Seismic Velocity Anomalies Research Articles

北海道南部の三次元的速度異常分布とそれに関連したローカル・サイスミシティ

北海道南部の三次元的速度異常分布とそれに関連したローカル・サイスミシティ

Three-dimensional seismic velocity anomalies and their relation to local seismicity

This is a brief review of recent results on three-dimensional seismic velocity structures, determined by the method of Aki, Christoffersson and Husebye, using the teleseismic and local earthquake P-time data. Particular attention was given to the relation between the velocity structure and local seismicity. We found that seismic activity may be associated with either low- or high-velocity anomalies, presumably depending on whether the structural weakness, which invites stress concentration, or the ductility, which cannot sustain the tectonic stress, is causing the velocity low.

Feasibility of earthquake prediction in Koyna region (India )

Seismcity alongwith various parameters, viz, tilt strain and geomagnetic field intensity has been observed over a long period in the active Koyna region. Geohydrological parameters like ground water levels, hot spring temperatures and discharge rates have also been observed. Study of these parameters alongwith the earthquakes has indicated feasibility of utilizing b-values, seismic wave velocity anomalies, ground tilts, in-siturock strains and geomagnetic field measurements for prediction of earthquakes. The anomaly (premonitory) periods observed for these parameters are in good agreement with those reported for other earthquakes. Extensive data over long period confirm the results obtained earlier by the authors regarding application of b-values, ground tilt and rock strain measurements for earthquake predictions.

Electrical conductivity anomalies in the earth

Anomalies of short-period geomagnetic variations have been found in various regions over the world. It is known that such anomalies arise from electromagnetic induction within an electrical conductivity anomaly or from local perturbation of induced electric currents by a conductivity anomaly. In order to investigate a regional electric state in the Earth, conductivity anomaly (CA) studies based on anomalous behaviors of geomagnetic variations have been extensively undertaken, as well as studies based on magnetotelluries in which induced currents are directly used. Some of the geomagnetic variation anomalies, however, turned out to be caused by surface conductors, such as sea water and sediments. Anomalies of this sort have been intensively studied and classified into coast, island, peninsula, and strait effects in the case of sea effects. Three-dimensional conduction or channelling of induced electric currents is sometimes observed in the cases of sediments and some crustal conductivity anomalies. However, anomalies of such surface origins often provide some information of the underground conductivity structure. Electrical conductivity anomalies can be classified into two types: anomalies originating in the crust and in the upper mantle. Many of crustal anomalies are well correlated with metamorphic belts, fracture zones, and hydrated layers, and magnetic and gravity anomalies are also often found over the conductivity anomalies. Most of mantle anomalies have been interpreted mainly in terms of high temperature and partial melting, since conductivity anomalies coincide well with anomalies in heat flow and seismic wave velocities.

Anomalies in the time‐averaged paleomagnetic field and their implications for the lower mantle

The proposal that the time‐averaged paleomagnetic field is not strictly that of a geocentric axial dipole but that of an axial dipole displaced slightly northward from the geocenter is examined in terms of spherical harmonic expansions. Standard procedures for spherical harmonic expansions such as are applied to the present instantaneous geomagnetic field are not necessarily applicable to paleomagnetic data. A technique is proposed that enhances the time‐averaging process that is a necessary part of determining the paleomagnetic field. This involves analyzing the inclination anomaly ΔI around latitude strips to determine the zonal harmonics and the declination anomaly ΔD around longitude sectors to determine the first nonzonal harmonics. The technique is applied to a carefully selected data set covering the last 5 m.y. All data are divided into normal or reversed sets providing 266 land‐based points (171 normal, 95 reversed) and 100 deep‐sea core results with inclination data only (50 normal, 50 reversed). These data demonstrate clearly that the time‐averaged field is not simply that of a geocentric axial dipole and also that it is different for the normal and reversed fields. With respect to the present field the zonal harmonics of the time‐averaged field are reduced less than the nonzonal harmonics, the indication being that westward and/or eastward drift of the nondipole field is dominant over 5 m.y. The magnitude of the second zonal harmonic suggests that on the average, paleomagnetic poles could be in error by about 3° when calculated by the usual geocentric axial dipole assumption. The data show clear asymmetries between the northern and southern hemispheres and possibly between the oceanic (Pacific) and continental hemispheres. Possible explanations of these asymmetries include variations in topography and/or temperature at the core‐mantle interface. Large‐scale asymmetries in the boundary conditions at this interface could affect the magnetic field through alteration of the convection pattern in the core. The asymmetries correlate with other global asymmetries such as the distribution of continents and oceans and the locations of subduction zones. They also apparently correlate with seismic velocity anomalies observed in the lower mantle. All these may reflect large‐scale inhomogeneities related to dynamic processes that have occurred in the lower mantle. Although all the Gaussian coefficients determined change sign when the axial dipole field reverses, there are significant differences between the normal and reversed fields, suggesting that the distribution of sources for these fields is not identical. This is consistent with the cyclonic convection models for reversal of the geomagnetic field proposed by Parker and Levy.

Travel-time anomalies in the mantle under the North Atlantic

Summary. Lateral heterogeneity exists in the Earth's mantle, and may result in seismic velocity anomalies up to several per cent. If convection cells and plumes extend down to the core, then these features may be associated with local inhomogeneities observed in the lower mantle. Published data for direct and core-reflected P-wave residuals are used to delineate velocity anomalies in the middle—lower mantle under the North Atlantic. Differential (PcP — P) residuals indicate travel-time anomalies near the core—mantle transition, and may be due to core topography or lateral variations in velocity. It is assumed that the anomalies occur near the midpoints of the ray paths. The main source of error in the data set may arise from phases which have been identified incorrectly. Hence trend-surfaces are fitted to the residual data to show only the large-scale trends in anomaly values, with wavelengths of the order of 1000 km. The Azores and Colorado hot spots occur in a region covered by the data. A possible interpretation of the trend maps is that an anomalous zone extends from a relatively fast region at the core boundary at 35° N, 50° W up to these hot spots, at about 30 degrees from the vertical. This may agree with the suggestion of Anderson that plumes are chemical rather than thermal in origin. If inclined plumes do exist, the deviation from the ideal vertical plume or convection cell boundary may imply that lateral shear or other distortion effects exist in the mantle.

DEPTH CONSTRAINTS ON DILATANCY INDUCED VELOCITY ANOMALIES

Dilatancy, the opening of cracks in rock under applied shear stress, has been proposed as a mechanism to explain seismic velocity anomalies prior to earthquakes. In recent years many studies have been made in search of precursory changes in Vp/Vs, but positive results have been extremely limited. Since dilatancy induced velocity anomalies require undersaturation of rock pores, we look carefully at porosity and fluid compressibility within the crust in an effort to determine the conditions that are most favorable for induced velocity anomalies. Simple porosity-temperature-pore pressure models of the crust are examined. Various dilatant strain distributions are imposed on the models and in. every case the most significant potential for velocity anomalies occurs within the upper 3 to 5km of the crust. Low porosity and high temperature at depth have the net effect of giving smaller velocity anomalies than in near surface regions. Actual crustal porosity, pore pressure and strain distributions prior to an earthquake are not well known. Nevertheless, our results suggest that searches for velocity anomalies should be concentrated in the upper few kilometers of the crust where the most significant anomalies are favored to occur.

Frequency of foreshocks

IN February 1975, the Chinese Seismological Bureau successfully predicted a magnitude 7.3 earthquake near Haicheng, Liaoning Province. From their reports, it is obvious that many methods such as radon count, abnormal animal behaviour, seismic velocity, and tilt and telluric current anomalies were considered in making the long term prediction1. The short term prediction, however, was based primarily on foreshock activity2,3. That foreshocks were important in the prediction raises the question of how common they are. We here analyse the frequency of foreshocks before shallow earthquakes with magnitudes greater than 7. Our findings show that 44% of all large shallow events in the world from 1950 to 1973 had fore-shocks large enough to be teleseismidally recorded, and that 21% of the large earthquakes that occurred in China between 1900 and 1949 had foreshocks large enough to be noted in local records.

Preliminary Results on the 3-dimensional Seismic Structure of the Lithosphere under the USGS Central California Seismic Array

About 1500 readings of teleseismic P-time residuals obtained from the US Geological Survey seismograph network in central California have been used to obtain a three-dimensional image of seismic velocity anomalies for this area by the method of Aki, Christoffersson & Husebye We found that the California network is less suitable than the LASA and NORSAR arrays for this kind of studies because of its greater proportion of peripheral blocks in which the resolution is very poor for the stochastic inverse solution and the random error effect is severe for the generalized inverse solution. Nevertheless, the resultant velocity anomalies show a remarkable correlation with the San Andreas fault zone to a depth of 75 km. The anomaly pattern changes drastically as the depth exceeds 75 km, suggesting that the asthenosphere has been reached.

Seismic wave attenuation and velocity anomalies in the eastern Scotia Sea

As part of a more general investigation of the structure and plate tectonics of the Scotia Sea early in 1972 a temporary seismographic station was installed at King Edward Point, the British Antarctic Survey station on South Georgia (see Fig. 1). The station (KEP) comprises a three-component set of short-period Willmore seismometers at position 54.28° S, 36.48° W, and two single channel (vertical) outstations with telemeter links to KEP; all five channels record on magnetic tape. Interest has been concentrated on a study of activity associated with the South Sandwich Islands subduction zone1.