Rupturing of the Awatere Fault during the 1848 October 16 Marlborough earthquake, New Zealand: Historical and present day evidence

Abstract

Evidence from newspaper reports, diaries and journals, related first‐hand information, an 1854 survey map of the lower part of the Awatere Valley, and reports by geologists between 1856 and 1890, indicates that surface rupturing occurred on the Awatere Fault during the 1848 October 16 earthquake and not on the Wairau Fault as previously inferred. The rupture was initially described as a “fissure”, “crack”, and later as a “rent”, and although it extended for c. 105 km (from the coast to Barefell Pass), it was not termed a fault because displacement of the land surface or strata across the rupture could not be determined. The coincidence of the 1848 earthquake “rent” and the Awatere Fault was first demonstrated by Alexander McKay in 1885. Present day evidence of the 1848 Awatere Fault rupture is indicated by a depression between 0.6–1.5 m wide and c. 0.3 m deep that has the appearance of an infilled fissure similar to that described in early reports. The smallest and freshest displacements along the fault (15 displacements; 12 localities, most within c. 20 km of the coast) range between 4 and 8 m dextral with both the southeast and northwest sides variably upthrown between 3.5 and 0.4 m. Dextral and vertical displacement appears to decrease towards the coast, consistent with a low rate of late Quaternary slip (<1.4 mm/ yr) and the inferred termination of the fault in Cook Strait. The 1848 rupture apparently bypassed the ENE‐striking Molesworth section of the Awatere Fault, which implies that the intersection (near Molesworth Station) between this strand and the eastern section of the fault that reaches the coast may be an important mechanical boundary controlling the location of earthquake rupture segments along the Awatere Fault. Reassessment of felt intensities (based on contemporary descriptions), minimum fault rupture length (105 km), and average dextral displacement (6 m), indicates a magnitude for the 1848 earthquake of M 7.4–7.5.