In recent days, we have experienced two earthquakes that many people have felt. On Sunday 12 January, Western Norway was struck by an earthquake measuring 4.3 in magnitude, and in the early hours of Wednesday, it was Andøya's turn.

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Earthquakes occur when a sudden natural rupture happens in the Earth's crust, sending out elastic waves. These waves are felt as tremors in the ground and/or measured with a seismometer. The strength of the tremors ranges from imperceptible to very strong, decreasing in intensity with increasing distance from the earthquake's epicentre.

Listen to our seismologist Annie explain more about earthquakes.

 

Earthquakes in Norway
Norway is actually the country north of the Alps with the most earthquakes. On average, 10-15 earthquakes that are felt by people occur each year. Most are small and happen offshore, but historically, large earthquakes have occurred. In 1904, Oslo was hit by an earthquake with a magnitude of 5.4.

Although Norway is not located on a plate boundary, where most earthquakes occur, this is no guarantee against earthquakes. The slow-moving changes in the Earth's interior build up stresses over thousands of years and can cause earthquakes. Risk analyses show that a larger earthquake can cause significant damage, especially in urban areas, and will particularly affect infrastructure. This emphasises the importance of securing constructions against earthquakes and monitoring seismic activity, including Norwegian oil installations along the coast.

Five main areas 
In Norway, there are five areas where earthquakes are most common. Western Norway and Nordland, which have recently experienced earthquakes, are among these. Earthquakes in Norway are often caused by stresses from the nearby mid-ocean ridge or land uplift after the last Ice Age. 

Nordland is one of the most active regions in Northern Europe regarding earthquakes. One of the largest earthquakes in Norway was recorded near Lurøy in 1819, with an estimated magnitude of 5.9. Smaller earthquakes in the Nordland area are common and often occur in swarms (see the report NEONOR2). 

Sunday's earthquake in Western Norway was caused by displacements along the Øygarden fault or its side faults. This area comprises coastal-parallel fault segments that separate the shelf from the mainland along Western Norway.

More about this week’s earthquakes can be found here.

The five main areas are shown in the map below.

kart med jordskjelvsoner

UtsattE områder

I

In Norway, there are mainly five zones that trigger the most earthquakes (see map):

1) Oslo Rift Zone (graben structure)

2) Western Norway, including Øygården (land uplift)

3) Norwegian Continental Shelf (graben structures)

4) Nordland Coast and Eggakanten (land uplift)

5) Mid-Atlantic Ridge including Svalbard

Is it possible to predict earthquakes?  
Lack of access to the Earth's interior makes it nearly impossible to predict earthquakes accurately. Earthquakes occur so deep within the Earth that scientists cannot directly observe the processes. Most of the data we have about the Earth's interior come from measurements and geological observations made on or near the surface. Nevertheless, scientists can estimate the probability of earthquakes in specific areas.

When do you feel an earthquake?
You can feel an earthquake when the energy from the seismic waves reaches the surface and causes ground tremors. The tremors can vary in strength, from barely noticeable to very strong, depending on the earthquake's size and distance from the epicentre.

Magnitude and intensity measure different characteristics of earthquakes. Magnitude measures the energy released at the earthquake's source and is determined from measurements on seismographs. Intensity measures the strength of the tremors caused by the earthquake at a specific location. Intensity is determined based on the effects on people, buildings, and the natural environment.

Intensity varies with the distance to the epicentre. The table below presents a summary of typical intensities and magnitudes (source:USGS). 

Magnitude

Typcial maximum modified Mercalli intensity

Mercalli intensity scale (abbreviated and modified)

1,0 - 3,0

I

I.I. Not felt, except by a very few under especially favourable conditions.

3,0 - 3,9

II - III

II. II. Felt only by a very few people at rest indoors.

 

 

III. III. Felt by some indoors. People at rest notice swaying or light shaking.

4,0 - 4,9

IV - V

IV. IV. Felt by many indoors, few outdoors. Some people awaken. Windows, doors and porcelain rattle.

 

  

V. V. Felt by most indoors, by few outdoors. Many are awakened. Some become frightened. Buildings shake throughout. Hanging objects swing significantly. Small objects are moved. Doors and windows swing open or shut.

5,0 - 5,9

VI-VII

VI. Many become frightened and run outside. Some objects fall. Many houses sustain minor, non-structural damages such as hairline cracks in brickwork and small damages to plaster.

6.0 – 6.9

VII - IX

VII. Most people become frightened and run outside. Furniture is moved and many objects fall from shelves etc. Many ordinary, well-built buildings suffer moderate damage: Minor cracks in walls, plaster loosens, damage to chimneys. Older buildings may develop larger cracks in the masonry.

7,0 og høyere

VIII eller høyere

VIII. Many have difficulty staying upright. Many houses develop large cracks in the walls. Some common, well-built buildings may suffer serious breaks in walls. Some weaker, older buildings may collapse.

 

 

IX. General panic. Many weak structures collapse. Even well-built structures sustain serious damage.

 

 

X. X. Many ordinary, well-built buildings collapse

 

 

XI. Most ordinary, well-built buildings collapse, some earthquake-resistant structures are destroyed.

 

 

XII. Nearly all buildings are destroyed.

 

More information about intensity and magnitude can be found here.

Contact person

Dr. Annie Jerkins, Seismolog

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