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A Railway-spotters Guide to Earthquakes: Coseismic Slip, Dynamic Strain and Ground Motion Intensity

Description: 

Railroads that cross active faults can provide a striking visual record of surface rupture in large earthquakes, similar to the offset fences that led Reid (1908) to develop elastic rebound theory. Railroads, once offset, may provide a precise measure coseismic offset long after they are repaired, since repairs are typically confined to the region of damage near the fault. We provide examples from a subset of railroads damaged by historical and recent earthquakes: the M > 7 1886 Charleston earthquake (doi.org/10.1785/0320230022), the ;M 6.8 1892 Chaman and M 7.7 1935 Quetta earthquakes in Pakistan (doi.org/10.1785/0220190148), the M 7.4 1944 Bolu, and the M 7.6 1999 Izmit and M 7.8 2023 Kharamanmaras earthquakes in Turkey. Railroads can, however, provide more than just a measure of coseismic offset. They act as distributed strainmeters in the meizoseismal region of an earthquake (doi.org/10.1785/0120240025). As an astute contemporary geologist deduced following the 1886 Charleston earthquake, railroad tracks respond to the transient strain associated with the outward propagation of surface waves. Prior to the implementation of continuously welded rail, early railroads incorporated expansion gaps sufficient to accommodate ~10^-4 of thermoelastic strain. Transient dynamic strain contraction exceeding this amount following an earthquake results in track buckling, and transient extension results in track separation. Once formed, buckled track cannot be straightened by the tensile phase of transient strain, since the forces required are typically far higher than those needed to create the buckle. Damaged railroad tracks can thus provide a max/min measure of dynamic strain in an earthquake. Because peak dynamic strain is proportional to peak ground velocity and conventional macroseismic shaking intensity at strong shaking levels is proportional to the logarithm of peak velocity, it follows that macroseismic intensity can be estimated from dynamic strain: 2x10^-4 strain is approximately equivalent to MMI 7.5, and 10^-3 strain is equivalent to MMI 9.

Session: Earthquakes, Lithospheric Structure, and Dynamics in Stable Continental Region - I

Type: Oral

Date: 4/17/2025

Presentation Time: 08:15 AM (local time)

Presenting Author: Roger

Student Presenter: No

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