from the tectonic regime. According to the data avail
able, the structural pattern of the Pacific region had not
suffered significant changes at least from the Oligocene
(see, for example, [8, 11]).
The question arises as to whether these estimates of
the erosional removal can be explained by the action of
the surface waves in the strongest earthquakes in the
Central American seismically active zone, bearing in
mind the noticeable seismic effect induced by this kind
of earthquakes on land (see above).
Let us assess the possible result of long-term action
of the surface waves from strongest earthquakes on the
sedimentary cover o f the region studied. For test area 2,
at distances of 30°, seismic impact is caused by the
earthquakes with M = 7.5-7.9 occurring in the north
western segm ent o f the Central American zone, since
we have no data on the amplitudes o f the Rayleigh
waves from distances smaller than 40°. In this case, the
maximum displacement o f the ocean floor is 0.5 mm
(see Table 2). A sim ilar oscillation amplitude should be
observed from the earthquakes with M = 8.2 in the
southeastern segment (Д = 39°), while from the earth
quakes with M = 7.5-7.9, ocean floor displacements
appear to be 1.25-fold sm aller (0.4 mm). The reason for
separation o f the seismic impacts with respect to the
magnitudes is explained by the different recurrence of
554 KUZIN, BARASH
the events with different magnitudes and, therefore, by
their different contribution to the total am plitude of the
ocean floor oscillations.
The situation with the seism ic impact within test
area 1 is similar. From the earthquakes with M = 7.5 -
7.9 in the northwestern segm ent (Д = 36°), the oscilla
tion amplitude in the Rayleigh waves reaches 0.45 mm
(see Table 2), From the earthquakes in the southeastern
segment (A = 45°), the strongest seismic impacts will be
caused by the seismic events with M = 8.2 (amplitude
0.5 mm), while the oscillation am plitude caused by the
earthquakes with M = 7.5-7.9 will be 0.37 mm.
In order to determine the integral effect of the seis
mic impacts one should account for the recurrence of
the earthquakes of different magnitude groups. For
example, for the K uril-K amchatka arc, the recurrence
of the earthquakes with M > 7.75 varies within 80-200 yr
or 140 + 60 years on average [16]. A close value char
acterizes the recurrence period of the earthquakes with
M = 8 for Central America (160 yr) [21].
One can suppose that the range of the recurrence
periods of 80-200 years, estimated for the Kuril-K am
chatka arc, is related to the combination of the earth
quakes with M = 7.5-7 .9 and 8 .0-8.4 into a single
group. Then, the minim um recurrence period equal to
80 year should correspond to the earthquakes with
M = 7.75 (magnitude group 7.5-7.9), while the period
value of 200 years should refer to the earthquakes with
M = 8.2 (magnitude group 8.0-8.4). Indeed, on the
basis of the recurrence (frequency-m agnitude) curve
for the K uril-Kamchatka area (angular coefficient
b — 0.9) [18], we can conclude that, in the recurrence
period for M = 7.75 of 80 years, the period for M = 8.2
should increase up to 200 years. Extrapolating the fre-
quency-m agnitude curve to the domain of even stron
ger earthquakes, we will obtain for M = 8.5 a value of
T = 380 yr.
Let us rely upon the data on the earthquake recur
rence for the magnitude groups 7.5-7.9, 8.0-8.4, and
greater than 8.5 in the Kuril-Kamchatka area as an ana
log for the earthquake frequency-magnitude depen
dence for the Central A merican zone and, with this
assumption, consider the situation in test area 2 as an
example. Let every earthquake, starting from M = 7.5-
7.9, cause a displacement, under the action of near-bot
tom “storms,” of the upper part of the sem iliquid sedi
mentary layer only 0.25 mm thick. At first glance, such
a negligible displacement seems hardly probable. How
ever, if we take into account that the rate of the bottom
sediment accum ulation is about 1-2 mm/ky [1-3], this
suggestion is not too incredible. Along with this, this
displacement amplitude, as follows from Table 2, can
be excited by an earthquake with M > 7.5 at a distance
of 40° and greater.
The distortion that precedes the removal of a fine
surface film of the sediments may have a stirring-up
character caused by the passing, first, of the faster Love
surface waves (v ~ 4.0 km/s); they should force the sed
iment particles to oscillate in the direction normal to the
direction of the propagation of these waves. The action
of the Love waves should destroy the horizontal links
between the sediment particles. Approximately one
minute and a half later, the Rayleigh waves should
arrive (v ~ 3.5 km/s), forcing the sedim ent particles to
perform mostly vertical oscillations, more favorable for
sediment removal.
Over 1000 years, earthquakes with M = 7.5-7.9 may
occur up to 12 times, causing an integral distortion of a
3-mm layer of the sediments; the respective values for
the earthquakes with M = 8 .0-8.4 and M > 8.5 are 5 and
3 times and 1.25 and 0.75 mm of the sediment. As a
result, the combined action of 20 earthquakes o f all
magnitude groups over 1000 years is able to distort a
sedimentary layer approximately 5 mm thick, if no con
solidation of the sediment occurs in the interval
between the earthquakes. Extrapolating this estimate of
the distortion of the surface layer of the sediments for a
period of 1 My, we obtain a value of the “seismic” ero
sion of 5 m.
In test area 1, the situation is similar to that in test
area 2.
Estimating the erosion value in both of the test
areas, one should have in mind that the erosion started
no earlier than 17 My BP. Then, assum ing the con
stancy of the seism ic regime, the estimated value of the
erosion caused by the near-bottom “storm s” excited by
the Love and R ayleigh surface waves should reach
85 m. This estimate coincides with the maximum ero
sional removal of the sediments suggested from the
stratigraphic data. However, if one assumes a shorter
erosional period, e.g., 1 My, the “seismological ero
sion” will provide a value that is a factor of 7-16 times
sm aller than the am plitude of erosion of the Tertiary
sediments.
Thus, the mechanism of direct impact of the surface
waves from the strongest Central American earth
quakes on the sedimentary cover of the Clarion-Clip
perton Province when applied to the explanation of the
total value of the sediment erosion observed still leaves
room for uncertainties. The principal obstacle for the
application o f this m echanism for explanation o f the
processes that occurred in the areas of the ferromanga
nese nodule concentration lies in the impossibility to
prove the feasibility of their displacement over the floor
and turning over under that small an oscillation ampli
tude. Since the size of an individual nodule reaches
(5 -1 0 cm), the maximum sea floor oscillations excited
by the Rayleigh waves from the strongest earthquakes
(up to 5 mm ) are 100-200 times as small as the charac
teristic nodule size. These data also suggest that the
Rayleigh waves cannot provide the vibrations required
by the screen separation effect, as the bottom accelera
tions produced at A = 0.5 mm and T = 18 s reach an
am plitude of only about 0.24 x 10~3 g.
In order to resolve the problem of the relationship
betw een the sediments and the ferromanganese nodules
OCEANOLOGY Vol. 42 No. 4 2002
ON THE INFLUENCE OF THE RAYLEIGH WAVES 555
in the Clarion-C lipperton Province, one should assess
the data on the near-bottom tsunami waves in the open
ocean induced by strong underwater earthquakes.
DISCUSSION
A seismological hypothesis is suggested to explain
the significant erosional cut of the Tertiary sediments
(3 5-80 m) and the resting of ancient massive ferroman
ganese nodules on the surface of the recent sediments
in the Clarion-C lipperton Province. In this paper, in
order to justify this hypothesis, we used the data on the
Rayleigh surface waves induced by strong earthquakes
from the Central American seismically active zone
exciting noticeable bottom oscillations. As an analog,
we used the data on the maximum amplitudes of the
vertical component of the Rayleigh wave oscillations
from the records obtained at the Petropavlovsk-Kam-
chatskii and Severo-KuriPsk seismic stations for the
earthquakes with M = 6.0 -8.2 from all seismically
active areas o f the Pacific Ocean at epicentral distances
from 5° to 83° (55 5-9200 km). The oscillation ampli
tudes at the registration poinfs are assumed to corre
spond, to a first approxim ation, to the bottom oscilla
tions at sim ilar epicentral distances. This approach was
applied due to the fact that the calculations o f the Ray
leigh wave amplitudes by the known formula lead to
their underestimation by an order o f magnitude.
For different m agnitude groups (M = 6.0-6.4,
6.5 -6.9, 7.0-7.4, 7.5 -7.9, and 8.2), attenuation of the
Rayleigh wave am plitudes with distance was deter
mined. A decrease in the amplitude attenuation of these
waves with growth in the magnitude of the earthquake
was revealed. Thus, upon the transition from the earth
quake group with M = 6 .0-6.4 to that with M = 8.2, the
attenuation coefficient decreases almost sixfold. This
result suggests the impossibility of extrapolation of the
oscillation amplitude value from one magnitude group
to another assuming a proportionality of the amplitude
to the square root from the energy.
At distances of 40°-80°, the displacem ent ampli
tudes for the earthquakes with M = 8.2, 7.5-7.9, and
7.0 -7.4 vary in the ranges 0.5— 0.38, 0.50-0.16, and
0.3 7-0.10 mm, respectively. For the earthquakes with
M = 6.5-6.9, the amplitudes reach a few tenths of a mil
limeter (0.10-0.20 mm) only at small epicentral dis
tances (10-30°). For the m agnitudes M = 6.0-6.4, the
displacements caused by the Rayleigh waves are negli
gible even at these distances, being only 0.02-0.05 mm
(see Table 2). Therefore, only the data for the earth
quakes with M = 7.5-7.9 and 8.2 were used in this
study.
In the considered ranges of magnitudes (M =
6.0-8.2) and epicentral distances (5.2-83°), we did not
find any dependence of the frequency composition of
the Rayleigh waves on the m agnitude and distance. The
dominating oscillation period in the Rayleigh waves is
18 ± 2 s.
The data on the oscillation amplitudes of the Ray
leigh waves were used to solve two problems:
(1) explanation of the erosional cut of the Tertiary
sediments (Oligocene-Early M iocene) in the test areas
in the Clarion-C lipperton Province due to the removal
sediments with a total thickness of 3 5-80 m over the
period from the Early M iocene to Quaternary
(1 7.5-1 .0 M y BP) and
(2) explanation of the possible reason for the occur
rence of dense old ferrom anganese nodules at the sur
face of unconsolidated recent sediments.
By the example of two test areas in the C larion-
Clipperton Province, it was shown that, under the con
dition of constancy o f the seism ic regime in the Central
American seismically active zone over the past 17 My,
the “seismological erosion” o f the bottom sediments
under the action o f the Rayleigh waves, assuming a
removal value caused by an individual earthquake with
M = 7.5-8.5 equal to 0.25 mm, should reach 85 m.
However, if one takes the minimum value of the dura
tion of erosion (~1 My), the erosional cut will be only
5 m, which is 7 times sm aller than the value determined
from the stratigraphic data (35 m from [3]). This
defines the uncertainty in the estim ate of the value of
the “seism ological erosion” produced by the Rayleigh
waves. In addition, due to the small amplitudes of the
ocean floor oscillations caused by these waves
(0.5 mm), one can hardly explain the displacement of
massive ferromanganese nodules by the action of the
Rayleigh waves.
Therefore, the results of the studies of the impact of
the Rayleigh surface waves on the ocean floor do not
provide a sufficient explanation of the features of the
observed interrelations between the sedim ents and fer
romanganese nodules in the Clarion-Clipperton Prov
ince. In order to resolve this problem, it is necessary to
assess the data on the near-bottom tsunam i waves from
the strongest earthquakes (M > 7.5) occurring in the
seismically active zone of Central America.
ACKNOWLEDGMENTS
This study was supported by the Russian Founda
tion for Basic Research, project no. 01-05-64263.
REFERENCES
1. Barash, M.S., Kruglikova, S.B., and Mukhina, V.V., On
the Stratigraphy of the Cenozoic Deposits in Two Test
Areas in the Clarion-Clipperton Province (Pacific
Ocean), Okeanologiya, 1993, vol. 33, no. 2,
pp. 276-283.
2. Barash, M.S. and Kruglikova, S.B., TrAnSl(Vozrast
radiolyarii iz zhelezomargantsevykh konkretsii provin-
tsii Klarion-Klipperton (Tikhii okean) i problema “nep-
otoplyaemosti” konkretsii), Okeanologiya, 1994, vol. 34,
no. 6, pp. 890-904.
3. Barash, M.S., Kruglikova, S.B., and Mukhina, V.V.,
Stratigraphic Features of the Sedimentary Deposits of
OCEANOLOGY Vnl АО ы.-. л ntvx n
556 KUZIN, BARASH
the Clarion-Clipperton Province (Eastern Equatorial
Pacific), Okeanologiya, vol. 40, no. 3, pp. 424-433.
4. Barenblatt, G.I. and Baturin, G.N., On the “Nonsinking”
Properties of Ferromanganese Nodules and Selected
Features of the Near-Bottom Layer of the Ocean, Dokl.
Akad. NaukSSSR, 1989, vol. 308, no. I, pp. 183-187.
5. Baturin, G.N., Rudy okeana (Ores of the Ocean), Mos
cow: Nauka, 1993.
6. Bolt, B.A., Horn, W.L., Macdonald, G.A., and
Scott, R.F., Geological Hazards, Heidelberg: Springer,
1977. Translated under the title Geologicheskie stikhii,
Moscow: Mir, 1978.
7. Bullen, K.E., An Introduction to the Theoiy o f Seismo
logy, Cambridge: 1963. Translated under the title Vvede-
nie v teoreticheskuyu seismologiyu, Moscow: Mir, 1966.
8. Okeanologiya. Geologiya okeana. Т. I. Osadkoobrazo-
vanie i magmatizm okeana (Oceanology. Ocean Geology:
Vol. I. Sedimentation and Magmatism in the Occan),
Bezrukov, P.L., Ed., Moscow: Nauka, 1979.
9. Zharkov, V.N., Vnutrennee stroenie Zemli i planet (Inter
nal Structure of the Earth and Planets), Moscow: Nauka,
1983.
10. Zharkov, V.N., Lyubimov, V.M., Dorofeeva, L.N., and
Dorofeev, V.M., Sample Distributions of the Dissipation
Function Q(l) in the E arth’s Mantle, Isv. Akad. Nauk
SSSR, Fiz. Zemli, 1974, no. 12, pp. 3-12.
11. Kropotkin, P.N. and Shakhvarstova, K.A., Geolog-
icheskoe stroenie Tikhookeanskogo podvizhnogo poyasa
(Geological Structure of the Pacific Mobile Belt), Mos
cow: Nauka, 1965.
12. Rautian, T.G., Energy of Earthquakes, Metody detal'nogo
izucheniya seismichnosti (Methods for Detailed Studies
of Seismicity), Moscow: USSR Academy of Sciences,
1960, pp. 75-114.
13. Riznichenko, Yu.V., Source Size of aCrustal Earthquake
and Seismic Moment, Issledovaniya po fizike zemletr-
yasenii (Investigations in the Physics of Earthquakes),
Moscow: Nauka, 1976, pp. 9-27.
14. Sorokhtin, O.G. and Ushakov, S.A., Global 'naya evoly-
utsira Zemli (Global Evolution of the Earth), Moscow:
Mosk. Gos. Univ., 1991.
15. Ulomov, V.I., Polyakova, T.P., Shumilina, L.S., et al..
Experience in the Mapping of Earthquake Foci, Seis-
michnost’ i seismiclieskoe raionirovanie Sevemoi
Evrazii (Seismicity and Seismic Zonation of North Eur
asia), Moscow: IFZ RAN, 1993, no. 1, pp. 99-108.
16. Fedotov, S.A., On Seismic Cycling: Possibilities of
Quantitative Seismic Zonation and Long-Term Seismic
Forecast, Seismiclieskoe raionirovanie SSSR (Seismic
Zonation of the USSR), Moscow: Nauka, 1968,
pp. 121-150.
17. Okeanologiya. Fizika okeana. Т. II. Gidrodinatnika
okeana (Oceanolbgy. Physics of the Ocean. Vol. II.
Hydrodynamics of the Ocean), Kamenkovich, V.M. and
Monin, A.S., Eds., Moscow: Nauka, 1978.
18. Chernysheva, G.V., Potapova, O.V., and Shumilina, L.S.,
Characteristics of the Seismicity of the Eastern Part of
North Eurasia from the Earthquake Catalogue for 1994,
Seismichnost’ i seismiclieskoe raionirovanie Sevemoi
Evrazii (Seismicity and Seismic Zonation of North Eur
asia), Moscow: OIFZ RAN, 1995, nos. 2-3, pp. 37 2-
386.
19. Aby, J., Zemletrvaseniya (Earthquakes), Moscow:
Nedra, 1982.
20. Gutenberg, B. and Richter, C.F., Seismicity of Earth and
Associated Phenomena, New Jersey: Princeton Univ.
Press, 1954.
21. Newmark, N.M. and Rosenblueth, E., Fundamentals of
Earthquake Engineering, Englewood Cliffs, New Jer
sey: Prentice-Hall, 1971, pp. 247-266.
22. Oliver, J., A Summary of Observed Seismic Surface
Wave Dispersion, Bull. Seismol. Soc. Am., 1960, vol. 52,
pp. 81-86.
OCEANOLOGY Vol. 42 No. 4 2002