Ordinary Earthquakes Research Articles

Mechanism of tsunami earthquakes

The mechanism of the Aleutian islands earthquake of 1946 and the Sanriku earthquake of 1896 is studied on the basis of the data on seismic waves from 5 to 100 s and on tsunamis. These earthquakes generated, despite their relatively small earthquake magnitude, two of the largest and most widespread tsunamis in history. The data obtained at different periods are interpreted in terms of the effective moment, M e. The effective moment at a certain period is defined as a seismic moment of a virtual step function dislocation that explains the observation at this period. The effective moment of the tsunami earthquakes increases rapidly towards 0.5 to 1.0 × 10 29 dyne · cm as the period increases while, for ordinary earthquakes, it is more or less constant. This dependence can be explained in terms of a source deformation having a time constant of about 100 s. The M c versus ƒ (frequency) diagram provides a diagnostic method of estimating the tsunami potential of earthquakes. If the M e −ƒ diagram for an earthquake has a steep upgrade towards low frequency implying an effective moment exceeding 10 28 dyne · cm at zero frequency, the earthquake has a high tsunami potential. Since the determination of the effective moment at various periods can be made by a simple procedure, this method could be incorporated in the tsunami warning system. The abnormal slow deformation at the source of the tsunami earthquakes may be a manifestation of viscoelasticity of a weak zone beneath the inner margin of the trenches. The weak zone which is implied by large normal-fault earthquakes such as the 1933 Sanriku and the 1929 Aleutian islands earthquakes may be a result of frictional heating at the interface between the oceanic and the continental lithospheres.

The Pamir Earthquake of 1911 February 18, in Relation to the Depths of Earthquake Foci.

SUMMARY Formulae have been derived for computing the energy in an earth-quake wave from seismographic records. These differ considerably from Galitzin's formula, especially with regard to the long-wave phase. An estimate is made of the fraction of the energy of a falling body that reappears as the energy of the seismic wave generated, the body being supposed broken up by the impact. With regard to the earth-quake under consideration, Galitzin computed the energy actually in the earthquake wave from seismological observations, and also found the energy set free by the impact of the falling mass on the ground. The two agreed. It is here shown that his estimate of the energy in the wave was about 250 times too high, but that only 1/300 of the energy of the impact would go into the wave : accordingly theory and observation still agree, and we can infer that the wave was actually produced by the landslip. The observations of this earthquake, whose focus must have been in the surface, are compared with those derived from ordinary earth-quakes, which have occurred at various depths. No systematic difference is found of the kind that would correspond to a difference in the depth of focus. Accordingly it is inferred that the foci of the earth-quakes used in the ordinary tables were not at depths greater than 120 km.

On the Measurement of Movements of the Earth, with Reference to Proposed Earthquake-Observations on Ben Nevis 1

MEASUREMENTS of earth-movements are of two distinct types. In one type the thing measured is the displacement, or one or more components of the displacement, of a point on the earth's surface. For this purpose the mechanical problem is to obtain a sieady point, to be used as an origin of reference, and this is effected by making use of the resistance which a mass opposes to any change of motion. This may be called the Inertia method of observing earth-movements. It is applicable to ordinary earthquakes, and also to the more minute earth-tremorswhich would pass unnoticed ifinstrumental means of detecting their presence were not employed. The steady point is to be obtained by suspending a heavy mass (with one, two, or three degrees of freedom) in such a manner that its equilibrium is very nearly neutral. Any moderately sudden displacement of the ground in the direction in which the mass has freedom to move leaves the mass almost undisturbed, and the displacement of the ground is therefore easily measured or recorded by a suitable autographic arrangement, which must be so designed as to introduce exceedingly little friction.