Paleoseismic Analysis Research Articles

Holocene surface-rupturing paleo-earthquakes along the Kachchh Mainland Fault: shaping the seismic landscape of Kachchh, Western India

This study explores the seismotectonics of Kachchh in western India, a region with a low-to-moderate strain rate and a history of significant earthquakes, notably the 1819, Mw 7.8 Allah Bund, and the 2001, Mw 7.6 Bhuj. Despite its substantial seismic risk, comprehensive studies on Kachchh’s seismogenic sources are scarce. This is attributed to the concealed nature of active structures, hindering definitive age constraints in paleoseismological research. Our research comprises a detailed paleoseismic analysis of the north-verging, reverse Jhura Fault underlying the Jhura anticline, a segment of the Kachchh Mainland Fault. This fault segment shows evidence of surface-rupturing earthquakes in the area south of the Great Rann of Kachchh. The investigation reveals three paleoseismic events: Event I before 9.72 ka B.P., Event II between 8.63–8.20 ka B.P., and Event III between 6.20–6.09 ka B.P. The elapsed time since the last event on this fault is > 8000 years, suggesting that the area is exposed to a significant earthquake hazard. This highlights the need for more precise characterization of individual seismogenic sources for future earthquake preparedness.

Paleoseismic Analysis of the Walanae Fault Zone in South Sulawesi, Indonesia

The most seismogenic fault of South Sulawesi region is Walanae Fault, which has a moderate seismicity history with magnitudes ranging from Mw 4 to Mw 5. The largest earthquake occurred in 1997 with a magnitude of Mw 5.9. After the Donggala-Palu earthquake assailed Sulawesi Island in 2018, the fault in this region were reactivated, including the Walanae Fault system that has pair parallel faults, West Walanae Fault (WWF) and East Walanae Fault (EWF). This fault zone has a 150-km-long and 3-km-wide fault traces, and should have seismicity potential to produce larger earthquakes than the events measured to date. The resulting radiocarbon ages obtained from the dating of organic samples collected on two tranches along with the EWF support that the Walanae Fault zone has a longer earthquake history in the ranges of 4,000 BP (3,050 cal BP and 3,990 cal BP) and 100-300 AD (101 AD pMC and 340 cal BP). The results of age evidence suggest that the Walanae Fault system is an active fault, and must remain of concern as an earthquake source with potential hazards in the region, comprising the West Walanae Fault (WWF) and the East Walanae Fault (EWF). Both faults are separated by a narrow lowland area of Walanae Depression (25 km). The lowland area sandwiched by these two faults harboring a potential seismic amplification that might be generated by thick sediment filled the lowland. Keywords: seismicity, earthquake, radiocarbon dating, Walanae Fault

The Gran Sasso fault system: Paleoseismological constraints on the catastrophic 1349 earthquake in Central Italy

We investigated the late Upper Pleistocene activity of the eight main faults that comprise the active, dip-slip Gran Sasso fault system (GSFS) in the Gran Sasso d'Italia Massif (central Italian Apennines; 2912 m a.s.l.). We carried out novel paleoseismological analyses at four sites of three different fault segments, and reviewed the data of the previous results for three other segments. We carried out several topographic profiles across the offset hillslopes, alluvial fans, glacial cirques, moraines, and valley floor. Through the dozen radiocarbon datings that were combined and cross-checked with many other ages published in previous studies, we have provided robust slip-rates and reconstructed the Holocene seismic history of this fault system. Paleoseismic analyses revealed the presence of three consecutive earthquakes since the onset of the Late Holocene, which were separated by 3.3 ky and 2.2 ky, respectively. The last one occurred in the 13th–14th century CE, a time-span that fits with the catastrophic 1349 seismic sequence.Our review of the macroseismic intensity distribution of this sequence indicates the existence of two distinct mesoseismic areas; a southern one that was already related to the Aquae Iuliae fault rupture (Abruzzi-Campania-Molise borders), and a northern one that robustly matches the hanging-wall of the Gran Sasso fault system. Given the length of this fault system, we estimated Mw 7 for its entire rupture, which accounts for the total destruction of L'Aquila and the neighboring villages. This also accounts for the strong effects and severe damage suffered by several settlements in the para-Tyrrhenian far field, especially to buildings characterized by long fundamental resonance periods, as seen for the monuments of Rome.Although characterized by long recurrence times (2.8 ± 0.5 ky), our results suggest that as for all of the main silent faults of the eastern set of active structures in the central Apennines, this fault system has one of the largest seismogenic potentials of the whole Apennine chain, with seismic risk implications extended even to its far-field.

Joint Interpretation of Geophysical Results and Geological Observations for Detecting Buried Active Faults: The Case of the “Il Lago” Plain (Pettoranello del Molise, Italy)

We report a geophysical study across an active normal fault in the Southern Apennines. The surveyed area is the “Il Lago” Plain (Pettoranello del Molise), at the foot of Mt. Patalecchia (Molise Apennines, Southern Italy), a small tectonic basin filled by Holocene deposits located at the NW termination of the major Quaternary Bojano basin structure. This basin, on the NE flank of the Matese Massif, was the epicentral area of the very strong 26 July, 1805, Sant’Anna earthquake (I0 = X MCS, Mw = 6.7). The “Il Lago” Plain is bordered by a portion of the right-stepping normal fault system bounding the whole Bojano Quaternary basin (28 km long). The seismic source responsible for the 1805 earthquake is regarded as one of the most hazardous structures of the Apennines; however, the position of its NW boundary of this seismic source is debated. Geological, geomorphological and macroseismic data show that some coseismic surface faulting also occurred in correspondence with the border fault of the “Il Lago” Plain. The study of the “Il Lago” Plain subsurface might help to constrain the NW segment boundary of the 1805 seismogenic source, suggesting that it is possibly a capable fault, source for moderate (Mw < 5.5) to strong earthquakes (Mw ≥ 5.5). Therefore, we constrained the geometry of the fault beneath the plain using low-frequency Ground Penetrating Radar (GPR) data supported by seismic tomography. Seismic tomography yielded preliminary information on the subsurface structures and the dielectric permittivity of the subsoil. A set of GPR parallel profiles allowed a quick and high-resolution characterization of the lateral extension of the fault, and of its geometry at depth. The result of our study demonstrates the optimal potential of combined seismic and deep GPR surveys for investigating the geometry of buried active normal faults. Moreover, our study could be used for identifying suitable sites for paleoseismic analyses, where record of earthquake surface faulting might be preserved in Holocene lacustrine sedimentary deposits. The present case demonstrates the possibility to detect with high accuracy the complexity of a fault-zone within a basin, inferred by GPR data, not only in its shallower part, but also down to about 100 m depth.

Evaluating variability in coseismic slips of paleoearthquakes from an incomplete slip history: an example from displaced terrace flights across the Kamishiro fault, central Japan

Examining the regularity in slip over seismic cycles leads to an understanding of earthquake recurrence and provides the basis for probabilistic seismic hazard assessment. Systematic analysis of three-dimensional paleoseismic trenches and analysis of offset markers along faults reveal slip history. Flights of displaced terraces have also been used to study slips of paleoearthquakes when the number of earthquakes contributing to the observed displacement of a terrace is known. This study presents a Monte Carlo-based approach to estimating slip variability using displaced terraces when a detailed paleoseismic record is not available. First, we mapped fluvial terraces across the Kamishiro fault, which is an intra-plate reverse fault in central Japan, and systematically measured the cumulative dip slip of the mapped terraces. By combining these measurements with the age of the paleoearthquakes, we estimated the amount of dip slip for the penultimate event (PE) and antepenultimate event (APE) to be 1.6 and 3.4 m, respectively. The APE slip was nearly three times larger than the most recent event of 2014 (Mw 6.2): 1.2 m. This suggests that the rupture length of the APE was much longer than that of the 2014 event and the entire Kamishiro fault ruptured with adjacent faults during the APE. Thereafter, we performed the Monte Carlo simulations to explore the possible range of the coefficient of variation for slip per event (COVs). The simulation considered all the possible rupture histories in terms of the number of events and their slip amounts. The resulting COVs typically ranged between 0.3 and 0.54, indicating a large variation in the slip per event of the Kamishiro fault during the last few thousand years. To test the accuracy of our approach, we performed the same simulation to a fault whose slip per event was well constrained. The result showed that the error in the COVs estimate was less than 0.15 in 86% of realizations, which was comparable to the uncertainty in COVs derived from a paleoseismic trenching. Based on the accuracy test, we conclude that the Monte Carlo-based approach should help assess the regularity of earthquakes using an incomplete paleoseismic record.

Neotectonic and Paleoseismic Analysis of the Northwest Extent of Holocene Surface Deformation along the Meers Fault, Oklahoma

ABSTRACT The Meers fault (Oklahoma) is one of few seismogenic structures with evidence for Holocene surface rupture in the stable continental region of North America. The 37-kilometer-long southeast section of the full 54-kilometer-long Meers fault is interpreted to be Holocene active. The 17-kilometer-long northwest section is considered Quaternary active, but not Holocene active. We reevaluate surface expression and earthquake timing of the northwest Meers fault to improve seismic source characterization. We use airborne light detection and ranging and historical stereopaired aerial photos to evaluate the fault scarp and local fault-zone geomorphology. In the northwest, complex surface deformation includes fault splays, subtle monoclinal warping, and a minor change in fault strike. We interpret that the along-strike transition from surface faulting on the southeast Meers fault to surface folding on the northwest Meers fault occurs at the lithologic contact between Permian Post Oak conglomerate and Hennessey shale. We excavated a paleoseismic trench to evaluate the timing of surface-deforming earthquakes on the northwest section of the fault. The excavation revealed weathered Permian Hennessey shale and an ∼1–2-meter-thick veneer of Holocene alluvial deposits that were progressively deformed during two surface-folding earthquakes likely related to blind fault rupture beneath the site. Repeated onlapping to overlapping stratigraphic sequences and associated unconformities are intimately related to folding events along the monocline. OxCal paleoearthquake age modeling indicates that earthquakes occurred 4704–3109 yr B.P. and 5955–4744 yr B.P., and that part of the northwest section of the Meers fault is Holocene active. We find the Holocene-active section of the Meers fault should be lengthened 6.1 km to the northwest, to a total Holocene-active fault length of 43 km. Empirical scaling relationships between surface rupture length and magnitude reveal that the fault could generate an Mw 7.0 earthquake.

Evidence for quaternary seismic activity of the La Alberca-Teremendo fault, Morelia region, Trans-Mexican Volcanic Belt

The La Alberca-Teremendo fault is a 26 km-long, complex fault composed of an en échelon array of short crustal fault segments, belonging to the Morelia-Acambay fault system. This fault system shows parallel scarps with morphological evidence of recent activity such as drainage alteration, maximum throws of 50 m and minimum throws of 1.4 m that displace the recent soils. The fault acted as a conduit for the formation of the La Alberca de Guadalupe maar (23000 to 21000 years ago) and displaced afterwards its phreatomagmatic sequences. The paleoseismic analysis indicates that the La Alberca-Teremendo fault moved three times in the past 23000 years (age of the maar); this activity caused an average vertical displacement of 87 cm, and might have generated earthquakes with magnitudes Mw between 6.6 and 7, as well as volcano-tectonic earthquakes with magnitudes Mw between 4 and 5.5. The displacements were identified on the fault through the superposition of soils differentiated by a disconformity and an anomalous increase in the percentage of clay and organic matter. The La Alberca-Teremendo fault has dominant dip slip with a minor left-lateral component, a slip rate of 0.114 mm/year, and an average recurrence interval of 7726 ± 68 years. According to scaling relations that use the surface rupture length, if we assume that the La Alberca-Teremendo fault moves tectonically, it could generate earthquakes with maximum magnitudes of Mw between 6.7 and 7.3, however because of the active volcanic processes in the area, we could expect moderate volcano-tectonic earthquakes (Mw 4–5.5) rather than catastrophic ones.

Neotectonic, morphotectonic and paleoseismologic analysis of the Las Chacras Fault System, Sierras Pampeanas Occidentales, San Juan, Argentina

Las Chacras Fault System is a Quaternary structure located in the western piedmont of Sierra de la Huerta in the Sierras Pampeanas Occidentales, in west-central Argentina. This fault system is subsidiary of the Valle Fértil Megafault System, the NW-striking master structure that affects the western flank of the Valle Fértil - La Huerta ranges. The Las Chacras Fault System comprise several sections including the Pan de Azúcar fault, which presents the most conspicuous evidence of tectonic Quaternary activity. The Pan de Azúcar fault affects three Quaternary alluvial surfaces (Q1, Q2 and Q3) along 26 km in the western piedmont of Sierra de La Huerta, where three natural exposures allowed us to observe Triassic rocks thrust over Quaternary deposits. The observation of different kinematic markers and the relative position of the units affected by the fault, allow us to define this fault as a reverse fault. This structure trends 325° and dips 25° NE. With a DGPS we carried out detailed topographic profiles perpendicular to the average strike of the fault, along interfluves and rivers that cross the structure, for the purpose of establishing the relationship between the topography and the slope of the terrain. This analysis indicates that Quaternary activity of the fault has played an essential role in controlling the piedmont geomorphology, generating west-facing scarps and drainage anomalies such as rivers with convex Hack profiles. Paleoseismic analyses indicate that the fault has moved after 18.360 ± 2990 yr BP. Furthermore, we calculated a net displacement for the Pan de Azúcar fault of ∼50 ± 0.2 m since the Late Pleistocene to Holocene, yielding a shortening rate and a slip rate of ∼0.16 ± 0.1 mm/yr. Thus, the Pan de Azúcar fault is a potential seismogenic source, which could produce a moderate earthquake between Mw = 6.4 and 7.0, with a recurrence interval between 2600 and 6900 years.

Active faults sources for the Pátzcuaro–Acambay fault system (Mexico): fractal analysis of slip rates and magnitudes <i>M</i><sub>w</sub> estimated from fault length

Abstract. The Pátzcuaro–Acambay fault system (PAFS), located in the central part of the Trans-Mexican Volcanic Belt (TMVB), is delimited by an active transtensive deformation area associated with the oblique subduction zone between the Cocos and North American plates, with a convergence speed of 55 mm yr−1 at the latitude of the state of Michoacán, Mexico. Part of the oblique convergence is transferred to this fault system, where the slip rates range from 0.009 to 2.78 mm yr−1. This has caused historic earthquakes in Central Mexico, such as the Acambay quake (Ms=6.9) on 19 November 1912 with surface rupture, and another in Maravatío in 1979 with Ms=5.6. Also, paleoseismic analyses are showing Quaternary movements in some faults, with moderate to large magnitudes. Notably, this zone is seismically active, but lacks a dense local seismic network, and more importantly, its neotectonic movements have received very little attention. The present research encompasses three investigations carried out in the PAFS. First, the estimation of the maximum possible earthquake magnitudes, based on 316 fault lengths mapped on a 15 m digital elevation model, by means of three empirical relationships. In addition, the Hurst exponent Hw and its persistence, estimated for magnitudes Mw (spatial domain) and for 32 slip-rate data (time domain) by the wavelet variance analysis. Finally, the validity of the intrinsic definition of active fault proposed here. The average results for the estimation of the maximum and minimum magnitudes expected for this fault population are 5.5≤Mw≤7. Also, supported by the results of H at the spatial domain, this paper strongly suggests that the PAFS is classified in three different zones (western PAFS, central PAFS, and eastern PAFS) in terms of their roughness (Hw=0.7,Hw=0.5,Hw=0.8 respectively), showing different dynamics in seismotectonic activity and; the time domain, with a strong persistence Hw=0.949, suggests that the periodicities of slip rates are close in time (process with memory). The fractal capacity dimension (Db) is also estimated for the slip-rate series using the box-counting method. Inverse correlation between Db and low slip-rate concentration was observed. The resulting Db=1.86 is related to a lesser concentration of low slip-rates in the PAFS, suggesting that larger faults accommodate the strain more efficiently (length ≥3 km). Thus, in terms of fractal analysis, we can conclude that these 316 faults are seismically active, because they fulfill the intrinsic definition of active faults for the PAFS.

Late Pleistocene-Holocene history of a tectonically active segment of the continental margin (Nekor basin, Western Mediterranean, Morocco)

In active basins, tectonics can segment the continental shelf and control its stratigraphic architecture and physiography. Segmentation can explain the local evolution and morphology of the continental shelf because of sea-level variations, local tectonic segmentation and hydrodynamic processes. Here we investigate the tectonically active Morocco continental margin (southern Alboran Sea) using high-resolution seismic profiles and multibeam bathymetric data. The active faults bounding the transtensive Nekor basin triggered the segmentation of the shelf into three sectors showing different subsidence rates: a western sector corresponding to an extensive fault relay, a central sector corresponding to the subsiding Al-Hoceima Bay and an eastern sector corresponding to the footwall of the Trougout senestral normal fault. Results show that the staircase morphology of the shelf corresponds to successive submarine terraces at the shelf edge (ST1), mid-shelf (ST2) and inner-shelf (ST3) around −110 m,-80 m, and −40 to −20 m deep, respectively. The terraces correspond to the top of prograding wedges seaward and are erosive landward. They are correlated with stillstand from the Last Glacial Maximum to the Holocene highstand. Above the terraces, sub-aqueous dune fields are interpreted as degraded and deposited during the post-glacial transgression. In the central sector, typical delta front seafloor undulations on the shelf and crescent shaped bedforms at the head of marine incisions on the upper slope denote a fluvial influence during the Holocene. Seismic stratigraphy analysis revealed the preservation of six seismic units bounded by polygenic regional unconformities (S1, S2, S3, S4 and S5). Based on comparison with other Mediterranean margins, S1 to S5 are attributed to 4th order maximum regressive surface. We discuss the local preservation of the system tracts as a function of the vertical motion and the physiography of this tectonically active domain. This study provides useful clues for future local paleo-seismic analysis and to advance our understanding of sedimentary processes in active areas. • We present new data of the morphology of the Mediterranean Moroccan shelf. • Marine terraces are present and correlated to local sea level variations. • The active tectonic locally segments the shelf and control the stacking pattern. • The relation between the stacking pattern and the morphology is investigated. • We propose a depositional model correlating the morphology and the stratigraphy of the shelf.

First evidence for Late Pleistocene to Holocene earthquake surface faulting in the Eastern Monferrato Arc (Northern Italy): Geology, pedostratigraphy and structural study of the Pecetto di Valenza site

The W Po Plain (Northern Italy) is commonly regarded as a region characterized by a low seismicity hazard, due to the lack of historical and instrumental record of strong earthquakes. Nevertheless, recent studies performed in the Monferrato hills provide evidence of active faulting and possible paleoseismicity occurred in Middle Pleistocene to Holocene times.Therefore, for verifying the seismic potential of this area, we firstly revised the available geological and geophysical information. We selected the area between Valenza and Alessandria (Eastern Monferrato Arc), which show the best structural, geomorphic, and stratigraphic setting for documenting recent tectonic deformation and faulting. Therein, we identified, for the first time, evidence for earthquake surface displacement in a Late Quaternary pedosedimentary sequence exposed at Pecetto di Valenza. The outcropping section has been logged and investigated according to an approach integrating sedimentological and micropedological data with structural analyses and radiocarbon dating. This allowed reconstructing the recent surface evolution and the paleoseismic history of the site.From the bottom, the pedosedimentary sequence consists of a Miocene marly bedrock, whose weathering started in Marine Isotopic Stage (MIS) 7 or 5 interglacials. A colluvial deposit follows, whose formation can be attributed to MIS 5e. The upper part of the sequence consists of two loess covers, showing different degrees of weathering, possibly occurred at ca. 30 ka BP for the deepest loess and in the Middle Holocene (ca. 4 ka BP) for the uppermost one. The loess accretion is therefore older and likely associated to the MIS 3 and MIS 2 glacial phases.Each deposit records the deformation induced by earthquake surface reverse faulting and warping, giving constraints to the sequence of events that characterized this site. In fact, the structural analyses and a 2D balanced retrodeformation of the section, integrating pedostratigraphic constraints, allowed identifying at least two different phases of deformation, and more than five fault scarp-forming events, which caused a total net displacement of ca. 4.8 m during the past ca. 40 ka. Our approach highlighted the interaction between the tectonically induced surface deformation and the aeolian deposition, allowing reconstructing the recent evolution of a small drainage basin.The results of this paleoseismic analysis reinforce the conclusion of previous Authors that the Monferrato Arc should be viewed as a seismic gap, characterized by strong earthquakes (Mmax ∼6.5) with long recurrence interval (in the order of several thousands years). This evidence has relevant implications for seismic hazard assessment, which must be checked with further trench investigations along the mapped Quaternary thrust faults affecting the western part of the Po Plain.

New insights on speleoseismology: The geothermal gradient and heat flow values in caves for the study of active faults

Normally in paleoseismology, the study of the tectonic slip-rate is performed in trenches on the fault scarp, or by the estimation of fault movements from the geomorphic features. In this work, we have carried out a paleoseismic analysis of the Benis Fault, located in southeast Spain, combined with a geothermal analysis inside a deep cave related to the fault (−350 m). Thus, we have estimated the last earthquake magnitude and time of occurrence from evidence of ceiling collapse and displaced carbonate blocks inside a cave, which is developed across the fault. The magnitude was obtained from the application of the empirical relationship of the fault parameters and coseismic vertical displacement, yielding a value ranging between M 5.9 and M 6.5. Moreover, we dated this paleoearthquake by the paleontological record of a “Lynx pardinus spelaea”, with an age of 65 ± 18 ka BP. Additionally, we have measured the thermal profile of the Benis Cave (−350 m of depth), from single rock point temperature measurements during 2 years. The temperature profile shows three different parts inside the cave, the shallow heterogeneous thermal zone till 50 m depth; the homogeneous thermal zone 150 m till with constant temperature and the hetero-thermal deep zone, deeper than 150 m and till the deepest zone (350 m). Furthermore, we have estimated the Vertical Geothermal Gradient, 1.85 °C/100 m for the deepest zone (−150; −290 m). The temperature increases with depth, showing a reverse thermal profile in comparison with normal gradients in deep caves. Finally, we have calculated the heat flux of 0.46 mWm2.

The Eastern Lower Tagus Valley Fault Zone in central Portugal: Active faulting in a low-deformation region within a major river environment

Active faulting in the Lower Tagus Valley, Central Portugal, poses a significant seismic hazard that is not well understood. Although the area has been affected by damaging earthquakes during historical times, only recently has definitive evidence of Quaternary surface faulting been found along the western side of the Tagus River. The location, geometry and kinematics of active faults along the eastern side of the Tagus valley have not been previously studied. We present the first results of mapping and paleoseismic analysis of the eastern strand of the Lower Tagus Valley Fault Zone (LTVFZ). Geomorphological, paleoseismological, and seismic reflection studies indicate that the Eastern LTVFZ is a left-lateral strike-slip fault. The detailed mapping of geomorphic features and studies in two paleoseismic trenches show that surface fault rupture has occurred at least six times during the past 10ka. The river offsets indicate a minimum slip rate on the order of 0.14–0.24mm/yr for the fault zone. Fault trace mapping, geomorphic analysis, and paleoseismic studies suggest a maximum magnitude for the Eastern LTVFZ of Mw~7.3 with a recurrence interval for surface ruptures ~1.7ka. At least two events occurred after 1175±95calyrBP. Single-event displacements are unlikely to be resolved in the paleoseismic trenches, thus our observations most probably represent the minimum number of events identified in the trenches.

Structural evolution of the El Salvador Fault Zone: an evolving fault system within a volcanic arc.

The El Salvador Fault Zone, firstly identified after the 13th February 2001 Mw 6.6 El Salvador earthquake, is a 150 km long, 20 km wide right-lateral strike-slip fault system. Ruptures along the ESFZ are thought to be responsible for most of the historical destructive earthquakes along the El Salvador Volcanic Arc, as well as for most of the current seismicity of the area. In this work, we focus on the geological setting of the fault zone by describing its geomorphology and structure, using field-based observations, digital terrain modelling, and aerial photograph interpretation with the aim at contributing to the understanding of the ESFZ slip behaviour. In particular, we address the ESFZ structure, kinematics and evolution with time. The ESFZ is a complex set of traces divided in major rupture segments characterized by different geometry, kinematics and geomorphic expressions. Natural fault exposures and paleoseismic trenches excavated along the fault show that the strike-slip deformation is distributed in several planes. Both geometry and kinematics of the fault zone are consistent with a transtensional strain regime. The estimated geological slip rate for the main fault segments by paleoseismic trenches and displaced geomorphic features implies a deficit in velocity of the fault compared to the available GPS velocities data. The high vertical scarps of some fault segments would require Quaternary slip rates not coherent neither with measured GPS velocities nor with slip rates obtained from paleoseismic analysis. This mismatch suggests a pre-existing graben structure that would be inherited from the previous regional roll back related extensional stage. We consider that the ESFZ is using this relict structure to grow up along it. As a result, we ropose a model for ESFZ development consistent with all these observations.

Paleoearthquakes as Anchor Points in Bayesian Radiocarbon Deposition Models: A Case Study from the Dead Sea

The Bayesian statistical method of the OxCal v 4.1 program is used to construct an age-depth model for a set of accelerator mass spectrometry (AMS) radiocarbon ages of organic debris collected from a late Holocene Dead Sea stratigraphic section (the Ein Feshkha Nature Reserve). The model is tested for a case where no prior earthquake information is applied and for a case where there is incorporation of known ages of 4 prominent historical earthquakes as chronological anchor points along the section. While the anchor-based model provided a tightly constrained age-depth regression, the “non-anchored” model still produces a correlation where most of the 68% or 95% age ranges of the 52 seismites can be correlated to historical earthquakes. This presents us with the opportunity for high-resolution paleoseismic analysis and comparison between various sites.

Quaternary earthquakes and landslides in the Sea of Galilee area, the Dead Sea Transform: Paleoseismic analysis and implication to the current hazard

Katz, O., Amit, R., Yagoda-Biran, G., Hatzor, Y.H., Porat, N., Medvedev, B. 2009/2010. Quaternary earthquakes and landslides in the Sea of Galilee area, the Dead Sea Transform: Paleoseismic analysis and implication to the current hazard. Isr. J. Earth Sci. 58: 275–294. A few destructive large (Mw >6) earthquakes occurred close to the Sea of Galilee (SOG) in the last millennia and, along with recorded current seismicity, underscore the seismic hazard to populated areas in the region. In this study we investigate the Late-Pleistocene–Holocene seismic history of the southeastern SOG area, around the northern extension of the Jordan Valley Fault. We integrate results from paleoseismic trenching of recent fault segments rupturing the surface with slope stability analysis of a large landslide that spatially coincides with the faults, to develop a better understanding of the seismic hazard. We have dated fault rupture as well as sliding events with optically stimulated luminescence (OSL). In a series of paleoseismic trenches we found evidence for at least five Mw >6 earthquakes that were dated to ca. 45, 40, 35, 10, and 5 ka. A younger event of 6 and Mw >6.5, respectively. Such large earthquakes are expected to result in high ground acceleration of up to 0.5 g and earthquake-induced landslides around the SOG.

Late Pleistocene to Holocene tectonic activity along the Nesher fault, Mount Carmel, Israel

ABSTRACT Zilberman, E., Greenbaum, N., Nahmias, Y., Porat, N., and Ashqar, L. 2008. Late Pleistocene to Holocene tectonic activity along the Nesher fault, Mount Carmel, Israel. Isr. J. Earth Sci. 57: 87-100. A paleoseismic analysis of the Nesher fault, a branch of the Yagur fault (the western seg­ment of the Carmel tectonic system), was conducted east of Nesher, at a site where the fault trace crosses a small, abandoned alluvial fan. A 30-m-Iong and 2-4 m-deep trench excavated in the alluvial fan across the fault trace exposed a steeply northeastward-tilted late Paleocene to Early Eocene chalky-marly sequence on the northern side of the fault and a thick (more than 4 m) alluvial-colluvial sequence on its southern side. Two periods of tectonic activity accompanied by surface deformation were identi­fied along the Nesher fault: I. Subsidence of a small basin south of the main fault manifested by colluvium that accumulated south of the fault trace and clay, unit I that accumulated in a small depression further to the south. The OSL age of unit 1 is 178

Late Holocene Paleoseismicity of the Southern Part of the Chelungpu Fault in Central Taiwan: Evidence from the Chushan Excavation Site

The geomorphic expression of the frontal Western Foothills in central Taiwan is usually defined by a late Holocene scarp that ranges from tens to hundreds of meters in height. This scarp is the product of displacement on a near-surface 20- 30 east-dipping thrust fault, the Chelungpu fault, which ruptured during the Chichi earthquake. The large scarp height may correspond directly to the accumulation of successive surface ruptures. The Chushan site is located on the southern part of this fault where the Chichi earthquake produced a scarp with a 1.7-m vertical offset for the total vertical separation. Based on core-boring estimates, the vertical displace- ment on both sides is 7 m along a 24 east-dipping thrust fault. The results from our paleoseismic analysis indicate that five large paleoearthquake events have caused the large offsets during the past 2 ka. The radiocarbon age constraints of the paleoearth- quakes suggest a clustering of 540-790 cal yr B.P. (E2), 710-950 cal yr B.P. (E3), 1380-1700 cal yr B.P. (E4), 1710-1930 cal yr B.P. (E5), and the 1999 Chichi earth- quake. Events E3 and E4 have not been reported in previous studies and we did not observe event E1 (300-430 cal yr B.P.) at the site. Based on displacement and fault segmentation from the geologic features, we argue that the two new events may have occurred along the northern part of the Chelungpu fault. The vertical slip rate is estimated to be at least 3.9 0.2 mm/yr over the past 2 ka, which is similar to the long-term estimation through a calculation of late Pleistocene-Holocene terrace el- evations on the hanging wall.

Giant earthquakes in South-Central Chile revealed by Holocene mass-wasting events in Lake Puyehue

Very high resolution reflection seismic profiling (3.5 kHz) revealed nine Holocene mass-wasting events in Lake Puyehue (South-Central Chile). These events are made up of numerous coeval mass-wasting deposits and some homogenites, which are radiocarbon-dated. The two youngest mass-wasting events could be attributed to the giant AD 1960 and AD 1575 Valdivia earthquakes. The most extensive event took place around 1660 cal yr BP. Evaluation of all possible slope failure processes led us to infer that giant earthquakes, roughly comparable to the AD 1960 Valdivia earthquake ( M w = 9.5), are the most likely trigger mechanisms of all mass-wasting events in Lake Puyehue. These occurred with a mean recurrence rate of 1000 yr although relatively aperiodically (ranging between 500 and 2000 yr). Quantitative comparison of mass-wasting events related to historically reported earthquakes (AD 1960 and AD 1575) showed significant differences although these earthquakes are assumed to have had a comparable strength. A lowered background sedimentation rate could be responsible for this variable earthquake recording, which highlights the importance of a thorough assessment of the depositional history before using lacustrine records for quantitative paleoseismic analysis.

Using uplifted Holocene beach berms for paleoseismic analysis on the Santa María Island, south‐central Chile

Major earthquakes (M > 8) have repeatedly ruptured the Nazca‐South America plate interface of south‐central Chile involving meter scale land‐level changes. Earthquake recurrence intervals, however, extending beyond limited historical records are virtually unknown, but would provide crucial data on the tectonic behavior of forearcs. We analyzed the spatiotemporal pattern of Holocene earthquakes on Santa María Island (SMI; 37°S), located 20 km off the Chilean coast and approximately 70 km east of the trench. SMI hosts a minimum of 21 uplifted beach berms, of which a subset were dated to calculate a mean uplift rate of 2.3 ± 0.2 m/ky and a tilting rate of 0.022 ± 0.002 °/ky. The inferred recurrence interval of strandline‐forming earthquakes is ∼180 years. Combining coseismic uplift and aseismic subsidence during an earthquake cycle, the net gain in strandline elevation in this environment is ∼0.4 m per event.