Aftershock detection

Aftershocks of the Darfield earthquake were recorded by 13 portable earthquake recorders that were deployed by the GeoNet seismic rapid response team on 5 and 6 September (co-ordinated with Victoria University of Wellington), plus 11 more deployed between the 7th and 10th September. Ten short-period seismometers were deployed west of Christchurch in the Canterbury Plains region between the Waimakariri and Rakaia Rivers. Seven strong-motion accelerometers were placed in buildings, three of these along the fault rupture, whilst 7 more were installed in Christchurch city. The instruments, complementing the permanent network, were retrieved after three and a half weeks. The aftershock data is used to image the subsurface fault structure and assess how strain in the earth was released following the Darfield earthquake.

In the day following the mainshock, most aftershocks clustered along the 22 km long, east-west fault trace produced during the mainshock. The aftershock zone then progressively expanded further to the east and west until it was over 60 km long. Such aftershock zone expansion is common for earthquake sequences on large, pre-existing faults. Following the mainshock, there is a discontinuity in strain at the lateral edges of the fault, between the fault patch that slipped and the surrounding fault patches that didn't. With time, this strain discontinuity is relieved by slow creep into these surrounding patches. As this creep expands outward from either end of the main rupture, rough spots on the fault loaded by the surrounding creep rupture, producing aftershocks. This process, together with the stress redistribution caused by the mainshock, leads to the expansion of the aftershock zone with time.

Aftershock modelling

Following a large earthquake, or mainshock, many smaller earthquakes, or aftershocks, occur with a decaying pattern in time, space and magnitude relative to the mainshock. When taken as a group, aftershock behaviour tends to follow predictable patterns, and we can have a good idea of how many aftershocks of a particular magnitude range we can expect and when they may occur; however, within that group, individual aftershocks can be random and unpredictable, which makes it impossible to know when or where any single aftershock will occur. We can estimate the average number of aftershocks we expect by looking at how aftershock sequences have behaved in the past in New Zealand.

For the Darfield earthquake, the fault length of the mainshock was estimated to be approximately 25 km and the aftershocks generally occurring within this distance from the mainshock. Below is a table that shows the number of aftershocks that occurred and how many were expected. The accompanying diagrams shows the modelled number of aftershocks of magnitude 4-4.9, and magnitude 5.0 and greater, per day for the month following the magnitude 7.1 Darfield earthquake. The black line is what GNS scientists modelled as the average daily expected number of aftershocks. On most days, the number of magnitude 5 or larger aftershocks should fall within the boundary of the light grey lines. The yellow stars show the number of actual aftershocks that occurred each day since the mainshock.

 

Canterbury region aftershock statistics and forecasts

Dates

Expected number of aftershocks of magnitude 4.0 - 4.9

Observed

Expected number of aftershocks of magnitude 5.0 and above

Observed

4 September43 - 731142 - 1218
5 September11 - 29190 - 51
6 - 12 September28 - 53371 - 94
13 - 19 September8 - 23200 - 50
20 - 26 September4 - 1690 - 30
27 September - 3 October2 - 1330 - 30
4 - 31 October
10 - 26150 - 43
1 - 28 November
5 - 17110 - 40
29 November - 26 December3 - 1330 - 30
27 December - 23 January2 - 1170 - 31
24 January - 20 February
1 - 950 - 20
21 - 22 February
0 - 200 - 11
February M6.3 sequence commences*
22 - 28 February12 - 29670 - 53
1 - 7 March
1 - 1040 - 21
8 - 14 March0 - 670 - 20
15 - 21 March
2 - 1110 - 20
22 - 28 March
0 - 720 - 20
29 March - 4 April0 - 520 - 10
5 - 11 April
0 - 410 - 10
12 - 18 April
0 - 410 - 11
19 April - 18 May
2 - 1170 - 22
19 May - 13 June
1 - 1050 - 23
June M6.4 sequence commences*
13 June - 12 July
11 - 28460 - 53
13 July - 12 August
1 - 1020 - 21
13 August - 12 September
1 - 890 - 20
13 September - 12 October
0 - 770 - 21
* The earthquakes of 22 February and 13 June generated significant aftershock sequences of their own, and forecasts were revised in light of that activity.
This table was last updated on 15 October 2013
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