Earthquake info ...
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| | Map of distribution of earthquake epicentres across New Zealand, colour-coded for focal depth (image courtesy IGNS website) | • The only part of the earth that can claim to be earthquake free is Antarctica | • Our (Top-of-the-South) region has been shaken by severe earthquakes 5 times over the past 150 years (MM VII or greater)
• Severe earthquake induced ground shaking can be expectedin our region on average every 25 years. The last such event, the Inangahua Earthquake,occurred in 1968
• Any major earthquake event will likely damage orsever lifeline services (ie water, sewage, power...) and disrupt air, road, and porttransportation. A major earthquake will stretch emergency resources and people are advised tobe prepared to look after themselves and their neighbours for up to 72 hours
• Aftershocks almost always follow the main event. The biggest aftershock is often a littlemore than a magnitude smaller than the main shock. In the month following the Magnitude 7 Inangahua earthquake in 1968, 15 aftershocks of Magnitude 5 or greater were recorded
• All earthquakes generate TWO shocks. You can guesstimate both the size and direction(location) of earthquakes by noting 1) the time lag between the P (Primary) and S (Secondary)waves and 2) the direction of ground motion. The P-wave always arrives first and the S-wavelags behind hence the bigger the time lag the further away the earthquake was
Typical seismic trace recorded for a moderatemagnitude earthquake
• The term “earthquake weather” commonly refers to unusual weather which is hot and muggy. While the term has its basis primarily in folklore it is based, in part, on fact - at least inCalifornia where minor earthquakes have been observed to coincide with sharp changes inbarometric pressure
• It is widely believed that a sharp increase in the number of small earthquakes heralds theapproach of a large one but this is the exception rather than the rule
• An earthquake of Magnitude 5.7 or greater occurs somewhere on the earth every 3 or 4 dayswith about 20 earthquakes of greater than Magnitude 7 occurring somewhere on the earth eachyear. A typical decade sees an average of 14 earthquakes of greater than Magnitude 8
• Nevertheless, some years are worse for earthquakes than others and 1960 is among the mosttragic in recent earthquake history (Chilean Magnitude 8.5, Pacific Basin Tsunami, others...).From the human perspective the Chinese Tang Shan earthquake of 1976 was the greatest naturaldisaster in modern times killing more than 1/2 million people
• The largest recorded event since instrument records have beenavailable (1903) had a measure of 8.9 on the Richter Scale (Columbia, 1906)
• No part of New Zealand is without its earthquakes. In NewZealand a “major” earthquake of Magnitude 7 or greater can be expected about once a decade.Both the historical and geologic record suggest that “great” earthquakes of Magnitude 8 or moredo not happen more than once a century
• The history of earthquake records in New Zealand began about 1460 with Maori traditiontelling of a large shock near Wellington known as Hao-whenua, the land swallower
Failure ofembankment fill, highway east of Inangahua PhotoCredit: N.Z.G.S, L.D Homer • The level of seismicity in New Zealand is very similar to that ofCalifornia. While Californians feel fewer shocks than New Zealanders the ones they do feelare more alarming because they are shallower (at around 30 km vs 100 km or more in NewZealand)
• The fresh fault breakage that occurred as a result of the 1906 SanFrancisco earthquake on the San Andreas fault is the longest on record for a single shockestimated at 300 km. The amount of displacement varied greatly with up to 7 metres ofhorizontal displacement 50 km northwest of San Francisco. Almost no vertical displacementoccurred • The fresh fault breakage that occurred as a result of the 1848 Marlborough earthquake onthe Awatere Fault measured more than 100 km. The amount of displacement varied with up to7 metres of horizontal displacement as well as a lesser vertical component 15 km south ofBlenheim • The 1855 Wairarapa (Wellington) earthquake is the only New Zealand shock in historictimes believed to have reached Magnitude 8 • The moon also experiences quakes referred to, logically enough, as moonquakes
Selected MAJOR Earthquakes in New Zealand( Bold entries have occurred within or had major impact on theTop-of-the-South) | Date | Name/Location
| Size (RM)(1) | Description(2) | | 1843, July 8 | Wanganui | 7.5 | (2) | | 1848, October 16 | Marlborough | 7.1 | (3) | | 1855, January 23 | South Wairarapa | 8.1 | (5) | | 1888, September 1 | North Canterbury | 7.0 | | | 1897, December 7 | Whanganui | 7.0 | | | 1901, November 17 | Cheviot | 7.0 | (1) | | 1921, June 19 | Hawkes Bay | 7.0 | | | 1929, June 16 | Murchison | 7.8 | (17) | | 1931, February 3 | Hawkes Bay | 7.9 | (258) | | 1934, March 5 | Pahiatua | 7.6 | (1) | | 1942, June 29 | South Wairarapa | 7.0 | | | 1950, February 5 | South of South Island | 7.0 | | | 1950, August 5 | South of South Island | 7.3 | | | 1953, September 25 | Bay of Plenty | 7.1 | | | 1960, May 29 | Fiordland | 7.0 | | | 1968, May 24 | Inangahua | 7.0 | (3) | | 1987, January 27 | Kermadec Islands | 7.3 | | 1997, May 25
| Kermadec Trench
| 7.8 | | 2004, 22 November
| Puysegar Trench
| 7.1 | | (1)Richter Magnitude (2) Number of known dead
* Note: The classification of a 'MAJOR' earthquake is for earthquakes of magnitude 7.0 or greater. Other earthquakes with RM <7.0, notably the Edgecumbe (Bay of Plenty) M 6.3 earthquake, have caused significant damage, but are not counted as 'major' earthquakes. See "Earthquakes" by G.A. Bibby, 1989, for further reading and a more extensive listing.
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