Ziro Suzuki

Earthquake generating stresses in a descending slab

Abstract A numerical study of stress within descending slabs beneath island arcs provides a general interpretation for the occurrence of deep- and intermediate-focus earthquakes. The computation of stress due to several factors, which have been proposed for the interpretation of the double-planed seismicity of intermediate-depth events, shows that thermal expansion and olivine-spinel phase change play an important role in earthquake generation. The stress field due to the combination of these two causes gives the following results; in the depth range 50–300 km, the compressional and tensional stress predominates in the upper and central part of the slab respectively. Maximum shear stress amounting to 1.6 GPa in compressional and 0.4 GPa in tensional fields occurs at a depth of 200 km. The stress is generally very small at around 300–350 km. Below 400 km, compressional stress exists at the central part of the slab and tensional stress appears near the upper and lower surfaces of the slab. Maximum shear stress is 2.8 GPa in compression and 1.6 GPa in tension at a depth of 500 km. The principal axes in these high stress fields align almost parallel to the dip direction of the slab. These stress features explain well such observed seismological facts beneath island arcs as the seismicity-depth relationship, planer configuration of seismic zone, existence or non-existence of double-planed seismicity and focal mechanisms of intermediate and deep focus earthquakes.

Double-planed deep seismic zone and anomalous structure in the upper mantle beneath northeastern Honshu (Japan)

Abstract Seismicity located by using the most recent data obtained from the high-gain seismograph network of Tohoku University shows that the deep seismic zone beneath northeastern Honshu, Japan, is composed of two thin planes which are parallel to each other and are 30–40 km apart. Focal mechanisms derived from the earthquakes in the upper plane are reverse-faulting, or, some of them, down-dip compression. As a contrast, those in the lower plane are down-dip extension. The location of the upper boundary of the descending lithospheric slab, inferred from the arrival-time difference between ScS and ScSp waves and from the travel-time anomaly of intermediate-depth earthquakes observed at the small-scale seismic array, coincides exactly with the upper plane of the double-planed deep seismic zone. Anelasticity (1/Q) structure of the upper mantle consists of three distinct zones: a high-Q (Qs− 1500) inclined lithospheric slab, an intennediate-Q (Qs−350) land-side mantle between the Pacific coast and the volcanic front, and a low-Q (Qs − 100) land-side mantle between the volcanic front and the coast of the Japan Sea. The evidence obtained here provides valuable information as to the definition of the type of mechanism producing the plate motion beneath island arcs.

Objective estimation of source parameters and local Q values by simultaneous inversion method

Abstract Accurate estimation of the physical parameters of earthquake sources requires an objective basis for determining the spectral parameters SO, the low-frequency level, fc, the corner frequency, and γ, the high-frequency decay rate. Knowledge of the appropriate Q value is also necessary. In most cases, however, the estimation of these parameters is rather subjective, being accomplished usually by visual fitting of some schematic spectral shape to the observations, and the average of Q over possible regional variations is sometimes used without sufficient evidence as representative of the local value. An objective method of simultaneous inversion is presented in this study for the estimation of source parameters and local Q values. According to theoretical predictions as well as to many observational results, an observed spectrum may be reasonably regarded as a sample from an ensemble of the form O(f)=SO[(f/fc)a+1]−bexp(− πftQ−1), where a and b are positive constants and γ = ab. For earthquakes occurring in a confined region and observed at a nearby station, the constants a and b are common and a single value may be used as the local value of Q. The observed spectra of K earthquakes at L frequency points give K × L data, while the number of unknowns to be estimated is 2 K + 3, that is, the SO and c values for the K earthquakes, and the values of a, b, and Q. For K × L > 2K +3, least-squares estimators are obtained so as to minimise s2=ϵϵ[ln(Okl)−ln(Skl)+πfltkQ−1]2. A numerical test using an artificial data set indicates that a stable solution is obtained, and the errors in the parameters SO and fc are less than 20%, for a data standard deviation of 0.2. This method has been applied to actual data sets for small earthquakes occurring near Miyako on the Pacific coast of the Northeastern part of Japan which were observed at a nearby station. The solution converges sufficiently at the third or fourth iteration step, the data standard deviation being 0.2. The source parameters and the Q value thus estimated are in good accordance with those obtained by another, independent method. The present method gives reliable data for an accurate scaling model, because the source parameters are obtained by a standard technique for a variety of earthquake sizes.

A scaling model for quantification of earthquakes in and near Japan

Abstract A simple method is developed to determine seismic moments of earthquakes. The method is qualified through criteria such as simplicity of calculations, coverage of wide magnitude range, and insensitivity to detailed instrumental response. The method is applied to 163 major earthquakes which occurred underneath Japan and the Japan Sea in the time from 1926 to 1977. Magnitudes of these earthquakes, which have been determined by the Japan Meteorological Agency, ( M JMA ) cover the range from 4.3 to 7.5. At first, source spectra are analyzed through a very simple way introducing two new parameters: characteristic period T c and seismic-moment factor M c . The former is defined as an average value of apparent periods of seismic waves with the maximum trace amplitude at many stations. The latter is an average of products of maximum trace amplitude and its apparent period multiplied by epicentral distance. It is shown that T c corresponds to the period of the corner frequency of an earthquake and M c to the seismic-moment density at the period of T c . A scaling model of earthquake source spectra is presented which satisfies the empirical relations between the surface-wave magnitude M s and M JMA , and M JMA and the body-wave magnitude m b . Those relations are independent of the Gutenberg and Richter relation between M s and m b , because M JMA is determined from maximum amplitudes of seismic waves with a period of about 4 sec. The static seismic moment of each earthquake can be estimated from calculated M c using the source spectra of the scaling model. Seismic moments of 18 earthquakes determined by conventional analyses from near- and/or far-field observations are consistent with static seismic moments thus estimated over the range from 2 × 10 23 to 3 × 10 27 dyne cm. This shows the potential in practice of the present method, especially in the routine processing of seismic data.

Moment tensors of earthquakes in and near the Tibetan Plateau and their scaling law

Moment tensors of eleven major earthquakes in the Tibetan Plateau and its surrounding region from 1966 to 1980 are estimated by generalized inverse technique. The seismic source time function and focal depth are immediately determined in the inversion. The results show that moment tensors of some events differ significantly from double couple, the deviation increases with decreasing plunge angle of null axis. All these events occurred in the upper crust, much shallower than those reported so far, for example, in NEIS Bulletin. Focal mechanism solution obtained from the moment tensors are consistent with the idea that the Indian plate collides northwards with the Eurasian plate and that an eastward spreading exists in the crust of the Tibetan Plateau. The stress drops for earthquakes of intraplate are systematically higher than those of earthquakes in suture zone. The source process duration becomes longer with seismic moment, but for the same seismic moment, the process duration for earthquakes in suture zone is about 1.4 times of those for intraplate event, these results suggest that the earthquakes near suture zone may be of a special characteristics in source process differently from those in intraplate.

The Fifth 5-Year Program for Earthquake Prediction in Japan

The fourth period of the Japanese national program for earthquake prediction will expire in March, 1984. In June of 1983, the fifth 5-year plan for the next period was proposed by the Geodetic Council, Ministry of Education, Japan. Prior to establishment of the new plan the results in the fourth period were critically reviewed. The new plan, as well as the review, is briefly described in this paper. Several precursors have been clearly observed before some earthquakes with magnitudes around 7, but, on the other hand, observations have also revealed a remarkably complicated appearance of precursors. Therefore, earthquake prediction should be based on a synthesis of various observed phenomena. Taking this into account, the new plan put its emphasis on the efforts to increase the accuracy of prediction, though the items of observations are not so different from those in the fourth period.