Earthquakes are generally regarded as the most destructive force of nature. While a long-term world-wide comparison shows that the number of deaths and the magnitude of economic losses caused by storms and floods by far exceed those by earthquakes, it is nevertheless true to say that no other natural phenomenon creates such a massive psychological shock. Since they are capable of causing severe damage over relatively large areas, earthquakes obviously have an enormous destructive potential. This was confirmed by the earthquakes in Northridge (California) in 1994 and in Kobe (Japan) in 1995. The losses were US$ 44 billion and US$ 100 billion respectively. The possible total cost of a major earthquake occurring today in California is estimated to be US$ 300 billion and in Tokyo even as high as US$ 1,000 to 3,000 billion. There are also a large number of regions with high concentrations of population and economic activity that are situated in zones of high seismic activity. For the insurance industry in particular, the problem of accumulated losses, which could threaten economic ruin, is therefore rapidly becoming a matter of great urgency. In view of this situation, it is essential to have an objective picture of the exposure to this hazard
Indeed, only on this basis can appropriate precautionary measures be taken – for example realistic premium calculations, accumulation control and the establishment of reserves or structural improvements to buildings and restrictions on land use.
More than 90% of earthquakes occur in regions where large tectonic plates meet. The relative motion of the adjacent plates is used to define three types of plate boundary:
It seems self-evident that the most severe earthquakes should occur in convergence zones where there is a large variation in the stress profile. Convergence zones are followed by transform faults and divergence zones. There are areas where volcanic activity occurs as an immediate consequence of plate movement. In this case too, there is a correlation with exposure. Volcanic activity in convergence zones is explosive (ash and glowing clouds), but is effusive in divergence zones (lava streams).
| Richter Magnitudes |
Earthquake Effects
|
| Less than 3.5 | Generally not felt, but recorded. |
| 3.5-5.4 | Often felt, but rarely causes damage. |
| Under 6.0 | At most slight damage to well-designed buildings. Can cause major damage to poorly constructed buildings over small regions. |
| 6.1-6.9 | Can be destructive in areas up to about 100 kilometers across where people live. |
| 7.0-7.9 | Major earthquake. Can cause serious damage over larger areas. |
| 8 or greater | Great earthquake. Can cause serious damage in areas several hundred kilometers across. |
Although each earthquake has a unique magnitude, its effects will vary greatly according to distance, ground conditions, construction standards, and other factors. Seismologists use a different Mercalli Intensity Scale to express the variable effects of an earthquake. Each earthquake has a unique amount of energy, but magnitude values given by different seismological observatories for an event may vary. Depending on the size, nature, and location of an earthquake, seismologists use several different methods to estimate magnitude. The uncertainty in an estimate of the magnitude is about plus or minus 0.3 units, and seismologists often revise magnitude estimates as they obtain and analyze additional data.
In seismology a scale of seismic intensity is a way of measuring or rating the effects of an earthquake at different sites. The Modified Mercalli Intensity Scale is commonly used in the United States by seismologists seeking information on the severity of earthquake effects. Intensity ratings are expressed as Roman numerals between I at the low end and XII at the high end. The Intensity Scale differs from the Richter Magnitude Scale in that the effects of any one earthquake vary greatly from place to place, so there may be many Intensity values (e.g.: IV, VII) measured from one earthquake. Each earthquake, on the other hand, should have just one Magnitude, although the several methods of estimating it will yield slightly different values (e.g.: 6.1, 6.3).
Ratings of earthquake effects are based on the following relatively subjective scale of descriptions:
Modified Mercalli Intensity Scale
| I | People do not feel any Earth movement. |
| II | A few people might notice movement if they are at rest and/or on the upper floors of tall buildings. |
| III | Many people indoors feel movement. Hanging objects swing back and forth. People outdoors might not realize that an earthquake is occurring. |
| IV | Most people indoors feel movement. Hanging objects swing. Dishes, windows, and doors rattle. The earthquake feels like a heavy truck hitting the walls. A few people outdoors may feel movement. Parked cars rock. |
| V | Almost everyone feels movement. Sleeping people are awakened. Doors swing open or close. Dishes are broken. Pictures on the wall move. Small objects move or are turned over. Trees might shake. Liquids might spill out of open containers. |
| VI | Everyone feels movement. People have trouble walking. Objects fall from shelves. Pictures fall off walls. Furniture moves. Plaster in walls might crack. Trees and bushes shake. Damage is slight in poorly built buildings. No structural damage. |
| VII | People have difficulty standing. Drivers feel their cars shaking. Some furniture breaks. Loose bricks fall from buildings. Damage is slight to moderate in well-built buildings; considerable in poorly built buildings. |
| VIII | Drivers have trouble steering. Houses that are not bolted down might shift on their foundations. Tall structures such as towers and chimneys might twist and fall. Well-built buildings suffer slight damage. Poorly built structures suffer severe damage. Tree branches break. Hillsides might crack if the ground is wet. Water levels in wells might change. |
| IX | Well-built buildings suffer considerable damage. Houses that are not bolted down move off their foundations. Some underground pipes are broken. The ground cracks. Reservoirs suffer serious damage. |
| X | Most buildings and their foundations are destroyed. Some bridges are destroyed. Dams are seriously damaged. Large landslides occur. Water is thrown on the banks of canals, rivers, lakes. The ground cracks in large areas. Railroad tracks are bent slightly. |
| XI | Most buildings collapse. Some bridges are destroyed. Large cracks appear in the ground. Underground pipelines are destroyed. Railroad tracks are badly bent. |
| XII | Almost everything is destroyed. Objects are thrown into the air. The ground moves in waves or ripples. Large amounts of rock may move. |
Generally, the intensity values refer to average subsoil conditions (firm sediments). Local subsoil conditions may result in exposure differences in areas too small to be visible on a world map. The following table states the mean change in intensity for various subsoil conditions. These changes only apply to particular sites. If used for larger areas, they should be reduced according to the type of subsoil generally found in the area.
| Subsoil |
Mean change in Intensity
|
| Rock (e.g. granite, gneiss, basalt) |
-1
|
| Firm sediments |
0
|
| Loose Sediments (sand, alluvial deposits) |
+1
|
| Wet sediments, artificially filled ground |
+2
|
The intensification effect of soft subsoil is partially due to a shift in ground motion to longer oscillations which are potentially more destructive in relation to buildings. This effect is greater further away from the epicenter than it is close to it. Depending on the thickness of the sediment layer, there may be resonance effects which amplify ground movements several times within a narrow frequency spectrum (well-known example: Mexico City)
