Minimum Slip Rate Research Articles

Active tectonics along the Cul-de-Sac – Enriquillo plain and seismic hazard for Port-au-Prince, Haiti

The active faults in Haiti were not well known and no detailed mapping of active fault traces was available before the 2010, M7.0 earthquake. The lack of detailed fault mapping hindered the interpretation of the event and the seismic hazard assessment. Here, using high-resolution LIDAR topography, aerial photographs, bathymetric charts, together with geological data complemented with field observations, we carried out a morphotectonic analysis at a variety of scales. We analyzed the drainage network at the northern front of the Massif de la Selle – Sierra de Bahoruco (MSB) by mapping the fans of the main rivers and their associated drainage basins. We found that the areas of all the fans were compatible with the sizes of their drainage basins, except the paleofan of Port-au-Prince. We interpret that this paleofan has been offset by 7.9±0.3km across the main strike-slip Enriquillo-Plantain Garden Fault (EPGF) and we estimate a minimum horizontal slip rate of ∼3mm/yr over the Pleistocene. Moreover, in the Cul-de-Sac – Enriquillo plain, within which the capital city of Port-au-Prince is located, we mapped numerous NW-SE to WNW-ESE-striking Quaternary folds and thrust faults. Some of the thrust faults are north-dipping and located to the southern front of the Matheux-Neiba Lower Miocene fold. Other thrusts, such as those near Ganthier, Jacquet, Nan Cadastre, and Port-au-Prince, are south-dipping and located along the northern front of the MSB Lower Miocene fold. Previously, Saint Fleur et al. (2015) showed that the 2010 Haiti earthquake activated both one of the young thrusts (Lamentin) and the EPGF, consistent with oblique convergence between the Caribbean and the North American plates.

Geodynamic importance of the strike-slip faults at the eastern part of the Anatolian Scholle: Inferences from the uplift and slip rate of the Malatya Fault (Malatya-Ovacık Fault Zone, eastern Turkey)

Controversy remains over when present-day configuration of the Anatolia boundary faults came into existence, and the issue of what are the driving forces of the Anatolian westward motion. The NW-striking dextral and NE-striking sinistral second-order strike-slip faults at the eastern part of the Anatolian Scholle play a crucial role within these long-lasting discussions, and the NE-striking sinistral Malatya–Ovacık Fault Zone (MOFZ) is particularly important in this ongoing debate. Although the MOFZ is defined as one of the intra-plate structures, it has been proposed that it was an inter-plate fault between the Anatolia and Arabian plates from the latest Miocene to mid-Pliocene and that it has been inactive during the last ca. 3.5 Ma. This study provides results from the first morphochronology-based uplift and slip rate estimates on the Malatya Fault within the southern section of the MOFZ. The cosmogenic isochron burial and cosmogenic depth burial of ages from the sinistrally offset Tohma River remnant terraces enabled us to calculate 1.0 ± 0.01 and 1.12 ± 0.01 mm/yr minimum and maximum horizontal slip rates, respectively, for the last 1.4 ± 0.1 Ma. Furthermore, we conclude that the 96 ± 11 m/Ma mean uplift has been driven by the Malatya Fault. Integrated interpretation of the findings of this study and available data on both the MOFZ and other strike-slip faults at the eastern part of the Anatolian Scholle support the hypothesis that they are plate-boundary related active deformation belts that originated from paleotectonic structures during the tectonic escape of the Anatolian Scholle.

The episodic rise, net growing rate and kinematics of radial faults of the Salinas de Oro diapir using paleoseismological techniques (NE Spain). Salt upwelling versus karstic subsidence

This paper explores the use of trenching and paleoseismological techniques to determine salt flow rates into a salt diapir for the first time in the literature. The Salinas de Oro diapir, located in the northern Spanish Pyrenees, is an oval-shaped Triassic-salt stock that extends vertically for ≫7 km down to the Paleozoic basement. Salt dissolution subsidence and diapir growth are coeval active processes. Karstification is responsible for the development of large sinkholes, a thick caprock and the monocline folding and ring faulting of the Cretaceous and Early Tertiary limestone rim. The evaporite karstic aquifer discharges high-concentration water of up to 137 g/l of total dissolved solids and a conductivity over 200 mS/cm into the Salado Creek, which drains the diapir top. The salinity monitoring of this drainage provides a maximum karstic surface lowering rate of around 2.8 mm/yr. Salt upwelling has caused the 150 m uplift of the annular limestone escarpment and the development of ≫3000 m long radial grabens with up to 90 m of vertical offset that disrupt drainages, displace Quaternary deposits and overprint concentric faults. The 260 cm drag folding of lacustrine facies exposed in a 42 m long and 6.5 m deep trench due to the creep motion of the western radial fault of Azanza Graben yielded a minimum short-term uplift slip rate of 1.75 mm/yr for the last 1485 years and provides a minimum salt supply rate of over 5 mm/yr considering karstification lowering. This value that is several hundred times higher than average, evidence the discontinuous growth of Salinas de Oro diapir.

Dextral, normal, and sinistral faulting across the eastern California shear zone–Mina deflection transition, California-Nevada, USA

AbstractStrike-slip faults commonly include extensional and contractional bends and stepovers, whereas rotational stepovers are less common. The Volcanic Tableland, Black Mountain, and River Spring areas (California and Nevada, USA) (hereafter referred to as the VBR region) straddle the transition from the dominantly NW-striking dextral faults that define the northwestern part of the eastern California shear zone into a rotational stepover characterized by dominantly NE-striking sinistral faults that define the southwestern Mina deflection. New detailed geologic mapping, structural studies, and 40Ar/39Ar geochronology across the VBR region allow us to calculate Pliocene to Pleistocene fault slip rates and test predictions for the kinematics of fault slip transfer into this rotational stepover. In the VBR, Mesozoic basement is nonconformably overlain by a Miocene sequence of rhyolite, dacite, and andesite volcanic rocks that yield 40Ar/39Ar ages between 22.878 ± 0.051 Ma and 11.399 ± 0.041 Ma. Miocene rocks are unconformably overlain by an extensive sequence of Pliocene basalt and andesite lava flows and cinder cones that yield 40Ar/39Ar ages between 3.606 ± 0.060 Ma and 2.996 ± 0.027 Ma. The Pliocene sequence is, in turn, unconformably overlain by Quaternary tuffs and sedimentary rocks. This sequence of rocks is cut by NS- to NW-striking normal faults across the Volcanic Tableland that transition northward into NS-striking normal faults across the Black Mountain area and that, in turn, transition northward into NW-striking dextral and NE-striking sinistral faults in the River Spring area. A range of geologic markers were used to measure offset across the faults in the VBR, and combined with the age of the markers, yield minimum ∼EW-extension rates of ∼0.5 mm/yr across the Volcanic Tableland and Black Mountain regions, and minimum NW-dextral slip and NE-sinistral slip rates of ∼0.7 and ∼0.3 mm/yr, respectively, across the River Spring region. In the River Spring area, our preferred minimum dextral slip and sinistral slip rates are 0.8–0.9 mm/yr and 0.7–0.9 mm/yr, respectively. We propose three kinematic fault slip models, two irrotational and one rotational, whereby the VBR region transfers a portion of dextral Owens Valley fault slip northwestward into the Mina deflection. In irrotational model 1, Owens Valley fault slip is partitioned into two components, one northeastward onto the White Mountain fault zone and one northwestward into the Volcanic Tableland. Slip from the two zones is then transferred northward into the southwestern Mina deflection. In irrotational model 2, Owens Valley fault slip is partitioned into three components, with the third component partitioned west-northwest onto the Sierra Nevada frontal fault zone. In the rotational model, predicted sinistral slip rates across the southwestern Mina deflection are at least 115% greater than our observed minimum slip rates, implying our minimum observed rates underestimate true sinistral slip rates. A comparison of summed geologic fault slip rates, parallel to motion of the Sierra Nevada block relative to the central Great Basin, from the Sierra Nevada northeastward across the VBR region and into western Nevada are the same as geodetic rates, if our assumptions about the geologic slip rate across the dextral White Mountain fault zone is correct.

Geomorphic evidence for the geometry and slip rate of a young, low-angle thrust fault: Implications for hazard assessment and fault interaction in complex tectonic environments

We present surface evidence and displacement rates for a young, active, low-angle (∼20°) reverse thrust fault in close proximity to major population centers in southern California (USA), the Southern San Cayetano fault (SSCF). Active faulting along the northern flank of the Santa Clara River Valley displaces young landforms, such as late Quaternary river terraces and alluvial fans. Geomorphic strain markers are examined using field mapping, high-resolution lidar topographic data, 10Be surface exposure dating, and subsurface well data to provide evidence for a young, active SSCF along the northern flank of the Santa Clara River Valley. Displacement rates for the SSCF are calculated over 103–104 yr timescales with maximum slip rates for the central SSCF of 1.9+1.0−0.5 mm yr−1 between ∼19–7 ka and minimum slip rates of 1.3+0.5−0.3 mm yr−1 since ∼7 ka. Uplift rates for the central SSCF have not varied significantly over the last ∼58 ka, with a maximum value of 1.7+0.9−0.6 mm yr−1 for the interval ∼58–19 ka, and a minimum value of 1.2±0.3 mm yr−1 since ∼7 ka. The SSCF is interpreted as a young, active structure with onset of activity at some time after ∼58 ka. The geometry for the SSCF presented here, with a ∼20° north dip in the subsurface, is the first interpretation of the SSCF based on geological field data. Our new interpretation is significantly different from the previously proposed model-derived geometry, which dips more steeply at 45–60° and intersects the surface in the middle of the Santa Clara River Valley. We suggest that the SSCF may rupture in tandem with the main San Cayetano fault. Additionally, the SSCF could potentially act as a rupture pathway between the Ventura and San Cayetano faults in large-magnitude, multi-fault earthquakes in southern California. However, given structural complexities, including significant changes in dip and varying Holocene displacement rates along strike, further work is required to examine the possible mechanism, likelihood, and frequency of potential through-going ruptures between the Ventura and San Cayetano faults. Confirmation of the SSCF in a previously well-studied area, such as southern California, demonstrates that identification of young faults is critical for accurate seismic hazard assessment. We suggest that many young, active faults remain undetected in other structurally complex and tectonically active regions globally, and that significant seismic hazards can be overlooked.

Evidence for Surface Faulting Earthquakes on the Montereale Fault System (Abruzzi Apennines, Central Italy)

AbstractWe conducted paleoseismic studies along the Montereale fault system (MFS; central Italy). The MFS shows geomorphological evidence of Late Quaternary activity and falls within the highest seismic hazard zone of central Apennines, between the epicentral areas of two recent earthquake sequences: 2009 L'Aquila and 2016–2017 central Italy. We excavated two trenches along the San Giovanni fault splay of the system, one intercepting the N140° striking bedrock main fault plane and the other cutting two subparallel fault scarps on the colluvial/alluvial deposits on the fault hanging wall. Excavations revealed repeated fault reactivation with surface faulting in prehistorical and historical times. We recognized and dated seven events in the last 26 kyr. The most recent ground‐rupturing event (evb1) possibly occurred 650–1,820 AD, consistent with one of the three main shocks that struck the area in 1,703 AD. A previous event (evb2) occurred between 5,330 bc and 730 bc, while older events occurred at 6,590–5,440 bc (evb3), 9,770–6,630 bc (evb4), and 16,860–13,480 bc (evb5). We documented two older displacement events (evb7 and evb6) between 23,780 bc and 16,850 bc. The minimum vertical slip rate at the trench site in the last 28–24 kyr is 0.3–0.4 mm/year. The inferred average recurrence interval for surface‐faulting events along the MFS is no longer than ~4 kyr. Based on the surface fault length ranging between 12 and 20 km, earthquakes with ≥M 6.0 are possible for the MFS. The MFS is an independent earthquake source, and its paleoseismic data are fully comparable with those known for faults in central Apennines.

Late Holocene slip rate and average recurrence interval of great earthquakes on the Shangzhi segment of the Yilan‐Yitong Fault Zone, northeastern China: Constraints from paleo‐earthquakes and historical written records

AbstractThe Yilan‐Yitong Fault Zone (YYFZ) is considered to be the key branch of the Tancheng‐Lujiang Fault Zone (TLFZ) in northeastern China. Although the Mesozoic and early Cenozoic deformation of the YYFZ has been studied intensively over the past century, few estimates of slip rate and recurrence interval of large earthquakes in the late Quaternary, which are the two most important parameters for understanding the potential seismic hazard of this crucial structure, were obtained. Based on integrated interpretations of high resolution satellite images and detailed geologic and geomorphic mapping, linear landforms were identified, including fault scarps and troughs, along the Shangzhi segment of the YYFZ, which exceeds 25 km in length. Synthesized results of trench excavations and differential GPS measurements of terrace surfaces indicate that two events (E1, E2) occurred along the Shangzhi segment during the late Holocene, which resulted in 3.2 ±0.1 m of total vertical co‐seismic displacement with clear features of thrust motion. 14C dating of samples suggests that event E1 occurred between 440 ±30 years BP and 180 ±30 years BP and that event E2 occurred between 4 090 ±30 years BP and 3 880 ±30 years BP, which indicates that the minimum vertical slip rate of the Shangzhi segment of the YYFZ has been approximately 0.8 ±0.03 mm/year during the late Holocene. Constraints from paleo events and the slip rate suggest that the average recurrence interval of major earthquakes on the YYFZ is 3 800 ±200 years. Historical documents in Korea show that event E1 possibly corresponds to the earthquake that occurred in AD 1810 (the Qing Dynasty in Chinese history) in the Ningguta area, which had surface‐wave magnitude (Ms) of 6.8–7.5. Studies of kinematics show that the right‐lateral strike‐slip with a reverse component has been dominant along the YYFZ during the late Holocene.

Impact of near-surface fault geometry on secular slip rate assessment derived from uplifted river terraces: implications for convergence accommodation across the frontal thrust in southern Central Bhutan

Vertical velocities obtained from uplifted river terrace dating near mountain fronts are commonly converted into overthrusting slip rates assuming simple geometry of the fault at depth. However, the lack of information on the dip angle of these shallow structures can lead to misinterpretation in the accommodation of convergence, and thus to erroneous conclusions on the transfer of shortening to the emergent thrust faults. Here, to assess the impact of fault geometry, we focus on the eastern Himalayan region in the south Central Bhutan, where the topographic frontal thrust (TFT) has been already documented by GPS, paleoseismic, geomorphic and geological studies. This study is based on high-resolution near-surface geophysical investigations, including electrical resistivity, seismic and gravity measurements. Using a similar stochastic inversion approach for all data sets, new quantitative constraints on both fault geometry and petrophysical parameters are obtained to image shallow depths, in the upper ca. 80 m. The combined results from both surface observations and geophysical measurement provide a TFT geometry that is dipping northwards with a shallow angle at the top (0–5 m), steeply dipping in the middle (5–40 m) and flattening at deeper depths (>40 m). Together, our new constraints on the fault geometry allow us to estimate a minimum overthrusting slip rate of 10 ± 2 mm yr−1, which is only a part of the ca. 17 mm yr−1 GPS convergence. This suggests that, in the study area, significant deformation partitioning on several faults including TFT and the Main Boundary Thrust cannot be ruled out. More importantly, assuming constant slip rate, the obtained dip angle variations lead to uplift rate changes with distance to the TFT. This underlines that taking into account uplift rate from terrace dating only at the front location and assuming a constant dip angle fault geometry based on surface observations may significantly bias the slip rate estimates.

Late Pleistocene slip rate of the northern Qilian Shan frontal thrust, western Hexi Corridor, China

AbstractWe derive a slip rate for a frontal thrust in the western Hexi Corridor along the northern Qilian Shan by combining topographic profiling and 10Be exposure dating. The active Yumen‐Beidahe thrust fault offsets late Pleistocene alluvial‐fan deposits, and a prominent north‐facing scarp is well preserved. To quantify the slip rate, we surveyed the uplifted terraces and sampled quartz‐rich pebbles on terrace surfaces and river channels to determine surface exposure ages and pre‐depositional inheritance. The minimum vertical slip rate of the fault is 0.73 ± 0.09 mm a−1. This represents a horizontal shortening rate of 1.26 ± 0.31 mm a−1 for a fault dip of 30 ± 5°. This estimated slip rate supports the inference made from previous geological and GPS constraints that NNE‐directed shortening across the western Qilian Shan and the Hexi Corridor is distributed on several active faults with a total shortening rate of 4–10 mm a−1.

Fault slip versus slope deformations: Experience from paleoseismic trenches in the region with low slip-rate faults and strong Pleistocene periglacial mass wasting (Bohemian Massif)

Successions of colluvia and loesses covering the faults in the eastern and central parts of the Bohemian Massif were exposed in six trenches (two trenches at each fault, up to 6 m deep) and studied in detail in terms of stratigraphy and deformation. Based on OSL and radiocarbon ages these sediments were dated as Weichselian Early Glacial to Younger Dryas/Holocene (∼120 ka to 11 ka). On the Kosíř fault (NW-SE strike), the offset strata bring evidence for Late Pleistocene oblique slip (normal and strike-slip components) on the main fault plane with minimum slip rate in order of 0.1 mm/a. Although the undisrupted topsoil suggests the absence of significant slip in Holocene and no clear evidence of paleoseismic events was found, this fault structure should be included in seismic hazard assessment. Conversely, on the Hluboká and Diendorf-Boskovice faults (NW-SE and NNE-SSW strikes, respectively), the tectonic slip is contradicted for the last 15–23 ka based on dating of undeformed strata sealing the fault planes. Multiple independent evidence suggests that these two faults were not active in Late Pleistocene at least and their prominent scarps is probably largely due to exhumation by differential denudation. Examples given document the mechanisms of slope-related deformations which may lead to destruction of the records of older faulting and occasionally produce deformation structures resembling the tectonic ones.

Lateral slip rate of Alhama de Murcia fault (SE Iberian Peninsula) based on a morphotectonic analysis: Comparison with paleoseismological data

To define the seismic potential of the left-lateral strike-slip Alhama de Murcia fault (SE Iberian Peninsula), we calculated its slip rate by measuring offset linear features of known age using a morphotectonic analysis. The Lorca-Totana section of the fault yielded a minimum slip rate of 1.0 ± 0.2 mm/a for the past 30 ka, based on a channel whose age is estimated by OSL technique. The minimum left-lateral slip rate of the Goñar-Lorca section is 1.6–1.7 mm/a for the past 200 ka, based on eight offset surface channels, previously mapped alluvial fans dated by TL, and by new U-series dating of pedogenic carbonate. The U-series technique was used here for first time in the Iberian Peninsula to date small amounts (mg) of pedogenic carbonate. According to the newly estimated slip rate values, the Alhama de Murcia fault is one of the most active faults in the Eastern Betics Shear Zone. These values are larger and have fewer uncertainties in comparison with previous slip rates estimations. In the Lorca-Totana section, the new lateral slip rate is compared with a slip rate calculated by means of a paleoseismic study showing good agreement between the values obtained with the two approaches. We encourage the combination of paleoseismology and morphotectonic analysis to obtain reliable slip rates for faults with scarce evidence of late Holocene slip.

Slip rate estimates and slip gradient for the Alpine Fault at Calf Paddock, Maruia River, New Zealand

ABSTRACTThe dextral–reverse Alpine Fault offsets alluvial terraces of the Maruia River at Calf Paddock. RTK‐GPS surveying of the faulted terraces yields detailed measurements of fault slip across terraces T1–T5. Pits were excavated to log the stratigraphy, date the deposits and to thereby constrain fault slip rates for this site. The mean of the four largest dextral offsets across terrace T2 is 12.0 ± 1.3 m. The corresponding mean vertical offset is 1.6 ± 0.6 m. A single charcoal date with an age of 1095–1275 cal yr BP is used to derive minimum dextral and reverse slip rates of 10 ± 2 and 1.3 ± 0.5 mm/yr, respectively. These rates are comparable with others observed nearby at Little and Lost Mary creeks and from previous studies at Calf Paddock. These new late Holocene slip rate estimates highlight the northeastward decrease in strike-slip and dip-slip rate along the northern section of the Alpine Fault as motion is partitioned onto faults of the Marlborough Fault System. The lack of aseismic creep observed across ‘Evison’s wall’ highlights that slip is achieved at Calf Paddock during metre-scale co-seismic displacements.

Diachronous uplift and cooling history of the Menderes core complex, western Anatolia (Turkey), based on new Zircon (U-Th)/He ages

New (U-Th)/He thermochronology data from the syn-extensional granitoids in the central part of the Menderes Massif in western Turkey reveal a minimum slip rate of 12.5km/Myr along the Alasehir detachment (~14° dip angle) and denudation rates between 1.75km/Myr and 3.25km/Myr between 4Ma and 2Ma. These values suggest relatively fast exhumation of the Central sub-massif, associated with cooling rates between 53°C/Myr and 128°C/Myr, which are higher than the estimated footwall cooling rates (60°C/Myr to 120°C/Myr) from the Northern sub-massif. Based on the initial crystallization ages of the syn-extensional granitoid intrusions and their exhumation–related cooling ages, our thermochronological findings suggest that the Central sub-massif in Menderes underwent accelerated uplift and faster exhumation in the latest Cenozoic than the Northern and Southern sub-massifs. This latest doming and rapid extension of the Central sub-massif was associated with the asthenospheric upwelling beneath the region and the related Na-alkaline, Kula volcanism. Our results indicate that the Menderes Massif has had a diachronous uplift and cooling history during its extensional tectonic evolution in the late Cenozoic. Thermal weakening of the young orogenic crust in western Anatolia via both lithospheric and asthenospheric melting episodes and magmatism produced higher than normal geothermal gradients and played a significant role in core complex formation.

Quaternary activity of the range front thrust system in the Longmen Shan piedmont, China, revealed by seismic imaging and growth strata

Reliable estimates of Quaternary or Cenozoic upper crustal shortening in the Longmen Shan fold-and-thrust belt are rare. In this paper, we report on our use of high-resolution 2-D and 3-D seismic reflection profiles at various scales, together with borehole data, to investigate the structural geometry of the Longmen Shan piedmont. The results reveal a thrust system beneath the Longmen Shan, termed the range front thrust system, which consists of the range front blind thrust and its upward splay faults. Moreover, on these faults we identified growth strata that provide an excellent opportunity for assessing the activity of this thrust system. Analyses of the growth strata reveal early to late Pleistocene activity on the range front blind thrust, with minimum dip slip and horizontal shortening rates of 1.1 ± 0.2 mm/yr and ~1 mm/yr. Accordingly, the maximum accumulated slip on the range front blind thrust is calculated to be 7.5 ± 0.3 km in the Longmen Shan. Using the new horizontal shortening rate and other published data, we also estimated that the long-term shortening rate across the Longmen Shan fold-and-thrust belt is 1–3 mm/yr, which is comparable to the short-term GPS rate. The similarity of these rates suggests that the Longmen Shan attained a steady state condition over the past 2 Myr. An additional highlight of our results is that we show Quaternary activity around the Tibetan Plateau to have been nearly synchronous in different regions, including in the Longmen Shan, the Himalayas, the western Kunlun Shan, and the northern Qilian Shan.

A palaeoseismic trench investigation of early Holocene neotectonic faulting along the Kango Fault, southern Cape Fold Belt, South Africa– part II: earthquake parameters

The location and estimated magnitude of a recent surface rupturing earthquake along the eastern part of the Kango Fault traversing the southern Cape Fold Belt, South Africa, is reported here, as well as the minimum recurrence interval and maximum slip rate. These palaeoseismic data are derived from analysis of the mapped logs of an 82 m long, 5 m deep trench excavated across the fault, supported by 12 optically stimulated luminescence (OSL) dates. The tectonic event that formed the observed 2.0 ± 0.06 m free-standing fault scarp at the trench site had a moment magnitude estimated between 6.97 ± 0.01 M and 7.18 ± 0.01 M. According to the recently established Environmental Scale of Intensity, the rupture was devastating (ESI Io = XI). It would have been accompanied by strong ground shaking felt across most of South Africa by the ancestors of the indigenous people groups who experienced the well-known 6.3 M Ceres-Tulbagh event of 29 September 1969 (currently South Africa’s largest and most damaging earthquake). This latter strike-slip event occurred along the western end of the same Ceres-Kango-Baviaanskloof-Coega (CKBC) fault system, but was not accompanied by significant surface deformation. In contrast, the Kango ‘Toorwater’ earthquake reported here displaced the land surface vertically, by over 2 m in places. While the fault scarp extends laterally for at least 84 km, from near the town of De Rust, east of Oudtshoorn, towards the start of the Baviaanskloof in the Eastern Cape, it is unlikely the entire surface rupture length originated during only the most recent event. The palaeoseismic data reported here have contributed to a seismic source characterization model used to update the existing seismotectonic model for South Africa, first established in the mid-1990’s, and revised at various intervals since. The presence of a potentially active, capable, seismogenic structure within a low-seismicity intraplate stable cratonic region suggests that the current 5.3 to 6.3 M m max for the south and south eastern Cape must be reviewed, should these data be included in a revised Probabilistic Sesimic Hazard Assessment (PSHA) for critical structures in the region.

Transition from collision to subduction in Western Greece: the Katouna–Stamna active fault system and regional kinematics

Transition from subduction to collision occurs in Western Greece and is accommodated along the downgoing plate by the Kefalonia right-lateral fault that transfers the Hellenic subduction front to the Apulian collision front. Here we present an active tectonic study of Aitolo-Akarnania (Western Greece) that highlights how such a transition is accommodated in the overriding plate. Based on new multi-scale geomorphic and tectonic observations, we performed an accurate active fault trace mapping in the region, and provide evidence for active normal and left-lateral faulting along the Katouna–Stamna Fault (KSF), a 65-km-long NNW-striking fault system connecting the Amvrakikos Gulf to the Patras Gulf. We further show that the Cenozoic Hellenide thrusts located west of the KSF are no longer active, either in field observation or in GPS data, leading us to propose that the KSF forms the northeastern boundary of a rigid Ionian Islands-Akarnania Block (IAB). Cosmic ray exposure measurements of 10Be and 36Cl were performed on a Quaternary alluvial fan offset along the KSF (~50 m left-lateral offset). A maximum abandonment age of ~12–14 ka for the alluvial fan surface can be determined, giving an estimated KSF minimum geological left-lateral slip rate of ~4 mm year−1, in agreement with high GPS slip rates (~10 mm year−1). Despite this high slip rate, the KSF is characterized by subdued morphological evidence of tectonic activity, a gypsum-breccia bedrock and a low level of seismicity, suggesting a dominantly creeping behavior for this fault. Finally, we discuss how the IAB appears to have been progressively individualized during the Pleistocene (younger than ~1.5 Ma).

In Search of the Source of the 1865–1866 Nicaraguan Earthquakes: Paleoseismic Data from the Cofradía Fault, Managua Graben (Nicaragua)

In 1865–1866, a series of catastrophic earthquakes struck Managua, Nicaragua. We present new data supporting the view that these earthquakes were caused by the seismogenic Cofradia fault. The data were collected at three paleoseismological sites: La Vaqueria (central‐northern area), El Cocal (central area), and Piedra Menuda (located at an antithetic strand of the southern fault segment). Coseismic evidence includes liquefaction features, offset layers, and colluvial wedges dated using radiocarbon ages and pottery fragments, providing relative cultural ages. The minimum event displacement observed at the central site (1 m) and the total length of the mapped geomorphological trace (39 km) are consistent with a maximum expected magnitude around 7. A minimum slip rate between 1.1 and 1.4 mm/yr is obtained from the new data, reinforcing earlier estimates. In line with paleoseismic chronology and compared with a Bayesian OxCal model of the stratigraphic sequence, at least three seismic events have occurred since A.D. 300, the last one occurring after A.D. 790 and the twentieth century. Thus, the earthquakes of 1865–1866 that resulted in surface alterations in the Tipitapa River probably correspond to the last paleoseismic event on the Cofradia fault. This assumption yields a range between 1511 and 1245 yr for the best estimated maximum recurrence interval between the ultimate and penultimate event. Online Material: High‐resolution photomosaics of the walls of the studied trenches.

Repeated large‐magnitude earthquakes in a tectonically active, low‐strain continental interior: The northern Tien Shan, Kyrgyzstan

AbstractThe northern Tien Shan of Kyrgyzstan and Kazakhstan has been affected by a series of major earthquakes in the late 19th and early 20th centuries. To assess the significance of such a pulse of strain release in a continental interior, it is important to analyze and quantify strain release over multiple time scales. We have undertaken paleoseismological investigations at two geomorphically distinct sites (Panfilovkoe and Rot Front) near the Kyrgyz capital Bishkek. Although located near the historic epicenters, both sites were not affected by these earthquakes. Trenching was accompanied by dating stratigraphy and offset surfaces using luminescence, radiocarbon, and 10Be terrestrial cosmogenic nuclide methods. At Rot Front, trenching of a small scarp did not reveal evidence for surface rupture during the last 5000 years. The scarp rather resembles an extensive debris‐flow lobe. At Panfilovkoe, we estimate a Late Pleistocene minimum slip rate of 0.2 ± 0.1 mm/a, averaged over at least two, probably three earthquake cycles. Dip‐slip reverse motion along segmented, moderately steep faults resulted in hanging wall collapse scarps during different events. The most recent earthquake occurred around 3.6 ± 1.3 kyr ago (1σ), with dip‐slip offsets between 1.2 and 1.4 m. We calculate a probabilistic paleomagnitude to be between 6.7 and 7.2, which is in agreement with regional data from the Kyrgyz range. The morphotectonic signals in the northern Tien Shan are a prime example of deformation in a tectonically active intracontinental mountain belt and as such can help understand the longer‐term coevolution of topography and seismogenic processes in similar structural settings worldwide.

Application of UAV Photography to Refining the Slip Rate on the Pyramid Lake Fault Zone, Nevada

The Pyramid Lake fault zone (PLFZ) is a 50‐km‐long, active northwest‐trending right‐lateral fault in the northern Walker Lane, located ∼30 km east of Reno, Nevada. Previous paleoearthquake and slip‐rate studies report that the Pyramid Lake fault has produced four surface‐rupturing paleoearthquakes since 15,475±720 cal B.P., three of which occurred after 8980±260 cal B.P., and has had an average minimum slip rate of 2.6±0.3 mm/yr since the late Pleistocene. These observations imply that coseismic offset for each paleoearthquake averaged 7–9 m, larger than expected from empirical scaling relationships for a strike‐slip fault of its length. To reconcile this discrepancy, we used a small camera‐mounted unmanned aerial vehicle to develop high‐resolution digital elevation models and interpret previously unreported right‐laterally offset geomorphic features along the northern section of the PLFZ. Offset measurements at seven sites range from 8 to 21 m. The ages of displaced features are interpreted from previous lake level and mapping studies of Lake Lahontan. From these observations, slip‐rate estimates at the sites range between 0.5 and 1.6 mm/yr. These lower values of slip rate require that coseismic displacements associated with previously reported paleoearthquakes average only 3–5 m, within the range that would be predicted from empirical scaling relationships of rupture length and coseismic displacement.

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.