Ho Hum another dreary Spring Day, 60kmph winds, drizzle turning to rain in the afternoon., no work today
Oh well, some time to do some more geology study at the desk
I was having a look at these two quakes and thought I would share
Two Mag 4.5 quakes yesterday UTC in New Zealand, one in the NE North Island (moderately deep) and one in the NW South Island (shallow). One had 3
aftershocks (the shallow one), the other didn't have any(the deep one).
Interactive NZ Map
The Max MMIntensity recorded of the shallow SI one was MM4, the deep NI one, was MM5
Why? You would think it would be the other way around eh.?
So I had a closer look at the geology of the two different areas.
The Quakes/Geology comparison
[color=F76541]Lake Waikaremoana Area, North Island
4.52ML, 3599623, -38.67447, 177.1405, 2011/10/24 17:15:19, 45, N of Lake Waikareiti, Urewera National Park 4195, Energy Released 90.867 tonnes of
Epicenter east of the Waimana Fault which is a dextral (right handed) fault with a recurrence Interval of 2000-3500 years and slip rate of 1
The geology (lithology) of the area where the quake occured is Calcareous sandstone and siltstone (partly alternating with graded sandstone),
pumiceous and andesitic tuff, limestone, of Early Miocene age, whereas on the west of the Fault the geology is Sandstone and siltstone (partly
alternating with graded sandstone), dark argillite, minor spilitic tuff and lava of Early Cretaceous age (146 to 100 million years ago)
[color=78C7C7]Kahurangi National Park, South Island
4.497ML, 3599335, -41.31572, 172.39122, 2011/10/24 4:30:60, 5, N of Mt. Kendall, Kahurangi National Park 7893, 83.928 tonnes of TNT
and 2 aftershocks
2.489ML, 3599336, -41.31299, 172.40227, 2011/10/24 4:38:37, 5, N of Mt. Kendall, Kahurangi National Park 7893, 0.082 tonnes of TNT
3.35ML, 3599340, -41.30635, 172.38499, 2011/10/24 4:49:43, 5, N of Mt. Kendall, Kahurangi National Park 7893,1.597 tonnes of TNT
Total energy released 85.607 tonnes of TNT
Epicenterswere between the White Creek fault which is a reverse (dip-slip, or up and down) and the Karamea Fault (no data) the geology is
predominantly granite and granodiorite; quartz diorite, diorite, aplite of Devonian-Cretaceous age (359-416 mya to 146 mya) .
The last big quake on the White Creek Fault (at the southern end 58km away) was the Inangahua1968 Mag 6.7, which interestingly enough had 6 x Mag 5.0
- 5.7 aftershocks within the first 3 days afterwards and further 6 x Mag 5.0 to 5.4 aftershocks within the 45 days following. (Hmm, might look into
that further, I remember it as a kid, first time helicopters were used to evacuate earthquake victims in NZ, seen it on (Black and White) TV News,
about a day later)
So sandstone (NI one) allows the shock wave to travel to the surface (to the intruments) with more intensity, despite having to travel 40km further up
than the SI quake.
The SI quake which only had 5km to travel, was through granite, and had aftershocks so there was more fracturing?
or maybe the difference is in the extra 5.26 tonnes of TNT @ the NI one
raises more questions than it answered.
(DB web Map)
Geological Map of NZ
Geonet Quake Search
Geological Time Scale
edit on 25-10-2011 by muzzy because: (no reason given)