Occurrence Of Mainshock Research Articles

Temporal change in scattering and attenuation associated with the earthquake occurrence—A review of recent studies on coda waves

The study of coda waves has recently attracted increasing attention from seismologists. This is due to the fact that it is viewed as a new means by which the stress accumulation stage preceding a large earthquake can be measured, since the scattering paths nearly uniformly cover a fairly large region around the focus and observation stations, compared with the direct ray paths. To date, we have had many reports on the temporal variation of the relation between coda duration and amplitude magnitude, and that of the coda attenuationQ −1 which is estimated from coda amplitude decay. Some of these have shown a precursor-like behavior; however, others seem to have shown a coseismic change. We have critically reviewed these reports, and discussed what these observational facts tell us about the change in the heterogeneous crust. We found significant temporal variations, not only in the mean but also in the scatter ofQ −1 , associated with the mainshock occurrence. The formation of new cracks, the reopening and growing of existing cracks, the interaction of these cracks, and the pore water movement through these cracks might correspond to such variations. In addition, we may expect an inhomogeneous distribution of crack clusters in a fairly large region, compared with the aftershock region. The gradual appearance of such crack clusters seems to be the most plausible mechanism by which coda decay gradients are caused to largely scatter in the stress accumulation stage.

A possible change in stress field orientation due to the 1976 Tangshan earthquake

Two techniques are used to study the stress field in the Tangshan region. First, the mean directions of principal stress axes in three subregions of the Tangshan aftershock region are determined from 29 focal mechanism solutions. In doing so a slip-fitting technique is applied which determines the mean stress tensor by minimizing the angle between the observed slip direction on every fault plane and the calculated shear stress resolved onto the fault planes. Each of the two nodal planes of the focal mechanism solutions are alternately considered as a possible fault plane. Second, by making a grid search over all possible orientations of stress axes, the meanP,B andT axes in the Tangshan region, both before and after the Tangshan mainshock, are found on the basis of firstP-wave polarity readings from a number of small earthquakes. The results indicate that there is a possible azimuthal change of the principal stress axes in the focal region due to the occurrence of the mainshock.

Foreshocks (1966-1980) in the San Andreas system, California

The spatial and temporal distributions of seismicity preceding moderate (ML ≧ 5.0) main shocks in the San Andreas fault system in California have been analyzed to recognize and characterize the patterns of foreshock occurrence. Of 20 main shocks in the San Andreas system, 7, or 35 per cent, have been preceded by immediate foreshock sequences that included events within 1 day and 5 km of the main shocks. A possible correlation of the rate of foreshock occurrence with type of faulting was found such that none of the four main shocks with reverse faulting had foreshocks while 44 per cent of the strike-slip earthquakes had foreshocks. Some enhanced seismic activity was also observed at relatively large distances from the main shock (13 to 30 km) 1 to 5 days before 40 per cent of the main shocks but this activity cannot be clearly distinguished from the background seismicity. Of the seven immediate foreshock sequences, only two had the swarm-like appearance of the class II foreshocks defined by Mogi. The other foreshock sequences appear to be single events (sometimes with their own aftershocks) preceding the respective main shocks. Four of these sequences are spatially correlated with distinct physical discontinuities in their faults between the hypocenters of the foreshock and main shock, and similar discontinuities may also be associated with the other sequences. The durations of the foreshock sequences were found to decrease as the depths of the main shocks increase from 3 to 11 km, which has been interpreted as a dependence on stress. To account for this stress dependence of the duration and the presence of discontinuities, a model for foreshocks occurrence is presented. This model proposes that foreshocks may represent a process of delayed multiple rupture and that the delay between occurrence of foreshock and main shock might represent the time needed for static fatigue to break the stronger rock at the discontinuity in the fault.

Generalised precursory swarm hypothesis.

The precursory swarm hypothesis as developed in New Zealand has been generalised on the basis of a systematic study of multiple earthquake events at moderate to large magnitudes in the Japan region. In the generalised hypothesis the precursory seismic sequence consists of one or more swarms followed by one or more mainshock events in the same sub-region. The predictor is the characteristic swarm magnitude, defined as the average magnitude of the three largest earthquakes in the swarm or swarms. The observed value of the predictor yields estimates of the magnitude and time of occurrence of the largest mainshock in the sequence; these parameters are estimated as soon as the first swarm occurs, and are revised on the occurrence of successive swarms in the sequence. Further statistical analysis of secondary mainshock events is required, after which a score-sheet of past successes, failures and false-alarms can be compiled, and the hypothesis can then be rigorously tested against future earthquake activity in the region.

「1978年伊豆大島近海の地震」の前震・余震の時空間分布

The characteristics of the space-time distribution of foreshocks and aftershocks of the Izu-Oshima-Kinkai Earthquake of 1978 are investigated, and tectonics in and around the Izu Peninsula is discussed.The overall distribution during the period investigated in this research is composed of three parts, the group I which spreaded in the sea near the Izu Oshima, the group II which extended to WNW direction in the Izu Peninsula and the group III which linked with the group II at the western end and stretched to the conjugate SSW direction. Among these three groups comparatively large aftershocks occurred in the last group. In the groups I and II aftershocks occurred just after the occurrence of main shock but there seems to have been some time till earthquake occurrence in the group III, though it is not certain when the activity of the group III accurately commenced.The aftershocks of the Izu-Hanto-Oki Earthquake of 1974 which extended to the Amagi mountain and those of the Oshima-Kinkai Earthquake of 1978 in the Izu Peninsula are not overlapped each other.In the Izu Peninsula, active faults of WNW-ESE direction are conspicuous and conjugate faults are also observed. And the earthquakes of magnitude larger than 5.0 which occurred in the Izu Peninsula in the last 50 years are all considered to be attributed to activities of these conjugate faults systems.

1970年秋田県南東部地震の余震の発生様式について

A study is made on the spatial and temporal changes in aftershock activity of the southeastern Akita earthquake of 1970, in relation to the characteristics of earthquake faulting in the upper crust. Aftershocks are found to have occurred almost exclusively in the so-called granitic layer of the crust. For the first 50 hours after the main shock occurrence the hypocenters of aftershocks were concentrated nearly on a plane dipping by 52° towards N64°E, which is in accordance with one of P-wave nodal planes in the focal mechanism solution of the main shock. Afterwards, the aftershock activity had a general tendency of upward migration, and spread in the northeast direction to constitute a group of shallow aftershocks which we call the eastern group. While shallow aftershocks in the western group were located approximately on the focal plane found for events in the first 50 hours, those in the eastern group are interpreted to have occurred along the other nodal plane in the mechanism solution of the main shock, indicating the existence of a conjugate fault system. Composite mechanism solutions are obtained from P-wave data for shallow aftershock groups. They are almost the same mechanism of reverse fault type for the main shock. Aftershocks deeper than 15km scattered around the horizontal boundary between the granitic and the basaltic layers. The composite mechanism solution for this group differs in its larger strike slip component from the other solutions, though it is of reverse fault type. If the nodal plane with a small dip angle is assumed to be the fault plane, the direction of slip motion agrees well with that of relative motion between the Eurasia and the Pacific plates.

1968年十勝沖地震に伴う岩手県北部の微小地震活動

A temporary observation of microearthquakes was carried out at Karumai, Iwate Prefecture (40°19′N, 141°32′E) after the occurrence of the Tokachi-oki Earthquake of May 16, 1968. It was made for about 100 days since 22 o'clock on May 18, about 60 hours later than the occurrence of main shock. Total number of observed earthquakes is as many as 52, 000. Most of them were, of course, the aftershocks in usual sense and their hypocenters were located within the aftershock area, of which the areal size is approximately accordant with the Utsu-Seki's law.Besides these normal aftershocks, however, an activity of microearthquakes with the S-P times less than 5 seconds at the station was clearly observed. These earthquakes, which are evidently located outside of the aftershock area, are the main concern of the present paper. The following natures of these events are found:a) The time distribution of these events is similar to the normal aftershock activity, and the number of shocks decreases in accordance with the hyperbolic law.b) The activity for these events is far lower than that of the aftershocks.c) The region of activity is estimated to lie on the northern part of Morioka-Shirakawa tectonic line, which is about 100km distant from the western boundary of the aftershock area.A similar phenomenon was reported by KURIHARA and HIKAWA (1968) to occur at the Tokachi-dake Volcano in Hokkaido, although the volcano is far from the aftershock area. A small activity in the northern part of Iwate Prefecture was found at the time of the great Sanriku Earthquake, 1933. This region may be near the place of the present activity. At the time of the Nankaido-oki Earthquake of 1946, activities in the middle part of Kyushu and in Kyoto were also reported. It should be noted that these activities apparently accompanied the great earthquakes and that the excited regions were on the tectonic lines.From these facts, it is concluded that the effect of a big earthquake is not limited within the so-called aftershock area but the occurrence of big shock sometimes causes the activity in the region far distant from it. It may be reasonable to consider that the microearthquake activity in the present case was resulted by the relative deformation of two crustal blocks of Kitakami mountaineous land and western area, Morioka-Shirakawa line being the boundary of the two blocks.