FAULT STRAIN AND SEISMIC COUPLING ON MIDOCEAN RIDGES

The contribution of extensional faulting to seafloor spreading along the East Pacific Rise (EPR) axis near 3-degrees-S and between 13-degrees-N and 15-degrees-N is calculated using data on the displacement and length distributions of faults obtained from side scan sonar and bathymetric data. It is found that faulting may account for of the order of 5-10% of the total spreading rate, which is comparable to a previous estimate from the EPR near 19-degrees-S. Given the paucity of normal faulting earthquakes on the EPR axis, a maximum estimate of the seismic moment release shows that seismicity can account for only 1% of the strain due to faulting. This result leads us to conclude that most of the slip on active faults must be occurring by stable sliding. Laboratory observations of the stability of frictional sliding show that increasing normal stress promotes unstable sliding, while increasing temperature promotes stable sliding. By applying a simple frictional model to mid-ocean ridge faults it is shown that at fast spreading ridges (greater-than-or-equal-to 90 mm/yr) the seismic portion of a fault (W(s)) is a small proportion of the total downdip width of the fault (W(f)). The ratio W(s)/W(f) is interpreted as the seismic coupling coefficient chi. and in this case chi almost-equal-to 0. In contrast, at slow spreading rates (less-than-or-equal-to 40 mm/yr), W(s) almost-equal-to W(f,), and therefore chi almost-equal-to 1, which is consistent with the occurrence of large-magnitude earthquakes (m(b) = 5.0 to 6.0) occurring, for example, along the Mid-Atlantic Ridge axis.