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Summary Anomalous high-frequency arrivals from deep-focus earthquakes have been observed for epicentral distances of 13–21 degrees in Japan, Tonga and South America. Their arrival times are up to a minute later than those of S, their durations are longer than those of the direct phases and they have an apparent phase velocity of 4.7 km s-1. Isacks and Barazangi have proposed a model which fits the data for arrivals of this type observed at NNA in South America. Their model assumes the existence of structure heretofore not predicted by independent observation. Assuming the uniqueness of their model in its ability to fit the data, they have used it as evidence for the existence of these structures. We have found, however, alternative models which fit the data for arrivals of this type observed at ARE, CUS and TRU in addition to those at NNA. According to these models, a P phase travels from the focus to some discontinuity where it is converted to an S phase which travels on to the station. The conversion points predicted by these models lie near a boundary of the inclined seismic zone which, we speculate, defines a dipping lithospheric slab. We conclude that these anomalous arrivals can be interpreted using seismic-ray calculations based on laterally homogeneous velocity-depth models and a dipping interface.

Summary Regional distribution of anelastic attenuation beneath the United States was investigated using amplitudes and dominant periods of short period P and S waves originating from deep focus earthquakes in South America and the Circumpacific seismic belt, and recorded at LRSM (Long Range Seismic Measurement) stations. The observed regional distribution pattern shows high attenuation in the western United States, including California, and a less pronounced higher attenuation region in the northeastern United States. This distribution pattern is similar to that reported by Solomon & Toksoz for long period S waves, but differs from it sufficiently to indicate lateral variations in the frequency dependence of the average crust-upper mantle attenuation across the United States.

The top of the olivine-spinel phase change in subducted oceanic lithosphere can be located by the travel times of seismic waves which have propagated through the slab. P-wave travel-time residuals from deep earthquakes in the Tonga island are observed at Australian seismic stations are grouped according to the depth of the earthquake. The change in mean residual with a change in earthquake depth is related to the velocity contrast between slab and normal mantle at that depth. The curve mean residual versus earthquake depth displays a region of markedly increased slope between earthquake depths of about 250 and 350 km. The most probable explanation of this observation is an elevation by 100 km of the olivine-spinel phase change within the relatively cooler slab. No evidence was found for vertical displacements within the slab of any deeper phase changes. A temperature contrast between slab and normal mantle of about 1,000°C at 250 km depth is implied. This finding confirms current thermal models for subducted lithosphere but is inconsistent with the global intraplate stress field unless only a few percent of the negative buoyancy force at subduction zones is transmitted to the surface plates.

An earthquake of magnitude 7.4 occurred off the Nemuro Peninsula on June 17, 1973. The crustal movements accompanying the earthquake can be interpreted as the result of elastic rebound caused by low-dipping underthrust of the Pacific Plate. The subsidence accompanying the earthquake has the same sense as that observed before the earthquake. At present it is clear neither whether the subsidence has increased or recovered, nor whether, in case of recovery, it was elastic or anelastic. An interesting phenomenon is the gravity decrease appearing in the subsided area at the occurrence of the earthquake. The relation between gravity and height changes is in opposite sense. A density decrease or mass transport in the upper mantle under eastern Hokkaido should have accompanied the earthquake.

The earthquake focus is represented by a bilateral bidirectional fault model in which the final fault area is rectangular. The transfer functions for body waves for this fault model have been derived. The directivity functions for body waves are defined by taking the ratio of the transfer functions corresponding to two observation stations. The sensitivity of the directivity function for P waves to the kinematic parameters and to the orientation of the fault plane is demonstrated. The S wave directivity function appears to be relatively insensitive to the change in the kinematic parameters. A deep focus earthquake (H = 600 km, mb = 7.0) has been investigated and the kinematic parameters have been successfully recovered using the modulus of the P wave directivity function. The kinematic parameters are as follows: The fault plane is the y-z nodal plane of the focal mechanism solution, the faulting is of the bilateral bidirectional type with total rupture along the y axis of 75 km and total rupture along the z axis of 29.5 km; the rupture speed along the y axis is 3.97 km/sec and along the z axis is 2.98 km/sec. The fault area is 2212 km2 and the dislocation is estimated to be 26 cm.

P-wave first motions, radiation patterns and amplitudes of long-period surface waves, relocated aftershock distributions, leveling and tsunami data indicate that the 1973 Nemuro-Oki earthquake is caused by a low-angle thrust-faulting, representing a rebound at the upper 50 km of the interface between the continental and oceanic lithospheres. Rebound, most likely aseismic, at depths below 50 km, is suggested to take place in the near future from a comparison of recent geologic crustal deformation with pre-seismic and co-seismic data. The estimated seismic moment is about 1 3 – 1 4 of that for the neighboring great earthquakes. The macro-seismic data suggest that the 1973 earthquake is smaller than the 1894 Nemuro-Oki earthquake, the last great earthquake in this region. The 1973 earthquake had been predicted on the basis of a seismic gap. Although the prediction was successful as to the location and nature of the faulting and partly as to the occurrence time, it is smaller than the predicted one. A part of the seismic gap may still remain. The difference between the observed seismic slip (1.6 m) and that predicted on the basis of the pre-seismic crustal deformation (3.0 m) indicates either (1) the 1973 earthquake relieved only a part of the strain accumulated in the upper 50 km, or (2) a significant amount of aseismic slip took place on the seismic fault and completely relieved the accumulated strain in the focal region of the 1973 earthquake. If the former is the case, the remaining strain, not only in the focal region, but also in the remaining seismic gap adjoining it, may be relieved in a larger earthquake in the future. The source parameters obtained are as follows: fault plane, dip direction = N40°W, dip angle = 27°; seismic moment = 6.7 · 1027 dyn cm; average slip dislocation, 1.6 m in N63°W direction; stress drop = 35 bars. In these calculations, the fault dimension and the rigidity are assumed to be 100 · 60 km2 and 7.0 · 1011 dyn/cm2, respectively.

This paper is a continuation of a previous publication (1938). The writers distinguish (1) shallow shocks, at depths not exceeding about 50 kilometers; (2) intermediate shocks, at depths from about 50 to 300 kilometers; (3) deep shocks. Separate maps are drawn for intermediate and for deep shocks, and new or revised determinations are presented and discussed. Previous conclusions as to mechanism and origin remain unmodified.

The source dimensions, stress-drops and apparent average stresses of 19 Tonga-Kermadec earthquakes with depth greater than 79 km were estimated from the amplitude spectra of body waves. The results for each event were based on a combination of spectra from short and long period records of several (an average of five to six) WWSSN stations. In all cases the focal mechanism was known. The radii for intermediate and deep earthquakes are approximately 10 km for mb (CGS) = 5.8 and the stress drops are between 4 and 70 b. The stress drops do not change much with depth, but appear to be slightly larger at intermediate depths than at great depths. The apparent stresses are larger at intermediate depths. These data suggest that the shear stress necessary to cause earthquakes may also be higher at intermediate depths. At greater depth, the increasing temperature and possible phase changes and dehydration may be factors that weaken the source material and that produce a change in the material properties within the slab near a depth of about 450 km.

A large deep-focus earthquake ( h = 577 km) in the western Brazil region that occurred on November 9, 1963, was followed by a relatively large aftershock. The long-period P wave records of the WWSSN for the main shock reveal three successive P phases. Regarding this earthquake as a triplet, the temporal and spatial relations among the three successive events are examined on the basis of the azimuthal distribution of the difference between the arrival times of the first and the second P phases and that of the first and the third P phases. The separation between the first shock and the aftershock in space and time is determined from the time differences of the first P arrivals which are read on the short-period seismograms of the WWSSN. The second and the third foci and the focus of the aftershock are found to be all situated on one of the P nodal planes of the first shock. This result almost definitely suggests that the earthquake occurred in the form of shear faulting. The primary P wave forms are obtained from the long-period P wave signals after eliminating the influence of the seismograph, crust and mantle. The seismic moment is accurately determined as 2.2 × 10 27 dyne · cm from their time integrations. The fault process is inferred from the spatial and temporal relations among the four shocks and from the primary P wave forms: the rupture which initiated at the first focus spread out with a uniform velocity and covered a more or less fan-shaped surface. The velocity of rupture propagation, faulted area, displacement discontinuity and the stress drop are estimated to be 2.2–2.5 km/s, 510–680 km 2 , 265–355 cm and 300–460 b. Some implications are discussed in the light of the plate tectonics. The predominant gap in seismic activity beneath northern Peru, western Brazil has been interpreted in terms of the detachment of the descending lithosphere. The presence of a large-scale normal faulting which extends over the entire thickness of the detached slab of lithosphere is suggested from the present results.

Horizontal and vertical crustal movements in regions where major earthquakes occur have a cyclic nature which can be correlated with the occurrence of major earthquakes. Recognizable phases in the cycle are: (1) slow continuous movement at a constant rate; (2) more rapid movement commencing from 10–20 years to a few hours or days prior to a major earthquake; (3) sudden movement at the time of the earthquake; and (4) rapid movement following the earthquake for a period of weeks to months. Movement at the time of the earthquake is usually produced by slip on a fault, which can be deduced from dislocation models and field evidence. It is always in the opposite sense to the slow continuous movement, which represents the accumulation of the elastic strain which results in, and is released by, the earthquake. The strain-accumulation field can be modeled as a virtual dislocation opposite to that of the earthquake. Rapid movements following the earthquake usually decelerate rapidly, often logarithmically, after the earthquake, and are often due to aseismic slow slip on the fault. Like aftershocks, these movements are adjustments to the stress state produced by the earthquake. They may be in the same direction as the movements during the earthquake if the earthquake slip was overdamped, or in the reverse sense if it was underdamped. Rapid movements prior to earthquakes are less well studied but may in some cases represent the beginning of fault slip in an aseismic, stable mode. The strain-accumulation field contains all information necessary to determine the maximum size of earthquakes that can be produced at any given time, whereas a sudden increase in rate of movement may give warning of the time of such an earthquake.

A systematic study has been made of the ratios of (displacementperiod) of PcP, PcS, ScS, and ScP to that of the corresponding incident wave {e.g.,(displacementperiod)PcP/(displacementperiod)P}, using intermediate and deep-focus earthquake seismograms. The results indicate that the observed ratios of the horizontal components of the waves that are reflected as P waves (i.e., PcP/P and ScP/S) and that of the vertical component of the waves that are reflected as S waves (i.e., ScS/S and PcS/P) at the mantle-core boundary are considerably larger than the theoretical ones, whereas the observed ratios of the vertical component of the first group and that of the horizontal component of the second group are in fairly good agreement with the theoretical values. Theoretical computations were based on the assumption that in the case of a longitudinal wave the vibration is in the direction of propagation and in the case of a transverse wave the vibration is perpendicular to the direction of propagation. It is further found that the behavior of the direct P and S waves is in accord with the theory, but the vibration of the ground is not in the direction of propagation for PcP and ScP and is not perpendicular to the direction of propagation for PcS and ScS.

In a previous paper* an account was given of a study of the characteristics of earthquakes with abnormal focal depth. It was shown that echoes should be produced by reflexion of waves at points near the epicentre and evidence in support of this idea was obtained from observations in the International Seismological Summary and also from seismograms registered at Kew and Eskdalemuir Observatories. To carry the work a step further, it was desirable that a detailed study should be made of records from a large number of stations of a well-observed deep focus earthquake ; this has been done in the present investigation. In the earlier paper, it was mentioned that it should be possible to recognise a deep focus earthquake from the records of a single station and to make an estimate of the depth. A favourable opportunity occurred on February 20, 1931, when a disturbance which was recorded at Kew Observatory showed quite definitely the abnormalities associated with a deep focus. On the day that the shock occurred, and from the Kew records alone, it was possible not only to locate the epicentre but also to obtain an estimate of the focal depth. The information so derived was published in the Kew Seismological Bulletin for February, 1931, and also in 'Nature.’ Reports subsequently obtained from other stations confirmed the fact that the shock was very deep seated, so this disturbance has been selected for the detailed study and records from a large number of stations have been examined.

Seismic and echo sounder profiles in the aftershock region of the 1964 Alaskan earthquake define a pre-existing zone of discontinuous faults in the area of maximum aftershock strain release. Steep reverse faults and possibly other types of steep faults occur in the zone; surface rupture has probably not been continuous along its full length during any Recent earthquake. The fault zone may represent a narrow area of maximum flexure and uplift in the broader area deformed during the 1964 earthquake. An anticline at the continental margin with local large structural relief was also uplifted during the earthquake.

The hypothesis has been advanced that major portions of the annual pole motion are caused by the seasonal transport of air masses. These pole motions in turn can act as trigger forces on earthquakes. This is tested by observations of northern hemisphere pressures, pole motion, and occurrence of deep focus earthquakes during the interval 1922 to 1935.

Summary and Conclusions In this paper the difficulties which attend any attempt to obtain the ground displacement which occurs during the passage of an earthquake wave have been illustrated. Reliable curves of ground displacement are difficult to obtain because the majority of seismographs, as operated, are poorly adapted for any purpose but the indication of the time of arrival of earthquake phases. This study has demonstrated, however, the practicability of an attempt to obtain ground displacement from seismograms, and has illustrated some of the uses which may be made of curves of ground displacement, when obtained. The use of the dynamic magnification to obtain the amplitude of ground displacement corresponding to an impulse on a seismogram was shown to give highly erroneous results. For long-period seismographs, calculation of displacement by methods in which the effect of transients was retained appeared to give fairly accurate results. The results of the study of the effects of a layered crust on the surface motion are to be considered only as preliminary, and indicative of the usefulness of a technique which hitherto has seldom been applied to seismic problems. Displacements were indicated in the horizontal components of ground displacement that were absent in the vertical, suggesting effects of crustal discontinuities. If the use of composite curves of surface displacement is valid, and the agreement between the observed and predicted curves of horizontal component of displacement is significant, the following are possible conclusions: oscillation in seismograms is a result of reflections and transformations of primary waves within the crust; the average thickness of the crust beneath the stations whose records were studied is 32 km.; the velocity change at the base of the crust takes place in a distance which is small compared to the wave length. An appropriate research for individual stations possessing good seismographs in three components would be a study of the thickness of the crust underlying each station. For this purpose the wave velocities as determined from local earthquakes or explosions, and the arrival time of P[S*S¯] for a few deep-focus earthquakes of simple initial form, would suffice. A knowledge of the curves of ground displacement would facilitate the solution of the following problems, among others: the effects of crustal discontinuities on the ground motion in the S phase; and the azimuthal variation of the form of the P wave, in relation to the process at the focus. The writer wishes to thank Professor L. B. Slichter, who suggested the problem, and the directors of the many seismological stations whose coöperation in providing seismograms made the investigation possible.

The dependence of stress drop on magnitude and distribution of first motions from long and short period seismographs suggests a trigger mechanism for deep focus earthquakes that might be different from the main dislocation process. Fracture experiments with two-dimensional plates of plastics were carried out to study a triggered fracture process. A high energy rubidium pulse laser served as source for the trigger energy. It was possible to initiate fracture in the stress range 0.2–0.8 of the strength of the material. Inhomogeneity of the stress field was found to be essential in order to obtain a fracture of finite extent. The velocity of crack propagation and the acceleration of the pulses radiated from the trigger phase and main phase were measured as a function of the applied stress.

During the years 1918–1931, there were 270 earthquakes with unusually deep foci, 167 in the Northern Hemisphere, 101 in the Southern, and two with epicentres on the equator. The normal depth of focus is assumed to be about 50 km. or ·008 of the earth's radius. The focal depths of the above earthquakes range from ·005 to ·090 of the earth's radius below the normal depth, or from 50 to 380 miles beneath the surface. Throughout this paper, the depth, when given in terms of the earth's radius, is referred to the normal depth; when given in miles, to the surface of the earth.

The attenuation of seismic waves is a direct measure of the absorption due to nonelastic processes in the earth. The well known difficulties in obtaining body wave amplitude decrement data have been avoided by studying the spectral ratios of multiple ScS and sScS phases from two deep focus earthquakes recorded at near normal incidence. The average Q, for shear, in the mantle is about 600 for the frequency range 0.015 to 0.07 cps. Assuming that equal radiation occurs upwards and downwards from the source the average Q for the upper 600 km of the mantle is determined to be about 200 and about 2200 for the rest of the mantle. The value for Q at the base of the mantle is at least 5000 for shear waves.

The strain produced by body waves in the vicinity of the earthquake is employed in the definition of the earthquake volume. The definition is applicable to artificial seismic events. Distances from the focus at which a given maximum strain was attained during the event, are presented as a function of magnitude and focal depth. The computed strains are compared with strains measured and reported for a number of earthquakes and nuclear explosions. The earthquake volume shows a distinct decrease with focal depth. The strains produced by body waves during the earthquake throw some light on the stress state in the vicinity of the focus, and are considered to be able to yield information of the rate of tectonic strain accumulation.

A large earthquake, magnitude 7.5, occurred at the western end of the Ryukyu Trench on March 12, 1966. The focal process of this earthquake is discussed in detail here on the basis of the moving lithosphere model. Its focal process and the features of the lithosphere in the Taiwan region are explained by means of a simple picture.The seismic parameters of this earthquake, which were obtained by analyzing two distinct phases in the initial P-wave group and by analyzing G waves, are as follows. Dip angle and dip direction of the fault plane are 82° and 215°, and the type of this fault is unilateral. The earthquake moment is 1.6×1027 dyne-cm; the average dislocation is 2.1×102cm; stress and strain drop are 6.6×10 bars and 1.8×10-4; strain energy is 1.4×1023 ergs; fault length in the strike direction is 2.7×10km; fault width is 2.8×10km; and rupture velocity is 1.9km·sec-1.

Summary (Abstract) Travel times of normal earthquakes are made the basis for calculated travel times for shocks at depths down to 800 km. These are presented in tabular form, together with certain auxiliary data. Theoretical discussion is given for the critical distances at which pP and PP, sP and SP, etc., coincide, and below which these phases should not occur. These distances are not focal points (caustics). Methods are given for determining epicenter, depth, and origin time, and are applied to a selected group of shocks. The results of different methods agree very well with one another and with the calculated travel-time data. A preliminary report is given on certain characteristics of the shocks studied. The mechanism of deep-focus earthquakes is discussed briefly, and it is concluded that normal and deep-focus earthquakes are probably brought about by the same forces.

Investigation of the magnitude-frequency realtion for earthquakes in island arcs has revealed lower values of the slope for deep earthquakes in New Hebrides, Solomon Islands and Japan as compared to the values of the slope for shallow and intermediate earthquakes. A very low value of the slope is revealed for the deep Andean earthquakes. The slope is the same for Tonga-Fiji for all depth ranges.

Two representative hypotheses on earthquake mechanism (so-called force types I and II) have been examined in comparison with seismograms for the earthquake of February 18, 1956 (south off Honshu, Japan; h = 450 km). On the basis of the fault-plane solution derived from P-wave data, one can predict polarity and relative amplitude of shear wave phases for a given station. The prediction by both of the hypotheses is compared with the observations at Kiruna and several other stations, where the principal seismic phases have been recorded clearly. The comparison has proved that the force type I does not fit the present case. The second type, on the other hand, explains the observations more consistently, although there are minor disagreements with respect to later phases. Reduction of the recorded wave form by an analog computer has shown that the original seismic disturbance (S) from the source is very simple in its wave form and harmonizes very well with Honda's theory. If we accept his theory, the radius of the origin sphere is estimated at 30-40 km for the present case.

Smylie & Mansinha (1971) have attempted to utilize static elastic dislocation theory in order to compute the change in the inertia tensor of a realistic spherically symmetric Earth model produced by a point tangential displacement dislocation. The present author has performed similar computations and has combined this theory together with an empirical earthquake moment-magnitude relation in order to estimate the total excitation of the Chandler wobble by all observed large earthquakes since 1904 (Dahlen 1971). The purpose of this note is to point out that there is significant disagreement between the final numerical results of Smylie & Mansinha (1971) and those of Dahlen (1971) and to suggest a possible explanation for this disagreement. In order to investigate the effect of the earthquake focal depth on the shift of the Earth's mean pole of rotation, Smylie & Mansinha (1971) have considered two point tangential displacement dislocation models for the faulting associated with the 1964 Alaskan earthquake. One model is a realistic shallow focus (depth h = lskm), shallow dip angle, thrust faulting mechanism; the other model has the same mechanism, but the focal depth h is taken to be 637 km. Table 1 shows a direct comparison of the results obtained for these two models by Smylie & Mansinha (1971) and the results obtained from my own computations (Dahlen 1971). The fault parameters used in all cases were those considered by Smylie and Mansinha (epicentral colatitude O,, = 29.0°, epicentral longitude 4,, = 213-0°, fault strike N42"E, fault dip 8"W, mechanism pure dip slip thrust, fault slip area 1-54 x 1OI2 m3). These fault parameters for the 1964 Alaskan earthquake differ slightly from those considered by Dahlen (1971). It can be seen that the computations of Smylie & Mansinha (1971) predict that the change in the inertia tensor produced by a deep focus earthquake will be nearly an order of magnitude larger than that produced by a shallow focus earthquake with the same fault parameters; my own computations predict that a deep focus earthquake will have a slightly smaller effect than a similar shallow focus earthquake. The major differences in the theoretical treatments of Smylie & Mansinha (1971) and Dahlen (1971) arise in the method used to deal with the fluid core of the Earth model. The reason why the treatment of a fluid core is not straightforward is that in the absence of both inertia and viscosity, individual fluid particles in a non-rotating fluid core encounter no resistance to a tangential (non-radial) displacement, and because of this the particle displacement field within the fluid core produced by a static tangential displacement dislocation (or any other static force system) located within the solid elastic mantle becomes indeterminate. In discussing this problem I

Summary Distance and time corrections for PcP, PKP and P phases with small angles of emergence are determined. The distance corrections permit extension of a ray path from the observing station through the source to the surface. This results in surface focus distances for the phases which permits more consistent comparison of amplitudes for phases from earthquakes at different depths. Application of the corresponding time corrections results in a surface focus travel-time curve.

Seismic velocity structure of the mantle beneath Japan has been determined on the basis of teleseismic explosion, long-period surface wave, and dΔ/dt data. A marked difference in structure is found across the deep seismic plane which dips from the Pacific Ocean side towards the continent. The mantle on the continental side of the seismic plane has such low velocity and Q (≈80) as to require a partial melting in addition to a possible compositional change. The low velocity seems to be associated with the underlying earthquake activity. It is found that the lower bound of the low velocity zone is relatively abrupt; this also provides a favorable evidence for the partial melting. A clear later phase branch observed at 15° < Δ < 20° suggests a rapid velocity increase at depths 375 to 400 km. This rapid velocity increase can be attributed to the onset of the olivine-spinel phase change of (MgFe)_2SiO_4, if the temperature at 400 km depth is about 1300 °C. This low temperature may call for a hydrous state in the upper mantle in order to cause the partial melting. The earthquake energy release has a notable maximum around 400 km depth in Japan region. The coincidence of this depth with that of the rapid velocity increase suggests that the olivine-spinel phase change is in some way related to the generation of deep focus earthquakes.

Early in the history of seismology it was occasionally suggested that, in addition to earthquakes with foci comparatively near the surface of the earth, shocks might also originate at depths of the order of several hundred kilometers. However, down to a comparatively recent date, all such conclusions were either purely speculative or were based on inadequate or misinterpreted data.

Summary and Conclusions Data were assembled of about 320 earthquakes, including nearly all those available for which the determinations of epicentral location, time of occurrence, and focal depth were considered reliable. They were analyzed collectively and in selected geographical groups by the method of periodogram analysis which appeared most suitable. Tests were made on periods of 6, 12, and 14 months and for correlation with twice the lunar and solar hour angles. None of the results could be interpreted as other than accidental. However, the Japanese group gave a slight indication of a correlation with twice the solar hour angle, and the South American earthquakes with twice the lunar hour angle. Many additional data must first be accumulated and tested before any special significance can be attached to these results. The manner whereby small fluctuations in stress might be expected to determine the exact time of occurrence of an earthquake was reviewed. A study was then made of three possible sources of such trigger stresses which could be effective at great depth, namely, tidal stresses in the solid earth, stresses due to changes in sea level, and stresses due to variations in barometric pressure. Tidal stresses alone were considered in detail. It was concluded that tidal stress variations approximate harmonic changes sufficiently well to warrant a search for lunar hour angle correlations by the methods of periodogram analysis, but they are nevertheless too irregular to make the statistical results of such studies valuable in supplying information concerning the mechanics and environment of deep-seated earthquakes. Periodicity investigations of this type, even if successful, can be expected to do little more than indicate the causes which can influence earthquake occurrence times. The importance of other agencies in causing stress variations within the earth was studied. It was concluded that oceanic tidal loads, and to a much less degree erratic barometric fluctuations, are capable of producing stress changes comparable with those due to tides in the earth. The stresses from the earth tides in general dominate the others, although the effect of ocean tidal loading may be of primary importance under special circumstances. The irregularity of both ocean loads and barometric pressure fluctuations renders them unsuitable subjects for study. For the reasons given, the only stress changes that appear to warrant study with reference to their effect on earthquakes are those due to earth tides. The best chance of finding a definite correlation world be from a study of data of a large number of earthquakes which had occurred within relatively small regions, as only then is it justifiable to assume that all the earthquakes had been similarly affected by the triggering stress. Data at present available are far from sufficient to provide a satisfactory basis for such a study.

The strain accumulation and release curves for shallow and deep focus earthquakes of Indian origin have been constructed for a span of sixty years, from 1905 to 1964. For shallow focus earthquakes, magnitudes 7.2 and above have been considered; for deep focus shocks, magnitudes 6.7 and above are used. Strain rebound characteristics yield a number of very interesting features; for example, the curve for shallow focus earthquakes shows two linear segments of strain accumulation. Deep focus shocks show a single cycle of strain accumulation. Comparison of these curves with similar curves from other regions given by Benioff are made.

A violent outburst of the Lopevi volcano in the central New Hebrides occurred on the 10th July, 1960. The eruption was preceded 4 months before by a deep earthquake (h=250 kms, Mag. 7 1/4), the focus of which was just under the volcano. An inventory of all shocks recorded in the Group since 1910 has been made and all informations about volcanic eruptions in this region have been collected. A close correlation appeared between these two phenomena. Each of the large volcanic eruptions recorded between 1910 and 1962 followed a deep focus earthquake of magnitude greater than 7. Moderate eruptions were preceded by earthquakes of magnitude between 5 3/4 and 6 3/4. The time between the tectonic shock and the climactic phase of the volcanic activity appears to be related to the distance between the focus and the volcano (i.e. the focal depth), the type of the volcano and the pattern of its eruption. It is of few months duration for the volcanoes in the Central group: Ambrym, Lopevi, the submarine volcano east of Epi and Karua. The authors tried to find the same correlations for others volcanoes in the world for which they have been able to collect dates of eruptions: Asama-Yama (Japan), Bezymiannyi (Kamtchatka), Paricutin and Izalco (Central America), Vesuve, Stromboli (Italy). Thus volcanic eruptions would appear to have their first origin in the mantle. A systematic survey of all volcanoes and deep regional earthquakes would bring evidence of this correlation and may permit a long term prediction of their eruptions.

The focal process of the Sanriku earthquake of March 2, 1933, is discussed in relation to the bending mechanism of the lithosphere. On the basis of the P times obtained at more than 200 stations, it is confirmed that the hypocenter of this earthquake is within the lithosphere beneath the Japan trench. The P wave fault plane solution, the amplitude of long-period (100 s) Love and Rayleigh waves and two near-field observations suggest, almost definitely, that the Sanriku earthquake represents a predominantly normal faulting on a plane dipping 45° towards N 90° W. A fault size of 185 × 100 km^2, in agreement with the size of the aftershock area, is required to yield a slip dislocation of 3.3 m, a value consistent with the tsunami data. This result suggests that the fracture took place over the entire thickness of the lithosphere, thereby precluding the possibility that the Sanriku earthquake merely represents a surface tensile crack due to the bending of the lithosphere. This large scale lithospheric faulting is presumably due to a gravitational pull exerted by the cold sinking lithosphere. The fracture probably took place on an old fault plane which had once fractured and healed up. The existence of this fracture zone which decouples, to some extent, the oceanic lithosphere from the sinking lithosphere accounts for the sharp bend of the lithosphere beneath oceanic trenches and also the abrupt disappearance of seismic activity across oceanic trenches. The sharp bend of the lithosphere is therefore a result, not the cause, of great earthquakes beneath oceanic trenches.

A new analytical method for the determination of velocity at the hypocenter of a deep earthquake has been developed making use of P- and S-wave travel times. Unlike Gutenberg's method which is graphical in nature, the present method makes use of the least square technique and as such it yields more quantitative estimates of the velocities at depth. The essential features of this method are the determination from the travel times of a deep-focus earthquake the lower and upper limits Δ1 and Δ2 respectively of the epicentral distance between which p = (dT/dΔ) in the neighborhood of inflection point can be considered stationary so that the travel-time curve there can be approximated to a straight line T = pΔ + a. From p = (1/v*) determined from the straight line least-square fit made on the travel-time observation points between Δ1 and Δ2 for various focal depths, upper-mantle velocity structure can be obtained by making use of the well known relation v = v*(r0 − h)/r0, h being the focal depth of the earthquake, r0 the radius of the Earth, v* the apparent velocity at the point of inflection and v the true velocity at that depth. This method not only gives an accurate estimate of p, at the same time it also yields quite accurate value of a which is a function of focal depth. Calibration curves can be drawn between a and the focal depth h for various regions of the Earth where deep focus earthquakes occur, and these calibration curves can then be used with advantage to determine the focal depths of deep earthquakes in those areas.

About 10 per cent of all major earthquakes are now known to originate below the "crust" of the earth at depths ranging up to 700 km. After a brief resume of the evidence for the existence of deep earthquakes, the paper describes the environment in which they occur under the headings of structure and composition, density and pressure, rigidity and bulk modulus, temperature, viscosity, and strength. The characteristics of deep earthquakes are then described under the headings of distribution (geographic and depth), energy released, aftershocks, periodicities, correlations, patterns of initial motion at the surface, and stress and strain near the focal region. Analysis of these data leads first to the conclusion that the type of process involved is probably a normal fracturing response to external stress differences, although other possibilities are considered. The possible known sources of stress appear to be inadequate to function as primary causes of deep earthquakes but may be important as "triggers." Th...

At least two different effects have been observed in the focal zone of strong earthquakes that are probably caused by variations in the state of the materials before and after the earthquake: (a) differences of source parameters of the main shocks and aftershocks that occur much later in time, (b) differences in attenuation of seismic waves in the focal zone before and after the earthquake. From many scientific and practical points of view, it is interesting to estimate how long after a strong earthquake occurs, the substance in the focal zone returns to its initial state before the earthquake. The study of the effects mentioned above as well as other phenomena appear to be useful in solving this problem.

Five years ago at the Pasadena meeting of the Seismological Society the writer ventured to state in a preliminary report of the evidence for a focal depth of 400 km for the Japanese earthquake of March 29, 1928, that the occurrence of earthquakes at a depth of 400 km may well have large significance for isostasy and related problems, as well as in regard to structure, constitution, and condition of the Earth. At that time deep foci enjoyed only a modicum of scientific respectability, but since then they seem to have found an accepted place in the family of scientific data, and interest has arisen in regard to their significance and implications. The present paper is an attempt to develop, to a little extent, some of these implications.