Sudden Stress Release Research Articles

Mode of the strain release along the Gibbs fracture zone, Mid-Atlantic ridge

The Gibbs fracture zone (52°N, 35°W) is one of the major transform faults in the Atlantic. A large earthquake ( m b = 5.8, M s = 6.9) occurred on October 16, 1974 on this fracture zone. This earthquake was preceded by similar large earthquakes along the fracture zone in 1967 ( m b = 5.5, M s = 6.5), 1954 ( M s = 6.5), 1941 ( M s = 6.25) a 1923 ( M s = 6.5). Detailed analyses of seismic body waves and surface waves over the period range from several seconds to 200 sec are made for the 1967 and 1974 events. The major conclusions are: 1. (1)|In addition to the obvious disparity in the m b vs. M s relation, the excitation of long-period waves by these earthquakes is also anomalously large. The seismic moments determined by using the long-period (40–200 sec) G 1 and R 1 waves are 3.4 · 10 26 and 4.5 · 10 26 dyne · cm for the 1967 and 1974 events, respectively. 2. (2)|Wave form analyses of body waves suggest a fault length of 60 km for the 1967 event and 70 km for the 1974 event. The point dislocation particle velocity of about 20 cm/sec obtained for both events is about an order of magnitude smaller than for other earthquakes of comparable magnitude. The large fault length and the slow particle velocity explain the disparity between m b and M s , and M s and seismic moment. 3. (3)|The amount of dislocation (about 170 cm) together with the fault length and the repeat time (13 years on the average) suggest a seismic slip rate of about 2.6 cm/year along the Gibbs fracture zone — a rate which is comparable to that for sea-floor spreading in the northern Atlantic. This agreement suggests that the slip along the fracture zone is primarily seismic at least to a depth of 10 km, the plates moving in jerks once every 13 years on the average, rather than in continuous creep. 4. (4)|Seismic activity along the ridge crest was very high during the 5-year period before the 1974 event, while activity along the fracture zone was very high during the 3-year period prior to the 1967 event. The seismic activity became very low after the 1967 event. This pattern may be interpreted in terms of an intermittent upwelling at the ridge which subsequently triggered, either directly or indirectly, the fracture zone events. The sudden release of stress in these events may lead to an initiation of the next episode of upwelling and eventually renewed seismic activity along the ridge—fracture zone system.

Earthquakes resulting from metastable phase transitions

Deep and intermediate earthquakes cannot be explained in terms of slow accumulation and sudden release of stress because relaxation rates in the earth's mantle are such that accumulation of stress over a long period of time does not appear likely. Consequently, sudden volume changes due to phase transitions in the mantle have been suggested as a possible energy source for these earthquakes. However, a phase transition at equilibrium pressure and temperature cannot result in the spontaneous release of energy because the rate of appearance of the new phase is determined by the rate at which energy enters or leaves the system. Thus a sudden release of free energy requires a spontaneous change from the metastable state. Slow currents in the earth's mantle contained between phase discontinuities could generate effects resulting in the transport of a mineral assemblage from one pressure-temperature environment to another. If the pressurevolume changes in these currents are essentially adiabatic the pressuretemperature environment near a phase discontinuity would be favourable for spontaneous phase transitions, even though the environment at the phase discontinuity in the surrounding mantle was that of equilibrium. Thus, the hypothesis that deep and intermediate earthquakes are triggered by sudden volume changes is consistent with thermodynamic considerations.

Differences in Chemical Composition and Mechanism of Generation between the Basalt Magmas in Japan and Hawaii

Basalt magmas in Japan range in composition from tholeiite, passing through high-alumina basalt, to alkali basalt. Picrite basalt related to either of these three basalt magmas is scarcely represented. In Hawaii, both tholeiite and alkali basalt are closely associated with picrite basalt having bulk compositions intermediate between the ordinary basalt and peridotite. The Japanese basalt magmas are supposed to be produced by sudden release of stress attending the generation of the intermediate to deep-focus earthquakes. In such a case, there may be a certain limitation of the heat energy supply for melting the mantle peridotite ; only a few per cent of the peridotite would be melted to form the basalt magmas.The Hawaiian basalt magmas are supposed to be produced by the heat transfer due to convection current within the mantle. In such a case, there would be less limitation of the heat energy supply for melting ; a greater per cent of the peridotite would be melted, resulting in the production of picrite basalt magmas.

Sonic Analysis of Fracture

The passage of stress waves of large amplitude through solid specimens often produces fractures which differ in several respects from those which can occur under any other conditions. Many of the observed phenomena can be accounted for in terms of the acoustical treatment of reflection and refraction of stress waves at the boundaries of the specimen and of diffraction effects in regions where the dimensions of the specimen become comparable with the wavelength of the disturbance. When a fracture occurs it results in a sudden release of stress, and consequently, secondary stress waves travel out from the fracture surfaces. The experimental investigation of these effects by means of high-speed photography, by electrical measurements of stress and strain, and by subsequent examination of the geometry and appearance of the fractured surfaces will be described, and the type of information which such investigations may give about the nature of the phenomenon of brittle fracture will be discussed.