By earthquake is meant a sudden natural rupture in the earth's crust which is of such a nature that elastic waves are emitted which makes motions in the ground. These are measured with a seismometer. The magnitude of the motions varies from imperceptible to very strong, and they decrease in intensity with distance from the center of the earthquake.
The motions that have occurred during the Natural Perils Pool's operating time has largely been within the stresses a building must withstand.
The Norwegian Natural Perils Pools has only registered a few claims reported as caused by earthquake. Almost every one has been repudiated, as no earthquake has had the strength which should cause damage.
Earthquakes are tremors under the surface of the earth which are caused by sudden release of stress in the earth crust. The point where the release takes place is named hypocenter, while the point on the surface directly above the hypocenter is named the epicenter.
During an earthquake elastic vibrations take place in the earth crust. The vibrations become weaker the further away you are from the center. Large earthquakes may cause extensive damages. During earthquakes at the seabed, large tsunamis may occur. Earthquakes take place where there are movements in the earth crust, and in particular at the boundaries between earth crust plates.
Link to the Norwegian National Seismic Network
There is more than one table for recording earthquakes:
| 1. |
Richter Magnitude Scale is based on instrument measuring (seismograph). The energy released in a quake is measured. Richter’s scale is logarithmic – e.g. a quake of magnitude 6 on Richter’s scale releases approximately 32 times more energy than an earthquake of a magnitude of 5 on the same scale. |
| 2. |
Mercalli’s Magnitude Scale is a scale bases on the consequences from 1 to 12. It is based on observations in the country side of the damages done and then next the extent of the damages is put on to the describing scale. |
A chart showing Mercalli’s scale can be found in enclosure 18.
The largest earthquake which has been recorded in Norway the last 100 years, was a quake in the Oslo area in 1904, which was recorded at Richter’s scale at 6 and transferred to magnitude 7 at Mercalli’s scale (“light” damages to buildings, personal chattels and glass together with wrecked chimneys).
Collection of facts in connection with reported damages shows that the main reason quite often is not
the earthquake, but has its natural explanation in:
| • | Subsidence cracks |
| • | Shrinkage cracks |
| • | Expansions |
| • | Heating of cold chimneys |
| • | “Broken” seal in windows/mist/leakage |
| • | Ground pressure |
| • | Cracks in laps |
| • | Weak fastening devices |
| • | Stress in floor, walls and ceiling |
| • | Weak strut-brace |
| • | Shrinkage of wall plates |
| • | Old roof covering |
| • | Frost and ice |
| • | Faulty foundation |
| • | Material dividers |
The list is not complete.
Be aware of the fact that the collection of facts in connection with reported damages caused by earthquakes, means very hard and demanding work for the surveyor, so in cases like these, only surveyors being in particular competent in this line of work shall be used.
All ordering of surveys on damages caused by earthquake shall be done through a main coordinator in the area where the damage has occurred.
If the case is being appealed, the reply from the company shall be sent The Pool for consideration before a final decision is taken.
Appeal cases:
| 291/90 | Residential house – over time/cracks | no natural damage. |
| 6/95 | Residential house – poor repair of old damage | no natural damage |
| 30/95 | Holiday home – no causal connection | no natural damage |
| 42/95 | Pump in well – no causal connection | no natural damage |