Significant Strike-slip Component Research Articles

Stress dynamics associated with the Nyasa / Malawi rift: Implication for the present-day East African Rift System dynamics

The Nyasa/Malawi rift (NMR), known for its poor magma and notable seismic activity, has sparked a debate regarding its stress kinematics. It is on one hand viewed as a transform fault, while on other hand as a rift structure characterized by normal faulting. In order to address this controversy, we conducted paleostress analysis that involved collecting fault slip data along the central to southern region of the rift. We integrated our findings with published kinematic data on focal mechanisms in the rift. Our results reveal that the central part of the rift experiences radial or sub-radial extension, while the southern half is subject to oblique NNE-SSW transtensive tectonic forces. The minimum horizontal principal stress axis aligns with an orientation of 020°. As we move further south, the extension direction changes by approximately 25°, resulting in a predominantly north-south opening with a minimum horizontal stress axis direction of 175° (Shmin = 175°). The degree of structural penetration and intensity of faulting indicate that the north-south opening is more significant and pronounced in the southern region compared to the northern region. Additionally, we observed that faults dipping to the east and trending NW-SE exhibit sinistral (left-lateral) movement, while faults dipping to the southwestern side display dextral (right-lateral) movement. This suggests that, regionally, the NMR primarily experiences a normal faulting regime, albeit with a significant strike-slip component, which accounts for the oblique kinematics observed. The tectonic regimes identified through our fault slip data encompass the crust and upper mantle, spanning a lithospheric scale.

A Review of Earthquake Source Parameters in the Main Ethiopian Rift

We assessed earthquake source parameters compiled from previous studies and international databases. In addition, moment tensor inversion is made from the broadband seismic data of two earthquakes that occurred in the region in 2017 and 2018 with magnitudes Mw 5.0 and 5.1, respectively. As a result, the two events’ reliable source parameters are retrieved. We found that earthquakes are distributed in the rift floor, at margins and adjacent plateaus. Because the majority of earthquakes occur on the rift floor, deformation is most likely caused by strain accumulation transferred from border faults to magmatic segments along the rift floor. Predominantly normal faulting is observed, but some strike-slip events are also observed. Normal faulting mechanisms are consistent with major plate divergence, whereas the strike-slip components observed in the region might be associated with the counterclockwise rotation of the Danakil microplate, and the mechanism would indicate an oblique-slip deformation between the Nubian plate and the Danakil microplate. However, the focal mechanism obtained from the moment tensor inversion for the Mw 5.1 event indicates dominant normal faulting accompanied by a minor strike-slip component at the western margin of Afar, whereas the Mw 5.0 event has a significant strike-slip component at the central part of MER. The majority of focal depths of earthquakes are found within the upper crust, including the 2017 (Mw 5.0) event with a focal depth of 9.7 km that was computed using moment tensor inversion. A significant number of earthquakes are also found within the lower crust, including the 2018 (Mw 5.1) event with a focal depth of 20.2 km. However, earthquakes with focal depths within the upper mantle are also found in the compiled international database, which may not be consistent with the previously published works in the region. The observed focal depth may suggest a widespread deformation throughout the upper and lower crusts, implying that magmatic intrusions and faulting play a central role in facilitating the seismicity of the main Ethiopian rift (MER). The current investigation will provide further information on the earthquake source parameters and seismogenic depth of earthquake occurrence in the MER.

Characteristics of the Earthquake Swarms in the Andaman Sea Region, India, from 1960 – 2020

The Andaman Sea region is the highly tectonically and seismically active region in India. This is the perfect site for studying earthquake swarms and how the swarms are reacting with the volcanic activities and tectonic processes. Using the International Seismological Centre (ISC) catalogue and USGS NEIC catalogue, we documented 17 earthquake swarms from 1960 to 2020. The hypocentre of the earthquake swarms was reported at 5 km. to 50 km and most of the incidents occurred below 20 km. depth. We examined the seismotectonic characteristics of these swarms. The moment tensor solutions of 6 swarms showing these events are controlled by pure normal faults with a significant strike-slip component, few are dominated by strike-slip faults but no thrust mechanisms are recorded. The b value distribution for the Andaman Sea region was calculated from the swarms’ data and the b value significantly varied between 1.4 –2.17. We conclude that the earthquake swarms in the Andaman and Nicobar Island region are probably the result of magma intrusion in the thin and hot oceanic crust. High b values, active volcanoes and cratered seamount support the volcanic background of these swarms.

Complex tectonic deformation in Circum-Tanzania Craton: East African Rift System

The reliable source parameters, the current deformation associated to the earthquakes and the current deformation in relationship with dynamic change of Victoria plate rotation are generally poorly understood owing to the lack of the proper techniques used in the Circum-Tanzania Craton. Therefore, the reliable estimated source parameters and the detail interpretation are very crucial. Here, moment tensor inversion is made from broadband seismic data for five earthquakes that occurred in the region in the years 2014 and 2016 with magnitude ranging from Mw 4.1 to 5.7. The 2014 sequence comprises four earthquakes where two of them occurred north of Kondoa with purely normal faulting mechanisms following the N–S oriented rift structure. The other two earthquakes from the 2014 group occurred north of Dodoma and are found to be normal faulting with significant strike-slip component with dextral sense of motion trending in the NW-SE direction which seems to agree with fault structure and aftershock distribution. On the other hand, focal mechanism of the 2016 earthquake in Kagera region with magnitude Mw 5.7 shows dominantly normal faulting trending WNW-ESE with strike-slip component and dextral sense of motion. The interpreted fault orientation for the Kagera earthquake seems to agree with macroseismic information and aftershock distribution which were recorded by seismic stations deployed immediately after the main shock (Mulibo, 2019). We therefore find that the observed normal, strike-slip and oblique slip deformation seems to be influenced by the extensional stress regime of the East African Rift System and the anti-clockwise rotation of the Victorian plate. On the other hand, it is not uncommon to observe deactivation of the NW-SE oriented weak zones in the East African Rift System which seems to agree with our fault mechanism results. In addition, the waveform inverted depths of the studied earthquakes vary from 12 to 26 km in the Circum-Tanzania Craton is implying relatively larger effective elastic thickness compared to the northern part of the East African Rift System in the Afar region.

BASEMENT FAULT REACTIVATION IN THE ZAKHO– SILOPI REGION, IRAQI KURDISTAN – TURKEY BORDER

The nature and orientation of underlying Late Precambrian basement structures, and their relationships to the seismic distribution and surface linear structures has provided a basis for studying the basement fault reactivation in the Zakho – Silopi region. This border region between Iraqi Kurdistan and Turkey is located in the north-eastern boundary of the Arabian plate and covers some parts of the Zagros fold-thrust belt as well as the high folded and foothill zones of Iraq. The Zakho – Silopi region is characterized by moderate-sized earthquakes of shallow-focus that caused mostly by the neotectonic reactivation of Late Precambrian thrust and strike-slip basement faults due to the on-going active continental collision between Arabia and Eurasia. Four potential NE- and NW-trending seismic lineaments were identified, based on the epicentral alignments of major earthquake and the orientation of nodal fault planes, to be the most seismically active basement faults in the Zakho - Silopi region. Any potential for the future damaging earthquakes throughout this region will be more likely associated with the reactivation of the basement faults along these active seismic lineaments. Therefore, a detailed seismic hazard assessment is recommended for the major cities and urban localities. The present-day stress-field orientation inferred from the inversion of fault plane solutions and lineament analysis indicates that the Zakho – Silopi region is undergoing a continuous state of nearly N-S (N05°E) compression. The trend analysis of lineament patterns and seismic lineaments reveals three dominant basement fault trends in the Zakho - Silopi region; roughly N-S, NE-SW, and NW-SE. These basement faults were probably initiated in Late Precambrian as normal faults, and subsequently inverted into thrust faults with a significant strike-slip component, and reactivated continuously from Late Cretaceous to present-day.

Neotectonic analysis, active stress field and active faults seismic hazard assessment in Western Crete

Within the framework of this study the complicated fault system of Western Crete was napped in detail and its kinematic and dynamic setting was analysed in order to distinguish 13 major active and possible active fault zones, the seismic potential of which was assessed. Moreover, kinematic data and striations were used to estimate the corresponding stress field geometry. Two stress phases were recognized: 1st the N-S extension phase (D1) in Mid-Upper Miocene to Lower Pliocene times forming E-W normal faults that bound the Neogene basins; 2nd the E-W extension phase (D2) in Late Pliocene-recent times forming N-S trending active normal faults. Smaller, mainly NE-SW trending faults, with significant strike-slip component, indicate a kinematic compatibility to the D2 phase, acting as transfer faults between larger N-S fault zones. The faults were incorporated in a detailed seismic hazard analysis together with the available seismological data, involving both probabilistic and deterministic approaches, for seismic hazard assessment of several selected sites (municipalities).

VÝZKUM POZDNĚ KVARTÉRNÍ AKTIVITY ZLOMU KOSÍŘE V HORNOMORAVSKÉM ÚVALU ANEB PROČ JE DOBRÉ STUDOVAT SVAHOVINY A SEISMOGRAMY

Preliminary results of a research into the late Quaternary slip of a major fault in the seismically active Upper Morava Basin are given. Three trenches, up to 6 m deep, were excavated at the foot of the Kosíř Fault scarp near Stařechovice and Čelechovice. The exposed complex sequences of colluvium and loess, now only partly dated by OSL and 14C, is heavily faulted. The faulting is explained by a tectonic slip at the Kosíř Fault and, in the Stařechovice trench, also by simultaneous slope deformations. None of the faults do off set the Holocene topsoil but the youngest of them were clearly active aft er the deposition of the youngest loess and indicate the slip of up to 1.4 m in Late Pleistocene. In Čelechovice trenches the minimum vertical throw of 4 m is indicated for the lower part of the sequence with assumed Late Pleistocene age. The geometry of the deformed zone suggests an oblique normal faulting with significant strike-slip component. The sense of shearing in the horizontal plane was not resolved. Minimum tectonic slip rate of 0.1‒0.3 mm/year in Late Pleistocene is suggested but this must be confi rmed by new dating. Our observations reveal surprisingly young and large deformation which may suggest a temporary increase of tectonic activity during Late Pleistocene.

Chapter 19 Geological history of the Kaladgi–Badami and Bhima basins, south India: sedimentation in a Proterozoic intracratonic setup

Abstract The Proterozoic Kaladgi–Badami and Bhima basins are intracratonic basins occurring over the Archaean Dharwar craton. The Kaladgi–Badami Basin contains arenites, shales and carbonates with minor cherts and conglomerates deposited in continental, transitional and shallow-marine environments presumably during the late Palaeoproterozoic/Mesoproterozoic to Neoproterozoic. The lower part of the succession (Bagalkot Group) is deformed into east–west-trending elongated doubly plunging synclines and anticlines. The upper part of the succession (Badami Group) is undeformed and unconformably overlies the lower part. The evolution of the Kaladgi–Badami Basin was controlled by movements along east–west-trending normal faults under an extensional stress regime. The Bhima Basin hosts mainly limestones with subordinate arenites and shales deposited in fluvial, deltaic and tidal flat environments possibly during the Neoproterozoic. These sediments are undeformed except along faults with significant strike-slip components. The basin is exposed in narrow strips arranged in an en echelon pattern and appears to be a pull-apart basin. Inadequate data exist on the age of the basin fills, the deep basinal architecture, subsidence history and tectonic controls for both of the basins. Future research may be directed towards these aspects which will have wide implications for understanding intracratonic basin formation, reconstructing Proterozoic supercontinents and studying the evolution of the atmosphere and primitive life forms.

Seismically-induced mass movements and volumetric fluxes resulting from the 2010 Mw = 7.2 earthquake in the Sierra Cucapah, Mexico

The observation that sediment flux from mountain ranges struck by high magnitude earthquakes can be strongly influenced by coseismic mass movements brings into question the nature of coseismic deformation as a net contributor to mountain building. To better constrain the role of high-magnitude earthquakes in orogenesis, high-resolution data of earthquake induced mass wasting is required for areas of differing tectonic, morphological, and climatic settings. Here we compare the erosional flux to the tectonic flux associated with the 2010 Mw=7.2 Sierra El Mayor earthquake in Mexico and examine the landslide patterning of coseismic mass movements associated with this event. The ruptured fault system has a significant strike-slip component with subsidence along the eastern flank and uplift on the western flank of the range. Peak ground acceleration was highest along the steepest sections of the range such that the frequency of landslide occurrence was strongly correlated to slope gradient. Both vertical and horizontal coseismic displacement demonstrated a strong control over landslide initiation. This result suggests that strike-slip systems experience very different landslide patterning to thrust faults during earthquakes. Based on interferometric analysis of synthetic aperture radar images, the earthquake resulted in a total uplifted volume of 41.6×106m3 and a loss of 95.2×106m3 due to subsidence. This suggests a net tectonic volumetric flux of −53.6×106m3. Sediment mobilisation by coseismic landslides is estimated at −2.7×106m3 derived from a manually mapped inventory using SPOT 5 multispectral imagery. Thus, the net volume loss through coseismic subsidence of the mountain range generated a strongly negative mass flux, which was only marginally enhanced by mass wasting.

Bayesian noise-reduction in Arabia/Somalia and Nubia/Arabia finite rotations since ∼20 Ma: Implications for Nubia/Somalia relative motion

Knowledge of Nubia/Somalia relative motion since the Early Neogene is of particular importance in the Earth Sciences, because it (i) impacts on inferences on African dynamic topography; and (ii) allows us to link plate kinematics within the Indian realm with those within the Atlantic basin. The contemporary Nubia/Somalia motion is well known from geodetic observations. Precise estimates of the past-3.2-Myr average motion are also available from paleo-magnetic observations. However, little is known of the Nubia/Somalia motion prior to ∼3.2 Ma, chiefly because the Southwest Indian Ridge spread slowly, posing a challenge to precisely identify magnetic lineations. This also makes the few observations available particularly prone to noise. Here we reconstruct Nubia/Somalia relative motions since ∼20 Ma from the alternative plate-circuit Nubia-Arabia-Somalia. We resort to trans-dimensional hierarchical Bayesian Inference, which has proved effective in reducing finite-rotation noise, to unravel the Arabia/Somalia and Arabia/Nubia motions. We combine the resulting kinematics to reconstruct the Nubia/Somalia relative motion since ∼20 Ma. We verify the validity of the approach by comparing our reconstruction with the available record for the past ∼3.2 Myr, obtained through Antarctica. Results indicate that prior to ∼11 Ma the total motion between Nubia and Somalia was faster than today. Furthermore, it featured a significant strike-slip component along the Nubia/Somalia boundary. It is only since ∼11 Ma that Nubia diverges away from Somalia at slower rates, comparable to the present-day one. Kinematic changes of some 20% might have occurred in the period leading to the present-day, but plate-motion steadiness is also warranted within the uncertainties.

Comparison of seismotectonic peculiarities of Altai Mountains and Mongolian Altai

The paleoseismogeological studies within the two representative segments of fold systems in the Altai mobile belt (Altai Mountains and Mongolian Altai) have been carried out. These studies revealed the primary seismodislocations (seismic ruptures) of both the ancient historic and prehistoric strongest earthquakes. Based on amplitudes of single displacements (reverse fault and normal fault motions were 0.5–1.6 m) and with the significant strike slip component (more than 1.5 m) taken into account, the magnitude of ancient events in the Altai Mountains was determined at approximately 7.5. The recurrence period is 1400 years on average for the M = 7.0 events and 2100 years for the M = 7.5 ones. In the western Mongolian Altai, reverse fault motions were up to 2 m, corresponding to an earthquake magnitude of about 8.0 (an analog with the 1931 Fuyun earthquake). The recurrence period for the strongest earthquakes in Mongolian Altai was longer than that for the Altai Mountains, about 3000 years. This can be explained by higher magnitude values for western Mongolian Altai. It also can be concluded that the seismic regimes of the Altai Mountains and Mongolian Altai remained almost unchanged during the entire Holocene.

Contrasting strike-slip motions on thrust and normal faults: Implications for space-geodetic monitoring of surface deformation

Recent global positioning system (GPS) records of surface deformation caused by earthquakes on intracontinental dip-slip faults revealed in unprecedented detail a significant strike-slip component near the fault tips that is markedly different for thrust and normal faults. In the hanging wall of the thrust fault ruptured during the A.D. 2003 Chengkung (Taiwan) earthquake, a divergent displacement pattern was recorded. In contrast, a convergent slip pattern was observed in the hanging wall of the normal fault that produced the 2009 L’Aquila (Italy) earthquake. Such convergent slip patterns are also evident in field records of cumulative fault slip from central Italy, which underlines the coseismic origin of cumulative displacement patterns. Here we use three-dimensional numerical modeling to demonstrate that the observed fault-parallel motions are a characteristic feature of the coseismic slip pattern on normal and thrust faults. Modeled slip vectors converge toward the center of normal faults, whereas they diverge for thrust faults; this causes contrasting fault-parallel displacements at the model surface. Our model also predicts divergent movements in normal fault footwalls, which were recorded for the first time during the L’Aquila earthquake. During the postseismic phase, viscous flow in the lower crust induces fault-parallel surface displacements, which have the same direction as the coseismic displacements but are distributed over a larger area that extends far beyond the fault tips. Therefore, detecting this signal requires GPS stations in the prolongation of the fault strike. Postseismic velocities vary over several orders of magnitude depending on the lower crustal viscosity and may reach tens of millimeters per year for low viscosities. Our study establishes the link between coseismic and cumulative slip patterns on normal and thrust faults and emphasizes that understanding fault-parallel slip components and associated surface displacements is essential for inferring regional deformation patterns from space-geodetic and fault-slip data.

Reassessment of source parameters for three major earthquakes in the East African rift system from historical seismograms and bulletins

Source parameters for three majo earthquakes in the East African rift are re-computed from historical seismograms and bulletins. The main shock and the largest foreshock of the August 25, 1906 earthquake sequence in the main Ethiopian rift are re-located on the eastern shoulder of the rift segment.The magnitude of the main shock is estimated to be 6.5 (Mw) from spectral analysis. The December 13, 1910 earthquake in the Rukwa rift (Western Tanzania) indicated a significant strike-slip component from teleseismcs body-waveform inversion for fault mechanism and seismic moment. The January 6, 1928 earthquake in the Gregory rift (Kenya) showed a multiple rupture process and unusually long duration for a size of 6.6(Mw). The May 20, 1990 earthquake in Southern Sudan, mentioned merely for the sake of comparison, is the largest of all instrumentally recorded events in the East African rift system. Despite the fact that the mode of deformation in the continental rift is predominantly of extensional nature, the three largest earthquakes known to occur in the circum-Tanzanian craton have shallow focal depths and significant strike-slip component in their fault mechanisms. This and similar works will enrich the database for seismic hazard assessment in East Africa.

Seismic moment tensors of the April 2009, L’Aquila (Central Italy), earthquake sequence

SUMMARY On 2009 April 6, the Central Apennines were hit by an Mw= 6.3 earthquake. The region had been shaken since 2008 October by seismic activity that culminated in two foreshocks with Mw > 4, 1 week and a few hours before the main shock. We computed seismic moment tensors for 26 events with Mw between 3.9 and 6.3, using the Regional Centroid Moment Tensor (RCMT) scheme. Most of these source parameters have been computed within 1 hr after the earthquake and rapidly revised successively. The focal mechanisms are all extensional, with a variable and sometimes significant strike-slip component. This geometry agrees with the NE–SW extensional deformation of the Apennines, known from previous seismic and geodetic observations. Events group into three clusters. Those located in the southern area have larger centroid depths and a wider distribution of T-axis directions. These differences suggest that towards south a different fault system was activated with respect to the SW-dipping normal faults beneath L’Aquila and more to the north.

Backward modeling of the rifting kinematics in the Upper Rhine Graben: insights from an elastic-perfect contact law on the restoration of a multi-bloc domain

This paper addresses the problem of volume restoration for 3-D sedimentary basin kinematic deformation. The primary purpose is methodological and concerns the use of contact mechanics with the finite element method, in order to deform a geological multi-bloc domain. This approach is applied to backward model the later stage of rifting of a segment of the southern Upper Rhine Graben (France–Germany border). Preliminary results from our modeling demonstrate the ability of the method not only to handle complex geometries, but also to successfully perform retro-deformation of a complex geological domain. In addition, they provide or confirm crucial information on the rifting evolution and tectonic features of this segment of the Upper Rhine Graben, such as the distribution of deformation, the asymmetry of the graben and a significant left-lateral strike-slip component of displacement.

Focal Mechanisms of Intermediate-depth Earthquakes Beneath Southeastern Hokkaido, Japan Implications of the Double Seismic Zone

In this study, the focal mechanism solutions and spatial distributions of 100 intermediate-depth earthquakes are examined and discussed. These earthquakes occurred from 1985 to 1993 mostly in the upper and lower planes of the double seismic zone beneath the southeastern part of Hokkaido. The focal-mechanism solutions along with the spatial distribution of their epicenters as a function of depth and the minimum- and maximum-compressive stress directions confirm the following essential features; A double-planed deep seismic zone between the depths from 60 to 120 km has been clearly revealed in this region and the number of earthquakes located in the upper plane are less than those for the lower one. Low-angle thrust fault mechanism is dominant to a depth of about 55–60 km in the zone. At depths deeper than about 60 km focal mechanism solutions are mostly characterized by down-dip extension type or down-dip compression type in both lower and upper planes of the zone. Most of the earthquakes deeper than 60 km have dip-slip components and some of the solutions have significant strike-slip components, as well. Although the down-dip extensional-type events are dominant, seven events that are located on the lower plane are constrained as down-dip compression mechanism. The T axes of earthquake focal mechanisms on the lower plane show extension at about N40°E and N35°W, and the dip-angle between 25° and 70°, almost coincide with the dip of the subducting Pacific Plate in this region. We also investigate the statistical features of the stress directions of the earthquakes occurring both on the lower and upper plane after converting all the events onto the plane having the same strike and dip angle as the subducting lithosphere in the region. Seventy-two percent of the minimum compressive stress directions are oriented mainly in the NS or NNE- SSW directions (within 45° of the dip direction) at the plate surface of the lower plane and are almost parallel to the dip of the local slab in this region. The number of earthquakes located in the upper plane is less than that for the lower one. Those which occurred on the upper plane show various types of mechanism solutions. Weak systematic orientation is found in the direction distribution of stress in the upper plane.

The nature of faulting and deformation in the Mineoka ophiolite, NW Pacific Rim

AbstractA belt of disrupted ophiolitic rocks occurs on the Boso Peninsula (Japan), currently located north of the oblique subduction boundary between the Philippine Sea and North American Plates, under which the Pacific Plate has been subducting westwards. This ophiolitic belt (Mineoka Belt) is composed of mafic-ultramafic rocks together with Tertiary chert and limestone and island-arc volcaniclastic rocks. Our detailed structural studies in and around the basaltic rock bodies within the ophiolite reveal three phases of deformation. The first phase is further divided into three stages, all related to oblique normal faulting associated with extensional tectonics at or near a spreading axis. Fluid pressures appear to have fluctuated in association with faulting and veining during this phase. The second phase of deformation is characterized by thrust-related shear zones with a significant strike-slip component and is probably related to the final emplacement of the ophiolite by oblique subduction-obduction processes. The third and final phase of deformation affected not only the ophiolite but also later terrigenous and island-arc pyroclastic rocks. This deformation involved large-scale transpressional dextral slip on forearc sliver faults, which are still active today.

The Skyros Earthquake (MW 6.5) of 26 July 2001 and Precursory Seismicity Patterns in the North Aegean Sea

The rupture zone of the M W 6.5 earthquake of 26 July 2001 in the North Aegean Sea (NAS) is oriented northwest-southeast and occupies the southwestern part of the seismic zone of NAS. The motion indicates reverse faulting with a significant sinistral strike-slip component. A review of the past NAS seismicity reveals that in the last 150 yr the seismicity is strongly clustered in time with 14 out of 15 pre-2001 events being members of a cluster. Only the 1912 earthquake is an isolated event. This implies that the probability of the 2001 earthquake being the first member of a new time cluster is 0.93, which is the probability for the next NAS strong earthquake to occur within a time interval equal to the mean interarrival time of cluster events: 2.9 ± 2.06 yr. The distribution of the earthquake rupture zones and the position of the 2001 event suggest that the next event may rupture one of the unruptured parts of NAS to the northeast of the 2001 earthquake.

Body wave moment tensor inversion of local earthquakes: an application to the South Iceland Seismic Zone

The focal mechanisms and scalar seismic moments of four local earthquakes are determined using a seismic moment inversion method for frequencies larger than those of dominant oceanic noise and lower than the corner frequency. The earthquakes have epicentral distances ranging from 9 to 42 km and were recorded by the South Iceland Lowland (SIL) seismic network. Synthetic seismograms are computed using the frequency–wavenumber integration technique for an elastic horizontally layered medium. Moment tensor inversion is then carried out using windows designed around P and S waveforms on individual components and at individual seismic stations. The waveforms within each window are then inverted in the time domain simultaneously on all components and at all available seismic stations. The trace elements of the seismic moment tensor are subject to the deviatoric constraint (zero trace element). The results of the inversion are classified as a sum of double couple and CLVD sources, and then compared with the results from the automatic amplitude spectra method routinely used in the SIL network. The comparison shows fairly good agreement between the two approaches, considering that the former uses both amplitude and phase information and the latter uses only amplitude information. The first three events studied have moment magnitudes in the range 2.3 ≤ Mw ≤ 3.1 and comprise two foreshocks and an associated main shock. The weighted least squares inversions for these events show dominant strike slip faulting with scalar seismic moments of the order of 0.285 × 1013 ≤ M0 ≤ 4.966 × 1013 N m. The fourth event is characterized by a moment magnitude Mw = 2.2 and scalar seismic moment M0 = 0.243 × 1013 N m. This event also shows a significant strike slip component.

Fault mechanisms and tectonic implication of the 1985-1987 earthquake sequence in south-western Ethiopia

Integrated inversions of short-period P, broadband P, and long-period P &s waves are done for fault mechanisms, focal depths, seismic moments, and source-time functions from the largest four earthquakes of the 1985 and 1987 earthquake sequence in south-western Ethiopia. These earthquakes had similar normal-faulting mechanisms. The general trends of the fault planes follow the Main Ethiopian Rift which is in agreement with foreshock-aftershock distribution, surface breaks and geology. Despite the morphological discontinuity of the Main Ethiopian Rift at its southern tip, the mode of deformation of the continental crust under study shows its extension southward. There are no significant strike-slip components trending NW–SE in all the mechanisms which would have been associated with the Aswa Fault Zone in southern Sudan or Anza Rift in northern Kenya. We also infer that the relatively broad fracture zone at the southern extreme of the Main Ethiopian Rift demonstrates the early stage of the break-up between the Nubia and Somalia plates in comparison with the Main Ethiopian Rift proper and the Afar Depression. The main shock of the sequence (Mw = 6.3) ruptured at a depth of 6.8 km, shallower than expected since the depth of earthquakes generally increase southward from the Afar Depression. The shallow depth of earthquake occurrence is supported by surface deformations with an overall trend in the direction of the Main Ethiopian Rift.