Ground ruptures of the 1974 and 1983 Kaoiki Earthquakes, Mauna Loa Volcano, Hawaii

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Abstract
The November 30, 1974, ML = 5.5 and November 16, 1983, ML = 6.6 earthquakes generated left‐stepping, en echelon ground cracks within the Kaoiki seismic zone, on the southeast flank of Mauna Loa volcano, Hawaii. The general trend of the ruptures, N48°–55°E, parallels a nodal plane of the main shocks' focal mechanisms. The ruptures themselves consist of short, predominantly extension cracks, which are up to 20 m long and strike roughly E‐W, 30°–50° clockwise from the overall trend of the zones. Some of the cracks are linked by secondary fractures and rubble breccia to form left‐stepping crack arrays, which are themselves linked to form longer en echelon systems of ground rupture. Geologic maps and field observations indicate that these features emerge from an underlying strike‐slip fault, and they form a “fracture‐process zone” above its tip. The maximum displacement measured across cracks in the 1983 rupture zone is 0.5 m. Trilateration data, however, suggests that the overall shear displacement was about 1.5 m at depth. Elastic solutions indicate that a region of significant tensile stress can exist above buried strike‐slip faults. We suggest that these stresses generated the extensional ground cracks and that shear displacements were transmitted to the Earth's surface by subsequent growth and linkage of these cracks into the observed arrays. We infer that the crack arrays accommodate increased displacement with depth and they merge downward into the “parent” strike‐slip fault at an estimated 1–2 km depth, where strike‐slip displacement was probably more or less continuous along the ∼7 km length of the rupture. In the Kaoiki region, only three major ground ruptures traverse a series of basaltic lava flows that date back 1500 years. This suggests that the recent ∼10‐year periodicity of moderate‐magnitude Kaoiki strike‐slip events may not have extended far into the past. The tectonic significance of strike‐slip faulting on Mauna Loa volcano remains enigmatic.