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58S. Toda, S. Matsumura / Tectonophysics 417 (2006) 53–68Fig. 3 (continued).during 1999–2000 to at least 2001–2002 (Fig. 3u–w,site A). The inferred stress decrease started northeastof Lake Hamana in 1999–2000 (Fig. 3u, site A) andexpanded to the north in 2000–2001 (Fig. 3v, site B).This evolution of the stress shadow resembles thegradual expansion of the slow slip area estimated byOzawa et al. (2002) from GPS analysis. In the plots ofthe stressing rate change, the stress shadow is morepronounced, and in addition to the Lake Hamanaregion (Fig. 4v–x, sites A and B), there is also afeature near the proposed source region of the futureTokai earthquake (Fig. 4v–x, site C). Moreover, theunloading or stress decrease phase under LakeHamana seems to have started in 1998–1999 (Fig.4t) that is not as clearly detected in Fig. 3 due tothe overprint of secular stress loading. It is also plausibleto infer that anomalous unloading had beengradually occurring at the Lake Hamana region andother areas since 1995–1996 (Fig. 4q).Several moderate-size earthquakes might have beentriggered by stress transfer from the aseismic slip underLake Hamana. An annulus of Coulomb stress increaseencloses the stress decrease area under Lake Hamana in1999–2000 (Fig. 3u), although the change in the stressingrate in Fig. 4 is not distinct, possibly due toaveraging. In the next time period, 2000–2001, threeM =5 shocks occurred in areas surrounding the stressshadow. This leads us to infer that the earthquakes were
S. Toda, S. Matsumura / Tectonophysics 417 (2006) 53–68 59promoted by stress increases that ringed the slow slipzone. Note that the stress steps and related stress loadingproduced by such moderate shocks lasted a fewyears, gradually shrinking (Fig. 4v–x).Our inversion analysis suggests that similar types ofaseismic slip might have occurred in two distinct periodsof 1981–1984 and 1987–1989. Compared to therelatively stable loading in the 1990s, significant stressshadows under the Lake Hamana region can be inferredin the 1980s (1981–1982, 1983–1984, and 1987–1988in Fig. 3; 1981–1985 and 1987–1989 in Fig. 4). Althoughthese stress decreases might not have lasted aslong as the one in the 2000s, the inferred stress changesare comparable in magnitude. Additionally, the locationsand NW-expansion pattern in Fig. 4 are similar tothose of the 2000s.Three inactive periods are identified on the LakeHamana time histories (Fig. 5) that resemble the subsequent2000–2004 slow slip event. Further, the initiationof the 2000 slow slip event can be dated back to thelate 1990s (at the earliest 1996 and at the latest 1999)before the GPS anomaly (Ozawa et al., 2002) began.The stress curve in cell 1 (colored blue) located west ofLake Hamana shows constant stress loading withoutany stress perturbations. Stresses in three cells (3, 4,and 5 in Fig. 5) underlying Lake Hamana demonstratesignificant stress fluctuations with distinctive stressdecreases and sluggish accumulations in the periodsFig. 4. Change in Coulomb stressing rate compared to the long-term rate averaged for the period 1980–2000 (note the log scale). Unlike the stressincrements in Fig. 3, the white color indicates average or secular stressing, whereas red and blue present a rate increase and decrease, respectively.
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