Earthquake Focal Depths Research Articles

An analysis of earthquake focal depths in the southeastern U. S.

Seven years (1977‐84) of seismic network monitoring in the southeastern U. S. have resulted in a catalog of 255 earthquakes (0 ≤ M ≤ 4.2) with ERZ ≤ 5 km. Focal depths from the Valley & Ridge and Blue Ridge provinces (mean depth 12 km) were combined for analysis as were those for the Piedmont and Coastal Plain provinces (mean depth 8 km). The mean focal depths between these two regions are shown to be statistically different. The 90% depths of earthquakes are also very different between the regions, implying a difference in the depth to the brittle‐ductile transition. First order models of crustal rheology based on estimates of composition, temperature and strain‐rate, indicate that the observed differences in earthquake focal depths may reflect differences in strain‐rate and/or composition.

Discussion of Dr Green's Paper

Discussion of Dr Green's Paper

Correlation of tsunami and sub-oceanic Rayleigh wave amplitudes possibility of the use of Rayleigh wave in tsunami warning system.

In the present tsunami warning system, the predominance of the approaching tsunami is estimated, taking account of the earthquake magnitude and focal depth. However the predominance of tsunami also depends on other focal parameters, e.g. the rise time of the source time function, fault length etc. Accordingly it is quite natural that great tsunamis due to earthquakes with relatively small magnitudes have been sometimes observed. For the improvement of the tsunami warning system, it is necessary to develop the method to find the focal parameters before the tsunami wave arrivals and to make clear the dependence of the generation of tsunami on the above parameters. Effects of the focal parameters on the generation of tsunami and Rayleigh waves are investigated in a previous and in the present paper respectively. If the focal parameters are pre-estimated by the use of early arrival Rayleigh waves, the predominance of tsunami can be predicted. Although some of the focal parameters have fairly complex effects on both waves, it is shown that there is some possibility of the improvement of the present warning system by the use of tectonic features and body waves in addition to sub-oceanic Rayleigh wave observation.

Crustal structure of the Rhinegraben area

Numerous ge ological and geophysical investigations within the past decades have shown that the Rhinegraben is the most pronounced segment of an extended continental rift system in Europe. The structure of the upper and lower crust is significantly different from the structure of the adjacent “normal” continental crust. Two crustal cross-sections across the central and southern part of the Rhinegraben have been constructed based on a new evaluation of seismic refraction and reflection measurements. The most striking features of the structure derived are the existence of a well-developed velocity reversal in the upper crust and of a characteristic cushion-like layer with a compressional velocity of 7.6–7.7 km/sec in the lower crust above a normal mantle with 8.2 km/sec. Immediately below the sialic low-velocity zone in the middle part of the crust, an intermediate layer with lamellar structure and of presumably basic composition could be mapped. It is interesting to note that the asymmetry of the sedimentary fill in the central Rhinegraben seems to extend down deeper into the upper crust as indicated by the focal depths of earthquakes. The top of the rift “cushion” shows a marked relief which has no obvious relation to the crustal structure above it or the visible rift at the surface.

阿寺断層付近の低い地震活動

An observation of microearthquakes was carried out near the Atera Fault for two and a half months, from July 14 to October 2, 1971, since the fault has been known as one of the most active faults in Japan.In the south-western side of the Atera Fault, the focal depths of earthquakes were deeper than 30km, that is, earthquakes occurred in the mantle but not in the crust. In the northeastern side, however, only a few earthquakes were detected. Although three direct analogue recording systems of high sensitivity were used near the fault, very few earthquakes could be located near the fault. A comparison of daily frequency of earthquakes near the NeoValley Fault with the present observation showed that the seismicity near the Atera Fault was undoubtedly lower than that near the Neo-Valley Fault. The same tendency has been also observed for larger earthquakes. Number of earthquakes detected at one of the three stations was exceptionally large, though the sensitivity was the same for all stations. But most of distant earthquakes were equally detected.Considering these facts, it might be concluded that the present seismic activity is very low and, in addition, seismic waves of high frequency are attenuated in the crust near the Atera Fault. Some focal mechanisms of microearthquakes were calculated. The pressure axis thus obtained was E-W direction as in the case studied by MATSUDA (1969) and ICHIKAWA (1971).

Broach (Gujarat) earthquake of March 23, 1970

An analysis of the seismicity of Broach region, Gujarat (India) has been made. The activity of the network of faults present in this region, perhaps, has been responsible for the present earthquake. Earthquake energy, magnitude and focal depth are connected by a single equation. An empirical relation for focal depth and magnitude has been derived for the area.

Comments on Paper by D. E. Smylie and L. Mansinha: 'The Elasticity Theory of Dislocations in Real Earth Models and Changes in the Rotation of the Earth'

Smylie & Mansinha (1971) have attempted to utilize static elastic dislocation theory in order to compute the change in the inertia tensor of a realistic spherically symmetric Earth model produced by a point tangential displacement dislocation. The present author has performed similar computations and has combined this theory together with an empirical earthquake moment-magnitude relation in order to estimate the total excitation of the Chandler wobble by all observed large earthquakes since 1904 (Dahlen 1971). The purpose of this note is to point out that there is significant disagreement between the final numerical results of Smylie & Mansinha (1971) and those of Dahlen (1971) and to suggest a possible explanation for this disagreement. In order to investigate the effect of the earthquake focal depth on the shift of the Earth's mean pole of rotation, Smylie & Mansinha (1971) have considered two point tangential displacement dislocation models for the faulting associated with the 1964 Alaskan earthquake. One model is a realistic shallow focus (depth h = lskm), shallow dip angle, thrust faulting mechanism; the other model has the same mechanism, but the focal depth h is taken to be 637 km. Table 1 shows a direct comparison of the results obtained for these two models by Smylie & Mansinha (1971) and the results obtained from my own computations (Dahlen 1971). The fault parameters used in all cases were those considered by Smylie and Mansinha (epicentral colatitude O,, = 29.0°, epicentral longitude 4,, = 213-0°, fault strike N42E, fault dip 8W, mechanism pure dip slip thrust, fault slip area 1-54 x 1OI2 m3). These fault parameters for the 1964 Alaskan earthquake differ slightly from those considered by Dahlen (1971). It can be seen that the computations of Smylie & Mansinha (1971) predict that the change in the inertia tensor produced by a deep focus earthquake will be nearly an order of magnitude larger than that produced by a shallow focus earthquake with the same fault parameters; my own computations predict that a deep focus earthquake will have a slightly smaller effect than a similar shallow focus earthquake. The major differences in the theoretical treatments of Smylie & Mansinha (1971) and Dahlen (1971) arise in the method used to deal with the fluid core of the Earth model. The reason why the treatment of a fluid core is not straightforward is that in the absence of both inertia and viscosity, individual fluid particles in a non-rotating fluid core encounter no resistance to a tangential (non-radial) displacement, and because of this the particle displacement field within the fluid core produced by a static tangential displacement dislocation (or any other static force system) located within the solid elastic mantle becomes indeterminate. In discussing this problem I

Differential Attenuation of P Waves as Derived from a Hindu-Kush Earthquake

Fourier analysis is made of P and pP waves radiated from a Hindu-Kush earthquake (focal depth; 207km) to study attenuation of the seismic waves in the mantle. The long-period seismograms are supplied by the U.S. Coast and Geodetic Survey.In order to reduce random error in the observed spectra due mainly to incomplete correction for the effect of crustal layering, the P and pP wave observations are divided into three and two groups, respectively, according to their epicentral distances, and the average spectral structure is obtained for each of the groups. Assuming that the intrinsic is independent of frequency in the period range involved (3-30sec for P and 3.75-15sec for pP), three independent values of the differential attenuation are derived from the spectral ratios of the average spectra. These observed values agree with the theoretical ones expected for the MM8-Qβ better than with those for the modified Qα-model by Teng (1966). This implies that is roughly equal to Qβ in the MM8-model throughout the mantle. To obtain a better agreement with the observations the MM8-model is modified. For the trial Qα-model thus obtained, we have Qα=340, Q'α=140, and Qα=2370, where Qα, Q'α, and Qα are values of the average throughout the whole mantle, for the upper mantle (h 0.14R), respectively.

大爆破による関東地方北部の地下構造

At 01h 05m, Nov. 14, 1954 and at 01h 05m, Aug. 15, 1955, large amounts of explosives were fired near the Lake Nozori, Gumma Prefecture. At 01h 05m, Dec. 5, 1956, 1 ton of explosives was fired near Hokota town, Ibaragi Prefecture. 18 temporary observation stations for the Nozori explosions and 15 stations for the Hokota explosion were set up along the line connecting the Nozori and Hokota shot points. Each station was equipped with high-sensitive electro-magnetic seismometers with amplifiers. By these explosion-experiments the most probable crustal structure of nearly east-west section in the northern part of Kwanto District was deduced. It is shown in Fig. 7a. The depth of the Mohorovicic discontinuity was determined to be 25-30km. The depth of earthquake foci near Mt. Tukuba is generally found to be greater than 30km. Consequently earthquakes near Tukuba evidently occur in the upper part of the mantle.

On Seismogram Types and Focal Depth of Earthquakes in the North Japan and Manchuria Region. Eijo VesanenOn the Gravity Field and the Isostatic Structure of the Earth's Crust in the East Alps. Paavo E. Holopainen

<i>On Seismogram Types and Focal Depth of Earthquakes in the North Japan and Manchuria Region</i>. Eijo Vesanen<i>On the Gravity Field and the Isostatic Structure of the Earth's Crust in the East Alps</i>. Paavo E. Holopainen

On the Focal Depth of Earthquakes and the Group of Seismic Waves

On the Focal Depth of Earthquakes and the Group of Seismic Waves

Depth of Earthquake Foci

Depth of Earthquake Foci

Further Note on the Depth of Xarthquake Foci.

SUMMARY 1. Galitzin's direct observations of the “angle of emergence” at Pulkovo led him to specify three new “critical surfaces” at the moderate depths 106 km., 232 km., and 492 km. below the surface, in addition to the surface much lower already indicated by Wiechert. 2. Results obtained for the relative depth of earthquake foci suggest that they also collect about three chief values, without, however, giving any indication as to the absolute depth of any. 3. If we identify the three corresponding depths of focus with those of Galitzin's three surfaces and revise the calculated velocities near the surface accordingly, we (a) can get a value for the surface velocity of P agreeing with that observed at the Oppau explosion; (b) can get the value 207 km. for the depth of the average focus corresponding to the adopted tables. The depths of the three principal seats of earthquakes are of course those given above from Galitzin; (c) The destructive earthquakes are probably those originating in the uppermost layer, at a depth of 106 km. = 66 miles.

The Variable Depth of Earthquake Foci

PROF. H. H. TURNER has given reasons (Mon. Not. R.A.S., Geophys. Suppt. No. 1) for believing that the depths of the foci of earthquakes differ among themselves by quantities up to about 300 km. It may be pointed out that this is precisely what may be expected from the theory of a cooling earth. The available information concerning the thermal state of the earth indicates that the rocks in the asthenosphere, at depths of 400 km. and more, must be very much weaker than those at the surface ; this is amply confirmed by the geodetic evidence collected by Barrell, which also suggests that the rocks at depths comparable with 100 km. are considerably stronger than those at the surface. Accordingly, whatever may be the cause of crustal deformation in the earth, yield will occur in the asthenosphere for smaller stresses than are necessary to produce it in the upper parts of the crust. Thus the earthquakes arising from fractures below 400 km. would be more numerous but much less violent than those occurring at higher levels, and the greatest earthquakes should have their foci at the depth of greatest strength. We should therefore expect that the depths of earthquake foci may range from zero to 200 or 300 km.