Stress Transfer and Aftershock Distribution of The Strong Earthquakes in The Thailand‑Laos‑Myanmar Border

This study aimed to investigate co-seismic stress and aftershock distribution along the Sumatra–Andaman subduction zone (SASZ). The fault parameters of six major earthquakes with an M ≥ 7 that occurred during 2010–2022 along the SASZ, were utilized to determine the Coulomb stress change using numerical modeling techniques calculated on the receiver faults with similar focal mechanisms of the mainshock, strike-slip, thrust, and normal faulting, respectively. The earthquake events were then classified to analyze the aftershocks of major earthquakes in the area. These aftershocks were mapped in order to determine the relationship between the aftershock distribution and the areas of increased or decreased stress. The relationship between the co-seismic stress and distribution of aftershocks in the SASZ was found to mainly depend on the focal mechanism of major earthquakes and the type of receiver fault used for calculation. After a major earthquake in the SASZ, there are two possible patterns that most aftershocks will be generated from in the areas of increased stress. First, a major earthquake is a type of thrust fault calculated on the receiver fault using the focal mechanism of the mainshock. Second, a major earthquake is a type of strike-slip fault calculated on the receiver fault with an optimum-oriented strike-slip fault. This relationship is likely to represent the specific pattern of the seismotectonic stress in the SASZ that can be used to evaluate the risk areas of aftershocks after a major earthquake has occurred. Furthermore, two earthquake events with large magnitudes were generated following the respective major earthquake in the SASZ that were located around the areas of increased stress, indicating that these two earthquake events were likely triggered in areas of increased stress following the respective major earthquake. Therefore, this study concluded that after a major earthquake occurrence in the SASZ, the areas of increased stress have a higher risk of generating both a large number of aftershocks and a new large-magnitude mainshock event. The investigation of co-seismic stress is very important to estimate areas of increased stress after a major earthquake, as this can be useful for monitoring both earthquake and tsunami hazards in the area.

Activation of major faults in Bulgaria and northern Greece presents significant seismic hazard because of their proximity to populated centers. The long recurrence intervals, of the order of several hundred years as suggested by previous investigations, imply that the twentieth century activation along the southern boundary of the sub-Balkan graben system, is probably associated with stress transfer among neighbouring faults or fault segments. Fault interaction is investigated through elastic stress transfer among strong main shocks (M ≥ 6.0), and in three cases their foreshocks, which ruptured distinct or adjacent normal fault segments. We compute stress perturbations caused by earthquake dislocations in a homogeneous half-space. The stress change calculations were performed for faults of strike, dip, and rake appropriate to the strong events. We explore the interaction between normal faults in the study area by resolving changes of Coulomb failure function (ΔCFF) since 1904 and hence the evolution of the stress field in the area during the last 100 years. Coulomb stress changes were calculated assuming that earthquakes can be modeled as static dislocations in an elastic half-space, and taking into account both the coseismic slip in strong earthquakes and the slow tectonic stress buildup associated with major fault segments. We evaluate if these stress changes brought a given strong earthquake closer to, or sent it farther from, failure. Our modeling results show that the generation of each strong event enhanced the Coulomb stress on along-strike neighbors and reduced the stress on parallel normal faults. We extend the stress calculations up to present and provide an assessment for future seismic hazard by identifying possible sites of impending strong earthquakes.

Supplemental Material: Three-dimensional finite-element modeling of Coulomb stress changes on normal and thrust faults caused by pore fluid pressure changes and postseismic viscoelastic relaxation

Supplemental Material: Three-dimensional finite-element modeling of Coulomb stress changes on normal and thrust faults caused by pore fluid pressure changes and postseismic viscoelastic relaxation

Supplemental Material: Three-dimensional finite-element modeling of Coulomb stress changes on normal and thrust faults caused by pore fluid pressure changes and postseismic viscoelastic relaxation

The well-recorded aftershocks and well-determined source model of the Noto Hanto earthquake provide an excellent opportunity to examine earthquake triggering associated with a blind thrust event. The aftershock zone rapidly expanded into a ‘butterfly pattern’ predicted by static Coulomb stress transfer associated with thrust faulting. We found that abundant aftershocks occurred where the static Coulomb stress increased by more than 0.5 bars, while few shocks occurred in the stress shadow calculated to extend northwest and southeast of the Noto Hanto rupture. To explore the three-dimensional distribution of the observed aftershocks and the calculated stress imparted by the mainshock, we further resolved Coulomb stress changes on the nodal planes of all aftershocks for which focal mechanisms are available. About 75% of the possible faults associated with the moderate-sized aftershocks were calculated to have been brought closer to failure by the mainshock, with the correlation best for low apparent fault friction. Our interpretation is that most of the aftershocks struck on the steeply dipping source fault and on a conjugate northwest-dipping reverse fault contiguous with the source fault. Since we found that the Coulomb hypothesis works well for the Noto Hanto sequence, we subsequently computed stress changes on the nearby active faults. Although the calculated stress changes were found to be negligible on the major faults south of the Noto Peninsula, several short active faults near the epicentral area were calculated to have been brought several bars closer to failure. Thus, the probability of strong shaking in and around the epicentral area may still be high due to the transfer of stress to the adjacent faults by a short blind thrust fault.

Stress transfer, aftershocks distribution and InSAR analysis of the 2005 Dahuieh earthquake, SE Iran

In order to investigate the possibility of forecasting aftershock distribution of the Iranian earthquakes, three strong earthquakes from the past 10 years were selected to calculate Coulomb stress changes and its correlation with the aftershock distribution. The common point of these earthquakes is their reverse focal mechanisms. Our results show a good triggering relationship between the aftershocks and main shocks. Moreover, though we selected earthquakes from three different seismotectonic provinces of Iran, the results were almost similar. In all cases, the utilization of specified oriented planes improved the correlation between the distribution of aftershock and stress-enhanced regions compared to the optimally oriented fault planes. And, the stress-increased regions are larger in the opposite side of the fault plane and the majority of aftershocks have been occurred in the same side. Our results in these cases show that faulting type may influence the aftershock distribution.

Western Turkey has a long history of destructive earthquakes that are responsible for the death of thousands of people and which caused devastating damage to the existing infrastructures, and cultural and historical monuments. The recent earthquakes of Izmit (Kocaeli) on 17 August, 1999 (M w = 7.4) and Duzce (M w = 7.2) on 12 November, 1999, which occurred in the neighboring fault segments along the North Anatolian Fault (NAF), were catastrophic ones for the Marmara region and surroundings in NW Turkey. Stress transfer between the two adjacent fault segments successfully explained the temporal proximity of these events. Similar evidence is also provided from recent studies dealing with successive strong events occurrence along the NAF and parts of the Aegean Sea; in that changes in the stress field due to the coseismic displacement of the stronger events influence the occurrence of the next events of comparable size by advancing their occurrence time and delimiting their occurrence place. In the present study the evolution of the stress field since the beginning of the twentieth century in the territory of the eastern Aegean Sea and western Turkey is examined, in an attempt to test whether the history of cumulative changes in stress can explain the spatial and temporal occurrence patterns of large earthquakes in this area. Coulomb stress changes are calculated assuming that earthquakes can be modeled as static dislocations in elastic half space, taking into account both the coseismic slip in large (M ≥ 6.5) earthquakes and the slow tectonic stress buildup along the major fault segments. The stress change calculations were performed for strike-slip and normal faults. In each stage of the evolutionary model the stress field is calculated according to the strike, dip, and rake angles of the next large event, whose triggering is inspected, and the possible sites for future strong earthquakes can be assessed. A new insight on the evaluation of future seismic hazards is given by translating the calculated stress changes into earthquake probability using an earthquake nucleation constitutive relation, which includes permanent and transient effects of the sudden stress changes.

Western Turkey has a long history of destructive earthquakes that are responsible for the death of thousands of people and which caused devastating damage to the existing infrastructures, and cultural and historical monuments. The recent earthquakes of Izmit (Kocaeli) on 17 August, 1999 (M w = 7.4) and Düzce (M w = 7.2) on 12 November, 1999, which occurred in the neighboring fault segments along the North Anatolian Fault (NAF), were catastrophic ones for the Marmara region and surroundings in NW Turkey. Stress transfer between the two adjacent fault segments successfully explained the temporal proximity of these events. Similar evidence is also provided from recent studies dealing with successive strong events occurrence along the NAF and parts of the Aegean Sea; in that changes in the stress field due to the coseismic displacement of the stronger events influence the occurrence of the next events of comparable size by advancing their occurrence time and delimiting their occurrence place. In the present study the evolution of the stress field since the beginning of the twentieth century in the territory of the eastern Aegean Sea and western Turkey is examined, in an attempt to test whether the history of cumulative changes in stress can explain the spatial and temporal occurrence patterns of large earthquakes in this area. Coulomb stress changes are calculated assuming that earthquakes can be modeled as static dislocations in elastic half space, taking into account both the coseismic slip in large (M ≥ 6.5) earthquakes and the slow tectonic stress buildup along the major fault segments. The stress change calculations were performed for strike-slip and normal faults. In each stage of the evolutionary model the stress field is calculated according to the strike, dip, and rake angles of the next large event, whose triggering is inspected, and the possible sites for future strong earthquakes can be assessed. A new insight on the evaluation of future seismic hazards is given by translating the calculated stress changes into earthquake probability using an earthquake nucleation constitutive relation, which includes permanent and transient effects of the sudden stress changes.Key wordsStress transferearthquake probabilitiesseismic hazard

Southeast of Iran experienced eight destructive earthquakes during 30 years from 1981 to 2011. Six of these events with M > 6.5 were fatal and caused great human and financial losses in the region. The 1981 July 28 (Mw 7.2) Sirch earthquake with 65 km surface rupture was the largest event in this region since 1877 and with other three earthquakes occurred in Golbaf-Sirch region during 17 years. The 26 December 2003 (Mw 6.6) Bam earthquake was one of the most destructive events in the recorded history of Iran. There were more than 26,000 killed, 30,000 to 50,000 injured people, and more than 100,000 were homeless. We calculated the static coulomb stress changes due to this earthquake sequence (four earthquakes) between 1981 and 1998 on the Golbaf-Sirch right-lateral fault and the Shahdad reverse fault and a slow slip on the Shahdad fault. Our calculations showed positive stress changes due to previous events on the ruptured plane of next earthquake. For example, the rupture plane of the 14 March 1998 (Mw 6.6) Fandoqa earthquake received a maximum positive stress change about 2.3 MPa. Also, some parts of the surrounding faults received positive stress changes due to these events. Stress changes on the planes of other four events until 2011 were calculated in this study. The 26 December 2003 (Mw 6.6) Bam earthquake and the 20 December 2010 (Mw 6.5) first Rigan earthquake received negligible (about thousandth (0.001)) negative stress changes in this sequence. The last event in our study area, the 27 January 2011 (Mw 6.2) second Rigan earthquake, experienced more than 0.5 MPa coseismic coulomb stress changes especially in its hypocenter and according to our calculations, it is mostly due to the first Rigan event. By using well-located aftershocks of the Rigan earthquake, we investigated the correlation between coulomb stress changes and aftershocks distribution. Calculated coulomb stress changes due to these two events on the optimally oriented strike-slip faults for the first event showed that most of the well-located seismicity occurred in regions of stress increase and majority of them concentrated near the ruptured plane where the stress changes are in the highest value. Based on our computation for the second event, it would be concluded that most of the aftershocks located in the places that imposed stress are positive and some of them are in places where the imposed stress changes are zero or very small. So, there is a good correlation between coulomb stress changes and aftershocks distribution for both Rigan events. Calculating imparted coulomb stress changes that resolved on the nodal planes of the Rigan first event aftershocks has also been considered to examine whether they were brought closer to failure or not by using different fault friction. Various values of effective coefficient of friction (0.2, 0.4, and 0.8) were used to find the best value of fault friction that produces the highest gain in positively stressed aftershocks. Based on these calculations, majority of aftershocks received positive stress changes by increasing the effective coefficient of friction.

This study focuses on the stress transfer using the Coulomb Failure Function (CFF) modeling and related poroelastic effect of four major earthquake sequences that occurred in different active zones of Africa: the May to July 1990 Sudan earthquake sequence, the May 2018 to June 2019 Mayotte-Comores earthquake swarm sequence, the 1980–2003 El Asnam-Zemmouri (Algeria) earthquake sequence, and the 1994–2016 Al Hoceima (Morocco) earthquake sequence. We observe the relationship between the stress transfer caused by mainshock fault ruptures and the post-seismic deformation controlled by the aftershock distribution. Based on other case studies, our hypothesis is that all seismic sequences are apparently controlled by the increase in pore fluid pressure caused by co-seismic phase and fluid-drained short-term post-seismic response. The poroelastic properties of any seismogenic zone appear to depend on the undrained and drained fluid conditions. The comparison between the 1990 Sudan sequence and the 1994–2016 Al Hoceima (Morocco) stress modeling shows that for most sequences the poroelastic response of the first mainshock play an important role in the occurrence of the second mainshock. Similar observations can be made for the 2018–2019 Mayotte-Comores and the 1980–2003 El Asnam-Zemmouri (Algeria) earthquake sequences. By comparing the case studies, we find that the value of fluid diffusivity controls the timing of earthquake sequences (e.g., more than three times larger between the Al Hoceima and Sudan earthquakes). The constraint of fault interactions with CFF modeling and fluid diffusivity allows a better estimate of the seismic hazard assessment.

Coseismic Coulomb stress changes caused by the Mw6.9 Yutian earthquake in 2014 and its correlation to the 2008 Mw7.2 Yutian earthquake

<p>Immediate after a large earthquake, accurate prediction of spatial and temporal distribution of aftershocks has a great importance for planning search and rescue activities. Currently, the most sophisticated approach to this goal is probabilistic aftershock hazard assessment (PASHA). Spatial distribution of the aftershocks fallowing moderate to large earthquakes correlate well with the imparted stress due to the mainshock. Furthermore the secondary static stress changes caused by smaller events (aftershocks) could have effect on the triggering of aftershocks and should be considered in the calculations. The 26 December 2003 (Mw 6.6) Bam earthquake with more than 26000 causalities is one of the most destructive events in the recorded history of Iran. This earthquake was an interesting event and was investigated in a majority of aspects. Good variable-slip fault model and precise aftershocks data enabled us to impart Coulomb stress changes due to mainshock and secondary static stress triggering on the nodal planes of aftershocks to learn whether they were brought closer to failure.</p><p>We used recently published high-quality focal mechanisms and hypocenters to reassess the role of small to moderate earthquakes for static stress triggering of aftershocks during the Bam earthquake. By imparting Coulomb stress changes due to the mainshock on the nodal planes of the 158 aftershocks we showed that 77.8% (123 from 158) of the aftershocks received positive stress changes at least in one nodal plane. We also calculated Coulomb stress changes imparted by the mainshock and aftershocks (1≤M≤4.1) onto subsequent aftershocks nodal planes and found that 81.6% (129 of 158) of aftershocks received positive stress changes at least in one nodal plane. In summary, 77.8% of aftershocks are encouraged by the main shocks, while adding secondary stress encourages 81.6%. Therefore, by adding secondary stress the Coulomb Index (CI), the fraction of events that received net positive Coulomb stress changes compared to the total number of events, increased from 0.778 to 0.816.</p>

The Kaikoura earthquake on November 14, 2016 is one of the largest and most complex earthquakes in New Zealand since 1947. Despite the fact that it has ruptured about 12 separate faults, triggered 2132 aftershocks within one week of the mainshock and induced considerable stress changes, few studies have been conducted to comprensively investigate the characteristics. The current study examines the horizontal and vertical displacements as well as the stress and gravity changes, aftershock distributions and also find out whether these changes affect the surrounding regions along the complex fault systems. The study covers the entire area affected by the Kaikoura event, which includes the northern part of the South Island and the southern part of the North Island. The dislocation theory was employed to evaluate the coseismic slip model on the multiple faults. The displacement results revealed that the maximum horizontal displacement is about 6 m and the vertical about 2 m, which are reasonably consistent with earlier study findings. Besides, the stress and gravity changes are quite complicated and inhomogeneous as evidenced by our coseismic model, demonstrating the complexity of the Kaikoura earthquake as well. Almost all the aftershocks are distributed in places where the stress and gravity change are found to be significant. In order to investigate the stability of our stress change models, we applied different friction coefficients and receiver fault parameters. The results justify the friction coefficient (μ=0.4) and the receiver fault parameters (230°, 70°, 150°) are suitable to define good stress change estimates. According to the stress change results at 15 km depth, the northern parts of the mainshock region, Hundalee fault, Humps fault and Jordan thrust areas together with the Wellington area are closer to failure and situated in a seismic risk zone. The multidimensional analysis adopted in this paper is helpful for making decisions and applications of stress and gravity change models in assessing seismic hazards.