Strong Aftershocks Research Articles

Reprint of: Rupture processes of the 2015 Mw 7.9 Gorkha earthquake and its Mw 7.3 aftershock and their implications on the seismic risk

The rupture processes of the 2015 April 25 Gorkha earthquake and its strongest aftershock occurred on May 12 in Nepal are investigated by joint inversion of seismological and geodetic data. Synthetic test shows that the sedimentary layers in the source region play an important role in the rupture process inversion. Our optimized model of the mainshock shows that the rupture has a unilateral propagation pattern. The dominant mechanism is pure thrust with maximum slip of 5.8m, the rupture scale extends ~60km along dip and ~150km along strike, and the largest static stress change is ~7.6MPa. The total seismic moment is 7.87×1020Nm, equivalent to Mw 7.9. Most seismic moment was released within 80s and the majority seismic moment was released at the first 40s. The rupture propagated in main slip asperity with a velocity of ~3.0km/s. The strong aftershock magnitude is about Mw 7.3, and the peak slip is about 5.0m, close to the peak slip of the mainshock. Moreover, the slips of the mainshock and the aftershocks are in good complementary, suggesting a triggering relationship between them. Considering the strain accumulation, the Gorkha earthquake ruptured only part of the seismic gap alone, thus still poses high earthquake risk, especially in the west side of the mainshock rupture zone.

Structural Health Monitoring (SHM) of Civil Structures

As newer and more reliable ways of construction were developed, civilization began to spread out further and retain functional infrastructure for longer periods of time.[...]

Rapid estimation of the hazard posed by strong aftershocks for Kamchatka and the Kuril Islands

This paper deals with the rapid estimation of the hazard posed by strong aftershocks for Kamchatka and the Kuril Islands based on the 12-hour aftershocks, namely, their rate, time of expectation, and maximum magnitude. The data set consists of main shocks and aftershocks as reported in the Catalog of the Kamchatka Branch of the Geophysical Survey, Russian Academy of Sciences (GS RAS) for 1968–2016. We used both the Gutenberg–Richter relation and the Omori–Utsu law to find that the aftershock rates in two time intervals are connected by a relationship of proportionality, with the constant of proportionality being independent of the lowest magnitude of these earthquakes. With this property in mind, we calculated the constants of proportionality for estimating the rate of aftershocks with a magnitude above threshold values based on the data for the first 12 hours after the main shock. We have derived easily remembered rules for estimating the aftershock rates that can be expected for 5 days and for 1 month with magnitudes above a fixed value based on the 12-hour aftershock observations. We also derived empirical regression relations to estimate the magnitude of the largest aftershock that can be expected to occur during 1 year after the main shock and the time of the last aftershock to occur whose magnitude is 5.2 or greater.

April 2016 Ecuador Earthquake of Moment Magnitude M<sub>w</sub>7.8: Overview and Damage Report

The northern coastal region of Ecuador was affected by a moment magnitude Mw7.8 strong earthquake on April 16, 2016, with a recorded PGA of 1.41 g close to the epicenter. The earthquake was named “Pedernales” because the epicenter was near this small town located in the coastal province of Manabí, with a hypocentral depth of 21 km. Two strong aftershocks of Mw6.7 and Mw6.9 were felt a month later and they continued the destruction caused by the main event. Ecuador is located in a seismic zone with a long history of major earthquakes that have caused a lot of destruction in the last six centuries, the main seismic source comes from the subduction of the Nazca Plate under the South American Plate, but there are also crustal earthquakes and volcanism on the Andes Mountains that cross the country from north to south. In the northern coastal province of Esmeraldas a powerful earthquake of moment magnitude Mw8.8 was recorded in 1906, with a rupture area that has caused strong earthquakes in the 20th and 21st centuries. This paper describes the seismological aspects and the recorded strong ground motions, the geotechnical and infrastructure performance, site amplification and liquefaction on a few places, structural damage and collapse of buildings, extensive non-structural damage (mainly large damage to masonry walls) at high-rise buildings in cities near (100 km) and far away (270 km) from the epicenter, due to local soil conditions and flexible framed reinforced concrete structures.

Which fault destroyed Fes city (Morocco) in 1755? A new insight from the Holocene deformations observed along the southern border of Gibraltar arc

In this paper, we present the first estimate of the Holocene deformation along the southern front of Gibraltar arc (Morocco) and the first field constraints on the local 1755CE Fes-Meknes surface rupturing earthquake which could be associated to the “Great Lisbon Earthquake” (M>8.5) in November 1st, 1755. Using satellite imagery, aerial photographs and field investigations, we carried out a morphotectonic study along the ~150km-long Southern Rif Front (SRF) to identify the most recent evidences of tectonic activity. Analyzed offset alluvial deposits confirm that (i) the last ~5ka cumulative deformation leading to a slip rate of ~3.5±1mm/yr for this segment of the SRF is consistent with the GPS derived horizontal shortening rate of 2–4mm/yr and (ii) a recent major earthquake ruptured a~30km-long segment along the SRF. Based on deposits dating and historical seismicity we propose that this seismic event occurred in 1755 as a local earthquake. Even though this 1755 local event cannot be considered as a strong aftershock of the main Lisbon seismic event (M>8.5), their temporal closeness, their occurrence under the same convergent stress regime (~NNW-SSE-oriented compression) and the fact that Fes-Meknes area was strongly shaken during the Lisbon earthquake, raises the question of the possible triggering of the Fes earthquake. Anyway, our new results suggest that most of the Nubia-Rif belt convergence is accommodated by the SRF, making it potentially the most destructive structure of the Rif.

The effect of mainshock-aftershock on the residual displacement of buildings equipped with cylindrical frictional damper

Recently, Friction dampers become popular due to the desirable performance in the energy dissipation of lateral loads. A lot of research which has been conducted on these dampers results in developing friction dampers with low sensitivity to the number of cycles and temperature increases. Friction dampers impose high residual drifts to the buildings because of low post-yield stiffness of the damper which results from increasing lateral displacement and period of buildings. This issue can be more critical under strong aftershocks which results in increasing of structural damages. In this paper, in addition to the assessment of aftershock on steel buildings equipped with friction dampers, methods for controlling residual drifts and decreasing the costs of retrofitting are investigated. Utilizing rigid connections as a lateral dual system and activating lateral stiffness of gravity columns by adding elastic braces are as an example of effective methods investigated in this research. The results of nonlinear time history analyses on the low to medium rise steel frames equipped with friction dampers illustrate a rise in residual drifts as the result of aftershocks. In addition, the results show that different slip loads of friction damper can affect the residual drifts. Furthermore, elastic stories in comparison to rigid connections can reduce residual drifts of buildings in an effective fashion, when most slip loads of friction dampers are considered.

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.

The smart cluster method

Earthquake clustering is an essential part of almost any statistical analysis of spatial and temporal properties of seismic activity. The nature of earthquake clusters and subsequent declustering of earthquake catalogues plays a crucial role in determining the magnitude-dependent earthquake return period and its respective spatial variation for probabilistic seismic hazard assessment. This study introduces the Smart Cluster Method (SCM), a new methodology to identify earthquake clusters, which uses an adaptive point process for spatio-temporal cluster identification. It utilises the magnitude-dependent spatio-temporal earthquake density to adjust the search properties, subsequently analyses the identified clusters to determine directional variation and adjusts its search space with respect to directional properties. In the case of rapid subsequent ruptures like the 1992 Landers sequence or the 2010–2011 Darfield-Christchurch sequence, a reclassification procedure is applied to disassemble subsequent ruptures using near-field searches, nearest neighbour classification and temporal splitting. The method is capable of identifying and classifying earthquake clusters in space and time. It has been tested and validated using earthquake data from California and New Zealand. A total of more than 1500 clusters have been found in both regions since 1980 with M m i n = 2.0. Utilising the knowledge of cluster classification, the method has been adjusted to provide an earthquake declustering algorithm, which has been compared to existing methods. Its performance is comparable to established methodologies. The analysis of earthquake clustering statistics lead to various new and updated correlation functions, e.g. for ratios between mainshock and strongest aftershock and general aftershock activity metrics.

SOURCE MECHANISM AND SIZE OF THE 24 APRIL 2002 ML 5.2 GNJILANE (KOSOVO) EARTHQUAKE

A b s t r a c t: The 24 April 2002 ML 5.2 Gnjilane earthquake was studied first through inversion of the Sg – Lg wave group displacement amplitude spectrum and P-nodal planes determination. The seismic moment, source spectrum corner frequency and Brune equivalent circular fault surface for this shock were obtained, respectively, as M0 = 6.48·1016 N·m, f0 = 0.59 Hz and Σs,eq = 15.2 km2. The P-nodal planes for the four strongest aftershocks and the distribution of other aftershocks’ epicentres were determined, too, and used in identifying the actual source mechanism of the main shock by a simple method that included also the vertical projections on the Earth’s surface of the main shock Σs,eq with the two main shock P-nodal planes as possible fault planes. It was found that the main shock was caused by a normal right lateral faulting in a plane which struck with an azimuth of 238° and dipped toward NNW under an angle of 22°. This faulting was associated with the shear stressed fault structure along the Pliocene-Quarternary sinking valley of Binačka Morava, and it led to activations of other ruptures as sources of a significant number of aftershocks.

Influence of modeling assumptions and aftershock hazard level in the seismic response of post-mainshock steel framed buildings

Seismic response under strong aftershocks has become an important topic in performance-based earthquake engineering recently. This paper examines the influence of the modeling approach in the response of a case-study four-story steel framed building designed with modern seismic provisions when subjected to two sets of aftershocks representing different aftershock-hazard levels. It was assumed that the building models experienced specific post-mainshock residual drifts prior of being subjected to the aftershocks. Results shown that including additional sources of stiffness and strength (i.e. inclusion of the interior gravity framing and participation of the slab in the beams) in the building analytical model lead to a different response under aftershocks in terms of maximum and residual interstory drifts than that of the analytical model commonly used in previous studies. Particularly, additional overstrength and post-yield stiffness ratio help to constrain residual drift demands under strong aftershocks, and decrease the probability of imminent demolition due to excessive post-mainshock residual drifts.

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

Strong Aftershock Study Based on Coulomb Stress Triggering—A Case Study on the 2016 Ecuador Mw 7.8 Earthquake

The 2016 Ecuador M 7.8 earthquake ruptured the subduction zone boundary between the Nazca plate and the South America plate. This M 7.8 earthquake may have promoted failure in the surrounding crust, where six M ≥ 6 aftershocks occurred following this mainshock. These crustal ruptures were triggered by the high coulomb stress changes produced by the M 7.8 mainshock. Here, we investigate whether the six M ≥ 6 aftershocks are consistent with the positive coulomb stress region due to the mainshock. To explore the correlation between the mainshock and the aftershocks, we adopt a recently published high-quality finite fault model and focal mechanisms to study the coulomb stress triggers during the M 7.8 earthquake sequence. We compute the coulomb failure stress changes (ΔCFS) on both of the focal mechanism nodal planes. We compare the ΔCFS imparted by the M 7.8 mainshock on the subsequent aftershocks with the epicenter location of each aftershock. In addition, the shear stress, normal stress, and coulomb stress changes in the focal sources of each aftershock are also computed. Coulomb stress changes in the focal source for the six M ≥ 6 aftershocks are in the range of −2.17–7.564 bar. Only one computational result for the M 6.9 aftershock is negative; other results are positive. We found that the vast majority of the six M ≥ 6 aftershocks occurred in positive coulomb stress areas triggered by the M 7.8 mainshock. Our results suggest that the coulomb stress changes contributed to the development of the Ecuador M 7.8 earthquake sequence.

Modeling the Southern Sakhalin earthquake sequences preceding strong shocks for short-term prediction of their origin time

Retrospective modeling of the sequences of shallow-focus weak earthquakes (M ~ 2.0–3.0) in Southern Sakhalin for the period of 2003–2014 is conducted using the method of self-developing process and the catalogue data of the local network. Mathematical models of the nonlinear increase of the cumulative shocks before strong (M = 4.6–6.2) events are constructed. Short-term forecasts of the parameter T0 (origin time of strong aftershocks) are obtained with a high degree of accuracy. The stability of the solutions obtained by varying the duration of the observation interval of catalogue data is shown. A gradual decrease in the error in the prediction of the T0 parameter is achieved as we approach the end of the processing interval to the time of the main shock. Although the errors in prediction in the retrospective version are no more than one day, the real evaluation of the accuracy can only be obtained in the practice of real predictions. Meanwhile, we have demonstrated the possibility in principle of the short-term prediction of strong shallow-focus earthquakes in Southern Sakhalin.

Variable anelastic attenuation and site effect in estimating source parameters of various major earthquakes including M w 7.8 Nepal and M w 7.5 Hindu kush earthquake by using far-field strong-motion data

Strong-motion records of recent Gorkha Nepal earthquake (Mw 7.8), its strong aftershocks and seismic events of Hindu kush region have been analysed for estimation of source parameters. The Mw 7.8 Gorkha Nepal earthquake of 25 April 2015 and its six aftershocks of magnitude range 5.3–7.3 are recorded at Multi-Parametric Geophysical Observatory, Ghuttu, Garhwal Himalaya (India) >600 km west from the epicentre of main shock of Gorkha earthquake. The acceleration data of eight earthquakes occurred in the Hindu kush region also recorded at this observatory which is located >1000 km east from the epicentre of Mw 7.5 Hindu kush earthquake on 26 October 2015. The shear wave spectra of acceleration record are corrected for the possible effects of anelastic attenuation at both source and recording site as well as for site amplification. The strong-motion data of six local earthquakes are used to estimate the site amplification and the shear wave quality factor (Qβ) at recording site. The frequency-dependent Qβ(f) = 124f0.98 is computed at Ghuttu station by using inversion technique. The corrected spectrum is compared with theoretical spectrum obtained from Brune’s circular model for the horizontal components using grid search algorithm. Computed seismic moment, stress drop and source radius of the earthquakes used in this work range 8.20 × 1016–5.72 × 1020 Nm, 7.1–50.6 bars and 3.55–36.70 km, respectively. The results match with the available values obtained by other agencies.

Residual seismic performance of steel bridges under earthquake sequence

A seismic damaged bridge may be hit again by a strong aftershock or another earthquake in a short interval before the repair work has been done. However, discussions about the impact of the unrepaired damages on the residual earthquake resistance of a steel bridge are very scarce at present. In this paper, nonlinear time-history analysis of a steel arch bridge was performed using multi-scale hybrid model. Two strong historical records of main shock-aftershock sequences were taken as the input ground motions during the dynamic analysis. The strain response, local deformation and the accumulation of plasticity of the bridge with and without unrepaired seismic damage were compared. Moreover, the effect of earthquake sequence on crack initiation caused by low-cycle fatigue of the steel bridge was investigated. The results show that seismic damage has little impact on the overall structural displacement response during the aftershock. The residual local deformation, strain response and the cumulative equivalent plastic strain are affected to some extent by the unrepaired damage. Low-cycle fatigue of the steel arch bridge is not induced by the earthquake sequences. Damage indexes of low-cycle fatigue predicted based on different theories are not exactly the same.

Stress history controls the spatial pattern of aftershocks: case studies from strike-slip earthquakes

Earthquake ruptures perturb stress within the surrounding crustal volume and as it is widely accepted now these stress perturbations strongly correlates with the following seismicity. Here we have documented five cases of the mainshock–aftershock sequences generated by the strike-slip faults from different tectonic environments of world in order to demonstrate that the stress changes resulting from large preceding earthquakes decades before effect spatial distribution of the aftershocks of the current mainshocks. The studied mainshock–aftershock sequences are the 15 October 1979 Imperial Valley earthquake (Mw = 6.4) in southern California, the 27 November 1979 Khuli-Boniabad (Mw = 7.1), the 10 May 1997 Qa’enat (Mw = 7.2) and the 31 March 2006 Silakhor (Mw = 6.1) earthquakes in Iran and the 13 March 1992 Erzincan earthquake (Mw = 6.7) in Turkey. In the literature, we have been able to find only these mainshocks that are mainly characterized by dense and strong aftershock activities along and beyond the one end of their ruptures while rare aftershock occurrences with relatively lower magnitude reported for the other end of their ruptures. It is shown that the stress changes resulted from earlier mainshock(s) that are close in both time and space might be the reason behind the observed aftershock patterns. The largest aftershocks of the mainshocks studied tend to occur inside the stress-increased lobes that were also stressed by the background earthquakes and not to occur inside the stress-increased lobes that fall into the stress shadow of the background earthquakes. We suggest that the stress shadows of the previous mainshocks may persist in the crust for decades to suppress aftershock distribution of the current mainshocks. Considering active researches about use of the Coulomb stress change maps as a practical tool to forecast spatial distribution of the upcoming aftershocks for earthquake risk mitigation purposes in near-real time, it is further suggested that the background mainshocks along the neighbouring faults should be taken into account in producing the stress change maps for commenting on aftershock occurrences.

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.

Typhoid versus typhus fever in post-earthquake Nepal.

Typhoid versus typhus fever in post-earthquake Nepal.

Resilient welded steel moment connections by enhanced beam buckling resistance

This study develops two (2) simple but effective techniques for enhancing buckling resistance of welded steel moment connections (WSMCs). The ANSI/AISC 358-10 prequalified connections satisfy the 4% interstory drift requirement, however experimental studies have shown that their strength degradation may initiate as early as 3% drift. This strength degradation has been observed to be initiated by buckling of the beam web which is followed by buckling of the beam flange and twisting of the beam. Consequently, buildings with the prequalified connections may sustain significant buckling damages under severe earthquakes and it is questionable as to whether these connections are capable of resisting gravity loads or lateral loads from strong aftershocks following a severe earthquake. To improve upon these shortcomings, two (2) performance enhancing techniques are proposed and investigated through finite element analysis (FEA). The more promising of the two involves reinforcing the beam web in the expected plastic hinge with a web reinforcement plate. Finite element analysis demonstrated that this reinforcement enhances the beam buckling resistance of WSMCs and thereby significantly reduces the beam buckling damages even at 5% interstory drift. The potential of this technique is analytically and experimentally demonstrated for the recently developed heat-treated beam section (HBS) WSMC. Test results confirm that the web reinforcement plate was effective in reducing local buckling damage and associated strength degradation, thereby improving the performance of HBS WSMCs. Areas for application and future development of the proposed techniques are identified.

Seismotectonics and seismogenesis of Mw7.8 Gorkha earthquake and its aftershocks

The April 25, 2015, Mw7.8 Gorkha earthquake in central Nepal was followed by intense aftershock activity, including Mw6.7 shock on April 26, 2015 and Mw7.3 shock on May 12, 2015. Synthesis of the focal mechanisms, space-time distribution of seismic activity in relation to previously imaged crustal velocity and resistivity structures reveals focusing of the Mw7.8 Gorkha earthquake near the upper surface of the thin fluid-filled low velocity and high conducting layer immediately above the plane of the detachment. On the geophysical sections, the detachment is identified as a sharp positive velocity interface. Modulation of frictional coupling and mechanical weakening by high-pore pressure fluids counteract the arc-normal stresses creating conditions for failure and nucleation of the Gorkha earthquakes on a plane sub-parallel with the detachment. Spatio-temporal patterns in aftershock activity indicate rapid alteration of main shock-induced stress fields, triggering a strong aftershock of Mw6.7. Large stress drop and increased energy released by the Mw6.7 event facilitates upward injection of high pore-pressure fluid fluxes into the hidden out-of-sequence thrust. It is suggested that decrease in shear strength along the hidden thrust plane due to the diffusion of high pore pressure fluids created conditions favourable to trigger Mw7.3 aftershock.