Low on the Richter scale, but highly respected in the lab

Registering details about earthquakes on cards led to the discovery by a Danish scientist that the Earth has a solid core

The newspaper headlines at the start of February 2008 said that northwestern Jutland had been hit by an earthquake. Despite reports of heavy tremors at 1:10 to 1:40pm on 1 February by several people in the Glyngøre-Salling-Nautrup area, seismologists at the Geological Survey of Denmark and Greenland (GEUS) in Copenhagen said the cause of the tremors was not an earthquake.

“The tremors did not register on our chain of seismographs, which register all larger tremors from the underground,” said Tine B Larsen, a GEUS seismologist and senior researcher. “We don’t know the source of the tremors (in the Glyngøre region), but judging from the lack of registration, the source must be in the top Earth layer – in the local area.”

Larsen explained that GEUS receives lots of calls about tremors that people have registered, but that are not earthquakes. “The tremors can come from many different sources,” she said. “The most common is piling, when poles or girders are force into the ground by a machine. But we also have examples of a neighbour’s washing machine in a housing complex being the cause.”

Jutland’s shoulder
Earthquakes occur when two tectonic plates move against each other, for instance along the Pacific Rim and in countries bordering the Mediterranean. Stresses in the centre of a tectonic plate can also result in earthquakes.

Denmark is hit by five to ten earthquakes a year, and 200 earthquakes were registered in Denmark between 1929 and 2004. Small earthquakes can occur in the Danish part of the Skagerrak, as it is the southern limit of an earthquake zone in western Norway, and they can also occur in the southern Kattegat, which is regarded as the southern limit of a zone in south-western Sweden. In central and south-western Denmark there are no earthquakes.

As the greatest probability for earthquakes is off the Jutland ‘shoulder’ and in the Kattegat north of Zealand, earth tremors are most likely to be felt in Thy in north-western Jutland and in northern Zealand.

Most earthquakes in Denmark are not powerful enough to be felt by people, as their strength on the Richter scale is between 1.5 and 4.5, while the large earthquakes we know from abroad are typically between 6 and 8 on the scale. Only a single recent earthquake in Denmark has caused minor damage to houses.

Ingen over Inge
Despite the weak earthquake activity in Denmark, seismological research carried out here led to the discovery that the Earth has a solid inner core.

Inge Lehmann (1888-1993) was head of the Seismological Department of the Royal Danish Geodetic Institute between 1928 and 1953.

Lehmann kept records of observations of the reflection and refraction of seismic waves generated by deep focus earthquakes that occur at depths of 600-700km beneath the Earth’s surface, which led her to conclude in 1936 that the Earth has a solid inner core 5121km below the surface of the Earth.

At the time of the discovery, the Earth was believed to have a homogeneous liquid core, but she pointed out that some hard-to-explain waves in the shadow zone (where secondary seismic waves are not detected because they cannot pass through the core of the Earth) could originate from reflection from a solid inner core. This also gave rise to theories about the Earth’s magnetic field.

“The inner core of the Earth is now thought to be a solid precipitate of the molten iron/nickel outer core,” wrote Professor Leon Knopoff of the University of California, Los Angeles (UCLA) in 1996. “One popular model has it that the condensation of the inner core is the heat source for driving convective motions in the outer core and hence is a mechanism for generating the Earth’s magnetic field.”

Solo for decades
Inge Lehmann participated in many other seismological discussions throughout her long life, and many of her publications deal with the Earth’s upper mantle.

Denmark’s only seismologist for two decades, Lehmann was remembered by her nephew Niels Groes, who wrote about her in the February 1994 issue of the Bulletin of the Seismological Society of America.

“I remember Inge one Sunday in her beloved garden … with a big table filled with cardboard oatmeal boxes,” wrote Groes.

“In the boxes were cardboard cards with information on earthquakes and the times for these and the times for their registration all over the world. This was before computer processing was available, but the system was the same.

With her cardboard cards and her oatmeal boxes, Inge registered the velocity of propagation of the earthquakes to all parts of the globe. By means of this information, she deduced new theories of the inner parts of the Earth.

It was not easy for a woman to make her way into the mathematical and scientific establishment in the first half of the 20th century.

As she said: ‘You should know how many incompetent men I had to compete with – in vain.’!”