Paleoearthquakes on the Porters Pass fault

Technical Abstract

The Porters Pass fault (PPF) is a prominent element of the Porters Pass-Amberley Fault Zone (PPAFZ) which forms a broad zone of active earth deformation ca 100 km long, 60-90 km west and north of Christchurch. The PPF is defined by a series of discontinuous Holocene active traces for a distance of ca 40 km between the Rakaia and Waimakariri Rivers.

Slip during paleoearthquakes is quantified by measuring 25 displaced geomorphic features (e.g., relict streams, stream channels and ridge crests) using either tape measure or surveying equipment. From south of Lake Lyndon to northeast of Porters Pass clustering of offset data are consistent with four to five earthquakes on the PPF during the Holocene with slip/event of ca 5-7 In/event and a slip rate of 2.5-3.4 mm/yr. To the southwest in the Lake Coleridge region only one Holocene rupture was unequivocally identified and produces a mean slip rate for this period of 0.5-0.7 mm/yr.

Information on the timing of prehistoric earthquakes on the PPF was obtained from four trenches excavated across the fault and two auger holes adjacent to the fault. Fourteen new wood, charcoal and peat samples from these sites were radiocarbon dated and used in conjunction with data from previous studies to constrain the timing and recurrence intervals of past earthquakes. At least four Holocene earthquakes were identified at; 8500 ± 200, 5300 ± 700, 2500 ± 200 and 1000 ± 100 calibrated years B.P. An additional event may also have occurred at 6200 ± 500 calibrated years B.P. The ca 1000, 5300 and 6200 years B.P. paleoearthquakes were previously unrecognised, while an earthquake ca 500 years ago previously inferred from rock-avalanches close to the fault was not confirmed by the trench data and is not included in the estimate of recurrence interval. The present data set produces average recurrence intervals for the Holocene of ca 2000-2500 years.

The combination of geometric and displacement/event data indicates moment magnitudes of prehistoric earthquakes on the PPF of 7.1 to 7.4 and 7.5 to 7.7 for the entire PPAFZ. These estimates are broadly consistent with previous magnitude estimates along the full length of the PPAFZ.