Coulomb stress transfer and fault interaction over millennia on non-planar active normal faults: the Mw 6.5-5.0 seismic sequence of 2016-2017, central Italy.
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In order to investigate the importance of including strike-variable geometry and the knowledge of historical and palaeoseismic earthquakes when modelling static Coulomb stress transfer and rupture propagation, we have examined the August–October 2016 A.D. and January 2017 A.D. central Apennines seismic sequence (Mw 6.0, 5.9, 6.5 in 2016 A.D. (INGV) and Mw 5.1, 5.5, 5.4, 5.0 in 2017 A.D. (INGV)).We model both the coseismic loading (from historical and palaeoseismic earthquakes) and interseismic loading (derived from Holocene fault slip-rates) using strike-variable fault geometries constrained by fieldwork. The inclusion of the elapsed times from available historical and palaeoseismological earthquakes and on faults enables us to calculate the stress on the faults prior to the beginning of the seismic sequence. We take account the 1316–4155 yr elapsed time on the Mt. Vettore fault (that ruptured during the 2016 A.D. seismic sequence) implied by palaeoseismology, and the 377 and 313 yr elapsed times on the neighbouring Laga and Norcia faults respectively, indicated by the historical record. The stress changes through time are summed to show the state of stress on the Mt. Vettore, Laga and surrounding faults prior to and during the 2016–2017 A.D. sequence.We show that the build up of stress prior to 2016 A.D. on strike-variable fault geometries generated stress heterogeneities that correlate with the limits of the main-shock ruptures. Hence, we suggest that stress barriers appear to have control on the propagation and therefore the magnitudes of theKeywords:
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Deformation characteristics, magnitude and time distributionof the aftershocks of the large earthquakes, occurred in the regionof Greece from 1926 till 1964 are investigated. An approximate relationbetween the number of aftershocks and the magnitude and focal depthof the main shock has been found. Also, an approximate relation has beenderived between the magnitude of the largest aftershock and the magnitudeand focal depth of the main sliok. The largest aftershock occurs withinfourteen days after the main shock. In many cases large " late aftershocks "occur one or more months after the main shock. One or more foreshocksof magnitude larger than 3.5 occurred in forty per cent of the cases. The probability for an earthquake to be preceded l>y a large foreshoek not muchsmaller than the main shock is 10%. It is shown that some properties ofthe Earth's material in the aftershock region can be derived by studying themagnitude distribution and deformation characteristics of the aftershocks.
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We present an analytical solution and numerical tests of the epidemic-type aftershock (ETAS) model for aftershocks, which describes foreshocks, aftershocks and mainshocks on the same footing. The occurrence rate of aftershocks triggered by a single mainshock decreases with the time from the mainshock according to the modified Omori law K/(t+c)^p with p=1+theta. A mainshock at time t=0 triggers aftershocks according to the local Omori law, that in turn trigger their own aftershocks and so on. The effective branching parameter n, defined as the mean aftershock number triggered per event, controls the transition between a sub-critical regime n<1 to a super-critical regime n>1. In the sub-critical regime, we recover and document the crossover from an Omori exponent 1-theta for t1 and theta>0, we find a novel transition from an Omori decay law with exponent 1-theta fot tt*. The case theta<0 yields an infinite n-value. In this case, we find another characteristic time tau controlling the crossover from an Omori law with exponent 1-theta for t
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The medium-strong earthquakes(MS≥5.0) from 886 A.D.to 2009 in Yunnan are classified as 3 sequential types—mainshock-aftershock,multiple mainshock,and single shock.Then the spatial distribution of the 3 types of the earthquakes is studied.We find that the mainshock-aftershock sequence is in the majority in Yunnan,and the multiple mainshock sequence takes the second place.These two types of sequence are widely distributed in Yunnan,and the single shock sequence is mainly distributed in the areas such as Dongchuan,Yuxi,Yinjiang,and so on.In the sub-region of Northwest Yunnan,the proportion of the 3 types of sequence is in accordance with the one in the whole Yunnan area.The multiple mainshock(MS≥6.0) sequence in a special part(called Xiaodianxi) of the West Yunnan is dominant,and the mainshock-aftershock sequence(MS≥6.0) reaches 90% of the total in Southwest Yunnan and 83% in Southeast Yunnan.In Northeast Yunnan,the proportion of the multiple mainshock sequence equals with that of the single earthquake sequence,meanwhile,the proportion of single earthquake(5.0≤M≤5.9) sequence is the highest in Yunnan.
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Aftershock activity of the 1987 Chiba-ken Toho-oki earthquake (M6.7) is investigated using JMA data. It is found that aftershocks during two weeks just after the main shock occurred mostly in a region to the east side of the fault plane which is nearly north-south direction with a steep dip to the east. However, aftershock activity in the area to the west side of the fault plane became high since the beginning of January 1988. The contrast between spatial distribution of aftershocks in December 1987 and that after January 1988 is conspicuous. The later activity was concentrated to a rather small area and the largest aftershock occurred on 16 January in the active region. The mechanism of the largest aftershock was reverse type in contrast to the mechanism of the main shock which was strike slip type. Further, pattern of temporal decrease of aftershock activity deviated notably from the Omori's formula when the later activity was started. All these characteristics suggest that most earthquakes which occurred in the region to the west of the fault plane of the main shock after January 1988 are not the so-called aftershocks in a narrow sense, but that they represent an appearance of a new fracture, which occurrence might be caused by the stress concentration due to the fault motion of the main shock. The phenomenon that aftershock activity in the either one side against a fault plane is higher than that in the other side is frequently observed, even for fault motions of strike slip type. It is interesting to note that seismicity before the main shock was also asymmetrical, i. e. it was active in the region to the west of the fault plane of the 1987 earthquake. The seismicity in the recent one year also seems to be active in the west region. These features may show that the western block to the fault plane has taken a positive part in the accumulation process of stress in the focal region of the 1987 Chiba-ken Toho-oki earthquake.
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Temporal features of the aftershock activity following some large shallow earthquakes of M≥7 in Japan have been studied quantitatively. The earthquakes concerned were accompanied by large aftershocks which triggered their own aftershock activity. The purpose of the present study is to seek any anomalous change in aftershock activity of the main shock before the occurrence of such large aftershocks. Aftershock activity shows an appreciable decrease from the level expected from the modified Omori formula before the occurrence of a large aftershock. The aftershock activity then recovers to the normal level or even increases beyond the normal level shortly before the occurrence of the large aftershock. The recovered activity generally occurs near the hypocenter of the forthcoming large aftershock. Such a feature has been recognized ha fourteen cases out of eighteen for which sufficient data are available. We have the possibility of predicting the occurrence of a large aftershock which might be as large and disastrous as the main shock, if we keep watch on the change of the aftershock activity immediately following the main shock. Moreover, a rough prediction of the place can be made by checking the hypocenter location of aftershocks occurring in the recovered stage.
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