Slide 1: Earthquake sequence (with colour coding around big events and subsequent period). Illustrates migration to the east initially into

Transcription

1 Slide 1: Earthquake sequence (with colour coding around big events and subsequent period). Illustrates migration to the east initially into Christchurch, but now moving away (23 December in particular).

2 Slide 2: The NZ National Seismic Hazard Model (NSHM) underpins building code requirements in New Zealand. Canterbury has always had earthquakes, and there has been damage in the past. The current sequence represents a much higher hazard than the long term average, but except for February 22 nd and June 13 th events in the City and Port Hills, the events have been close to code requirements. The different elements of the model are comparable earthquakes and GPS strain look similar and we know the Alpine fault will produce large earthquakes approximately 100 times more frequently than beneath the Canterbury Plains. Central panel shows all of the M>3 earthquakes for a year prior to September 4 th

3 Slide 3: This is the 2002 hazard model for 500 year return period Peak Ground Acceleration (PGA) on class C (average ground). Christchurch is about 30% of gravity, comparable with Nelson or New Plymouth and more than Dunedin, Hamilton, and Auckland. 3

4 4

5 5

6 6

7 7

8 8

9 9

10 10

11 11

12 12

13 13

14 14

15 15

16 16

17 17

18 18

19 19

20 20

21 Slide 21: Comparison of PGA s for successive events February and June were really big in the city, December very much smaller (close to code), but further significant damage occurred in some cases because of progressive accumulated damage. 21

22 Slide 22: Shows very active early phase following each major event but activity dies away quickly. 22

23 Slide 23: The blue line is the theory of a perfectly behaved mainshock and aftershock in terms of numbers of aftershocks >3 following the M7.1 Darfield earthquake. The red line is the Canterbury sequence significantly lower than the blue line especially after September 4 th. It seems like we are now getting back to being normal but this is occurring via the rejuvenation events spaced many months apart. The pink or mauve lines are the 2 standard deviations on the perfect curve. The small steps in the standard deviation curve is an artefact due to binning the data. The cumulative number scale is x

24 Slide 24: Offshore survey lines they reveal some faults but with very low average movement rates, and decreasing activity rate from northern Pegasus Bay toward the south. There are no major features (red lines) directly east of Christchurch, but there are a lot of faults in the greywacke bedrock below the red line in the lower panel. The location of this particular seismic line is shown in the upper right panel. 24

25 Slide 25: Shows that the December 23 earthquakes (now re-assessed as M5.8 followed by M 5.9) did not impact very much on the forecast probabilities. This comparison uses the old model we were using last year note the 17% in bold in the left panel. 25

26 Slide 26: These probabilities use the refined model formulation and parameter the 17% from the old model method transforms into 13% in the refined model. So models from last year are now thought to have been a bit conservative (by about 20%, depending on which magnitude range is being looked at). Note also the expected average number and the range corresponding to each probability. For low probabilities it is certainly possible that an earthquake of this size will not occur in the period. 26

27 Slide 27: Same comments as for slide 26. The 50 year model means that behind the scenes some parameter choices and some statistical models perform better over long periods compared with short term ( 1 year ) in the previous slide. There is much more uncertainty about the stability of these results for the long term and of course they will change as the model is trained by earthquakes that are still to occur. 27