Eysteinn Tryggvason

Vertical Crustal Movement in Iceland

The rift zones of Iceland are subsiding at relatively constant rate. The absolute subsidence has not been determined but the rate of tilt of the flanks of the rift zones is 0.2 to 0.7 microradians per year and the width of the zone that is being tilted appears to be more than 40 kilometers. It is estimated that the central part of the rift zone is subsiding 0.5 to 1.0 centimeter per year relative to surrounding areas. The ground deformation associated with this subsidence is characterized by gentle bending of the earth’s crust with no measureable fault displacement except during earthquakes and possibly during a period of one or two years after earthquakes.

Surface Deformation in Iceland and Crustal Stress over a Mantle Plume

The horizontal radial flow of plastic material near the top of a mantle plume excerts stresses on the lower lithosphere. The velocity of this flow is a function of the distance from the center of the plume. The radial stress in the lithosphere will be tensional over the plume but compressional outside of the plume, while the tangential stress is tensional throughout the whole region of the radial flow. The difference between radial and tangential stress represents a shear stress. This has a maximum a short distance outside of the plume. If both radial and tangential stress is tensional, as above the plume, the surface deformation will be related to normal faults. High volcanic activity is expected but seismic activity will be low and only small earthquakes will occur because of the low tensile strength of crustal rocks. Where the radial stress is compressional and the tangential stress is tensional, as in areas outside of the plume, the ground de-formation will be largely associated with strike-slip faults. Volcanic activity is expected to be low, but seismic activity may be high with occasional large earthquakes because of the relatively great shear strength of crustal rocks.

Amplitudes of explosion-generated seismic waves

SummaryRecords of underwater explosions in Iceland in 1959 and 1960, mainly performed for crustal studies, have been investigated with regard to amplitudes. The amplitudes ofP2 in the 1959 explosions and of the first arrivingP waves in the 1960 explosions were found to be proportional to the first power of the charge weight, whereas amplitudes ofP1 andS1 in the 1959 explosions were proportional to the 3/4-power of the charge up to 200 kg, but increased much slower for larger charges. The influence of water depth of shot point on the charge weights, required to obtain a certain amplitude, has been determined and it was found that the logarithm of the charge weight has a linear relation to water depth down to about 8 meters. The amplitudes of first arrivingP waves decrease as the inverse 2.2-power of the distance up to about 30 km. For greater distances an exponential decrease of the form (const./Δ) exp (−βΔ) is valid. ForP2 waves with a frequency of 10 cps we found β=0.027±0.003 km−1 for a profile across central Iceland and β=0.009±0.004 km−1 for a profile in the western part of Iceland.ZusammenfassungIn den Jahren 1959 und 1960 wurden auf Island Unterwasser-Explosionen ausgelöst, deren Hauptziel Untersuchung der Erdkruste waren. Die Aufzeichnungen der Explosionen werden hier in Hinsicht auf die Amplituden der Wellen untersucht. DieP2-Amplituden in den Explosionen von 1959 und die Amplituden der zuerst ankommendenP-Wellen in den Explosionen von 1960 waren proportional der ersten Potenz des Ladungsgewichtes, während diePl-undSl-Amplituden in den Explosionen von 1959 proportional der 3/4-Potenz des Ladungsgewichtes bis 200 kg waren. Für höhere Ladungsgewichte wuchsen die Amplituden langsamer an. Es wurde der Einfluss der Tiefe des Schusspunktes unterhalb des Wasserspiegels auf das für eine gewisse Amplitude benötigte Ladungsgewicht bestimmt. Es ergab sich dabei, dass der Logarithmus des Ladungsgewichtes in einer linearen Beziehung zur Tiefe, bis etwa 8 m, steht. Die Amplituden der zuerst ankommendenP-Wellen sind umgekehrt proportional der 2.2-Potenz der Entfernung, bis etwa 30 km. Für grössere Entfernungen ist die Abnahme der Amplituden exponential und kann durch (Konst./Δ) exp (−βΔ) dargestellt werden. FürP2 mit einer Frequenz von 10 cps haben wir β=0.027±0.003 km−1 für ein Profil in Zentral-Island und β=0.009 ± 0.004 km−1 für ein Profil in West-Island gefunden.

Fault Displacement and Ground Tilt During Small Earthquakes

A program of repeated precision leveling at selected locations in Iceland over a period of seven years shows an annual rate of tilting of less than one microradian per year on the flanks of the active rift zones. Earthquakes of magnitude 5 to 6 are associated with measurable fault displacement at distances up to 25 kilometers from the epicenters and the area around the active fault is abnormally deformed during the earthquakes. During a period of one or two years after an earthquake the rate of ground deformation is higher than normal. This is explained by plastic flow at depth which tends to restore the surface to the position before the sudden deformation.