Paleoseismological Trenches Research Articles

Paleo-earthquake evidence and earthquake recurrence for Düzce fault, Turkey

Paleoseismological trenching was performed along the Düzce fault providing some preliminary insight about its seismogenic behavior. Dating was based on radiocarbon analysis of peat samples collected from the trenches and suggested seven earthquakes have occurred since 1740 BC. Integrating date constraints of events exposed in the trenches suggests a periodical earthquake recurrence model. According to a linear sequential event serial that has minimum misfit determined by considering the probability curve limits of the sample dates, the earthquake recurrence interval is between 384 and 460 years (or possibly between AD 394 and 400). A probability curve was also calculated for the date of the last earthquake (1999 Düzce earthquake) considering the probability distributions of sample dates based on the same event serial. This probability-distribution-based method, similarly, predicted that the 1999 Düzce earthquake occurred between 1933 and 2005 (±36 years) with a 68% probability. After this verification, using this method, it was estimated that the next earthquake along the Düzce fault has a 68% probability of occurring between 2328 and 2392. According to this calculation, the earthquake recurrence interval is about 391 ± 34 years with a 68% probability and the AD 967 historical earthquake likely ruptured the Düzce fault. Assuming an average slip of 350 cm (the average slip of the 1999 earthquake), the slip rate was estimated to be between 8.7 and 11.2 mm/a.

New trenching results along the \u0130znik segment of the central strand of the North Anatolian Fault (Turkey): an integration with preexisting data

This paper provides a new contribution to the construction of the complex and fragmentary mosaic of the Late Holocene earthquakes history of the İznik segment of the central strand of the North Anatolian Fault (CNAF) in Turkey. The CNAF clearly displays lower dextral slip rates with respect to the northern strand however, surface rupturing and large damaging earthquakes (M > 7) occurred in the past, leaving clear signatures in the built and natural environments. The association of these historical events to specific earthquake sources (e.g., Gemlik, İznik, or Geyve fault segments) is still a matter of debate. We excavated two trenches across the İznik fault trace near Mustafali, a village about 10 km WSW of İznik where the morphological fault scarp was visible although modified by agricultural activities. Radiocarbon and TL dating on samples collected from the trenches show that the displaced deposits are very recent and span the past 2 millennia at most. Evidence for four surface faulting events was found in the Mustafali trenches. The integration of these results with historical data and previous paleoseismological data yields an updated Late Holocene history of surface-rupturing earthquakes along the İznik Fault in 1855, 740 (715), 362, and 121 CE. Evidence for the large M7 + historical earthquake dated 1419 CE generally attributed to this fault, was not found at any trench site along the İznik fault nor in the subaqueous record. This unfit between paleoseismological, stratigraphic, and historical data highlights one more time the urge for extensive paleoseismological trenching and offshore campaigns because of the high potential to solve the uncertainties on the seismogenic history (age, earthquake location, extent of the rupture and size) of this portion of NAFZ and especially on the attribution of historical earthquakes to the causative fault.

Late Miocene to Quaternary Development of the Jiujing Basin, Southern Beishan Block, China: Implications for the Kinematics and Timing of Crustal Reactivation North of Tibet

AbstractWe present results from a multidisciplinary investigation of the Jiujing fault (JJF) system and adjacent Jiujing Basin in the southern Beishan block, western China. Structural and geomorphological fieldwork involving fault and landform investigations, remote sensing analysis of satellite and drone imagery, analysis of drill-core data, paleoseismological trench studies, and Quaternary dating of alluvial sediments suggest the JJF is a late Pleistocene to Holocene oblique sinistral-slip normal fault. Satellite image analysis indicates that the JJF is a connecting structure between two regional E-W-trending Quaternary left-lateral fault systems. The Jiujing Basin is the largest and best developed of three parallel NE-striking transtensional basins within an evolving sinistral transtensional duplex. Sinistral transtension is compatible with the orientation of inherited basement strike belts, NE-directed SHmax, and the modern E-NE-directed geodetic velocity field. Cosmogenic 26Al/10Be burial dating of the deepest sediments in the Jiujing Basin indicates that the basin began to form at ~5.5 Ma. Our study reveals a previously unreported actively deforming domain of transtensional deformation 100 km north of Tibet in a sector of the Beishan previously considered tectonically quiescent. Recognition of latest Miocene-Recent crustal reactivation in the Jiujing region has important implications for earthquake hazards in the Beishan and western Hexi Corridor/North Tibetan foreland sectors of the Silk Road Economic Belt. Additionally, we compare the timing of latest Miocene-Recent crustal reactivation in the southern Beishan with the documented onset of reactivation in other deforming regions north of Tibet.

Slip rates and paleoearthquakes along the east segment of the Guanggaishan–Dieshan fault zone, West Qinling Range, NW China

The WNW-trending Guanggaishan–Dieshan fault (GDF) is located in the West Qinling Range at a site of tectonic junction between the Tibetan plateau, the Sichuan basin and the Ordos block. Tectonically, this region is bounded by the East Kunlun fault to the southwest, the West Qinling fault to the north, and the Longmenshan fault to the southeast. The activity of the GDF is essentially related with the northeastward expansion of the Tibetan Plateau and associated strain partition between the East Kunlun fault and the West Qinling fault. This study reported the sense of fault movement, rate of fault slip, and the timing of three paleoseismic events on the east segment of the Guanggaishan–Dieshan fault (EGDF) with which no historical earthquake has been documented before. We focused our investigation on three branch faults in the central part of the EGDF using a range of study methods, e.g. structural mapping, paleoseismological trenching, unmanned aircraft vehicle photogrammetry, differential GPS survey, as well as radiocarbon and optically stimulated luminescence dating. Our new results highlighted: (1) the EGDF is dominated by the left-lateral sense of movement with a minor component of dip slip, (2) the range-bounding branch is characterized by a maximum left-lateral slip rate of 0.72 ± 0.34 mm/year and a maximum vertical-slip rate of 0.13 ± 0.03 mm/year, respectively. Kinematics of the EGDF is similar to many other active faults in the West Qinling Range, together they play a joint role in accommodating and transferring the residual left-lateral slip of the East Kunlun Fault. Three paleoseismic events were identified on the range-bounding fault based on two types of event markers: cut-cover relation and colluvial deposition. The ages of the most recent two events are estimated to be 2765–3320 cal. year BP and 12,775–13,005 cal. year BP and the age of the oldest event is as early as 18,495–32,950 cal. year BP. In contrast, there is no compelling evidence to show paleoseismic event along the rest two branches, and further work is needed to demonstrate this.

Adyr Faults: Generators of Strong Earthquakes in the Lake Issyk-Kul Depression (a Case Study of the Kultor Fault Zone)

We have studied the eastern end of the Kultor adyr (foothill) fault in northern Lake Issyk-Kul region. Morphologically, the fault zone is a staircase of seismic fault scarps formed by movements along few adjacent ruptures gently tilted under the adyrs (northward). Numerous Saka-Wusun, Turkic and Karakhanid archeological monuments ended up to the slope parts of the fault scarps and were damaged or destroyed. Logging of paleoseismological trenches excavated across southernmost and youngest fault scarps has made it possible to determine the minimum age, recurrence period, and energy of strong historical earthquakes. Strong earthquakes along the studied part of the adyr fault occurred during the following periods: 60–170 AD; 530–620 AD; 1160–1250 AD; 1410–1460 AD. The time intervals among dates (strong earthquake reccurrence along the fault) is 200–600 years. The obtained parameters of the fault scarps were used to calculate the magnitudes of historical seismic events: M = 7.2 and M = 6.9 according to the heights of the studied fault scarps. These magnitudes can produce seismic oscillations in the Earth’s surface with I0 ≥ IX according to the MSK-64 Scale. We have confirmed our earlier supposition that the southern parts of the Issyk-Kul Depression were involved in uplift. A lateral longitudinal increase in the area of the adyrs occurs during strong earthquakes. These conclusions are important for a new seismic hazard assessment of the northern Lake Issyk-Kul region, the most populated part of Issyk-Kul Oblast, Kyrgyz Republic, as well as for other regions of the earth evolving in a postplatform orogenic regime.

Recurrence of large paleo-earthquakes in Kashmir Himalaya seismic gap (Riasi area, India)

Abstract In Kashmir Himalaya, the Medlicott-Wadia Thrust is a main active fault responsible for the crustal accretionary prism building during the Late Quaternary. Because of the long seismic silence during the last five centuries, it is a key structure to be studied in order to estimate the regional seismic hazard. In the Riasi area, the analysis of two paleoseismological trenches allowed us identifying and measuring several seismic ruptures over the last ~3500 years. We determined that the oldest rupture occurred around 1600–1000 BC, while the youngest occurred after 1470 AD. The latest event is compatible with the great 1555 AD (Mw > 7.5) Kashmir earthquake, whose evidence at the surface had not yet been recognized. Our results show that despite the long-lasting seismic gap, the Medlicott-Wadia Thrust is a main seismogenic structure in the region, able to produce large earthquakes. Their recurrence interval ranges between 500 and 700 years, implying that a main seismic event could occur in the next decades. These seismic ruptures are localized over gently-dipping fault branches associated with decameter-scale scarps. The morphologies observed are due to a significant non-localized component of the deformation in Quaternary sediments.

Title Paleoseismological and Neotectonic Characteristics of Çaldıran Fault Around Çaldıran District

Sağ yanal atımlı Çaldıran Fayı Türkiye’nin Doğu Anadolu bölgesindeki önemli aktif faylarındandır. Fayın 24 Kasım 1976 tarihinde ürettiği yıkıcı deprem önemli hasar ve can kaybına neden olmuş ve 50-55 km yüzey kırığı oluşturmuştur. Çaldıran ilçesi dolayında yüzey deformasyonları ve paleosismolojik kazı esasına dayalı olarak gerçekleştirilen bu çalışmada son depremde oluşan yüzey kırıklarının korunduğu kesimlerin olduğu saptanmıştır. Yüzey ve paleosismolojik çalışmalar Holosen’de bölgede yüzey kırığı oluşturan (M≥7) en az üç depremin olduğunu göstermektedir. Çaldıran dolayında, son depremde 1,5 m sağ yanal ve 30-40 cm düşey ötelenme saptanmıştır. Ayrıca,1696’da Çaldıran bölgesinde meydana gelen depremi [18] Çaldıran Fayı’nın ürettiği kabul edilerek tekrarlanma aralığının 280 yıl; kısa dönem yatay kayma hızının 5,36 mm/yıl-1 düşey kayma hızının ise 1,43 mm/yıl-1 olduğu saptanmıştır.

A composite rupture model for the great 1950 Assam earthquake across the cusp of the East Himalayan Syntaxis

Although the M=w8.7, 1950 Assam earthquake endures as the largest continental earthquake ever recorded, its exact source and mechanism remain contentious. In this paper, we jointly analyze the spatial distributions of reappraised aftershocks and landslides, and provide new field evidence for its hitherto unknown surface rupture extent along the Mishmi and Abor Hills. Within both mountain fronts, relocated aftershocks and fresh landslide scars spread over an area of ≈330 km by 100 km. The former are more abundant in the Abor Hills while the later mostly affect the front of the Mishmi Hills. We found steep seismic scarps cutting across fluvial deposits and bounding recently uplifted terraces, some of which less than two thousand years or even a couple centuries old, at several sites along both mountain fronts. They likely attest to a minimum 200 km-long 1950 surface rupture on both the Mishmi and Main Himalayan Frontal Thrusts (MT and MFT, respectively), crossing the East Himalayan Syntaxis. At two key sites (Wakro and Pasighat), co-seismic surface throw appears to have been over twice as large on the MT as on the MFT (7.6 ± 0.2 m vs. >2.6 ± 0.1 m), in keeping with the relative, average mountain heights (3200 m vs. 1400 m), mapped landslide scar numbers (182 vs. 96), and average thrust dips (25–28° vs. 13–15°) consistent with relocated aftershocks depths. Corresponding average slip amounts at depth would have been ≈17 and ≈11 m on the MT and MFT, respectively, while surface slip at Wakro might have reached ≈34 m. Note that this amount of superficial slip would be out of reach using classic paleo-seismological trenching to reconstruct paleo-earthquake history. Most of the 1950 first arrivals fit with a composite focal mechanism co-involving the two shallow-dipping thrust planes. Their intersection lies roughly beneath the Dibang Valley, implying forced slip parallel to GPS vectors across the East Himalayan Syntaxis. Successive, near-identical, terrace uplifts at Wakro suggest near-characteristic slip during the last two surface rupturing earthquakes, while terrace boulder ages may be taken to imply bi-millennial return time for 1950-size events. As in Nepal, East-Himalayan mega-quakes are not blind and release most of the elastic, interseismic shortening that accumulates across the range.

The effect of stress changes on time-dependent earthquake probabilities for the central Wasatch Fault Zone, Utah, USA.

Summary Static and quasi-static Coulomb stress changes produced by large earthquakes can modify the probability of occurrence of subsequent events on neighboring faults. This approach is based on physical (Coulomb stress changes) and statistical (probability calculations) models, which are influenced by the quality and quantity of data available in the study region. Here, we focus on the Wasatch Fault Zone (WFZ), a well-studied active normal fault system having abundant geologic and paleoseismological data. Paleoseismological trench investigations of the WFZ indicate that at least 24 large, surface-faulting earthquakes have ruptured the fault's five central, 35–59-km long segments since ∼7 ka. Our goal is to determine if the stress changes due to the youngest paleoevents have significantly modified the present-day probability of occurrence of large earthquakes on each of the segments. For each segment, we modeled the cumulative (coseismic + postseismic) Coulomb stress changes (∆CFScum) due to earthquakes younger than the most recent event on the segment in question and applied the resulting values to the time-dependent probability calculations. Results from the Coulomb stress modeling suggest that the Brigham City, Salt Lake City, and Provo segments have accumulated ∆CFScum larger than 10 bars, whereas the Weber segment has experienced a stress decrease of 5 bars, in the scenario of recent rupture of the Great Salt Lake fault to the west. Probability calculations predict high probability of occurrence for the Brigham City and Salt Lake City segments, due to their long elapsed times (> 1–2 ka) when compared to the Weber, Provo, and Nephi segments (< 1 ka). The range of calculated coefficients of variation (CV) has a large influence on the final probabilities, mostly in the case of the Brigham City segment. Finally, when the Coulomb stress and the probability models are combined, our results indicate that the ∆CFScum resulting from earthquakes postdating the youngest events on each of the five segments substantially affects the probability calculations for three of the segments: Brigham City, Salt Lake City, and Provo. The probability of occurrence of a large earthquake in the next 50 years on these three segments may therefore be underestimated if a time-independent approach, or a time-dependent approach that does not consider ∆CFS, is adopted.

A Late Glacial surface rupturing earthquake at the Peel Boundary fault zone, Roer Valley Rift System, the Netherlands

Paleoseismological trenching studies constrain recurrence times and magnitudes of faulting events and earthquakes on active faults. In a trench along the central part of the Peel Boundary fault zone (PBFZ), southeastern Netherlands, evidence was found for such a large faulting event that occurred around 14 ka. The event caused a fault scarp in unconsolidated sediments of ∼1 m height. A colluvial wedge was formed next to the scarp. A second faulting event offsets this colluvial wedge by 0.2–0.1 m. This event can be tentatively dated at ∼13 ka. During or immediately after the second event, a large clastic dyke intruded along the fault plane. The dyke is not faulted, but its emplacement did cause some minor thrust faulting around the injection.The sudden character of the main faulting event, the brittle deformation style of loam layers, the lack of growth faulting in the colluvial wedge, the clastic dykes and the flame structures demonstrate that the main faulting event was a surface rupturing earthquake. Based on the scarp height, the estimated moment magnitude is about 6.8 ± 0.3. Similar observations in a previous trench site suggest that the length of the surface rupture was at least 32 km. The earthquake took place during the Weichselian (Würmian) Late Glacial. This timing corresponds to the start of the glacio-isostatic forebulge collapse in the Netherlands. Glacio-isostatic movements have been invoked before to explain earthquake events in the Roer Valley Rift System in which the PBFZ is situated, and in northern Germany and Denmark. If these earthquakes can indeed be attributed to a collapsing forebulge, their ages should show a decrease in the direction of ice-sheet retreat. This might indeed be the case, as the ages decrease from 14 ka and 13 ka in this trench via 13–16 ka at the Osning Thrust Zone, NW Germany, to 12–14 ka in northern Denmark.

Analysis of uncertainties associated with different factors used to determine the age of sediments with the Optically Stimulated Luminescence method

In this work, a sediment sample from an excavated paleoseismological trench was collected and dated following the Optically Stimulated Luminescence (OSL) dating method, using the Riso TL/OSL DA-20 reader. Chemically purified quartz, from the sample, was analysed following a single-aliquot regenerative-dose (SAR) protocol for the equivalent dose (De) determination. Also, to estimate dose rates, the natural radioactivity of soil from the surroundings of the original sample location was measured, using gamma spectrometry. Since the application of the OSL dating method involves a number of intermediary factors and processes, all being the sources of uncertainties propagating to the total uncertainty, an exhaustive analysis of the involved uncertainties is presented and the implications to the derivation of the final ages are discussed.

Use of the Optically Stimulated Luminescence Dating and X-Ray Fluorescence Spectrometry Methods as Tools in Paleoseismology

We have investigated the application of luminescence dating to sediment and pottery samples from a paleoseismological trench excavated in the Gyrtoni Fault, Tyrnavos Basin, Central Greece. The samples were dated following the optically stimulated luminescence (OSL) dating method, using the Riso TL/OSL DA-20 reader. The OSL ages were obtained from chemically purified quartz and a single-aliquot regenerative-dose (SAR) protocol was followed for the equivalent dose (De) determination. Additionally, samples were collected and analyzed with the method of X-ray Fluorescence (XRF) spectrometry, in order to assess their elemental composition. Radioisotope sources (109Cd and 241Am) were used for sample excitation, while X-ray spectra were acquired using a Si(Li) detector coupled with standard electronics. The XRF data were submitted to principal component analysis (PCA). This statistical handling aimed to distinguish from which part of the upthrown fault block scarp-derived colluvium and alluvial deposits, parts of the downthrown block were derived and thus estimate the displacement. The results indicated that both the OSL dating method and the XRF analysis combined with PCA can serve as useful tools for paleoseismological investigations.

Paleoseismological trenching and detailed mapping of the surface rupture appeared along the Kita-amagi fault during the 2016 Kumamoto earthquake at Shimada, Mashiki Town, Kumamoto Prefecture, southwest Japan

Paleoseismological trenching and detailed mapping of the surface rupture appeared along the Kita-amagi fault during the 2016 Kumamoto earthquake at Shimada, Mashiki Town, Kumamoto Prefecture, southwest Japan

Earthquake Geology of the Lembang Fault, West Java, Indonesia

The Lembang Fault is a major fault in western Java that skirts the northern edge of Bandung, one of Indonesia's largest cities, just south of the active Tangkuban Perahu volcano. Although it has no recorded or historical large earthquakes, the Lembang Fault shows obvious geomorphic evidence of recent activity and has long been thought to be active. In this study, we use geomorphic analysis to unequivocally establish that the fault has a dominantly sinistral sense of movement with a slip rate of 1.95–3.45 mm/yr. This proves that the fault is accommodating trench parallel slip resulted from a slight obliquity in plate convergence at the JavaTrench. With a length of 29 km, this suggests that the Lembang Fault could produce a Mw 6.5–7.0 earthquake with a recurrence time of 170–670 years. We also conducted paleoseismological trenching of the Lembang Fault and found evidence for at least 3 earthquakes in the 15th century, 2300–60 BCE and 19,620–19,140 BP. The 2300–60 BCE earthquake had a measurable vertical displacement of 40 cm, which is consistence with a Mw 6.5 earthquake. This is the first mapping of a source of crustal earthquakes in Java, Indonesia, the world's most densely populated island in one of its most tectonically active areas. The Lembang Fault and other faults in Java are likely to pose substantial risk to not only Bandung but many of Java's major urban agglomerations.

Holocene paleoseismology of the Norcia fault system (Central Italy)

We investigated the Late Quaternary activity of the Norcia fault system, one of the most hazardous seismogenic structures of Italy, responsible over the centuries for nucleating several Mw 6 earthquakes, possibly up to Mw 6.9 as on 14th January 1703. This fault is located only 10 km away from the Mount Vettore fault system, which was responsible for the deadly Mw 6.6 central Italy earthquake in 2016. Using evidence gathered from ten paleoseismological trenches, we reconstructed the history of the strongest earthquakes sourced by the Norcia faults during the past 4 ky, and in previous time-windows back to 20 ka. In particular, we found indications of the historical 1703 and 99 BCE events, as well as a previously unknown one in the Bronze Age. We also obtained a robust estimate of the slip-rate for the post Last Glacial Maximum period (1 ± 0.2 mm/yr), and a rough 1.8 ky recurrence time for ~Mw 6.9 events during the late Holocene.

Slip deficit and temporal clustering along the Dead Sea fault from paleoseismological investigations

Temporal distribution of earthquakes is key to seismic hazard assessment. However, for most fault systems shortness of large earthquake catalogues makes this assessment difficult. Its unique long earthquake record makes the Dead Sea fault (DSF) exceptional to test earthquake behaviour models. A paleoseismological trench along the southern section of the DSF, revealed twelve surface-rupturing earthquakes during the last 8000 years, of which many correlate with past earthquakes reported in historical chronicles. These data allowed us building a rupture scenario for this area, which includes timing and rupture length for all significant earthquakes during the last two millenaries. Extending this rupture scenario to the entire DSF south of Lebanon, we were able to confirm the temporal-clustering hypothesis. Using rupture length and scaling laws, we have estimated average co-seismic slip for each past earthquake. The cumulated slip was then balanced with long-term tectonic loading to estimate the slip deficit for this part of DSF over the last 1600 years. The seismic-slip budget shows that the slip deficit is similarly high along the fault with a minimum of 2 meters, which suggests that an earthquake cluster might happen over the entire region in the near future.

PALAEOSEISMOLOGICAL INVESTIGATION OF THE GYRTONI FAULT (THESSALY, CENTRAL GREECE)

Two paleoseismological trenches were excavated across the Gyrtoni Fault in NE Thessaly and studied in order to understand the recent seismotectonic behavior of this structure. Twenty five fluvial-colluvial sediment and pottery samples from both the upthrown and the downthrown fault blocks were investigated. Optically Stimulated Luminescence (OSL) dating has been applied to date both sedimentary depositspalaeosoils and pottery fragments. Paleoseismological analysis of the two trenches indicates evidence of three surface faulting events in the time span between 1.42 ±0.06 ka and 5.59 ± 0.13 ka. The observed vertical displacement per event of ~0.50 m corresponds to an Mw 6.5 ± 0.1 earthquake. An average fault slip rate of 0.41 ± 0.01 mm/yr and an average recurrence of 1.39 ± 0.14 ka for earthquakes were estimated. The results documented the activity of the fault and since the return period from the most recent event (minimum age 1.42 ± 0.06 ka) has expired, the possibility for reactivation of this active structure in the near future should be included in Seismic Hazard Assessment.

APPLICATION OF OSL METHOD IN DATING PAST EARTHQUAKES

The aim of this study was to establish a chronological frame of paleoseismic events of Gyrtoni Fault, (Thessaly, Central Greece), with the use of OSL dating method. The Gyrtoni Fault, defines the north-eastern boundary of the Middle-Late Quaternary Tyrnavos Basin, and was previously investigated with geological methods. Twenty five fluvial-colluvial sediment and pottery samples were collected from two paleoseismological trenches, excavated along the Gyrtoni Fault, from both the upthrown and the downthrown fault blocks. Optically Stimulated Luminescence (OSL) dating was applied to coarse grain quartz using the single-aliquot regenerative-dose (SAR) protocol. Investigations of luminescence characteristics using various tests confirmed the suitability of the material for OSL dating using the SAR protocol. Radioactivity measurements were performed in order to estimate the annual dose rateof the surrounding soils to which the quartz grains were submitted during the burial period of the collected samples. The estimated OSL ages agreed well with the available stratigraphical data, and archaeological evidence. The occurrence of three surface faulting events in a time span between 1.42 ± 0.06 ka and 5.59 ± 0.13 ka was revealed while an earlier faulting event (fourth) was also recognized to be older than 5.59 ± 0.13 ka.

Geoarchaeology and paleoseismology blend to define the Fucino active normal fault slip history, central Italy

We first describe the late Holocene slip history of one of the major segments of the Fucino active normal fault, in central Italy, by combining geoarchaeological investigations with paleoseismological trenching. The Fucino fault system released a Mw 7 earthquake in 1915 (with many other events with decimetre and/or metre-size palaeoseismic slip events in the past), that is the strongest seismic shock occurred in this portion of the Italian territory over at least the past millennium. We dug trenches across the investigated tectonic structure; then, the sedimentary sequence and its relation with the exposed fault planes have been analysed “vertically”, as typically made in paleoseismological investigations, but also “horizontally”, by deepening the excavations “step-by-step” while digging, i.e. performing archaeological-type stratigraphic excavations. Such a procedure permitted the recognition of different displacement events of the fault, and the progressive surveying of different cultural levels, since the Neolithic Period, interposed with or cut into natural levels. The reconstruction of the interplay between human occupation of the site and the local geomorphic evolution – framed by the late Holocene climatic changes – permitted us to gain reliable chronological data for constraining the fault slip history in the last 5500 yr. Our analyses also confirmed that the investigated structure activated during the 1915 earthquake. Four previous displacement events were recognised: a first event, prior to the 1915 one, occurred slightly after the Roman Period (probably during the 5th–6th century AD); two preceding events occurred between the Late Neolithic and the Roman period, the older of the two during the late Neolithic, while the later during the Late Bronze Age-Early Iron Age. The oldest event predates the Neolithic Period. No evidence of a Late Middle Ages faulting event found by others researchers along another branch of the Fucino fault was found in our trenches. From a methodological viewpoint, the results of our study mark the effectiveness of adopting joint geoarchaeological/paleoseismological approach in terms of chronological constraints for active faulting studies in such contexts where long human occupation took place, where the natural and “human” events rhythmically interplay.

Late Quaternary paleoearthquakes along the northern segment of the Nantinghe fault on the southeastern margin of the Tibetan Plateau

The strong earthquake behaviors of faults are significant for learning crustal deformation mechanisms and for assessing regional seismic risk. To date, faults that bound tectonic blocks have attracted considerable concern and many studies; however, scant attention has been paid to faults within blocks that can also host devastating earthquakes. The Nantinghe fault is a left-lateral strike-slip fault within the Southwestern Yunnan Block, and it slips at ∼4mm/yr suggesting strong activity in the late Quaternary. Nevertheless, no earthquake greater than 6has ever been recorded along it, except for the 1941M ∼7 earthquake near the Myanmar-China border region. In contrast, many earthquakes have occurred in the near region, delineating a seismic gap near the Nantinghe fault. Although several studies have been conducted upon it, the activity of its northern segment is confusing, and whether this fault segment has loaded sufficient stress to fail remains debatable. Furthermore, previous work failed to conduct any paleoseismological studies bringing out great uncertainty in learning its activity and faulting behavior, as well as in assessing the regional seismic risk. To solve these problems, we mapped the fault traces utilizing high-resolution satellite images and aerial photographs, and conducted three paleoseismological trenches along the northern segment of the Nantinghe fault. The trench excavations revealed a ∼45,000-year incomplete paleoearthquake history and confirmed that this fault segment has been active since the late Pleistocene but was not ruptured during the 1941 earthquake. Additionally, at least five paleoearthquakes are identified with their respective age ranges of before 39,030BCE; 38,500–37,220BCE; 28,475–5445BCE; 3535BCE–800CE; and 1320–1435CE based on radiocarbon dating. Among the paleoearthquakes, the latest is suggested to have generated a surface rupture much longer than 14km with a magnitude likely up to Ms 7.0. Furthermore, based on the elapsed time since the latest paleoearthquake and the sinistral slip rate along the fault, it is proposed that the northern segment of the Nantinghe fault has accumulated a seismic energy equivalent to Ms 7.0, and it is in a high seismic risk along this fault segment and in the neighboring area.