Article

Earthquake hazard in the Kenya Rift: the Subukia earthquake 1928

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Abstract

The Subukia Valley earthquake in East Africa, magnitude 6.9, occurred on 1928 January 6. This is a little known earthquake associated with a 38 km long surface break that showed normal faulting with a small component of left lateral motion. Maximum throw was 240 cm with an average along the rupture of less than 100 cm. The earthquake triggered rockfalls and minor landslides and exhibited long-period effects at large distances. Considering the magnitude of the earthquake, the amount of damage it caused was not as great as it could have been. This is directly attributable to the sparsity of dwellings and the inherent resistance to earthquake shaking of the local type of huts. In the process of studying this earthquake, the seismicity of the (Gregory) Kenya Rift Valley has been re-evaluated. It is shown that relatively large but infrequent earthquakes do occur in the Kenya (Gregory) Rift Valley, a slowly extending region of low apparent seismicity, and that the seismic hazard of this part of East Africa, deduced from data of the last 95 years, is significant.

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... By measuring the offsets of landforms and fault scarps, the earthquake-induced surface displacement along the fault can be determined, which can provide information about 5 the rupture and slip history on the fault (e.g., Wallace, 1968;Sieh, 1978;Zielke et al., 2012;Ren et al., 2016), and be used to identify structural segmentation (e.g., Watterson, 1986;Giba et al., 2012;Manighetti et al., 2015) and the presence of linking structures (e.g., Soliva and Benedicto, 2004;Nicol et al., 2010). For faults whose component segments remain unconnected at the surface, the distribution of displacement along a fault can also provide clues to the future structural development (e.g., Walsh and Watterson, 1988;Cowie and Scholz, 1992;Dawers et al., 1993;Dawers and Anders, 1995;Peacock, 2002) by 10 indicating soft-linkages between segments (Willemse et al., 1996;Hilley et al., 2001), such as relay ramps. Over time, these segments may hard-link , i.e. a connection establishes between segments via rupture of discrete fault planes (e.g., Childs et al., 2017). ...
... The morphology and geometry of the scarp, however, varies along strike (Hodge et al., 2018a) and is therefore typical of a large, structurally segmented normal fault (e.g., Schwartz and Coppersmith, 1984;Wesnousky, 1986;Peacock and Sanderson, 1991). The fault is suggested to comprise six, major (first-order) segments, varying 10 in length from 13 km to 38 km, and the distribution of scarp height is of two symmetrical bell-shaped profiles separated by the Citsulo segment (Hodge et al., 2018a). The relatively coarse measurement resolution of the former studies along the BMF have meant that secondary (second-order) segments were unable to be identified or characterised, i.e. subordinate segments that have a length of the same order of magnitude as the major segment they exist within (Manighetti et al., 2015). ...
... Profile A has a high signal-to-noise ratio, and a large, wide scarp; however, the gradient of the scarp is not constant, leading to large slope derivative values (Fig. 2a). Profile B has a low signal-to-noise ratio, caused by vegetation or 10 other topographical features; this noise creates local variability in slope θ, yet the gradient on the scarp itself is fairly constant (Fig. 2b). Profile C has a low signal-to-noise ratio, the scarp width is small and the magnitude of the change in slope at the fault scarp is not large; it is therefore difficult to accurately identify the scarp from the footwall topography (Fig. 2c). ...
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Article
Along-strike variation in scarp morphology reflects differences in a fault's geomorphic and structural development and can thus indicate fault rupture history as well as mechanical segmentation. Parameters that define scarp morphology (height, width, slope) are typically measured or calculated manually. The time-consuming manual approach reduces the density and objectivity of measurements, and can lead to oversight of small-scale morphological variations that occur at a resolution impractical to capture. Furthermore, inconsistencies in the manual approach may also lead to unknown discrepancies and uncertainties between, and also within, individual fault scarp studies. Here, we aim to improve the efficiency, transparency and uniformity of calculating scarp morphological parameters by developing a semi-automated Scarp PARameTer Algorithm (SPARTA). We compare our findings against a traditional, manual analysis and assess the performance of the algorithm using a range of elevation model resolutions. We then apply our new algorithm to a 12 m resolution DEM for four southern Malawi fault scarps, located at the southern end of the East African Rift System: the Bilila-Mtakataka fault and three previously unreported scarps – Thyolo, Muona and Malombe. All but Muona exhibit first-order structural segmentation at their surface, and by using a 5 m resolution DEM derived from high-resolution stereo satellite imagery for the Bilila-Mtakataka fault scarp, we are able to quantify secondary structural segmentation. Our scarp height calculations from all four fault scarps suggests that if each scarp was formed by a single, complete rupture, the slip-length ratio for each fault exceeds the maximum typical value observed empirically in historical normal faulting earthquakes around the world, implying that their structural histories are more complex. The distribution of vertical displacement at the surface implies the structural segments of both the BMF and Thyolo fault have merged via rupture of discrete faults (hard-links) through several earthquake cycles, and the segments of the Malombe fault have connected via distributed deformation zones (soft-links). For all faults studied here, the length of earthquake ruptures may therefore exceed the constitutive length of each segment. Thus, our findings shed new light on the seismic hazard in southern Malawi, indicating evidence for a number of large (MW 7–8) prehistoric earthquakes, as well as providing a new semi-automated methodology (SPARTA) for calculating scarp morphological parameters, which can be used on other fault scarps to infer structural development.
... The most significant earthquakes in Kenya up to now have been the M s 6.9 earthquake at Subukia area in the central part of the Kenya Rift in 1928 and an aftershock M s 6.0 which occurred 4 days later and the 1913 Turkana region earthquake having a surface wave magnitude M s 6.0. According to Ambraseys (1991), the 1928 Subukia earthquake is a little-known earthquake that occurred in an area of relatively sparse seismicity but high tectonic activity. It was associated with a 38-km-long surface break that showed normal faulting with a small component of left lateral motion. ...
... It triggered rock falls and minor landslides and exhibited Chapter 19 Seismic Hazard 273 long-period effects at large distances within a radius of 460 km. Considering the magnitude of the Subukia earthquake, the amount of damage it caused was not as great as it could have been, and this is directly attributed to the sparsity of dwellings and the inherent resistance to earthquake shaking of the local type of huts (Ambraseys, 1991). Except these three significant earthquakes, therefore, no M > 5 earthquakes in the Kenya Rift have been reported since 1928 and this explains why Kenya has been relatively free from seismicity (earthquake) hazards over the years. ...
... The seismicity in Kenya for the period 1906-1963 is mainly based on macroseismic data and the magnitudes are expressed as surface wave magnitudes, M s , for example, Shah (1986) and Ambraseys (1991). The seismicity Chapter 19 Seismic Hazard 275 from 1963 to 2010 is based on data from instrumental recordings and the magnitudes are expressed as local magnitudes, M l . ...
Chapter
This chapter begins with general overview of the seismicity in Kenya from the 1900s to the present. Seismicity in Kenya up to 1963 is mainly based on macroseismic data while that from 1963 to the present is based on data from instrumental recordings. In the past, a number of microseismic and seismicity studies in Kenya have previously been undertaken and the results from these studies are rather disjointed. In this chapter, we have made an attempt to merge all the existing results into one database from which the general seismicity, and subsequently seismic hazard in Kenya has been evaluated. The main goal of this chapter is to bring into focus the area(s) in Kenya more prone to seismic hazards either due to ground shaking occasioned by an earthquake or due to tsunami as a result of earthquakes occurring along the Davie Ridge.
... In contrast with the magmatically active Kenya rift, higher seismicity levels with historical magnitudes as large as M 7:4 characterize the virtually amagmatic western branch of the East African rift and the Tanzania rift zone ( Fig. 1a; e.g., Shudofsky [1985]; Ambraseys [1991]; Parsons and Thompson [1991]; Jackson and Blenkinsop [1993]; Girdler and McConnell [1994]). Amongst these are the recent M 6:8 Kalemie earthquake (2005), M 7:0 Machaze earthquake (2006), and M 6:0 Cyangugu earthquake (2008). ...
... (b) Main surface breaks of the Subukia Valley earthquake. The plus and minus signs along the surface rupture indicate the upthrown and downthrown side (after Ambraseys [1991]). The inserted rectangle outlines the footprint of Figure 3 and the arrow indicates the trench site location (Fig. 4). ...
... It is the largest instrumentally recorded earthquake in the Kenya rift, resulting in a discontinuous surface rupture more than 38 km long. This normal faulting event had an average throw of about 1 m, a maximum throw of 2.4 m, and a small oblique slip component (McCall, 1967;Ambraseys, 1991). Detailed reports on this earthquake were published in the appendices of Richter's classic seismology textbook (Richter, 1958), McCall's (1967) geological report, and by Ambraseys (1991). ...
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Article
The seismicity of the Kenya rift is characterized by high-frequency low-magnitude events concentrated along the rift axis. Its seismic character is typical for magmatically active continental rifts, where igneous material at a shallow depth causes extensive grid faulting and geothermal activity. Thermal overprinting and dike intrusion prohibit the buildup of large elastic strains, therefore prohibiting the gen-eration of large-magnitude earthquakes. On 6 January 1928, the M S 6:9 Subukia earthquake occurred on the Laikipia–Marmanet fault, the eastern rift-bounding struc-ture of the central Kenya rift. It is the largest instrumentally recorded seismic event in the Kenya rift, standing in contrast to the current model of the rift's seismic character in which large earthquakes are not anticipated. Furthermore, the proximity of the rup-tured fault and the rift axis is intriguing: The rift-bounding structure that ruptured in 1928 remains seismically active, capable of generating large-magnitude earthquakes, even though thermally weakened crust and better oriented structures are present along the rift axis nearby, prohibiting any significant buildup of elastic strain. We excavated the surface rupture of the 1928 Subukia earthquake to find evidence for preceding ground-rupturing earthquakes. We also made a total station survey of the site topography and mapped the site geology. We show that the Laikipia–Marmanet fault was repeatedly activated during the late Quaternary. We found evidence for six ground-rupturing earthquakes, includ-ing the 1928 earthquake. The topographic survey around the trench site revealed a degraded fault scarp of ≈7:5 m in height, offsetting a small debris slide. Using scarp-diffusion modeling, we estimated an uplift rate of U ˆ 0:09–0:15 mm=yr, con-straining the scarp age to 50–85 ka. Assuming an average fault dip of 55°–75°, the preferred uplift rate (0:15 mm=yr) accommodates approximately 10%–20% of the recent rate of extension (0:5 mm=yr) across the Kenya rift.
... The slip-length ratios for the normal-faulting 2008 Yutian and 2006 Mozambique earthquakes were 1-2 × 10 −4 although both were blind earthquakes which did not rupture the surface (Elliot et al., 2010;Copley et al., 2012). The only well-documented surfacerupturing event with a recorded slip-length ratio within the EARS was for the ∼ M s 6.8 1928 Kenya earthquake (Ambraseys and Adams, 1991), whose 1 m scarp could be traced for ∼ 38 km at the surface (Ambraseys, 1991b), resulting in a ratio of ∼ 2.8 × 10 −5 . ...
... Even the subsurface rupture lengths of these events have been modelled to be just ∼ 26 and ∼ 16 km, respectively (Moussa, 2008), significantly smaller than the total lengths of each of the fault scarps in this study. In addition, one of the few recorded surface ruptures for a large-magnitude event along the EARS, the ∼ M s 6.9 1928 earthquake on the Laikipia-Marmanet fault in Kenya -the largest instrumentally recorded earthquake in the Kenya rift -resulted in just a ∼ 38 km long surface rupture (Ambraseys, 1991b). ...
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Article
Along-strike variation in scarp morphology reflects differences in a fault's geomorphic and structural development and can thus indicate fault rupture history and mechanical segmentation. Parameters that define scarp morphology (height, width, slope) are typically measured or calculated manually. The time-consuming manual approach reduces the density and objectivity of measurements and can lead to oversight of small-scale morphological variations that occur at a resolution impractical to capture. Furthermore, inconsistencies in the manual approach may also lead to unknown discrepancies and uncertainties between, and also within, individual fault scarp studies. Here, we aim to improve the efficiency, transparency and uniformity of calculating scarp morphological parameters by developing a semi-automated Scarp PARameTer Algorithm (SPARTA). We compare our findings against a traditional, manual analysis and assess the performance of the algorithm using a range of digital elevation model (DEM) resolutions. We then apply our new algorithm to a 12m resolution TanDEM-X DEM for four southern Malawi fault scarps, located at the southern end of the East African Rift system: the Bilila–Mtakataka fault (BMF) and three previously unreported scarps – Thyolo, Muona and Malombe. All but Muona exhibit first-order structural segmentation at their surface. By using a 5m resolution DEM derived from high-resolution (50cmpixel⁻¹) Pleiades stereo-satellite imagery for the Bilila–Mtakataka fault scarp, we quantify secondary structural segmentation. Our scarp height calculations from all four fault scarps suggest that if each scarp was formed by a single, complete rupture, the slip–length ratio for each earthquake exceeds the maximum typical value observed in historical normal faulting earthquakes around the world. The high slip–length ratios therefore imply that the Malawi fault scarps likely formed in multiple earthquakes. The scarp height distribution implies the structural segments of both the BMF and Thyolo fault have merged via rupture of discrete faults (hard links) through several earthquake cycles, and the segments of the Malombe fault have connected via distributed deformation zones (soft links). For all faults studied here, the length of earthquake ruptures may therefore exceed the length of each segment. Thus, our findings shed new light on the seismic hazard in southern Malawi, indicating evidence for a number of large (Mw 7–8) prehistoric earthquakes, as well as providing a new semi-automated methodology (SPARTA) for calculating scarp morphological parameters, which can be used on other fault scarps to infer structural development.
... frequent, low-magnitude seismicity has been associated with dyke intrusions (e.g., Tongue et al., 1994), but fault ruptures during large earthquakes have occurred without coupled volcanism (e.g., Ambraseys, 1991; Biggs et al., 2010; Zielke and Strecker, 2009). Thus, despite of its overall magmatic character, strain release in the EARS is not necessarily associated with dyke intrusions and magmatism. ...
... This pattern is mimicked by a southward increase in crustal thickness estimated from seismic-refraction profiles (KRISP, 1991), compatible with a decrease in the magnitude of finite extension. Shallow earthquakes of M > 5 have occurred along the EARS during the past century, with only a few large events associated with surface ruptures (Abdallah et al., 1979; Ambraseys, 1991; Parsons and Thompson, 1991). It is unlikely that seismic slip on crustal faults accounts for all the extension across the rift, unless very large events with long recurrence times are missing from historical catalogs (e.g., Zielke and Strecker, 2009). ...
Article
A comparison of deformation rates in active rifts over different temporal scales may help to decipher variations in their structural evolution, controlling mechanisms, and evolution of sedimentary environments through time. Here we use deformed lake shorelines in the Suguta and Turkana basins in northern Kenya as strain markers to estimate deformation rates at the 103–104 yr time scale and compare them with rates spanning 101–107 yr. Both basins are internally drained today, but until 7 to 5 kyr lake levels were 300 and 100 m higher, respectively, maintained by the elevation of overflow sills connecting them with the Nile drainage. Protracted high lake levels resulted in formation of a maximum highstand shoreline — a distinct geomorphic feature virtually continuous for several tens of kilometers. We surveyed the elevation of this geomorphic marker at 45 sites along > 100 km of the rift, and use the overflow sills as vertical datum. Thin-shell elastic and thermomechanical models for this region predict up to ~ 10 m of rapid isostatic rebound associated with lake-level falls lasting until ~ 2 kyr ago. Holocene cumulative throw rates along four rift-normal profiles are 6.8–8.5 mm/yr, or 7.5–9.6 mm/yr if isostatic rebound is considered. Assuming fault dips of 55–65°, inferred from seismic reflection profiles, we obtained extension rates of 3.2–6 mm/yr (including uncertainties in field measurements, fault dips, and ages), or 3.5–6.7 mm/yr considering rebound. Our estimates are consistent, within uncertainties, with extension rates of 4–5.1 mm/yr predicted by a modern plate-kinematic model and plate reconstructions since 3.2 Myr. The Holocene strain rate of 10− 15 s− 1 is similar to estimates on the ~ 106 yr scale, but over an order of magnitude higher than on the ~ 107 yr scale. This is coherent with continuous localization and narrowing of the plate boundary, implying that the lithospheric blocks limiting the Kenya Rift are relatively rigid. Increasing strain rate under steady extension rate suggests that, as the magnitude of extension and crustal thinning increases, the role of regional processes such as weakening by volcanism becomes dominant over far-field plate tectonics controlling the breakup process and the transition from continental rifting to oceanic spreading.
... In 1928, Kenya was hit by an earthquake, of magnitude 6.9 at Subukia. The earthquake's epicenter lied within the rift valley boundaries [2]. In 2019, a 4.8 magnitude earthquake hit Kenya, Wundanyi as the epicenter. ...
... The largest known event in the region is the 13 December 1910 M S 7.4 Rukwa (Tanzania) event that badly cracked all European-style houses in towns on the eastern shore of Lake Tanganyika (Midzi and Manzunzu 2014;Ambraseys 1991a;Ambraseys and Adams 1991). A M S 6.9 earthquake that occurred on 6 January 1928 in the Subakia Valley (part of the Kenya Rift, some 200 km northwest of Nairobi) produced a 38 km long surface rupture with a maximum throw of 2.4 m and destroyed, or damaged beyond repair, all European-style houses within 15 km of the rupture, fortunately without causing casualties (Ambraseys 1991b). ...
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Article
The East African Rift System is the major active tectonic feature of the Sub-Saharan Africa region. Although the seismicity level of this divergent plate boundary can be described as moderate, several damaging earthquakes have been reported in historical times, and the seismic risk is exacerbated by the high vulnerability of the local buildings and structures. Formulation and enforcement of national seismic codes is therefore an essential future risk mitigation strategy. Nonetheless, a reliable risk assessment cannot be done without the calibration of an updated seismic hazard model for the region. A major limitation affecting the assessment of seismic hazard in Sub-Saharan Africa is the lack of basic information needed to construct source and ground motion models. The historical earthquake record is sparse, with significant variation in completeness over time across different regions. The instrumental catalogue is complete down to sufficient magnitude only for a relatively short time span. In addition, mapping of seismogenically active faults is still an on-going task, and few faults in the region are sufficiently constrained as to allow them to be directly represented within the seismic hazard model. Recent studies have identified major seismogenic lineaments, but there is substantial lack of kinematic information for intermediate-to-small scale tectonic features, information that is essential for the proper calibration of earthquake recurrence models. In this study, we use new data and Global Earthquake Model (GEM) computational tools such as the Hazard Modeller’s Toolkit and the OpenQuake engine to perform a pilot study of the seismic hazard associated with the East African Rift. The hazard model obtained has been created using the most recent information available from scientific literature, global bulletins and local earthquake catalogues, including those from AfricaArray projects. In this report, in accordance with the GEM philosophy, we describe in detail all working assumptions, main processing steps, data analyses and interpretations used for the model setup.
... Recent fault movements and subsidence are common throughout the Baringo–Bogoria region. Although seismic activity is common (Young et al., 1991; Tongue et al., 1994), the last major local earthquake (Subukia: M 6AE9) occurred in 1928 (McCall, 1967; Ambraseys, 1991). Evidence for neotectonic tilting of strata on the Loboi Plain is seen 15 km north-west of the wetland (Le Turdu et al., 1995, their fig. ...
Article
Loboi Swamp is situated near the equator on the western fault-bounded margin of an asymmetric half-graben within the East African Rift valley. The freshwater wetland is ~ 3km2 and developed during mid to late Holocene on the low relief floodplain of the axial Loboi River. The swamp is groundwater-fed by several springs and seeps associated with the border fault system. Spring waters are ~35°C, with pH ~6.4-6.9 and the water compositions suggest that the sources are shallow, and dominated by meteoric water with little contributed by deep re-circulating fluids. The climate is semi-arid. P is ~700 mm/yr on the valley bottom and 1200mm/yr in the adjacent highlands; ET is estimated to be ~2500 mm/yr. Variation in precipitation occurs on a range of time scales: semi-annual monsoonal rains in Nov. and April; El Nino and La Nina periods every 5-7 years; and long term variations in climate are also likely, such as, orbitally-forced Precession cycles (~20ka). The modern swamp is dominated by Typha domingensis Pers. (~80%) and Cyperus papyrus L. (20%), a crocodile habitat. The stratigraphy revealed in a soil pit and 8 piston cores (1.5-4 m long) records the formation, evolution and maybe the beginning of the demise of the wetland. Basal sediments are floodplain (sandy silts) that fine upward to f. silt and clay and are capped with organic-rich sediment (peat). Subparallel siderite concretion horizons in the silts indicate that Fe-reducing conditions developed as the basal sediments were flooded by the developing wetland. The peat is thickest (1.5 m) in the spring-proximal area near the fault and thins to 0.30m in the spring-distal areas. The appearance and expansion of peat indicates moister climate, however preliminary pollen analyses reveals that Cyperaceae and Tpyha are less abundant now than earlier suggesting a change from moister to drier conditions after the development of the swamp. Surface and porewater compositions in the swamp are modified by processes of evaporation and decay of organic matter. Soils and paleosols developed on the periphery of the wetland reveal evidence for dramatic fluctuations in hydrologic budget, as indicated by formation of siderite and redoximorphic features during wetter phases, and vertic (shrink-swell) and clay illuviation features during drier phases. The combined records of sedimentology, soils, and pollen suggests a gradual change (over few thousands years) to wetter conditions and then to generally drier conditions with superimposed shorter term wet-dry cycles (hundreds of years to decades?).
... According to Yang and Chen (2010), the source mechanisms of earthquakes (Mb≥ 4.5) recorded in the East-African rift system over the period 1964-2008 show mostly normal faulting. Up to 2.4 m normal-fault displacements were observed during the Subukia Valley earthquake (M S 6.9) at a total fault length of about 38 km (Ambraseys, 1991). ...
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This paper provides a cumulative review of important specific features in the formation and development of Proval Bay (Lake Baikal) as a large seismic dislocation element. This bay appeared during one of the largest historical earthquakes in Siberia (MLH 7.5) on January 12, 1862. As a result, more than 230 km2 of the shore was submerged. The paper considers the formation of Proval Bay in the context of analysis conducted on general morphological characteristics of the Baikal rift and in terms of the occurrence of the main elements in the mechanism of its neotectonic development. It is precisely these seismotectonic phenomena, associated with the subsidence of large tectonic blocks, which primarily cause the growth of Lake Baikal basin. In spite of the fact that the northwestern side of the rift has a more pronounced morphological structure, whose general elements are high and steep monolithic tectonic escarpments, major lithospheric extension and its associated extension of the rift, thinning and rearrangement of blocks in the upper lithosphere slab occur on the more gently sloping eastern side.
... Recent fault movements and subsidence are common throughout the Baringo-Bogoria region. Although seismic activity is common ( Young et al., 1991;Tongue et al., 1994), the last major local earthquake (Subukia: M 6AE9) occurred in 1928 ( McCall, 1967;Ambraseys, 1991). Evidence for neotectonic tilting of strata on the Loboi Plain is seen 15 km north-west of the wetland ( Le Turdu et al., 1995, their fig. ...
Article
Loboi Swamp is a 1·5 km2 freshwater wetland situated near the equator in the Kenya Rift Valley. The climate is semi-arid: precipitation is ≈ 700 mm year−1, and evapotranspiration is ≈ 2500 mm year−1. Some of the wetland water is currently used for irrigation. An interdisciplinary study was conducted on the geology, hydrology, pedology and biology of the wetland to determine its origin and history and to assess its longevity under present hydrological conditions. Sedimentary records from two piston cores (1·8 and 4 m long) indicate that the present wetland developed during the late Holocene on a low-relief alluvial plain. Floodplain deposits (sandy silts) are capped with wetland sediments (organic-rich clay and peat), which began to form at ≈ 700 BP. The swamp is dominated by Typha domingensis Pers. (≈ 80%) and floating Cyperus papyrus L. (20%). It is fed by warm springs (T ≈ 35 °C; pH ≈ 6·4–6·9) emanating from grid faults of the rift floor. Water compositions suggest that sources are dominated by shallow meteoric water, with little contribution from deeper geothermal fluids. Siderite concretions in the floodplain silts reflect the Fe-reducing conditions that developed as the surface became submerged beneath the water table. The pollen record captured both local and more regional vegetation, showing the prevailing dry rift valley climate despite development of the wetter conditions on the valley floor. The diatom record also suggests a dramatic change in local hydrology. The combined biological records of this semi-arid wetland indicate an abrupt change to wetter conditions, most probably as a result of a regional change in climate. Rift tectonics provided accommodation space, maintained the wetland at or below the water table and enabled spring recharge. The size of the modern wetland has been reduced by about 60% since 1969, which suggests that the system may now be under hydrological stress due to anthropogenic impacts from land-use change.
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Seismic hazard assessment in slow straining regions is challenging because earthquake catalogues only record events from approximately the last 100 years, whereas earthquake recurrence times on individual faults can exceed 1000 years. Systematic mapping of active faults allows fault sources to be used within probabilistic seismic hazard assessment, which overcomes the problems of short-term earthquake records. We use Shuttle Radar Topography Mission (SRTM) data to analyse surface deformation in the Luangwa Rift in Zambia and develop the Luangwa Rift Active Fault Database (LRAFD). The LRAFD is an open-source geospatial database containing active fault traces and their attributes and is freely available at https://doi.org/10.5281/zenodo.6513691. We identified 18 faults that display evidence for Quaternary activity, and empirical relationships suggest that these faults could cause earthquakes up to Mw 8.1, which would exceed the magnitude of historically recorded events in southern Africa. On the four most prominent faults, the median height of Quaternary fault scarps varies between 12.9 ± 0.4 and 19.2 ± 0.9 m, which suggests they were formed by multiple earthquakes. Deformation is focused on the edges of the Luangwa Rift: the most prominent Quaternary fault scarps occur along the 207 km long Chipola and 142 km long Molaza faults, which are the rift border faults and the longest faults in the region. We associate the scarp on the Molaza Fault with possible surface ruptures from two 20th century earthquakes. Thus, the LRAFD reveals new insights into active faulting in southern Africa and presents a framework for evaluating future seismic hazard.
Chapter
According to the seismicity and the historical records of the previous considerably large magnitude earthquakes, Kenya can be regarded as a country of low to medium seismicity. Tectonically, the country is, however, cut longitudinally by the seismically active East Africa Rift. The most significant earthquakes have occurred along the rift and adjoining areas causing minor to major damages in the region. Based on seismicity records and the seismotectonics of the region, the current work presents a probabilistic assessment of seismic hazard in Kenya with the aim of bringing into focus the cities/towns that are more prone to seismic hazard due to ground shaking occasioned by earthquakes along the region. The potential seismic source zones are identified, and the resultant seismic ground motion parameters are evaluated. An appropriate ground motion prediction attenuation equation is adopted. The seismic hazard has been computed using the probabilistic approach under “Openquake Version 2.0.1” program. The results show that the seismic hazard levels in PGA range from 0.32 to 0.02 g and 0.176 to 0.016 g for the 2% and 10% probability of exceedance in 50 years, respectively. Cities/towns located in south-western regions are more vulnerable to high seismic hazards than the cities located in the north-eastern part of the study area. The results from this research have shown that the high hazard levels are associated with very active tectonic activity within the region.
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Coseismic surface faulting is a significant source of hazard for critical plants and distributive infrastructure; it may occur either on the principal fault or as distributed rupture on nearby faults. Hazard assessment for distributed faulting is based on empirical relations which, in the case of normal faults, were derived almost 15 years ago using a dataset of US earthquakes. We collected additional case histories worldwide, for a total of 21 earthquakes, and calculated the conditional probability of distributed faulting as a function of distance from the principal fault. We found no clear dependency on the magnitude nor the time of occurrence of the earthquakes, but our data consistently show a higher probability of rupture when compared with the scaling relations currently adopted in engineering practice. We derive updated empirical regressions and show that the results are strongly conditioned by the averaging of earthquakes effectively generating distributed faulting at a given distance and those which did not generate faulting; thus, we introduce a more conservative scenario that can be included in a logic tree approach to consider the full spectrum of potential ruptures. Our results can be applied in the framework of probabilistic assessment of fault displacement hazard.
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Preprint
Coseismic surface faulting is a significant source of hazard for critical plants and distributive infrastructures; it may occur either on the primary fault, or as distributed rupture on nearby faults. Hazard assessment for distributed faulting is based on empirical relations which, in the case of normal faults, were derived almost 15 years ago on a dataset of US earthquakes. We collect additional case histories worldwide, for a total of 21 earthquakes, and we calculate the conditional probability of distributed faulting as a function of distance from the primary fault. We found no clear dependency on the magnitude nor the time of occurrence of the earthquakes, but our data consistently show a higher probability of rupture when compared to the scaling relations currently adopted in engineering practice. We derive updated empirical regressions and show that results are strongly conditioned by the averaging of earthquakes effectively generating distributed faulting at a given distance and those which did not generate faulting; thus, we introduce a more conservative scenario, which can be included in a logic tree approach to consider the full spectrum of potential ruptures. Our results can be applied in the framework of probabilistic assessment of fault displacement hazard.
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Conference Paper
Material from design earthquake studies for infrastructures projects worldwide is presented, grouped in the two broad subjects of PSHA and site response analysis (SRA). Emphasis is given in the first group to seismotectonic complexity occurring where stable continental zones are contiguous to active ones, and uncertainty in ground shaking hazard assessment increases because of data scarcity worldwide for continental crust. Using the Persian Gulf region as an example, the difficulty in establishing seismogenic source models and earthquake activity rates is highlighted. In the same context, the selection of GMPEs in such complex settings is also discussed, with examples from Near East and Africa. Regarding SRA, discussed first are the comparative merits of the type of approach used, i.e. hybrid (probabilistic + deterministic) vs. rigorous probabilistic (Bazzurro & Cornell 2004). Lastly, deep borehole data from a real project site are used to investigate whether the lack of deep seismic velocity data at deep soft soil sites plays or not a significant role in SRA.
Conference Paper
Material from design earthquake studies for infrastructures projects worldwide is presented, grouped in the two broad subjects of PSHA and site response analysis (SRA). Emphasis is given in the first group to seismotectonic complexity occurring where stable continental zones are contiguous to active ones, and uncertainty in ground shaking hazard assessment increases because of data scarcity worldwide for continental crust. Using the Persian Gulf region as an example, the difficulty in establishing seismogenic source models and earthquake activity rates is highlighted. In the same context, the selection of GMPEs in such complex settings is also discussed, with examples from Near East and Africa. Regarding SRA, discussed first are the comparative merits of the type of approach used, i.e. hybrid (probabilistic + deterministic) vs. rigorous probabilistic (Bazzurro & Cornell 2004). Lastly, deep borehole data from a real project site are used to investigate whether the lack of deep seismic velocity data at deep soft soil sites plays or not a significant role in SRA.
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This book presents a unique, interdisciplinary approach to disaster risk research, combining cutting-edge natural science and social science methodologies. Bringing together leading scientists, policy makers and practitioners from around the world, it presents the risks of global hazards such as volcanoes, seismic events, landslides, hurricanes, precipitation floods and space weather, and provides real-world hazard case studies from Latin America, the Caribbean, Africa, the Middle East, Asia and the Pacific region. Avoiding complex mathematics, the authors provide insight into topics such as the vulnerability of society, disaster risk reduction policy, relations between disaster policy and climate change, adaptation to hazards, and (re)insurance approaches to extreme events. This is a key resource for academic researchers and graduate students in a wide range of disciplines linked to hazard and risk studies, including geophysics, volcanology, hydrology, atmospheric science, geomorphology, oceanography and remote sensing, and for professionals and policy makers working in disaster prevention and mitigation.
Article
Kenya has had a seismic station since 1963 as part of the World Wide Standardized Seismograph Network (WWSSN). In 1990, the University of Nairobi in collaboration with GeoForschungsZentrum (GFZ) started to build up a local seismological network, the Kenya National Seismic Network (KNSN), which operated for about ten years between 1993–2002. This, however, experienced a myriad of problems ranging from equipment breakdown, vandalism and lack of spares. Kenya is seismically active since the Kenya rift valley traverses through the country from north to south bisecting the country into eastern and western regions. In the central part, the Kenya rift branches to form the NW-SE trending Kavirondo (Nyanza) rift. The Kenya rift valley and the Kavirondo (Nyanza) rift are the most seismically active where earthquakes of local magnitude (Ml) in the order of ⩽2.0–5.0 occur. Furthermore, historical records show that earthquakes of magnitudes of the order of Ml ⩾ 6.0 have occurred in Kenya. Such large magnitude earthquakes include the January 6, 1928 Subukia earthquake (Ml 7.1) and an aftershock (Ml 6.2) four days later, as well as the 1913 Turkana region earthquake (Ml 6.2). Since early 1970’s, numerous seismic investigations have been undertaken in Kenya in order to understand the formation and structure of the Kenyan part of the East African rift valley. Earthquake data from these studies is, however, rather disorganized and individual datasets, including that acquired during the period 1993–2002, cannot furnish us with comprehensive information on the seismicity of Kenya for the past ∼100 years. The purpose of this paper is, therefore, to review the seismicity in Kenya for the period 1906–2010 by utilizing data and results from different sources. The general seismicity of Kenya has been evaluated using historical data, data recorded by local seismic networks, the United States Geological Survey catalogue as well as earthquake data from the numerous seismic investigations by different individuals and research groups. On the basis of earthquake data from these sources, the entire N–S trending Kenya rift valley and the NW-SE trending Nyanza (Kavirondo) rift are characterized by a high rate of seismicity, and the USGS network has been effective in detecting local M > 3.0 earthquakes. A peculiar trend is exhibited by earthquakes of Ml ⩾ 5.1 in that these occur along the N-S and NW-SE trending Kenya rift valley and the Kavirondo (Nyanza) rift zone respectively. Earthquake data from the various sources for the period 1906–2010 is complete for Ml ⩾ 4.4 earthquakes with a b-value of 0.79 which is characteristic of tectonic active regions like rifts. There is need to revive and extend the KNSN for a greater coverage and effective seismic monitoring in Kenya.
Article
Travertine is present at 20% of the ca 60 hot springs that discharge on Loburu delta plain on the western margin of saline, alkaline Lake Bogoria in the Kenya Rift. Much of the travertine, which forms mounds, low terraces and pool‐rim dams, is sub‐fossil (relict) and undergoing erosion, but calcite‐encrusted artefacts show that carbonate is actively precipitating at several springs. Most of the springs discharge alkaline (pH: 8·3 to 8·9), Na‐HCO3 waters containing little Ca (−1) at temperatures of 94 to 97·5°C. These travertines are unusual because most probably precipitated at temperatures of >80°C. The travertines are composed mainly of dendritic and platy calcite, with minor Mg‐silicates, aragonite, fluorite and opaline silica. Calcite precipitation is attributed mainly to rapid CO2 degassing, which led to high‐disequilibrium crystal morphologies. Stratigraphic evidence shows that the travertine formed during several stages separated by intervals of non‐deposition. Radiometric ages imply that the main phase of travertine formation occurred during the late Pleistocene (ca 32 to 35 ka). Periods of precipitation were influenced strongly by fluctuations in lake level, mostly under climate control, and by related changes in the depth of boiling. During relatively arid phases, meteoric recharge of ground water declines, the lake is low and becomes hypersaline, and the reduced hydrostatic pressure lowers the level of boiling in the plumbing system of the hot springs. Any carbonate precipitation then occurs below the land surface. During humid phases, the dilute meteoric recharge increases, enhancing geothermal circulation, but the rising lake waters, which become relatively dilute, flood most spring vents. Much of the aqueous Ca2+ then precipitates as lacustrine stromatolites on shallow firm substrates, including submerged older travertines. Optimal conditions for subaerial travertine precipitation at Loburu occur when the lake is at intermediate levels, and may be favoured during transitions from humid to drier conditions.
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Article
Although the morphology and dimensions of continental rift basins vary considerably worldwide, one aspect is similar; tectonicall active rifts are bordered on one or both sides by relatively long (tens of kilometres) normal fault systems (termed borde faults) that largely control basin morphology. We compile data constraining the geometry of border faults within the tectonicall active East African Rift system, and evaluate these results with respect to variations in thickness of the elastic lithosphere. Border–fault lengths greater than 75 km occur in regions with deep crustal seismicity and relatively high estimates of effectiv elastic thickness (Te derived from forward and inverse models of gravity and topography data (Te > 25 km). Most East African border faults cross–cut pre–existing structures and basement foliations, although segments o the longest faults (greater than 80 km) reactivate Precambrian shear zones or structural fabrics. From observations in Eas Africa, comparisons with data from the Aegean and Baikal Rifts, and considerations of the rheology of continental lithosphere we propose that the elastic lithosphere determines the length, width and style of faults within East Africa, and perhaps othe continental rifts.
Article
Maps of earthquake epicenters are presented for East Africa, the Gulf of Aden and the Arabian and Red Seas for the period January 1955 to March 1964. Many of these epicenters were located with an accuracy of about 10 km; errors of 100 km or more were common in previous studies. Several tectonic features can be resolved with the new epicenters. In the Gulf of Aden and in the Arabian Sea, the epicenters are confined to narrow linear segments. A large fracture zone that intersects the mid-oceanic ridge near 58°E is clearly delineated with the new epicenters. In the Gulf of Aden regions of high seismic activity are found where NNE-SSW trending faults intersect the median ridge. The seismic activity in the Red Sea is less than that in the Gulf of Aden and the Arabian Sea. A large number of the earthquakes in East Africa are associated with various branches of the rift system. However, many well-recorded earthquakes were not located along the rift valleys. The l~rge areal extent of seismic activity in East Africa differs from the narrow linear pattern of activity that is associated with the mid-oceanic ridge.
Article
Magnitudes of earthquakes determined using traditional methods show systematic deviations, dependent on tectonic setting, from accurate estimates of earthquake size. Magnitudes are overestimated for most of the continental earthquakes, underestimated for mid-oceanic ridges and differences of varying sign are obtained for subduction zones. This is important in evaluating the seismic risk or consideration of effects such as the seismic versus aseismic strain release as seismic moments determined from magnitudes can be wrong by as much as a factor of four.
Article
The seismicity of Africa has been studied for the period January 1963 to December 1970. Epicentres have been relocated using the method of Joint Epicentral Determination (Douglas). Magnitude-frequency graphs suggest that since 1963, earthquakes with body wave magnitude mb ≽ 4.8 are being well determined for the northern half of Africa and with mb ≽ 4.7 for the southern half of Africa. The epicentres have been plotted on new geological maps and for the East African rift system there is a good correlation between seismicity and recent faulting. The study confirms that the western rift is more seismically active than the eastern rift and the eastern rift is most active south of about 3° S. Special studies have been made of the series of events associated with the 1966 March 20 and May 17 events in Uganda, with the filling of the Kariba Dam, with deep mining near Johannesburg and with the Ceres earthquake of 1969 September 29. A comparative study has been made of P wave arrivals for seismic stations near and away from the rift system and an attempt made to map the region of P slowing down associated with the rifting. Fourteen fault plane solutions are now available for Africa including the Red Sea and Gulf of Aden. These suggest that the tensional stress field is directed NE-SW for the Red Sea and Gulf of Aden and WNW-ESE for the regions of rifting in Africa.
Article
SummaryA detailed gravity survey of part of the Gregory Rift Valley in Kenya has shown a positive Bouguer anomaly over the rift floor between 0.25° N and 1.25° S. The anomaly is between 40 and 80 km wide and has an amplitude of 300-600 g.u. (30-60 mgal). Attempts to account for the anomaly with shallow mass-distributions lead to geologically un-reasonable models. The geophysical and geological data are best satisfied if the anomaly is due to a dense intrusion. The required body must be about 20 km wide, and its upper surface must be less than 3 km below the land surface. Other evidence in favour of this interpretation is presented, and it is shown that the models developed here are consistent with the crustal models of the area deduced from long-wavelength gravity anomalies by other workers. An intrusion of the magnitude inferred in this paper represents extreme thinning of the lithosphere below the rift valley.
Article
We have re-examined those earthquakes in Africa south of 20°N, in the period 1900–1930, that appear from instrumental or macroseismic evidence to have a magnitude of 5 3/4 or greater. We identify more than 50 such events, about twice as many as listed by Gutenberg and Richter (1954). We find that the combined use of early instrumental readings and macroseismic information gleaned from previously untapped sources gives the best control of location. Instrumental relocation is difficult because of the lack of stations in Africa and the very uneven global distribution. For the low-gain, medium-period instruments then in use, the best control often comes from using the maximum Airy phase of surface waves. Similarly, there is a lack of sources of macroseismic information, and the simple building practice makes it difficult to assess intensity. We have recalculated magnitude Ms uniformly using the Prague formula. We discuss these problems and show that it is likely that our list is complete only down to magnitude about 6, and that the seismic record for Africa before this century will probably remain incomplete for events of all magnitudes. Of the 54 events in our list 20 are between magnitude 6 and 7, and the largest is the Rukwa earthquake of 1910 in Tanzania (Ms 7.4). The only other African event known to rival it in size is that in southern Sudan on 20 May 1990 (Ms 7.2).
Article
The Kenya Rift Valley, part of the East African Rift System, which extends from Lake Turkana in the north to Lake Magadi in the south, is seismically active. It is surprisingly free of teleseismic events compared to the Western Rift, but experiences considerable microseismic activity releasing the elastic strain energy. This is consistent with the crust having a low tensile strength, probably due to raised geotherms beneath the rift valley arising from lithospheric aattenuation. The microseismicity suggests presently active rifting occurs as far north as 2.5°N. This is consistent with recent seismic reflection results in this region which show deep half-grabens beneath Lake Turkana. There is also a broad zone of seismicity subparalleling the rift and displaced about 150 km to the east. This may be associated with a second culmination of the lithosphere-asthenosphere boundary. In order to obtain a better understanding of the rifting process in Kenya it is suggested that a microseismic study be carried out in the Turkana region, whose near surface 3-dimensional morphology has already been examined via the seismic reflection method.
Article
The seismicity of the East African Rift system within the region bounded by latitudes 2°N and 12°S and longitudes 28°E and 40°E has been studied as far as all available instrumentally based material permits. An earthquake catalogue is presented and the data contained therein are used for tectonophysical investigations, including frequency—magnitude relations and time and space distribution of the seismicity within the region. In addition, earthquake engineering aspects are discussed.
Subukia Valley earthquake, Kenya, MS report & map
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Gilbert, C. D., 1928. Subukia Valley earthquake, Kenya, MS report & map, pp. 12, Files Survey Dept. Kenya, Nairobi.
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Report of the Kenya Land Commission, 1934. Reportfor September 1933, HMSO, London. kchter, C., 1958. Elementary Seismology, Freeman, San Francisco.
Erdbebengeographie, in B. Gutenberg's Handbuch der Geophysik
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Sieberg, A., 1932. Erdbebengeographie, in B. Gutenberg's Handbuch der Geophysik, vol. 4, Berlin.
Information concerning the effects produced in the various districts, Reports, Correspondence, press-cuttings, questionnaire, Archives East Afr Seismicity of the Tanzania region
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Seismicity of the western African Rift System from
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Ndontoni, Z., 1976. Seismicity of the western African Rift System from 1956 to 1970, Individual Studies, vol. 12, pp. 121-140, Intern. Inst. Seism. Earthq. Eng., Tokyo.
The underground water resources of Kenya colony
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Notes taken in the Nakuru area in connection with the Subukia Valley earthquake of 1928
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A re-evaluation of the Subukia Valley earthquake of 1928, Appendix B, UNESCO Seismological and Geophysical Survey Mission to Africa, NS/53/6
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Summary of the field work carried out in 1928
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A catalogue of felt earthquakes in Kenya, 1892-1969
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Survey of the area affected by the Subukia Valley earthquake 1928, MS notes, sketches and photos
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Stardardization of magnitude scales
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M S & Drawing of Field Trip 3-7 Feb, p. 8 , Public Works
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Record of earthquakes experienced at Springs Valley estate between 6.1.28 and 16.428
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Documentation of faulting associated with earthquakes
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Tremors recorded at Entebbe in connection with the Subukia earthquake of 6
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Anonymous, 1928. Tremors recorded at Entebbe in connection with the Subukia earthquake of 6 January 1928 to 12 January, Unpublished Report, p. 6, Geological Survey, Seismological Observatory, Entebbe.
Studies in comparative seismology; East African plateaus and rift valleys
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Willis, B., 1936. Studies in comparative seismology; East African plateaus and rift valleys, Carnegie I n~t. Publ. No. 470, pp. 321-328, Washington, DC.
Some notes on earth tremors in East Africa
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Henderson, J., 1953. Some notes on earth tremors in East Africa, Tech. Note 4, East African Meteorolog. Dept., Nairobi.
Source mechanisms and focal depths of East African earthquakes
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Shudofsky, G. N., 1985. Source mechanisms and focal depths of East African earthquakes, Geophys. 1. R. astr. SOC., 83, 27, 353-379. 563-614.
Examination of the fault line between Baringo Lake and Solai
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Champion, A. M., 1928. Examination of the fault line between Baringo Lake and Solai, Letter-Reports Turkana Provincial Commirsioner, Secretariat Colony & Protectorate of Kenya, Nairobi.
The seismicity of Kenya: procedure for compilation of the map
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Shah, E., 1982. The seismicity of Kenya: procedure for compilation of the map, Tech. Rep. Seirm. Dept., University of Uppsala.
Records of the earthquake of 1928 January 6 from districts in Kenya, MS & sketch maps
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Further note on the African Rift Valley
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Record of earthquake shocks in 1928, MS notes and photos
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Seismicity of Africa
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Seismicity of the western African Rift System from
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Extracts from notes on the Subukia earthquake
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The East African earthquake of 1928 January 6
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Report on the Subukia Valley earthquake of
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Information concerning the effects produced in the various districts, Reports, Correspondence, press-cuttings, questionnaire
  • J Barton
  • Tectonophysics
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Barton, J., 1928. Earthquake of January 1928, Colon. Secrer. Circular No. 5, Information concerning the effects produced in the various districts, Reports, Correspondence, press-cuttings, questionnaire, Archives East Afr. Met. Dept., Nairobi. Bath, M., 1975. Seismicity of the Tanzania region, Tectonophysics, Bowker, H., 1928. Record of earthquake shocks in 1928, MS notes and photos, pp. 16, Survey Dept., Entebbe, Uganda.
Documentation of faulting associated with earthquakes
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  • J. Tchalenko
Earthquake of January 1928
  • J. Barton