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Abstract—This paper analyzes the use of the automated aftershock hazards assessment system (AFCAST) through the example of a series of aftershocks of the Mw 7.8 earthquake in Turkey of February 6, 2023 (the Pazarcik earthquake). The paper presents automated estimates of the aftershock activity area, the magnitude of the strongest aftershock, and the duration of the hazardous period, yielded using data on the main shock and on the first aftershocks. [ABSTRACT FROM AUTHOR]

A relative decrease in the fraction of large aftershocks in the first days after earthquakes in subduction zones is demonstrated, and a connection of this phenomenon with aseismic stress release is shown. [ABSTRACT FROM AUTHOR]

Continuing the series of publications on aftershock hazard assessment, we consider the problem of estimating the time interval after a strong earthquake that is prone to aftershocks which may pose an independent hazard. The distribution model of this quantity, which depends on three parameters of the Omori–Utsu law, is constructed. With the appropriate averaged parameter estimates, the model fairly closely fits the real (empirical) distributions of this quantity on the global and regional scale. A key parameter in the model is the expected number of aftershocks of a given magnitude. This number broadly varies from earthquake-to-earthquake, which determines the wide confidence variant of the estimates based on the averaged parameters. Therefore, for forecasting the duration of the hazardous aftershock-prone period, we propose to use two variants of the estimates. The first variant is based only on the averaged parameter estimates for the region under study and on the value of the magnitude of the earthquake. This variant is applicable immediately after a strong earthquake. The second variant employs information about the aftershocks that occurred during the first few hours after an earthquake, which improves the forecast considerably. [ABSTRACT FROM AUTHOR]

This paper considers the global statistics of times of largest aftershocks relative to the times of the corresponding main shocks. A large data set was used to show that the time-dependent distribution of largest aftershocks obeys a power law distribution. This is analogous to the Omori law for the sequence of all aftershocks. It is also shown that the times of the second, etc., largest aftershocks obey the same distribution. Thereby, we have confirmed the hypothesis that the times and magnitudes in an aftershock sequence are independent and make a good case for the Reasenberg–Jones representation of the aftershock process as a superposition of the Omori–Utsu law and the Gutenberg–Richter relation. Events that are smaller than the largest in an aftershock sequence show no delay relative to the largest event; this rejects the idea of the aftershock process as a direct failure cascade involving gradual transitions from larger to lesser scales, which imposes certain restrictions on the widely popular stochastic models of aftershock generation as branching processes. The above result is important in practice for prediction of aftershock activity and for assessing the hazard of large aftershocks. [ABSTRACT FROM AUTHOR]

The issue of whether tidal forces really affect seismicity has been raised many times in the literature. Nevertheless, even though there seems to be a kind of consensus that such effects do exist, no quantitative estimates are available to relate tide parameters to changes in the level of seismic activity. Such estimation for aftershocks of large earthquakes near Kamchatka is the goal of the present study. We consider the influence on seismicity due to ocean tides only, because their effects are stronger than those of solid earth tides. Accordingly, we only consider earthquakes that occurred in the ocean. One important feature that distinguishes the present study from most other such research consists in the fact that we study the height of ocean tides and its derivative rather than tidal phases as the decisive factors. We considered 16 aftershock sequences of earthquakes near Kamchatka with magnitudes of 6 or greater. We also examined shallow background earthquakes along the coast of Kamchatka. Our basic model of aftershock rate was the Omori–Utsu law. The background seismicity distribution was assumed to be uniform over time. In both of these cases we used the actual distributions in space. The heights of sea tides were estimated using the FES 2004 model (Lyard et al., 2006). The variation in activity from what the basic model assumes in relation to tidal wave height and its time derivative was estimated by the method of differential probability gain. The main practical result of this study consists in estimates of averaged differential probability gain functions for aftershock rate with respect to both of the considered factors. These estimates can be used for earthquake hazard assessment from aftershocks with ocean tides incorporated. The results of our analysis show a persistent tendency of aftershock rate increasing during periods when the ocean tide decreased at a high rate. For the background events, we found a typical tendency of events rate increasing when the ocean tide decreased with high tidal amplitudes. The difference in the main factors that affect aftershocks and background seismicity suggest the inference that the effects of tides on aftershocks are more likely to be direct dynamic initiation of events during high strain rates, while the effects on the background events were static in character. [ABSTRACT FROM AUTHOR]

In seismology according to Båth’s well-known law, the magnitude of the strongest aftershock is on average by unity lower than the magnitude of the main shock. At the same time, most of the strongest aftershocks typically occur within a few hours after the main shock. From the practical standpoint, this activity is quite naturally perceived as a direct continuation of the main earthquake. The subsequent strong aftershocks occur against the rarer background shocks, are less expected, and therefore constitute a separate hazard. The average difference in magnitudes between the main shock and the strongest aftershock that occurs a certain time after the main shock gradually increases. In this work, we consider the problem of estimating the magnitudes of the strongest future aftershock at the successive instants of time after the main shock without taking into account the information about the aftershocks that have already occurred before a given time. For these estimates, we construct the theoretical distributions whose shape proves to be independent of time, whereas the time dependence of the shift in the magnitude proves to be known a priori. The predetermination of these dependences at the moment of the strong earthquake gives us grounds to characterize the constructed theoretical model as Båth’s dynamic law. [ABSTRACT FROM AUTHOR]

This paper shows that the number of aftershocks with a relative magnitude does not depend on the magnitude of the main shock, and, in global and regional consideration, it is characterized by an exponential distribution that is similar to the Gutenberg-Richter repeatability law. This type of distribution makes it possible to give a simple and adequate interpretation of Bath’s law. [ABSTRACT FROM AUTHOR]