Earthquakes and Earth structure: a perspective since Hutton and Lyell

Earthquakes and Earth structure: a perspective since Hutton and Lyell

B R U C E A. BOLT

Department of Geology and Geophysics, University of California, Berkeley, CA 94720, USA

Abstract: Lyell's interest in earthquakes as part of the Principles of Geology continues to be justified many fold. A quarter century after Lyell's death, seismology began to open the window on the contemporary structure and tectonic deformation of the Earth. Detailed non-biased observations of the global distribution of earthquakes played a crucial role in the attack on pre- plate theories of Earth dynamics. There were three critical seismological assault tools: reliable hypocentre catalogues, uniform magnitude estimates, and fault source mechanisms. Previously used as evidence for plate tectonics, seismicity is now often taken as predicted by it. Nevertheless, earthquake occurrence remains unforecastable in definite temporal terms. Interplate and intraplate spatial patterns show complexity in macro-crustal and micro-crustal structures. In particular, the mechanism and dynamic implications of deep-focus earthquakes and subduction remain a challenge.

Local and global seismographic networks are increasingly enhanced by broadband digital seismometry. This modern instrumentation provides high resolution of strong ground shaking and crustal and deeper interior structure. Second-order structural variations are now being mapped in the upper mantle and more detailed boundary conditions for convection models are being resolved in the lithosphere and in the D" mantle--core layer. Recently, seismological evidence for scattering anomalies throughout the mantle has become persuasive.

It is well known that Charles Lyell's Principles of Geology (1875) contains considerable descriptive material on earthquakes and links them with uplift and other deformation of the Earth's surface. Only eight years after its publication, Professor John Milne, then working in Japan, surmised (see Bolt 1993) that 'it was not unlikely that every large earthquake might with proper appliances be recorded at any point of the globe'.

This prediction was fulfilled in 1889 by the German physicist E. Von Rebeur Paschwitz, who 'was struck by the coincidence in time' between the arrival of singular waves which were registered by delicate horizontal pendulums at Potsdam and Wilhelmshaven in Germany and the time of a damaging earthquake that shook Tokyo at 2:07 am Greenwich Mean Time on 18 April. His conclusion was that 'the disturbances that were noticed in Germany were really due to the earthquake in Tokyo'. The significance of this identification - an early example of remote sensing - was that earthquakes in inhabited and uninhabited parts of the world alike could be monitored uniformly, and thus patterns of geological activity could be mapped without bias; an era in the quantitative study of earthquakes and geology not known to Lyell then began.

A principal aim of this paper is to provide historical illustrations and a short commentary on

major ongoing problems in seismology. In tracing the historical evolution of knowledge in seismology and related tectonics from Lyell's day, I have been forced to select only four central topics on earthquakes: their tectonic causes; their wave motion: their prediction in time and location; and their use to image the three-dimensional structure of the deep interior. Even these subjects, each of interest to my own research, must be considered very briefly, with a narrow focus on recent debates. My textual reference to Lyell's writings is, for brevity only, the twelfth (and last) edition of Principles of Geology. Each successive edition of this seminal treatise incorporated ' important additions and corrections' . Nevertheless, Lyell comments that although between the first and twelfth editions numerous descriptions of recent earthquakes had been published, he doubted that they illustrated new principles.

Lyell's accounts of earthquakes

James Hutton wrote little on earthquakes (Bailey 1967). He did describe processes that had led to land surfaces above the sea surface. He concluded that 'the land in which we dwell ' has been elevated 'by extreme heat and expanded with amazing force'. This belief led in turn to a consideration of volcanoes, active and extinct, with slight reference

BOLT, B. A. 1998. Earthquakes and Earth structure: a perspective since Hutton and Lyell. In: BLUNDELL, D. J. & SCOTT, A. C. (eds) Lvell: the Past is the Key to the Present. Geological Society, London, Special Publications, 143, 349-361.

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to earthquakes, neither of which were within his personal experience. He considered volcanic eruptions to be safety valves 'in order to prevent the unnecessary elevation of land and fatal effect of earthquakes'.

In contrast, Lyell emphasized the value of earthquake studies for geology. In the twelfth edition of the Principles he discusses volcanoes and earthquakes as constructive forces. These accounts reflect the prevailing view of a common underlying cause and intimate physical connections. Neverthe- less, they still read well today, with many case histories and arguments based on the very limited geophysical measurements available. Lyell begins by regretting the deficiency of accounts of ancient earthquakes, almost all descriptions being restricted to damage and injury. His interest was in the geological aspect of earthquakes, particularly the coseismic changes in the Earth's crust that accompanied them.

By 1875, Lyell had available reports by Robert Mallet and the catalogues of Alexis Perry and others. There is little doubt that he developed a strong interest in seismology and he summarized published reports of major earthquakes in such widely distributed places as Jamaica (1692), Java (1699), Chile (1751), Lisbon (1755), Calabria (1783), Sicily (1790), Bengal (1792), Quito (1797), and New Madrid, Missouri (1811-1812). He took any opportunity to converse with engineers and others who had been eyewitnesses and these second-hand accounts are of continuing value.

There was no surface evidence of fault rupture genesis of many of the earthquakes discussed by Lyell. We see in his writings only the beginnings of the accumulated field evidence for the uniformi- tarianism of the seismic source of most tectonic earthquakes. It is of interest that the separate classification 'volcanic earthquakes' persisted well into this century in text books. Now they are regarded as also immediately produced by sudden elastic strain release in fractured rocks around the volcanic tubes and chambers.

A number of nineteenth century earthquakes described at length in the Principles of Geology have been the subject of much recent research. We might mention the 1835 elevations along the Chilean coast (nowadays described as being associated with subduction earthquakes) and the coseismic uplift along the coast during the prototype intraplate earthquake in the Rann of Kutch, India on 6 June 1819. In the latter, land rose by up to 10 feet over an area of radius 50 miles. The woodcuts in the Principles of Geology (p. 100) showing Sundree Fort before and after this earthquake are classics. Other intraplate earthquakes in an area specially visited by Lyell are the New Madrid earthquakes of 1811 and 1812: in

March 1846 he had an opportunity to visit the disturbed region of the Mississippi embayment and talk with eye-witnesses. The main geological conclusion reached by Lyell in his study of earthquakes was the contravention of the belief that significant changes of relative levels of land and sea had ceased: 'in the face of so many striking facts, it is vain to hope that this favourite dogma will be shaken'.

His celebrated description of an 1883 Italian earthquake series (pp. 113-144), which lasted for many months, continues to have a prominent place among seismological studies. These earthquakes in Calabria were powerful enough to destroy over 180 towns and villages and kill 30 000 people. They were accompanied by many striking geological phenomena, and furnished examples of many seismic effects common to earthquakes around the world. A special importance of the 1883 Calabrian earthquakes was, as Lyell states (p. 113), that they afforded 'the first example of a region visited during and after the convulsions, by men possessing sufficient leisure, zeal and scientific information to enable them to collect and describe with accuracy such physical facts as throw light on geological questions'. Lyell relied on the extensive field report of the Neapolitan Academy of Sciences to whom goes the credit for appointing the first scientific commission to investigate a great earthquake. He also quotes D. Vivenzio, who wrote the first monograph devoted to an earthquake disaster, and the report of the French geologist D6odat Gratet de Dolomieu. Some authors (Yeats et al. 1997) suggest that Gratet de Dolomieu who described a fissure several feet wide over 10 miles along the contour margin of the Aspromote massif, may have been the first to discover surface faulting which had led to an earthquake.

It should not be overlooked that Lyell includes some of the best descriptions of widespread liquefaction in his Calabria case analysis, including drawings and descriptions of sand 'blows' and 'boils'. Typically, he does not speculate on their physical basis in terms of the modern explanation involving shear strength and pore pressure of soils. Lyell remarks that the shocks caused no eruption of either of the nearby volcanoes Etna and Stromboli. He acutely concluded that therefore the 'sources of the Calabrian convulsions and the volcanic fires of Etna and Stromboli appear to be very independent of each other'.

Causes of tectonic earthquakes

As a result of direct geological and geodetic field measurements after the 1906 San Francisco earthquake, H. F. Reid propounded an elastic rebound theory of earthquake genesis: strains build



EARTHQUAKES AND EARTH STRUCTURE 351

up in the faulted rocks until a failure point is reached; rupture then takes place in the strained rock; each side of the fault rebounds under the elastic stress field until the strain is largely or wholly relieved. On this theory there is no direct connection between volcanic activity and the sudden emission of seismic waves; rather so-called volcanic earthquakes, often in swarms, may be associated with the movement of magma in subterranean ducts from one chamber to another. Conversely, large earthquakes in a volcanic region may produce seismic P and S waves energetic enough at regional sites to stimulate volcanic activity by means of shaking of the magma in underground chambers with consequent activation of superheated steam.

The theory of plate tectonics was the first to provide a global physical reason for the uneven geographical pattern of significant seismicity around the world. In brief, it explains why most earthquakes occur along the edges of the interacting tectonic plates (interplate earthquakes), and why the Wadati-Benioff zones along the ocean trenches coincide with the plate convergence that results in crustal rocks subducting into the mantle. In addition, the convergence rates match the seismic energy budget derived from the standardized earthquake observatory catalogues of the last 50 years (Bolt 1993). These show that earthquakes at convergent plate boundaries contribute more than 90 per cent of the Earth's release of seismic energy for shallow earthquakes, as well as most of the energy for intermediate and deep-focus earthquakes (down to 680 km depth). Most of the Earth's largest earthquakes (such as the 1960 and 1985 Chile earthquakes, the 1964 Alaskan earthquake, and the 1985 Mexican earthquake) originate in subduction zones. The high rate of seismicity occurring along undersea faults along the mid-oceanic ridges (unknown to Hutton and Lyell), is the consequence of the construction of tectonic plates by volcanic processes. Collision margins (such as the Himalayas and Caucasus) also generate energetic earthquakes with thrust mechanisms.

One example must suffice to illustrate how the present geological knowledge extends far beyond Lyell's scope. He spent considerable space (Chapter 28) discussing earthquakes in New Zealand: 'in no country, perhaps, have earthquakes, or to speak more correctly, the subterranean causes to which such movements are due, been so active in producing changes of geological interest as in New Zealand.' Yet tectonics and seismogenesis in New Zealand have occasioned considerable controversy over the years and research continues on a tectonic synthesis based on regional plate tectonic models (Berryman et al. 1992).

Two great earthquakes are addressed: 19 October

1848 and 23 January 1855. Secondhand accounts of the two are given in the Principles o f Geology. Sir E Weld informed Lyell that he had seen in 1848, in the northeast South Island, fissures extending for 60 miles, striking north-northeast in a line parallel to the mountain chain. Circumstantial evidence is that at least part of the 'great rent' described by Weld was fresh displacement on the Awatere Fault. Verification is complicated by the Marlborough earthquake of 1888 (M= 7.3) which is now ascribed to rupture of the Hope fault. Like the Awatere Fault, the latter branches from the major Alpine Fault of the South Island of New Zealand, but some hundred kilometres to the south. These large seismic sources are part of a rather unusual trench-trench boundary (Yeats et al. 1997), an active transform system of which the Alpine Fault is a part, although the main trace of the Alpine Fault has not generated a major earthquake since at least 1840 when European settlement began.

The most complete description by Lyell was of the West Wairarapa earthquake in 1855, which according to Lyell was felt by ships at sea 150 miles from the coast, with a strongly shaken area estimated at 360 000 square miles, 'an area three times as large as the British Isles.' In the vicinity of Wellington (Fig. 1) in the North Island, a tract of land comprising 4600 square miles was supposed to have been 'permanently' upraised by 1-9 feet. He repeats eye-witness descriptions of changes in geomorphology. These contain what may be the first instance of observed faulting generally known (Yeats et al. 1997). Later field work by M. Ongley and others maps the Wairarapa Fault as passing in the northeast direction about 15 miles east from Wellington, which suffered serious damage in the 1855 earthquake. The source rupture is now estimated to have uplifted the country to the west with an upthrow along the fault scarp of 3-10 feet (Ongley 1943).

The New Zealand geologist Alexander McKay, carrying a copy of the Principles o f Geolog3, made an immediate field investigation of the subsequent and also damaging I September 1888 earthquake (see Richter 1958). Towards the northern end of the South Island, McKay observed new fault rupture giving freshly disturbed ground and shifting fences near the Hope River more than 2.6 m out of line, with the northern side displaced with left-lateral offsets. McKay is credited with being one of the first geologists to document strike-slip on a fault. The kinematics of the Alpine fault system accommodates the plate motion by right lateral offsets along the Alpine Fault and subsidiary faults, such as the Awatere and Hope Faults, together with shortening of geological structures to the east of the main fault system in response to the general convergence (Berryman et al. 1992).



352 B.A. BOLT

Fig. 1. Aerial photo looking northwest into Wellington Harbour, New Zealand. The Wellington Fault strikes to the west of the harbour. (Courtesy DSIR Geology and Geophysics.)

Finally, it is of interest that while Lyell was not fortunate enough to visit the scenes of earthquakes where surface faulting was evident, he did correlate the 1855 earthquake in New Zealand with what he defined as a fault. He comments (p. 88), 'The geologist has rarely enjoyed so good an opportunity of observing one of the steps by which those great displacements of the rocks called 'faults' may be brought about.' His following remarks make clear, however, that he still held to the then usual belief that fault displacement is the result of the earthquake rather than the cause of it.

Predictions of ground shaking

We find in the Principles of Geology various accounts of the intensity of seismic ground shaking, but Lyell refrains from attempting quantitative dynamical explanations. Lyell was ever wary of what he called 'the spirit of exaggeration in which the vulgar are ever ready to indulge'. Very rarely is he physically gullible and his description of the passage of strong earthquake waves in the Calabrian earthquakes is sound. An exception is his recounting a secondhand story that rents and

chasms in the ground opened and closed alterna- tively 'so that houses, trees, cattle and men were first engulfed in the instant and then the sides of the fissures coming together again no vestige of them was to be seen on the surface'. In his account of the Calabrian earthquake he mentions observations of pavement stones 'bounding into the air' and coming down with their sides reversed and he ventures a dynamical argument for the occurrence. Typically, on matters of mechanics, he quotes Mallet who had much deeper engineering knowledge.

As with the other parts of earthquake studies, it was the operation of specially-designed seismographs to record on-scale the strongest shaking in earthquakes that led to the modem advances in strong motion seismology (see Fig. 2). In the 1970s and 1980s, particularly as wave interpretation of the recorded strong seismic waves became more reliable, mathematical work began on ways to solve the basic inverse problem of strong motion seismology: namely, the prediction of strong ground motions given the seismic source and site (Bolt 1996). An associated trend is a return to the prevailing concept in Lyell's time that engineers




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and geologists share a common interest in earthquakes. It recognized that there is a significant overlap in expertise, even though it is not to be expected that seismologists and geologists will be specialists in engineering analysis or mechanics. The common physical understanding of strong ground motions and their effect on structures comes from a shared training in basic mechanics.

One geological aspect of recent studies in strong motion seismology recalls Lyell's interest in earthquake intensity distribution. It has now been demonstrated that deep crustal structure as well as surficial soil and sediments affects the distribution of the high seismic intensity around the earthquake source. The 1989 Lama Prieta earthquake (M = 6.9) in California provided perhaps the firmest yet quantitative evidence. The damage patterns and strong motion recordings showed significant spatial variations in the shaking (Lomax & Bolt 1992) around San Francisco Bay. Large coherent shear wave displacements observed at certain distances were in the period range of 2-5 s (see Fig. 2). These waves were amplified by factors of two within San Francisco and Oakland at distances of 50-60 km from the earthquake source when compared with similar wave amplitudes at much shorter distances.

Theoretical wave modelling with a realistic three- dimensional crustal structure demonstrated that the lateral refraction of the shear waves by the particular geological structures in the San Francisco Bay Area was responsible. The wave focusing was a consequence of the different rock types across the San Andreas Fault and deep sedimentary basins at the south end of San Francisco Bay. The study illustrated the present ability of realistic propa- gation path modelling to explain variations in shaking which are so clearly described in historical earthquakes by Lyell.

Global seismicity and its inferential value

The construction of the plate tectonic model of terrestrial deformation depended to a crucial extent on two seismological products: the uniform global mapping of earthquake foci and the estimation of source mechanisms. This information flowed from the punctilious earthquake surveillance work at seismographic stations around the world (see Bullen & Bolt 1985). By the 1950s, the enlightened proposals by Mallet and later by Milne had led to the Worldwide Standardized Seismographic Network of about 120 stations distributed in 60



354 B.A. BOLT

Fig. 3. Map showing the location (1996) of seismographic stations (triangles) with broadband digital instruments belonging to the Global Seismic Network. Crosses plot 979 shallow earthquakes with M > 5.7 occurring during 1988-1994 (Astiz et al. 1996).

countries (see Fig. 3). Global earthquake measurements, together with the concurrent introduction of high-speed digital computers, meant that seismologists were in the fight place at the fight time (see Oliver 1996). Precise hypocentre locations, magnitude estimates and source mechanisms became almost routine. By 1960 it was possible to have a broad global classification of seismogenesis caused by crustal convergence and divergence. These results were basic to the development of plate tectonics. Some peculiarities, however, remained and remain for additional detailed investigation (Yeats et al. 1997). Why on the plate tectonic description do many earthquakes, including major damaging ones, occur far from plate boundaries? Indeed, hinterland seismic activity occurs in all continents except Greenland (Bolt 1993). As mentioned already, Lyell described a number of these intraplate earthquakes.

There is one special class of earthquake which continues to attract research ingenuity. Deep-focus earthquakes have sources well below the crust (the deepest have foci at depths of about 680 km) and hypocentres within or along the boundary of the relatively cold subduction slab. The great pressures at such depths (up to 240 kbars) make it difficult to envisage their genesis by elastic rebound along a discontinuous surface in brittle rocks. Various alternative mechanisms have been suggested to settle this longtime dilemma, such as sudden dilatational change in volume of the rocks, perhaps from a sudden change in the phase state of the con-

stituent minerals. Another requires that the water of crystallization becomes mobilized at the high ambient temperatures and pressures and migrates throughout the rock pores; fractures are thus lubricated, allowing slip to become coherent. A third is that the phase transitions are localized to boundaries between rock lenses, where fluid conditions are particularly favourable to sudden transitions. Along these pre-existing grain boundaries, the crystal structures change rapidly, thus weakening the bonds across the discontinuities.

It is instructive to consider the extraordinary deep earthquake that occurred on 9 June 1994 at a depth of 637 km within the Nasca subduction slab under Bolivia. This earthquake (M = 8.3) is the largest earthquake at these extreme depths ever recorded. Strong ground motion was felt over much of Bolivia, but there were no deaths and relatively minor damage. One remarkable feature was that 10 to 20 minutes after the deep energy release, seismic waves were felt by many people in the Caribbean and in North American cities as distant as Chicago, Illinois and Toronto. The location of the focus placed it in 'the grand jog' of the subduction slabs between the Wadati-Benioff zone under Argentina near southern Bolivia and the zone under Peru and Brazil. These zones are nearly parallel, but offset from each other by 1000 km in the vicinity of this earthquake focus.

Cross-correlation between seismic wave forms recorded at a number of the newly positioned global digital seismographic stations (see Fig. 3)




now allows theoretical wave forms computed from fault models to be compared directly with the observations. An analysis of this type (Antolik 1996) demonstrated that the source mechanism for this very deep earthquake source (with magnitude exceeding that of the 1906 San Francisco earthquake) is the same as that for shallower earthquakes in the crust. The solution in this study was a near- horizontal nodal plane (strike 302 degrees, dip 11 degrees), an average rupture velocity of 1.7 km/s over a roughly ellipsoidal area with major axis 12 km long. The average slip throughout the rupture area was about 5 m with a peak value of 16.5 m. Unfortunately, no uncertainties on these values are given and the solution is not unique.

The subduction slabs defined by deep-focus earthquakes bear in a critical way on one of the foremost geophysical questions, extending back to at least Lyell's time (Brush 1979), namely, the effect of viscous hydrodynamical conditions in the Earth's mantle. In recent times, the kinematics of plate tectonics led to the adoption of large-scale mantle convection as the driving force for subduction, mountain building, volcanic activity, and major earthquake genesis. The older question of rigidity versus fluidity (Brush 1979) shifted to a long-running debate on whether the mantle convects as a whole or whether it convects in a two shell system.

Recent special studies of earthquake waves have indicated a way to discriminate between the two models. Statistical reanalysis of the standard catalogues of earthquake locations and, hence,

reported travel times has provided a significantly more precise sample of seismic travel times for tomographic imaging. From such a database, inversions to the three-dimensional spherical structure of the mantle paths have indicated linear patterns of velocity anomalies which extend down- wards from subduction slabs in the lithosphere (Van der Hilst et at. 1997). These linear patterns that map regions of relatively fast seismic waves can be interpreted as slabs extending at least 1000km below the 400 km boundary in the upper mantle (see Table 1) at places around the Pacific. The consequence of the confirmation of such models would be that subduction slabs sink to the core boundary. There they may accumulate and perhaps remelt, mix and rise again as molten plumes to the surface.

E a r t h q u a k e p r e d i c t i o n

The hallmark of a scientific theory is its predictive power. Indeed, the attraction of the Newtonian dynamical theory, modified appropriately for relativity, and of the Darwinian theory of evolution are their ability to forecast beyond the available observations and descriptions. From early times, a belief in forerunners to large earthquakes and reported eye-witness accounts of them have been recorded. Lyell only refers to such forerunners in passing (p. 81). He mentions reported irregularities in the seasons preceding shocks, animals evincing extraordinary alarm, violent rains and other

Table 1. Bullen's 1942 specification of average internal shells of the Earth

Region Level Depth (km)

- Outer surface A

Base of crustal layers - (distance R from the 33

Earth's centre) B

- 0.94R 413 C - 0.85R 984* D - 0.548R = R 1 2898 E - 0.40R 1 4982 F - 0.36R 1 5121 G - Earth's centre 6371

Features of region

Crustal layers

Steady positive P and S velocity gradients

Transition region*

Steady positive P and S velocity gradients

Steady positive P velocity gradient

Negative P velocity gradient +

Small positive P velocity gradient

* Now estimated to be 660 km. In 1962 shown incorrect (Bolt 1982).



356 B.A. BOLT

occurrences. Nevertheless, he confines his text 'almost entirely to the changes brought about by earthquakes and in the configurations of the Earth's crust'.

It is generally accepted that a serious earthquake prediction must also specify the location, origin time, and magnitude within specific known limits. Such certainty is impossible to justify because prediction is always based on a limited number of measurements, themselves imprecise. Conse- quently, scientific predictions must state the probability of the occurrence.

A great amount of research has been carried out on earthquake prediction in a number of countries, particularly Japan, the United States, the Soviet Union, China, and Italy in recent decades (Mogi 1985). Comparison of the earthquake predictions published in earthquake journals with the actual seismological record indicates no proclaimed method can be taken as proved or effective. In my view, the failure of the modern research on earthquake prediction raises doubt whether forecasts within strict bounds of time and place will ever be possible for most earthquakes, particularly the large damaging ones. Two contemporary cases give the flavour of the present hiatus in prediction pro- grammes. The first was the inability of the Japanese seismological prediction programme, often regarded as a model, to give forewarnings of the devastating Kobe earthquake of 17 January 1995. The failure was exacerbated by the numerous public statements made over decades in Japan that 'precursors' had been recorded after earlier damaging earthquakes (Geller 1997). For example, within hours of the Kobe earthquake, there were announcements from some seismological quarters that instruments had recorded several small earthquakes with almost the same epicentre as the main shock, a few hours to weeks before it. There are, however, no scientific grounds that allowed any of these small earthquakes to be identified in advance as a forerunner of a major earthquake. Sharp criticisms in Japan of such hindsights based on coincidence were reflected in the public press.

The second illustration comes from ongoing debate of the validity of the predictions of earthquakes in Greece (Varotsos et al. 1996). These predictions used variations in the electrical field within a specified region to trigger alarms. The proponents argue strongly that their predictions are more successful than those provided by chance coincidences. For example, between 1987 and 1989 the number of events with magnitudes greater or equal to 4.7 was 39. During this interval, the method yielded 23 predictions with a claimed success of 38 per cent. The nominal duration of the alarm period was 23 days. There are many critics of these claims (e.g. Mulargia & Gasperini 1992).

Some doubt the instrumental ability to even detect, above the ambient electrical noise, the tiny electro- magnetic variations due to static elastic straining in the seismogenic area; substantial electric signals of all kinds are prevalent in modern populated areas. Another evaluation approach is to accept the published observed correlations and subject them to strict statistical tests (Stark 1996, 1997). The most detailed tests along these lines to date indicate that the success of the claimed predictions is unsup- ported by the evidence.

My own inclination is to be skeptical of most prediction claims based on 'abnormal' seismicity, geodetic, geological or geophysical observations, or unexpected deviations from average regional parameters. One source of doubt is that the physical genesis of tectonic earthquakes denies the assumption that origin times, locations, and magnitudes of earthquakes are jointly independent. Most statistical analyses of seismicity catalogues assume the constant rate of a memoryless Poisson distribution; yet the elastic rebound theory states that there is an evolution of the stress field, interrupted by sudden strain decrease at the time of fault slip (Bullen & Bolt 1985). So, too, after the substantial rupture of a fault, like the San Andreas fault in California, the frictional distribution on the contiguous fault surfaces is likely to be irrevocably changed so that the traction memory is lost or weak. Although the mechanism of earthquake genesis is understood physically, the concomitant geological complex- ities and remoteness of the rupture surfaces have limited prediction abilities to a stage not much beyond the time of Lyell (see Geller 1997).

The three-dimensional geological structure of the Earth

Although many of his case histories describe large seismic events in regions where there are no volcanoes, Lyell continued to link them together. 'The regions convulsed by violent earthquakes include within them the site of all the active volcanoes. Earthquakes sometimes local, sometimes extending over vast areas, often precede volcanic eruptions.' He argues that the common cause is the passage of heat from the interior to the Earth's surface. His discussion of the physical condition of the Earth's interior uses arguments from mechanics, such as experiments with pendulums and the attraction of the Earth to the Moon. These investigations 'have shown that our planet is not an empty sphere, but on the contrary that its interior, whether solid or fluid, has a higher specific gravity than the exterior'. He also accepts that vibrations of the Moon indicate that the Earth




has an increase of density from the surface towards the centre, the average value being about 5.5. It is actually 5.52 g/cm 3. He quotes Young on the effect of compression at the Earth's centre, which would compress steel into a quarter of its volume using the assumption that a terrestrial nucleus may be metallic with a specific gravity of iron of about 7. He is acute in judging that such extrapolations are uncertain because compressibilities of bodies differ from values obtained in the laboratory.

Finally, Lyell adopts the notion of a solid crust and refers to the effect of the attraction of the Moon on the Earth's precession. Lyell admitted that the inside of the Earth was hot, but in accordance with his uniformitarian view, denied its temperature had changed significantly in the course of geological history (pp. 210-240). He differed from the argument for an Earth model with a solid shell over a liquid nucleus, favoured by William Hopkins, the founder of British physical geology, who often opposed the uniformitarian school (see Fisher 1889). Lyell points out that theoretical precessional orbits do not agree with the astronomical observations Hopkin's relied upon unless the minimum thickness of the crust of the globe 'cannot be less than 1/4 or 1/5 of the Earth's radius'. In fact, the rigid mantle is about one half of the radius. Debates of this kind defined the status of global Earth structure at the end of Lyell's life (Brush 1979). Seismological tomography (Iyer 8,: Hirahara 1993) was needed to estimate the detailed elastic structure and physical properties of the Earth's interior.

The worldwide network of seismographic stations allowed the production of standard travel times of the main seismic body waves through the Earth. The most notable estimates were obtained by Jeffreys and Bullen in 1940. These tables (Fig. 4) provided the solution of two basic geophysical inverse problems. Problem 1: if the location of an earthquake's epicentre is known, times of travel to observatories at any site can be calculated; the actual problem is to locate an epicentre anywhere in the world using readings of arrival times of earthquake waves at observatories. Problem 2: if the seismic wave velocity distribution at each depth in the Earth is known, one can calculate the travel time T(D) to distance D. The inverse question is: given T(D), calculate the variation in velocity v(r) in the Earth.

The use of the Jeffreys-Bullen travel-time tables in solving Problem 2 provided robust estimates of the unknown velocity structure but on the assumption of radial symmetry (Bullen & Bolt 1989). Their construction, accompanied by application of probability and inference theory, was so successful that the tables have been used widely up to today even though regional anomalies in the average

times were known and were shown to be correlated with deviations from interior radial symmetry.

Estimation of the variation of seismic wave velocities, Vp and V s, within the Earth provided the key to structure (Fig. 5). Bullen went on to define a nomenclature for radial shells based on the velocities (Bullen & Bolt 1985). The shells range from layer A, the crust, to layer G, the inner core of the Earth (see Table I). The layers B and C incorporate what has become known as the upper mantle of the Earth where the largest deviations from spherical symmetries occur. The layer D is the solid lower mantle and E the liquid outer core.

As more earthquake observatories with better equipment were put into operation, particularly the broadband digital seismographs of the 1980s and 1990s, regional deviations from the radial depen- dence became more sharply focused. These deviations entailed significant heterogeneities in deep Earth structure, some of which correlate well with surface geology. (Three-dimensional maps of this heterogeneous structure really require colour projections of the globe (see for example Dziewonski & Woodhouse 1987; Bolt 1993).) Nevertheless, deviations from spherical symmetry become less marked, and are generally of the second order, with depth because of the strong influence of pressure on the mineralogy of the rocks. Below a low velocity lithosphere layer in which S and perhaps P velocities decrease some- what, the velocities increase by about a percent or so down to depth of 220 kin, often called the Lehmann discontinuity, where there is a jump of about 3 per cent. The depth of this discontinuity varies from region to region, and it is not detected in some studies in some places. The 410 and 660 km depth discontinuities are features of the upper mantle that have been observed worldwide. They have been confirmed by underside reflections which occur as precursors to the reflective core waves, although summarizing the structural trends of these discontinuities has proved difficult.

The next zone of interest is near the mantle-core boundary (MCB). High frequency (about 1 Hz) P and S waves are both recorded that are reflected from the upper side of the MCB; P waves are also reflected from its underside. These common observations are convincing evidence that the MCB is quite sharp. In the last seven years, much effort has been made to map the structural details of the layer called D", at the mantle base. Bullen differentiated this sub-shell because the 1939 Jeffreys P and S velocity curves flattened for about 200 km above the core boundary. In fact, the constant slope was an artifact of the smoothing by Jeffreys of the inversion process, but the suggestion led to attempts to find more direct observational evidence for a transition shell. This partly comes from



358 B.A. BOLT

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PKPPK$

PKPPKP

Pggs

sr~s

.c 2s

. _

i . - -

PKIKP

I .. L I I _ _ I l 0 20 40 60 80 I00 i20

360 340 320 300 280 260 240 Distance (degrees o f a r c )

L I ...1 140 160 180 220 200

Fig. 4. Travel-time curves of various seismic wave types for a surface source to various angular distances on the Earth's surface (after Jeffreys & Bullen 1940).

diffracted earthquake waves (of both P and SH types), which run around the mantle core boundary (Bolt, 1982). It is now known that a radially spherical inversion for D" structure does not yield a realistic description. There are relatively large- scale deviations in elastic properties within the layer, which affect waves according to their wave length. These azimuth and wave-length dependent effects are difficult to tie down and a number of competing solutions have been given. The most recent studies of high frequency core waves appear to require heterogeneous (non-radially symmetric) structures both in D" and well above this layer in the mantle (Hedlin et al. 1997).

Future work

At present, the research situation in seismology is excellent. The worldwide network of broadband digital stations and the easy exchange of digital recordings of earthquakes through the Incorporated Research Institutions for Seismology (IRIS) programme has made accessible to researchers, in a straightforward way, very high quality seismic wave-form data. Even the debilitating uneven sampling for tomographic studies of the deep interior due to the predominantly continental locations of observatories is being addressed. The next decade will see more ocean-bottom seismographic stations at critical distances.




Fig, 5. A cross-section of a radially symmetric Earth model based on seismological evidence. The paths taken by three major kinds of earthquake waves are shown (Bolt 1982).

The digital earthquake data of the present seismographic networks are easily downloaded onto personal computers. On the interpretation side, there can be a drawback. The paper photographic records of yesteryear, that displayed 24 hour signals continuously, enabled students to examine the whole of an earthquake record, including the back- ground noise in which it is embedded. Because the present methods tend to be restricted to short runs of the seismic evolutary time series, a narrow focus on the expected wave forms results rather than scanning for abnormal observations.

The picking by eye of the first onsets of seismic waves recorded on seismograms, such as P, S and their multiples, was the major tool at earthquake observatories to provide the basic data for the construction of travel-time curves for waves throughout the Earth's interior. A problem was always that at many observatories the selection of phases was highly correlated with the available charts of travel-time curves. Readers sought to find the appropriate identification by reference to available curves such as Jeffreys-BuUen, rather than picking values using the assumption of ignorance. More and more in the last decades, reported readings in the seismological catalogues were

therefore biassed to the better known branches of the travel-time tables (see Fig. 4).

An alternative method now coming into vogue is to make use of the wave forms on the seismograms, rather than picking the first wave onsets. The idea is to superimpose ('stack') the digital seismograms provided by the global digital networks at common source-receiver distances and to average the result to obtain a composite record. Figure 6 gives an example from a recent paper that discusses the procedure (Astiz et al. 1996). The figure shows the stacking of transverse components of the recorded seismic ground motions after the signals were passed through a 30 s low-pass filter before the superposition was made. In this case, the procedure enhances the horizontally polarized shear waves (SH) passing through the Earth relative to the compressional P waves. It is instructive to compare the concentration of wave energy on this plot with the predicted theoretical times for the S and P phases in Fig. 4. Outstanding problems on Earth structure may achieve better resolution using such methods. An example of such a problem is the need to obtain a sharp resolution of the longitudinal velocity in the upper part of the outer core (Murtha 1984).



360 B.A. BOLT

Fig. 6. A composite photograph produced by superimposing (stacking) over 33 000 digital seismograms. Only the transverse components of the wave motions have been used after 30 s low-pass filtering of each trace (Astiz et al. 1996).

Of course, much more than the velocity structure of the Earth's interior can be inferred from earthquake wave probes of the Earth. The attenuation of earthquake waves with distance gives a direct measure of the non-elastic (viscous) properties of the interior rocks. Wave polarization allows their anisotropic elastic parameters to be estimated. Strict bounds are also placed on the density distribution by considering the complete seismic response spectrum of the Earth. For example, from short period reflections (see Bolt 1991), it was demonstrated in 1970 that the average inner core density could not exceed about 14 g/cm 3.

There is much reason to be grateful to Hutton and, particularly, Lyell for their recognition of the crucial evidence on global tectonic processes provided by earthquakes. The geologically ephemeral duration of earthquakes did not preclude them informing the past. Lyell had no profound

theories on the Earth's interior dynamics and his views on the immediate causal link between volcanoes and earthquakes were erroneous. Never- theless, his emphasis on the value of field studies immediately after earthquakes and his judicious description of them remain important educationally even today.

I am grateful to D. Drager and E Shearer for assistance with figures.

References

ASTIZ, L., EARLE, P. & SHEARER, P. 1996. Global stacking of broadband seismograms, Seismological Research Letters, 67, 8-18.

ANTOLm, M. S. 1996. New Results from Studies of Three Outstanding Problems in Local, Regional, and Global Seismology. PhD thesis. University of California, Berkeley.




BAILEY, E. B. 1967. James Hutton - the Founder of Modern Geology. Elsevier, Amsterdam.

BERRYMAN, K. R., BEANLAND, S., COOPER, A., CUTIEN, H., NORRIS, R. & WOOD, E 1992. The Alpine fault, New Zealand: variation of quaternary structural style and geomorphic expression. Annalis Tectonicae, (Suppl.), 6, 126-163.

BOLT, B. A. 1982. Inside the Earth. W. H. Freeman, New York.

- - 1991. The precision of density estimation deep in the Earth. Quarterly Journal of Royal Astronomical Society, 32, 367-388. 1993. Earthquakes and Geological Discovery.

Scientific American Library, New York. 1996. From earthquake acceleration to seismic

displacement, the Fifth Mallet-Milne Lecture. Society for Earthquake and Civil Engineering Dynamics. John Wiley & Sons, Chichester.

BRUSH, S. G. 1979. Nineteenth century debates about the inside of the Earth: solid, liquid or gas? Annals of Science, 35, 225-254.

BULLEN, K. E. & BOLT, B. A. 1989. Introduction to the Theory of Seismology. 4th edn. Cambridge University Press, Cambridge, 1985.

DZIEWONSKI, A. M. & WOODHOUSE, J. H. 1987. Global images of the Earth's interior. Science, 236, 37-48.

FISHER, O. 1889. Physics of the Earth's Crust. Macmillan, London.

GELLER, R. J. 1997. Predictable publicity, astronomy and geophysics. Journal of the Royal Astronomical Society, 38, 1 6-18.

HEDLIN, M. A. H., SHEARER, P. M. & EARLE, P. S. 1997. Seismic evidence for small-scale heterogeneity throughout the Earth's mantle. Nature, 387, 145-150.

IYER, H. M. & HIRAHARA, K. (eds) 1993. Seismic Tomography: Theory and Practice. Chapman & Hall, London.

JEFFREYS, H. & BULLEN, K. E. 1940. Seismological Tables. British Association Gray-Milne Trust, Edinburgh.

LOMAX, A. & BOLT, B. A. 1992. Broadband waveform modelling of anomalous strong ground motion in the 1989 Loma Prieta earthquake using three- dimensional geologic structures. Geophysical Research Letters, 19, 1963-1966.

LYELL, C. 1875. Principles of Geology, 12th edn. 4 vols, London.

MoGI, K. 1985. Earthquake Prediction. Academic Press, Tokyo.

MULARGIA, E & GASPERINI, P. 1992. Evaluating the statistical validity of 'VAN' earthquake precursors. Geophysical Journal International, 111, 32--44.

MURTHA, P. E. 1984. Seismic Velocities in the Upper Part of the Earth's Core. PhD thesis. University of California, Berkeley.

OLIVER, J. 1996, Shocks and Rocks: Seismology in the Plate Tectonics Revolution. American Geophysical Union, Washington.

ONGLEY, M. 1943. Surface trace of the 1855 earthquake. New Zealand Transactions, 73, 84--89.

RICHTER, C. 1958. Elementary Seismology. W. H. Freeman, New York.

STARK, P. B. 1996. A few considerations for ascribing statistical significance to earthquake predictions. Geophysics Research Letters, 23, 1399-1402.

STARK, P. B. 1997. Earthquake prediction: the null hypothesis. Geophysical Journal International, (in press).

VAN DER HILST, R. D., WIDIYANTORO, S, t~ ENGDAHL, E. R. 1997. Evidence for deep mantle circulation from global tomography. Nature, 386, 578-584.

VAROTSOS, P., EFTAXIAS, K., VALLIANATOS, F. & LAZARIDOU, M. 1996. Basic principles for evaluating an earthquake prediction method. Geophysical Research Letters, 23, 1295-1298.

YEATS, R. S., SIEH, K. & ALLEN, C. R. 1997. The Geology of Earthquakes. Oxford University Press, Oxford.



Earthquakes and Earth structure: a perspective since Hutton and Lyell

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