Paleoseismic Data Research Articles

A case of paleoseismic evidence of normal fault capable of triggering an M>8 earthquake − study on Sertengshan range-front fault, north margin of Hetao Basin, China

The Sertengshan Range-front Fault (SRF) and Langshan Range-front Fault (LRF) constitute a normal fault system on the northern boundary of the Linhe Depression in its northwest, and an investigation of the LRF–SRF is significant for understanding the seismic activities of normal faults. We set four trenches at three study sites to reveal paleo-earthquakes in the western segment of the SRF (W-SRF) in this system, and established 5 surface rupture events along W-SRF since the Late Pleistocene. By integrating paleoseismic data from 27 trenches on multiple fault segments, we reconstructed the paleoseismic sequence for the LRF–SRF region since 15 ka, and identified 10 paleoseismic events with corresponding rupture segments and magnitudes. The slip rates at the three study sites decreased gradually from the west to east on the W-SRF, by comparing with previous studies, further evidencing that the tectonic activities of the two faults have gradually synchronized. Besides, based on the timing of the latest paleoseismic event (1.88−2.03 ka BP), earthquakes of > M 8 surface ruptures of >200 km can occur in the northwestern Ordos Block and its seismic behavior revealed the seismogenic possibility of normal faults triggering an M > 8 earthquake.

Contrasting sedimentary and long-lasting geochemical imprints of seismic shaking in a small, groundwater-fed lake basin (Klopeiner See, Eastern European Alps)

In slowly deforming tectonic settings (e.g., European Alps), large earthquakes occur too infrequently to be adequately represented in instrumental and historical records. This leads to uncertainties and inaccuracies of seismic hazard estimations. To extend the seismic record, lacustrine paleoseismologists usually resort to the sedimentary archive of large lakes where earthquakes can be recorded as mass-transport deposits and associated turbidites. The imprint of seismic shaking is generally more subtle and poorly understood in small lakes (<2 km2) with small catchments and therefore such sediment-starved basins are often neglected for paleoseismology. However, these basins might harbour additional information about past earthquakes, thus constituting a valuable supplement to other paleoseismic data. Here, we present the 18 ka-long paleoseismic record of Klopeiner See, a small and rather shallow groundwater-fed lake in the Eastern European Alps. Reflection seismic profiles and sediment cores reveal that several large earthquakes led to extensive mass-wasting in early Late-Glacial times when sedimentation rates were very high (~10 mm/yr). In the Early and Middle Holocene, low sedimentation rates (~0.2-0.5 mm/yr) may have decreased the lake´s sensitivity for recording seismic shaking and no imprints were found for paleo-earthquakes inferred from other records in the region. A short succession of turbidites at ca. 3160 cal BP suggests a burst of strong paleoseismic activity. This may have caused permanent modifications of inflowing ground water systems, archived as a permanent shift in the geochemical signal of the sediment. Such a period of enhanced paleoseismic activity was also inferred from the nearby Lake Wörthersee, but it remains unclear whether these represent the same earthquakes or migrating paleoseismicity. This study highlights the unexpected potential and peculiarities of paleoseismology on small ground-water fed lakes.

Earthquake forecasting from paleoseismic records

Forecasting large earthquakes along active faults is of critical importance for seismic hazard assessment. Statistical models of recurrence intervals based on compilations of paleoseismic data provide a forecasting tool. Here we compare five models and use Bayesian model-averaging to produce time-dependent, probabilistic forecasts of large earthquakes along 93 fault segments worldwide. This approach allows better use of the measurement errors associated with paleoseismic records and accounts for the uncertainty around model choice. Our results indicate that although the majority of fault segments (65/93) in the catalogue favour a single best model, 28 benefit from a model-averaging approach. We provide earthquake rupture probabilities for the next 50 years and forecast the occurrence times of the next rupture for all the fault segments. Our findings suggest that there is no universal model for large earthquake recurrence, and an ensemble forecasting approach is desirable when dealing with paleoseismic records with few data points and large measurement errors.

Late Quaternary activity of the NW Cardrona Fault, Otago, New Zealand

ABSTRACT We use new paleoseismic data and lidar to reassess late Quaternary activity of the NW (northwest) Cardrona Fault, a ∼60 km long range-bounding fault in Otago. Paleoseismic investigations of the NW Cardrona Fault were conducted in the 1980s, but findings were limited by a paucity of materials suitable for dating. Here, re-exposure and re-assessment of a 1980s trench at Macdonalds Creek provide stratigraphic evidence for two surface rupturing earthquakes on the NW Cardrona Fault. Through Optically Stimulated Luminescence dating and OxCal modelling, we estimate these earthquakes occurred at (mean ± 2 standard deviations, σ) 20.5 ± 7.8 ka and 8.7 ± 6.6 ka. The timings of these earthquakes are consistent with those proposed from re-interpretation of another 1980s trench on the NW Cardrona Fault in the Kawarau River valley, ∼25 km farther south. The slip rate and recurrence interval derived from the Macdonalds Creek trench data are 0.13 ± 0.05 mm/yr and 11,800 ± 5100 years (2σ error) respectively. These estimates imply lower slip rates and longer earthquake recurrence intervals on the NW Cardrona Fault than previously assessed.

Paleoseismology of the Sawtooth Fault and Implications for Fault Behavior in the Epicentral Region of the 2020 Mw 6.5 Stanley, Idaho, Earthquake

Abstract The 2020 moment magnitude (Mw) 6.5 Stanley, Idaho, earthquake raised questions about the history and extent of complex faulting in the northwestern Centennial Tectonic Belt (CTB) and its relation to the Sawtooth normal fault and Eocene Trans-Challis fault system (TCFS). To explore faulting in this area, we excavated a paleoseismic trench across the Sawtooth fault along the western margin of the CTB, and compared an early Holocene (9.1 ± 2.1 ka, 1σ) rupture at the site with lacustrine paleoseismic data and fault mapping in the 2020 epicentral region. We find: (1) a history of partial to full rupture of the Sawtooth fault (Mw 6.8–7.4), (2) that shorter ruptures (Mw≤6.9) are likely along distributed and discontinuous faults in the epicentral region, (3) that this complex system that hosted the 2020 earthquake is not directly linked to the Sawtooth fault, (4) that the northeast-trending TCFS likely plays a role in controlling fault length and rupture continuity for adjacent faults, and (5) that parts of the TCFS may facilitate displacement transfer between normal faults that accommodate crustal extension and rotation. Our results help unravel complex faulting in the CTB and imply that relict structures can help inform regional seismic hazard assessments.

Discovery of an Active Forearc Fault in an Urban Region: Holocene Rupture on the XEOLXELEK‐Elk Lake Fault, Victoria, British Columbia, Canada

AbstractSubduction forearcs are subject to seismic hazard from upper plate faults that are often invisible to instrumental monitoring networks. Identifying active faults in forearcs therefore requires integration of geomorphic, geologic, and paleoseismic data. We demonstrate the utility of a combined approach in a densely populated region of Vancouver Island, Canada, by combining remote sensing, historical imagery, field investigations, and shallow geophysical surveys to identify a previously unrecognized active fault, the XEOLXELEK‐Elk Lake fault, in the northern Cascadia forearc, ∼10 km north of the city of Victoria. Lidar‐derived digital terrain models and historical air photos show a ∼2.5‐m‐high scarp along the surface of a Quaternary drumlinoid ridge. Paleoseismic trenching and electrical resistivity tomography surveys across the scarp reveal a single reverse‐slip earthquake produced a fault‐propagation fold above a blind southwest‐dipping fault. Five geologically plausible chronological models of radiocarbon dated charcoal constrain the likely earthquake age to between 4.7 and 2.3 ka. Fault‐propagation fold modeling indicates ∼3.2 m of reverse slip on a blind, 50° southwest‐dipping fault can reproduce the observed deformation. Fault scaling relations suggest a M 6.1–7.6 earthquake with a 13 to 73‐km‐long surface rupture and 2.3–3.2 m of dip slip may be responsible for the deformation observed in the paleoseismic trench. An earthquake near this magnitude in Greater Victoria could result in major damage, and our results highlight the importance of augmenting instrumental monitoring networks with remote sensing and field studies to identify and characterize active faults in similarily challenging environments.

Channel Profiles Reveal Fault Activity along the Longmen Shan, Eastern Tibetan Plateau

Assessing fault activity in regions lacking Quaternary sedimentary constraints remains a global challenge. In this study, we used channel slope distribution to examine variations in rock uplift along faults. By comparing channel steepness with published low-temperature thermochronology and paleo-seismic data, we identified deformation changes both perpendicular to and along the Longmen Shan at various time scales. Our data revealed distinct fault segments displaying distinct thrust activities along the Longmen Shan’s strike. In the southern segment, the Dachuan fault exhibited the highest activity, and its movement had persisted for millions of years. In the central segment, the Wenchuan fault was active during theearly Quaternary but has become dormant since the late Pleistocene. Within the past millions of years, the Yingxiu and Pengguan faults displayed significant vertical displacement. Fault activity in the northern Longmen Shan was relatively weak, with the Qingchuan fault transitioning from thrust movement during the Neogene to pure strike-slip activity since the Pleistocene. Overall, the Dachuan and Huya faults exhibited deformation patterns similar to the Yingxiu fault during the Quaternary. Similar to the Yingxiu fault, which triggered the Wenchuan earthquake, the Dachuan and Huya faults possess the capacity to produce significant earthquakes in the future. The variations in deformation perpendicular to and along the Longmen Shan fault system underscore the importance of upper crustal shortening in shaping the rock uplift patterns and topography of the eastern Tibetan Plateau margin.

The New Zealand Paleoseismic Site Database, Version 1.0

AbstractSite-specific paleoseismic data are key datasets underpinning the parameterization of large (Mw∼7+) earthquakes on faults in seismic hazard models. For the New Zealand National Seismic Hazard Model 2022 Revision Project (NZ NSHM 2022), a new database, the NZ Paleoseismic Site Database (NZ PSDB), was developed that contains paleoseismic (slip rate, earthquake timings, recurrence interval [RI], and single-event displacement [SED]) data for crustal (upper plate) active faults. The first edition version 1.0 (v.1.0) was modeled on datasets underpinning the Third Uniform California Earthquake Rupture Forecast (UCERF3) and builds on and significantly improves previous NZ compilations. Key improvements include documentation of the data underlying these parameters, such as offset measurements and ages for slip rate, and assignment of quality rankings for key attributes. The NZ PSDB v.1.0 contains 2811 records, subdivided into slip rate (871), earthquake timings (953), and SED (987); the earthquake timings dataset also includes 320 last event and 98 RI records. Because of time constraints, the most effort went into compiling the slip-rate records, in priority from highest to lowest slip rates, and the least amount of time was spent on the SED dataset, prioritizing records from historical earthquakes. The compilation also aimed to obtain as much geographical spread as possible, but the records only lie on a relatively small (maximum, 24%) number of faults and fault sections in the NZ Community Fault Model v.1.0. The most significant spatial gaps are offshore and in the southern and western South Island. The NZ PSDB v.1.0 contains both published and unpublished data, and many sites were relocated using high-resolution basemaps. However, radiocarbon ages were not recalibrated using a consistent calibration curve. Developing a process to efficiently recalibrate radiocarbon ages, as well as filling key data gaps, are current focuses of work toward v.2.0.

First paleoseismic data from the Balkan Range

The normal faults along the Balkan range front integrate great tectonic relief, prominent basin-and-range morphology, historic seismic quiescence, and ongoing dilatation rate varying near zero. The official standards prioritize the instrumental seismicity in a random model of earthquake occurrence, and the region is assessed at low seismic hazard levels. The need of geologic data on earthquake occurrence motivated us to excavate the first paleoseismological trench in the Balkan Range. We trenched the Zlatitsa normal fault, which occupies the westernmost position within a zone of localized strain along the Central Balkan Range. The trench exposure revealed two surface-rupturing earthquakes generated at a long-term slip rate lower than 0.04 mm/yr, probably about 0.02 mm/yr for the past 42 thousand years. A stratigraphic hiatus between the earthquake records, coincident with the Last Glacial Maximum, results in large uncertainties in the estimated inter-event time, yielding 13.2–33.4 kyr (95 % confidence interval). The new data suggest that the strain is released by regionally distributed faulting. The Zlatitsa fault may form spatial clusters with the neighboring faults, analogous to the two M6.8–7.1 earthquake pairs in 1904 and 1928, south the studied location. The time elapsed since the most recent earthquake on the Zlatitsa fault is 1–15 % from the duration of the previous seismic cycle. The geodetic dilatation rate in the fault vicinity most likely exceeds several times the paleoseismic dilatation rate. However, the geodetic rate eastward from the fault is nearly identical to the paleoseismic rate obtained in the trench. Assuming exponentially decreasing fault reloading, the Zlatitsa fault may still undergo rapid stress restoration, whereas the time elapsed since the most recent earthquakes on faults to the east may be longer than the corresponding relaxation time.

The 1673 Tianzhen Earthquake and Rupture Behavior of the Yanggao‐Tianzhen Fault in the Northern Shanxi Graben, China

AbstractStudying the causative structure, extent, and magnitude of historical earthquakes is essential for regional seismic hazard assessment. There has been some debate in the literature regarding the epicenter, magnitude, and seismogenic source of the 1673 Tianzhen earthquake, one of the few documented strong earthquakes that occurred in northern Shanxi Graben, China. The Yanggao‐Tianzhen Fault (YTF) has long been recognized as a seismogenic fault; however, previous studies have suggested that the fault has not ruptured at the surface for several thousand years. In this work, we investigate fresh surface ruptures along the YTF based on field investigations, unmanned aerial vehicle photogrammetry, and ground‐penetrating radar surveys. Scarp formation at Yanjiabu is bracketed to the last 1.4 ± 0.6 kyr using optically stimulated luminescence dating. More importantly, we found that the Great Wall at Li'erkou (dated from 1546 CE) was likely faulted, which led us to conclude that the YTF is the causative fault of the 1673 event. Based on our measured fault parameters, the magnitude of the 1673 event was re‐estimated to Mw 6.8–7.2, suggesting that previous magnitudes were underestimated. Four trenches were excavated to investigate paleoearthquakes on the fault. Together with previous paleoseismic data, we suggest a bimodal rupture model in which the fault can produce multi‐segment rupturing events with large magnitudes, such as the 1673 event, and smaller partial or single‐segment rupturing events. Our results shed light on the reassessment of seismic hazards in the densely populated Yangao‐Tianzhen Basin and its nearby regions.

Channel Incision Ages to the Rescue: An Improved Age for the Penultimate Earthquake That Ruptured the Carrizo Section of the South-Central San Andreas Fault

ABSTRACT A primary step toward assessing the time and size of future earthquakes is identifying earthquake recurrence patterns in the seismic record. The San Andreas fault (SAF) is one of the most studied active faults in the world. However, there is no unequivocal interpretation of paleoseismic data to determine the timing and rupture extent of the earthquakes that occurred prior to the historical 1857 and 1906 ruptures. The penultimate earthquake is the least well-dated earthquake along the Cholame, Carrizo, and Big Bend sections of the SAF. The main reason for this poor determination is because the past few hundred years have seen large natural fluctuations in atmospheric C14 concentration. These fluctuations mean that a single radiocarbon date may yield a calibrated age consisting of several possible age ranges. At sites along frequently rupturing faults with historical ruptures, such as the SAF and the North Anatolian fault in Turkey, determining the incision age of channels displaced only by the most recent earthquake can place a tighter minimum limit on the possible age range of the penultimate earthquake. In our study, we dated five sandy fill units with the post-infrared infrared-stimulated luminescence method on single feldspar grains of channel Sieh 31 in the Carrizo Plain. The data indicate the channel Sieh 31, offset ∼6 m during the 1857 earthquake, incised before ∼1740 ± 30 C.E. (1σ). This new result trims the age constraint of the penultimate earthquake that ruptured the Carrizo section of the SAF determined at the nearby Bidart Fan site from 1640–1857 to 1631–1745 C.E., tightening the age constraint by nearly 80 yr. The revised mean recurrence interval for surface rupturing earthquakes along the Carrizo section of the south-central SAF is 117 yr (95% confidence interval 62–255 yr). This approach can improve paleoearthquake age and slip-per-earthquake constraints.

Improved Geological Slip Rate Estimations in the Complex Alhama de Murcia Fault Zone (SE Iberia) and Its Implications for Fault Behavior

AbstractFault slip rate is one of the most crucial parameters to characterize earthquake occurrence in fault‐based seismic hazard assessments (SHA). Accordingly, paleoseismic studies have increasingly focused on constraining this parameter in active faults worldwide. We present a comprehensive paleoseismic study in the Alhama de Murcia Fault (AMF), one of the most active faults in SE Spain and source of destructing earthquakes such as the 2011 Mw 5.2 Lorca event. Contrasting with previous studies, we integrate paleoseismic data from four fault strands in the AMF and, based on trench slip analysis and numerical dates, we derive slip rate estimates of each strand over the whole transect and assess their time variability. The AMF has a minimum net slip rate between 1.35+0.16/−0.10 and 1.64+0.16/−0.11 mm/yr for the past 18 ± 1 to 15.2 ± 1.1 ka. These results prove the importance of accounting for the complete sections of a geological structure as they are almost twice the previous estimates for a single fault branch. Slip rate variability is identified in the AMF, with cyclic acceleration‐quiescence patterns that could be related to stress field changes driven by fault interaction or synchronicity with neighboring faults (e.g., Carrascoy). We hope that the data presented here motivates their inclusion into forthcoming fault‐based SHAs. In this regard, limitations related to the lack of paleoseismic data for one fault strand, along with poor characterization of the strike component of slip and insufficient age control of the units for another strand are highlighted and need to be accounted for by modelers.

Mid- to Late-Quaternary Geomorphic and Paleoseismic Event History, Cheraw Fault, Southeastern Colorado

ABSTRACT Despite its subdued expression and isolated location within the Great Plains of southeastern Colorado, the 80 km long Cheraw fault may be one of the most active faults in North America east of the Rocky Mountains. We present geomorphic analyses, geochronology, and paleoseismic trenching data to (1) document the rupture history of the ∼45 km long southwestern section of the Cheraw fault over the past ∼19 ka, and (2) evaluate slip rate changes for the entire fault over the past ∼200 ka. Results from new trenches excavated at the Old Ranch site show evidence of four surface rupture events since ∼19 ka, each with an average vertical displacement of 0.75 ± 0.15 m. An additional event is likely only slightly older than ∼19 ka. Evidence for relatively small displacements at and near the Old Ranch site indicates that most of these earthquakes were magnitude (M) 7 or less and likely did not rupture the full length of the Cheraw fault. Since ∼19 ka, the average slip rate is ∼0.16 ± 0.3 mm/yr near the Old Ranch site with an average interevent time of 3–5 ka. New geochronologic data for mid- to late-Quaternary geomorphic surfaces cut by the Cheraw fault imply rapid incision by local Arkansas River tributaries from ∼145 to ∼100 ka. Maximum vertical offsets of 7–9 m for these surfaces indicate that from ∼19 to >200 ka the average slip rate was no greater than ∼0.03 mm/yr. The accelerated slip rate since ∼19 ka indicates a possible response to rapid erosional unloading and/or a limited late Cenozoic, <40 ka, paleoseismic history for the Cheraw fault.

Characterization of Fault Kinematics based on Paleoseismic Data in the Malbang area in the Central Part of the Ulsan Fault Zone

Characterization of Fault Kinematics based on Paleoseismic Data in the Malbang area in the Central Part of the Ulsan Fault Zone

First Paleoseismic Data from the Balkan Range

First Paleoseismic Data from the Balkan Range

The Pamir Frontal Thrust Fault: Holocene Full‐Segment Ruptures and Implications for Complex Segment Interactions in a Continental Collision Zone

AbstractThe Pamir Frontal Thrust (PFT) of the Trans‐Alai Range in Central Asia is the principal active fault of the intracontinental convergence zone between the Pamir and Tien Shan. Its northward propagation is reflected by frequent seismic activity and ongoing crustal shortening. Recent and historic earthquakes exhibit complex rupture patterns within and across seismotectonic segments bounding the Trans Alai, challenging our understanding of fault interaction and seismogenic potential. We provide paleoseismic data from five trenches along the central PFT segment (cPFT) and interpret five and possibly six paleoearthquakes that have ruptured since ∼7 ka and 16 ka, respectively. Our results indicate that at least three major earthquakes ruptured the full‐segment length and possibly crossed segment boundaries with a recurrence interval of ∼1.9 kyr and potential magnitudes of up to Mw 7.4. We did not find evidence for great (i.e., Mw ≥8) earthquakes. However, discrepancies between slip and rupture extent during apparent partial segment ruptures in the western half of the cPFT, combined with significantly higher scarp offsets, indicate a more mature fault section with potential for future fault linkage. We estimate an average rate of horizontal motion for the cPFT of 4.1 ± 1.5 mm/yr during the past ∼5 kyr, which does not fully match the GNSS‐derived present‐day shortening rate of ∼10 mm/yr. This suggests a complex distribution of strain accumulation and potential slip partitioning between the cPFT and additional faults and folds within the Pamir that may be associated with a partially locked regional décollement.

Paleoseismic history of the Manisa fault zone, Western Anatolia

The 45-km-long, E-W to NW-SE-striking Manisa Fault Zone, which constitutes the western section of the Gediz Graben, is characterized by a pure normal sense of motion with a minor strike-slip component. Even though there are numerous historical earthquakes have been listed in Western Anatolia Graben System, a few studies have been addressed on seismic sources of the earthquakes so far. According to existing literature, the western segment of the Manisa Fault Zone is well known characterized; however, the eastern segment of it has rarely been addressed. To decipher the Holocene seismotectonic behavior of the eastern segment of Manisa Fault Zone, trench-based paleoseismological analyses for the first time were performed along with it. To constrain the timing and frequency of past earthquakes, and elapse time from the last activation using the optically stimulated luminescence (OSL) and radiocarbon (14C) dating, we collected twenty-six colluvial and paleosol samples from the trenches. The obtained paleoseismic data show that (i) Manisa Fault Zone is responsible for six surface rupturing earthquakes since late Pleistocene-Holocene, which is occurred at 30.6 ± 8.8 ka (E1), 15.0 ± 5.0 ka (E2), 6.6 ± 1.3 ka (E3), 2.9 ± 1.3 ka (E4), 0.8 ± 0.4 ka (E5), and 0.1 ± 0.1 ka (E6), (ii) the recurrences interval of destructive earthquakes on Manisa Fault Zone was found between 0.95 kyr to 3.8 kyr for Holocene, and (iii) the elapsed time since the most recent surface ruptured earthquake on the Manisa Fault Zone is 159 year.

Latest Pleistocene–Holocene Incremental Slip Rates of the Wairau Fault: Implications for Long‐Distance and Long‐Term Coordination of Faulting Between North and South Island, New Zealand

AbstractWe use high‐resolution lidar microtopographic data and luminescence dating to constrain incremental Holocene–latest Pleistocene slip rates for the Wairau fault, a major dextral strike‐slip fault in the Marlborough Fault System, South Island, New Zealand. Our data come from two closely spaced study areas along the structurally simple, central portion of the fault: The well‐known Branch River terrace flight, and a previously undated series of offset risers and channel features several km to the east that we refer to as the Dunbeath site. Field work and mapping using lidar‐derived topography yields revised or novel measurements of nine fault offsets. We date those features using a post‐IR50‐IRSL225 infrared stimulated luminescence dating method, and a stratigraphically informed Bayesian age model. The dated slip history of the Wairau fault is further constrained using newly cataloged offset measurements collected along a ∼35 km stretch of the fault, and available paleoseismic data. Incremental slip rates are precisely computed using a Monte Carlo resampling scheme. Our results provide a nearly earthquake‐by‐earthquake record of incremental slip, with pronounced variations in incremental slip rate spanning multiple millennia and tens of m of slip. These extreme, multi‐millennial variations in fault slip rate have basic implications for earthquake occurrence, plate boundary lithosphere behavior, and probabilistic seismic hazard assessment.

Modeling of earthquake chronology from paleoseismic data: Insights for regional earthquake recurrence and earthquake storms in the Central Apennines

Among the geologic data, trenching records of paleoearthquakes represent an important input for the development of seismic hazard evaluations and, specifically, for the characterization of fault rupture behavior in time.Based on the available paleoseismological trenching data in the Central Apennines (Italy) we have compiled a new database of surface faulting earthquakes for 10 faults. The compilation contains 109 surface faulting events, occurred in the past ~28 kyr. Events younger than 7 kyr (59 dated events) are much better constrained and, therefore, are the basis for most of the analyses. Through a quantitative multistep method, we integrate paleoseismic trench data and treat them statistically, providing relevant improvement needed for trenching-based seismic hazard evaluation. Indeed, the combined analysis of trenching data from multiple sites on the same fault led to a reduction of the approximations and uncertainties in the rupture history of individual faults. The procedure was also applied on specific fault systems and indicated that the single faults may have occasionally ruptured simultaneously, or close in time, in the past. The whole set of age ranges was also statistically analyzed to produce regional earthquake scenarios for a period much longer than the millennial historical record. The built scenarios for the last 7 kyr define a regional mean inter event time (IET) of 230–240 yr, comparable with the average recurrence time of about 200 yr calculated for the region. We also identify the possible occurrence of earthquake storms, i.e., concentration of surface faulting earthquakes in the region within time periods of 200 yr, suggesting a variability in time of the seismic behavior of the faults, with alternation of peaks of activity with more “quiet” periods. Even though the paleoseismic data from the Apennines contains uncertainties, the results appear quite stable and promising for future applications in earthquake-hazard assessment.

Paleoseismological Findings at a New Trench Indicate the 1714 M8.1 Earthquake Ruptured the Main Frontal Thrust Over all the Bhutan Himalaya

The 1714 Bhutan earthquake was one of the largest in the Himalaya in the last millennium. We show that the surface rupture caused by this earthquake extended further to the east than previously known, it was at least 175 km long, with slip exceeding 11 m at our study site. The age of the surface rupture was constrained by a combination of radiocarbon and traditional optically stimulated luminescence dating of affected river sediments. Computations using empirical scaling relationships, fitting historical observations and paleoseismic data, yielded a plausible magnitude of Mw 8.1 ± 0.4 and placed the hypocentre of the 1714 Bhutan earthquake on the flat segment of the Main Himalayan Thrust (MHT), the basal décollement of the Himalayan orogen. Calculations of Coulomb stress transfer indicate that great earthquakes along the leading part of the MHT would cause surface rupture. In contrast, distal earthquakes may not immediately trigger surface rupture, although they would increase the stresses in the leading part of the MHT, facilitating future surface-rupturing earthquakes. Frontal earthquakes would also transfer stress into the modern foreland basin facilitating southward propagation of the MHT as a blind basal décollement. In conclusion, studies of surface-rupturing events alone likely underestimate the seismic slip along the Himalayan megathrust.