Variability of single event slip and recurrence intervals for large magnitude paleoearthquakes on New Zealand’s active faults A. Nicol R. Robinson R. J. Van Dissen A. Harvison GNS Science Report 2012/41 December 2012 BIBLIOGRAPHIC REFERENCE Nicol, A.; Robinson, R.; Van Dissen, R. J.; Harvison, A. 2012. Variability of single event slip and recurrence intervals for large magnitude paleoearthquakes on New Zealand’s active faults, GNS Science Report 2012/41. 57 p. A. Nicol, GNS Science, PO Box 30368, Lower Hutt 5040, New Zealand R. Robinson, PO Box 30368, Lower Hutt 5040, New Zealand R. J. Van Dissen, PO Box 30368, Lower Hutt 5040, New Zealand A. Harvison, PO Box 30368, Lower Hutt 5040, New Zealand © Institute of Geological and Nuclear Sciences Limited, 2012 ISSN 1177-2425 ISBN 978-1-972192-29-0 CONTENTS LAYMANS ABSTRACT ....................................................................................................... IV TECHNICAL ABSTRACT ..................................................................................................... V KEYWORDS ......................................................................................................................... V 1.0 INTRODUCTION ........................................................................................................ 1 2.0 GEOLOGICAL EARTHQUAKES ................................................................................ 2 2.1 Data Sources ................................................................................................................. 2 2.2 Sampling artefacts ......................................................................................................... 6 3.0 SYNTHETIC EARTHQUAKES ................................................................................. 11 4.0 DATA ANALYSIS ..................................................................................................... 13 5.0 RESULTS ................................................................................................................. 17 5.1 Recurrence Intervals ................................................................................................... 17 5.1.1 Histograms and Probability Density Functions (PDF) ..................................... 17 5.1.2 Coefficient of Variation (COV) ......................................................................... 19 5.2 Single event slip .......................................................................................................... 21 5.2.1 Histograms ...................................................................................................... 22 5.2.2 Coefficient of Variation (COV) ......................................................................... 24 6.0 APPLICATION TO SEISMIC HAZARD ASSESSMENT ........................................... 27 7.0 CONCLUSIONS ....................................................................................................... 30 8.0 ACKNOWLEDGEMENTS ......................................................................................... 31 9.0 REFERENCES ......................................................................................................... 31 GNS Science Report 2012/41 i FIGURES Figure 2.1 Active fault map of New Zealand showing the locations of faults (thick black lines) listed in Table 2.1. Fault numbers correspond to those given in the left-hand column of Table 2.1. Fault locations form the GNS Science active faults database (GNS open access, August 2012). Red boxes show regions of synthetic seismicity models. ......................... 5 Figure 2.2 Displacement profile along the surface rupture of the Greendale Fault 2010 Mw 7.1 Darfield earthquake (modified from Quigley et al., 2012). Blue and red lines show displacement variations using the ‘best’ measurements for the west (W) and east (E) fault strands, respectively. The different types of measurement are: max, maximum of multiple measurements; best, preferred measurement from multiple measurement; indiv, one measurement; min, minimum of multiple measurements. ..................................................... 7 Figure 2.3 Probability of surface rupture for earthquakes beneath the New Zealand landmass since 1840 (solid line) (data from Downes, in press) and from the Wells and Coppersmith (1994) global compilation of historical earthquakes from 1954-1994 (dashed line). ..................... 8 Figure 2.4 Recurrence interval histograms for individual active faults with 7 or more recorded surface-rupturing earthquakes generated using the data in Appendix 1 and the Monte Carlo method outlined in the “Data Analysis” section. ................................................................ 10 Figure 4.1 Probability Density Functions (PDF) for the 7 faults presented in Figure 2.4. To enable comparison of the PDFs for faults with different mean recurrence intervals the curves have been normalised to their mean (i.e. mean=1). The thick black line is the arithmetic mean of the 7 faults presented. Note that for the examples presented the most common recurrence interval is typically less than or equal to the average, while recurrence intervals a factor of 2-4 times the average are common. ............................................................ 14 Figure 4.2 Positive relationship between the incompleteness magnitude (Mc) and the mean recurrence interval for synthetic earthquakes that ruptured anywhere on the Wairau Fault (i.e. includes events that do not rupture the ground surface or the entire fault). Note the similarity of mean synthetic recurrence for Mc 7.2 (~2450 yrs) and mean recurrence for geological paleoearthquakes (2260 yrs) which, using the data on the graph, would correspond with an Mc of about 7.15. ........................................................................................ 15 Figure 5.1 Recurrence interval PDFs for synthetic earthquakes on six faults (Awatere, Wellington, Wairau, Wairarapa, Hope, and Paeroa faults). For each fault events included in the plots are ≥Mc (for further discussion see section 2.2) and recurrence interval normalised to the mean (i.e. mean=1). Combined curve is the arithmetic mean of the curves for the individual faults. .......................................................................................................................... 18 Figure 5.2 Variation in COV over time for sample windows of 7 synthetic earthquakes (≥ Mc ~7.5) on the Wairau Fault. This figure illustrates that the COV on a fault may change temporally. .................................................................................................................................. 19 Figure 5.3 COV histograms for recurrence interval of geological earthquakes (A) and synthetic earthquakes (B) and for slip of geological earthquakes (C). ....................................................... 20 Figure 5.4 COV of recurrence interval vs slip rate for geological and synthetic large magnitude earthquakes. ............................................................................................................................... 21 Figure 5.5 Histograms of single event slip for 10 active faults with 4 or more recorded earthquake slip values generated using the data in Appendix 1 and the Monte Carlo method outlined in the text. Red lines indicate the mean single event slip. For description of the faults see Table 2.1. Numbers beside fault name correlate with numbers assigned in Table 2.1 and Appendix 1. ................................................................................................................................ 23 ii GNS Science Report 2012/41 Figure 5.6 Histograms of single event slip from synthetic earthquake models for 5 of the active faults including the Wairau, Paeroa and Whirinaki faults shown in Figure 5.5. In each case the Single event slip has been normalised to the mean. .................................................... 24 Figure 5.7 Single event slip histograms for synthetic earthquakes on the Wairau Fault. (A) Average slip for all earthquakes that ruptured the ground surface in the central section of the fault trace, (B) average slip for earthquakes of Mc 7.2 that ruptured anywhere on the fault surface (i.e. they need not rupture the ground surface or the central section of the fault) and, (C) average slip for earthquakes of Mc 6.5 that ruptured anywhere on the fault surface (i.e. they need not rupture the ground surface or the central section of the fault). Inclusion of smaller events shows a significant decrease in the modes between the upper two graphs and the lower histogram................................................................................. 25 Figure 6.1 (A) Average recurrence PDF for geological data (red bars) plotted with best-fit log-normal and Weibull distributions. (B) Average PDF for synthetic recurrence intervals from six faults (Figure 5.1) plotted with normal, log normal and Weibull best-fit distributions. ................. 28 TABLES Table 2.1 Summary of faults and paleoearthquake information from geological datasets in this study. Timing of events and recurrence rounded to nearest 10 years. Single event slip (and uncertainty) rounded to nearest 10 cm. See Appendix 1 for raw paleoearthquake data and Figure 2.1 for fault locations. † Slip rates calculated for approximately the duration of the paleoearthquake record. In single event slip column “*” denotes mean calculated for two or three events only. Alpine Fault data excludes Hokuri Creek data of Berryman et al. (2012) as