Detecting Earthquakes and Nuclear Explosions

8 February 2012

Dr Cyril Isenberg began the branch 2012 London lecture season at the IOP on 1 February with his talk highlighting earthquake and nuclear explosion detection.

Detecting Earthquakes and Nuclear Explosions

Earthquakes can be devastating and significantly disruptive, with human casualties, collapse of buildings, disruption of traffic via opening fissures on the roads, and many other damaging phenomena. 

Best known examples are the San Francisco earthquake in 1906, the more recent 17 October 1989 and another one affecting the West Coast of America in 1995. 

This region is prone to earth tremors, as is situated on the St Andreas Fault. Other earthquakes include the one affecting Alaska in 1964 and in Kobe, Japan, in 1995 (7.2 magnitude of the Richter scale).

Earthquakes result from a build-up of stresses within the rocks until they are strained to the point beyond which they will fracture. 

The pressure builds up around the boundaries, with consequent release of elastic energy in the form of seismic waves which cause the damage. 

Seismic phenomena occurs in narrow continuous belts of activity, which correspond with the junction of lithospheric plates, this including the circum-Pacific belt, the Alpine-Himalayan belt, and the ocean ridges. 

The Richter scale of magnitude is based on the amplitude of the seismic wave from the Earth's centre, amplitude depending exponentially on the magnitude.

There are 4 varieties of seismic waves: 1-P waves, a longitudinal wave due to the elastic nature of the mantle. They are fast waves; 2- S waves or transfer waves, slower then P waves; 3- Love waves, that are transverse waves and slower than S waves and 4- Rayleigh waves, the slower of all of them.

Seismographers are instruments that measure motions of the ground (either seismic, volcanic in origin and other sources such as nuclear explosions) and transversal movement of the building with a recording graph.

Inertial seismographs have levers, a weight (the internal mass) that can move relative to the instrument frame. Any motion of the ground moves the frame. 

The mass tends not to move because of the inertia, and by measuring the motion between the frame and the mass, the motion of the ground can be determined. Early instruments used optical levers or mechanical linkages to amplify the motions involved, then recording on paper. 

Modern seismographs use electronics. In some systems, the mass is held nearly motionless relative to the frame by an electronic feedback loop. The motion of the mass relative to the frame is measured, and the feedback loop applies a magnetic or electrostatic force to keep the mass nearly motionless. 

The voltage needed to produce this force is the output of the instrument, which is recorded digitally. In other designs the weight is allowed to move, and the motion produces a voltage in a coil attached to the mass and moving through the magnetic field of a magnet attached to the frame.

Seismometers were sent to the Moon in the Apollo mission and moonquakes were detected. 

These instruments have played a significant role in the detection and specification of the Earth's of internal structures. 

Hopefully, they will also play a pivotal part in the building of structures aimed at withstanding earthquakes.