Process in the Source Region for a Great Earthquake

Nagamune, Tomeo

doi:10.4294/zisin1948.22.2_104

Cited by 7 publications

(6 citation statements)

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“…In the Tokachi-Oki earthquake, arrivals of large-amplitude phases about 20 to 50 s after the initial $ phases were observed for many stations in northern Japan. Interpreting those prominent phases as being emitted from a major subevent, Nagamune [1969] determined its hypocenter to be located on the landward side of the core zone. The epicenters of the initial rupture (I(õ8)) and the major subevent (Hr(a8)) of the Tokachi-Oki earthquake are also shown in Fig.1.…”

Section: Discussionmentioning

confidence: 99%

“…WWSSN records of the Tokachi-Oki earthquake, identifying two subevents that radiated large high-frequency waves. The earlier subevent was determined to be located very close to the major subevent determined by Nagamune [1969]. If the major subevent determined by Nagamune [1969] is regarded as the high-frequency rupture, the location and origin time of the high-frequency rupture relative to the initial rupture are very similar to those of the Sanriku-Oki earthquake.…”

Section: Mori and Shimazaki [1984] Investigated Short-periodmentioning

confidence: 91%

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Three‐stage rupture process of the 28 December 1994 Sanriku‐Oki Earthquake

Sato

Imanishi

Kosuga

1996

Geophysical Research Letters

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We investigated the rupture process of the 28 December 1994 Sanriku-Oki earthquake (Mw = 7.7) using broadband seismograms recorded at local distances. The earthquake rupture nucleated at the eastern end of the aftershock area. As it propagated towards the west, a major subevent occurred near the center of the aftershock area about 26 s after the initial rupture, releasing most of the seismic energy of this earthquake. About 24 s later, another subevent followed at the western end of the aftershock area, emitting large high-frequency waves. The whole rutpure time is about 55 sec. The three-stage rupture process is very similar to the earlier-stage rupture process of the 1968 Tokachi-Oki earthquake (M• -8.2). The difference is that the rupture of the Tokachi-Oki earthquake further propa, gated towards the north in the following stage instead that of the 1968 Tokachi-Oki earthquake (M• -8.2), a similar interplate event that occurred 26 years earlier. It surprises us that the same segment slips after such a short time interval in this part of the plate boundary where the recurrence time interval of large earthquakes is thought to be about 100 years [Utsu, 1987].In this paper, we describe the rupture process of the 1994 Sanriku-Oki earthquake as revealed from broadband seismograms recorded at local distances. Comparing the rupture process with that of the 1968 Tokachi-Oki earthquake, we also discuss the mode of strain release associated with large earthquakes in this area. o Three-Stage Rupture Process Figure 2 shows different types of seismograms recorded of terminating at the western end. The occurrence of at TMR, located about 200 km from the epicenter the Sanriku-Oki earthquake is critical to the validity of (Fig. l). The onset of weaker vibration P1 is registered the characteristic earthquake so far believed to exist in on the high-gain short-period velocity seismogram (SP) this part of the plate boundary.

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Section: Discussionmentioning

confidence: 99%

Section: Mori and Shimazaki [1984] Investigated Short-periodmentioning

confidence: 91%

Three‐stage rupture process of the 28 December 1994 Sanriku‐Oki Earthquake

Sato

Imanishi

Kosuga

1996

Geophysical Research Letters

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“…Such places of increased strength, either of geometrical origin or of some other causes, are generally called asperities. The importance of asperities in various failure processes was recognized in laboratory studies (Byerlee, 1970;Scholz and Engelder, 1976) and the concept of asperity has been frequently used in seismology, either explicitly or implicitly, to explain non-uniform se:lsmicity along fault zones (Wesson and Ellsworth, 1973;Bakun et al, 1980) complex events (Wyss and Brune, 1967;Nagamune, 1971Nagamune, , 1978Kanamori and Stewart, 1978;Lay and Kanamori, 1980a;Das and Aki, 1977;Aki, 1979), seismic clustering Kanamori, 1978, 1980), and certain aspects of seismicity patterns (Mogi, 1977, Tsumura, 1979Katsumata and Yoshida, 1980;Lay and Kanamori, 1980b). interpreted preseismic clustering of events near the main shock epicenter in terms of stress concentration around a strong asperity due to failure of weaker asperities surrounding it.…”

Section: Asperity Modelmentioning

confidence: 99%

The Nature of Seismicity Patterns Before Large Earthquakes

Kanamori

2013

Maurice Ewing Series

217

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Abstract. Various seismicity patterns before major earthquakes have been reported in the literature. They include foreshocks (broad sense), preseismic quiescence, precursory swarms, and doughnut patterns. Although many earthquakes are preceded by all, or some, of these patterns, their detail differ significantly from event to event. In order to examine the details of seismicity patterns on as uniform a basis as possible, we made space-time plots of seismicity for many large earthquakes by using the NOAA and JMA catalogs. Among various seismicity patterns, preseismic quiescence appears most common, the case for the 1978 Oaxaca earthquake being the most prominent.Although the nature of other patterns varies from event to event, a common physical mechanism may be responsible for these patterns; details of the pattern are probably controlled by the tectonic environment (fault geometry, strain rate) and the heterogeneity of the fault plane. Here a simple asperity model is introduced to explain these seismicity patterns. In this model, a fault plane with an asperity is divided into a number of subfaults. The subfaults within the asperity are, on the average, stronger than those in the surrounding weak zone. As the tectonic stress increases, the subfaults in the weak zone break in the form of background small earthquakes. If the frequency distribution of the strength of the subf aults has a sharp peak, a precursory swarm occurs. By this time, most of the subfaults in the weak zone are broken and the fault plane becomes seismically quiet. As the tectonic stress increases further, eventually the asperity breaks and sympathetic displacement occurs on the entire fault zone in the form of the main shock. Foreshocks do or do not occur depending upon the distribution of the strength of the subfaults within the asperity. Since the spatio-temporal change in the stress on the fault plane is most likely to dictate the change in seismicity patterns, detailed analysis of seismicity patterns would provide a most direct clue to the state of stress in the fault zone. However, because of the large variation from event to event, seismicity pattern alone is not a definitive tool for earthquake prediction; measurements of other physical parameters such as the spectra, the mechanism and the wave forms of the background events should be made concurrently.

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“…Although the extent of the aftershock area suggests a bilateral propagation of the fault rupture, they concluded that the main rupture had propagated unilaterally between the two subevents. The location of the first subevent is very close to the epicenter of I-phase determined by Nagamune (1971), which is the epicenter of a very strong phase recorded on seismograms at observation stations operated by the Japan Meteorological Agency (JMA). Also, asperities obtained by Kikuchi and Fukao (1985) are located at almost the same places as the two subevents.…”

Section: Fault Plane Of the Largest Aftershockmentioning

confidence: 99%

Fault extent of the largest aftershock of the 1968 Tokachi-Oki, Japan, earthquake and an interpretation of the normal faulting focal mechanism

Izutani

2011

Earth Planet Sp

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The focal mechanism of the 1968 Tokachi-Oki earthquake (M J = 7.9) was thrust faulting but that of the largest aftershock (M J = 7.5) was normal faulting. The tectonic implication of the reversal of the focal mechanisms of the two events has not been clarified yet. In order to investigate this, we ought to take into account the relative location of the two fault planes. In the present study, the length and the direction of rupture propagation for the largest aftershock is derived from the azimuth dependence of the duration of observed strong ground motion. If we assume that the rupture initiation point is the hypocenter location determined by the Japan Meteorological Agency (JMA), the location of the fault plane is considerably further north of the tsunami source area as estimated previously, and at the northern end of the aftershock area within 24 hours after the main shock. The fault plane of the largest aftershock is deeper than that of the main shock and the two fault planes are nearly parallel to each other. The thrust faulting of the main shock and the normal faulting of the largest aftershock indicate that the part of the Pacific plate between the two fault planes moved relatively further northwestward due to the two events than the deeper part beneath the fault plane of the largest aftershock.

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Process in the Source Region for a Great Earthquake

Cited by 7 publications

References 0 publications

Three‐stage rupture process of the 28 December 1994 Sanriku‐Oki Earthquake

Three‐stage rupture process of the 28 December 1994 Sanriku‐Oki Earthquake

The Nature of Seismicity Patterns Before Large Earthquakes

Fault extent of the largest aftershock of the 1968 Tokachi-Oki, Japan, earthquake and an interpretation of the normal faulting focal mechanism

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