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The Earth's background free oscillations at ∼3.7 and ∼4.4 mHz resonantly couple with the atmospheric acoustic modes and thus energy cross‐talk between the earth‐atmosphere system is maximum at these frequencies. The present study proposes resonant coupling between the Earth's surface and atmosphere during the 11 April 2012 Sumatra doublet earthquake and offer a possible explanation to this occurrence. Following both these earthquakes, prolonged ionospheric oscillations centered at frequency of ∼4 mHz were observed in GPS (Global Positioning System) derived total electron content (TEC) towards north‐northeast of the epicenters. We scrutinize these oscillations in terms of the manifestations of plausible non‐tectonic and tectonic forcing mechanisms surrounding the epicentral region. Non‐tectonic forcing such as the geomagnetic field coupling factor and observation geometry played a critical role in determining the amplitude anisotropy of resonant ionospheric signatures. Further, the Rayleigh waves of the first earthquake (Mw 8.6) were already characterized by an excess of energy at ∼4 mHz. We propose this could make the Mw 8.6 earthquake particularly efficient to excite the 4 mHz resonance in the atmosphere. The resonant ionospheric signatures after the second earthquake (Mw 8.2) were observed to be closely associated with the Earth's free oscillations caused by R2 Rayleigh wave train of the Mw 8.6 earthquake event. Together, all the above point to a scenario where the resonant ionospheric signatures during the Sumatra doublet event were indeed related to the seismic source. Therefore, resonant co‐seismic ionospheric signatures could provide additional information on the low‐frequency features of seismic ruptures. Key Points: Resonant ionospheric signatures observed after Sumatra 2012 doublet earthquake could be linked to seismic sourcesResonant ionospheric signatures can provide more insight into rupture process of large earthquakesEffects of geomagnetic field coupling factor and satellite geometry on co‐seismic ionospheric signals can be inconclusive [ABSTRACT FROM AUTHOR]

Ionospheric perturbations in possible association with a major earthquake (EQ) (M = 8.5) which occurred in India-Oceania region are investigated by monitoring subionospheric propagation of VLF signals transmitted from the NWC transmitter (F = 19.8 kHz), Australia to a receiving station at Varanasi (geographic lat. 25.3°N, long 82.99°E), India. The EQ occurred on 11 April 2012 at 08:38:35 h UT (magnitude ≈ 8.5, depth = 10 km, and lat. = 2.3°N, long. = 93.0°E). A significant increase of few days before the EQ has been observed by using the VLF nighttime amplitude fluctuation method (fixed frequency transmitter signal). The analysis of total electron contents (TEC) derived from the global positioning system (GPS) at three different stations namely, Hyderabad (latitude 17.38°N, longitude 78.48°E), Singapore (latitude 1.37°N, longitude 103.84°E) and Port Blair (latitude 11.62°N, longitude 92.72°E) due to this EQ has also been presented. Significant perturbation in TEC data (enhancements and depletion) is noted before and after the main shock of the EQ. The possible mechanisms behind these perturbations due to EQ have also been discussed. [ABSTRACT FROM AUTHOR]

In this paper we present evidence of earthquake dynamic triggering in southeast Africa. We analysed seismic waveforms recorded at 53 broad-band and short-period stations in order to identify possible increases in the rate of microearthquakes and tremor due to the passage of teleseismic waves generated by the Mw8.6 2012 Indian Ocean earthquake. We found evidence of triggered local earthquakes and no evidence of triggered tremor in the region. We assessed the statistical significance of the increase in the number of local earthquakes using ß-statistics. Statistically significant dynamic triggering of local earthquakes was observed at 7 out of the 53 analysed stations. Two of these stations are located in the northeast coast of Madagascar and the other five stations are located in the Kaapvaal Craton, southern Africa. We found no evidence of dynamically triggered seismic activity in stations located near the structures of the East African Rift System. Hydrothermal activity exists close to the stations that recorded dynamic triggering, however, it also exists near the East African Rift System structures where no triggering was observed. Our results suggest that factors other than solely tectonic regime and geothermalism are needed to explain the mechanisms that underlie earthquake triggering. [ABSTRACT FROM AUTHOR]

The Wharton Basin, off southwest Sumatra, ruptured to a large intraplate left-lateral strike-slip Mw 7.8 earthquake on 2016 March 2. The epicentre was located ∼800 km to the south of another similar-mechanism intraplate Mw 8.6 earthquake in the same basin on 2012 April 11. Small tsunamis from these strike-slip earthquakes were registered with maximum amplitudes of 0.5-1.5 cm on DARTs and 1-19 cm on tide gauges for the 2016 event, and the respective values of 0.5-6 and 6-40 cm for the 2012 event. By using both teleseismic body waves and tsunami observations of the 2016 event, we obtained optimum slip models with rupture velocity (Vr ) in the range of 2.8-3.6 kms-1 belonging to both EW and NS faults. While the EW fault plane cannot be fully ruled out, we chose the best model as the NS fault plane with a Vr of 3.6 km s-1, a maximum slip of 7.7 m and source duration of 33 s. The tsunami energy period bands were 4-15 and 7-24 min for the 2016 and 2012 tsunamis, respectively, reflecting the difference in source sizes. Seismicity in the Wharton Basin is dominated by large strike-slip events including the 2012 (Mw 8.6 and 8.2) and 2016 (Mw 7.8) events, indicating that these events are possible tsunami sources in theWharton Basin. Cumulative number and cumulative seismic-moment curves revealed that most earthquakes are of strike-slip mechanisms and the largest seismic-moment is provided by the strike-slip earthquakes in this basin. [ABSTRACT FROM AUTHOR]

On April 11, 2012, two strike-slip Sumatra earthquakes with moment magnitudes higher than 8 generated strong, preferentially horizontal, motions. If only body and surface waves are inverted, their centroid-moment-tensor (CMT) parameters producing vertical motions, in particular the M rr components, are poorly resolved. Independent constraints can be obtained from observations of the radial free-oscillation modes. The signal of radial modes is acquired from four unperturbed superconducting gravimeter records with low noise levels in submillihertz frequency range. We show that the observed signal substantially differs from the synthetic calculations for most of the published CMTs except for the Global CMT solution, which agrees better. We perform modal inversions considering uncertainties in centroid times and calculate marginal posterior probability density function of the M rr components. The amplitude-spectrum inversion is robust enough to estimate the intervals of admissible M rr values. Finally, we incorporate also a phase information and reduce the trade-off between the M rr components of the two studied events. [ABSTRACT FROM AUTHOR]

The main purpose of an earthquake risk assessment is to assess the potential loss of buildings in earthquake prone regions and to provide enough information to employ suitable mitigation strategies. The M w 9.1 great Sumatra earthquake in 2004 and the M w 9.0 Japan earthquake in 2011 have highlighted the importance of a multi-hazard risk assessment framework that includes the effects of both earthquake and its associated hazards such as tsunami. The multi-hazard risk assessment of buildings requires several stages including earthquake and tsunami hazard analyses, the vulnerability analysis of buildings, building inventory as well as the risk estimation of buildings in the investigated area, which are discussed in this paper. The method on how to incorporate these stages into a multi-hazard risk assessment framework is also presented. The developed risk assessment framework is then used for the city of Padang that has a considerably high earthquake and tsunami hazard with great exposure to population and infrastructures. The estimated risk obtained in this study is expressed in terms of Pure Risk Premium (PRP) for different building categories commonly found in Padang including unreinforced brick masonry (UBM), confined brick masonry (CBM), reinforced concrete frame with masonry infill (RCI) and steel structures. The earthquake PRP(s) obtained in this study are comparable with the latest insurance rates released by MAIPARK. MAIPARK is an insurance company specialized in earthquake reinsurance, which also provides earthquake statistics and sets a benchmark for earthquake insurance pricing in Indonesia. Tsunami PRP(s) for different building categories in the area are also presented in this paper; however, no comparison can be made due to the unavailability of tsunami insurance premium data for Indonesia. Based on the results, it can be concluded that building risk associated with tsunami in Padang is considerably lower than that of earthquake, due to infrequent occurrence of tsunamis in the area. The earthquake and tsunami risk of buildings in Padang is predicted to be £54.5 million and £30.8 million per annum, respectively. [ABSTRACT FROM AUTHOR]

To investigate the coupling relationship between earthquake and ionosphere perturbation, we process the global total electron contents (TECs) estimated from GPS data of IGS stations by CODE and analyze TEC anomalies before M 8.6 Sumatra earthquake and M 6.7 Mexico earthquake on April 11, 2012. The sliding interquartile method with the sliding window of 27 days is used to process data of SSN, F10.7 solar flux, Dst and Kp to eliminate the effects of solar activity and the geomagnetic activity. The results show that there are positive and negative TEC anomalies over epicenter on the 11th day and the 3rd day prior to the Sumatran earthquake respectively. But the decrement and increment of TEC anomalies over epicenter occurred on the 11th day and the 3rd day prior to the Mexico earthquake. The global TEC anomalies are analyzed on the 11th day and the 3rd day before these two great earthquakes, respectively. The magnitudes of TEC anomaly occurred on the 3rd day before earthquakes are larger than that occurred on the 11th day before earthquakes. This indicates the magnitude of anomaly may be related to the number of days prior to earthquake. But the magnitude of TEC anomaly near Sumatran is lower than that near Mexico, and the duration of anomalies occurred on 11th day is longer than those on 3rd day prior to earthquake, which are different from previous results. The TEC anomaly of Sumatran earthquake mainly occurs in the afternoon and night local time, but the TEC anomaly of Mexico earthquake mainly occurs in the morning local time. The TEC anomaly peak regions before these two earthquakes appeared on the south of epicenters instead of the vertical projection of the epicenter. Corresponding ionospheric anomalies are also observed in the magnetically conjugated regions. These can be guidance for the ionosphere monitoring in earthquake study. [ABSTRACT FROM AUTHOR]

This study discusses the evaluation of Hara's model to estimate seismic moment magnitude (MW) by using teleseismic waveform data, and then presents the development of an extended Hara model. Both models use the maximum amplitude of displacement and epicenter distance, as well as the duration of high-frequency energy radiation, of the vertical component of earthquake P-wave records. Nineteen moderate-magnitude (5.0 ≤ MW ≤ 7.0), shallow (depths ≤ 70 km), Sumatra subduction megathrust earthquake data sets recorded by the KAPI seismograph station (Kappang, South Sulawesi) in 2010 and 2011 were used in this study. The analysis is performed to obtain the maximum amplitude of displacement, epicenter distance, and the duration of high-frequency energy radiation on the first arriving P-wave. The main results show that Hara's model (2007) overestimates MW to be less than 7.0 compared with that obtained from the Global Centroid Moment Tensor (CMT) catalog. The extended Hara model was developed with the use of the same basic equation, and the resulting coefficients are a = 0.538792, ß = 0.783840, = 0.242616, and d = 4.929095. The mean and standard deviation of the difference between the extended Hara model and the Global CMT catalog are 0.01 and 0.14, respectively. [ABSTRACT FROM AUTHOR]

Sun and Dong (2014) studied the co-seismic geo-center movement using dislocation theory for a spherical earth model. However, they incorrectly considered the maximum vertical co-seismic displacement as the rigid geo-center motion (i.e., they did not separate the rigid shift and elastic deformation). In this paper, we correct Sun and Dong (2014) by using a new approach. We now define the geo-center motion as a shift of the center of figure of the Earth relative to the center of mass of the Earth. Furthermore, we derive new formulas to compute the co-seismic geo-center and inner core's center movements caused by huge earthquakes. The 2004 Sumatra earthquake and the 2011 Tohoku-Oki earthquake changed the geo-center by 1–4 mm and about 2 mm, respectively, and caused the inner core's center to displace by about 0.05 mm and 0.025 mm, respectively. [ABSTRACT FROM AUTHOR]

We conduct a systematic search for remotely triggered earthquakes in South-Central Tibet following the 2004 December 26Mw 9.1 Sumatra and 2005 March 28Mw 8.6 Nias earthquakes. We apply a Network Waveform Matched Filter Technique (NWMFT) to continuous seismic data recorded by the Hi-CLIMB array to obtain more complete earthquake catalogues in this region. Local earthquakes with magnitudes up to 4 were triggered during the large-amplitude surface waves, and most of them occurred in Gaize immediately north of the Bangong-Nujiang Suture Zone separating the Lhasa and Qiangtang Terranes. The triggered seismicity lasted 50 hr following the Sumatra and a few hours following the Nias main shocks, respectively. The difference in triggering durations could be explained by the fact that the Sumatra main shock excited long-duration surface waves with cumulative energy density 15 times more than that of the Nias main shock. In both cases, the transient increase in seismicity is followed by a moderate transient decrease, likely reflecting a 'dynamic shadow effect', where there is a temporary lack of seismic events following the remotely triggered seismicity. In comparison, we do not find any clear evidence of dynamic triggering (with a magnitude of completeness Mc = 1.7) in the Zhongba region, where a Mw 6.3 normal faulting earthquake occurred 10 d after the 2005 Nias event. [ABSTRACT FROM AUTHOR]

During the decade from mid-2004 to mid-2014 18 great ( M w ≥ 8.0 ) earthquakes occurred globally (∼1.8 per year), compared to 71 from 1900 to mid-2004 (∼0.68 per year), yielding a short-term rate increase of 265%. Six events had M w ≥ 8.5 , larger than any prior event since the 1965 Rat Islands earthquake. The December 26, 2004 M w 9.2 Sumatra earthquake had the longest recorded rupture length of 1300+ km and a rupture duration exceeding 450 s. The largest recorded strike-slip earthquake ( M w 8.7) occurred in the Indo–Australian plate on April 11, 2012. The largest recorded deep focus earthquake ( M w 8.3) occurred under the Sea of Okhotsk on May 24, 2013. While this overall surge of activity has not been demonstrated to be causally linked, regional spatio-temporal clustering is clearly evident for great events along the Sumatra, Kuril and Tonga subduction zones, and longer-range interactions have been established for global seismicity and seismic tremor at lower magnitudes following some of the events. This recent decade of intense great earthquake activity coincided with vastly expanded global networks of seismometers, GPS stations, tsunami gauges, and new satellite imaging capabilities such as InSAR and LandSAT interferometry and gravity measurements by GRACE and GOCE, enabling unprecedented analyses of precursory, co-seismic and post-seismic processes around the subduction zone environments where most of the events occurred. Individual events such as the March 11, 2011, Tohoku, Japan M w 9.0 earthquake produced more ground motion and tsunami recordings than available for all great earthquakes of the last century collectively. Joint inversion and modeling of the diverse data sets exploit complementary sensitivity of the signals to different aspects of the earthquake processes. Major advances have been achieved in quantifying frictional locking and strain accumulation prior to some great events and in relating it to co-seismic slip heterogeneity. Many surprising aspects of these well-quantified great earthquakes have been manifested, associated with their rupture dimensions, tectonic location, compound faulting, triggering interactions, slow slip and foreshock migration precursors, aftershock complexity, and depth-varying seismic radiation characteristics. Regions with potential for near-future great ruptures include mature seismic gaps along the Mentawai Islands and northern Chile, as well as western North America and the Himalayan front, so more great earthquake activity can certainly be anticipated. [ABSTRACT FROM AUTHOR]

We studied ionospheric responses to the 2012 April 11 Mw 8.6 North Sumatra earthquake using total electron content (TEC) measurements with the regional Global Navigation Satellite System network. This earthquake ruptured the oceanic lithosphere off the Indian Ocean coast of North Sumatra, and is known as the largest strike-slip earthquake ever recorded. Coseismic ionospheric disturbances (CIDs) with rapid TEC enhancement of a few TEC units propagated northward with a speed of acoustic waves (-1km s-1). Resonant atmospheric oscillation with a frequency -4mHz have been found as monochromatic oscillation of TEC lasting for an hour after the main shock and the largest aftershock. We compared CID amplitudes of 21 earthquakes world-wide with moment magnitudes (Mw) 6.6-9.2. They roughly obeyed a law such that CID amplitude increases by two orders of magnitude for the Mw increase of three. The 2012 North Sumatra earthquakes slightly deviated negatively from the trend possibly reflecting their strike-slip mechanisms, that is small vertical crustal movements for their magnitudes. [ABSTRACT FROM AUTHOR]

On 26 December 2004 and 28 March 2005 two large earthquakes occurred between the Indo-Australian and the southeastern Eurasian plates with moment magnitudes Mw=9.1 and Mw=8.6, respectively. Complete data (mb≥4.2) of the post-1993 time interval have been used to apply Poisson Hidden Markov models (PHMMs) for identifying temporal patterns in the time series of the two earthquake sequences. Each time series consists of earthquake counts, in given and constant time units, in the regions determined by the aftershock zones of the two mainshocks. In PHMMs each count is generated by one of m different Poisson processes that are called states. The series of states is unobserved and is in fact a Markov chain. The model incorporates a varying seismicity rate, it assigns a different rate to each state and it detects the changes on the rate over time. In PHMMs unobserved factors, related to the local properties of the region are considered affecting the earthquake occurrence rate. Estimation and interpretation of the unobserved sequence of states that underlie the data contribute to better understanding of the geophysical processes that take place in the region. We applied PHMMs to the time series of the two mainshocks and we estimated the unobserved sequences of states that underlie the data. The results obtained showed that the region of the 26 December 2004 earthquake was in state of low seismicity during almost the entire observation period. On the contrary, in the region of the 28 March 2005 earthquake the seismic activity is attributed to triggered seismicity, due to stress transfer from the region of the 2004 mainshock. [ABSTRACT FROM AUTHOR]

On 11 April 2012, a strong earthquake of magnitude Ms8.6 occurred near the west coast of Northern Sumatra, Indonesia. In this paper, we investigated the morphological characteristics of anomalous variations in Global Positioning System Total Electron Content (GPS TEC) prior to the earthquake by the method of the statistical analysis. It was found the TEC anomaly was firstly decreased, then, it became more enhanced, finally, it decreased, the peak of anomaly enhancement arose from 13:00-17:00 LT on April 5 lasted for ∼4 hours and the anomalous ionospheric regions extended to ∼40 in longitude and ∼20 in latitude, its location did not coincide with the vertical projection of the epicenter, but lies at the north and south of the geomagnetic equator, meanwhile, corresponding ionospheric anomalies are also observed in the magneto conjugate region. Potential causes of these results are discussed,eliminating the ionospheric anomalies that may be caused by solar activities and magnetic storms, it can be concluded that the observed obvious anomalous variation in GPS TEC on April 5 were possibly related to the earthquake. [ABSTRACT FROM AUTHOR]

Earthquakes are one of the most destructive and harmful natural disasters, especially in recent years, the 2008/5/12 Wenchuan M7.9 earthquake, the 2011/3/11 Tohoku M9.0 earthquake and the 2012/4/11 Sumatra M8.6 earthquake have caused a significant impact to the human life. In this paper, we make a study of the temporal and spatial distribution of the Global Positioning System Total Electron Content (GPS TEC) anomalies prior to the three strong earthquakes by the method of statistical analysis. Our results show that the pre-earthquake ionospheric anomalies are mainly positive anomalies and take the shape of a double-crest structure with a trough near the epicenter. The ionospheric anomalies do not coincide with the vertical projection of the epicenter of the subsequent earthquake, but mainly localize in the near-epicenter region and corresponding ionospheric anomalies are also simultaneously observed in the magnetic conjugate region prior to the three earthquakes. In addition, the amplitude and scale-size of the ionospheric ΔTEC are different with the magnitude of the earthquake, and the horizontal scale-size of the greatest anomalies before the Tohoku M9.0 earthquake is ∼30 in longitude and ∼10 in latitude, with the maximum amplitude of TEC disturbances reaching ∼20 TECu relative to the background. The peak of anomaly enhancement usually occurs in the afternoon to sunset (i.e. between 14:00 and 18:00 local time) which lasts for approximate 2 hours. Possible causes of these anomalies are discussed, and after eliminating the effect of solar activities and magnetic storms it can be concluded that the detected obvious and regular anomalous behavior in TEC within just a few days before the earthquakes is related with the forthcoming earthquakes with high probability. [ABSTRACT FROM AUTHOR]

The magnitude 8.6 and 8.2 earthquakes off northwestern Sumatra on 11 April 2012 generated small tsunami waves that were recorded by stations around the Indian Ocean. Combining differential travel-time modeling of tsunami waves with results from back projection of seismic data reveals a complex source with a significant trench-parallel component. The oblique plate convergence indicates that ~20-50 m of trench-parallel displacement could have accumulated since the last megathrust earthquake, only part of which has been taken up by the Great Sumatran fault. This suggests that the remaining trench-parallel motion was released during the magnitude 8.6 earthquake on 11 April 2012 within the subducting plate. The magnitude 8.6 earthquake is interpreted to be a result of oblique subduction as well as a reduction in normal stress due to the occurrence of the Sumatra-Andaman earthquake in 2004. [ABSTRACT FROM AUTHOR]

Most studies of afterslip distribution consider only elastic media. However, the effects of poroelastic rebound in the upper crust and viscoelastic relaxation in the asthenosphere are part of the observed post-seismic deformation. Therefore, these effects should be removed to give a more reliable and correct afterslip distribution. We developed a method for calculating an afterslip distribution in elastic, poroelastic and viscoelastic media, and we applied this method to the case of the 2007 southern Sumatra earthquake (Mw 8.5). To estimate the coseismic slip and time evolution of the afterslip distribution, we applied Akaike's Bayesian Information Criterion (ABIC) inversion method of coseismic displacement, and analysed 15 months of GPS post-seismic deformation data in 3-month observation periods. To calculate afterslip in each period, we considered not only viscoelastic responses to coseismic slip but also viscoelastic responses to afterslip in the preceding periods. We used viscoelastic model to compute post-seismic deformation models every 3 months during the 15 months after the earthquake. The viscosity value for the asthenosphere layer is a crucial unknown parameter. To overcome this problem, we used a grid search method to determine the best-viscosity value, and we found that the best viscosity for the Sumatra subduction zone was 2.5 × 1018 Pa·s. After removing the poroelastic and viscoelastic responses, we obtained maximum afterslip of 0.5 m during the 15-month investigation (the same as maximum afterslip estimated using the elastic medium only), but the poroelastic and viscoelastic responses brought the afterslip distribution to a shallower depth than the main coseismic rupture area. The results showed that the poroelastic and viscoelastic responses added significant corrections to the afterslip distribution. Compared with the traditional method, this method improved the determination of the afterslip distribution. We conclude that consideration of poroelastic and viscoelastic behaviours is essential for calculating the afterslip distribution. We propose that these parameters should be considered to obtain more reliable and correct afterslip distribution models following earthquakes. [ABSTRACT FROM PUBLISHER]

Abstract: The equatorial Indian Ocean is a well known place of active intraplate deformation defying the conventional view of rigid plates separated by narrow boundaries where deformation is confined. On 11 April 2012, this region was hit in a couple of hours by two of the largest strike-slip earthquakes ever recorded (moment magnitudes Mw=8.6 and 8.2). Broadband seismological observations of the Mw=8.6 mainshock indicate a large centroid depth (∼30km) and remarkable rupture complexity. Detailed study of the surface-wave directivity and moment rate functions clearly indicates the partition of the rupture into at least two distinct subevents. To account for these observations, we developed a procedure to invert for multiple-point-source parameters. The optimum source model at long period consists of two point sources separated by about 209km with magnitudes Mw=8.5 and 8.3. To explain the remaining discrepancies between predicted and observed surface waves, we can refine this model by adding directivity along the WNW–ESE axis. However, we do not exclude more complicated models. To analyze the Mw=8.2 aftershock, we removed the perturbation due to large surface-wave arrivals of the Mw=8.6 mainshock by subtracting the corresponding synthetics computed for the two-subevent model. Analysis of the surface-wave amplitudes suggests that the Mw=8.2 aftershock had a large centroid depth between 30km and 40km. This major earthquake sequence brings a new perspective to the seismotectonics of the equatorial Indian Ocean and reveals active deep lithospheric deformation. [Copyright &y& Elsevier]

The Indo-Australian plate is undergoing distributed internal deformation caused by the lateral transition along its northern boundary-from an environment of continental collision to an island arc subduction zone. On 11 April 2012, one of the largest strike-slip earthquakes ever recorded (seismic moment magnitude Mw 8.7) occurred about 100-200?kilometres southwest of the Sumatra subduction zone. Occurrence of great intraplate strike-slip faulting located seaward of a subduction zone is unusual. It results from northwest-southeast compression within the plate caused by the India-Eurasia continental collision to the northwest, together with northeast-southwest extension associated with slab pull stresses as the plate underthrusts Sumatra to the northeast. Here we use seismic wave analyses to reveal that the 11 April 2012 event had an extraordinarily complex four-fault rupture lasting about 160?seconds, and was followed approximately two hours later by a great (Mw 8.2) aftershock. The mainshock rupture initially expanded bilaterally with large slip (20-30?metres) on a right-lateral strike-slip fault trending west-northwest to east-southeast (WNW-ESE), and then bilateral rupture was triggered on an orthogonal left-lateral strike-slip fault trending north-northeast to south-southwest (NNE-SSW) that crosses the first fault. This was followed by westward rupture on a second WNW-ESE strike-slip fault offset about 150?kilometres towards the southwest from the first fault. Finally, rupture was triggered on another en échelon WNW-ESE fault about 330?kilometres west of the epicentre crossing the Ninetyeast ridge. The great aftershock, with an epicentre located 185?kilometres to the SSW of the mainshock epicentre, ruptured bilaterally on a NNE-SSW fault. The complex faulting limits our resolution of the slip distribution. These great ruptures on a lattice of strike-slip faults that extend through the crust and a further 30-40?kilometres into the upper mantle represent large lithospheric deformation that may eventually lead to a localized boundary between the Indian and Australian plates. [ABSTRACT FROM AUTHOR]

In this article, the author presents information on the aspects of earthquake by a geophysicist. He states that Ross Stein, a geophysicist with the U.S. Geologi­cal Survey (USGS) in Menlo Park, California has put an effort to build an open-source digital network of databases and tools, which are intended to focus on seismic dangers around the world. He further mentions about Sumatra-Andaman earthquake and tsunami, which killed more than 230,000 people in 14 countries in 2012.

The article focuses on the two earthquakes that occurred in the northeastern Indian Ocean on April 11, 2012 with a magnitude of 8.6 and 8.2. It says that the earthquakes will mark in the annals of seismology due to its unusual high magnitudes, unexpected event in the strike-slip faulting mechanism, and the complexity of the rupture for intervening plate. It mentions that the event has shown the failure of oceanic lithosphere.

Seismological observations of the 2012 moment magnitude 8.6 Sumatra earthquake reveal unprecedented complexity of dynamic rupture. The surprisingly large magnitude results from the combination of deep extent, high stress drop, and rupture of multiple faults. Back-projection source imaging indicates that the rupture occurred on distinct planes in an orthogonal conjugate fault system, with relatively slow rupture speed. The east-southeast-west-northwest ruptures add a new dimension to the seismotectonics of the Wharton Basin, which was previously thought to be controlled by north-south strike-slip faulting. The rupture turned twice into the compressive quadrant, against the preferred branching direction predicted by dynamic Coulomb stress calculations. Orthogonal faulting and compressional branching indicate that rupture was controlled by a pressure-insensitive strength of the deep oceanic lithosphere. [ABSTRACT FROM AUTHOR]

The article reports on an 8.5 magnitude earthquake that struck near the coast of Sumatra, Indonesia on April 11, 2012, noting that it was one of the largest intraplate earthquakes in recorded history and one of a series of large strike-slip quakes in the ocean lithosphere. Scientists are trying to determine how large earthquakes can occur through vertical faults in the ocean plate, suggesting that heat caused by strains in shear zones can lead to fault zone failure. Other topics include extensional earthquakes, the risks presented by ocean intraplate earthquakes, and the number and types of faults that can be implicated in a single earthquake.