Forum

GSA Annual Meeting in Seattle, Washington, USA - 2017

Paper No. 62-3
Presentation Time: 4:20 PM

A FIVE YEAR FORECAST FOR INCREASED GLOBAL SEISMIC HAZARD (Invited Presentation)


BILHAM, Roger, Geological Sciences and CIRES, University of Colorado at Boulder, UCB 399, Boulder, CO 80309-0399 and BENDICK, Rebecca, Department of Geosciences, University of Montana, Missoula, MT 59812, Roger.Bilham@colorado.edu

On five occasions in the past century a 25-30% increase in annual numbers of Mw≥7 earthquakes has coincided with a slowing in the mean rotation velocity of the Earth, with a corresponding decrease at times when the length-of-day (LoD) is short. The correlation between Earth's angular deceleration (d[LoD]/dt) and global seismic productivity is yet more striking, and can be shown to precede seismicity by 5-6 years, permitting societies at risk from earthquakes an unexpected glimpse of future seismic hazard. The cause of Earth's variable rotation is the exchange of angular momentum between the solid and fluid Earth (atmospheres, oceans and outer core). Maximum LoD is preceded by an angular deceleration of the Earth by 6-8 years corresponding to a π/4 phase lag of the 24-33 year peak-to-peak period of multidecadal oscillations of Earth's rotation. We show delayed global seismic productivity is most pronounced at equatorial latitudes 10°N-30°S. Two mechanisms may be responsible: (1) decreased oblateness (a reduction in J2) that attends a slowing of Earth's rotation and (2) lithospheric overshoot, a process whereby the equatorial lithosphere sluggishly overrides the decelerating underlying mantle westward, much as a loose cannon slides upon the deck of a rolling ship. The observed relationship is unable to indicate precisely when and where these future earthquakes will occur, although we note that most of the additional Mw>7 earthquakes have historically occurred near the equator in the West and East Indies. A striking example is that since 1900 more than 80% of all M≥7 earthquakes on the eastern Caribbean plate boundary have occurred 5 years following a maximum deceleration (including the 2010 Haiti earthquake). Calculations show the asthenosphere to have an appropriate viscosity to account for the delay between deceleration and subduction zone seismicity, however, a geodetic test of the anticipated westward overshoot would be of utility. Whatever the mechanism, the 5-6 year advanced warning of increased seismic hazards afforded by the first derivative of the LoD is fortuitous, and has utility in disaster planning. The year 2017 marks six years following a deceleration episode that commenced in 2011, suggesting that the world has now entered a period of enhanced global seismic productivity with a duration of at least five years.