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During the past decade, satellite technol-ogy has spawned a revolution in mea-suring the surface deformation caused by earthquakes. Yet as Bilham and England1 show on page 806 of this issue, classical ground-based surveying methods in use as long as 150 years ago can still yield astound-ing results.
The Global Positioning System (GPS) constellation of satellites allows repeated measurements of ground-survey markers that are precise to a few millimetres. Comple-menting GPS, interferometric synthetic aper-ture radar (InSAR) imaging uses repeat-pass data from radar satellites to obtain complete ground-displacement maps of 100 km by 100 km regions with 100-metre spatial resolution that are accurate to 10 millimetres or better. I use both of these methods and understand their principles. Yet it still seems absurd that a satellite 800 km, or more above Earth's surface, travelling at 7 km per second, should be able to provide such precise measurements of surface displacements. In contrast, groundbased triangulation, involving repeated angle measurements made with a theodolite, can typically resolve relative motions of only hun-dreds of over tens of kilometres.
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Figure 1 Aftermath: devastation in Muktagachha, photographed in 1897 by G. E. Grimes. |
How then can the humble surveyor's theodolite be mightier than the latest satellite technology? The answer, of course, is that, until time travel is perfected, the archive of theodolite-derived historical data is unique. It can be mined to study the rare large earth-quakes that have by chance been recorded using methods that are now largely extinct.
Using triangulation surveys carried out between 1862 and 1936, Bilham and England show that the great Assam earthquake of 12 June 1897 occurred on a previously unknown fault, which they christen the Old-ham fault. The earthquake was of magnitude 8.1. The fault is located near the northern edge of the Shillong plateau in northeastern India (see map on page 806), and did not rupture the Earth's surface. Its existence can, in retrospect, be seen as necessary to explain the enigmatic existence of the plateau, an iso-lated, uplifted block of crust lying 200 km south of the Himalayas. This block has a steep southern flank bounded by the Dauki fault, which may be able to sustain an earth-quake as large as the 1897 event, thus threat-ening the city of Dhaka, Bangladesh, with a population of over three million, that lies 150 km to the south of the plateau.
The 1897 earthquake holds a special place in the history of seismology. R.D. Oldham, an Englishman who led the Geological Survey of India, studied its effects and published an extensive, well-illustrated monograph2 of his observations that has been much cited by succeeding generations of seismologists3. The earthquake was particularly destructive, and devastated a huge region of northeastern India (Fig 1).Observations that heavy objects such as large boulders had been thrown clear of the ground were the first documented evidence that earthquake ground motions can exceed the force of gravity. Contemporary reports in Nature are summarized in Box I.
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What was unusual about this earthquake was its location. Most earthquakes of this magnitude occur at the boundaries of the 15 or so large plates, some 100 km thick, that comprise the Earth's surface. Although frictional forces lock their boundaries most of the time, continued relative motion of the plates builds up forces that eventually exceed the frictional strength of the boundary faults, producing sudden fault slips that manifest themselves in the form of roughly periodic earthquakes.
One such plate boundary occurs 200 km north of the 1897 event, where the Indian plate dives down beneath the Eurasian plate at the Himalayan mountain front. Billiam and England explain the Shillong plateau as an effect of two unusual local circumstances. The weight of the Himalayas to the north and the thick sediments of the Bengal fan to the south create a bow-shaped flexural bend of the Indian plate in Assam. These bending stresses, superposed on the compressed Indian plate, are relieved by the Shillong plateau 'popping up' by earthquake slip on its bounding faults (see Fig. 3 on page 808).
Events such as that of 1897 are compara-tively rare because the earthquake slip is large and long-term fault slip rates are low. With an estimated slip of 18 m and a guesstimated fault slip rate of 2-- 4 mm per year, events like that the Assam earthquake should recur only every 4,000-8,000 years or so, according to Bilham and England's calculations. At plate boundaries, slip rates are an order of magni-tude greater and earthquake slip is generally less, so great earthquakes occur on each fault segment every few hundred years.
Despite their idiosyncratic causes and relative infrequency, earthquakes such as the Assam event are receiving increasing atten-tion, both because of their intrinsic interest and because of the hazards they pose to densely populated regions. Although these events are not quite plate-boundary earth-quakes, they ultimately owe their existence to the dynamism of the boundary region. Plate- boundary processes generate stresses and create and destroy topography, producing local forces that must be balanced by deformation and earthquakes. Three examples illustrate the diversity of these processes.
The Seattle fault4 is one of a series of structures oriented east-west lying landward of the north-south-oriented Cascadia subduction zone, the boundary between the Juan de Fuca and North American plates in the northwestern United States. The east-west- oriented faults result from compression aris-ing from northward movement of the small Oregon Coast microplate impinging on the rigid buttress of the British Columbia Coast Ranges5. The Seattle fault last slipped in about AD 900, and it ruptures much less frequently and causes smaller earthquakes than does the Cascadia subduction boundary. But as its name implies, this fault cuts directly through downtown Seattle, and a repeat of the AD900 event poses a greater hazard than a larger Cas-cadia event (The magnitude 6.9 earthquake of 28 February 2001 was a different type of event again -it occurred at a depth of 52 km in the subducting Juan de Fuca plate that dives beneath Seattle.)
Much of California's Los Angeles basin is underlain by faults that, like the Oldham fault, do not yet penetrate to the Earth's sur-face6. They result from compression due to an easterly bend of the San Andreas fault, which takes up most of the relative motion between the Pacific and North American plates. Again, the proximity of these buried faults to metropolitan Los Angeles makes them arguably more hazardous than Cali-fornia's proverbial 'big ones' on the San Andreas.
Finally, the Bhuj earthquake, which occurred in India on 26 January2001, under-lines the importance of better understanding the causes of potentially destructive earth-quakes happening near plate boundaries. This event, of magnitude 7.8, occurred on one of a series of east-west faults which, like the Oldham fault and those of the Los Angeles basin, did not reach the Earth's surface. The origin of this fault zone is uncertain. But its location several hundred kilometres south of the Himalayas, and 200 km north of the thick fan sediments of the Indus delta, suggests that forces similar to those invoked to explain the Shillong plateau may be involved.
Wayne Thatcher, is at the US Geological Survey,
345 Middlefield Road, Menlo Park. California 94025, USA
e-mail thatcher@usgs.gov
1. Bilham, R. & England, P., Nature 410, 806-809 (2001)
2. Oldham, R. D. Mem. Geol. Soc. India 29, 1-379 (1899)
3. Richter, C. F. Elemetary Seismology 49-56 (Freeman, San Francisco, 1957)
4. Bucknam, R. C. et al. Science 258, 1611-1614 (1992)
5. Wells, R. E. et al. Geology 26, 759-762 (1998)
6. Dolan, J.F. et al. Science 267, 199-205 (1995)
