• Hawaii;
  • flexure;
  • earthquakes

[1] On October 15, 2006, two large earthquakes rocked the northwest coast of the island of Hawaii six minutes apart: the Kiholo Bay and Mahukona events, with magnitudes Mw = 6.7 and 6.0, respectively. Their close proximity in space and time suggests a common origin, but sharp contrasts in mechanism and depth present an unusual fault-aftershock relationship. Here I account for the October 15th earthquakes as the divergent outcomes of a single process: downward flexing of the lithosphere in response to loading by Hawaiian volcanoes. Viscoelastic finite element models of lithospheric flexure reveal that a strong stiffness contrast between crustal and mantle materials produces peak upper lithosphere stresses at the top of the stiffer mantle. High compression at the mantle top explains the tendency to trap magmas near the base of the crust: the underplating observed seismically at older Hawaiian volcanoes. These phenomena produce peak stress regions at depths consistent with that of the unexpectedly deep Mahukona event.