Аннотация:The differential probability gain approach is used to estimate quantitatively the change inaftershock rate at various levels of ocean tides relative to the average rate model. Anaftershock sequences are analyzed from two regions with high ocean tides, Kamchatkaand New Zealand. The Omori-Utsu law is used to model the decay over time,hypothesizing an invariable spatial distribution. Ocean tide heights are consideredrather than phases. A total of 16 sequences of M ≥6 aftershocks off Kamchatka and15 sequences of M ≥6 aftershocks off New Zealand are examined. The heights of theocean tides at various locations were modeled using FES 2004. Vertical stress changesdue to ocean tides are here about 10–20 kPa, that is, at least several times greater than theeffect due to Earth tides. An increase in aftershock rate is observed by more than two timesat high water after mainM≥6 shocks in Kamchatka, with slightly less pronounced effect forthe earthquakes of M 7.8, December 15, 1971 and M 7.8, December 5, 1997. Forthose two earthquakes, the maximum of the differential probability gain function is alsoobserved at low water. For New Zealand, we also observed an increase in aftershock rateat high water after thrust type main shocks with M ≥6. After normal-faulting main shocksthere was the tendency of the rate increasing at low water. For the aftershocks of the strikeslipmain shocks we observed a less evident impact of the ocean tides on their rate. Thissuggests two main mechanisms of the impact of ocean tides on seismicity rate, anincrease in pore pressure at high water, or a decrease in normal stress at low water, bothresulting in a decrease of the effective friction in the fault zone.
