geodynamics term project
On the original of intraplate earthquakes
Author: Jia Zhang
Supervisor: Dr. Eunseo Choi
Center of Earthquake and Research Information
University of Memphis
July 29, 2017
The origin of intraplate earthquakes remains one of the major questions in seismology. The occurrence of intraplate earthquakes has long been considered as problematic since the accumulation and release of strain within the interiors of oceanic and continental plates are too weak to generate earthquakes according to the plate tectonics’s prediction.
However, about 5% of earthquakes are observed worldwide to take place within a plate, away from plate boundaries. And some of intraplate earthquakes are catastrophically large. For example, three large earthquakes in North America occurred in 1811-1812 in the Mississippi River valley of the central United States and a large event in 1886 severely damaged Charleston, South Carolina. These intraplate earthquakes present fundamental challenges to our understanding of how strain accumulates/releases and what the associated seismic potential and hazard are for intraplate regions, which is especially significant for the densely populated area to prepare for seismic shaking.
Several hypotheses have been proposed over the years for the mechanisms of intraplate earthquakes (Abhijit. Gangopadhyay 2003), including a time-dependent mechanical model for the generation of repeated large intraplate quakes (Kenner S. J. 2000), lateral seismical transition in the upper mantle (H. 2000) and a geodynamic model for stress concentration (F. 2001). Herein, the commonly accepted assumption is that a weakness in the crust, which could be a deeply buried causative fault or ancient failed rifts, can easily slip to accommodate regional tectonic strain. The earthquakes can occur within the pre-existing zones of weakness by three ways, a localized build-up of stress on the potentially seismogenic feature due to the ambient stress field, the superposition of a triggering stress and the reduction of strength of the feature by mechanical and/or chemical means ((1989) 1989).
Nowadays, new seismological observations, in combination with numerical modelling and geodynamic calculations update the views of the origin and interpretation of intraplate earthquakes.
In this project, I will review some seismological observations concerning intraplate earthquakes in two regions, which are north China and New Madrid Seismic Zone and discuss several theories and modelling for their mechanism. The analyses of the roles of fault intersections, postglacial rebound in localizing stress build-up, can show how these mid-plate earthquakes occur in response to stresses emanating from plate boundaries. And a unified model to explain how these stresses build up on discrete structures and lead to such mid-plate earthquakes will also be discussed here.