Paleoseismological Investigations Research Articles

Can flexural-slip faults related to evaporite dissolution generate hazardous earthquakes? The case of the Grand Hogback monocline of west-central Colorado

A paleoseismological investigation of flexural-slip faults related to interstratal evaporite dissolution suggests that such gravitational structures might have the potential to generate earthquakes with damaging magnitude. The Carbondale collapse center, in the southern Rocky Mountains of Colorado, is a morpho-structural depression of ~1200 km 2 where Miocene volcanic rocks are downdropped as much as 1200 m due to interstratal dissolution of halite-bearing evaporites. On the western margin of the collapse center, the debuttressing effect related to active evaporite dissolution drives unfolding of the steeply dipping late Laramide Grand Hogback monocline, accompanied by displacement on bedding-parallel faults. These flexural-slip faults rupture unconformable Miocene basalts and Quaternary mantled pediments, generating conspicuous half-graben depressions bounded by antislope fault scarps parallel to the underlying strata of the monocline. Two trenches dug across flexural-slip fault scarps developed in each stratigraphic marker (basalt cap, mantled pediment) revealed unexpected evidence of multiple late Quaternary faulting events (e.g., faulted colluvial wedge, sharp unconformities), with displacement-per-event values of ≥1 m. Three faulting events were inferred from the trench dug in the pediment ( w ) around 6.

Deep-seated gravitational slope deformation, large-scale rock failure, and active normal faulting along Mt. Morrone (Sulmona basin, Central Italy): Geomorphological and paleoseismological analyses

Active faulting is one of the main factors that induce deep-seated gravitational slope deformations (DGSDs). In this study, we investigate the relationships between the tectonic activity of the NW–SE normal fault system along Mt. Morrone, central Apennines, Italy, and the evolution of the associated sackung-type DGSD. The fault system is considered to be the source of M 6.5–7 earthquakes. Our investigations have revealed that the DGSD began to affect the Mt. Morrone SW slope after the Early Pleistocene. This was due to the progressive slope instability arising from the onset of the younger western fault, with the older eastern fault acting as the preferred sliding zone. Paleoseismological investigations based on the excavation of slope deposits across gravitational troughs revealed that the DGSD was also responsible for the displacement of Late Pleistocene–Holocene sediments accumulated in the sackung troughs. Moreover, we observed that the investigated DGSD can evolve into large-scale rock slides. Therefore, as well as active normal faulting, the DGSD should be considered as the source of a further geological hazard. Overall, our approach can be successfully applied to other contexts where active normal faults control the inception and evolution of a DGSD.

Recurrence of strong earthquakes in the active Hovd Fault Zone, Mongolian Altay

A geological, geomorphic, and paleoseismological study of paleoseismodislocation systems has been carried out. Numerous indications of prehistoric strong earthquakes expressed in sediments deposited in the fault-line region due to the formation of transient barrier lakes were revealed in the active Hovd Fault Zone on the eastern slope of the Mongolian Altay during a paleoseismological study of young near-surface sediments in trenches and dug pits. The paleosoils buried by these sediments were radiocarbon-dated. It has been established that the strongest earthquakes with magnitudes of ∼8, which took place here 7200, 6600, 1500, and later than 700 years ago, substantially changed the topography in the epicentral zone. By analogy with the results of paleoseismological investigations performed in the Gorny Altay and the western Mongolian Altay, the data obtained shed light on the recurrence of the strongest seismic events in the Great Altay.

Integrated near surface geophysics across the active Mount Marzano Fault System (southern Italy): seismogenic hints

Here, we describe an original geophysical multi-method approach applied to the Mount Marzano Fault System. This is one of the most hazardous seismogenic faults of the Apennines (Irpinia, southern Italy), and it was responsible for the 1980, Mw 6.9, earthquake, along with many others before. We carried out electrical resistivity tomography (ERT), ground penetrating radar (GPR) measurements, and horizontal-to-vertical spectral ratio (HVSR) microtremor analysis along several common transects designed across the potential and/or certain fault traces. The data obtained from these non-invasive, inexpensive, expeditious methods mutually integrate with and complement each other, providing a valuable subsurface image of the near surface fault architecture. ERT depicts the general shallow image of the fault zone and of the fault-controlled sedimentary basin, with the depth of the buried bedrock cross-correlated through ambient-noise HVSR results. GPR delineates the very shallow geometry of the fault and of the associated deformation. Coupled with previous paleoseismological studies, these data allow the evaluation of some fault parameters and the precise locating of the fault trace, to aid future paleoseismological investigations aimed at seismic risk reduction programs.

Outlook for use of laser scanning in paleoseismological studies in Siberia

A large-scale survey to select a path for the East Siberia-Pacific Ocean (ESPO) pipeline has shown the urgency and high demand for detailed paleoseismological investigations using a new technology based on laser scanning. The laser scanning results allowed us not only to identify and thoroughly map numerous fault-line scarps, but also to estimate their morphometric parameters (height, steepness) and distinguish scarps that were formed as a result of single or multifold seismogenic displacements. The high efficiency of the laser scanning results makes it necessary to involve this tool in paleoseismological investigations.

Numerical simulation of dynamic mechanisms of the 2008 Wenchuan Ms8.0 earthquake: implications for earthquake prediction

The sudden and unexpected Wenchuan earthquake (Ms = 8.0) occurred on the Longmen Shan Fault, causing a large number of casualties and huge property loss. Almost no definite precursors were reported prior to this event by Chinese scientists, who made a first successful prediction of the 1975 Haicheng earthquake (M = 7.3) in China. Does the unsuccessful prediction of the Wenchuan earthquake mean earthquake prediction is inherently impossible? In order to answer this question, the paper simulated inter- and co-seismic deformation, and recurrence of strong earthquakes associated with the Longmen Shan listric thrust fault by means of viscoelastic finite element method. The modeling results show that the computed interseismic strain accumulation in the lower crust beneath the Eastern Tibet is much faster than that in the other regions. In particular, the elastic strain energy density rate accumulates very rapid in and around the Longmen Shan fault in the depth above ~25 km that may explain why the great Wenchuan earthquake occurs in the region of such a slow surface deformation rate. The modeled coseismic displacements around the fault are consistent with surface rupture, aftershock distribution, and GPS measurement. Also, the model displays the slip history on the Longmen Shan fault, implying that the average earthquake recurrence interval on the Longmen Shan fault is very long, 3,300 years, which is in good agreement with the observed by paleoseismological investigations and estimates by other methods. Moreover, the model results indicate that the future earthquake could be evaluated based on numerical computation, rather than on precursors or on statistics. Numerical earthquake prediction (NEP) seems to be a promising avenue to a successful prediction, which will play an important part in natural hazard mitigation. NEP is difficult but possible, which needs well supporting.

Paleoseismological investigations on a slow-moving active fault in central Anatolia, Tecer Fault, Sivas

Tecer Fault is a N60˚-70˚E-trending, left-lateral, strike-slip fault to the south of the town of Sivas, Turkey. This fault is considered as the eastward continuation of Deliler Fault, which was classified as a probably active, left-lateral fault on the Active Fault Map of Turkey. We investigated the field characteristics and paleoseismic history of Tecer Fault in detail. After analyzing aerial photographs and satellite images, we mapped the exact fault trace on a 1/25,000 scale topographic map, between the towns of Deliilyas in southwest and Bogazdere in northeast. Tecer Fault is characterized by morphological features such as offset streams and gullies, linear depressions and scarps, and elongated hills. Four paleoseismological trenches were excavated on the northeastern extent of the fault. Two past earthquakes were identified in these trenches, and the dates of the collected charcoal samples suggested that the first of these earthquakes occurred about 8000 B.C. while the more recent event took place around 3500 B.C. Field observations and paleoseismic data indicate that Tecer Fault is an active, pure sinistral, strike-slip fault, and that there is about a 4500 years time span between the two earthquakes. It is also clear that there has not been any surface-ruptured faulting over the last 800 years. Compared with the earthquake characteristics of other strike-slip fault zones in Turkey in terms of time-slip relations, the slip rate can be estimated as about 1 mm/yr on Tecer Fault.

Historical earthquakes and variable kinematic behaviour of the 2009 L'Aquila seismic event (central Italy) causative fault, revealed by paleoseismological investigations

Paleoseismological investigations has been carried out along the causative fault of the Mw 6.1 2009 L'Aquila earthquake. We dug three trenches in two different sites on the Paganica fault to understand its Holocene kinematic behavior. Paleoseismological results show a minimum of three faulting events preceding the 2009 earthquake. The most recent recognized event occurred after 1420–1610A.D., tentatively attributed to the 1703 (February 2) seismic shock; a previous event happened between 1st–2nd century A.D. and 7th–9th century A.D., probably in the interval 4th–6th century A.D., while another identified previous event occurred prior to 1st–2nd century A.D. The observed minimum displacement per event varies from 0.25m to 0.50m consistent with a fault length of about 22–27km, from which we infer a maximum expected Mw 6.6–6.7. This implies that the Paganica fault may occasionally rupture during 2009-like seismic events as well as during larger earthquakes, activating contemporaneously to nearby active faults kinematically and structurally linked to the Paganica fault. More specifically, on the basis of our data and of recent paleoseismologic studies, we hypothesize a linkage of the investigated fault with the Upper Aterno Valley Fault system. In this perspective, the Paganica Fault can rupture as an independent fault segment with recurrence interval of about 3 hundreds of years and together with the Upper Aterno Valley Fault system with recurrence interval that varies between 1000 and 1500years.

Coseismic subsidence recorded in the Holocene delta sequence of the Nobi plain at the footwall side of the Yoro fault

The Gafsa fault is the longest and most active structure of the fold-and-thrust belt achieving southeastward propagation of the Atlas belt of Eastern North Africa onto the Saharan platform. The Gafsa fault is a 75-km long dextral-oblique basement fault ramp that poses a sizable challenge in earthquake hazard assessment because the post-Paleozoic sedimentary cover is decoupled from its basement above the basement fault. In this study, we combine seismic lines interpretation, tectonic geomorphology and paleoseismological investigations to assess the level of seismic hazard of this fault and evaluate its role in the geodynamic framework of the Central Mediterranean. We show that despite a moderate instrumental and historical seismicity, the fault has produced M ≥ 6 earthquakes with a return period of ca. 500–5000 years during the Late Quaternary. The latest large event having produced a surface rupture on the fault occurred around 8000 yr BP, suggesting an M ≥ 6 earthquake is overdue on the fault. The fault has a minimum reverse component of slip rate of 0.21–0.34 mm/yr over the past 50 Ka. The occurrence of M ≥ 7 paleoearthquakes on the fault may be suspected but not established. Such very strong earthquakes would require transient coseismic linkage of the buried basement fault with the overlying listric fault ramping off the décollement layer. The level of seismic hazard may be underestimated on the Gafsa fault. Indeed, given the geometry of the basement–cover fault system, a number of earthquakes generated in the basement would have led to coseismic surface folding instead of to surface rupture. The Gafsa fault is a major structure accommodating eastward extrusion / spreading of the Atlas belt onto the Saharan and Pelagian plateforms above the retreating Ionian lithospheric slab.

Arshan palaeoseismic feature of the Tunka fault (Baikal rift zone, Russia)

The traditional concept of the rift development of flank depressions in the Baikal rift zone is now doubted in view of some indicators for compression deformations identified by the seismogeological and geodetic methods. Besides, the paleoseismological investigations revealed seismogenic strike-slips and reverse faults in the Tunka fault zone that is a major structure-controlling element of the Tunka rift depression. However, a detailed study of the upslope-facing scarp in the Arshan paleoseismogenic structure zone has shown that its formation might be due to rift mechanism of basin formation. Age estimation has been made for the previously unknown pre-historic earthquake whose epicentral area coincides with the western flank of the Arshan paleoseismogenic structure. Judging from previously determined ages of paleoearthquakes, the mean recurrence period for faulting events on the central Tunka fault is 2780–3440years.

Geological evidence of pre-2012 seismic events, Emilia-Romagna, Italy

<p>In May 2012, two moderate (-to-strong) earthquakes that were associated with a noticeable aftershock sequence affected the eastern sector of the Po Plain, Italy, in correspondence with a buried portion of the Apennines thrust belt. The Provinces of Ferrara, Modena and Bologna (Emilia Romagna Region), Mantua (Lombardy Region), and Rovigo (Veneto Region) were affected to different extents. The first shock (Ml 5.9 according to the Istituto Nazionale di Geofisica e Vulcanologia (INGV; National Institute of Geophysics and Volcanology), and Mw 6.1 according to the US Geological Service) occurred on May 20, 2012, at 2:03 a.m. (GMT); this was the strongest of the sequence, and it was followed by several aftershocks (up to Ml 5.1). This first event produced secondary ground deformation effects, which were mainly associated with liquefaction phenomena that were spread across the broader epicentral region, and particularly in the western sector of the Ferrara Province [Papathanassiou et al. 2012, this volume]. A few weeks after the earthquake, a paleoseismological trench was excavated south of San Carlo village, where earthquake-induced effects were widely documented. This report presents the preliminary results of the paleoseismological investigation and documents the occurrence in the same area of paleo-events older than the May 2012 earthquakes. […]</p>

Late Pleistocene and Holocene large magnitude earthquakes along Himalayan Frontal Thrust in the Central Seismic Gap in NW Himalaya, Kala Amb, India

The Himalayan Frontal Thrust (HFT) forms the southernmost active tectonic mountain front of the Himalaya. To understand the ongoing tectonics further, paleoseismological study has been carried out in the vicinity of the HFT system along the Himalayan Front near Kala Amb, India. The trench excavation survey conducted across an explicit surface exposure of the HFT exhibits two distinct faults considered to be associated with the reactivation of the HFT where the Middle Siwalik rocks (Late Miocene) have repeatedly thrust over the Late Pleistocene and Holocene sediments. Presence of large-sized coseismically induced sand-injection feature and its disposition recognized in the trench also suggest occurrence of large magnitude earthquakes in this region. An uplifted and upwarped strath terrace, 3 to 5m thick alluvium, resting over the 15m high Middle Siwaliks, abruptly truncated by the HFT indicates its latest activity. Optically Stimulated Luminescence dating techniques were employed to constrain the chronology of events. The long term slip rate of the abandoned terraces due to the activity of the HFT is estimated to be 3.4mm/yr or greater since Late Holocene. The paleoseismological investigations have provided unambiguous evidences of at least two large magnitude earthquakes occurred in this region where an earthquake with 12m or larger surface displacement and magnitude 7.5 or greater hit this region in the period between 29.3ka and 17ka in the Late Pleistocene and another great earthquake recurred with 20–22m or more surface displacement and magnitude of 7.7 or greater between 5.8ka and 2ka in the Holocene. The present study is the first time report of multiple large magnitude paleoearthquakes in the northwestern part of the Frontal Himalaya during Late Pleistocene and Holocene. The repeated reactivation of HFT substantiates high seismic potential of the Frontal Himalaya and calls for more extensive study of paleoearthquakes of this vastly populous mountainous region.

Evidence for surface faulting events along the Paganica fault prior to the 6 April 2009 L'Aquila earthquake (central Italy)

[1] We performed paleoseismological investigations at four sites across the normal Paganica fault (PF) (source of the 2009 Mw 6.3 L'Aquila earthquake), with the goal of reconstructing the rupture history and of contributing to the evaluation of the maximum event expected along the PF. We recognized five distinct surface faulting earthquakes (including the 2009) in the trenches. The age of the penultimate event is consistent with the 1461 earthquake; the third event back occurred around 1000 AD. The two oldest events have larger uncertainties and occurred in the interval 760 BC–670 AD and 2900–760 BC, respectively. The along-strike vertical displacement for each paleoevent has a limited variability consistently with the fairly homogeneous slip observed in 2009 along the northern part of the rupture. Conversely, the throws change between distinct events and range between 0.15 m in 2009 (maximum estimate) and close to 0.4 (lower bound estimate) in earlier events. These paleorecords and the high fault escarpments imply that earthquakes larger than 2009 occurred on the PF, with implications for the level of hazard. Recurrence intervals also reflect a change with time, the average interval before ∼1000 AD is longer compared to that after this date. Two events occurred in the 2000–4000 years preceding ∼1000 AD, while three events occurred since ∼1000 AD. The age uncertainties affecting the interpreted events prevent the evaluation of a unique value for interevent interval; the older events appear closely spaced in time or far apart depending on the upper or lower boundary of the age interval. We tentatively assign an average interevent time of ∼500 years for the three youngest events, whereas the time elapsed between the previous ones could be larger, in the order of 1000–2000 years. We calculate a late Pleistocene dip-slip rate for the PF of 0.2–0.4 mm/yr, consistent with 0.25–0.5 mm/yr for the early Pleistocene. Using age and throw of individual events, we calculate a similar late Holocene average dip-slip rate of ∼0.3–0.4 mm/yr. This suggests that the portion of the PF where the 2009 continuous surface faulting occurred has fairly a constant average slip release since late Pleistocene. Finally, we discuss different rupture scenarios and alternative models of occurrence compatible with our data and their variability.

The 2009 L'Aquila earthquake (Italy): What's next in the region? Hints from stress diffusion analysis and normal fault activity

This work aims at defining the contribution, in terms of earthquake probability assessment, of the integration of Coulomb stress diffusion analysis related to an earthquake with geological studies on fault activities, investigating the case of the April 6, 2009, L'Aquila (central Italy) earthquake (Mw 6.3). The analysis of the Coulomb stress diffusion induced by this earthquake has revealed a stress increase along two poorly-investigated active normal faults in the Apennines: the Subequana fault and the Middle Aterno Valley fault. No strong seismic events have been attributed to these tectonic structures over the past 800–1000yr, and they have therefore been considered as probable seismic gaps. Geological and paleoseismological investigations have since indicated that these tectonic structures belong to the same 25–30-km-long fault system that ruptured twice during the late Holocene. The last activation occurred between the 4th–1st century B.C. and the past millennium (probably during the 2nd–1st century B.C.), with the penultimate between 6381±30BP and 3511±37BP. The data obtained indicate that this fault system might rupture in up to magnitude 6.8 earthquakes and that the 2009 seismic event has brought these tectonic structures about 200yr closer to failure.

Revisiting the earthquake sources in the Himalaya: Perspectives on past seismicity

The ~ 2500 km-long Himalaya plate boundary experienced three great earthquakes during the past century, but none of them generated any surface rupture. The segments between the 1905–1934 and the 1897–1950 sources, known as the central and Assam seismic gaps respectively, have long been considered holding potential for future great earthquakes. This paper addresses two issues concerning earthquakes along the Himalaya plate boundary. One, the absence of surface rupture associated with the great earthquakes, vis-à-vis the purported large slip observed from paleoseismological investigations and two, the current understanding of the status of the seismic gaps in the Central Himalaya and Assam, in view of the paleoseismological and historical data being gathered. We suggest that the ruptures of earthquakes nucleating on the basal detachment are likely to be restricted by the crustal ramps and thus generate no surface ruptures, whereas those originating on the faults within the wedges promote upward propagation of rupture and displacement, as observed during the 2005 Kashmir earthquake, that showed a peak offset of 7 m. The occasional reactivation of these thrust systems within the duplex zone may also be responsible for the observed temporal and spatial clustering of earthquakes in the Himalaya. Observations presented in this paper suggest that the last major earthquake in the Central Himalaya occurred during AD 1119–1292, rather than in 1505, as suggested in some previous studies and thus the gap in the plate boundary events is real. As for the Northwestern Himalaya, seismically generated sedimentary features identified in the 1950 source region are generally younger than AD 1400 and evidence for older events is sketchy. The 1897 Shillong earthquake is not a décollement event and its predecessor is probably ~ 1000 years old. Compared to the Central Himalaya, the Assam Gap is a corridor of low seismicity between two tectonically independent seismogenic source zones that cannot be considered as a seismic gap in the conventional sense.

Guest editorial: L’Aquila earthquake: seismic sequence of 6th April 2009, Abruzzo, Italy

On April 6th 2009, a Mw 6.3 earthquake struck the town of L’Aquila and surroundings in Central Italy, causing 308 deaths. The mainshock was preceded by a swarm which started at the end of 2008; the largest earthquake of this swarm had magnitude 4.1 and occurred on March 30th 2009. The mainshock was followed by thousands of aftershocks, some of which with magnitudes greater than Mw 5.0 (Chiarabba et al. 2009). Since the first hours after the mainshock, the national and international scientific community approached the L’Aquila earthquake from various points of view and using different methodologies, in order to achieve a complete understanding of the phenomenon as soon as possible. After more than one year from the earthquake, it is possible to outline some first research milestones reached through the work carried out by the scientific community since that 6th April and some future possible advances, resulting from more in depth studies exploiting data collected after the event. From a seismotectonic point of view, in addition to the classical geological, geomorphological and paleoseismological investigations (e.g., EMERGEO Working Group 2009), which have reached a very high level of accuracy, the L’Aquila earthquake has been especially characterized by a massive and mature use of technology. Fully developed seismic and strong motion national networks have provided huge, highquality datasets available in quasi-real time, allowing for a rapid response of the seismologists’ community, also suitable for operational and decisional purposes. Moreover, a great contribution has come from the satellite technology. Apart from the GPS-based estimates of coseismic ground displacement, which are well-established (Anzidei et al. 2009), in this case also interferometric investigations have reached a very high level of accuracy for the Italian

Active oblique ramp faulting in the Southern Tunisian Atlas

The Gafsa fault is the longest and most active structure of the fold-and-thrust belt achieving southeastward propagation of the Atlas belt of Eastern North Africa onto the Saharan platform. The Gafsa fault is a 75-km long dextral-oblique basement fault ramp that poses a sizable challenge in earthquake hazard assessment because the post-Paleozoic sedimentary cover is decoupled from its basement above the basement fault. In this study, we combine seismic lines interpretation, tectonic geomorphology and paleoseismological investigations to assess the level of seismic hazard of this fault and evaluate its role in the geodynamic framework of the Central Mediterranean. We show that despite a moderate instrumental and historical seismicity, the fault has produced M ≥ 6 earthquakes with a return period of ca. 500–5000 years during the Late Quaternary. The latest large event having produced a surface rupture on the fault occurred around 8000 yr BP, suggesting an M ≥ 6 earthquake is overdue on the fault. The fault has a minimum reverse component of slip rate of 0.21–0.34 mm/yr over the past 50 Ka. The occurrence of M ≥ 7 paleoearthquakes on the fault may be suspected but not established. Such very strong earthquakes would require transient coseismic linkage of the buried basement fault with the overlying listric fault ramping off the décollement layer. The level of seismic hazard may be underestimated on the Gafsa fault. Indeed, given the geometry of the basement–cover fault system, a number of earthquakes generated in the basement would have led to coseismic surface folding instead of to surface rupture. The Gafsa fault is a major structure accommodating eastward extrusion / spreading of the Atlas belt onto the Saharan and Pelagian plateforms above the retreating Ionian lithospheric slab.

Detecting young, slow‐slipping active faults by geologic and multidisciplinary high‐resolution geophysical investigations: A case study from the Apennine seismic belt, Italy

The Southern Apennines range of Italy presents significant challenges for active fault detection due to the complex structural setting inherited from previous contractional tectonics, coupled to very recent (Middle Pleistocene) onset and slow slip rates of active normal faults. As shown by the Irpinia Fault, source of a M6.9 earthquake in 1980, major faults might have small cumulative deformation and subtle geomorphic expression. A multidisciplinary study including morphological‐tectonic, paleoseismological, and geophysical investigations has been carried out across the extensional Monte Aquila Fault, a poorly known structure that, similarly to the Irpinia Fault, runs across a ridge and is weakly expressed at the surface by small scarps/warps. The joint application of shallow reflection profiling, seismic and electrical resistivity tomography, and physical logging of cored sediments has proved crucial for proper fault detection because performance of each technique was markedly different and very dependent on local geologic conditions. Geophysical data clearly (1) image a fault zone beneath suspected warps, (2) constrain the cumulative vertical slip to only 25–30 m, (3) delineate colluvial packages suggesting coseismic surface faulting episodes. Paleoseismological investigations document at least three deformation events during the very Late Pleistocene (<20 ka) and Holocene. The clue to surface‐rupturing episodes, together with the fault dimension inferred by geological mapping and microseismicity distribution, suggest a seismogenic potential of M6.3. Our study provides the second documentation of a major active fault in southern Italy that, as the Irpinia Fault, does not bound a large intermontane basin, but it is nested within the mountain range, weakly modifying the landscape. This demonstrates that standard geomorphological approaches are insufficient to define a proper framework of active faults in this region. More in general, our applications have wide methodological implications for shallow imaging in complex terrains because they clearly illustrate the benefits of combining electrical resistivity and seismic techniques. The proposed multidisciplinary methodology can be effective in regions characterized by young and/or slow slipping active faults.

Left-lateral active deformation along the Mosha–North Tehran fault system (Iran): Morphotectonics and paleoseismological investigations

The Mosha and North Tehran faults correspond to the nearest seismic sources for the northern part of the Tehran megacity. The present-day structural relationships and the kinematics of these two faults, especially at their junction in Lavasanat region, is still a matter of debate. In this paper, we present the results of a morphotectonic analysis (aerial photos and field investigations) within the central part of the Mosha and eastern part of the North Tehran faults between the Mosha valley and Tehran City. Our investigations show that, generally, the traces of activity do not follow the older traces corresponding to previous long-term dip–slip thrusting movements. The recent faulting mainly occurs on new traces trending E–W to ENE–WSW affecting Quaternary features (streams, ridges, risers, and young glacial markers) and cutting straight through the topography. Often defining en-echelon patterns (right- and left-stepping), these new traces correspond to steep faults with either north- or south-dipping directions, along which clear evidences for left-lateral strike–slip motion are found. At their junction zone, the two sinistral faults display a left-stepping en-echelon pattern defining a positive flower structure system clearly visible near Ira village. Further west, the left-lateral strike–slip motion is transferred along the ENE–WSW trending Niavaran fault and other faults. The cumulative offsets associated with this left-lateral deformation is small compared with the topography associated with the previous Late Tertiary thrusting motion, showing that it corresponds to a recent change of kinematics.

Paleoseismological investigation of the oblique‐normal Ekkara ground rupture zone accompanying theM6.7–7.0 earthquake on 30 April 1954 in Thessaly, Greece: Archaeological and geochronological constraints on ground rupture recurrence

Observations in trenches excavated across the Ekkara rupture zone (ERZ), the longest of the sporadic ground ruptures that accompanied theM6.7–7.0 earthquake of 30 April 1954 on the Domokos fault zone (DFZ), identify a left‐lateral oblique‐normal fault with a slip vector aligned with the regional extension direction (verifying the tectonic origin of the ERZ). The paleoseismological interpretation of stratigraphic, soil‐stratigraphic, and tectonic features is discussed with emphasis on the issues related to strongly oblique fault kinematics. Two pre‐1954 events of ground rupture are recognized (E1 and E2) and an event of ground cracking only (E1a, shortly after E1). Event E2 was accompanied by larger displacement than E1 and 1954, but this does not necessarily imply a stronger earthquake. Archaeological dating of transported ceramic sherds constrains E1 between 6750 and 4450 B.P. (more likely, 6450–5750 B.P.). Luminescence dating of colluvial deposits and14C dating of pedogenic carbonates (stone coatings) place E2 at ∼17,500 +− ∼2500 B.P. (preferred age). In the (hazard‐wise) most conservative interpretation, recent recurrence intervals of ERZ activations exceed 3195 years. The sporadic nature of the 1954 ruptures precludes certainty on whether the ERZ has recorded every past activation of the DFZ unless data are also collected from other DFZ rupture zones. Minimum‐limiting estimates of 0.3–0.5 mm/yr slip rate are derived for the ERZ, and the true slip rate is not expected to exceed 1 mm/yr. Slip rate estimates for the ERZ may be minima for the DFZ slip rate, however, because coseismic deformation in the past may have been distributed to more than one rupture zones at the surface.