Strike-slip Mechanisms Research Articles

Source parameters, focal mechanisms and stress tensor inversion from moderate earthquakes and its relationship with subduction Zone

Nine earthquakes occured between 2005 and 2009, with magnitudes between 4.9 and 5.5, at the zone that correspond to the triple junction of the Cocos, Rivera and North America plates were relocated. Records from RESCO seismic stations were used together with records from SSN seismic network. Source parameters were estimated: source dimension, seismic moment, stress drop, average displacement and source time.The seismic moment tensor inversion was used to estimate focal mechanism and seismic moment to compare then whit short and long period estimation of these parameters.A stress tensor inversion was also performed in order to evaluate stress state of the area of study. The results indicate a compressive stress regime, which is characteristic of a subduction zone. It is interesting however, that plunge of ?1 goes from a moderate angle in the northwest to sub-horizontal in the southeast. The existence of normal and strike-slip mechanisms is also interesting in a subduction zone. Their existence was interpreted in terms of the interaction of Cocos and Rivera plates in this tectonically complicate zone.

Kinematics of the 2012 Ahar–Varzaghan complex earthquake doublet (Mw6.5 and Mw6.3)

On 2012 August 11, an earthquake doublet (Mw6.5 and Mw6.3), separated in time by 11 min, occur in the northwest of Iran. The hypocentres of these earthquakes are close (∼6 km) and located near the cities of Ahar and Varzaghan. The rupture process of both main shocks is retrieved by inverting the near-field strong motions data and using the elliptical subfault approximation method. Our calculations show that the two earthquakes are occurring on two distinct fault planes: the first main shock (M1) has nucleated at a depth of ∼8.5 km, and is located ∼4 km east of the eastern termination of the E-W trending surface rupture. The slip reaches the ground surface west of the hypocentre on an E-W striking fault (N88°E) that dips almost vertically (80°S). This earthquake exhibits a right-lateral strike-slip mechanism. The entire slip is imaged on a single patch that ruptures with an average speed of 2.4 km s−1. The rupture duration is ∼5.6 s and the earthquake releases a seismic moment of ∼8.41E + 18 N·m. The slip reaches the surface with a right-lateral dislocation value of ∼1 m, which is consistent with the observed surface rupture. About 11 min later, the second main shock (M2) nucleates ∼5 km to the west and 4 km to the north with respect to the hypocentre of the M1, and at a depth of ∼16.5 km. The M2 rupture evolves toward shallower depths and to the west on an ENE-WSW oriented fault plane (strike ∼256°) with a dip of ∼60° northward. The slip is essentially distributed on two distinct patches with strike-slip and reverse mechanisms, respectively. The first patch has a pure right-lateral strike-slip mechanism, and ruptures at a relatively fast speed of over 2.8 km s−1, and last for about 2.6 s until it reaches the second patch. The latter has a reverse mechanism (rake∼112°) and extends the rupture toward shallow depths, and to the west at a speed of ∼2.5 km s−1, and its rupture lasts for ∼2.5 s. The top of the slip distribution of M2 stops at a depth of ∼8 km. We observe that aftershocks surround the M1 and most of the M2 slip models. They are not distributed in the region of high slip (∼3.1 m) of M1. We show that the rupture of M2 is controlled by the static Coulomb stress changes caused by M1, with the maximum slip of M2 located in the positive Coulomb stress caused by M1. The M2 rupture stops where it reaches the area of high negative Coulomb stress change (over −10 bars). The cumulative Coulomb stress fields of both main shocks show a transfer of positive static Coulomb stress change of >0.1 bars on the eastern segment of the North Tabriz Fault. This segment did not rupture since the 1721 M∼7.6–7.7 event that has destroyed the city of Tabriz, and that currently hosts 2 million people. The occurrence of this earthquake doublet with different mechanisms reveals the slip partitioning of the oblique convergence regime of NW Iran on the Ahar–Varzaghan complex fault system.

Seismic b-value anomalies in the Sumatran region: Seismotectonic implications

The spatial variations in the seismic b-value, seismic moment, Bouguer gravity change and fault-plane solution are estimated in the Sumatran region based on homogeneous seismicity data from June 2005 to March 2012 to investigate the seismotectonic characteristics and tectonic activity between the 2004 Sumatran earthquake and the 2012 East Indian Ocean earthquakes. Based on changes in the seismicity rate, this time period is divided into five intervals with different earthquake distributions and faulting styles. The seismic b-values, faulting mechanisms and seismic moment release values are mapped throughout the Sumatran region, and the gravity change every two years between the 2004 event and the 2012 events are determined from satellite (GRACE) gravity data. The b-values vary from 0.45 to 1.59, and an inverse correlation is observed between the b-values and convergence rates. Low b-values are located at depths between 20 km and 40 km in the subduction zone and between 20 km and 35 km in the oceanic plate; the lowest b-values are located at a depth of approximately 28 km in both regions, which indicates a homogeneous fault system between them. The areas with low b-values are associated with megathrust and strike-slip mechanisms, while the area with the lowest b-values is dominated by a megathrust mechanism. Seismically active regions have low b-values associated with a high stress accumulation and high seismic moment release. Low b-value areas are located mainly along the Andaman-Sunda Trench and Andaman-Nicobar-Sumatra forearc. Temporal variations in the seismicity distribution and faulting style are recognized with temporal variations in the b-values in each period. Relationships exist between the epicenter of the 2004 event and the location of the maximum Bouguer gravity change in 2012 and between the epicenters of the 2012 events and the location of the maximum Bouguer gravity change in 2004. Furthermore, a zone of continuous 2-year positive gravity change is located near the Nicobar Islands. The 2004 event boost the 2012 events and intraplate seismicity via stress transfer. The seismicity distributions in individual periods within the studied timeframe highlight the active and locked portions of the fault systems.

The Large Andaman Islands Earthquake of 26 June 1941: Why No Significant Tsunami?

We present a modern seismological study of the earthquake of 26 June 1941 in the Andaman Islands, the largest pre-2004 event along that section of the India-Burma plate boundary. Despite a large conventional magnitude (\(M_{\text {PAS}} \,=\,8.1\)), it generated at best a mediocre tsunami for which no definitive quantitative reports are available. We show that the 1941 earthquake took place under the Andaman accretionary prism and consisted of a composite event, whose nucleating phase had a strike-slip mechanism incompatible with a data set of spectral amplitudes of mantle Rayleigh and Love waves. Combining this initial phase with a larger normal faulting mechanism can reconcile them with P-wave first motions, reports of subsidence on the eastern coast of the Andaman Islands and the small amplitudes of any putative tsunami. The small tsunami results from a combination of that mechanism and of a source located under the islands themselves and in shallow water, implying a reduction in amplitude under Green’s law when transitioning to a deeper basin.

Focal mechanism of the 5.1Mw 2014 Lloja earthquake, Bolivia: Probing the transition between extensional stresses of the central Altiplano and compressional stresses of the sub-Andes

The 2014 Lloja earthquake with 5.1 Mw occurred in the Eastern Cordillera of Bolivia between the Subandean belt (characterized by reverse and thrust faulting) and the Altiplano (where extensional stresses due to gravitational collapse dominate). Determination of the Lloja focal mechanism, therefore, is important to sample the transition between these two Andean stress provinces. In addition, two different moment tensor inversions had been published: a strike-slip solution with a normal component (NEIC-USGS) and a purely strike-slip solution (GCMT). A detailed relocation of the epicenter was carried out using two data sets: a) only the local stations of the La Paz network with a 1D model appropriate for the Central Andean plateau, and b) local + regional stations with the 3D RSTT model. Both epicenters agree within their uncertainties of about +- 5 km each. P-wave polarity of the local and regional stations, together with the S-wave polarity at LPAZ station, allowed the focal mechanism to be determined as a strike-slip solution with a small normal component, similar to the USGS solution. One nodal plane strikes NNE-SSW and was interpreted as the seismogenic fault because of its consistency with a) the distribution of the relocated aftershocks, and b) the NE-SW trending local Caracato - Rio La Paz geological fault near the epicenter. The strike-slip mechanism with a small component of normal faulting is consistent with a gradual transition between compressional stresses in the sub-Andean belt and extensional stresses in the Altiplano.

Structural analysis using three-component acceleration time histories caused by shallow crustal fault earthquakes with a maximum magnitude of 7 Mw

Research on the improvement of Indonesian seismic hazard maps has already been carried out by the National Center for Earthquake Studies in 2017. One important global data obtained from this research related with all earthquake sources mechanism which might be used for building design and evaluation. Based on this research there are two important and closest earthquake sources to Semarang, Lasem fault (strike slip mechanism) and Semarang fault (reverse mechanism). This paper presents the dynamic structural evaluations of the hospital building (approximately 49 meters height) located in Semarang, Indonesia, by conducting two seismic load functions, two-component surface spectral acceleration (X and Y directions) developed from Indonesian Seismic Code SNI:1726-2012 and three-component (North-South, East-West and Vertical) of surface acceleration time histories. All acceleration time histories were modified from three earthquake events which represents Semarang fault earthquake scenarios and developed from earthquake events with magnitude from 6.3 to 6.9 Mw and the maximum distance of building to earthquake epicentre 15 Km. Based on the floor deformation and the drift ratio results calculated using dynamic analysis, the building is predicted has the capability on resisting earthquake scenarios up to a maximum magnitude 7 Mw with minimum epicentre distance 10 Km.

Regional Seismic Activity after 2012 M8.6 Sumatra Earthquake

The Sumatra subduction zone, one of the most active plate tectonic margins in the world, is characterised by the Indo-Australia Plate subducting beneath the Sunda plate and Andaman micro plate, causing seismic activity along the plate boundary. There had been five major earthquakes of magnitude greater than 8.0 in this region from 2004 to 2014. Three of them are dip-slip and rest of the two is strike-slip type events. Regional earthquake activity after the occurrence of those five events was analyzed. Hypocentral data obtained from the Data Management Center at the Incorporated Research Institutions for Seismology for the period from January 2000 to December 2014 of magnitude 3.0 were used for the analysis. A statistical analysis was carried out to know whether earthquake activity has increased after the major five events and the analysis was carried out both qualitatively and quantitatively. The results of the analysis show that the number of earthquakes in the region has increased considerably after the occurrence of April, 2012 magnitude 8.6 and 8.2 strike-slip events. Further results show that there is no change in the regional earthquake activity after the occurrence of other three major dip-slip type events. Present study results reasonably agree with the results obtained by the other studies carried out with different methods. In the context of Sri Lanka, strike-slip type focal mechanism of the 2012 two major events may be the reason for increasing of activity in the region, especially in Eastern part of Sri Lanka near Maduruoya, Highland-Wijayan boundary and Wadinagala area of Ampara District. Keywords: seismology, Sumatra subduction zone, regional seismicity, earthquake, Sri Lanka

GMPE based Shakemap Generation of Peak Ground Motion and Intensity Maps for Pidie Jaya Earthquake

An algorithm based on GMPE (Ground Motion Prediction Equation) is used for Generating standard and officially Shakemap as early information for disaster mitigation quick respond. We investigate the method developed by USGS (US Geological Survey) namely ShakeMap for generating Peak Ground Motion and Intensity Maps. The 2016 Mw 6.5Pidie Jaya Earthquake which strike-slip mechanism was selected for the scenario earthquake. The appropriate GMPE (i.e.Zhao06_crustal) was selected for this earthquake, which is located at the crust with focal depth of13km and strike-slip mechanism. Strong ground motion on bedrock, without sediment amplification, generate PGA (Peak Ground Acceleration) and PGV (Peak Ground Velocity) spherically uniform. The algorithm able to consider the site effect only for thin sedimentary layer Vs30 without considering deeper structural layer. The amplification effect is calculated based on Vs30 without considering the nonlinear effect and independently each site-class. Because of unavailable spatial distribution Vs30 measurement, the Vs30 was estimated from the topographic gradient and the method is commonly used. Strong ground motion on sediment layer will be amplified following the Vs30 distribution. The intensity map, which is easier to read, shown in MMI (Modified Mercalli Intensity) and SIG-BMKG (Indonesia Earthquake Intensity Scale). The GMPE based shakemap has bias error because of the general equation has not considering complete structural model and radiation pattern of a focal mechanism. An advance method for thefuture development will be much more realistic with the shakemap based on synthetics seismogram calculation.

A Multi-Peak Evolutionary Model for Stochastic Simulation of Ground Motions Based on Time-Domain Features

ABSTRACT This paper introduces an efficient stochastic method to produce fully nonstationary records having multiple peaks in power spectrum. The zero-crossing characteristics of the acceleration, velocity, and displacement time histories of the output signal are used to estimate the model parameters. This procedure is utilized to resimulate 252 non-pulse-like, horizontal near-field records with rupture distance of less than 10 km and strike–slip mechanism. The model parameters are regressed against moment magnitude, rupture distance, hypo-central depth, and shear wave velocity, enabling the scenario-based simulation of the records. The response spectra of the simulated records are compared with NGA-West2 ground motion prediction equations.

Study on the relationship between multi-stage strike-slip mechanism and basin evolution in Fangzheng fault depression

Fangzheng fault depression is controlled by the northern of the Tan-Lu fault zone. It undergoes multi-stage strike-slip, extrusion modification, and erosion of the thermal uplift, forming a tectonic pattern of uplifts connected with sags. Through the regional dynamic analysis, the study of the activity law of the western Pacific plate has clarified the formation and transformation of the regional tectonic stress field. Under the background of the multi-stage of the strike-slip mechanism in the northern part of the Tan-lu fault, the Fangzheng fault depression has a characteristic of the “left-lateral strike-slip pull-apart basin, right-lateral strike-slip extrusion transformation.” According to the difference of the strike-slip, the Fangzheng fault depression has divided into two parts: the East fault depression and the West fault depression. The seismic data, seismic attribute analysis, and geological modeling techniques have applied to analyze the two fault depressions, the East fault depression has actively controlled by the strike-slip activity, and the structure is complex. The seismic data quality is poor; the structure of the West Fault Depression is the opposite and structural characteristics of asymmetrical difference strike-slip in the East and West fault depressions. Interpretation of seismic sections through a slippery background, the strike-slip attributes of the whole fault depression from south to north are segmented, and the strike-slip mechanism from east to west is different. Under the control of the multi-stage strike-slip mechanism, the Fangzheng fault depression is divided into six stages of strike-slip evolution, corresponding to the six different stages of the strike-slip control basin, the formation process of the asymmetric difference strike-slip fault basin is clarified, which provides a reference for the study of the strike-slip pull-apart basin with multi-stage structure.

Mainshocks/aftershocks study of the August 2012 earthquake doublet on Ahar-Varzaghan complex fault system (NW Iran)

Seismotectonic features of the Ahar-Varzaghan complex fault system are reviewed by the mainshocks/aftershocks study of the 2012 Ahar-Varzaghan earthquake doublet (Mw = 6.5 & Mw = 6.3). The early aftershocks were accurately monitored by a dense local seismological network. The mainshocks hypocenters were relocated using a new velocity model for the area. The regional displacement waveforms of the mainshocks and large aftershocks were jointly inverted for their moment tensors and centroids.Precisely located aftershocks are mostly distributed in three distinct clusters. These E-W trending clusters are situated to the north of the surface rupture over a ∼30 km long and 10 km wide zone, extending from the ground surface down to the depth of ∼15 km. Based on our results, the first mainshock (M1) with Mw = 6.5 nucleated 3 km east of the surface rupture at a depth of 10 km. The rupture evolved toward the west and shallower depths on an almost E-W striking right-lateral strike-slip vertical fault plane and produced ∼12 km surface rupture. Its main energy was released at about 5 km west of the hypocenter at an average depth of 5 km. About 11 min later, the second mainshock (M2) with Mw = 6.3 nucleated 5 km northwest of the M1 hypocenter and at a depth of 15 km. It occurred on an ENE-WSW striking, north-dipping (65° to 70°) fault plane with a dominant reverse mechanism and strike-slip component. The M2 related rupture also expanded to the west and to shallower depths and released most of its energy ∼5 km west of the respective hypocenter at an average depth of 11 km. The eastern aftershocks mainly show the right-lateral strike-slip mechanism on almost E-W trending fault, close to the location of the first main centroid (M1). The northern aftershocks mostly include reverse mechanisms with strike-slip component near the M2 centroid. A big gap of aftershock activity is observed close to the M1 centroid location, most likely associated with the area of maximum slip. We do not observe any N-S lineament of aftershocks neither near the M2 hypocenter and nor its centroid, thus making the suggestion of some previous investigators that M2 ruptured an N-S trending fault unlikely.

Source characteristics of the 18 September 2011 Sikkim earthquake and zoning

Using detailed waveform analysis of Pn, sPn depth phases from the nearby seismic stations, attempts were made to resolve the focal depth of the 2011 Sikkim earthquake. Focal depth of 46.8 km thus determined was found closer to the GCMT source mechanism solution as compared to ISC solution (29.6 km). Re-examination of its source mechanism with the spatial/ depth distribution of its aftershocks sug- gested that both the nodal planes oriented WNW or ENE, initially ruptured but the orientation of meizoseismal area and larger concen- tration of its aftershocks parallel to the Tista lineament conformed WNW striking nodal plane relatively more active. The large value of the stress drop derived from S-wave spectra based on the data of Indian stations was attributed to its strike-slip mechanism and deeper focal depth. The stress drop of its foreshock was much lower than that the main shock. The b-value also showed a decrease during the last decade prior to the main earthquake.The recent Sikkim earthquake (Mw 6.9) has brought out the limitations of the microzoning of the Sikkim region attempted earlier.

Study of fault plane solutions and stress drop using local broadband network data: The 2011 Sikkim Himalaya earthquake Mw 6.9 and its aftershocks

Fault plane solutions of the September 18, 2011 Sikkim Himalaya earthquake Mw 6.9 and its four aftershocks (Mw>4.0) are studied by waveform inversion using the local broadband network data. The solutions show pure strike-slip mechanisms; one aftershock show thrust faulting with strike-slip component. Strike-slip mechanisms indicate predominant transverse tectonics in Sikkim, in the eastern Himalaya region, unlike predominant thrust tectonics in the western Himalaya. The 2011 main shock occurred at a much deeper depth (~47 km) compared to the shallower (<20 km) thrust events in the western Himalaya. Further, analysis of ground acceleration spectra reveals low stress drop (14-38 bars) in agreement with the relatively long source duration and small co-seismic slip of the main shock as well as the aftershocks. We interpret the low stress drop in terms of lower energy release due to reactivation of the Tista fault in opposite sense to that of the Himalayan thrust tectonics. The low stress drop also indicates a pre-existing brittle zone or fault zone at deeper depth or mantle depth.

Strike-Slip Fault Deformation and Its Control in Hydrocarbon Trapping in Ketaling Area, Jambi Subbasin, Indonesia

Geologically, Ketaling area consists of a local high considered as flexure margin of Tempino-Kenali Asam Deep in west part and graben in east part also known as East Ketaling Deep. Numerous proven plays were established in Ketaling area with reservoir in early Miocene carbonate and middle Miocene sand. This area underwent several major deformations. Faults are developed widely, yet their geometrical features and mechanisms of formation remained so far indistinct, which limited exploration activities. With new three-dimensional seismic data acquired in 2014, this area evidently interpreted as having strike-slip mechanism. The objective of this study is to examine characteristic of strike slip fault and its affect to hydrocarbon trapping in Ketaling Area. Structural pattern and characteristic of strike slip fault deformation was examined with integration of normal seismic with variance seismic attribute analysis and the mapping of Syn-rift to Post-rift horizon. Seismic flattening on 2D seismic cross section with NW-SE direction is done to see the structural pattern related to horst (paleohigh) and graben. Typical flower structure, branching strike-slip fault system and normal fault in synrift sediment clearly showed in section. An echelon pattern identified from map view as the result of strike slip mechanism. Detail structural geology analysis show the normal fault development which has main border fault in the southern of Ketaling area dipping to the Southeast-East with NE-SW lineament. These faults related to rift system in Ketaling area. NW-SE folds with reactive NE-SW fault which act as hydrocarbon trapping in the shallow zone. This polyphase tectonic formed local graben, horst and inverted structure developed a good kitchen area (graben) and traps (horst, inverted structure). Subsequently, hydrocarbon accumulation potentials such as basement fractures, inverted syn-rift deposit and shallow zone are very interesting to explore in this area.

Heterogeneous subduction structure within the Pacific plate beneath the Izu-Bonin arc

The Izu-Bonin subduction zone is a subduction system formed in early Eocene. The structure of the subduction zone becomes complicated with the evolution of the surrounding plate motion, and many aspects are still unkown or ambiguous. The geodynamic implications are further investigated in related to published seismic observations and geochemical characters of the Izu-Bonin subduction zone.As indicated by seismic tomography and epicentral distributions, the dip angle of the plate beneath the segment to the south of 29°–30°N (the southern Izu-Bonin) is much steeper than the northern one (the northern Izu-Bonin). Deep focus events in the southern segment extend to the depth of ∼600 km, whereas in the northern section deep events just terminate at 420–450 km. Particularly, tomographic images show an obvious boundary between the northern and southern Izu-Bonin at depths of 150–600 km neglected in the previous studies. The northern and southern segments are even separated by a wide range of low-velocity anomaly in P and S wave tomography at 380 km and 450 km depths. In this depth range, three events near 30°N are characterized by strike-slip mechanisms with slab parallel σ1 and horizontally north-south trending σ3, which differ with the typical down-dip compression mechanisms for neighboring events. These events could be attributed to an abrupt change of the morphology and movement of the slab in the transition segment between the northern and southern Izu-Bonin. Indicated by the focal mechanisms, the northern and southern Izu-Bonin exhibits an inhomogeneous stress field, which is closely related to age differences of the downgoing slab. Because of the reheating process, the thermal age of the Pacific plate entering the Izu-Bonin trench in the past 10 Ma, is only 60–90 ± 20 Ma, along with the younger plate subducting in the northern segment. The seismic anisotropy implies that mantle wedge flow orientation is between the motion direction of the Pacific plate and trench strike, which may be caused by the viscous coupling to the subducting plate and along-trench N-S shearing. The NE splitting direction oblique or perpendicular to the NW-NNW movement of the Pacific plate beneath the transition segment results from the “tearing” of the slab, which is also confirmed by the slab-related velocity anomalies, a sharp change in the dip angle of the Wadati-Benioff zone, the tectonic stress characteristics and along-arc variations of Sr-Nd-Pb isotope ratios in the transition segment.

Seismic b-values and its correlation with seismic moment and Bouguer gravity anomaly over Indo-Burma ranges of northeast India: Tectonic implications

b-value is one of the most significant seismic parameters for describing the seismicity of a given region at a definite time window. In this study, high-resolution map of the Gutenberg-Richter b-value, seismic moment-release, Bouguer gravity anomaly and fault-plane solutions containing faulting styles are analyzed in the Indo-Burma ranges of northeast India using the unified and homogeneous part of the seismicity record in the region (January 1964–December 2016). The study region is subdivided into few square grids of geographical window size 1° × 1° and b-values are calculated in each square grid. Our goal is to explore the spatial correlations and anomalous patterns between the b-value and parameters like seismic moment release, Bouguer gravity anomaly and faulting styles that can help us to better understand the seismotectonics and the state of present-day crustal stress within the Indo-Burma region. Most of the areas show an inverse correlation between b-value and seismic moment release as well as convergence rates. While estimating the b-value as a function of depth, a sudden increase of b-value at a depth of 50–60 km was found out and the receiver function modeling confirms that this depth corresponds to the crust-mantle transition beneath the study region. The region is also associated with negative Bouguer gravity anomalies and an inverse relation is found between Gravity anomaly and b-value. Comparing b-values with different faulting styles, reveal that the areas containing low b-values show thrust mechanism, while the areas associated with intermediate b-values show strike-slip mechanism. Those areas, where the events show thrust mechanism but containing a strike-slip component has the highest b-value.

A recent Mw 4.3 earthquake proving activity of a shallow strike-slip fault in the northern part of the Western Desert, Egypt

The Mw 4.3 earthquake of September 2015 is the first felt earthquake since 1900 A.D in the northern part of the Western Desert, Egypt, south of the El-Alamein City. The available waveform data observed at epicentral distances 52–391 km was collected and carefully evaluated. Nine broad-band stations were selected to invert full waveforms for the centroid position (horizontal and vertical) and for the focal mechanism solution. The first-arrival travel times, polarities and low-frequency full waveforms (0.03–0.08 Hz) are consistently explained in this paper as caused by a shallow source of the strike-slip mechanism. This finding indicates causal relation of this earthquake to the W-E trending South El-Alamein fault, which developed in Late Cretaceous as dextral strike slip fault. Recent activity of this fault, proven by the studied rare earthquake, is of fundamental importance for future seismic hazard evaluations, underlined by proximity (∼65 km) of the source zone to the first nuclear power plant planned site in Egypt. Safe exploration and possible future exploitation of hydrocarbon reserves, reported around El-Alamein fault in the last decade, cannot be made without considering the seismic potential of this fault.

Tsunamis from strike-slip earthquakes in the Wharton Basin, northeast Indian Ocean: March 2016Mw7.8 event and its relationship with the April 2012Mw 8.6 event

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 km s−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 the Wharton 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.

Near-source high-rate GPS, strong motion and InSAR observations to image the 2015 Lefkada (Greece) Earthquake rupture history

The 2015/11/17 Lefkada (Greece) earthquake ruptured a segment of the Cephalonia Transform Fault (CTF) where probably the penultimate major event was in 1948. Using near-source strong motion and high sampling rate GPS data and Sentinel-1A SAR images on two tracks, we performed the inversion for the geometry, slip distribution and rupture history of the causative fault with a three-step self-consistent procedure, in which every step provided input parameters for the next one. Our preferred model results in a ~70° ESE-dipping and ~13° N-striking fault plane, with a strike-slip mechanism (rake ~169°) in agreement with the CTF tectonic regime. This model shows a bilateral propagation spanning ~9 s with the activation of three main slip patches, characterized by rise time and peak slip velocity in the ranges 2.5–3.5 s and 1.4–2.4 m/s, respectively, corresponding to 1.2–1.8 m of slip which is mainly concentrated in the shallower (<10 km) southern half of the causative fault. The inferred slip distribution and the resulting seismic moment (M0 = 1.05 × 1019 N m) suggest a magnitude of Mw 6.6. Our best solution suggests that the occurrence of large (Mw > 6) earthquakes to the northern and to the southern boundaries of the 2015 causative fault cannot be excluded.

DETERMINATION OF ASEISMIC DEFORMATION ON NORTH ANATOLIAN FAULT IN ISMETPASA AND DESTEK REGIONS USING GPS DATA, TURKEY

The North Anatolian Fault Zone (NAFZ) is one of the most destructive fault in the eastern Mediterranean region. After Izmit and Düzce earthquakes, the projects on monitoring the fault motion increase using instrumental tools like GPS, InSAR, LIDAR, creepmeter, etc. The eastern and central part of the NAFZ from Karlıova to Vezirköprü has almost strike slip mechanism. The western part of the central NAFZ from Vezirköprü to Bolu has transpressive character. The aseismic fault deformation (creep) is also important phenomena for these two sections. The InSAR and LIDAR studies showed that the Ismetpasa and Destek regions have creep motions. For this purpose, the new project has been started to proof this phenomena with GPS data and to determine quantitatively the rate of convergence and its variation along segment of the NAF between Bolu and Çorum. The main aim of this study is determination of creep rate with geodetic measurements and combination of the data obtained from seismology, geodesy and geophysics to understand fault mechanism. Therefore, in this paper we discuss tectonic phenomena on the central part of the NAFZ and present the first results of the project.