We propose that two seismic gaps exist in southern Hawaii. From their dimensions and tectonic setting, we estimate that the south Kona gap may be capable of a mainshock in the magnitude range of to , while the east Kona gap will probably not produce earthquakes larger than . During historic time (1832 to the present), no mainshocks have occurred in these gaps. Since there is no evidence that fault creep relieves stress in these areas, we expect earthquakes to fill these gaps. The boundaries of the south Kona gap are defined by the approximate rupture extent of the earthquakes of 2 April 1868 (M = 7.9) and 21 August 1951 (M = 6.9), those of the east Kona gap by the latter mainshock and the extent of the seismogenic zone.
The seismotectonic model on which we base our gap hypothesis defines a major seismic zone in the southern half of the island of Hawaii. We propose that most major and large earthquakes within this zone are of a decollement type, in which the upper 10 km of the crust is pushed seaward along a near-horizontal plane of weakness, provided by the oceanic sediments buried beneath the volcanic edifice. The direction of slip is away from the volcano summits and perpendicularly away from the volcano rifts.
Several recurrence intervals are known for the Kaoiki area, where M = 6 ± 0.6 mainshocks occur every 10.4 ± 1.5 years. For the Kalapana area, one recurrence interval of 108 years is known for earthquakes with 7 < M < 8. In most of the major seismic zone in Hawaii, the recurrence times are not known, and no information on strain accumulation rate is available. It is therefore not possible to estimate when the portion of the 1868 rupture that has not re-ruptured yet (southern Kau) will produce another large mainshock. The potential for a large earthquake in this area is estimated to be intermediate. The potential in the Kalapana, the central Kona, and northern Kona areas are estimated to be low for the next several decades.