Triggered/migrated seismicity due to the 2001 M w7.7 Bhuj earthquake, Western India

Paper describes triggered seismicity to 200?km distance and for a decade due to the 2001 M _w7.7 Bhuj earthquake. The Kachchh region is seismically one of the most active intraplate regions of the World due to the occurrence of two large earthquakes 1819 (M _w7.8) and 2001 (M _w7.7). Though, it has high hazard but was known to have low seismicity in view of the occurrence of fewer smaller shocks. However, the status seems to have changed after 2001. Besides the strong aftershock activity for over a decade, seismicity has spread to nearby faults in Kachchh peninsula and at several places southward for 200?km distance in Saurashtra peninsula. Beyond the rupture zone of the 2001 Bhuj earthquake, more than 40 mainshocks of M _w?~?3?C5 have occurred at 20 different locations, which is unusual. The increased seismicity is inferred to be caused by stress perturbation due to the 2001 Bhuj earthquake by viscoelastic process. In Saurashtra, over and above the viscoelastic stress increase, the transient stress increase by water table rise in monsoons seems to be affecting the timing of mainshocks and associated sequences of earthquakes.     

Seismic source characteristics in Kachchh and Saurashtra regions of Western India: b-value and fractal dimension mapping of aftershock sequences

Seismic source characteristics in the Kachchh rift basin and Saurashtra horst tectonic blocks in the stable continental region (SCR) of western peninsular India are studied using the earthquake catalog data for the period 2006–2011 recorded by a 52-station broadband seismic network known as Gujarat State Network (GSNet) running by Institute of Seismological Research (ISR), Gujarat. These data are mainly the aftershock sequences of three mainshocks, the 2001 Bhuj earthquake (M _w 7.7) in the Kachchh rift basin, and the 2007 and 2011 Talala earthquakes (M _w ≥ 5.0) in the Saurashtra horst. Two important seismological parameters, the frequency–magnitude relation (b-value) and the fractal correlation dimension (D _c) of the hypocenters, are estimated. The b-value and the D _c maps indicate a difference in seismic characteristics of these two tectonic regions. The average b-value in Kachchh region is 1.2 ± 0.05 and that in the Saurashtra region 0.7 ± 0.04. The average D _c in Kachchh is 2.64 ± 0.01 and in Saurashtra 2.46 ± 0.01. The hypocenters in Kachchh rift basin cluster at a depth range 20–35 km and that in Saurashtra at 5–10 km. The b-value and D _c cross sections image the seismogenic structures that shed new light on seismotectonics of these two tectonic regions. The mainshock sources at depth are identified as lower b-value or stressed zones at the fault end. Crustal heterogeneities are well reflected in the maps as well as in the cross sections. We also find a positive correlation between b- and D _c-values in both the tectonic regions.

     

Seismic source characteristics in Kachchh and Saurashtra regions of Western India: <Emphasis Type="Italic">b</Emphasis>-value and fractal dimension mapping of aftershock sequences

Seismic source characteristics in the Kachchh rift basin and Saurashtra horst tectonic blocks in the stable continental region (SCR) of western peninsular India are studied using the earthquake catalog data for the period 2006–2011 recorded by a 52-station broadband seismic network known as Gujarat State Network (GSNet) running by Institute of Seismological Research (ISR), Gujarat. These data are mainly the aftershock sequences of three mainshocks, the 2001 Bhuj earthquake (M _w 7.7) in the Kachchh rift basin, and the 2007 and 2011 Talala earthquakes (M _w ≥ 5.0) in the Saurashtra horst. Two important seismological parameters, the frequency–magnitude relation (b-value) and the fractal correlation dimension (D _c) of the hypocenters, are estimated. The b-value and the D _c maps indicate a difference in seismic characteristics of these two tectonic regions. The average b-value in Kachchh region is 1.2 ± 0.05 and that in the Saurashtra region 0.7 ± 0.04. The average D _c in Kachchh is 2.64 ± 0.01 and in Saurashtra 2.46 ± 0.01. The hypocenters in Kachchh rift basin cluster at a depth range 20–35 km and that in Saurashtra at 5–10 km. The b-value and D _c cross sections image the seismogenic structures that shed new light on seismotectonics of these two tectonic regions. The mainshock sources at depth are identified as lower b-value or stressed zones at the fault end. Crustal heterogeneities are well reflected in the maps as well as in the cross sections. We also find a positive correlation between b- and D _c-values in both the tectonic regions.     

Crustal heterogeneities beneath the 2011 Talala,Saurashtra earthquake,Gujarat, India source zone: Seismological evidence for neo-tectonics

During the 1st decade of the 21st century, the study area of Talala, Saurashtra of western India witnessed three damaging earthquakes of moderate magnitude, year 2007 [Mw 5.0; Mw 4.8] and in the year 2011 [Mw 5.1] that generated public panic in the region. The last damaging moderate earthquake of the 20th October 2011 in Talala region (21.09°N;70.45°E), located at about 200 km south to the devastating 2001 Bhuj (23.412°N, 70.232°E) mainshock (Mw 7.6), jolted the entire Saurashtra region of Gujarat. A long series of aftershocks followed hereafter, recorded at nine seismograph/accelerograph stations. Hypocenters of aftershocks were relocated accurately using absolute and relative travel time (double-difference) method. In this study, we, for the first time, determined 3-D tomographic images of the upper crust beneath the 2011 Talala earthquake source zone by inverting about 1135 P and 1125 S wave arrival time data. Estimates of seismic velocities (Vp, Vs) and Poisson’s ratio (σ) structures offer a reliable interpretation of crustal heterogeneities and their bearing on geneses of moderate earthquakes and their aftershock sequences beneath the source zone. It is found that the 2011 Talala mainshock hypocenter depth (6 km) is located near the boundary of the low and high velocity (Vp, Vs) and the source zone is associated with low-σ anomalies guarded by the prominent high-σ anomalies along the active fault zone having strike-slip motion beneath the earthquake source zone. The pattern of distribution of (Vp, Vs, σ) and its association with occurrences of aftershocks provide seismological evidence for the neo-tectonics in the region having left lateral strike-slip motion of the fault.     

Geomorphic evidences of tectonic instability during the Late Quaternary Period along southern Saurashtra,western India

The coastal tract of southern Saurashtra has been studied for spectacular Miliolite Formation—its origin and depositional environment. The region had witnessed two moderate earthquakes of M_w 5.0 and 5.1 during 2007 and 2011. However, there have been limited studies which pronounce the geological and geomorphic evidences of neotectonics in southern Saurashtra. Also, there exists little to no information regarding the presence of faults from the region. The paper presents geological and geomorphic evidences of neotectonics from region between Somnath to Jafrabad, western India. A geospatial dataset was used to highlight the neotectonically active nature of the region, as well as support the claim with field evidences and drainage network anomalies. The morphometric parameters like longitudinal river profile, drainage basin asymmetry, and hypsometric integral suggest neotectonic activeness of the southern Saurashtra region. The southern Saurashtra coastal alluvial plains show presence of knick zone, ravines, coastal marine notches, and offset channels/ridges, which testifies the neotectonically active nature of the region. Presence of these features in Miliolites, Middle to Late Pleistocene age, suggests rejuvenation of the landscape post this period.     

Evidence for right-lateral strike-slip environment in the Kutch basin of northwestern India from moment tensor inversion studies

The Kutch region located in northwestern part of India is an ancient rift basin that was active until Cretaceous period. The region falls close to the India–Arabia and the India–Eurasia plate boundaries and has experienced devastating earthquakes in the past, namely the 1819 Allah Bund earthquake, the 1956 Anjar earthquake and the 2001 Bhuj earthquake. To understand the tectonics of this region with respect to the adjacent plate boundaries, we invert seismic waveform data of 11 earthquakes in this region recorded by a network of the Institute of Seismological Research (ISR) during 2007–2009. The study yields focal mechanism solutions of reverse fault and strike-slip type mechanism. The inferred fault planes correlate well with the local trends of the known tectonic faults while the principal stress directions derived from stress inversion based on a linearized least squares approach, trend agreeably with the ambient stress field directions. A consistently right-lateral sense of shear is found on all the local faults as derived from each of the matching planes of the focal mechanism solutions computed in the present study. It is inferred that in the Kutch region a right-lateral strike-slip environment prevails along predominantly EW to NW-SE oriented deep-seated pre-existing faults in an otherwise compressive stress regime. This, in conjunction with the left-lateral movements along the Girnar mountain in southern Saurashtra, inferred from previous studies, indicates a westward escape of the Kutch–Saurashtra block as a consequence of the northward collision of the Indian plate with respect to the Eurasian landmass.     

Andean earthquakes triggered by the 2010 Maule,Chile (Mw 8.8) earthquake: Comparisons of geodetic,seismic and geologic constraints

The Maule, Chile, (M_w 8.8) earthquake on 27 February 2010 triggered deformation events over a broad area, allowing investigation of stress redistribution within the upper crust following a mega-thrust subduction event. We explore the role that the Maule earthquake may have played in triggering shallow earthquakes in northwestern Argentina and Chile. We investigate observed ground deformation associated with the M_w 6.2 (GCMT) Salta (1450 km from the Maule hypocenter, 9 h after the Maule earthquake), M_w 5.8 Catamarca (1400 km; nine days), M_w 5.1 Mendoza (350 km; between one to five days) earthquakes, as well as eight additional earthquakes without an observed geodetic signal. We use seismic and Interferometric Synthetic Aperture Radar (InSAR) observations to characterize earthquake location, magnitude and focal mechanism, and characterize how the non-stationary, spatially correlated noise present in the geodetic imagery affects the accuracy of our parameter estimates. The focal mechanisms for the far-field Salta and Catamarca earthquakes are broadly consistent with regional late Cenozoic fault kinematics. We infer that dynamic stresses due to the passage of seismic waves associated with the Maule earthquake likely brought the Salta and Catamarca regions closer to failure but that the involved faults may have already been at a relatively advanced stage of their seismic cycle. The near-field Mendoza earthquake geometry is consistent with triggering related to positive static Coulomb stress changes due to the Maule earthquake but is also aligned with the South America-Nazca shortening direction. None of the earthquakes considered in this study require that the Maule earthquake reactivated faults in a sense that is inconsistent with their long-term behavior.     

Estimating tsunami potential of earthquakes in the Sumatra–Andaman region based on broadband seismograms in India

In this paper, we report that the ratio of broadband energy (0.01?C2?Hz) to high-frequency energy (0.3?C2?Hz), E _r, estimated from regional seismograms of India, might be a useful parameter in estimating tsunami potential of earthquakes in the Sumatra?CAndaman region. E _r is expected to be sensitive to the depth as well as to the source characteristics of an earthquake. Since a shallow and slow earthquake has a greater tsunamigenic potential, E _r may be a useful diagnostic parameter. We base our analysis on broadband seismograms of the great earthquakes of Sumatra?CAndaman (2004, M _w?~?9.2) and Nias (2005, M _w 8.6), 41 of their aftershocks, and the earthquakes of north Sumatra (2010, M _w 7.8) and Nicobar (2010, M _w 7.4) recorded at VISK, a station located on the east coast of India. In the analysis, we also included the two recent, great strike-slip earthquakes of north Sumatra (2012, M _w 8.6, 8.2) recorded at VISK and three south Sumatra earthquakes (2007, M _w 8.5; 2007, M _w 7.9; 2010, M _w 7.8) recorded at PALK, a station in Sri Lanka. We find that E _r is a function of depth; shallower earthquakes have higher E _r values than the deeper ones. Thus, E _r may be indicative of tsunamigenic potential of an earthquake. As M _w and E _r increase so does the tsunami potential. In addition to the parameter E _r, the radiated seismic energy, E _s, may be estimated from the regional seismograms in India using empirical Green??s function technique. The technique yields reliable E _s for the great Sumatra and Nias earthquakes. E _r and E _s computed from VISK data, along with M _w and focal mechanism, may be useful in estimating tsunami potential along the east coast of India from earthquakes in the Sumatra?CAndaman region in less than ~20?min.     

Seismicity of Gujarat

Paper describes tectonics, earthquake monitoring, past and present seismicity, catalogue of earthquakes and estimated return periods of large earthquakes in Gujarat state, western India. The Gujarat region has three failed Mesozoic rifts of Kachchh, Cambay, and Narmada, with several active faults. Kachchh district of Gujarat is the only region outside Himalaya-Andaman belt that has high seismic hazard of magnitude 8 corresponding to zone V in the seismic zoning map of India. The other parts of Gujarat have seismic hazard of magnitude 6 or less. Kachchh region is considered seismically one of the most active intraplate regions of the World. It is known to have low seismicity but high hazard in view of occurrence of fewer smaller earthquakes of M????6 in a region having three devastating earthquakes that occurred during 1819 (M _w7.8), 1956 (M _w6.0) and 2001 (M _w7.7). The second in order of seismic status is Narmada rift zone that experienced a severely damaging 1970 Bharuch earthquake of M5.4 at its western end and M????6 earthquakes further east in 1927 (Son earthquake), 1938 (Satpura earthquake) and 1997 (Jabalpur earthquake). The Saurashtra Peninsula south of Kachchh has experienced seismicity of magnitude less than 6.     

Characteristics of earthquake-induced landslides in a heavy snowfall region—landslides triggered by the northern Nagano prefecture earthquake, March 12, 2011, Japan

The northern Nagano Prefecture earthquake, M_JMA?6.7 (Mw?6.2), which is inferred to have been triggered by the huge (Mw?9.0) March 11, 2011 Tohoku earthquake, occurred on March 12, 2011, in northern Nagano Prefecture, an area in Japan famous for heavy snowfall. A large number of landslides were triggered by the 12 March earthquake, and it caused building damage in the area of the epicenter. To clarify characteristics of the distribution and dynamic behavior of these landslides, we analyzed aerial photographs and conducted field surveys in and around the epicentral area. Large-scale landslides with long distance run-outs are a remarkable characteristic of the landslides induced by this earthquake. The long travel distance is considered to be related to the thick snowpack at the time the earthquake occurred. Moderate scale deep-seated landslides and shallow landslides were also observed in the study area. Based on an analysis of landslides with the active fault on which the earthquake is believed to have occurred, most of these landslides were distributed on the hanging wall of the active fault, within a distance of 12?km from the fault.     

The 2007 Talala,Saurashtra, western India earthquake sequence: Tectonic implications and seismicity triggering

A damaging and widely felt moderate (M_w 5.0) earthquake occurred in the Talala region of Saurashtra, Gujarat (western India) on November 6, 2007. The highly productive sequence comprised about 1300 micro earthquakes (M > 0.5) out of which 325 of M ? 1.5 that occurred during November 6, 2007–January 10, 2008 were precisely located. The spatial aftershock distribution revealed a NE–SW striking fault in accordance with the centroid moment tensor solution, which in turn implies left-lateral motion. The orientation and sense of shear are consistent with similarly orientated geological fault identified in the area from satellite imagery and field investigation.The aftershocks temporal decay, b-value of frequency–magnitude distribution, spatial fractal dimension, D, and slip ratio (ratio of the slip occurred on the primary fault to the total slip) were examined with the purpose to identify the properties of the sequence. The high b-value (1.18 ± 0.01) may be attributed to the paucity of the larger (M ? 4.0) aftershocks and reveals crustal heterogeneity and low stress regime. The high p-value (1.10 ± 0.39), implying fast decay rate of aftershocks, evidences high surface heat flux. A value of the spatial fractal dimension (D) equal to 2.21 ± 0.02 indicates random spatial distribution and source in a two-dimensional plane that is being filled-up by fractures. A slip ratio of 0.42 reveals that more slip occurred on secondary fault systems.The static Coulomb stress changes due to the coseismic slip of the main shock, enhanced off fault aftershock occurrence. The occurrence of a moderate earthquake (M_w 4.3) on October 5, 2008 inside a region of positive Coulomb stress changes supports the postulation on aftershock triggering. When the stress changes were resolved on a cross section including the stronger (M4.8) foreshock plane that is positioned adjacent to the main fault, it became evident that the activity continued there due to stress transfer from the main rupture.     

Modeling of strong motion generation areas of the 2011 Tohoku,Japan earthquake using modified semi-empirical technique

Modification in the semi-empirical technique for the simulation of strong ground motion has been introduced to incorporate the strong motion generation areas (SMGA) in the modeled rupture plane. Strong motion generation areas identified within the rupture plane of the Tohoku earthquake of March 11, 2011 (M _w = 9.0), have been modeled using this modified technique. Two different source models having four and five SMGAs, respectively, are considered for modeling purpose. Strong motion records using modified semi-empirical technique have been simulated at two near-field stations located at epicentral distance of 137 and 140 km, respectively, using two different source models. Comparison of the observed and simulated acceleration waveforms is made in terms of root mean square error (RMSE) at both stations. Minimum root mean square error of the waveform comparison has been obtained at both the stations for source model having five SMGAs. Simulations from same rupture model have been made at other four stations lying at epicentral distance between 154 and 249 km. Comparison of observed and simulated records has been made in terms of RMSE in acceleration records, velocity records and response spectra at each six station. Simulations have been made at six other stations to obtain distribution of peak ground acceleration and peak ground velocity with hypocentral distance. Peak ground acceleration and velocity from simulated and observed records are compared at twelve stations surrounding the source of Tohoku earthquake. Comparison of waveforms and parameters extracted from observed and simulated strong motion records confirms the efficacy of the developed modified technique to model earthquake characterized by SMGAs.     

Strong ground motion during the largest aftershock (Mw=5.8) of the 1999 Izmit earthquake, Turkey

We present a preliminary study of strong ground motion during the largest aftershock (M_w 5.8) of the 1999 Izmit earthquake (M_w 7.4), Turkey, at 11:55 on 13 September 1999. The peak ground acceleration observed near the epicentre of this aftershock was in agreement with that predicted by standard empirical prediction equations. Its spectral source parameters of the largest aftershock are also typical for a M_w 5.8 earthquake. At greater epicentral distances, there is an order-of-magnitude in scatter in peak ground acceleration values for this aftershock, which is attributed to site effects. The presence of thick layers of low-velocity sediments caused significant amplification of S-waves in the Avcılar district of Istanbul, at frequencies of 1 Hz, explaining the observed concentration of damage there as a result of the Izmit mainshock.     

Aftershock activity of Bhuj earthquake of january 26th, 2001

Following a large-sized Bhuj earthquake (M _s = 7.6) of January 26th, 2001, a small aperture 4-station temporary local network was deployed, in the epicentral area, for a period of about three weeks and resulted in the recording of more than 1800 aftershocks (-0.07 ≤M _L <5.0). Preliminary locations of epicenters of 297 aftershocks (2.0 ≤M _L <5.0) have brought out a dense cluster of aftershock activity, the center of which falls 20 km NW of Bhachau. Epicentral locations of after-shocks encompass a surface area of about 50 × 40 km² that seems to indicate the surface projection of the rupture area associated with the earthquake. The distribution of aftershock activity above magnitude 3, shows that aftershocks are nonuniformly distributed and are aligned in the north, northwest and northeast directions. The epicenter of the mainshock falls on the southern edge of the delineated zone of aftershock activity and the maximum clustering of activity occurs in close proximity of the mainshock. Well-constrained focal depths of 122 aftershocks show that 89% of the aftershocks occurred at depths ranging between 6 and 25 km and only 7% and 4% aftershocks occur at depths less than 5 and more than 25 km respectively. The Gutenberg-Richter (GR) relationship, logN = 4.52 - 0.89M_L, is fitted to the aftershock data (1.0<-M _L<5.0) and theb-value of 0.89 has been estimated for the aftershock activity.     

Postseismic relaxation due to Bhuj earthquake on January 26, 2001: possible mechanisms and processes

Earthquakes cause static stress perturbations in the nearby crust and mantle. Obeying rheological laws, this stress relaxes in a time frame of months to years with the spatial extent of few km to hundreds of km. While postseismic relaxation associated with major inter-plate earthquakes is well established, there have been few opportunities to explore its occurrence following intraplate earthquakes. The M _w 7.6 Bhuj earthquake on January 26, 2001 in western India is considered to be an intraplate event and provided a unique opportunity to examine post-earthquake relaxation processes sufficiently away from plate boundaries. To study the characteristics of transient postseismic deformation, six Global Positioning System campaigns were made at 14 sites. The postseismic transients were delineated after removing plate motions from the position time series. Postseismic deformation has been observed at all the sites in the study area. During 2001?C2007, the site closest to the epicenter exhibited postseismic deformation of about 30 and 25?mm in the north and east components, respectively. Time series of the NS and EW components of the postseismic transients can be fitted to both logarithmic and exponential functions. Close to the epicenter, the logarithmic function fits well to the initial transient, and an exponential function fits well to the later phases. The remaining sites (located east and west of the epicentral region) exhibited significantly diminished north?Csouth relaxation. Rapidly decaying afterslip and poroelastic mechanisms seem to be responsible for postseismic relaxation in the vicinity of epicenter during the initial period subsequent to the Bhuj earthquake. Postseismic relaxation by viscoelastic flow below the seismogenic zone seems to affect displacements across the entire Bhuj region. This paper presents the characteristics of postseismic transients and deformation processes in the scenario of the highly heterogeneous crust in the Bhuj region.     

A review on advances in seismology in Mexico after 30 years from the 1985 earthquake

The 19 September 1985 (M_w8.1) earthquake, located on the Michoacán coast, Mexico, generated great damage in Mexico City, more than 300 km away from the epicentral area. Other important cities near the coast and in central Mexico also suffered severe damage. Thirty years after this important event, the National Autonomous University of Mexico (UNAM), the Centro de Investigación Científica y de Educación Superior de Ensenada, Baja California (CICESE) and other institutions organized a conference to discuss the scientific advances, particularly in seismology, that had taken place in Mexico since then.     

The 2007 Bengkulu earthquake, its rupture model and implications for seismic hazard

The 12 September 2007 great Bengkulu earthquake (M _w 8.4) occurred on the west coast of Sumatra about 130 km SW of Bengkulu. The earthquake was followed by two strong aftershocks of M _w 7.9 and 7.0. We estimate coseismic offsets due to the mainshock, derived from near-field Global Positioning System (GPS) measurements from nine continuous SuGAr sites operated by the California Institute of Technology (Caltech) group. Using a forward modelling approach, we estimated slip distribution on the causative rupture of the 2007 Bengkulu earthquake and found two patches of large slip, one located north of the mainshock epicenter and the other, under the Pagai Islands. Both patches of large slip on the rupture occurred under the island belt and shallow water. Thus, despite its great magnitude, this earthquake did not generate a major tsunami. Further, we suggest that the occurrence of great earthquakes in the subduction zone on either side of the Siberut Island region, might have led to the increase in static stress in the region, where the last great earthquake occurred in 1797 and where there is evidence of strain accumulation.     

Annular seismicity structures and the March 11, 2011, earthquake (Mw = 9.0) in Northeast Japan

The characteristics of seismicity prior to the series of eight very strong earthquakes (M _w = 7.0–9.0) in Northeast Japan are discussed. Ring seismicity structures that appeared prior to all eight events in two depth ranges of 0–33 and 34–70 km are identified. The epicenters of the main shocks were located near areas of crossing or touching of shallow and deep rings. It was shown that the sizes of shallow rings and threshold magnitudes corresponding to seismicity rings grow with the energy of the main shocks. It was noted that the prognosis with respect to the place and magnitude of the catastrophic earthquake on March 11, 2011, had been made before it based on the data obtained prior to July 1, 2009. Use of the new data obtained prior to March 10, 2011, enabled us to specify this prognosis significantly. We obtained correlation dependences of threshold magnitudes on the energy of the main shocks (with a high correlation coefficients). It was shown that the duration of the period for seismicity rings to emerge in the considered region nearly did not depend on magnitude. The nature of annular structures and the possibility of application of their parameters for prognosis of strong earthquakes were discussed.     

Earthquakes in Switzerland and surrounding regions during 2011

This report of the Swiss Seismological Service summarizes the seismic activity in Switzerland and surrounding regions during 2011. During this period, 522 earthquakes and 92 quarry blasts were detected and located in the region under consideration. With a total of only 10 events with M _L????2.5, the seismic activity in the year 2011 was far below the average over the previous 36?years. Most noteworthy were the earthquake sequence of Sierre (VS) in January, with two events of M _L 3.3 and 3.2, the M _L 3.3 earthquake at a depth of 31?km below Bregenz, and the M _L 3.1 event near Delémont. The two strongest events near Sierre produced shaking of intensity IV.     

Tectonic structures across the East African Rift based on the source parameters of the 20 May 1990 M7.2 Sudan earthquake

Earthquakes in Kenya are common along the Kenya Rift Valley because of the slow divergent movement of the rift and hydrothermal processes in the geothermal fields. This implies slow but continuous radiation of seismic energy, which relieves stress in the subsurface rocks. On the contrary, the NW-SE trending rift/fault zones such as the Aswa-Nyangia fault zone and the Muglad-Anza-Lamu rift zone are the likely sites of major earthquakes in Kenya and the East African region. These rift/fault zones have been the sites of a number of strong earthquakes in the past such as the M _w = 7.2 southern Sudan earthquake of 20 May 1990 and aftershocks of M _w = 6.5 and 7.1 on 24 May 1990, the 1937 M _s = 6.1 earthquake north of Lake Turkana close to the Kenya-Ethiopian border, and the 1913 M _s = 6.0 Turkana earthquake, among others. Source parameters of the 20 May 1990 southern Sudan earthquake show that this earthquake consists of only one event on a fault having strike, dip, and rake of 315°, 84°, and ?3°. The fault plane is characterized by a left-lateral strike slip fault mechanism. The focal depth for this earthquake is 12.1 km, seismic moment M _o = 7.65 × 10¹⁹ Nm, and moment magnitude, M _w = 7.19 (?7.2). The fault rupture started 15 s earlier and lasted for 17 s along a fault plane having dimensions of ?60 km × 40 km. The average fault dislocation is 1.1 m, and the stress drop, ?σ, is 1.63 MPa. The distribution of historical earthquakes (M _w ≥ 5) from southern Sudan through central Kenya generally shows a NW-SE alignment of epicenters. On a local scale in Kenya, the NW–SE alignment of epicenters is characterized by earthquakes of local magnitude M _l ≤ 4.0, except the 1928 Subukia earthquake (M _s = 6.9) in central Kenya. This NW–SE alignment of epicenters is consistent with the trend of the Aswa-Nyangia Fault Zone, from southern Sudan through central Kenya and further southwards into the Indian Ocean. We therefore conclude that the NW–SE trending rift/fault zones are sites of strong earthquakes likely to pose the greatest earthquake hazard in Kenya and the East African region in general.