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Asprities Recognized from Seismicity Pattern Change Preceding Megathrust Earthquakes

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Asprities Recognized from Seismicity Pattern Change Preceding Megathrust Earthquakes ASPERITIES RECOGNIZED FROM SEISMICITY PATTERN CHANGE PRECEDING INTERPLATE MEGATHRUST EARTHQUAKES Shozo Matsumura National Research Institute for Earth Science and Disaster Prevention Abstract Recently, rupture processes of large earthquakes can be analyzed in detail to discriminate largely slipping area, that is, asperity by using various kinds of observation data. However such a manner is valid only to the earthquake that has already happened. In the view of seismic disaster prevention, we want to know where the asperity of a future earthquake should be located before its breakage. Replying to this request, we treat a case of the Tokai assumed earthquake. Analyzing the seismicity rate change, we could discriminate a patch-like pattern covering the assumed seismogenic zone. We interpret this pattern to reflect inhomogeneity in strength such that a stronger part can stand as it is, while a weaker part should slip to release the corresponding load. Then, the stress concentration might focus on the former one, which can be distinguished as an activated area in seismicity rate. And so, we recognize it as an asperity. If this is the case, we can extract a potential asperity for a future earthquake. Introduction The Tokai district, located around the central part of the Japan islands is well known as one of the most dangerous places in Japan since 1976, when Ishibashi(1976) gave a warning to the public that a mega-thrust earthquake had been impending around the Suruga bay, that is the next Tokai earthquake. This earthquake is regarded to belong to the historical Tokai earthquake series, the latest one of which was the 1944 Tonankai earthquake. The problem is that the fault area of the latest one did not reach inside the Suruga Bay. Based on this evidence, Ishibashi concluded that the Tokai area around the Suruga Bay was left as a potential seismogenic area, having been accumulating stress since the 1854 Ansei Tokai earthquake, and as a result, had been grown enough to arouse M8 earthquake. According to Ishibashi’s warning, the government organization, the Central Disaster Management Council (CDMC) started to establish a national program to promote a countermeasure act, one of the Figure 1. Seismogenic zone assumed for main subjects in which is to make an the next Tokai earthquake assessment of the influence brought by the earthquake. Earthquake model Ishibashi proposed his original prospective view for the seismogenic zone of the next Tokai earthquake by investigating the historical evidences that appeared with the occurrence of the 1854 Ansei Tokai earthquake. This zone has a rectangle shape enclosing the west Suruga Bay and the coastal area of the Shizuoka prefecture (see Fig.1). This was adopted as an official model by CDMC in 1978. Based on this model, various kinds of countermeasure acts had been set up by the government, the local autonomous bodies, and the public offices. However, it has already passed more than 20 years without any occurrence of the focused one. During the while, various kinds of observation networks have been developed covering the Tokai area in order to monitor and catch some anomalous signals even if it might be very small. One of representatives is a high sensitive seismic network, which was constructed and has been operated by our institute, the National Research Institute for Earth Science and Disaster Prevention (NIED). Another is the GPS network constructed and managed by the Geographical Survey Institute (GSI). Both observation networks could provide remarkable results, which led us to a new point of view for the impending Tokai earthquake. This resulted in a new version of the potential seismogenic zone as shown in the figure, an egg-plant like zone enclosed by the broken line. Figure 2. Asperity model The next step is to evaluate the intensity of the placed inside the strong ground motion generated due to the earthquake. In assumed seismogenic order to estimate it through a numerical procedure, it is zone necessary to know the information respecting the asperities, that is, locations and sizes of the asperities. The committee of CDMC has proposed a procedure to set it up, that is called “the Irikura’s recipe”. In case of the Tokai earthquake, the whole seismogenic zone is divided into three segments, for each of which a couple of asperities are placed both in the inland zone, and in the oceanic zone, respectively. The total area of asperities is adjusted to be 30% of the whole. The result is shown in Fig.2. This was utilized for evaluation of strong motions and tsunami intensities. Then, some problems will be brought on; How much practical is such a model ? Is it impossible to recognize the asperities before occurrence of the main earthquake ? 2003 Tokachi-Oki Earthquake We have a good example to answer to the proposed problem. The 2003 Tokachi-Oki earthquake (M8.0) happened around the off-coastal region of south-eastern Hokkaido on Sep. 26 in 2003, as a succeeding one of the 1952 Tokachi-Oki earthquake (M8.2), both of which belong to the sequence of interplate megathrust earthquakes between the Pacific and the North-American plates. Figure 3 shows the epicenter and the major asperities of the earthquake, analyzed and shown by Yagi(2004), and Tohoku University(2004). Although any significant crustal movement such as slow-slip or pre-slip was never detected, a clear change of the seismicity could be recognized preceding the main earthquake. The seismicity rate inside the fault zone of the main earthquake indicated a slight but clear quiescence progressing since the middle of 1998, about 5 years preceding the occurrence (Katsumata and Kasahara, 2004). Figure 4(a) shows hypocentral distribution of microearthquakes occurred before the main earthquake, and (b) Figure 3. Slip distribution of the 2003 magnitude distribution for those inside the Tokachi-Oki earthquake. Yagi’s result and forward slip analysis based on GPS measurement. The star indicates the epicenter. fault area. It is recognized that those earthquakes of M2.5 and greater are almost uniformly detected. Figure 5 shows the cumulative earthquake frequency counted from 1990, where earthquakes of M2.5 and greater and depth less than 100km are sampled, and processed through declustering. It is found that a clear depression of the rate commenced in 1998, and is somewhat recovered toward the occurrence. We consider that this is an epoch when some change has started in the locking state, and has been extending toward the entire fault zone. In order to investigate the spatial pattern of the change, we depicted a contour map of rate change between both seismicities before and after the epoch. Figure 6 shows the result, where 120 activated zones (shaded), and 150% zones (black-painted) are extracted. Comparing this with Fig.3, we are impressed in that there is a good correspondence of both locations between the activated zones and the asperities. Then, is there any necessity in such correspondence ? Figure 4. (a) Hypocenter distribution, and (b) magnitude distribution before the occurrence of 2003 Tokachi-Oki earthquake. Figure 5. Cumulative frequency of earthquakes (>M2.5) inside the fault zone of the Tokachi-oki earthquake. We interpret this as follows. It might be probable that there is intrinsic Figure 6. Activated zones derived from inhomegeneity in strength of locking state on rate change between both seismicities the fault zone. However, it is not distinctive in before and after the epoch in 1998. the early stage of stress accumulation. In approaching the critical stage, some stress release will progress partly in the portions where strength is relatively weak, due to the rise of stress level. Even in this stage, the remaining strong portions will retain this slip to prevent an overall slip. At that time when most of the stress is supported by particular portions of strong locking, stress concentration will progress, and is going to appear. Then, this can be reflected on the seismicity change, and the asperity corresponding to the strong locking will be recognized as activated zones caused by stress concentration. This means that the potential asperity will be discovered by analyzing the seismicity pattern change inside the fault zone preceding the occurrence of the main earthquake. Tokai Region Again, we will discuss on the locking state in the Tokai region. We have been performing microearthquake observation and continuing to monitor the seismicity change covering this area more than two decades. In recent years, some delicate but significant changes of the seismicity were detected as similar like the Tokachi-oki case. Figure 7 shows hypocentral distribution in the Tokai region, where several blocks are selected for monitoring the seismicity. The first two, (a) and (b) are the top and the bottom of the layered activities, respectively. A small Figure 7. Hypocenter distribution in the rectangular area (c), surrounding the northeast Tokai region. Blocks a, b, c, and d are coast of Lake Hamana is also sampled. referred in Fig.8. Adding these, we sampled a distorted rectangular area extending to the northwest of Lake Hamana (d). The last one was selected as a reference to monitor the motion of the plate convergence. For these four blocked areas, cumulative number of earthquakes were counted, and displayed in Fig.8, where counting started from June 1986. Here, earthquakes of magnitude M1.5 and greater were sampled, and declusterd in order to reject step-like changes due to earthquake swarms and aftershocks. In these four graphs, we can recognize quiescence in recent years commonly for blocks (a), (b), and (c), while cannot find it only for (d), which is belonging to the extended portion of the subducted slab. The focal mechanism of earthquakes in (d) shows predominantly the normal fault type sustained by the stress with approximate EW extension (Matsumura, 1997).
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