Roger Bilham: Earthquakes and Plate Boundary Processes

Urban earthquakes in the developing nations. 12th Mallet-Milne lecture 2009 pdf

By the year 2025 more than 5500 million people will live in cities - more than our entire 1990 combined rural and urban population. The growth of these giant urban agglomerations is a new experiment for life on Earth. Tragically, a significant fraction of the largest of these agglomerations (supercities and megacites) are located close to regions of known seismic hazard. With few exceptions (Tokyo 1923; Tangshan, 1976), recent large earthquakes (M>7.5) have spared the world's major urban centers, but this will not persist indefinitely. In the next millennium several megacities will be damaged by significant earthquakes. We are most certain of the fate of those cities near plate boundaries, however, mid-continent earthquakes also occur, albeit infrequently (c.f. M>7.6 events in the eastern US and India in the early 18th century. One of the most intractable problems in saving future lives from earthquakes in the developing nations is the prevalance of corruption in the building industry.

Flexure of India

The northern edge of the Indian plate is flexed downwards more then 20 km by the weight of the Himalaya and by the compressive forces of India's collision with Asia. The 6-km-deep flexural depression near the Himalayn foothills has filled with sediments from the Ganges, raising a 450 m high bulge in central India. These same flexural forces form a 40 m depression between Bombay and Hyderabad. This flexed surface is responsible for the general stress regime that drives earthquakes in India. Shallow reverse faulting occurs near this depression (e.g. the Latur earthquake), deep reverse faulting occurs beneath the central Indian Bulge and the Ganges plain (e.g. the Jabalpur earthquake).

Himalayan earthquakes (more info)

Great earthquakes in the Himalaya have occurred repeatedly in the past and will continue to do so in the future. We do not know precisely when and where they will occur although in several locations we can calculate how much elastic energy is currently stored ready to drive a future earthquake. The map shows graphically how large these future earthquake in the Himalaya might be should a great earthquake occur today. The 2005 Mw7.6 Kashmir earthquake occurred 5 months after this figure was published at the extreme western end of the region. The westernmost M8 event, which last occurred in 1555 in SE Kashmir has yet to recur

Deformation Sensors

GPS geodesy is accurate to roughly 1 mm, but strain-meters, tilt-meters and creepmeters effectively monitor signals that are much smaller. The tiltmeter (left) in Pozzuoli Italy measures relative vertical motions over a distance of 300 m with a precision of 0.1 microns, one ten-thousandth of the precision of GPS. Three biaxial tiltmeters operate near Seattle and one monitors inflation of the Long Valley caldera, California. Creep-meters on the San Andreas and Hayward faults monitor motions of the fault each minute to a precision of 10 microns.

principle of a creepmeter

tiltmeter operation.

Earthquakes and Tectonic Plate motions

Roger Bilham

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