Geodetic, hydrologic and seismological signals associated with precipitation and infiltration in the central Southern Alps, New Zealand
The Southern Alps of New Zealand is an actively deforming mountain range, along which collision between the Pacific and Australian plates is manifest as elevated topography, orographic weather, active contemporary deformation, and earthquakes. This thesis examines interactions between surface processes of meteorological and hydrological origin, the ground surface deformation, and processes within the seismogenic zone at depth. The two main objectives of the thesis are a better understanding of the reversible repetitive ground surface deformation in the central Southern Alps and the analysis of the evolution of the rate of microseismicity in the area to explore relationships between seismicity rates and the hydrologic cycle. Surface deformation in the central Southern Alps is characterised by a network of 19 continuous GPS stations located between the West Coast (west) and the Mackenzie Basin (east), and between Hokitika (north) to Haast (south). These show repetitive and reversible movements of up to ∼55mm on annual scales, on top of long-term plate motion, during a 17 year-long period. Stations in the high central Southern Alps exhibit the greatest annual variations, whereas others are more sensitive to changes following significant rain events. Data from 22 climate stations (including three measuring the snowpack), lake water levels and borehole pressure measurements, and numerical models of solid Earth tides and groundwater levels in bedrock fractures, are compared against geodetic data to examine whether these environmental factors can explain observed patterns in annual ground deformation. Reversible ground deformation in the central Southern Alps appears strongly correlated with shallow groundwater levels. Observed seasonal fluctuation and deformation after storm events can be explained by simple mathematical models of groundwater levels. As a corollary, local hydrological effects can be accounted for and ameliorated during preprocessing to reduce noise in geodetic data sets being analysed for tectonic purposes. Two catalogues of earthquakes (containing 38 909 and 89 474 events) in the area spanning the period 2008–2017 were built using a matched-filtered detection technique. The smaller catalogue is based on 211 template events, each of known focal mechanism, while the latter is based on 902 templates, not all of which have focal mechanisms, providing greater temporal resolution. Microseismicity data were examined in both time and frequency domains to explore relationships between seismicity rates and the hydrologic cycle. Microseismicity shows a pronounced seasonality in the central Southern Alps, with significantly more events detected during winter than during summer. These changes cannot be easily accounted for by either acquisition or analysis parameters. Two models of hydrologically-induced seasonal seismicity variations have been considered — surface water loading and deep groundwater circulation of meteoric fluids — but neither model fully explains the observations, and further work is required to explain them fully. An observed diurnal variation in earthquake detection rate is believed to originate mostly from instrumental effects, which should be accounted for in future seismological studies of earthquake occurrence in the central Southern Alps. Relationships and correlations observed between hydrological, geodetic, and seismological data from the central Southern Alps provide clear indications that surface processes exert at least some degree of influence on upper-crustal seismicity adjacent to the Alpine Fault.