ALBERT

Description: As the first application of the method of Hashida and Shimazaki (J. Phys. Earth, 32, 299–316, 1984), a three-dimensional seismic attenuation structure and source strengths are estimated by inversion of seismic intensity data for earthquakes that occurred in the Tohoku district, Japan. We carefully selected 1,630 intensity data from 101 earthquakes so that the intensities are consistent with accelerations converted from Kawasumi's relation and so that a well-resolved attenuation structure is obtained. A reasonable fit of the formula proposed in our preceding paper to the actual intensity data guarantees that a systematically biased attenuation structure is not obtained. A comparison of the obtained attenuation structure with velocity structures estimated by previous studies shows that high (low) Q nearly corresponds to high (low) V. The correlation of both structures indicates that the attenuation structure estimated by the proposed method is reliable. The resultant attenuation structure shows a remarkable contrast in the attenuation coefficient and two prominent features. The first feature is low-Q zones down to a depth of 90 km, which corresponds to the distribution of volcanoes. The second is high-Q zones that correspond to the subducting Pacific slab. The high-Q slab is in contact with the high-Q zone in a depth range of 30–60 km, which lies on the east side of the volcanic front. The presence of high-stress earthquakes in this depth range, such as the 1978 Miyagi-ken-oki earthquake, is explained by a model in which the contact of the underthrusting Pacific plate with the surface high-Q zone accumulates higher stress and thus causes stronger seismic coupling. The estimated source strength, which is expressed as a point source acceleration, correlates well with earthquake magnitude. Normalized source acceleration, which is an average acceleration over a source area, is estimated. The acceleration suggests that the stress drop of an earthquake becomes higher with magnitude and with depth. The relation between JMA magnitude (M1) and seismic intensity at a hypocentral distance of 100 km (I100) is found to be I100= 1.5 M1 — 6.5 for crustal events. This I100is in agreement with the value reported by Utsu (Bull. Earthq. Res. Inst., 59, 219–233, 1984) which was determined from events excluding those which show anomalous distributions of intensity data. This agreement suggests that our method of estimating earthquake magnitude from intensity data is effective for removing the effect of structure. © 1987, The Seismological Society of Japan, The Volcanological Society of Japan, The Geodetic Society of Japan. All rights reserved.

Description: Seismic intensity data, which measure the degree of ground shaking, contain information on attenuation along the path from the source to the station and earthquake source strength. With some reasonable assumptions about the seismic intensity data, a properly formulated damped least squares estimation procedure can be used to determine simultaneously both three-dimensional attenuation structure and source strength. By means of this method, sufficiently distributed earthquakes with a large amount of seismic intensity data may provide information on the crust and upper mantle Q structure in a region where a spatial variation of attenuation is large enough to affect the distribution of seismic intensity. To check the validity of the method, numerical experiments were undertaken for artificial data including errors comparable to those expected from using seismic intensity. The results obtained for a two-dimensional island arc structure model show the method to be practical. The reliability of the obtained solution for Q structure can be measured by the corresponding diagonal element of the resolution matrix. By using a large number of seismic intensity data up to about one thousand and selecting solutions with high resolution, a Q structure with an accuracy of 1.5 x 10~3 in 1/Q and a source strength with an accuracy of 0.1 in magnitude scale can be obtained. This method is extendable to many regions including island arcs and continents because large amounts of intensity data are easily attainable. © 1984, The Seismological Society of Japan, The Volcanological Society of Japan, The Geodetic Society of Japan. All rights reserved.

Description: Near the junction of two arc systems, the state of stress within the earth's crust often appears to differ from the regional trend. The internal deformation of the continental plate at arc-arc junctions is probably controlled by geometrical configurations and the forces acting on the plate edges. If forces acting on the two neighbouring plate boundaries which intersect at a junction are not parallel to each other, convergence (or divergence) or shearing would take place at the junction. At a plate edge, four types of force (or displacement) can be assumed, whose direction is constrained by the strike of the edge. Thus four types of deformation can be expected within the continental plate near an arc-arc junction. One of these four basic types appears to be dominant in the real earth, probably because of the limitation on possible combinations of force (or displacement) types acting on the two neighbouring plate boundaries. The anomaly in the state of stress found near the junctions of the Kurile and Japan arcs and of the Aleutian and Kamchatka arcs may be related to a shearing force acting on the obliquely converging plate boundaries. © 1978, The Seismological Society of Japan, The Volcanological Society of Japan, The Geodetic Society of Japan. All rights reserved.

Description: Along arcs, a belt-like area can be identified where shallow seismicity within the continental plate is extremely low compared with other parts of the plate margin. Examples are in the Tohoku, Hokkaido, Kurile, Kamchatka, Aleutian, Peru-Chile, New Hebrides and Tonga arcs. The present paper proposes to call this inactive (or less-active) area the “aseismic belt,” which seems to be a typical feature of arcs. The aseismic belt is some tens of kilometers wide and is located, generally speaking, along the frontal non-volcanic arc between the volcanic front and the aseismic front. This belt can be explained as a mechanically unstrained area on the basis of a plausible model of plate subduction. Geodetic data and seismological results obtained in Japan are incorporated into this model in the framework of plate tectonics. © 1978, The Seismological Society of Japan, The Volcanological Society of Japan, The Geodetic Society of Japan. All rights reserved.

Description: The logarithmic number of aftershocks which occur within one month after a large shallow earthquake in Japan is found to be proportional to the logarithm of the seismic moment of the main shock. The proportional constants for interplate and intraplate earthquakes are different, but it turns out to reflect a similar difference in scaling relations of the fault area to the seismic moment between interplate and intraplate earthquakes. Thus we can derive the fundamental relationship that the aftershock number is proportional to the fault area of the main shock. This is consistent with the hypothesis that aftershocks are generated by unbroken strong patches on the main-shock fault, if the patch density is constant. Combining this new result with the results of previous studies on aftershocks, we propose that a rate of aftershock occurrence is given by where n(t) indicates the number of aftershocks which occur t days after the main shock, S indicates the fault area of the main shock in km2, Mth is the threshold magnitude, b is the b-value of the Gutenberg-Richter relationship, and c and p are the c- and p-values, respectively, of the modified Omori formula. The constant k amounts to 13.4 for an interplate earthquake and to 31.7-63.5 for an intraplate earthquake depending on the assumption on its fault width. Apparently the areal density of aftershocks on the fault plane is higher for an intraplate earthquake than for an interplate earthquake. Further examination of the results suggests that the areal density of the aftershocks of a continental intraplate earthquake may be systematically higher than that of an oceanic intraplate earthquake. These differences may indicate some intrinsic difference in rupturing process among a continental intraplate, an oceanic intraplate, and an interplate events. © 1990, The Seismological Society of Japan, The Volcanological Society of Japan, The Geodetic Society of Japan. All rights reserved.

Description: The effectiveness of existing strong-motion arrays for source inversion analysis is investigated by estimating the accuracy of the inversion solution using simplified theoretical seismograms. The resolving power is examined for different subfault sizes and for different array configurations for four existing array networks. They are for the 1979 Imperial Valley and the anticipated Parkfield, California earthquakes, and the 1968 Tokachi-Oki and the anticipated Tokai, Japan earthquakes. We use the method previously developed by Miyatake, Iida, and Shimazaki in 1986 based on the Wolberg's prediction analysis. The main results are: (1) The array used for an analysis of the 1979 Imperial Valley earthquake is not suitable for source inversion; especially the E1 Centro array, a linear array crossing perpendicularly the fault trace, is ineffective. (2) The array installed for the anticipated Parkfield earthquake by the end of 1985 seems to be satisfactory because of the intensive installation of many stations. However, the resolving power of the whole array will further increase by adding a few stations in the northwestern part of the fault or in distant areas for a better azimuthal coverage in the northwest of the fault. (3) Detailed source inversion analysis cannot be expected for the 1968 Tokachi-Oki, Japan, earthquake because of both the large fault area and lack of offshore stations. Strong-motion, ocean bottom instruments within the fault area are required for a further improvement of the inversion analysis for this type of earthquake. (4) An addition of several land stations on the west and north sides of the fault area is desirable to the present network for the anticipated Tokai, Japan earthquake. The resolving power of the whole array for this earthquake is strongly dependent on a rupturing direction because station coverage of the southern oceanic part of the fault tends to be poor. © 1990, The Seismological Society of Japan, The Volcanological Society of Japan, The Geodetic Society of Japan. All rights reserved.

Description: Predictions of seismic intensities are attempted based on a three-dimensional attenuation structure and amplifying factors at stations obtained in the previous study for the Tohoku district, Japan. An empirical relation between ‘earthquake size’ related to the intensity and JMA magnitude MJis also used. The first prediction is to map spatial distribution of the minimum magnitude Msof an earthquake which causes intensity 5 at a specific JMA station. The result shows that the effects of regional variation of attenuation and local site condition at the station are important in assessing the seismic hazard. Another attempt is made to predict an annual number of felt earthquakes at each JMA station based on the JMA earthquake catalogue for a period from 1963 to 1984. A comparison of predicted number with the observed one shows a good coincidence between them for each year. This may suggest that the proposed method can successfully predict intensities. However, a comparison of the predicted cumulative number for each intensity grade with the observed one shows that the cumulative numbers for intensities 2, 3, and 4 are underestimated by the proposed method. This result may suggest a nonlinearity between the JMA intensity scale and the logarithm of acceleration, which has been suggested by other evidences. © 1987, The Seismological Society of Japan, The Volcanological Society of Japan, The Geodetic Society of Japan. All rights reserved.

Description: The focal mechanism of the 1975 central Oita, Japan, earthquake (M=6.4) is reexamined, and the rupture process is investigated by comparing synthetic and observed P-waveform data of WWSSN long-period seismograms. The obtained focal mechanism is not a normal fault type as was previously reported, but a strike-slip fault type with dip-slip component. The tensional axis of this solution is consistent with the tectonic stress field in this region. The rupture time, the seismic moment, the fault displacement, and the stress drop are estimated to be 6.7 s, 2.2 × 1025 dyn·cm, 85cm, and 78bar, respectively. However, these only show a general or average feature of this earthquake. Based on a complicated feature of observed seismograms, we found that this event is a multiple shock and has a shallow initial rupture with short source duration. A distinct later phase of seismograms suggests a change of source mechanism during the rupture process, which is consistent with the distribution of the mapped late Quaternary faults in the focal area. The trend and sense of the fault of this earthquake and active faults around there, and strain field of this region agree with the pattern for oblique rifting, proposed by Withjack and Jamison in 1986. It is suggested that oblique rifting, which appears to be related to a right-lateral displacement of the Median Tectonic Line lying east of and subparallel to the inferred rift axis, takes place in this region. The relative displacement between opposite sides of the rift is estimated to be directed in NESW, consistent with the geodetic data. © 1988, The Seismological Society of Japan, The Volcanological Society of Japan, The Geodetic Society of Japan. All rights reserved.