1. Reappraisal of single station locations reported by the SANSN
10th Inkaba yeAfrica/!Khure Africa (AEON) Conference/Workshop
Lord Milner Hotel, Matjiesfontein - Karoo
29 September – 3 October 2014
Reappraisal of single station locations reported by the SANSN
Ian Saunders1 and Stoffel Fourie2
1. Council for Geoscience, Private Bag X112, Silverton, Pretoria, South Africa, 0001.
2. Environmental, Water and Earth Science Department, Faculty of Science, Tshwane University of Technology, Private Bag X460, Pretoria, South Africa, 0001.

2. OUTLINE
Introduction
Methodology
Results
Conclusion
References

3. Introduction
We critically evaluate the epicentral solution of earthquakes located through the three-component single-station methodology (Roberts et al., 1989) as implemented in the SEISAN software (Ottemöller et al., 2012), during the period October 2000 to December 2005.
The practice of evaluating earthquake epicenters at the Council for Geoscience (CGS) with the single-station method was introduced following reports of an increase in felt earthquake activity in the town of Ceres, located in the Western Cape Province of South Africa, during the last quarter of 2000.

4. Introduction

5. Methodology
Visual inspection for quality and to determine accurate primary and secondary phase onsets.
Amplitude Signal-to-Noise Ratio (ASNR) to detect the signal above ambient noise. An ASNR≥1.5 (Diehl and Kissling , 2007) was accepted as the threshold.
Waveform data from the International Monitoring Stations (IMS) was obtained from the Incorporated Research Institutes for Seismology (IRIS).
Primary and secondary phases identified on these waveforms were used with existing phase readings to obtain epicentral solutions with the HYPOCENTER software (Lienert and Havskov, 1995).
Epicentral solutions (Root Mean Square (RMS)) of travel time residuals ≤2 seconds were accepted and included in the database.
Magnitudes determined during this study were from the maximum amplitude of the S-Lg wave complex. The calibrated local magnitude (Saunders et al., 2013) was adopted for uniformity.

6. Methodology

7. Results
A total of 110 seismic events could be confirmed and located with phase reading data from three or more stations.
47 Events could be attributed to mining related activity (labeled as 1 to 7 on the map), while the remaining 63 events were of a tectonic nature.  
A further 237 events could be confirmed by phase readings from the IMS stations, five of which had been recorded with three stations but reliable epicenter solutions could not be determined due to indistinct phase onsets.
Three events that had originally been identified as having a regional origin coincided with teleseismic signals identified from the IMS waveforms.

8. Results

9. Conclusions
This study critically reviewed regional events located with the single-station location technique by the SANSN during the period 2000 to 2005.
The results of this short dataset (5 years) indicate a large number of earthquakes located with the single-station technique could not be confirmed with observations at seismological stations of the IMS.
This apparent discrepancy may result from many localized small earthquakes that can only be discerned at very short distances.
A large number of events relocated with the least square iterative method, had epicenter differences of 50km or less from the original epicentral solution determined with the single-station technique.
This error remains large even for regional studies. This study agrees with the remark of Frolich and Pulliam (1999) who cautions that the current available single-station location methodology is inadequate for routine use at regional distances (1 -10°).

10. References
Diehl, T and E. Kissling (2007). Users guide for consistent phase picking at local to regional scales. Institute of Geophysics. Eidgenőssische Technische Hochshule, Zurich, Switzerland. 21 pp.
Lienert, B.R.E. and J. Havskov (1995). A computer program for locating earthquakes both locally and globally, Seismological Research Letters, 66, 5, 26–36.
Mine Health and Safety Act, No. 29 of 1994.
Roberts, R. G., A. Christoffersson, and F. Cassidy (1989) . Real time events detection, phase identi­fication and source location estimation using single-station component seismic data and a small PC, Geophysical Journal International, 97, 471–480.
Saunders, I.,L. Ottemöller, M.B.C. Brandt, and C.J.S. Fourie (2013) Calibration of an MLscale for South Africa using tectonic earthquake data recorded by the South African National Seismograph Network: 2006 to 2009, Journal of Seismology, 17, 2,