Abstract
Abstract. The water level of the Waiwera geothermal reservoir is mainly governed by the production operations due to a commonly strong pumping signal. However, in the hours and days after the Kaikoura earthquake on 14 November 2016, it increased by more than 0.5 m, indicating that seismic events can have an influence as well. In a continuous time series consisting of monthly means of water level data and pumping rates starting in 1986, we tried to determine if events above a certain strength alter groundwater dynamics at Waiwera in general. We applied an empirical equation and calculated the on-site seismic energy density from earthquake magnitudes and distances. All recorded earthquakes with a magnitude above 4 within a radius of about 1500 km around New Zealand have been taken into account. A clear correlation cannot be proven but none of the recorded earthquakes led to such a high energy density in Waiwera as the Kaikoura event did. For the future, it is recommended to increase the resolution of the metred water production rates to daily averages to improve the detectability of water level changes following earthquakes.
Highlights
The warm water reservoir of Waiwera on the North Island of New Zealand has been known to the indigenous people, the Maori, for many centuries
Hydrological impacts of earthquakes on water levels in wells of reservoirs have been documented for a long time (Wang and Manga, 2010). They seem to be associated with seismic waves and can be estimated empirically for the near and intermediate field from the on-site seismic energy density, which is defined as the maximum energy e available in a unit volume to do work on rocks or sediments calculated from magnitude M and distance r from the earthquake epicentre to the observed event as given in Eq (1), published by Wang and Chia (2008): log (r) − 0.48M + 1.4 log (e) = −
The diagonals of equal seismic energy density indicate that about 0.3 % of all recorded earthquakes (n = 56) are above the threshold for water level changes with e = 10−3 J m−3
Summary
The warm water reservoir of Waiwera on the North Island of New Zealand has been known to the indigenous people, the Maori, for many centuries It is located about 40 km north of Auckland below the village (Fig. 1a). Wang and Manga (2010) compiled and documented observed hydrologic changes of this kind and showed that they occur mostly in the shallow subsurface (< 1 km depth) Because the water level in the monitoring well at Waiwera increased by more than 0.5 m in the hours and days after the Kaikoura earthquake on 14 November 2016 (Fig. 2) and artesian flow was temporarily observed from various production wells, as reported by residents, we decided to explore the existing data sets of water level and production rates again. For the investigations presented here, we took into account all recorded earthquakes with a magnitude above 4 since 1986 around New Zealand, consistent with the available data sets of water level measurement and metred production rates
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