Recent Research - Earth Related Projects
A. Structure and Evolution of Large Oceanic Intraplate Volcanoes
(1) Flexural stresses, earthquakes, and magma ascent.
The large Hawaiian earthquakes of October 15, 2006 offered insights into the state of stress in the lithosphere, as generated by flexural response to Hawaiian volcano loading. The Kiholo Bay earthquake (Mw 6.7) occurred 39 km below sea level according to the Hawaiian Volcano Observatory. The reported focal mechanisms (from the Global CMT catalog) indicate a mixed normal/strike-slip mode of faulting. Six minutes later, the nearby Mahukona Mw 6.0 event occurred at depth 19 km, with a reverse faulting mechanism. The apparently inconsistent fault mechanisms are difficult to reconcile with a conventional fault/aftershock relationship. However, the mechanisms and depths are consistent with the expected stress-vs.-depth (“strength”) profile of concave-upward flexed lithosphere: a compressional maximum in the upper lithosphere and an extensional one in the lower lithosphere (see the red curve in the figure below). There is a significant complication: the depth of the Mahukona quake places it well beneath the predicted zone of maximum differential stress at the top of a uniform lithospheric plate.
I modeled the state of stress in the mechanical lithosphere (crust and upper mantle) induced by Hawaiian volcano loading using a viscoelastic finite element code. I found that loading by the older edifices of Hawaii (Hualalai and Kohala) produced a stress state consistent with the observed earthquakes, provided that the zone of peak differential stresses occurs within the upper lithosphere at the top of the mantle, rather than at the top of the crust (and lithosphere). This condition is satisfied by models (blue curve in figure above) with a stiffness contrast between crustal and mantle rocks (the latter being significantly stiffer). This zone of peak stresses also presents a formidable barrier to magma ascent, contributing to the stalling and emplacement of magma at the base of the crust: the “underplating” seen in seismic surveys of the Hawaiian Islands.
Relevant Papers:
- McGovern and Solomon, JGR-Planets,1993.
- McGovern, Ph.D. Thesis, MIT, 1996.
- McGovern and Solomon, JGR-Planets,1998.
- McGovern, GRL, 2007.
