Earthquake Information Worth Knowing

Did You Know ….. ??

• 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 expected in our region on average every 25 years. The last such event, the Inangahua earthquake, occurred in 1968

• Any major earthquake event will likely damage or sever lifeline services (ie water, sewage, power…) and disrupt air, road, and port transportation. A major earthquake will stretch emergency resources and people are advised to be 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 little more 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

• 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 in California where minor earthquakes have been observed to coincide with sharp changes in barometric pressure

• It is widely believed that a sharp increase in the number of small earthquakes heralds the approach 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 days with about 20 earthquakes of greater than Magnitude 7 occurring somewhere on the earth each year. 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 most tragic 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 natural disaster in modern times killing more than 1/2 million people

• The largest recorded event since instrument records have been available (1903) had a measure of 8.9 on the Richter Scale (Columbia, 1906)

• No part of New Zealand is without its earthquakes. In New Zealand 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 more do not happen more than once a century

• The history of earthquake records in New Zealand began about 1460 with Maori tradition telling of a large shock near Wellington known as Hao-whenua, the land swallower

• The level of seismicity in New Zealand is very similar to that of California. While Californians feel fewer shocks than New Zealanders the ones they do feel are more alarming because they are shallower (at around 30 km vs 100 km or more in New Zealand)

• 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 shock estimated at 300 km. The amount of displacement varied greatly with up to 7 metres of horizontal displacement 50 km northwest of San Francisco. Almost no vertical displacement occurred

• The fresh fault breakage that occurred as a result of the 1848 Marlborough earthquake on the Awatere Fault measured more than 100 km. The amount of displacement varied with up to 7 metres of horizontal displacement as well as a lesser vertical component 15 km south of Blenheim

• The 1855 Wairarapa (Wellington) earthquake is the only New Zealand shock in historic times believed to have reached Magnitude 8

• The moon also experiences quakes referred to, logically enough, as moonquakes

Failure of embankment fill, highway east of Inangahua
PhotoCredit: N.Z.G.S, L.D Homer

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