D A Rhoades, C Mueller, R Buxton, M C Gerstenberger, GNS Science (EQC funded project 10/592)
Disturbances in the total electron content (TEC) of the ionosphere have been reported as proposed short-term precursors to earthquakes in some regions, with a lead time of several days. In this project we computed TEC in the New Zealand region, with a view towards testing the value of TEC anomalies for short term earthquake forecasting. We adapted software produced by the University of Texas to compute TEC from GPS network observations. We mapped TEC in the ionosphere on a spatial grid covering the whole of New Zealand, at half-minute intervals, for several weeks before, and a few days after, four recent major earthquakes – the 2010 Darfield, 2011 Christchurch, 2013 Seddon and 2013 Lake Grassmere earthquakes.
TEC has strong daily fluctuations, with high values during daytime and near-zero values at night-time. Also, TEC averaged over the whole region was found to have anomalously high spikes preceding three of the four earthquakes, with lead times ranging from 7–11 days. However, the spatial distributions of TEC at the times of these spikes suggest that they are probably due to solar or geomagnetic effects and not related to earthquake occurrence. They are different from anomalies in the literature, which show local perturbations of the daily fluctuations of TEC close to the location of forthcoming earthquakes.
A local correlation index was defined for each point in the spatial grid, to find TEC anomalies that are more likely to be related to earthquakes. Most values of the index are very close to one. A value much less than one is anomalous. The index showed an anomalously low value near to the location of the Christchurch earthquake three days before its occurrence. The lead time of three days is typical for proposed ionospheric earthquake precursors. The index showed no local anomaly in the lead up to the Darfield, Seddon and Lake Grassmere earthquakes.
A short-term precursor can be quite informative for earthquake forecasting, even if it occurs only for some of the major earthquakes. Future studies should extend the database of TEC observations to a much longer period. The local correlation index should then be used as one possible systematic input to short-term earthquake forecasting models. Its information value can best be measured in combination with other inputs, which include what we already know about how the likelihood of earthquake occurrence varies in time and space.
Anecdotal studies suggest that ionospheric disturbances sometimes occur as short-term precursors to earthquakes with a precursor time of several days. This study initiates an examination of possible ionospheric earthquake precursors in New Zealand, with a view to systematically testing their value for short-term earthquake forecasting.
GPSTk software produced by the University of Texas at Austin to compute Total Electron Content (TEC) in the ionosphere from GPS network observations has been adapted to the New Zealand region. It has been used to estimate the TEC in the ionosphere on a spatial grid of locations surrounding the New Zealand region for selected periods preceding and following four recent major earthquakes – the M 7.1 2010 Darfield earthquake, M6.3 2011 Christchurch earthquake, M 6.5 2013 Seddon earthquake and M 6.6 2013 Lake Grassmere earthquake. The software gives snapshots of the spatial variation of TEC over New Zealand at 30 second intervals.
The time series of maximum and mean regional TEC, in the lead-up to the four earthquakes, was studied in a search for precursory anomalies. TEC has strong daily fluctuations, with high values during daytime and near-zero values at night-time. Significant higher-than-normal TEC anomalies were found for the Darfield, Christchurch and Lake Grassmere earthquakes with precursor times ranging from 7–11 days. Despite the similarities of the precursor times, the spatial distributions of TEC at the times of these anomalies suggest that they are unrelated to the earthquake concerned and more likely due to solar or geomagnetic effects. They are different from proposed ionospheric precursors in the literature, in which the anomalies are seen as local variations to the pattern of TEC variation close to the location of forthcoming earthquakes.
A local correlation index was defined to identify TEC anomalies which are more likely to be related to earthquake occurrence. This is a daily index that compares the variation of TEC within a day at each point of the grid with the average daily variation at nearby surrounding grid points. It is designed to distinguish local variations in TEC from the large scale variations caused by solar and geomagnetic influences. Most values of the index are very close to one. A value much less than one is anomalous.
The local correlation index was plotted for the same time periods before and after the four selected earthquakes, for the grid point closest to each earthquake source. For the Christchurch earthquake, the index showed an anomalously low value with a precursor time of three days. For the other earthquakes no precursor anomaly was observed.
A short-term precursor can be quite informative for earthquake forecasting, even if it occurs only for some of the major earthquakes. It is proposed that future studies should extend the database of TEC observations to a much longer period. Then the local correlation could be used as a systematic input for the formation of hybrid short-term earthquake forecasting models. In that way its information value for earthquake forecasting, over and above what is already known about how the likelihood of earthquake occurrence varies in time and space, could be determined.