THE HINDU KUSH EARTHQUAKE on October 26, 2015 with Mw=7.5, I0~7: PRELIMINARY SEISMICITY and AFTERSHOCK SEQUENCE

On October 26, 2015, a strong Hindu Kush earthquake with KR=17.0, Mw=7.5 occurred in the Afghan Pamir-Hindu Kush subzone at a depth of hpP=217 km. Shakes of varying intensity caused by this earthquake were recorded in settlements of 14 states: Afghanistan, Tajikistan, Pakistan, Turkmenistan, India, Kyrgyzstan, Uzbekistan, Kazakhstan, China, Iran, Nepal, United Arab Emirates, Russia, Qatar and Bangladesh with a total area of S=14106 km2. The earthquake was preceded by three large (KR=12.5, 12.1, 14.0) foreshocks and was accompanied by a series of more than 1400 aftershocks unprecedented for aftershocks of deep earthquakes with KR=9–13. The energy step between the mainshock and the maximum foreshock is Kfor=3.0, between the mainshock and the maximum (KR=12.8) aftershock – Kaft=4.2. The aftershock recurrence graph has a slope =–0.67, which in absolute value is higher than the average value in the region =0.50. The attenuation para-meter  of the Omori law in the initial phase of attenuation, =–1.26, in absolute value is also higher than the average =1.0 for strong earthquakes in the World. Based on the results of a joint analysis of the focal me-chanism solutions of different agencies and vertical sections along and across the aftershock cloud, it was con-cluded that an upthrust movement occurred in the source along a steep east-south-east nodal plane, dipping to the south. The reason for the activity at the site of the earthquake is the movement of the Indian continent to the north and its collision with Eurasia, as a result of which the separation and subduction of the Hindu Kush plate continue. The Hindu Kush earthquake on October 26, 2015, and its aftershocks are just one of the events of successive deformation and stress relief in the latitudinal zone, marked in 2015 by the migration of earthquake epicenters with KR=13–17 from east to west.

Sequence-based hazard analysis for Italy considering a grid seismic source model

Earthquakes are usually clustered in both time and space and, within each cluster, the event of highest magnitude is conventionally identified as the mainshock, while the foreshocks and the aftershocks are the events that occur before and after it, respectively. Mainshocks are the earthquakes considered in the classical formulation of the probabilistic seismic hazard analysis (PSHA), where the contribution of foreshocks and aftershocks is usually neglected. In fact, it has been shown that it is possible to rigorously, within the hypotheses of the model, account for the effect of mainshock-aftershocks sequences by means of the sequence-based PSHA (i.e., SPSHA). SPSHA extends the usability of the homogeneous Poisson process, adopted for mainshocks within PSHA, to also describe the occurrence of clusters maintaining the same input data of PSHA; i.e., the seismic rates derived by a declustered catalog. The aftershocks’ occurrences are accounted for by means of conditional non-homogeneous Poisson processes based on the modified Omori law. The seismic source model for Italy has been recently investigated, and the objective of the study herein presented is to include and evaluate the effect of aftershocks, by means of SPSHA, based on a new grid model. In the paper, the results of PSHA and SPSHA are compared, considering the spectral and return periods that are of typical interest for earthquake engineering. Finally, a comparison with the SPSHA map based on a well- established source model for Italy is also provided.

The early postseismic phase of Tohoku-Oki earthquake (2011) from kinematics solutions: implication for subduction interface dynamics

<p>Earthquakes are usually followed by a postseismic phase where the stresses induced by the earthquakes are relaxed. It is a combination of different processes among which aseismic slip on the fault zone (called afterslip), viscoelastic deformation of the surrounding material, poroelastic relaxation and aftershocks. However, little work has been done at the transition from the co- to the postseismic phase, and the physical processes involved.</p><p>We study the 2011 Mw 9.0 Tohoku-Oki earthquake, one of the largest and most instrumented recent earthquake, using GEONET GPS data. We focus on the few minutes to the first month following the mainshock, a period dominated by afterslip.</p><p>Based on the method developed by Twardzik et al. (2019), we process 30-s kinematic position time series and we use it to characterize the fast displacements rates that typically occur during the early stages of the postseismic phase. We quantify precisely the co-seismic offset of the mainshock, without including early afterslip, and we also characterize the co-seismic offset of the Mw 7.9 Ibaraki-Oki aftershock, which occurred 30 minutes after the mainshock. We analyze the spatial distribution of the co-seismic offsets for both earthquakes. We also use signal induced by the postseismic phase over different time windows to investigate the spatio-temporal evolution of the postseismic slip. We determine the redistribution of stresses to estimate the regional influence of the mainshock and aftershock on postseismic slip.</p><p>From a detailed characterization of the first month of postseismic kinematic time series, we find that the best-fitting law is given by an Omori-like decay. The displacement rate is of the type v<sub>0</sub>/(t+c)<sup>p</sup> with spatial variation for the initial velocity v<sub>0</sub> and for the time constant c. We find a consistent estimate of the p-value close to 0.7 over most of the studied area, apart from a small region close to the aftershock location where higher p values (p~1) are observed. This p value of 0.7 shows that the evolution of the Tohoku-Oki early afterslip is not logarithmic. We discuss about the implications of these observations in terms of subduction interface dynamics and rheology. We also discuss about the different time-scales involved in the relaxation, and how this model, established for the early postseismic phase over one month, performs over longer time scales (by comparison with daily time series lasting several years).</p><p>Twardzik Cedric, Mathilde Vergnolle, Anthony Sladen and Antonio Avallone (2019), doi.org/10.1038/s41598-019-39038-z</p><p><strong>Keywords: </strong>Early Postseismic, Afterslip, GPS, Kinematic, Omori Law</p>

Seismicity clusters in Central Chile: investigating the role of repeating earthquakes and swarms in a subduction region

SUMMARY Seismicity along subduction interfaces is usually dominated by large main-shock–aftershock sequences indicative of a continuum distribution of highly coupled large asperities. In the past decades, however, the increased resolution of seismic catalogues at some subduction zone seems to indicate instead a more complex rheological segmentation of the interface. Large and megathrust earthquake ruptures seem interspersed among regions of low seismic coupling and less stress buildup. In this weaker zone, the strain is primarily released via a combination of moderate-size swarm-like seismicity and aseismic slip. Along the Chilean subduction zone, the densification of the seismic network allowed for the identification of localized seismic clusters, some of them appearing in the form of swarms before megathrust earthquakes. The origin and driving processes of this seismic activity have not yet been identified. In this study, we follow a systematic approach to characterize the seismicity at two persistent clusters in Central Chile, one located offshore Navidad and one inland, at ∼40 km depth beneath Vichuquén, which occurred throughout ∼20 yr. We investigated these clusters, by deriving high-resolution hypocentral locations and moment tensors and performing a detailed analysis of spatio-temporal patterns, magnitude and interevent time distributions of the clustered earthquakes. Both clusters are characterized by weak to moderate seismicity (below Mw 6) and stand out as clear seismicity rate and Benioff strain anomalies. At the Navidad cluster, seismicity occurs in the form of swarms, with a characteristic duration of 2–7 d and location and thrust mechanisms compatible with activity on the slab interface. Conversely, we find at Vichuquén activity dominated by thrust earthquakes occurring as repeaters on the slab interface, with a slip rate of approximately ∼5.0 cm yr−1. We attribute these clusters to local features of the subducting plate: the Navidad swarms are likely driven by repeated high pore pressure transients along a pre-fractured patch of the slab, while the seismicity at the Vichuquén cluster is interpreted as the result of a subducting seamount. Both clusters have been active before and after the Mw 8.8 Maule earthquake and persisted afterwards with the seismicity decay following the Omori law. These interactions are especially evident for the Vichuquén cluster, where the seismicity rate increased considerably after the Maule earthquake and continues to be an area of clearly elevated seismicity rate compared to its surroundings.

Quantifying evolution of aftershocks sequences by Delta/Sigma method: a parametric analysis

<p>The Delta/Sigma method is applied in this study to investigate the seismic sequences following major earthquakes, with the aim to understand whether they fit the classical laws of aftershocks occurrence, such as the classical Omori Law and its recent variants, and to explore whether observed deviations from these laws may provide some statistically significant information about the possible occurrence of further large aftershocks. Specifically, the Delta/Sigma method is based on the observation of possible anomalies in the temporal decay of an aftershock sequence. In fact, given the number of events actually observed within a time window U (e.g. 1 day), its difference (Delta) with respect to theoretical number of events, and its standard deviation (Sigma), it was found that, before the occurrence of large aftershocks, the Delta/Sigma ratio may reach rather high values (e.g. above 2-2.5), which can be possibly followed within few days by some relevant aftershocks. The investigation area (referred as "Box") is defined as a rectangular sector, with dimensions proportional to the magnitude M of the mainshock, and with barycenter computed based on aftershocks occurred during the first Tb days from the mainshock.</p><p>To investigate the performances of the Delta/Sigma method various earthquake sequences are selected from different regions worldwide, including those associated with recent destructive earthquakes in Italy and Iran. The input data necessary for this study are extracted from global datasets (ANSS-USGS and ISC catalogs) and regional catalogs (e.g. ISIDE bulletins for the Italian territory). A wide set of parametrics tests is carried out in order to verify if this method could forecast the moderate and large aftershocks, which occurred in the region surrounding the mainshocks epicenter. Different input parameters are considered, in order to check the stability and statistical significance of the obtained results.</p><p>The preliminary results suggest that the application of Delta/Sigma method could highlight the possible occurrence of several significant aftershocks. Careful assessment of forecasting capability is essential, in order to provide relevant information for mitigation of risks associated with large aftershocks occurrence.</p>

Maximum Earthquake Size and Seismicity Rate from an ETAS Model with Slip Budget

ABSTRACT We define a seismicity model based on (1) the epidemic-type aftershock sequence model that accounts for earthquake clustering, and (2) a closed slip budget at long timescale. This is achieved by not permitting an earthquake to have a seismic moment greater than the current seismic moment deficit. This causes the Gutenberg–Richter law to be modulated by a smooth upper cutoff, the location of which can be predicted from the model parameters. We investigate the various regimes of this model that more particularly include a regime in which the activity does not die off even with a vanishingly small spontaneous (i.e., background) earthquake rate and one that bears strong statistical similarities with repeating earthquake time series. Finally, this model relates the earthquake rate and the geodetic moment rate and, therefore, allows to make sense of this relationship in terms of fundamental empirical law (the Gutenberg–Richter law, the productivity law, and the Omori law) and physical parameters (seismic coupling, tectonic loading rate).

A project of creating atlas aftershocks of strong earthquakes

The Omori Law, which describes the repeated underground shocks after a strong earthquake, is written in the form of a nonlinear differential equation. An idea of the focal deactivation coefficient after the main shock is introduced. Two advantages of the new wording of the Omori Law are given. Firstly, there is an interesting possibility to naturally take into account exogenous and endogenous triggers affecting the earthquake source. Endogenous triggers in the form of round-the-world seismic echo and free oscillations of the Earth, excited by the main shock, are especially noted. The second advantage is that the differential aftershock equation makes it possible to put the reverse problem of the earthquake source physics. The essence of the inverse problem is to determine the deactivation coefficient from the data on the observed aftershock frequency. Examples of inverse problem solution are given. The project of creation of the Atlas of aftershocks on the basis of the solution of the inverse problem of the source, cooling down after a strong earthquake is offered.

Ensembles of ETAS Models Provide Optimal Operational Earthquake Forecasting During Swarms: Insights from the 2015 San Ramon, California Swarm

Earthquake swarms, typically modeled as time‐varying changes in background seismicity, which are driven by external processes such as fluid flow or aseismic creep, present challenges for operational earthquake forecasting. Although the time decay of aftershock sequences can be estimated with the modified Omori law, it is difficult to forecast the temporal behavior of seismicity rates during a swarm. To explore these issues, we apply the epidemic‐type aftershock sequence (ETAS) model (Ogata, 1988) to the 2015 San Ramon, California swarm, which lasted several weeks and had almost 100 2≤M≤3.6 earthquakes. We develop three‐day forecasts during the swarm based on an ETAS model fit to all prior seismicity in the region as well as an ETAS model fit only to previous swarms in the region, which is better at capturing the higher background rate during the swarm. We also explore forecasts in which the background rate is updated periodically during the swarm using data over different lookback windows and find generally these models perform better than the models in which the background rate is fixed. Finally, we construct ensemble forecasts by combining the different models weighted according to their performance. The ensemble forecasts outperform all of the individual models and allow us to avoid making arbitrary choices at the outset of a swarm as to which single model will perform the best.

Influence of the Structure of a Gouge-Filled Fault on the Parameters of Acoustic Emission

Presented are the results of laboratory experiments on investigating manifestations of acoustic emission (AE) of a gouge-filled fault during stick-slip. The laboratory experiments were held at the slider-model setup, when a granite block slides along a rough granite base under normal and shear loads. In the course of experiments we altered the structure of the two-component filler of the fault and focused on variations of the AE parameters. The kinematic parameters of fault slip events in all the realizations remained approximately the same. The eff ect of gouge structure on the statistics of AE has been revealed. An alteration of proportion of quartz sand / glass beads in the filler of the fault was accompanied by an alteration of the b-value of frequency-energy distribution from 0.53 to 0.85, and the p-value of Omori law from 1.00 to 2.06. Also, it has been demonstrated that the nucleation of a slip event is accompanied by an alteration of the mechanism of AE generation – at the initial stage the 'tensile crack' signals prevailed, while at the final stage – the 'shear crack' signals did. The alteration of AE genesis manifested vividly in a corresponding alteration of the emitted waveforms for all the realizations.