Aftershock Process Research Articles

Global statistics of aftershocks following large earthquakes: independence of times and magnitudes

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 after- shocks. 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 after- shock 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.

BLOCK STRUCTURE OF THE SOUTHERN KURAI BASIN OF GORNY ALTAI ACCORDING TO GEOELECTRIC DATA COMPARED TO THE DISTRIBUTION OF EARTHQUAKE EPICENTERS

The Kurai Basin of Gorny Altai is located in the area of high seismic activity, which is involved in the focal zone of the 2003 M7.3 Chuya earthquake. Its aftershock process has not ceased yet, and shows the likelihood of major seismic events. The seismic monitoring records of the last 15 years after the devastating earthquake show the state of stress in the basin. A comprehensive field database has been consolidated from the studies of direct and alternating currents by electromagnetic methods, including transient electromagnetic sounding, vertical electrical sounding, and electric field tomography. Using a combination of the research techniques and the method of interpretation based on data inversion, it becomes possible to select optimal models, ensure higher reliability, and improve the contents of the study. The available seismological data are used to identify the zones of concentration of seismic events in the southern mountain frame of the Kurai Basin. Our study aims to determine and clarify the geoelectric structure of the southern troughs. The first section of the Southwestern trough is constructed, and the latitudinal fault boundaries of the Eshtykel graben are confirmed. A comparison of electromagnetic and seismic data for 2015 shows that the earthquake epicenters were mainly confined to the submeridional fault zones. In our study of the Southwestern trough, it is established that most of the earthquakes concentrated in active fault structures separating the blocks of varying depths, which are identified from the geoelectric data. The southern piedmont troughs of the Kurai Basin are cut by numerous faults of latitudinal and submeridional strikes.

Математические методы анализа и прогноза афтершоков землетрясений: необходимость смены парадигмы

Анализ и прогноз афтершоков сильных землетрясений в мировой практике в настоящее время основан исключительно на стохастических моделях развития афтершокового процесса. Это дает возможность использования статистических методов анализа, а также применять в прогнозе ”сценарный” подход путем многократного генерирования случайных последовательностей афтершоков и подсчета частоты повторения интересующих событий. Исследования по проекту РНФ ”Создание информационной системы автоматической оценки сейсмической опасности после сильных землетрясений по данным геофизического мониторинга” в 2016-2018 гг. показали однако, что эффективность таких подходов имеет существенные ограничения. В статье дается критический обзор статистических методов анализа и прогноза афтершоков, интерпретируются пределы эффективности прогнозов при использовании стандартных подходов, приводится обоснование необходимости смены парадигмы. В качестве одного из направлений поиска предлагается применение методов Дискретного математического анализа (ДМА), разрабатываемых академиком А.Д. Гвишиани и его научной школой. Очевидное преимущество такого подхода продемонстрировано на примере простого алгоритма идентификации афтершоков с использованием аппарата нечетких сравнений.

The MW 7.8 Earthquake of July 17, 2017 off the Commander Islands and Other Large Earthquakes at the Western Segment of the Aleutian Island Arc

This paper discusses the tectonic setting, seismological characteristics, and the aftershock process for the great (MW = 7.8) Near-Islands Aleutian earthquake of July 17, 2017 off the Commander Islands. Our analysis showed that the rupture zone as inferred from the distribution of aftershock epicenters in the shape of an elongate narrow zone approximately 400 km long completely covered the north slope of the Commander Islands uplift and was confined to the Bering Fault. The rupture zone involved this entire earthquake-generating area as far as a transverse structure west of the Near Islands group (Attu Island). The fault-plane solutions and the inferred displacements in the rupture zones of the main shock and of the largest foreshocks and aftershocks revealed the rupture to be a practically pure right lateral movement. The aftershock process of the July 17 earthquake was occurring at rather a sluggish pace for such a large earthquake. In addition, the process involved two other features to distinguish it from the aftershock processes of most Kuril–Kamchatka earthquakes: (1) a low release of the cumulative scalar seismic moment (M0cum aft), which amounted to between 0.75% and 1.0% of the mainshock seismic moment (M0me), according to several estimates; and (2) a very slow increase in the deficit of seismic moment (M0) release. At the same time, the duration of the quasi-stationary phase in the release of M0cum by aftershocks, which is estimated to have lasted approximately half a year and which occupied much of the entire duration of the aftershock process for this earthquake seems to be unusually long. These features in the aftershock process of the Near-Islands Aleutian earthquake of July 17, 2017 distinguish the process from those common to most large Kuril–Kamchatka earthquakes. Overall, the rupture zone can be regarded as a transform feature between two Benioff zones (The Aleutian and the Kuril–Kamchatka ones) rather than as a subduction event, as is typical of the two latter arcs.

Global Statistics of Aftershocks Following Large Earthquakes: Independence of Times and Magnitudes

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.

Ring-Shaped Seismicity Structures Forming before Large Earthquakes and the Great Earthquakes in the Western and Eastern Pacific

We consider the characteristics of seismicity before large earthquakes and the great earthquakes in the periphery of the Pacific Ocean. It is found that earthquakes with Mw = 7.0–9.2 that occurred in 1964–2016 were preceded by the formation of ring-shaped seismicity structures, usually in two depth ranges: 0–33 and 34–70 km. We obtain correlations between sizes of shallow (L) and deep (l) seismicity rings, threshold magnitudes (Mth1 and Mth2, respectively), and the time of their formation (T1 and T2, respectively) on magnitudes Mw of the mainshocks. It is shown that the sizes of ring-shaped structures at any given Mw for earthquakes in the western margin of the Pacific are significantly smaller than for those in the eastern margin. However, the values of Mth1 and Mth2 are close for these two regions. Parameters T1 and T2 vary considerably depending on the event, but on average they are ~27–30 years. It is supposed that the formation of ring-shaped structures is related to the migration of deep-seated fluids, while the difference between characteristics of these structures in the western and eastern margins of the Pacific are caused by different contents of fluids in the crust and upper mantle of the respective regions. This conclusion agrees with the available data on the peculiarities of aftershock processes of large earthquakes in the considered regions.

Modeling the Stress-Strain State in the Epicentral Zone of a Strong Earthquake in Iran (December 26, 2003, Mw = 6.6)

The results of modeling the stress-strain state for the epicentral zone of the earthquake that occurred on December 26, 2003 in the southeast of Iran in the city of Bam, Kerman province, before and after the rupture are presented. It is shown that the source of the earthquake is located in the zone of high stresses, whereas the emerged rupture traces this zone on the surface and corresponds to its length. The aftershocks are localized in the area of the maximal stress drop after the formation of the rupture. The drop in the stress stimulates the release of the accumulated tectonic stresses during the subsequent aftershock process. The obtained results can be useful for the deterministic approach to seismic hazard assessment and forecasting, as well as for setting up the geophysical observations intended for forecasting the strong crustal earthquakes in the continental regions.

Strong Earthquakes in 2009–2016 in Central Italy: Tectonic Position, Seismic History, and Aftershock Processes

Based on the seismological, geodynamic, and seismotectonic data on the strongest series of earthquakes in 2009–2017 in Central Italy, which were collected by many researchers, mostly Italian, the tectonic position of these events is determined and the seismic history of the region over more than 2000 years of observations of seismic manifestations in Italy is traced in the context of the strong events of the beginning of the 21st century. The aftershock processes of these earthquakes are investigated and, as a result, the possibility of a series of strong aftershocks of the earthquake of October 30, 2016 (MW = 6.6) is predicted in advance of the actual occurrence of these events on January 18, 2017.

Aftershocks following crash of currency exchange rate: The case of RUB/USD in 2014

The dynamical behavior of the currency exchange rate after its large-scale catastrophe is discussed through a case study of the rate of Russian rubles to US dollars after its crash in 2014. It is shown that, similarly to the case of the stock market crash, the relaxation is characterized by a power law, which is in analogy with the Omori-Utsu law for earthquake aftershocks. The waiting-time distribution is found to also obey a power law. Furthermore, the event-event correlation is discussed, and the aging phenomenon and scaling property are observed. Comments are made on (non-)Markovianity of the aftershock process and on a possible relevance of glassy dynamics to the market system after the crash.

Об афтершоковых процессах, сопровождающих умеренные и слабые землетрясения на территории Бишкекского геодинамического полигона и в его окрестностях

Об афтершоковых процессах, сопровождающих умеренные и слабые землетрясения на территории Бишкекского геодинамического полигона и в его окрестностях

Tectonic position, geological manifestations of sources, and aftershock processes for a series of strong earthquakes of 2016–2017 in central Italy

Based on seismological, geodynamic, and seismotectonic data on a series of strong earthquakes which occurred in 2016–2017 in central Italy, the tectonic positions and geological manifestations of these events are determined. The aftershock processes of these earthquakes are studied. It is found that epicentral clouds of mainshocks and aftershocks almost overlapped each other, so these earthquake sources in aggregate can be classified as a high-magnitude earthquake swarm.

The Tolud Burst of Seismicity and the Earthquake of November 30, 2012 (MC = 5.4, MW = 4.8) that Accompanied the Start of the 2012‒2013 Tolbachik Eruption

This paper reports a study of the Tolud earthquake sequence; the sequence was a burst of shallow seismicity between November 28 and December 7, 2012; it accompanied the initial phase in the Tolbachik Fissure Eruption of 2012‒2013. The largest earthquake (the Tolud earthquake of November 30, 2012, to be referred to as the Tolud Earthquake in what follows, with KS = 11.3, ML = 4.9, MC = 5.4, and MW = 4.8) is one of the five larger seismic events that have been recorded at depths shallower than 10 km beneath the entire Klyuchevskoi Volcanic Cluster in 1961‒2015. It was found that the Tolud earthquake sequence was the foreshock–aftershock process of the Tolud Earthquake. This is one of the larger seismicity episodes ever to have occurred in the volcanic areas of Kamchatka. Data of the Kamchatka seismic stations were used to compute some parameters for the Tolud Earthquake and its largest (ML = 4.3) aftershock; the parameters include the source parameters and mechanisms, and the moment magnitudes, since no information on these is available at the world seismological data centers. The focal mechanisms for the Tolud Earthquake and for its aftershock are consistent with seismic ruptures at a tension fault in the rift zone. Instrumental data were used to estimate the intensity of shaking due to the Tolud Earthquake. We discuss the sequence of events that was a signature of the time-dependent seismic and volcanic activity that took place in the Tolbachik zone in late November 2012 and terminated in the Tolud burst of seismicity. Based on the current ideas of the tectonics and magma sources for the Tolbachik volcanic zone, we discuss possible causes of these earthquakes.

Forecasting aftershock activity: 2. Estimating the area prone to strong aftershocks

The technique for forecasting the spatial domain where fairly intense aftershocks should be expected after a strong earthquake is considered. The paper presents the task of estimating the area prone to the strong future aftershocks using the data for the first 12 h after the main shock. The existing aftershock identification techniques are inapplicable to this task because they either analyze the distributions of the epicenters of the aftershock process that has been already completed or only consider the parameters of the main shock and only provide rough estimates. Using the developed criteria of estimating the quality of the prediction, we quantitatively compared quite a few different candidates. The latter included the main known techniques and their modifications suggested by us. In these modifications, we took into account the results of the recent studies on the dynamics of the aftershock process. This enabled us to select the optimal procedure which demonstrated the best results of the quantitative tests for more than 120 aftershock sequences with the magnitudes starting from 6.5 all over the world. This procedure can be used in the seismological monitoring centers for forecasting the area prone to the aftershock activity after a strong earthquake based on the data of operative processing.

Abnormal geophysical and seismotectonic processes observed during the period of preparation and development of the earthquake with a magnetude of 8,8 Maule 2010 (Chile)

Focal zone, basic shock, jerking of strongest aftershocks and progress of seismic energy release during the aftershock process of the earthquake Maule 27.02.2010 (M W =8,8) in Chile have been analyzed. Macroseismic manifestations, tectonic position of the focus and reconstruction of acting plane position in the focus have been considered. Geophysical treatment of anomalous gravimagnetic disturbances and seismic precursors induced in variations of magnetic field and geoacoustic fields of the Earth during preparation of the basic shock and of three strongest iterative shocks on March 11 —16 2010 has been done for the first time. It has been shown that application of present-day cosmic observations, which allow to analyze gravity disturbances (so far only coseismic), reflected on the Earth’s surface makes possible to consider the precursors of seismic shocks of the arthquake Maule and other seismic events in a new way. Long-, middle- and short-term approaches to prognostication of strong seismic events both on land and sea have been stated

Post-seismic relaxation from geodetic and seismic data

We have examined the aftershock sequence and the post-seismic deformation process of the Parkfield earthquake (2004, M = 6, California, USA) source area using GPS data. This event was chosen because of the possibility of joint analysis of data from the rather dense local GPS network (from SOPAC Internet archive) and of the availability of the rather detailed aftershock sequence data (http://www.ncedc.org/ncedc/catalog-search.html). The relaxation process of post-seismic deformation prolongs about the same 400 days as the seismic aftershock process does. Thus, the aftershock process and the relaxation process in deformation could be the different sides of the same process. It should be noted that the ratio of the released seismic energy and of the GPS obtained deformation is quite different for the main shock and for the aftershock stage. The ratio of the released seismic energy to the deformation value decreases essentially for the post-shock process. The similar change in the seismic energy/deformation value ratio is valid in a few other strong earthquakes. Thus, this decrease seems typical of aftershock sequences testifying for decrease of ratio of elastic to inelastic deformation in the process of post-shock relaxation when the source area appears to be mostly fractured after the main shock occurs, but the healing process had no yet sufficient time to develop.

STRESS FIELDS REVEALED BY AFTERSHOCKS OF THE STRONGEST EARTHQUAKES OF TIEN SHAN

This study focused on the state of stress in the aftershock areas of the strongest earthquakes registered in Tien Shanon 11 August 1974 (M S =7.3), 24 March 1978 (M S =7.1), 01 November 1978 (M S =6.8), 19 August 1992 (M S =7.3), and 05 October 2008 (M S =6.9). These earthquakes occurred in similar geodynamic conditions in the zone of the nearly N–S oriented compression caused by the India-Eurasia collision. The parameters of the state of stress were determined by Yu.L. Rebetsky method of cataclastic analysis (MCA) of displacements on faults, based on the focal mechanisms of aftershocks. In the first stage of MCA, we determined the orientations of the principal stress axes, the Lode-Nadai coefficients, and the geodynamic types of the state of stress in the study area. The stress fields reconstructed from the aftershocks of different energy levels show that the principal deviatoric stress axes pattern becomes less orderly as the magnitude of seismic events decreases. A peculiar feature of the aftershock process is the misalignment of the axes of maximum compression in the mechanisms of the weakest aftershocks and the compression axis in the focal mechanism of the main seismic event. Most of the aftershocks were recorded west of the location of the main earthquake.

Tectonic position and geological and seismic manifestations of the Gorkha earthquake of April 25, 2015, in Nepal

The characteristics of sources of the Gorkha earthquake’s mainshock (April 25, 2015, Nepal) and strongest aftershock are given. Macroseismic data and examples of seismic dislocations are provided. The course of seismic energy release during the aftershock process is analyzed. The data on seismological precursors of the mainshock and the strongest aftershock of May 12, 2015, are presented, which allowed the aftershock to be predicted in a short-term interval.

The Gorkha earthquake of April 25, 2015 in Nepal: Tectonic position, aftershock process, and possibilities of forecasting the evolution of seismic situation

The characteristics of the foci for the main shock and strongest aftershocks of the Gorkha earthquake of April 25, 2015 in Nepal are described. The macroseismic data and examples of seismic dislocations are discussed. The progression of the release of seismic energy by the aftershock process is analyzed. The data for the geophysical and seismological precursors of the main shock and the strongest aftershock of May 12, 2015 are presented. These data allowed us to formulate the short-term forecast of this event.

Forecasting aftershock activity: 1. Adaptive estimates based on the Omori and Gutenberg–Richter laws

The method for forecasting the intensity of the aftershock processes after strong earthquakes in different magnitude intervals is considered. The method is based on the joint use of the time model of the aftershock process and the Gutenberg–Richter law. The time model serves for estimating the intensity of the aftershock flow with a magnitude larger than or equal to the magnitude of completeness. The Gutenberg–Richter law is used for magnitude scaling. The suggested approach implements successive refinement of the parameters of both components of the method, which is the main novelty distinguishing it from the previous ones. This approach, to a significant extent, takes into account the variations in the parameters of the frequency–magnitude distribution, which often show themselves by the decreasing fraction of stronger aftershocks with time. Testing the method on eight aftershock sequences in the regions with different patterns of seismicity demonstrates the high probability of successful forecasts. The suggested technique can be employed in seismological monitoring centers for forecasting the aftershock activity of a strong earthquake based on the results of operational processing.

The aftershock processes of strong earthquakes in the Western Caucasus

The aftershock processes of the four strong earthquakes that occurred in the Western Caucasus from 1991 to June 2013 are considered. The main shocks of these earthquakes include the first Racha earthquake (April 29, 1991, Ms = 6.9); second Racha earthquake (June 15, 1991, Ms = 6.2); Oni earthquake (September 7, 2009, Ms = 5.8); and East Black Sea earthquake (December 23, 2012, Ms = 5.6). Based on the simulations with the LPL relaxation model and the ETAS model of triggered seismicity, the differences in the properties of the aftershock processes and the characteristics of the fault zones accommodating the main shocks are revealed. The nonrelaxation character of the aftershocks from the East Black Sea earthquake is established. It is hypothesized and validated that this is a result of the violation of the fluid-dynamic equilibrium in the fault zone due to the destruction of the gas hydrate layer by the main shock and strong aftershocks.