Times and distances between events from selected global earthquake sequences; comparison with the 2010-2012 Canterbury earthquake sequence

N. J. Litchfield K. R. Berryman GNS Science Report 2012/43 January 2013

© Institute of Geological and Nuclear Sciences Limited, 2012

ISSN 1177-2425 ISBN 978-1-972192-31-3

N. J. Litchfield, GNS Science, PO Box 30368, Lower Hutt K. R. Berryman, GNS Science, PO Box 30368, Lower Hutt

BIBLIOGRAPHIC REFERENCE

Litchfield, N. J.; Berryman, K. R. 2013. Times and distances between events from selected global earthquake sequences; comparison with the 2010-2012 Canterbury earthquake sequence, GNS Science Report 2012/43. 20 p.

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CONTENTS

ABSTRACT .......................................................................................................................... III

KEYWORDS ......................................................................................................................... III

1.0 INTRODUCTION ........................................................................................................ 1

2.0 1811-1812 NEW MADRID .......................................................................................... 3

3.0 1876 OAMARU ........................................................................................................... 5

4.0 1883-1892 TASMANIA ............................................................................................... 7

5.0 1929-1968 BULLER ................................................................................................... 8

6.0 1988 TENNANT CREEK .......................................................................................... 10

7.0 1992-1999 LANDERS ............................................................................................... 12

8.0 2010-2012 CANTERBURY ....................................................................................... 13

9.0 DISCUSSION............................................................................................................ 15

10.0 CONCLUSIONS ....................................................................................................... 18

11.0 REFERENCES ......................................................................................................... 19

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FIGURES

Figure 2.1 Isoseismal map of the 1811-1812 New Madrid earthquake sequence shown with respect to the density of population at the time (Johnston and Schweig, 1996). ....................................... 3

Figure 2.2 Epicentres of ≤M 5.0 earthquakes recorded in the area of the 1811-1812 New Madrid sequence between 1974 and 1993. These are inferred to be ongoing small aftershocks (Johnston and Schweig, 1996). .................................................................................................... 4

Figure 3.1 Felt intensity maps for the 1876 Oamaru Earthquakes (M. Reyners unpubl. data). The numbers are Modified Mercalli Intensities (scale 1-12, 12 being the highest) compiled from felt reports; F is information insufficient to assign intensity. Adapted from Downes (1999). .......................................................................................................................................... 6

Figure 4.1 Isoseismal maps for the three largest earthquakes of the 1883-1892 Tasmania earthquake sequence. Adapted from Michael-Leiba (1989). ........................................................ 7

Figure 5.1 Epicentral locations of the 1929-1968 Buller earthquake sequence (red). Adapted from Hincapie et al. (2005). .................................................................................................................. 8

Figure 6.1 Locations of the 1988 Tennant Creek mainshocks (red), largest foreshocks (crosses), and selected aftershocks (circles and diamonds). Adapted from Bowman and Dewey (1991). ........................................................................................................................................ 10

Figure 6.2 Histogram of earthquakes recorded by the Warramunga seismic array (located in Figure 6.1), showing the 1987 foreshock sequence (F) and the 22 January 1988 Tennant Creek (TC) mainshocks. Ticks within the bars show earthquakes of ML 3 or greater recorded at the Alice Springs array. Some of the earthquakes prior to 1987 are from mines. Adapted from Bowman (1992). ................................................................................................................. 11

Figure 7.1 Locations of the 1992-1989 Landers earthquake sequence. From Haukkson et al. (2002). ........................................................................................................................................ 12

Figure 8.1 Locations of the (ongoing) 2010-2012 Canterbury earthquake sequence. Compiled from Geonet (http://www.geonet.org.nz/) by R. Langridge and W. Ries. ............................................ 14

Figure 9.1 Times between successive earthquakes in the sequences compiled in this report. The frequency is the number of inter-event times within each interval. A) All earthquakes. Note the horizontal axis is truncated on the right hand side. B) Only those earthquakes less than 500 days apart. ........................................................................................................... 15

Figure 9.2 Distances between successive earthquakes in the sequences compiled in this report. The frequency is the number of inter-event distances within each interval................................. 16

Figure 9.3 Time between successive earthquakes compared with earthquake magnitude. A) All earthquakes. B) Only those earthquakes less than 500 days apart. The inter-event times are shown as the time to the next earthquake, rather than time since the previous earthquake in Figure 9.1, as the aim of these plots was to test if triggering of the next earthquake in the sequence is magnitude-dependent. ............................................................... 17

Figure 9.4 Distance between successive earthquakes compared with earthquake magnitude. The inter-event distances are shown as the distance to the next earthquake, rather than distance from the previous earthquake in Figure 9.2, as the aim of these plots was to test if triggering of the next earthquake in the sequence is magnitude-dependent............................ 17

TABLES

Table 1.1 Summary of key features of the global earthquake sequences compiled in this report, including inter-event times and distances. .................................................................................... 2

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ABSTRACT

The lengths of times (days) and distances (km) between large events in selected global earthquake sequences are summarised and compared with the 2010-2012 (and ongoing) Canterbury earthquake sequence. The motivation was to address the question “Is the Canterbury sequence typical?” The global earthquake sequence inter-event times and distances were compiled from published scientific papers and maps.

The selection of the earthquake sequences is not statistically robust, but instead is simply a set of examples that we consider may have some similarities with the tectonic setting in which occurred the Canterbury earthquake sequence. The sequences reviewed are the pre-instrumental 1811-1812 New Madrid, 1876 Oamaru, 1883-1892 Tasmania, and 1929-1968 Buller sequences, and the instrumentally-recorded 1988 Tennant Creek, and 1992-1999 Landers earthquake sequences.

Inter-event times for the global earthquake sequences vary from 0 to 14,192 days (39 years) and inter-event distances from 3 to 110 km respectively. This wide variation suggests that there is no such thing as a typical earthquake sequence in terms of inter-event times and distances in low strain rate zones such as Canterbury.

The Canterbury earthquake sequence inter-event times and distances are 171, 111, and 193 days and 42, 5, and 5 km respectively. These are clearly within, but are generally at the shorter end of the global sequence ranges, which suggests that the Canterbury sequence is not atypical.

Plots of inter-event times and distances against magnitude are not well correlated, indicating earthquake magnitude cannot be used as an indication of likely times and distances to subsequent earthquakes. Instead, earthquake sequences are more likely to reflect the state of stress on faults in surrounding areas, such that earthquakes are triggered only if faults are ready to fail.

KEYWORDS

Earthquake, earthquake sequence, aftershock, inter-event time, inter-event distance, Christchurch, Christchurch Earthquake

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1.0 INTRODUCTION

This report presents an analysis of the times and distances between events in selected global earthquake sequences and then compares them with the 2010-2012 (and ongoing) Canterbury earthquake sequence. The motivation was to address the question “Is this typical?”, which was being asked by members of the public after the 23rd December 2011 aftershock. The answer turned out to be, as expected, that there is no such thing as typical sequences in regions similar to the Canterbury Plains. On the other hand, the Canterbury earthquake sequence is shown to be not atypical. Therefore, the main use of this report is most likely to simply provide examples of other sequences which can be referred to when such questions are asked in the future.

The selection of the earthquake sequences documented here are based on a broad regional characterisation of the long historical record rather than the instrumental record. Interrogation of instrumental catalogues for sequences similar to the Canterbury sequence is ongoing, but preliminary analyses also showed that the Canterbury sequence is not atypical (A. Christophersen pers. comm. 2012). We document examples that we consider may have some similarities to the Canterbury earthquake sequence tectonic setting. Some of these examples are well known and well documented (e.g. 1811-1812 New Madrid; 1929-1968 Buller; 1988 Tennant Creek; 1992-1999 Landers). Others are less well known (1876 Oamaru; 1883-1892 Tasmania), but are included because their tectonic settings are potentially similar to the Canterbury earthquake sequence. Because of the small dataset we do not undertake any detailed analysis of the tectonic setting or kinematics (e.g., between regional sequences and triggered sequences on individual fault systems).

The review focuses on the length of times (inter-event time) and distances (inter-event distance) between “large” earthquakes in the sequences. “Large” is nominally defined as greater than Magnitude1 6.0, but does include the slightly smaller (M 5.75) 1876 Oamaru earthquakes since they are one of the few New Zealand examples of an earthquake sequence. Furthermore, pre-instrumental (1811-1812 New Madrid; 1876 Oamaru, 1883-1892 Tasmania, 1929 Buller) earthquake magnitudes, are estimated from felt reports of damage, so have considerable uncertainty compared with the instrumentally recorded earthquakes (1968 Inangahua; 1988 Tennant Creek; 1992-1999 Landers; 2010-2012 Canterbury). The New Madrid and Tasmania sequences may also have large aftershocks missing from the records.

Inter-event times are simply the time between successive earthquakes in the sequence, measured in hours, days, months, or years as appropriate. Inter-event distances are measured as the distance between the epicentres of successive earthquakes in the sequence, taken from published maps. For the older, pre-instrumental, earthquakes there is considerable uncertainty in the location of these epicentres, which are highly dependent on the spread of the population, and hence documentation, in the vicinity of the earthquakes.

In sections 2-8 we briefly describe each earthquake sequence in chronological order. The Canterbury earthquake sequence is then summarised in section 9, following the same methodology. The results are summarised in Table 1.1 and are compared and discussed in section 10.

1 Where possible, the reported magnitude is Moment Magnitude Mw. However, for some of the older earthquakes, other types

of magnitude (ML- Richter, or Ms - Surface) are the only types available.

Table 1.1 Summary of key features of the global earthquake sequences compiled in this report, including inter-event times and distances.

Sequence name Earthquake name

Earthquake date

Earthquake Magnitude

Time to the next earthquake (days)

Distance to the next earthquake (km)

1811-1812 New Madrid 1 D1 16/12/1811 7.6 38 85

J1 23/01/1812 7.5 15 35

F1 7/02/1812 7.8

1876 Oamaru Oamaru I 25/02/1876 5.75 0 ?

Oamaru II 25/02/1876 5.75 46 ?

Oamaru III 25/02/1876 5.75

1883-1892 Tasmania Tasmania 1 13/07/1884 6.2 303 110

Tasmania 2 12/05/1885 6.5 2450 110

Tasmania 3 26/01/1892 6.9

1929 – 1968 Buller Buller 16/06/1929 7.3 3 21

B2 19/06/1929 6.3 3 45

B3 22/06/1929 6.5 0 48

B4 22/06/1929 6.4 23 60

B5 15/07/1929 6.3 14,192 38

Inangahua 23/05/1968 7.2

1988 Tennant Creek Tennant Creek 1 22/01/1988 6.3 0 3

Tennant Creek 2 22/01/1988 6.4 0 6

Tennant Creek 3 22/01/1988 6.4

1992-1999 Landers Joshua Tree 13/04/1992 6.1 76 30

Landers 28/06/1992 7.3 0 37

Big Bear 28/06/1992 6.2 2666 75

Hector Mine 16/10/1999 7.1

2010-2012 Canterbury Darfield 4/09/2010 7.1 171 42

Christchurch 22/02/2011 6.2 111 5

June 13/06/2011 6 193 5

December 23/12/2011 6

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2.0 1811-1812 NEW MADRID

The 1811-1812 New Madrid earthquake sequence occurred over about 8 weeks in the southeastern central USA, near the small town of New Madrid in southeast Missouri (Figure 2.1) (Johnston and Schweig, 1996; Nuttli, 1973). Being located in the middle of the American Plate, this is a true intraplate (mid-continental) sequence.

The New Madrid earthquake sequence is generally described in terms of the three largest earthquakes Mw 7.6 (16 December 1811), Mw 7.5 (23 January 1812), and Mw 7.8 (7 February 1812) (Bakun and Hopper, 2004), which occurred approximately 5.5 weeks and 2 weeks apart respectively. However aftershocks “strong enough to be felt occurred through the year 1817” (6 years) (Nuttli, 1974), which extends the sequence to at least 6 years. Small earthquakes occur in the area today and many people infer these are small aftershocks, 200 years after the start of the sequence (e.g., Johnston and Schweig, 1996) (Figure 2.2).

Figure 2.1 Isoseismal map of the 1811-1812 New Madrid earthquake sequence shown with respect to the density of population at the time (Johnston and Schweig, 1996).

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Figure 2.2 Epicentres of ≤M 5.0 earthquakes recorded in the area of the 1811-1812 New Madrid sequence between 1974 and 1993. These are inferred to be ongoing small aftershocks (Johnston and Schweig, 1996).

The successive distances between the three largest earthquakes are approximately 85 and 35 km respectively, but as noted in the introduction, the uncertainties of the epicentre locations for these pre-instrumental earthquakes are likely to be large. In this particular case, the earthquakes occurred on the western edge of settlements with newspaper accounts, and so the isoseismal contours derived from felt reports are only constrained on one side (Figure 2.1). Aftershocks up to M 6.3 are documented in the first three days, and >M 6.0 aftershocks may also have occurred in the following 6 years, but their epicentres have not been determined.

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3.0 1876 OAMARU

The 1876 Oamaru earthquakes are a poorly-understood sequence of three strong earthquakes near Oamaru, in northeast Otago, southeastern South Island New Zealand (Figure 3.1). The location approximately 220 km south-southwest of Christchurch and on the edge of the New Zealand plate boundary means it is probably the most analogous sequence to the Canterbury earthquake sequence.

The first two earthquakes occurred, 6 hours apart, on 25th February 1876, and were followed 6.5 weeks later by a third earthquake on 11th April 1876 (Downes, 1999). Some reports suggest the third earthquake was the strongest, but the distribution of felt reports (Figure 3.1) suggest each earthquake may have been of similar magnitude, M 5.75 (M. Reyners unpubl. data in Downes, 1999). Each earthquake is reported to have been followed by a rich aftershock sequence, with those after the third earthquake occurring until at least the end of May. This suggests a sequence length of at least 3.5 months.

The limited distribution of felt reports (Figure 3.1) means no reliable epicentre locations, and hence distances between the three earthquakes, can be calculated. For example, the limited distribution of settlements along the coast means that it is not clear if these earthquakes occurred on land or under the sea.

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Figure 3.1 Felt intensity maps for the 1876 Oamaru Earthquakes (M. Reyners unpubl. data). The numbers are Modified Mercalli Intensities (scale 1-12, 12 being the highest) compiled from felt reports; F is information insufficient to assign intensity. Adapted from Downes (1999).

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4.0 1883-1892 TASMANIA

The 1883-1892 Tasmania earthquake sequence occurred in the vicinity of the Cape Barren and Flinders islands off the northeast coast of Tasmania (Michael-Leiba, 1989) (Figure 4.1). This location within the Australian Plate means the sequence occurred in an intraplate setting similar to the New Madrid sequence. About 2000 earthquakes have been catalogued from newspaper felt reports and indicate that earthquake activity (number of earthquakes) peaked in 1884, but the ground shaking intensity peaked in 1892 (Michael-Leiba, 1989; Ripper, 1963). The sequence is noted to have ended abruptly in 1892, but earthquakes have occurred in the area since (Leonard, 2008; Michael-Leiba and Jensen, 1993).

Isoseismal maps are available for three of the largest earthquakes (Figure 4.1) (Michael-Leiba, 1989). A ML 6.2 earthquake on 13th July 1884 was followed 10 months later by a ML 6.5 earthquake on 12th May 1885, and then 6 years and 9 months later by a ML 6.9 earthquake on 26th January 1892.

The successive distances between these earthquakes are approximately 110 km each, but the age and submarine location means there are large uncertainties in the epicentre locations.

Figure 4.1 Isoseismal maps for the three largest earthquakes of the 1883-1892 Tasmania earthquake sequence. Adapted from Michael-Leiba (1989).

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5.0 1929-1968 BULLER

The 1929-1968 Buller earthquake sequence is defined for the purposes of this report as starting with the 1929 Buller (Murchison) Earthquake and ending with the 1968 Inangahua Earthquake, in the Buller district in the northwestern South Island, New Zealand (Figure 5.1). However, small aftershocks still occur in the region today (M. Reyners pers. comm. 2011; www.geonet.org.nz). A causative relationship between these earthquakes is not clear however, as static stress modelling suggests that the 19th June 1929 Buller Earthquake may have actually delayed the 1968 Inangahua Earthquake (Hincapie et al., 2005). The location west of the main New Zealand plate boundary means its location has some similarities to the Canterbury earthquake sequence (which is located in a similar position east of the main plate boundary).

Figure 5.1 Epicentral locations of the 1929-1968 Buller earthquake sequence (red). Adapted from Hincapie et al. (2005).

The 16th June 1929 Mw 7.3 Buller Earthquake was followed by four >Mw 6.0 aftershocks (Anderson et al., 1993; Doser et al., 1999). The first occurred 3 days after the mainshock, a Mw 6.3 earthquake on 19th June 1929. This was followed by a Mw 6.5 earthquake on 22nd June 1929 (3 days later), then 3 hours later by a Mw 6.4 earthquake on 22nd June 1929. The final large aftershock was a Mw 6.3 earthquake on 15 July 1929 (3.5 weeks after the 22nd June earthquakes).

The Mw 7.2 Inangahua Earthquake occurred on 23rd May 1968, 38 years and 10 months after the 15th July 1929 aftershock (Anderson et al., 1994). The Ingangahua Earthquake was not followed by any ≥Mw 6.0 aftershocks, but as mentioned above, small earthquakes still occur in the region today, which may be ongoing small aftershocks.

Two other relatively well known earthquakes in the region (Figure 5.1), the 1962 Westport (WE) and 1991 Hawks Crag (H) Earthquakes were of Mw 5.7 and Mw 5.9 respectively. These magnitudes are below the Mw 6.0 minimum cut-off generally used in this report, and so are not included in the Buller sequence in this report. If they were, the sequence length would have been 62 years (compared with 39 years).

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The earthquake locations generally cluster around the 1929 Buller Earthquake (Figure 5.1). The successive distances between these large earthquakes are 21, 45, 48, 60, and 35 km respectively. The distance between the Buller and Inangahua Earthquakes is 30 km.

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6.0 1988 TENNANT CREEK

The 1988 Tennant Creek earthquake sequence occurred near the town of Tennant Creek, Northern Territory, northern central Australia (Figure 6.1). Like the Tasmaina sequence, the location within the Australian plate makes it a true intraplate setting sequence, similar also to the New Madrid sequence.

Figure 6.1 Locations of the 1988 Tennant Creek mainshocks (red), largest foreshocks (crosses), and selected aftershocks (circles and diamonds). Adapted from Bowman and Dewey (1991).

The Tennant Creek sequence is slightly different to the other sequences in this report in that it includes foreshocks for approximately 1 year (Figure 6.2) before three large earthquakes on one day (Bowman, 1992; Jones et al., 1991). Aftershocks still occur today, but none have been >M 6.0 (Bowman, 1992; Jones et al., 1991; Leonard, 2008).

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Figure 6.2 Histogram of earthquakes recorded by the Warramunga seismic array (located in Figure 6.1), showing the 1987 foreshock sequence (F) and the 22 January 1988 Tennant Creek (TC) mainshocks. Ticks within the bars show earthquakes of ML 3 or greater recorded at the Alice Springs array. Some of the earthquakes prior to 1987 are from mines. Adapted from Bowman (1992).

The first large earthquake (M 6.3) was at 10.06 am on the 22nd January 1988. This was followed 3.5 hours later by a M 6.4 earthquake at 1.27 pm, and then a M 6.4 earthquake, at 9.35 pm (8 hours later). Thus the total sequence of large earthquakes lasted approximately 12 hours.

The three mainshocks occurred tightly clustered together near the centre of the faults which ruptured (Figure 6.1), with inter-event distances of approximately 3 and 6 km respectively.

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7.0 1992-1999 LANDERS

The Landers sequence is defined for the purposes of this report as starting with the 1992 Joshua Tree earthquake, a foreshock to the 1992 Landers Earthquake, and ending with the 1999 Hector Mine Earthquake, in southern California, USA (Figure 7.1). The location is on the eastern side of the North America plate boundary, in an area of strike-slip faulting, which has some similarities to the location of the Canterbury earthquake sequence.

Figure 7.1 Locations of the 1992-1989 Landers earthquake sequence. From Haukkson et al. (2002).

The Mw 6.1 Joshua Tree Earthquake occurred on 23rd April 1992, 9.5 weeks before the Mw 7.3 Landers Earthquake on 28th June 1992 (Hauksson et al., 1993). Aftershocks detectable above the background seismicity occurred for approximately 3 years (Gross and Kisslinger, 1997), but the only aftershock greater than Mw 6.0 was the Mw 6.2 Big Bear Earthquake, which occurred 3 hours after the Landers mainshock (Hauksson et al., 1993).

The Mw 7.1 Hector Mine Earthquake occurred on 16th October 1999, 7 years and 3.5 months after the Big Bear Earthquake (Hauksson et al., 2002). Less aftershocks occurred after the Hector Mine Earthquake than the Landers Earthquake (Wiemer et al., 2002), and none were >Mw 6.0 (Hauksson et al., 2002). Static stress modelling indicates the Landers Earthquake most likely triggered the Hector Mine Earthquake (e.g., Pollitz and Sacks, 2002).

The earthquakes are generally spread out along the main faults which ruptured, with the Landers aftershocks occurring over wider distances (Figure 7.1). The distances between these large earthquakes are 30, 37, and 75 km respectively. The distance between the Landers and Hector Mine Earthquakes is 50 km.

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8.0 2010-2012 CANTERBURY

The Canterbury earthquake sequence started with the 2010 Darfield Earthquake, near Christchurch, Canterbury, New Zealand (Figure 8.1), and is currently ongoing. The location is at the eastern edge of the South Island collisional plate boundary and so can be considered to be transitional between interplate and intraplate.

The Mw 7.1 Darfield Earthquake occurred on 4th September 2010 (e.g., Gledhill et al., 2011) and was followed by three earthquakes greater than Mw 6.0. The Mw 6.2 Christchurch Earthquake occurred 5.5 months after the Darfield Earthquake, on the 22nd February 2011 (Kaiser et al., 2012). A Mw 6.0 earthquake occurred nearly 4 months later, on 13th June 2011, and was followed approximately 6 months later by a Mw 5.9 earthquake on 23rd December 2011 (http://www.geonet.org.nz/).

The earthquake locations are spread out along the main faults which ruptured in the Darfield Earthquake, but a large number, including the three large aftershocks and their own aftershocks, occurred distinctly further east (Figure 8.1). The distances between the large earthquakes are 42, 5, and 5 km respectively.

Figure 8.1 Locations of the (ongoing) 2010-2012 Canterbury earthquake sequence. Compiled from Geonet (http://www.geonet.org.nz/) by R. Langridge and W. Ries.

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9.0 DISCUSSION

The inter-event times and distances compiled for selected earthquake sequences show considerable variation, ranging from 0 to 14,192 days (39 years) and from 3 to 110 km respectively (Table 1.1, Figure 9.1 and Figure 9.2). As shown in Figure 9.1, the majority of inter-event times cluster at less than 200 days, but there is no clear clustering of inter-event distances in Figure 9.2. Bearing in mind that this is a small sample of global earthquake sequences, the variation nevertheless leads to the conclusion noted in the introduction that there is no such thing as a typical earthquake sequence in terms of inter-event times and distances.

Figure 9.1 Times between successive earthquakes in the sequences compiled in this report. The frequency is the number of inter-event times within each interval. A) All earthquakes. Note the horizontal axis is truncated on the right hand side. B) Only those earthquakes less than 500 days apart.

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Figure 9.2 Distances between successive earthquakes in the sequences compiled in this report. The frequency is the number of inter-event distances within each interval.

The Canterbury earthquake sequence inter-event times of 171, 111, and 193 days and the inter-event distances of 42, 5, and 5 km are clearly within the ranges of global earthquake sequence inter-event times and distances (Table 1.1, Figure 9.1 and Figure 9.2). In general terms, the Canterbury earthquake sequence inter-event times are at the shorter end of the global range (Figure 9.1A), but they are in fact longer than the majority compiled here; 12 of the 16 global inter-event times are shorter than the Canterbury earthquake sequence inter-event times (Figure 9.1B). The Canterbury earthquake sequence inter-event distances are also generally at the shorter end of the global range, particularly the two 5 km distances between the large aftershocks. Bearing in mind that this is a small sample of global earthquake sequences, the comparisons nevertheless lead to the conclusion that the Canterbury sequence inter-event times and distances are not atypical. A similar conclusion was drawn by preliminary analysis of instrumental earthquake catalogues (A. Christophersen pers. comm. 2012).

Figure 9.3 and Figure 9.4 show the inter-event times and distances plotted against earthquake magnitude. The purpose of these figures were to see if there was a causative relationship, such as larger earthquakes trigger earthquakes after longer times and over greater distances. If this were the case, then the graphs would show a positive trend. These graphs show that, at least for the global earthquake sequences compiled here, there is no such relationship, or in fact any relationship at all. That is, they show that larger magnitude earthquakes trigger earthquakes after a range of times and at a range of distances. This suggests that earthquake magnitude cannot be used as an indication of likely times and distances to subsequent earthquakes. Instead, earthquake sequences are more likely to reflect the state of stress on faults in surrounding areas, such that earthquakes are triggered only if faults are ready to fail.

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Figure 9.3 Time between successive earthquakes compared with earthquake magnitude. A) All earthquakes. B) Only those earthquakes less than 500 days apart. The inter-event times are shown as the time to the next earthquake, rather than time since the previous earthquake in Figure 9.1, as the aim of these plots was to test if triggering of the next earthquake in the sequence is magnitude-dependent.

Figure 9.4 Distance between successive earthquakes compared with earthquake magnitude. The inter-event distances are shown as the distance to the next earthquake, rather than distance from the previous earthquake in Figure 9.2, as the aim of these plots was to test if triggering of the next earthquake in the sequence is magnitude-dependent.

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10.0 CONCLUSIONS

Inter-event times and distances calculated from a selection of global earthquake sequences vary from 0 to 14,192 days (39 years) and from 3 to 110 km respectively. This wide variation suggests that there is no such thing as a typical earthquake sequence.

The 2010-2012 Canterbury earthquake sequence inter-event times and distances are 171, 111, and 193 days and 42, 5, and 5 km respectively, which are generally at the shorter end of the global sequence ranges. This suggests that the Canterbury sequence is not atypical.

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11.0 REFERENCES

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Anderson, H., Beanland, S., Blick, G., Darby, D., Downes, G., Haines, J., Jackson, J., Robinson, R. and Webb, T., 1994. The 1968 May 23 Inangahua, New Zealand, earthquake: an integrated geological, geodetic, and seismological source model. New Zealand Journal of Geology and Geophyics, 37: 59-86.

Bakun, W.H. and Hopper, M.G., 2004. Magnitudes and locations of the 1811-1812 New Madrid, Missouri, and the 1886 Charleston, South Carolina, Earthquakes. Bulletin of the Seismological Society of America, 94: 64-75.

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Bowman, J.R. and Dewey, J.W., 1991. Relocation of teleseismically recorded earthquakes near Tennant Creek, Australia: Implications for midplate seismogenesis. Journal of Geophysical Research, 96(B7): 11973-11979.

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Hincapie, J.O., Doser, D.I. and Robinson, R., 2005. Stress changes induced by earthquakes and secular stress accumulation in the Buller Region, South Island, New Zealand (1929-2002). Pure and Applied Geophysics, 162: 291-310.

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Avalon

PO Box 30368

Lower Hutt

New Zealand

T +64-4-570 1444

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