Dynamic interaction of seismic and volcanic activity of the Nazca plate edges

Physics of the Earth and Planetary Interiors, 9 (1974) 175—182 © North-Holland Publishing Company, Amsterdam — Printed in The Netherlands

DYNAMIC INTERACTION OF SEISMIC AND VOLCANIC ACTIVITY OF THE NAZCA PLATE EDGES EDUARD BERG and GEORGE H. SUTTON Department of Geology and Geophysics, University of Hawail at Manoa, Honolulu (Hawaii) Accepted for publication September 13, 1974 Cumulative seismic strain release, during the period from 1964 through 1972, along the East Pacific and Chile rise edges of the Nazca plate is related to cumulative strain release of shallow earthquakes along the South American sinking edge: periods of high strain release along the rises alternated systematically with periods of high strain release along the sink. Volcanic activity (number of volcanoes in eruption per year between 1900 and 1968) migrates from Central to South America at some 900 km/year. High volcanic activity in Central America usually precedes large-magnitude earthquakes in Chile by several years, and would appear to be dynamically interrelated with the alternating strain release along the Nazca plate edges.

1. Introduction A fundamental premise of instantaneous plate tectonics is that of rigid lithospheric plates. On a global scale, the pattern of tectonic activity and of deformation along plate boundaries is rapidly being established. However, the detailed nature of forces interacting at plate boundaries is far from clear, If we accept the basic premise of rigidity for the Nazca plate, the question arises whether or not the present tectonic activity along the rising and sinking edges in the form of earthquakes and volcanism shows a temporal dynamic relationship. If such a relation exists, tectonic activity should be indicative of stress (or stress release) transmission through the plate. This paper examines earthquakes from 1964 to 1972 and volcanic eruption data from 1900 to 1973 to show that such a relationship exists and is statistically meaningful. Earthquake data for the ridges are incomplete prior to the operation (1964) of the Worldwide Standard Seismograph Networkin South America and therefore have not been used. It is felt that the data from 1964 to 1972 inclusive form a homogeneous data set, mainly because the smaller earthquakes that might have been missed contribute little to strain release or slip and the regularities found for the alternating high strain release along the rising and sinking edges would be much weaker. —

—

The original idea, that earthquake energy release along the rising and that along the sinking edges of plate boundaries are correlated as a consequence of plate rigidity, was presented by Walker (1971) and Walker et al. (1972). In the later report it was noted that the energy released annually along the eastern edge of the Nazca plate decreased prior to the 1970 Peruvian earthquake, and so did the energy released annually along the East Pacific and Chile rise edges of the plate, with minimum activity in both areas immediately preceding the earthquake. Similar corroborating observations according to Walker et al. (1972) apply to the 1960 Chilean earthquake. Using data from 1964 to 1970 inclusive, correlation of energy release between rising and sinking edges of other plate boundaries was also found.

2. Stresses in the lithosphere Many geophysical and geological observations mdicate the presence of large stresses in the lithosphere. The stresses vary from the rising to the sinking edge of the tectonic plates, depending on crustal and uppermantle relief, thickness, and density (Artyushkov, 1973). They have been determined from seismic data in many areas including areas of oceanic crust (Wyss and Brune, 1968; Wyss, 1970; Hanks, 1971). Since the vis-

176

cosity (s~)of the plate material is high, the relaxation time (i~/E)(where E = Young’s modulus) for such stresses is very large (order of thousands of years) cornpared to periods of years considered here. It is therefore expected that stress or strain changes of such short time periods will be transmitted “instantaneously” through the plate and therefore a close relation of cumulative seismic strain release between the rising and sinking edges of the Nazca plate is expected and indeed observed. This instantaneous stress (or strain) transmission through the plate from one boundary to the other requires an important extension to the recent theoretical findings of Bott and Dean (1973). Their paper examines the application (or removal) of stress on one side of the elastic plate, underlain by a fluid highly viscous asthenosphere, that is itself underlain by a rigid boundary. The lithosphere plate itself is held fixed at the other end. With a thickness of 80plate,and km and Young’s modulus 2 for the a thickness of 250 of 1012 dyn/crn of 2 1021 P for the asthenosphere, km and viscosity their stress penetration depth (attenuation by a factor e~)for periods of one year or less is 10 km or less, a conclusion that would not allow mantle Rayleigh waves to propagate. However, since the asthenosphere is also elastic (the extension proposed here to Bott and Dean’s model), stress application at one boundary of the lithospheric plate propagates “instantaneously” (with P-wave velocity) through the plate,interface applying that shearisstress at the lithosphere—asthenosphere subjected to relaxation with the type of time constants etc., discussed in their paper. Therefore direct interaction between the rising and sinking parts of the plate is expected. In fact the 3.5 year interaction period found here for the plate boundaries confirm someNazca points of the Bott andwould Dean appear paper, iftoone allows for an elastic—viscous asthenosphere. Fault-plane mechanisms along the rises are not known here, hence no distinction between dipslip on the rise and strikeslip along nearby fracture zones could be made. Therefore the cumulative seismic strain release was used rather than the cumulative slip. However, time variations of the cumulative strain release along the South American border of the plate parallel the time variations of the cumulative slip (compare Figs. 2B and C) consistent with the fact that fault-plane solutions for wide areas along the sink indicate underthrusting of the oceanic plate, that is, unidirectional slip. An

E. BERG AND G.H. SUTTON

additional new type of direct evidence for the underthrusting has recently been obtained from seismic reflection and refraction data across the Peru—Chile trench (Kulm et al., 1973, 1974). These data clearly trace the down-dipping oceanic crust some 20—40 km from the trench axis toward the continents. 3. Data The data used are the epicenters, depth and magnitudes from the USCGS/NOAA catalogue. Body-wave magnitudes m~have been converted to surface-wave magnitudesM~by: M5 = 1.59 m~ 3.97 —

(Richter, 1958, p. 690)

and where surface-wave magnitudes were also given, averages were taken. The strain release of an individual earthquake wasthe taken as where E, the energy, was obtained from surface-wave average magnitude by log 10E = 11.4 + 1.5 M (E in ergs; Richter, 1958, p. 366). For comparison, the slip along the South American edge was calculated (as in Davies and Brune, 1971; and Brune, 1968) as: slip = Mo/pA (cm) where Mo = earthquake moment, p = shear modulus, 2), with: A = slipzone surface (in cm log 10Mo = M + 19.9 M ~ 7.9 log10Mo/2 = M + 19.9 M ~ 8.0 (Mo in dyn cm) 2 p = 3.3 L .11 1011 dynfcm A= = 4.8 l0~km2 sina where H = 60 km depth, a = 45°,L = length of plate edge 5600 km. Data considered for the sinking edge were limited to a depth of 60 km and the following geographical coordinates: —

0—20°Sand 65—85°W 20—45°Sand 65—77°W Data for the East Pacific and Chile rises were limited

SEISMIC AND VOLCANIC ACTIVITY OF THE NAZCA PLATE EDGES

10

177

0

I0~ERG10

EAST PACIFiC 9196 AND CI4ILE RISE

.l&~ERG

1::

_____

I 1965

1967

~97Q

CUR4JLATIVE STRAIN FLEA96

ID ERG

SOUTH AMERICA

CM

SOUTH AMERICA

1973

NAZCA PLATE

h~NJ

~

N Fig. 1. Nazca plate. A. Cumulative strain release East Pacific and Chile rises. B. Cumulative strain release South America sink. Despite the change in rate the data are considered homogeneous (see Introduction).

2 -

C 1964

1967

19,0 NAZCA

by the following geographical coordinates: 0

0

0—38 S and 95—115 W

973

PLATE

Fig. 2. Nazca plate. A. Differential cumulative strain release, East Pacific and Chile rises. B. Differential cumulative strain release, South America sink. C. Differential cumulative slip, South America sink.

35—45°Sand 78—95°W All data were taken between 1 January 1964 and 31 December 1972.

4. Strain release relation The cumulative strain release for the rising and sinking edges of the Nazca plate is presented in Fig. 1 A and B. Total slip over the 9 years amounted to 9.5 cm, considerably less than the rates obtained by Davies and Brune (1971) averaged over a much longer time period (4.5 cm/year 6 N—18 S, 6.1 cm/year 18—46 S). The average slip rate from 1900 to 1964 was also recalculated for the eastern edge of the Nazca plate; using Duda’s (1965) catalogue (and other parameters given earlier) the average obtained was 5.0 cm/year, which is slightly lower than Davies and Brune’s average (5.3 cm/year), probably due to our neglect of earthquakes with magnitudes less than 7. It appears therefore that

the seismic slip and strain release over the last 10 years has been very low when compared to the total for the earlier part of this century. The differential cumulative strain (defined below) for the rises and the differential slip for the South American edge are presented in Figs. 2A, B, and C for the period 1964 to 1972 inclusive. The differential cumulative strain release as used here is the difference between the average strain release (the straight line connection (0, 0) and the end point of the cumulative plot) and the actual cumulative strain released (similarly for the slip). A least-square approach to the data presented in Figs. lA and B could have been used, but would not significantly alter the conclusions reached. The data show that on the rises and on the sink, low strain releases correlate closely in the first half of the time period, and high strain releases, in the second half, especially evident in the differential plots. To demonstrate the relation differently, the cumulative strain release along the sinking edge (y-axis) versus the cumulative strain release on the rising edges (x-axis)

178

E. BERG AND G.H. SUTTON

1~ 1O~ER& 20 ~TH AMERICA SINK

The results should be viewed with some caution since years of data do plate. not necessarily reflect total nine seismic regime of the In the light of the Kelleher’s work (Kelleher, 1972; Kelleher et al., 1973) and Michael’s (1973) volcano investigations discussed below, 30 or 60 years would seem more appropriate to cover a full rupture propagation period in Chile, but seismicity data for the rise are not available for so long a time period. However Fig. 3 (as well as Fig. 4) sup-

NAZCA PLATE CUMULATIVE STRAIN RELEASE 1964-1972

15

~J

[I

875 765 MAGNITUDE SINK

•

• 5.5 -.

MONTHLY YEARLY

MAGNITUDE RISE

6.—. 7 1

EAST PACIFiC AND CHILE RISE 2

3

4

ports thelimited hypothesis that the strain fluctuations strongly on both risesrelease and the sink of theare plate. Fig. 4 is derived from Fig. 3; it shows the variation of the strain release—strain release curve around the average line as a function of time (that is, the difference between they-component to the straight line of Fig. 3 versus time):

;0~1ERG12

Fig. 3. Nazca plate. Cumulative strain release East Pacific and

t

strain release

(t)

=

[

~ —

r—t

2(r)

—

4.79

5E~

0 2

Chile rises (x-axis) versus cumulative strain release South America sinkrise (y-axis), 1964—1972. Note alternation of release between and sink. Nçite difference in scale factors. Least-square line slope is: 4.79 erg~i2(sink)/erg1/2(rise)and offset is

(t

—0.71

0 = 1964; S = sink; R = rise). Positive values indicate that more strain has been released along the sink than

-

1011 ergh/2.

is plotted in Fig. 3. The data points correspond to cumulative strains at the end of calendar months. For these end-of-the-month cumulative strains, a linear regression line was obtained (by computer) and is also 2(sink)/ergh/2 shown. The slope of the line is 4.79 ergsl/I (rise), with standard deviation of 0.07. The correlation between the differential cumulative strain release of Figs. 2A and B for zero time lag has a correlation coefficient of 0.775 and slope of 4.31 ±0.34, which is highly significant (Crow et al., 1955, p.:159 and table on p.241). It is evident from Fig. 3 that relative strain release along the rises (portions of the curve predominantly parallel to the x-direction) alternates with relative strain release (curve predominantly parallel to the y-direction) along the sink, oscillating along the average relationship. The strain released along the sinking edge (y-axis) during one oscillation is equivalent to that of a single magnitude 8.0—8.1 earthquake. The strain release along the rising edges (x-axis) during one oscillation is equivalent to that of a single magnitude 7.2—7.4 earthquake. The largest earthquakes recorded on the rises were of magnitude 7.3 (24 September 1910) and magnitude 7.0 (20 March 1920) (Duda, 1965).

+

0.71 1011 erg~

along the rise as compared to the average; negative values indicate the converse. Again the peak-to-peak limitation shows clearly, implying that after reaching a minimum corresponding to a strain release deficit, a major release depending along the sink willtime follow within years, perhaps on the history (ortwo steepness of the preceding negative-going portion of the curve and/or the value of that deficit). The period of some 3.5 years for the strain release variations relates closely to the migration time of the volcanic activity along the sinking edge. The 3.5 year periodicity is clearly established by x1011 ERG~2

~

YEAR

A STRAIN RELEASE NAZCA PLATE

Fig. 4. Nazca plate. A strain release, that is: difference (my) between data point and least-square line of Fig. 3 as function of time.

SEISMIC AND VOLCANIC ACTIVITY OF THE NAZCA PLATE EDGES

the autocorrelation of the time series of Fig. 4 (correlation coefficient 0.66 at 41 months lag time, 64 degrees of freedom). The critical correlation coefficient for a 1% probability that there is no such correlation between the original and the time-lagged data (for 60 degrees of freedom) is only 0.325 (Crow et al., 1955).

179

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In a recent Ph.D. dissertation, Michael (1973) exammed the annual number of volcanoes in eruption for the circum-Pacific belt from 1900 to 1968, using data from the Catalogue of Active Volcanoes and a listing obtained through the courtesy of Dr. John H. Latter, Wellington, New Zealand. In 16 out of 21 areas, Michael found that fluctuations proved statistically significant and that the periods of high activity were 17 and 34 years apart (approximately) for the western and eastern margins, re. . spectively. Large pulses of activity tend to migrate from north to south with time, this tendency being more pronounced in the eastern part of the margin, with a slowness of 0.136 years per degree (as measured from the north pole). Michael also found a correlation between the annual number of volcanoes in eruption and the annual, shallow large-magnitude (M> 7) earthquake energy release in South America, and attributes this correlation mainly to the two years (1906, 1960) of simultaneous high volcanic activity and high energy release (correlation coefficient 0.54). For the purposes of this paper, the data were reexamined with a particular view on: (1) migration of vol. canic activity along the Central and South American volcanic chain (rather than using distance from the north pole); and (2) the relation of the migration of volcanic activity to the large-magnitude Chilean earthquakes and their migration (after Kelleher) at the southeastern border of the Nazca plate. Fig. 5 presents the number of volcanoes in eruption in a given year from 1900 to 1968 as used by Michael (1973) for Mexico, Central America, Colombia—Ecuador, Peru, and northern, central and southern Chile. Each of these areas corresponds to the one used in the catalogue of active volcanoes, a division also maintained by Michael and retained here for convenience. Data for 1969—1973 were obtained from the Smithsonian geophysical event cards. The bars under the eruption time plots for Peru and ~

,~

.

CENTRAL AMERICA

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PERU

—

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2

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NORTHERN —

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SOUTHERN 1925

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~

~

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NUMBER CF SOLCANCES IN ERUPTION PER TEARS OF INCREASE BY 2 ~ MORE

I T

CHILE

YEAR

B

N ~ 6 OCCURRENCE CF LARGE EARTHQUAKES

1~ 1st ~ 2nd RUPTURE C’ItLE

Fig. 5. Number of volcanoes in eruption for Central and South America in relation to occurrence of large shallow earthquakes along the Nazca plate subduction zone. Rupture cycle refers to north—south progression of fracture zone of the large earthquakes (after Kelleher, 1972). X represents eruptions that have been disputed (1972 in central Chile, 1973 in southern Chile).

central Chile indicate the years of occurrence of large earthquakes (M> 8) of shallow focal depth along the Nazca plate subduction zone (Duda, 1965). Michael (1973) cross-correlated the number of volcanoes in eruption in Central America with those of the other east Pacific margin areas and listed the lag times for the highest correlation coefficient obtained, or two such lag times if the corresponding correlation coefficients were within 0.02 of each other. In Fig. 6 these lag times (y-axis) are plotted versus the central distance of each area from the reference area (Central America) along the volcanic chain (rather than the distance from the north pole as done by Michael). Circles indicate that the correlation coefficient indicated only a 5% or less probability that there was no correlation, crosses mdicating that there was (statistically) no meaningful correlation except for northern Chile, where the value of 0.24 was just below the critical value of 0.25 for 5% probability of no correlation (Crow et al., 1955). The

180

E. BERG AND G.H. SUTTON

large (M~8) Peruvian quakes followed eruptions in

YEARS 8

is mentioned, southern Perubecause within 2—3 veryyears. little attention This possible has been relation CENTRAL

CHILE

NORTHERN

4

I-RI

FRU

COLOMBIA ECUASOR

/616.5%

-

~

CRIIIRAL AMERICA

/~“O

ME9EO

_______________________________ I I I

0

MIGRATION

5000

KM

OF VOLCANIC ACTIVITY

Fig. 6. Lag times of number of volcanoes in eruption (after Michael, 1973) with respect to Central America as function of distance from Central America. o lag times are statistically meaningful; X lag times are statistically not meaningful (see text); all symbols were used to calculate the regression line.

lag times given in parentheses are those secondary lag times that fall outside the graph (northern Chile, Co. lombia—Ecuador). Since most data points fall nearly on a straight line, all (except those in parentheses) have been used for the least-square fit, resulting in a slope of 0.00108 years/km with a standard error of about 7.0%. Taking the inverse indicated a migration velocity of 924 km/year (2.5 km/day) with a 95% confidence interval of 16.5%. This migration velocity is not significantly different from the 7 degrees/year obtained by Michael; however the 95% confidence interval has been narrowed from about 75% to 16% by using the geophysically more appropriate distance along the volcanic chain. This tendency of southward migration is visible in Fig. 5, with high activity starting in Mexico or Central America in 1902, 1920, 1952, and corresponding to peak activity in central or southern Chile in 1906/1907, 1927—1929, and 1959—1961. It also appears (Fig. 5), however tenuous, that the large shallow Chilean earthquakes seem to coincide with or follow increased volcanic activity in central Chile (with exceptions). Similarly, at least two of the three

paid to the interaction of strain fields of large-magnitude earthquakes and volcanic activity. Somewhat clearer isofthe relation quakes: volcanic whenever activity (number thehowever number of volcanoes volcanoes in eruption) inbetween eruption in Central America and Chilean large shallow-focus earthincreases by two or more over the preceding year (see arrows in Fig. 5) a large shallow earthquake follows in Chile within ten or so years. The delay time (happens to) correspond closely to the migration time of the volcanic activity from Central America to Chile as found by Michael (1973; see Fig. 6). Only the 1939 earthquake was preceded by a smaller increase (in 193 1/1932), perhaps due to a masking effect of the very high number of volcanoes in eruption during the period from 1920 to 1930. Also the 1961 increase was not followed by an earthquake of magnitude greater than 8; however one shock of magnitude 7.7 followed in Peru in 1970, one in the Ecuador—Peru border area in 1970 (M = 7.3) and one in central Chile in 1971 (M = 7.5). (The 1950 earthquake was inland and is not directly related to this study.) Table I compares years of increases in the number of volcanoes in eruption by two or more in Central America with large shallow earthquakes in Chile along the Nazca plate edge.

TABLE 1

Comparison of years of increased volcanic activity in Central America with earthquakes in Chile Year of increased number Year of Chilean (by two or more) of vol- earthquake, and

Delay in

canoes erupting in Central magnitude years America _________ ____________________________ 1902 1914 1919—1920 1939 1952 1961

1906 1922 1928 1943 1960 1971

(8.6) (8.4) (8.3) (8.3) (8.3) (7.5)

4 8 9 4 8 10

(1930—1931)?

1939 (8.3)

9

SEISMIC AND VOLCANIC ACTIVITY OF THE NAZCA PLATE EDGES

6. Discussion The volcanic migration from the northeastern edge of the Nazca plate (Colombia—Ecuador in Fig. 6) to the Chile area takes about 3—4 years, which corresponds closely to the 3.5 year strain-release cycling discussed earlier, so that the volcanic migration follows the strain cycling of the Nazca plate. Unfortunately this cannot be demonstrated directly since the detailed earthquake data from 1964 to 1972 hardly overlap sufficiently with the volcanic data. However the strain-release relation clearly demonstrates that a major release along the South American subduction zone is: (1) only possible during times of a strain-release deficit (Figs. 3 and 4); and (2) almost always preceded by a considerable increase in the number of volcanoes in eruption (two or more) in Central America. Since the volcanic activity migrates from Mexico to Chile, a strong dynamic coupling must exist between the Nazca and Cocos plates. It might be pointed out that the first Kelleher rupture cycle of this century for large Chile earthquakes from north to south (1906, 1928, 1939 and 1960 earthquakes) was preceded by much higher Central American and accompanied by higher central Chilean volcanic activity than the second rupture cycle (1922, 1943, 1971). Use of the migration of the number of volcanoes in eruption and the dynamic strain release relation, together with Kelleher’s (1972) considerations for seismicity gaps and large-earthquake fracture zone propagation, results in a longer lead time for earthquake prediction than the methods based on such changes as tilts (Karmaleeva, 1960), “b” slopes (Mogi, 1962; Suyehiro, 1966; Suyehiro et al., 1964; Berg, 1968), or V~/V5 due to dilatancy of rock under compression (Semenov, 1969; Berg and Pulpan, 1970; Scholz et al., 1973).

the average relationship between cumulative strain release of the sink versus the rise appear strongly limited, and show a periodicity of some 3.5 years, a period that spans about the same time as it takes the volcanic activity to migrate from north to south along the South American edge of the plate. Since this plate edge is only a portion of the volcanic migration pattern from Central America to southern Chile, a dynamic coupling between the Nazca and Cocos plates should exist. In addition a marked increase in the number of volcanoes in eruption in Central America is usually followed by a large Chile earthquake with a time corresponding closely to the volcanic activity migration time. Since large strain releases along the sink follow relatively larger strain releases on the rises, the volcanic activity migration together with the strain-release status of the plate seems to allow large lead times (several years) for anticipating the shallow depth strain release in the Chilean subduction zone. Acknowledgements We express appreciation to Charles McCreery, Allyson Kam, and Christopher Berg for computer programming and data preparation; and to D. Walker, L. Kroenke and William M. Adams for discussion. This research was supported in part by NSF Grants GA 371 18X and GX 28674A 2. This paper constitutes Hawaii Institute of Geophysics Contribution No. 586. References Artyushkov, E.V., 1973. Stresses in the lithosphere caused by crustal thickness inhomogeneities. J. Geophys. Res., 78 (32): 7675—7708. Berg, E., 1968. Relation between earthquake foreshocks, stress *

7. Conclusion Cumulative seismic strain release along the rising and sinking edges of the Nazca plate between 1964 and 1972 support the concept of rigid plates. Episodes of higher than average strain release alternate between the rising and sinking portions of the plate. The amount of strain release in one such episode is roughly equal to one magnitude 7.0—7.3 earthquake on the rises and from an 8.1—8.2 earthquake along the sink. The excursions

181

and mainshocks. Nature, 219 (5159): 1141 —1 143. Berg, E. and Pulpan, H., 1970. Earth tilts in connection with

crustal failure, A study in Alaska. Annual Progress Report, Geophys. Inst., Univ. Alaska, College, Alaska, 16 pp. Bott, M.I-I.P and Dean, D.S., 1973. Stress diffusion from plate boundaries. Nature, 243: 339—341. Brune, J.N., 1968. Seismic moment, seismicity, and rates of slip along major fault zones. J. Geophys. Res., 73 (2): 777— 784. Crow, E.L., Davies, F.A. and Maxfield, M.W., 1955. Statistics Manual. Publications, NewRegional York.Sci., 1ip rates from seismicity. Phys. 229 (4): fault 101— Davies, G.F. Dover and Brune, J.N., Nature 1971. and global s10~i.

182 Duda, S.J., 1965. Secular seismic energy release in the circumPacific belt. Tectonophysics, 2 (5): 409—452. Hanks, T.C., 1971. The Kuril trench—Hokkaido rise system: Large shallow earthquakes and simple models of deformstion. Geophys. J.R. Astron. Soc., 23: 173—189. Karmaleeva, R.M., 1960. An attempt to forecast the time of near earthquakes. Bull. Acad. Sci. U.S.S.R., Geophys. Ser., 1960: 467—474. Kelleher, J.A., 1972. Rupture zones of large South American earthquakes and some predictions. J. Geophys. Res., 77 (11): 2087—2103. Kelleher, J., Sykes, L. and Oliver, J., 1973. Possible criteria for predicting earthquake locations and their application to major plate boundaries of the Pacific and Caribbean. J. Geophys. Res., 78 (14): 2547—2585. Kuim, L.D., Scheidegger, K.F., Prince, R.A., Dymond, J., Moore, T.C., Jr., and Hussong, D.M., 1973. Tholeiitic basalt ridge in the Peru trench. Geology, 1(1): 11—14. Kulm, L.D., Resig, J.M., Moore, T.C. and Rosato, V.J., 1974. Transfer of Nazca ridge pelagic sediments to the Peru continental margin. Geol. Soc. Am. Bull., 85: 769 —7 80. *Michael, MO., 1973. Fluctuations in Circum-Pacific Volcanic Activity and in Seismicity of South America. Thesis, Univ. of Hawaii, Hawaii Inst. Geophys. Rep. HIG-73-1 1, 96 pp. Mogi, K., 1962. The fracture of a semi-infinite body caused by an inner stress origin and its relation to the earthquake phenomena (First paper). Bull. Earthquake Res. Inst., Tokyo

Univ., 40, Part 2: 815—830 (in English). Richter, C.F., 1958. Elementary Seismology. Freeman, New York.

Scholz, C.H., Sykes, L.R. and Aggarwal, Y.P., 1973. Earthquake prediction: A physical basis. Science, 181 (4102): 803— 810.

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Semenov, A.N., 1969. Variations in the travel-time of transverse and longitudinal waves before violent earthquakes. Phys. Solid Earth (Engi. Edit.), 4: 245—248. Suyehiro, 5., 1966. Difference between aftershocks and foreshocks in the relationship of magnitude to frequency of occurrence for the great Chilean earthquake of 1960. Bull. Seismol. Soc. Am., 56 (1): 185—200. Suyehiro, S., Hsada, T. and Ohtake, M., 1964. Foreshocks and aftershocksaccompanying a perceptible earthquake in central Japan, on the peculiar nature of foreshocks. Pap. * Meteorol. Geophys., XV: 1. Walker, D.A., 1971. Evidence of a relation between the seismic energy released along spreading and sinking edges of the Nazca plate. Hawaii Inst. Geophys., Univ. of Hawaii Rep., * HIG-71-16, 3 pp. Walker, D.A., Sutton, G.H., Woollard, G.P., LeTourneau, N.J. and Kausel, E., 1972. Easter Island seismograph observations indicative of sea-floor spreading; plate-edge seismicity relationships and the prediction of earthquakes along the west coast of the Western Hemisphere. Hawaii Inst. Geophys., Univ. of Hawaii, Rep., HIG-72-2, 25 pp. Wyss, M., 1970. Stress estimates for South American shallow and deep earthquakes. J. Geophys. Res., 75 (8): 1529— 1544. Wyss, M. and Brune, J.N., 1968. Seismic moment, stress, and source dimensions for earthquakes in the California—Nevada

region. J. Geophys. Res., 73 (14): 4681 —4694. *

Reports available on request.