Velocity anomalies in the lower mantle

1969, Phys. Earth Planet. Interiors 2, 1-10, North-Holland Publishing Company, Amsterdam

VELOCITY ANOMALIES IN T H E L O W E R M A N T L E

M I C H A E L A. C H I N N E R Y

Department o f Geological Seiences Brown University Providence, Rhode Island, U.S.A. Received 5 December 1968

Observations of the slope of the made on the P-wave arrivals at Array from about 400 events at a the array. A velocity structure for

travel time curve have been the Large Aperture Seismic Northwesterly azimuth from the lower mantle is deduced

using the Wiechert-Herglotz formula. Regions of anomalous velocity change are found to occur in the vicinity of 700, 1150 and 2000 km depth, and there is a possibility of an additional anomaly at about 2500 kin.

The presence of inhomogeneities in the upper mantle has now been clearly indicated by a variety of methods (see for example, ToKsOz et al., 1967), and it is natural that interest should turn to the properties of the lower mantle. For our purposes, we shall define the lower mantle to be that portion of the mantle largely inaccessible to conventional surface wave studies. A reasonable boundary between the upper and lower mantle might therefore be placed at a depth of about 1000 km. To provide continuity with upper mantle results, we shall discuss in this paper the Pwave velocity structure below a depth of about 700 km. Traditionally, the lower mantle has been regarded as homogeneous, and changes of velocity with depth have been attributed to the effects of temperature and pressure alone. More recently, however, there have been suggestions that this part of the Earth may contain regions where physical properties depart significantly from those expected for a homogeneous material (CHINNERYand ToKs6z, 1967). These inhomogeneities are "small" in the sense that they produce departures from the standard travel time curve of only a few seconds. On the other hand, one of these anomalous regions appears to affect the velocity-depth curve over a depth range of about 400 km and from this point of view it might reasonably be called a "large" effect. Apart from studies of the free oscillations of the Earth, which have not yet produced any very detailed

information about the lower mantle, techniques for investigating this region are at present limited to the use of body waves. This is hardly a limitation, of course, as the short periods available make these methods potentially of extremely high resolution. It has turned out, however, that it is hard to achieve this resolution by the observation of the absolute travel times of body waves. The reason for this is that it is extremely difficult to separate the contributions to the travel times from the crust and upper mantle from those due to velocity structure deep in the Earth. Nevertheless, recent investigations (e.g. HERRIN et al., 1968) show clear departures of observed travel times from the standard tables of Jeffreys and Bullen. The use of seismic arrays to determine the velocities of P-waves within the Earth was pioneered by NIgzI and ANDERSON (1965) for the upper mantle, and has been further elaborated for the same region by JOHNSON (1967). The application of these techniques to the lower mantle have been described by CHINNERY and ToI~s6z (1967) and JOHNSON (1967). The advantage of these methods is that the measurement of relative times of arrival at an array turns out to be extremely sensitive to the velocity gradient at the lowest point of the path, and is much less sensitive to crust and upper mantle structure. Since the first preliminary data were presented in CmNNERY and TOKS6Z (1967), a considerable amount of new information has been obtained. This has necessitated some revisions in the earlier interpretation, and it is now possible to locate the position of the

1. Introduction

2

MICHAEL A. CHINNERY

anomalous regions in the lower mantle with some confidence and precision. The purpose of the present paper is to review these results and to discuss the velocity distribution beneath one profile on the Earth's surface (a great circle passing through central North America, the Aleutian and Kuril Islands, Japan and Indonesia). There are two very interesting problems that have emerged from the study of array travel time data and which will be the subjects of future papers. The first is concerned with the structure of the crust and upper mantle beneath the array, as revealed by travel time anomalies at the various stations in the array (see, for example, GREENFIELD and SHEPPARD,1968). The second deals with the accumulating evidence for significant lateral variations in the lower mantle (CHINNERY, 1967). Both of these studies require an appropriate velocity distribution against which observations can be compared, and this will be described in the results that follow. 2. P r e v i o u s w o r k on the l o w e r m a n t l e

The presence of anomalous regions within the lower mantle has been suggested by the results of a number of authors. Although individually these studies are not conclusive, together a pattern seems to emerge which, as we shall see, is in broad agreement with the conclu-

sions of the present paper. We summarize some of these previous investigations briefly below since they provide a basis for an assumption that we must make in the interpretation of the data to be presented later. Analyses of the departures of P-travel wave times from the standard Jeffreys-Bullen Tables have been made by a number of authors, including CARDER(1964), CLEARY and HALES (1966), and HERRIN et al. (1968). The results of CLEARY and HALES (1966), which were obtained from a study of the travel times of P-waves from 25 earthquakes to stations in North America, are shown by the solid curve in fig. 1. Other investigations, including the very comprehensive work in HERRIN et al. (1968), have produced very similar data when travel times are averaged for many different paths. Also shown in fig. 1 are the travel time residuals from the J-B tables for the P-waves from Longshot (CHINNERYand TOKSOz, 1967), with station corrections applied. The large residuals from this event are well known, and indicate a large source term, due presumably to an unusually fast upper mantle velocity beneath the Aleutian Islands. However, changing the base line for these points does not produce a good agreement with the solid "world average" curve. There is a distinct suggestion that the peak-to-peak amplitude of the Longshot residuals is about 2 1/2 or 3 seconds, which is much larger than those found by other studies.

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3

VELOCITY ANOMALIES IN THE LOWER MANTLE

This suggests that lateral variations within the mantle may produce significant departures from a "world average" curve. There are, however, points of similarity between all travel time studies of this kind. The J-B residuals are generally small in the distance range 40°-50 °, large in the range 60°-70 ° , and then decrease fairly rapidly between 70°-80 °. These variations must indicate structure within the deeper layers of the mantle. Since the Jeffreys-Bullen velocity distribution is close to the curve expected for a homogeneous mantle, any structure of the kind we have indicated must show the presence of inhomogeneity. Other authors have used absolute travel time information to suggest anomalous regions within the mantle. GUTENBER~ (1958) suggested that relatively straight segments of the travel time curve occurred in the distance ranges 40°-44 ° and 53°-63 ° . These correspond to a small increase in velocity with depth between 900 and 1000 km, and between 1400 and 1500 km, respectively. BUGAYEVSKIY(1964), by fitting sections of the travel time curve with a second order polynomial, found that the derivative of the travel time curve showed several extrema in the distance ranges 35 ° to 38 °, 50 ° to 54 °, and 70 ° to 72 °. He attributed these effects to the presence of hypothetical boundaries in the mantle at depths of 900, 1200 and 1800 kin. In an interesting paper, VVEDENSKAYAand BALAKINA (1959) observed the relative amplitudes of P-waves and SH waves as a function of distance. At distances of about 39 ° , 52 ° and 70 ° anomalous readings were obtained. The authors attributed this effect to double refraction within layers of the Earth's mantle laying at depths of about 950, 1250, 1800, and about 2200 km. The number and position of these anomalous regions makes an interesting comparison with the results to be presented later. Much useful information on the structure of the mantle must be contained within observations of the amplitude of teleseismic P-wave arrivals as a function of distance. Gutenberg is well known for his efforts in this field, though the scatter of the observations makes the extraction of reliable information extremely difficult. Recently, CARPENTER et al. (1967) have produced such an amplitude distance curve derived from explosion data, and their data are shown graphically in fig. 2. In spite of the rather wide confidence

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limits that must be applied to their data, the rather abrupt increases in amplitude that occur in the neighborhood of 33 ° , 48 ° and 73 ° seem to be real. The increase near 96 ° is probably somewhat less reliable. Again these variations must be attributed to some kind of structure within the mantle. It seems reasonable to make a tentative conclusion from these pieces of information that there is indeed structure within the lower mantle. In particular, within the depth range 1000 km to the core mantle boundary there are probably 2 and perhaps 3 levels at which the most important parts of this structure are located. 3. M e t h o d o f data analysis

The data used in this study consists of the arrival times of P-wave at the eight outer sub-arrays and center sub-array of the large aperture seismic array in Montana. The layout and identification of the various sub-arrays are shown schematically in fig. 3. The onset of P-waves from an earthquake is extracted visually from the 16 m m film recordings supplied by the LASA data service, Alexandria, Virginia. Times are recorded to the nearest 0.01 seconds, and in most cases appear to be accurate to about 0.05 seconds or better. F r o m these observations it is comparatively easy to obtain, by a standard least squares technique, the apparent azimuth of the event and the corresponding slope of the travel time curve dT/dd (CHINNERY and TOKS(~Z, 1967). It is, in addition, possible to estimate the second derivative of the travel time curve d2T/dA 2, though the scatter in the data has so far made it

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array. This includes events from the Aleutian Islands, Kuril Islands, Japan and Indonesia, and covers an azimuth range from the array of between 290 ° and 330 ° . The distribution of these events in azimuth and distance is shown schematically in fig. 4. Comparatively few events at distances closer than 25 ° have been observed along this direction, but the coverage beyond this distance is very good. In the next section we shall show the observations of d T / d A as a function of distance for earthquakes lying within this azimuth range. The problem which remains is to invert this information to obtain the velocity distribution within the mantle, and in order to do this we have used the Wiechert-Herglotz formula (see, for example, NIAZI and ANDERSON, 1965). After this inversion, the resulting velocity distribution was processed to determine travel times, travel time derivatives and differences from the Jeffreys Bullen tables. In order to do this an adaptation of a method described BULLEN (1960) was used, in which, by interpolation, the mantle was subdivided into approximately 5000 thin layers and the velocity in each was approximated by formula of the type v ~ ar b. The calculated values of d T / d A were found to agree with the input observations to within 0.02 seconds/degree. This provided a useful check on the numerical integration involved in the Wiechert-Herglotz formula. In order to evaluate the formula it is necessary to have a complete knowledge of the variation of d T / d A with distance out to a given distance. Since data closer than about 25 ° was not available in this study, the crust and upper mantle of the Earth were approximated by

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the model CIT 204 (JoHNsoN, 1967). Although this model was determined for a slightly different region, it was found to produce values of dT/dA that join very smoothly onto the data observed with the LASA array. Small errors in the assumed velocity distribution for the upper mantle should not effect our derivation of velocities in the lower mantle appreciably but may impose a systematic increase or decrease in travel times which should vary very little with distance. As we shall see, there is some evidence that the upper mantle velocities appropriate for the data presented below should be slightly faster than the model CIT 204.

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4. Observations To date about 400 events have been detected at the array shown in fig. 3 from distances of 25 ° to 100 ° along the Northwesterly azimuth from the array with sufficient amplitude for accurate timing. Because of the amount of data, the distance range concerned has been divided into four sections which are discussed separately below. In each case several curves have been

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put through the data, in an attempt to indicate the variety of velocity structures which are consistant with the observations.

the velocity variation between these three models is quite small. The validity of curve C is very qtestionable, and may simply be due to a few unreliable points. It is included however, in order to illustrate the range of possibilities available. The "knee" in velocity at about 700 km shown by curve A is of course the same as that to be found in JOHNSON'S (1967) model CIT 204. ANDERSON (1967) has interpreted this anomaly to be the result of a phase change in the fayalite-forsterite system.

(i) D i s t a n c e R a n g e 23 ° to 4 2 ° Observations of d T / d A in the distance range 23 ° to 42 ° are shown in fig. 5a. The main feature of these data is a very pronounced difference in slope between the points in the range 25 ° to 30 °, and those from 37 ° and 42 °. This difference in slope implies a region within the mantle where the velocity changes anomalously slowly with depth. The actual form of this structure is hard to determine exactly because of the scattered data between 30 ° and 37 ° . Three curves have been put through the data labeled A, 13, C. Throughout this paper, curve A will correspond to the smoothest possible variation consistent with the observations, while B and sometimes C will represent more extreme changes in velocity gradient. The velocity distributions corresponding to these curves are shown in fig. 5b. Clearly

(ii) D i s t a n c e R a n g e 39 ° to 61 ° Observations in the distance range 39 ° to 61 ° are shown in fig. 6a. The departure of the observed values of d T / d A from those derived from the Jeffreys-Bullen distribution is clear in the distance range 40 ° to 50 °. This implies that the real velocities increase more rapidly than the standard curve. As before, curve A is a smooth mean of the observations, while 13 and C represent more detailed possibili-

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well documented. Notice in particular the anomalous slope of the data points in the distance range 65 ° to 75 °. The velocity distributions corresponding to the curves A and B are shown in fig. 7b. In both cases an anomalously low change of velocity with depth is found at a depth of 2000 km. This is perhaps the most interesting anomaly in the lower mantle, and is certainly the largest in the sense that it causes the largest change in absolute travel times. This point will be discussed further below. (iv) D i s t a n c e R a n g e 78 ° to 99 °

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ties. The triplication shown in curve B between 47 ° and 49 ° is perhaps the most important difference between the present results and those described in a preliminary way in CHINNERY and TOKSOZ (1967). The presence of this feature is substantiated by amplitude observations at the array (GREENFIELD and SHEPPARD, 1968), and where events have been relocated by U.S.C.G.S. the associated points tend to move closer to curve B. The anomaly shown in curve C at 50 ° is not considered reliable; it is included only for comparison. The velocity distributions corresponding to curves A, B, and C are shown in fig. 6b. Note again the very small differences between the absolute velocities for these different possibilities. The anomaly in curve B clearly corresponds to a slow variation of velocity with depth at about 1100 km. (iii) D & t a n c e R a n g e 5 9 ° to 81 ° Observations in the distance range 59 ° to 81 ° are shown in fig. 7a. The departures of the observations from the Jeffreys-Bullen curve are pronounced and

For reasons which are not clear at present, the data points within the distance range 78 ° to 99 °, shown in fig. 8a, are more scattered than elsewhere. Events at these distances occur over a range of azimuths: to emphasize this events towards the extremes of the range 290 ° to 330 ° have been given different symbols in figs. 4, and 8. Clearly, apart from the few events from the interior of China which are indicated by triangles, there is no very significant difference between the re, maining data points. Because of the scatter we hat,~ put only one curve, labeled A, through the aata and the corresponding velocity distribution is shown in fig. 8b. Control in this region is not very good, and cannot at present be used to define the properties c f the region close to the core-mantle boundary. As fig. 8b shows, the data is consistent with the Jeffreys-Bullen radius of the core and velocity at the core-mantle boundary of 13.64 km/sec. The data do not suggest the presence of a decrease in velocity in the neighborhood of the core-mantle boundary as has been suggested by DAHM (1936) and MACELWANE (1951). The results are also inconsistent with the CARDER (1964) estimate for a P-wave velocity of 13.92 km/sec at the core-mantle boundary. They would however, admit a core-mantle boundary velocity of as little as 13.59 km/sec, and if further information were to indicate a velocity anc._v,a[y in the region of 85 ° (where some poor anomalous arrivals have been observed) it may be possible to reduce this velocity further to about 13.55 km/sec. This would permit an increased core radius of 10 to 20 km, as seems to be required by several recent lines of argument (BUCHBINDER, 1965 and others). 5. Discussion

There is one basic problem with the data that have been presented in this paper. This arises because we

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have omitted so far to consider the possible effects of near surface structure on the observations. It seems reasonable to suggest that corrections for the structure in the vicinity of the source will be extremely small. By observing the relative times of arrival of waves at an array from events at teleseismic distances, we are concerned with the differences between the paths taken by a very closely spaced group of rays emitted from the source. The approximate angular width of this bundle of rays at the source may be estimated from the dimensions of the array (approximately 2 °) and the standard tables of angles of incidence (see for example, RICHTER, 1958). For a bundle of arrays emerging at a distance of about 60 °, the angular diameter is approximately 0.5 ° . It would take an extraordinary structure in the neighborhood of an earthquake to produce a significant difference between the end-members of this bundle of rays. On the other hand, the effects of the crustal structure beneath the array are not negligible. Events from teleseismic distances traverse the upper mantle and crust

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at a steep angle, and the end members of the bundle of rays that we have mentioned are likely to traverse very different geological structures. In principle, if the geological structure is known beneath the array, its effect may be removed from the data (JOHNSON, 1967). In the case of the large aperture seismic array, crustal seismic surveys (e.g. MEYER e t a l . , 1961) indicate only gentle crustal structure beneath the array. On the other hand, the travel time anomalies indicated by this and other studies (e.g. TELEDYNE REPORT No. LL-6, 1967) show a high degree of complexity, and vary as functions of both azimuth and distance from the array. We must therefore provide some justification for our assumption that the regions of anomalous velocity change that we have discussed in the preceding section are in fact due to velocity structure within the mantle and not to some anomalous effect of the geology near the array. It can be argued, for example, that, since the angle of incidence at the array changes only slowly with the distance to an event, structure beneath the array is

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suit is probably fortuitous, since there is evidence for considerable structural effects on data obtained from other azimuths (GREENFIELD and SHEPPARD, 1968). The large negative residuals shown in fig. 9 at about 70 ° are clearly related to the similar dip in the data of CLEARY and HALES (1966: see fig. 1) which occurs at about 60 °. It does not seem to be possible to bring these two features into coincidence by any reasonable manipulation of the present data. This must therefore be taken to be an indication of lateral variations within the structure of the lower mantle, a subject which will be discussed in more detail in a later paper.

Acknowledgements unlikely to produce the rapid changes in dT/dd that occur in the regions where we have interpreted triplications. Perhaps a better justification is obtained by comparing our results with those of previous authors, outlined in the first section of this paper. The presence of anomalous portions of the travel time curve in the ranges 32°-37 °, 460-48 °, and 65°-75 ° has now been indicated by a variety of different techniques, and must be considered established. We can also obtain some useful information about the mantle velocities derived from the present data. Fig. 9 shows the computed travel time residuals between models A and B and the Jeffreys-Bullen travel time tables. Though these curves are displaced vertically, they have the same general form as the results obtained from absolute travel time studies (fig. 1). In particular, curve B can be brought into excellent agreement with the Longshot arrival times by displacing it upwards by about half a second. This could easily be accomplished by a minor change in the upper mantle velocity distribution from the model CIT 204 which we assumed. This indicates that the effect of local structure on the computed dT/dA values must be very small. Even slight displacement or distortions of the dT/dA curve will give rise to large changes in the calculated travel time residuals, and these are not permitted if the results are to agree with the Longshot residuals. We conclude, therefore, that the measured values of dT/dA for events along the Northwesterly azimuth are reliable. This re-

This research was supported by the Advanced Research Projects Agency and was monitered by the Air Force Office of Scientific Research under contract AF-F-44620-67-C-0006.

References ANDERSON, D. L. (1967), Science 157, 1165. BUCHBINDER, G. B. R. (1965), Bull. Seismol. Soc. Am. 55, 441. BUGAYEVSKIY, G. N. (1964), Akad. Nauk SSSR Sibirskoy Otdeleniye Trudy 18, 151. BULLEN, K. E. (1960), Geophys. J. 3, 258. CARDER, O. S. (1964), Bull. Seismol. Soc. Am. 54, 2271. CARPENTER, E. W . , P. O . MARSHALL a n d A . DOUGLAS (1967), Geophys. J. 13, 61. CHINNERY, M. A. (1967), Trans. Am. Geophys. Union 48, 194. CHINNERY, M. A. and M. N. TOKS6Z (1967), Bull. Seismol. Soc. Am. 57, 199. CLEARY, J. and A. L. HALES (1966), Bull. Seismol. Soc. Am. 56, 467. DAHM, C. G. (1936), Bull. Seismol. Soc. Am. 26, 159. GREENFIELD, R. J. and R. M. SHEPPARD (1968), Bull. Seismol. Soc. Am. (in press). HERRIN, E., W. TUCKER, J. TAGGART, D. W. GORDON and J. L. LOBDELL (1968), Bull. Seismol. Soc. Am. 58, 1273. JOHNSON, L. R. (1967), J. Geophys. Res. 72, 6309. MACELWANE, J. B. (1951), in: Internal Constitution o f the Earth (2nd Ed. Dover Publications, New York) p. 227. MEYER, R. P., J. S. STEINHART and W. E. BONINI (1961), in: Explosion Studies o f Continental Structure (Carnegie Institution of Washington) p. 305. NIAZI, M. and D. L. ANDERSON(1965), J. Geophys. Res. 70, 4633. RICHTER, C. F. (1958), Elementary Seismology (W. H. Freeman, San Francisco). TOKS/3Z, i . N., M. A. CHINNERY and D. L. ANDERSON(1967), Geophys. J. 13, 31. VVEDENSKAYA, A. V. and L. M. BALAKINA (1959), IZV. Acad. Nauk SSSR Ser. Geofiz. 7, 1138.