Earthquakes and Earth Structure: a Perspective Since Hutton and Lyell

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Earthquakes and Earth structure: a perspective since Hutton and Lyell

BRUCE 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 major ongoing problems in seismology. In tracing Geology (1875) contains considerable descriptive the historical evolution of knowledge in seismology material on earthquakes and links them with uplift and related tectonics from Lyell's day, I have been and other deformation of the Earth's surface. Only forced to select only four central topics on eight years after its publication, Professor John earthquakes: their tectonic causes; their wave Milne, then working in Japan, surmised (see Bolt motion: their prediction in time and location; and 1993) that 'it was not unlikely that every large their use to image the three-dimensional structure earthquake might with proper appliances be of the deep interior. Even these subjects, each of recorded at any point of the globe'. interest to my own research, must be considered This prediction was fulfilled in 1889 by the very briefly, with a narrow focus on recent debates. German physicist E. Von Rebeur Paschwitz, who My textual reference to Lyell's writings is, for 'was struck by the coincidence in time' between the brevity only, the twelfth (and last) edition of arrival of singular waves which were registered by Principles of Geology. Each successive edition of delicate horizontal pendulums at Potsdam and this seminal treatise incorporated 'important Wilhelmshaven in Germany and the time of a additions and corrections'. Nevertheless, Lyell damaging earthquake that shook Tokyo at 2:07 am comments that although between the first and Greenwich Mean Time on 18 April. His conclusion twelfth editions numerous descriptions of recent was that 'the disturbances that were noticed in earthquakes had been published, he doubted that Germany were really due to the earthquake in they illustrated new principles. Tokyo'. The significance of this identification - an early example of remote sensing - was that earth- Lyell's accounts of earthquakes quakes in inhabited and uninhabited parts of the world alike could be monitored uniformly, and thus James Hutton wrote little on earthquakes (Bailey patterns of geological activity could be mapped 1967). He did describe processes that had led to without bias; an era in the quantitative study of land surfaces above the sea surface. He concluded earthquakes and geology not known to Lyell then that 'the land in which we dwell' has been elevated began. 'by extreme heat and expanded with amazing A principal aim of this paper is to provide force'. This belief led in turn to a consideration of historical illustrations and a short commentary on volcanoes, active and extinct, with slight reference

BOLT, B. A. 1998. Earthquakes and Earth structure: a perspective since Hutton and Lyell. 349 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. Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021

350 B.A. BOLT to earthquakes, neither of which were within his March 1846 he had an opportunity to visit the personal experience. He considered volcanic disturbed region of the Mississippi embayment and eruptions to be safety valves 'in order to prevent the talk with eye-witnesses. The main geological unnecessary elevation of land and fatal effect of conclusion reached by Lyell in his study of earthquakes'. earthquakes was the contravention of the belief that In contrast, Lyell emphasized the value of earth- significant changes of relative levels of land and quake studies for geology. In the twelfth edition of sea had ceased: 'in the face of so many striking the Principles he discusses volcanoes and earth- facts, it is vain to hope that this favourite dogma quakes as constructive forces. These accounts will be shaken'. reflect the prevailing view of a common underlying His celebrated description of an 1883 Italian cause and intimate physical connections. Neverthe- earthquake series (pp. 113-144), which lasted for less, they still read well today, with many case many months, continues to have a prominent place histories and arguments based on the very limited among seismological studies. These earthquakes in geophysical measurements available. Lyell begins Calabria were powerful enough to destroy over 180 by regretting the deficiency of accounts of ancient towns and villages and kill 30 000 people. They earthquakes, almost all descriptions being restricted were accompanied by many striking geological to damage and injury. His interest was in the phenomena, and furnished examples of many geological aspect of earthquakes, particularly the seismic effects common to earthquakes around the coseismic changes in the Earth's crust that world. A special importance of the 1883 Calabrian accompanied them. earthquakes was, as Lyell states (p. 113), that they By 1875, Lyell had available reports by Robert afforded 'the first example of a region visited Mallet and the catalogues of Alexis Perry and during and after the convulsions, by men others. There is little doubt that he developed a possessing sufficient leisure, zeal and scientific strong interest in seismology and he summarized information to enable them to collect and describe published reports of major earthquakes in such with accuracy such physical facts as throw light on widely distributed places as Jamaica (1692), Java geological questions'. Lyell relied on the extensive (1699), Chile (1751), Lisbon (1755), Calabria field report of the Neapolitan Academy of Sciences (1783), Sicily (1790), Bengal (1792), Quito (1797), to whom goes the credit for appointing the first and New Madrid, Missouri (1811-1812). He took scientific commission to investigate a great any opportunity to converse with engineers and earthquake. He also quotes D. Vivenzio, who wrote others who had been eyewitnesses and these the first monograph devoted to an earthquake second-hand accounts are of continuing value. disaster, and the report of the French geologist There was no surface evidence of fault rupture D6odat Gratet de Dolomieu. Some authors (Yeats et genesis of many of the earthquakes discussed by al. 1997) suggest that Gratet de Dolomieu who Lyell. We see in his writings only the beginnings of described a fissure several feet wide over 10 miles the accumulated field evidence for the uniformi- along the contour margin of the Aspromote massif, tarianism of the seismic source of most tectonic may have been the first to discover surface faulting earthquakes. It is of interest that the separate which had led to an earthquake. classification 'volcanic earthquakes' persisted well It should not be overlooked that Lyell includes into this century in text books. Now they are some of the best descriptions of widespread regarded as also immediately produced by sudden liquefaction in his Calabria case analysis, including elastic strain release in fractured rocks around the drawings and descriptions of sand 'blows' and volcanic tubes and chambers. 'boils'. Typically, he does not speculate on their A number of nineteenth century earthquakes physical basis in terms of the modern explanation described at length in the Principles of Geology involving shear strength and pore pressure of soils. have been the subject of much recent research. We Lyell remarks that the shocks caused no eruption of might mention the 1835 elevations along the either of the nearby volcanoes Etna and Stromboli. Chilean coast (nowadays described as being He acutely concluded that therefore the 'sources of associated with subduction earthquakes) and the the Calabrian convulsions and the volcanic fires of coseismic uplift along the coast during the Etna and Stromboli appear to be very independent prototype intraplate earthquake in the Rann of of each other'. Kutch, India on 6 June 1819. In the latter, land rose by up to 10 feet over an area of radius 50 miles. The Causes of tectonic earthquakes woodcuts in the Principles of Geology (p. 100) showing Sundree Fort before and after this As a result of direct geological and geodetic field earthquake are classics. Other intraplate earth- measurements after the 1906 San Francisco quakes in an area specially visited by Lyell are the earthquake, H. F. Reid propounded an elastic New Madrid earthquakes of 1811 and 1812: in rebound theory of earthquake genesis: strains build Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021

EARTHQUAKES AND EARTH STRUCTURE 351 up in the faulted rocks until a failure point is 1848 and 23 January 1855. Secondhand accounts of reached; rupture then takes place in the strained the two are given in the Principles of Geology. Sir rock; each side of the fault rebounds under the E Weld informed Lyell that he had seen in 1848, in elastic stress field until the strain is largely or the northeast South Island, fissures extending for wholly relieved. On this theory there is no direct 60 miles, striking north-northeast in a line parallel connection between volcanic activity and the to the mountain chain. Circumstantial evidence is sudden emission of seismic waves; rather so-called that at least part of the 'great rent' described by volcanic earthquakes, often in swarms, may be Weld was fresh displacement on the Awatere Fault. associated with the movement of magma in Verification is complicated by the Marlborough subterranean ducts from one chamber to another. earthquake of 1888 (M= 7.3) which is now Conversely, large earthquakes in a volcanic region ascribed to rupture of the Hope fault. Like the may produce seismic P and S waves energetic Awatere Fault, the latter branches from the major enough at regional sites to stimulate volcanic Alpine Fault of the South Island of New Zealand, activity by means of shaking of the magma in but some hundred kilometres to the south. These underground chambers with consequent activation large seismic sources are part of a rather unusual of superheated steam. trench-trench boundary (Yeats et al. 1997), an The theory of plate tectonics was the first to active transform system of which the Alpine Fault provide a global physical reason for the uneven is a part, although the main trace of the Alpine Fault geographical pattern of significant seismicity has not generated a major earthquake since at least around the world. In brief, it explains why most 1840 when European settlement began. earthquakes occur along the edges of the interacting The most complete description by Lyell was of tectonic plates (interplate earthquakes), and why the West Wairarapa earthquake in 1855, which the Wadati-Benioff zones along the ocean trenches according to Lyell was felt by ships at sea 150 miles coincide with the plate convergence that results in from the coast, with a strongly shaken area crustal rocks subducting into the mantle. In estimated at 360 000 square miles, 'an area three addition, the convergence rates match the seismic times as large as the British Isles.' In the vicinity of energy budget derived from the standardized Wellington (Fig. 1) in the North Island, a tract of earthquake observatory catalogues of the last 50 land comprising 4600 square miles was supposed to years (Bolt 1993). These show that earthquakes at have been 'permanently' upraised by 1-9 feet. He convergent plate boundaries contribute more than repeats eye-witness descriptions of changes in 90 per cent of the Earth's release of seismic energy geomorphology. These contain what may be the for shallow earthquakes, as well as most of the first instance of observed faulting generally known energy for intermediate and deep-focus earthquakes (Yeats et al. 1997). Later field work by M. Ongley (down to 680 km depth). Most of the Earth's largest and others maps the Wairarapa Fault as passing in earthquakes (such as the 1960 and 1985 Chile the northeast direction about 15 miles east from earthquakes, the 1964 Alaskan earthquake, and the Wellington, which suffered serious damage in the 1985 Mexican earthquake) originate in subduction 1855 earthquake. The source rupture is now zones. The high rate of seismicity occurring along estimated to have uplifted the country to the west undersea faults along the mid-oceanic ridges with an upthrow along the fault scarp of 3-10 feet (unknown to Hutton and Lyell), is the consequence (Ongley 1943). of the construction of tectonic plates by volcanic The New Zealand geologist Alexander McKay, processes. Collision margins (such as the carrying a copy of the Principles of Geolog3, made Himalayas and Caucasus) also generate energetic an immediate field investigation of the subsequent earthquakes with thrust mechanisms. and also damaging I September 1888 earthquake One example must suffice to illustrate how the (see Richter 1958). Towards the northern end of the present geological knowledge extends far beyond South Island, McKay observed new fault rupture Lyell's scope. He spent considerable space giving freshly disturbed ground and shifting fences (Chapter 28) discussing earthquakes in New near the Hope River more than 2.6 m out of line, Zealand: 'in no country, perhaps, have earthquakes, with the northern side displaced with left-lateral or to speak more correctly, the subterranean causes offsets. McKay is credited with being one of the to which such movements are due, been so active in first geologists to document strike-slip on a fault. producing changes of geological interest as in New The kinematics of the Alpine fault system Zealand.' Yet tectonics and seismogenesis in New accommodates the plate motion by right lateral Zealand have occasioned considerable controversy offsets along the Alpine Fault and subsidiary faults, over the years and research continues on a tectonic such as the Awatere and Hope Faults, together with synthesis based on regional plate tectonic models shortening of geological structures to the east of the (Berryman et al. 1992). main fault system in response to the general Two great earthquakes are addressed: 19 October convergence (Berryman et al. 1992). Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021

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 chasms in the ground opened and closed alterna- fortunate enough to visit the scenes of earthquakes tively 'so that houses, trees, cattle and men were where surface faulting was evident, he did correlate first engulfed in the instant and then the sides of the the 1855 earthquake in New Zealand with what he fissures coming together again no vestige of them defined as a fault. He comments (p. 88), 'The was to be seen on the surface'. In his account of the geologist has rarely enjoyed so good an opportunity Calabrian earthquake he mentions observations of of observing one of the steps by which those great pavement stones 'bounding into the air' and displacements of the rocks called 'faults' may be coming down with their sides reversed and he brought about.' His following remarks make clear, ventures a dynamical argument for the occurrence. however, that he still held to the then usual belief Typically, on matters of mechanics, he quotes that fault displacement is the result of the Mallet who had much deeper engineering know- earthquake rather than the cause of it. ledge. As with the other parts of earthquake studies, it Predictions of ground shaking was the operation of specially-designed seismographs to record on-scale the strongest shaking in We find in the Principles of Geology various earthquakes that led to the modem advances in accounts of the intensity of seismic ground shaking, strong motion seismology (see Fig. 2). In the 1970s but Lyell refrains from attempting quantitative and 1980s, particularly as wave interpretation of dynamical explanations. Lyell was ever wary of the recorded strong seismic waves became more what he called 'the spirit of exaggeration in which reliable, mathematical work began on ways to solve the vulgar are ever ready to indulge'. Very rarely is the basic inverse problem of strong motion he physically gullible and his description of the seismology: namely, the prediction of strong passage of strong earthquake waves in the ground motions given the seismic source and site Calabrian earthquakes is sound. An exception is his (Bolt 1996). An associated trend is a return to the recounting a secondhand story that rents and prevailing concept in Lyell's time that engineers Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021

EARTHQUAKES AND EARTH STRUCTURE 353

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TREASURE ISLAND (98 km from the epicenter)

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Fig. 2. Comparison of strong ground accelerations recorded at Treasure Island (soil) and Yerba Buena Island (rock) in the 1989 Lama Prieta earthquake, California.

and geologists share a common interest in Theoretical wave modelling with a realistic three- earthquakes. It recognized that there is a significant dimensional crustal structure demonstrated that the overlap in expertise, even though it is not to be lateral refraction of the shear waves by the partic- expected that seismologists and geologists will be ular geological structures in the San Francisco Bay specialists in engineering analysis or mechanics. Area was responsible. The wave focusing was a The common physical understanding of strong consequence of the different rock types across the ground motions and their effect on structures comes San Andreas Fault and deep sedimentary basins at from a shared training in basic mechanics. the south end of San Francisco Bay. The study One geological aspect of recent studies in strong illustrated the present ability of realistic propa- motion seismology recalls Lyell's interest in gation path modelling to explain variations in earthquake intensity distribution. It has now been shaking which are so clearly described in historical demonstrated that deep crustal structure as well as earthquakes by Lyell. surficial soil and sediments affects the distribution of the high seismic intensity around the earthquake Global seismicity and its inferential value source. The 1989 Lama Prieta earthquake (M = 6.9) in California provided perhaps the firmest yet The construction of the plate tectonic model of quantitative evidence. The damage patterns and terrestrial deformation depended to a crucial extent strong motion recordings showed significant spatial on two seismological products: the uniform global variations in the shaking (Lomax & Bolt 1992) mapping of earthquake foci and the estimation of around San Francisco Bay. Large coherent shear source mechanisms. This information flowed from wave displacements observed at certain distances the punctilious earthquake surveillance work at were in the period range of 2-5 s (see Fig. 2). These seismographic stations around the world (see waves were amplified by factors of two within San Bullen & Bolt 1985). By the 1950s, the enlightened Francisco and Oakland at distances of 50-60 km proposals by Mallet and later by Milne had led to from the earthquake source when compared with the Worldwide Standardized Seismographic similar wave amplitudes at much shorter distances. Network of about 120 stations distributed in 60 Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021

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 measure- stituent minerals. Another requires that the water of ments, together with the concurrent introduction of crystallization becomes mobilized at the high high-speed digital computers, meant that seis- ambient temperatures and pressures and migrates mologists were in the fight place at the fight time throughout the rock pores; fractures are thus (see Oliver 1996). Precise hypocentre locations, lubricated, allowing slip to become coherent. A magnitude estimates and source mechanisms third is that the phase transitions are localized to became almost routine. By 1960 it was possible to boundaries between rock lenses, where fluid have a broad global classification of seismogenesis conditions are particularly favourable to sudden caused by crustal convergence and divergence. transitions. Along these pre-existing grain bound- These results were basic to the development of aries, the crystal structures change rapidly, thus plate tectonics. Some peculiarities, however, weakening the bonds across the discontinuities. remained and remain for additional detailed investi- It is instructive to consider the extraordinary gation (Yeats et al. 1997). Why on the plate tectonic deep earthquake that occurred on 9 June 1994 at a description do many earthquakes, including major depth of 637 km within the Nasca subduction slab damaging ones, occur far from plate boundaries? under Bolivia. This earthquake (M = 8.3) is the Indeed, hinterland seismic activity occurs in all largest earthquake at these extreme depths ever continents except Greenland (Bolt 1993). As recorded. Strong ground motion was felt over much mentioned already, Lyell described a number of of Bolivia, but there were no deaths and relatively these intraplate earthquakes. minor damage. One remarkable feature was that 10 There is one special class of earthquake which to 20 minutes after the deep energy release, seismic continues to attract research ingenuity. Deep-focus waves were felt by many people in the Caribbean earthquakes have sources well below the crust (the and in North American cities as distant as Chicago, deepest have foci at depths of about 680 km) and Illinois and Toronto. The location of the focus hypocentres within or along the boundary of the placed it in 'the grand jog' of the subduction slabs relatively cold subduction slab. The great pressures between the Wadati-Benioff zone under Argentina at such depths (up to 240 kbars) make it difficult to near southern Bolivia and the zone under Peru and envisage their genesis by elastic rebound along a Brazil. These zones are nearly parallel, but offset discontinuous surface in brittle rocks. Various from each other by 1000 km in the vicinity of this alternative mechanisms have been suggested to earthquake focus. settle this longtime dilemma, such as sudden Cross-correlation between seismic wave forms dilatational change in volume of the rocks, perhaps recorded at a number of the newly positioned from a sudden change in the phase state of the con- global digital seismographic stations (see Fig. 3) Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021

EARTHQUAKES AND EARTH STRUCTURE 355 now allows theoretical wave forms computed from reported travel times has provided a significantly fault models to be compared directly with the more precise sample of seismic travel times for observations. An analysis of this type (Antolik tomographic imaging. From such a database, 1996) demonstrated that the source mechanism for inversions to the three-dimensional spherical this very deep earthquake source (with magnitude structure of the mantle paths have indicated linear exceeding that of the 1906 San Francisco earth- patterns of velocity anomalies which extend down- quake) is the same as that for shallower earthquakes wards from subduction slabs in the lithosphere (Van in the crust. The solution in this study was a near- der Hilst et at. 1997). These linear patterns that map horizontal nodal plane (strike 302 degrees, dip 11 regions of relatively fast seismic waves can be degrees), an average rupture velocity of 1.7 km/s interpreted as slabs extending at least 1000km over a roughly ellipsoidal area with major axis 12 below the 400 km boundary in the upper mantle km long. The average slip throughout the rupture (see Table 1) at places around the Pacific. The area was about 5 m with a peak value of 16.5 m. consequence of the confirmation of such models Unfortunately, no uncertainties on these values are would be that subduction slabs sink to the core given and the solution is not unique. boundary. There they may accumulate and perhaps The subduction slabs defined by deep-focus remelt, mix and rise again as molten plumes to the earthquakes bear in a critical way on one of the surface. foremost geophysical questions, extending back to at least Lyell's time (Brush 1979), namely, the effect of viscous hydrodynamical conditions in the Earthquake prediction Earth's mantle. In recent times, the kinematics of plate tectonics led to the adoption of large-scale The hallmark of a scientific theory is its predictive mantle convection as the driving force for sub- power. Indeed, the attraction of the Newtonian duction, mountain building, volcanic activity, and dynamical theory, modified appropriately for major earthquake genesis. The older question of relativity, and of the Darwinian theory of evolution rigidity versus fluidity (Brush 1979) shifted to a are their ability to forecast beyond the available long-running debate on whether the mantle con- observations and descriptions. From early times, a vects as a whole or whether it convects in a two belief in forerunners to large earthquakes and shell system. reported eye-witness accounts of them have been Recent special studies of earthquake waves have recorded. Lyell only refers to such forerunners in indicated a way to discriminate between the two passing (p. 81). He mentions reported irregularities models. Statistical reanalysis of the standard in the seasons preceding shocks, animals evincing catalogues of earthquake locations and, hence, extraordinary alarm, violent rains and other

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

Region Level Depth (km) Features of region

- Outer surface A Crustal layers Base of crustal layers - (distance R from the 33 Earth's centre) B Steady positive P and S velocity gradients - 0.94R 413 C Transition region* - 0.85R 984* D Steady positive P and S velocity gradients - 0.548R = R 1 2898 E Steady positive P velocity gradient - 0.40R 1 4982 F Negative P velocity gradient + - 0.36R 1 5121 G Small positive P velocity gradient - Earth's centre 6371

* Now estimated to be 660 km. In 1962 shown incorrect (Bolt 1982). Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021

356 B.A. BOLT occurrences. Nevertheless, he confines his text Some doubt the instrumental ability to even detect, 'almost entirely to the changes brought about by above the ambient electrical noise, the tiny electro- earthquakes and in the configurations of the Earth's magnetic variations due to static elastic straining in crust'. the seismogenic area; substantial electric signals of It is generally accepted that a serious earthquake all kinds are prevalent in modern populated areas. prediction must also specify the location, origin Another evaluation approach is to accept the time, and magnitude within specific known limits. published observed correlations and subject them to Such certainty is impossible to justify because strict statistical tests (Stark 1996, 1997). The most prediction is always based on a limited number of detailed tests along these lines to date indicate that measurements, themselves imprecise. Conse- the success of the claimed predictions is unsup- quently, scientific predictions must state the ported by the evidence. probability of the occurrence. My own inclination is to be skeptical of most A great amount of research has been carried out prediction claims based on 'abnormal' seismicity, on earthquake prediction in a number of countries, geodetic, geological or geophysical observations, particularly Japan, the United States, the Soviet or unexpected deviations from average regional Union, China, and Italy in recent decades (Mogi parameters. One source of doubt is that the physical 1985). Comparison of the earthquake predictions genesis of tectonic earthquakes denies the assump- published in earthquake journals with the actual tion that origin times, locations, and magnitudes of seismological record indicates no proclaimed earthquakes are jointly independent. Most statis- method can be taken as proved or effective. In my tical analyses of seismicity catalogues assume the view, the failure of the modern research on earth- constant rate of a memoryless Poisson distribution; quake prediction raises doubt whether forecasts yet the elastic rebound theory states that there is an within strict bounds of time and place will ever be evolution of the stress field, interrupted by sudden possible for most earthquakes, particularly the large strain decrease at the time of fault slip (Bullen & damaging ones. Two contemporary cases give the Bolt 1985). So, too, after the substantial rupture of flavour of the present hiatus in prediction pro- a fault, like the San Andreas fault in California, the grammes. The first was the inability of the Japanese frictional distribution on the contiguous fault seismological prediction programme, often surfaces is likely to be irrevocably changed so that regarded as a model, to give forewarnings of the the traction memory is lost or weak. Although the devastating Kobe earthquake of 17 January 1995. mechanism of earthquake genesis is understood The failure was exacerbated by the numerous physically, the concomitant geological complex- public statements made over decades in Japan that ities and remoteness of the rupture surfaces have 'precursors' had been recorded after earlier limited prediction abilities to a stage not much damaging earthquakes (Geller 1997). For example, beyond the time of Lyell (see Geller 1997). 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 The three-dimensional geological structure main shock, a few hours to weeks before it. There of the Earth are, however, no scientific grounds that allowed any of these small earthquakes to be identified in Although many of his case histories describe large advance as a forerunner of a major earthquake. seismic events in regions where there are no Sharp criticisms in Japan of such hindsights based volcanoes, Lyell continued to link them together. on coincidence were reflected in the public press. 'The regions convulsed by violent earthquakes The second illustration comes from ongoing include within them the site of all the active debate of the validity of the predictions of volcanoes. Earthquakes sometimes local, some- earthquakes in Greece (Varotsos et al. 1996). These times extending over vast areas, often precede predictions used variations in the electrical field volcanic eruptions.' He argues that the common within a specified region to trigger alarms. The cause is the passage of heat from the interior to the proponents argue strongly that their predictions are Earth's surface. His discussion of the physical more successful than those provided by chance condition of the Earth's interior uses arguments coincidences. For example, between 1987 and 1989 from mechanics, such as experiments with the number of events with magnitudes greater or pendulums and the attraction of the Earth to the equal to 4.7 was 39. During this interval, the Moon. These investigations 'have shown that our method yielded 23 predictions with a claimed planet is not an empty sphere, but on the contrary success of 38 per cent. The nominal duration of the that its interior, whether solid or fluid, has a higher alarm period was 23 days. There are many critics of specific gravity than the exterior'. He also accepts these claims (e.g. Mulargia & Gasperini 1992). that vibrations of the Moon indicate that the Earth Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021

EARTHQUAKES AND EARTH STRUCTURE 357 has an increase of density from the surface towards times were known and were shown to be correlated the centre, the average value being about 5.5. It is with deviations from interior radial symmetry. actually 5.52 g/cm 3. He quotes Young on the effect Estimation of the variation of seismic wave of compression at the Earth's centre, which would velocities, Vp and Vs, within the Earth provided the compress steel into a quarter of its volume using the key to structure (Fig. 5). Bullen went on to define a assumption that a terrestrial nucleus may be nomenclature for radial shells based on the metallic with a specific gravity of iron of about 7. velocities (Bullen & Bolt 1985). The shells range He is acute in judging that such extrapolations are from layer A, the crust, to layer G, the inner core of uncertain because compressibilities of bodies differ the Earth (see Table I). The layers B and C from values obtained in the laboratory. incorporate what has become known as the upper Finally, Lyell adopts the notion of a solid crust mantle of the Earth where the largest deviations and refers to the effect of the attraction of the Moon from spherical symmetries occur. The layer D is the on the Earth's precession. Lyell admitted that the solid lower mantle and E the liquid outer core. inside of the Earth was hot, but in accordance with As more earthquake observatories with better his uniformitarian view, denied its temperature had equipment were put into operation, particularly the changed significantly in the course of geological broadband digital seismographs of the 1980s and history (pp. 210-240). He differed from the 1990s, regional deviations from the radial depen- argument for an Earth model with a solid shell over dence became more sharply focused. These devia- a liquid nucleus, favoured by William Hopkins, the tions entailed significant heterogeneities in deep founder of British physical geology, who often Earth structure, some of which correlate well with opposed the uniformitarian school (see Fisher surface geology. (Three-dimensional maps of this 1889). Lyell points out that theoretical precessional heterogeneous structure really require colour orbits do not agree with the astronomical obser- projections of the globe (see for example vations Hopkin's relied upon unless the minimum Dziewonski & Woodhouse 1987; Bolt 1993).) thickness of the crust of the globe 'cannot be less Nevertheless, deviations from spherical symmetry than 1/4 or 1/5 of the Earth's radius'. In fact, the become less marked, and are generally of the rigid mantle is about one half of the radius. Debates second order, with depth because of the strong of this kind defined the status of global Earth influence of pressure on the mineralogy of the structure at the end of Lyell's life (Brush 1979). rocks. Below a low velocity lithosphere layer in Seismological tomography (Iyer 8,: Hirahara which S and perhaps P velocities decrease some- 1993) was needed to estimate the detailed elastic what, the velocities increase by about a percent or structure and physical properties of the Earth's so down to depth of 220 kin, often called the interior. Lehmann discontinuity, where there is a jump of The worldwide network of seismographic about 3 per cent. The depth of this discontinuity stations allowed the production of standard travel varies from region to region, and it is not detected times of the main seismic body waves through the in some studies in some places. The 410 and 660 Earth. The most notable estimates were obtained by km depth discontinuities are features of the upper Jeffreys and Bullen in 1940. These tables (Fig. 4) mantle that have been observed worldwide. They provided the solution of two basic geophysical have been confirmed by underside reflections inverse problems. Problem 1: if the location of an which occur as precursors to the reflective core earthquake's epicentre is known, times of travel to waves, although summarizing the structural trends observatories at any site can be calculated; the of these discontinuities has proved difficult. actual problem is to locate an epicentre anywhere in The next zone of interest is near the mantle-core the world using readings of arrival times of boundary (MCB). High frequency (about 1 Hz) P earthquake waves at observatories. Problem 2: if and S waves are both recorded that are reflected the seismic wave velocity distribution at each depth from the upper side of the MCB; P waves are also in the Earth is known, one can calculate the travel reflected from its underside. These common obser- time T(D) to distance D. The inverse question is: vations are convincing evidence that the MCB is given T(D), calculate the variation in velocity v(r) quite sharp. In the last seven years, much effort has in the Earth. been made to map the structural details of the layer The use of the Jeffreys-Bullen travel-time tables called D", at the mantle base. Bullen differentiated in solving Problem 2 provided robust estimates of this sub-shell because the 1939 Jeffreys P and S the unknown velocity structure but on the assump- velocity curves flattened for about 200 km above tion of radial symmetry (Bullen & Bolt 1989). the core boundary. In fact, the constant slope was Their construction, accompanied by application of an artifact of the smoothing by Jeffreys of the probability and inference theory, was so successful inversion process, but the suggestion led to that the tables have been used widely up to today attempts to find more direct observational evidence even though regional anomalies in the average for a transition shell. This partly comes from Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021

358 B.A. BOLT

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I .. L I I __I l L I ...1 0 20 40 60 80 I00 i20 140 160 180 360 340 320 300 280 260 240 220 200 Distance (degrees ofarc)

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 Future work types), which run around the mantle core boundary At present, the research situation in seismology is (Bolt, 1982). It is now known that a radially excellent. The worldwide network of broadband spherical inversion for D" structure does not yield a digital stations and the easy exchange of digital realistic description. There are relatively large- recordings of earthquakes through the Incorporated scale deviations in elastic properties within the Research Institutions for Seismology (IRIS) layer, which affect waves according to their wave programme has made accessible to researchers, in a length. These azimuth and wave-length dependent straightforward way, very high quality seismic effects are difficult to tie down and a number of wave-form data. Even the debilitating uneven competing solutions have been given. The most sampling for tomographic studies of the deep recent studies of high frequency core waves appear interior due to the predominantly continental to require heterogeneous (non-radially symmetric) locations of observatories is being addressed. The structures both in D" and well above this layer in next decade will see more ocean-bottom seismo- the mantle (Hedlin et al. 1997). graphic stations at critical distances. Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021

EARTHQUAKES AND EARTH STRUCTURE 359

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 seis- therefore biassed to the better known branches of mographic networks are easily downloaded onto the travel-time tables (see Fig. 4). personal computers. On the interpretation side, An alternative method now coming into vogue is there can be a drawback. The paper photographic to make use of the wave forms on the seismograms, records of yesteryear, that displayed 24 hour signals rather than picking the first wave onsets. The idea continuously, enabled students to examine the is to superimpose ('stack') the digital seismograms whole of an earthquake record, including the back- provided by the global digital networks at common ground noise in which it is embedded. Because the source-receiver distances and to average the result present methods tend to be restricted to short runs to obtain a composite record. Figure 6 gives an of the seismic evolutary time series, a narrow focus example from a recent paper that discusses the on the expected wave forms results rather than procedure (Astiz et al. 1996). The figure shows the scanning for abnormal observations. stacking of transverse components of the recorded The picking by eye of the first onsets of seismic seismic ground motions after the signals were waves recorded on seismograms, such as P, S and passed through a 30 s low-pass filter before the their multiples, was the major tool at earthquake superposition was made. In this case, the procedure observatories to provide the basic data for the enhances the horizontally polarized shear waves construction of travel-time curves for waves (SH) passing through the Earth relative to the throughout the Earth's interior. A problem was compressional P waves. It is instructive to compare always that at many observatories the selection of the concentration of wave energy on this plot with phases was highly correlated with the available the predicted theoretical times for the S and P charts of travel-time curves. Readers sought to find phases in Fig. 4. Outstanding problems on Earth the appropriate identification by reference to avail- structure may achieve better resolution using such able curves such as Jeffreys-BuUen, rather than methods. An example of such a problem is the need picking values using the assumption of ignorance. to obtain a sharp resolution of the longitudinal More and more in the last decades, reported velocity in the upper part of the outer core (Murtha readings in the seismological catalogues were 1984). Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021

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 theories on the Earth's interior dynamics and his of the Earth's interior can be inferred from earth- views on the immediate causal link between quake wave probes of the Earth. The attenuation of volcanoes and earthquakes were erroneous. Never- earthquake waves with distance gives a direct theless, his emphasis on the value of field studies measure of the non-elastic (viscous) properties of immediately after earthquakes and his judicious the interior rocks. Wave polarization allows their description of them remain important educationally anisotropic elastic parameters to be estimated. even today. Strict bounds are also placed on the density distribution by considering the complete seismic I am grateful to D. Drager and E Shearer for assistance with figures. response spectrum of the Earth. For example, from short period reflections (see Bolt 1991), it was demonstrated in 1970 that the average inner core References density could not exceed about 14 g/cm 3. ASTIZ, L., EARLE, P. & SHEARER,P. 1996. Global stacking There is much reason to be grateful to Hutton of broadband seismograms, Seismological Research and, particularly, Lyell for their recognition of the Letters, 67, 8-18. crucial evidence on global tectonic processes ANTOLm, M. S. 1996. New Results from Studies of Three provided by earthquakes. The geologically Outstanding Problems in Local, Regional, and ephemeral duration of earthquakes did not preclude Global Seismology. PhD thesis. University of them informing the past. Lyell had no profound California, Berkeley. Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021

EARTHQUAKES AND EARTH STRUCTURE 361

BAILEY, E. B. 1967. James Hutton - the Founder of JEFFREYS, H. & BULLEN, K. E. 1940. Seismological Modern Geology. Elsevier, Amsterdam. Tables. British Association Gray-Milne Trust, BERRYMAN, K. R., BEANLAND, S., COOPER, A., CUTIEN, Edinburgh. H., NORRIS, R. & WOOD, E 1992. The Alpine fault, LOMAX, A. & BOLT, B. A. 1992. Broadband waveform New Zealand: variation of quaternary structural modelling of anomalous strong ground motion in style and geomorphic expression. Annalis the 1989 Loma Prieta earthquake using three- Tectonicae, (Suppl.), 6, 126-163. dimensional geologic structures. Geophysical BOLT, B. A. 1982. Inside the Earth. W. H. Freeman, New Research Letters, 19, 1963-1966. York. LYELL, C. 1875. Principles of Geology, 12th edn. 4 vols,

-- 1991. The precision of density estimation deep in London. the Earth. Quarterly Journal of Royal Astronomical MoGI, K. 1985. Earthquake Prediction. Academic Press, Society, 32, 367-388. Tokyo. 1993. Earthquakes and Geological Discovery. MULARGIA, E & GASPERINI, P. 1992. Evaluating the Scientific American Library, New York. statistical validity of 'VAN' earthquake precursors. 1996. From earthquake acceleration to seismic Geophysical Journal International, 111, 32--44. displacement, the Fifth Mallet-Milne Lecture. MURTHA, P. E. 1984. Seismic Velocities in the Upper Part Society for Earthquake and Civil Engineering of the Earth's Core. PhD thesis. University of Dynamics. John Wiley & Sons, Chichester. California, Berkeley. BRUSH, S. G. 1979. Nineteenth century debates about the OLIVER, J. 1996, Shocks and Rocks: Seismology in the inside of the Earth: solid, liquid or gas? Annals of Plate Tectonics Revolution. American Geophysical Science, 35, 225-254. Union, Washington. BULLEN, K. E. & BOLT, B. A. 1989. Introduction to the ONGLEY, M. 1943. Surface trace of the 1855 earthquake. Theory of Seismology. 4th edn. Cambridge New Zealand Transactions, 73, 84--89. University Press, Cambridge, 1985. RICHTER, C. 1958. Elementary Seismology. W. H. DZIEWONSKI, A. M. & WOODHOUSE, J. H. 1987. Global Freeman, New York. images of the Earth's interior. Science, 236, STARK, P. B. 1996. A few considerations for ascribing 37-48. statistical significance to earthquake predictions. FISHER, O. 1889. Physics of the Earth's Crust. Macmillan, Geophysics Research Letters, 23, 1399-1402. London. STARK, P. B. 1997. Earthquake prediction: the null GELLER, R. J. 1997. Predictable publicity, astronomy and hypothesis. Geophysical Journal International, (in geophysics. Journal of the Royal Astronomical press). Society, 38, 16-18. VAN DER HILST, R. D., WIDIYANTORO, S, t~ ENGDAHL, E. HEDLIN, M. A. H., SHEARER, P. M. & EARLE, P. S. 1997. R. 1997. Evidence for deep mantle circulation from Seismic evidence for small-scale heterogeneity global tomography. Nature, 386, 578-584. throughout the Earth's mantle. Nature, 387, VAROTSOS, P., EFTAXIAS, K., VALLIANATOS, F. & 145-150. LAZARIDOU, M. 1996. Basic principles for IYER, H. M. & HIRAHARA, K. (eds) 1993. Seismic evaluating an earthquake prediction method. Tomography: Theory and Practice. Chapman & Geophysical Research Letters, 23, 1295-1298. Hall, London. YEATS, R. S., SIEH, K. & ALLEN, C. R. 1997. The Geology of Earthquakes. Oxford University Press, Oxford.