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Earthquakes and Earth Structure: a Perspective Since Hutton and Lyell

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Earthquakes and Earth Structure: a Perspective Since Hutton and Lyell Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021 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.
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