Structure and segmentation of the Ovindoli – Piano di Pezza – Campo Felice fault system (Central Apennines, Italy): Evolution and reactivation of inherited faults

2020 ◽  
Author(s):  
Matthieu Ferry ◽  
Stéphanie Gautier ◽  
Stéphane Mazzotti ◽  
Fabio Villani ◽  
Eric Stell ◽  
...  
Keyword(s):  
Surface Deformation ◽  
Slip Rate ◽  
Fault System ◽  
Normal Faults ◽  
Reverse Fault ◽  
Earthquake Rupture ◽  
Central Apennines ◽  
Active Deformation ◽  
Fault Surface ◽  
Cumulative Length

<p>Active deformation in the Central Apennines is mostly accommodated by NW-SE normal faults systems that produce moderate to large earthquakes at shallow depth. Recent examples include the 1915 Mw≈7 Avezzano earthquake (Fucino basin) and the 2009 Mw=6.1 L’Aquila earthquake (Aterno basin) which were both associated with major loss of life and massive damage to buildings and infrastructure. Here, we study the 40-km-long Ovindoli – Piano di Pezza – Campo Felice – Monti d’Ocre (OPCM) fault system, a major NNW-SSE system that potentially links the Fucino and the Aterno fault-systems. The OPCM exhibits linear and arcuate sections with four main segments and borders the eastern margin of the Aterno basin. Paleo-earthquake rupture data on the Piano di Pezza (PPF) and Campo Felice (CFF) faults exhibit remarkable synchronicity with the Fucino fault system, with the most recent surface-rupturing earthquake likely occurring in the XIV<sup>th</sup> century. In order to better understand the relationships between and earthquake rupture scenarios, we focus on the basin geometry and fault surface expression of the Piano di Pezza fault combining geomorphology and subsurface geophysics. We map the fault trace with unprecedented detail using terrestrial laser scanner surveys and quantify surface deformation affecting alluvial fans as well as glacial moraines. We obtain a mean vertical offset of 2.5 m +/- 0.3 m for the most recent features, well in agreement with paleoseismological data. Furthermore, we document slip distributions at different time scales along strike with a maximum value at the connection between the PPF and the OF. Beneath the scarp, geophysical data reveal a complex faulting geometry with several parallel strands and two minor blind splays. Electrical resistivity tomography images show a cumulative vertical offset of ~ 15 m affecting an interface attributed to the Last Glacial Maximum and confirm the high vertical slip across the fault zone. Gravimetric anomalies across the basin also indicate the sedimentary fill has recorded a maximum finite cumulative throw of the PdP fault system of 110-140 m. This suggests a maximum vertical slip rate of 0.2-0.3 mm/year since the Pleistocene, which contrasts with the high post-LGM slip rate estimated from trenches. Overall, our observations suggest that the arcuate PPF originally formed as a reverse fault during the Mio-Pliocene compressive stage and is now reactivated as an extensional horsetail-like feature by ruptures along a major strike-slip fault (OF). This finding points to the PPF as mostly built through ruptures along the OF leaking onto an inherited structure. The time-varying slip rates may also denote an episodic behavior marked by short periods of high seismic activity (a few centuries) and long intervals of seismic quiescence (a few millennia). Furthermore, possible earthquake rupture scenarios along the OPCM may encompass the whole OPCM fault system (cumulative length ca. 40 km) or rupture termination along the PPF (cumulative length ca. 15-20 km) with significantly different impacts over the populated Fucino and Aterno basins.</p>

scholarly journals Methodology for Earthquake Rupture Rate estimates of fault networks: example for the Western Corinth Rift, Greece

2017 ◽  
Author(s):  
Thomas Chartier ◽  
Oona Scotti ◽  
Hélène Lyon-Caen ◽  
Aurélien Boiselet
Keyword(s):  
Active Faults ◽  
Slip Rate ◽  
Seismic Hazard Assessment ◽  
System Level ◽  
Fault System ◽  
Normal Faults ◽  
Accurate Estimation ◽  
Probabilistic Seismic Hazard Assessment ◽  
Earthquake Rupture ◽  
Multiple Faults

Abstract. Modelling the seismic potential of active faults is a fundamental step of probabilistic seismic hazard assessment (PSHA). An accurate estimation of the rate of earthquakes on the faults is necessary in order to obtain the probability of exceedance of a given ground motion. Most PSHA studies consider faults as independent structures and neglect the possibility of multiple faults or fault segments rupturing simultaneously (Fault to Fault -FtF- ruptures). The latest Californian model (UCERF-3) takes into account this possibility by considering a system level approach rather than an individual fault level approach using the geological , seismological and geodetical information to invert the earthquake rates. In many places of the world seismological and geodetical information long fault networks are often not well constrained. There is therefore a need to propose a methodology relying only on geological information to compute earthquake rate of the faults in the network. In this methodology, similarly to UCERF-3, a simple distance criteria is used to define FtF ruptures and consider single faults or FtF ruptures as an aleatory uncertainty. Rates of earthquakes on faults are then computed following two constraints: the magnitude frequency distribution (MFD) of earthquakes in the fault system as a whole must follow an imposed shape and the rate of earthquakes on each fault is determined by the specific slip-rate of each segment depending on the possible FtF ruptures. The modelled earthquake rates are then confronted to the available independent data (geodetical, seismological and paleoseismological data) in order to weigh different hypothesis explored in a logic tree. The methodology is tested on the Western Corinth Rift, Greece (WCR) where recent advancements have been made in the understanding of the geological slip rates of the complex network of normal faults which are accommodating the ~15 mm/yr North-South extension. Modelling results show that geological, seismological extension rates and paleoseismological rates of earthquakes cannot be reconciled with only single fault rupture scenarios and require hypothesising a large spectrum of possible FtF rupture sets. Furthermore, in order to fit the imposed regional Gutenberg-Richter MFD target, some of the slip along certain faults needs to be accommodated either with interseismic creep or as post-seismic processes. Furthermore, individual fault’s MFDs differ depending on the position of each fault in the system and the possible FtF ruptures associated with the fault. Finally, a comparison of modelled earthquake rupture rates with those deduced from the regional and local earthquake catalogue statistics and local paleosismological data indicates a better fit with the FtF rupture set constructed with a distance criteria based on a 5 km rather than 3 km, suggesting, a high connectivity of faults in the WCR fault system.

scholarly journals Structural characterization of fault damage zones in carbonates (Central Apennines, Italy)

2021 ◽  
Author(s):  
Miriana Chinello ◽  
Michele Fondriest ◽  
Giulio Di Toro
Keyword(s):  
Fault Zone ◽  
Hanging Wall ◽  
Damage Zone ◽  
Normal Fault ◽  
Vertical Displacement ◽  
Fault System ◽  
Normal Faults ◽  
Central Apennines ◽  
Fault Surface ◽  
Fracture Intensity

<p>The Italian Central Apennines are one of the most seismically active areas in the Mediterranean (e.g., L’Aquila 2009, Mw 6.3 earthquake). The mainshocks and the aftershocks of these earthquake sequences propagate and often nucleate in fault zones cutting km-thick limestones and dolostones formations. An impressive feature of these faults is the presence, at their footwall, of few meters to hundreds of meters thick damage zones. However, the mechanism of formation of these damage zones and their role during (1) individual seismic ruptures (e.g., rupture arrest), (2) seismic sequences (e.g., aftershock evolution) and (3) seismic cycle (e.g., long term fault zone healing) are unknown. This limitation is also due to the lack of knowledge regarding the distribution, along strike and with depth, of damage with wall rock lithology, geometrical characteristics (fault length, inherited structures, etc.) and kinematic properties (cumulative displacement, strain rate, etc.) of the associated main faults.</p><p>Previous high-resolution field structural surveys were performed on the Vado di Corno Fault Zone, a segment of the ca. 20 km long Campo Imperatore normal fault system, which accommodated ~ 1500 m of vertical displacement (Fondriest et al., 2020). The damage zone was up to 400 m thick and dominated by intensely fractured (1-2 cm spaced joints) dolomitized limestones with the thickest volumes at fault oversteps and where the fault cuts through an older thrust zone. Here we describe two minor faults located in the same area (Central Apennines), but with shorter length along strike. They both strike NNW-SSE and accommodated a vertical displacement of ~300 m.</p><p>The Subequana Valley Fault is about 9 km long and consists of multiple segments disposed in an en-echelon array. The fault juxtaposes pelagic limestones at the footwall and quaternary deposits at the hanging wall. The damage zone is < 25 m  thick  and comprises fractured (1-2 cm spaced joints) limestones beds with decreasing fracture intensity moving away from the master fault. However, the damage zone thickness increases up to ∼100 m in proximity of subsidiary faults striking NNE-SSW. The latter could be reactivated inherited structures.</p><p>The Monte Capo di Serre Fault is about 8 km long and characterized by a sharp ultra-polished master fault surface which cuts locally dolomitized Jurassic platform limestones. The damage zone is up to 120 m thick and cut by 10-20 cm spaced joints, but it reaches an higher fracture intensity where is cut by subsidiary, possibly inherited, faults striking NNE-SSW.</p><p>Based on these preliminary observations, faults with similar displacement show comparable damage zone thicknesses. The most relevant damage zone thickness variations are related to geometrical complexities rather than changes in lithology (platform vs pelagic carbonates).  In particular, the largest values of damage zone thickness and fracture intensity occur at fault overstep or are associated to inherited structures. The latter, by acting as strong or weak barriers (sensu Das and Aki, 1977) during the propagation of seismic ruptures, have a key role in the formation of damage zones and the growth of normal faults.</p>

Quantifying fault reactivation risk in the western Gippsland Basin using geomechanical modelling

2013 ◽  
Vol 53 (1) ◽  
pp. 255 ◽  
Author(s):  
Ernest Swierczek ◽  
Cui Zhen-dong ◽  
Simon Holford ◽  
Guillaume Backe ◽  
Rosalind King ◽  
...  
Keyword(s):  
Structural Model ◽  
Strike Slip ◽  
Fault Reactivation ◽  
Fault System ◽  
Co2 Injection ◽  
Normal Faults ◽  
Surface Geometry ◽  
Northern Margin ◽  
Fault Surface ◽  
Gippsland Basin

The Rosedale Fault System (RFS) bounds the northern margin of the Gippsland Basin on the Southern Australian Margin. It comprises an anastomosing system of large, Cretaceous-age normal faults that have been variably reactivated during mid Eocene-Recent inversion. A number of large oil and gas fields are located in anticlinal traps associated with the RFS, and in the future these fields may be considered as potential storage sites for captured CO2. Given the evidence for geologically recent fault reactivation along the RFS, it is thus necessary to evaluate the potential impacts of CO2 injection on fault stability. The analysis and interpretation of 3D seismic data allowed the authors to create a detailed structural model of the western section of the RFS. Petroleum geomechanical data indicates that the in-situ stress in this region is characterised by hybrid strike-slip to reverse faulting conditions where SHmax (40.5 MPa/km) > SV (21 MPa/km) ~ Shmin (20 MPa/km). The authors performed geomechanical modelling to assess the likelihood of fault reactivation assuming that both strike-slip and reverse-stress faulting regimes exist in the study area. The authors’ results indicate that the northwest to southeast and east-northeast to west-southwest trending segments of the RFS are presently at moderate and high risks of reactivation. The authors’ results highlight the importance of fault surface geometry in influencing fault reactivation potential, and show that detailed structural models of potential storage sites must be developed to aid risk assessments before injection of CO2.

Comb-veins as a marker for crustal-scale fluid circulation: insight from geochronological (U-Th dating), geochemical, and field to microstructural analyses along the seismogenic Val Roveto Fault (central Apennines, Italy)

2020 ◽  
Author(s):  
Luca Smeraglia ◽  
Stefano M. Bernasconi ◽  
Fabrizio Berra ◽  
Andrea Billi ◽  
Chiara Boschi ◽  
...  
Keyword(s):  
Active Faults ◽  
Shallow Groundwater ◽  
Optical Microscope ◽  
Maximum Temperature ◽  
Normal Faults ◽  
Fluid Circulation ◽  
Central Apennines ◽  
Study Region ◽  
Fault Surface ◽  
Geochemical Signature

<p>Comb-veins are mineral-filled fractures oriented perpendicular to fault surfaces, with their intersection with the fault surface generating lineations that are perpendicular to the downdip slip direction. Despite the large occurrence along normal faults within seismogenic extensional tectonic settings (i.e. Greece, Turkey, Italy), their origin, geochemical signature, and kinematics are still poorly constrained. Here we present the first multidisciplinary study, combining field to microscale observations (optical microscope and cathodoluminescence) with geochemical-geochronological analyses (U-Th dating, stable-clumped isotopes, Strontium isotopes, whole-rock geochemistry, and fluid inclusions), on calcite-filled comb-veins cutting through the principal surface of the seismogenic Val Roveto Fault in the central Apennines, Italy. We show that comb-veins precipitated in Late Pleistocene time (between 300 ky and 140 ky) below the present-day outcrop level at a maximum depth of ∼350 m and temperatures between 32 and 64°C from deep-seated fluids modified by reactions with crustal rocks and with a mantle contribution (up to ∼39%). The observed geochemical signature and temperatures are not compatible which those of cold meteoric water and/or shallow groundwater (maximum temperature of 12 °C) circulating within shallow aquifers (≤ 500 m depth) in the study region. Therefore, we propose that deep-seated crust/mantle-derived warm fluids were squeezed upward during earthquakes and were hence responsible for calcite precipitation at shallow depths in co-seismic comb fractures. As comb-veins are rather common, particularly along seismogenic normal faults, we suggest that further studies are necessary to test whether these veins are often of co-seismic origin. If so, they may become a unique and irreplaceable tool to unravel the seismic history and crustal-scale fluid circulation of active faults.</p>

scholarly journals An integrated structural and GPS study of the Jalpatagua fault, southeastern Guatemala

Geosphere ◽  
2020 ◽  
Author(s):  
Bridget Garnier ◽  
Basil Tikoff ◽  
Omar Flores ◽  
Brian Jicha ◽  
Charles DeMets ◽  
...  
Keyword(s):  
El Salvador ◽  
Global Positioning System ◽  
Slip Rate ◽  
Fault Analysis ◽  
Fault System ◽  
Normal Faults ◽  
Central Section ◽  
Global Positioning ◽  
Major Fault ◽  
Fault Trace

The Jalpatagua fault in Guatemala accommodates dextral movement of the Central America forearc. We present new global positioning system (GPS) data, minor fault analysis, geochronological analyses, and analysis of lineaments to characterize deformation along the fault and near its terminations. Our data indicate that the Jalpatagua fault terminates at both ends into extensional regions. The western termination occurs near the Amatitlan caldera and the southern extension of the Guatemala City graben, as no through-going structures were observed to continue west into the active volcanic arc. Along the Jalpatagua fault, new and updated GPS site velocities are consistent with a slip rate of 7.1 ± 1.8 mm yr–1. Minor faulting along the central section of the fault includes: (1) N-S–striking normal faults accommodating E-W elongation; and (2) four sets of strike-slip faults (oriented 330°, 020°, 055°, and 295°, parallel to the Jalpatagua fault trace). Minor fault arrays support dextral movement along a major fault in the orientation of the Jalpatagua fault. GPS and fault data indicate that the Jalpatagua fault terminates to the east near the Guatemala–El Salvador border. Data delineate a pull-apart basin southeast of the fault termination, which is undergoing transtension as the Jalpatagua fault transitions into the El Salvador fault system to the east. Within the basin, minor faulting and lineations trend to the NW and accommodate NE-directed elongation. This faulting differs from E-W elongation observed along the Jalpatagua fault and is more similar to minor faults within the El Salvador fault system.

36Cl exposure dating of post-glacial features along the Mt Vettore Fault (Central Apennines, Italy) constraining fault slip rate and last glacial advance.

2021 ◽  
Author(s):  
Lea Pousse-Beltran ◽  
Lucilla Benedetti ◽  
Jules Fleury ◽  
Paolo Boncio ◽  
Valery Guillou ◽  
...  
Keyword(s):  
Slip Rate ◽  
Fault Slip ◽  
Fault System ◽  
Central Apennines ◽  
Climate Conditions ◽  
Last Glacial ◽  
Exposure Dating ◽  
Fault Systems ◽  
Fault Slip Rate ◽  
Exposure Ages

<p>In the Central Apennines (Italy), up to now, no absolute dating directly based on the moraines has been carried out to constrain glacial oscillation. However, climatic constrains are often used in the Central Apennine to estimate long term (> 10 ka) fault slip rate. In addition slip rate assessments based on offset morphotectonic markers on the main branches of fault systems and encompassing several seismic cycles (> 10 ka) are sparse. This is particularly true for the Monte Vettore-Monte Bove fault system which triggered the 2016-2017 seismic sequence. We thus provide new assessment for the vertical slip rates along the Mt Vettore-Mt Bove fault system.  Offset measurements were made using a 5-cm resolution DEM obtained through a drone survey and constrain a fault scarp height of 15.5 ± 1.4 m and a cumulative offset of 32-40.5 m. Samples were collected from the Valle Lunga terminal moraine at 1710 m asl and yield <sup>36</sup>Cl exposure ages of 12.7 + 2.2/-1.9 ka while the flat, abraded surface located on top of the tectonic scarp yield <sup>36</sup>Cl exposure ages of 23.4 + 5.3/-4.3 ka. Assuming the offset started to accumulate when climate conditions allow its preservation, thus once the surface was abandoned, we constrain a vertical slip rate of 1.2 ± 0.2 mm/yr along the master branch of the Mt Vettore normal fault.  This rate is higher than the ones previously obtained from trenches along secondary splays of the Mt Vettore-Mt Bove and on the Norcia fault systems. Besides, the yielded chronology for the last glacial maximum in that area at ~23 ka is in good agreement with the timing previously proposed for the LGM in the Apennines.</p>

scholarly journals The Ulakhan fault surface rupture and the seismicity of the Okhotsk–North America plate boundary

Solid Earth ◽  
2019 ◽  
Vol 10 (2) ◽  
pp. 561-580 ◽  
Author(s):  
David Hindle ◽  
Boris Sedov ◽  
Susanne Lindauer ◽  
Kevin Mackey
Keyword(s):  
North America ◽  
Plate Tectonics ◽  
Plate Boundary ◽  
Slip Rate ◽  
Field Work ◽  
Alluvial Fan ◽  
Age Dating ◽  
Aerial Photographs ◽  
Fault System ◽  
Fault Surface

Abstract. New field work, combined with analysis of high-resolution aerial photographs, digital elevation models, and satellite imagery, has identified an active fault that is traceable for ∼90 km across the Seymchan Basin and is part of the Ulakhan fault system, which is believed to form the Okhotsk–North America plate boundary. Age dating of alluvial fan sediments in a channel system that is disturbed by fault activity suggests the current scarp is a result of a series of large earthquakes (≥Mw 7.5) that have occurred since 11.6±2.7 ka. A possible channel feature offset by 62±4 m associated with these sediments yields a slip rate of 5.3±1.3 mm yr−1, in broad agreement with rates suggested from global plate tectonics. Our results clearly identify the Ulakhan fault as the Okhotsk–North America plate boundary and show that tectonic strain release is strongly concentrated on the boundaries of Okhotsk. In light of our results, the likelihood of recurrence of Mw 7.5 earthquakes is high, suggesting a previously underestimated seismic hazard across the region.

Laramide Uplift near the Ray and Resolution Porphyry Copper Deposits, Southeastern Arizona: Insights into Regional Shortening Style, Magnitude of Uplift, and Implications for Exploration

2020 ◽  
Vol 115 (1) ◽  
pp. 153-175
Author(s):  
Daniel A. Favorito ◽  
Eric Seedorff
Keyword(s):  
Porphyry Copper ◽  
Black Hills ◽  
Fault System ◽  
Normal Faults ◽  
Reverse Fault ◽  
Geologic Mapping ◽  
Porphyry Copper Deposits ◽  
Kinematic Modeling ◽  
Copper Deposits ◽  
Strike Direction

Abstract This study integrates new geologic mapping and structural analysis with previous work near Walnut Canyon and Telegraph Canyon to address the style and magnitude of shortening and the relationship between contractional structures and porphyry preservation and localization between the Ray and Resolution porphyry copper deposits. Cenozoic extensional structures were superimposed on earlier contractional structures formed during the Laramide orogeny, which dates from ~80 to 50 Ma. This superposition requires that Cenozoic normal faults be restored prior to analysis of Laramide contractional structures and their relationship to nearby porphyry copper deposits. Five distinct sets of normal faults within the study area progressively tilted the region 65° east. The amount of extension was 10.3 km or 276%. Using key constraints such as offset strata, cutoff angles between faults and various units, and Laramide fault geometries, the study area was structurally reconstructed and verified using 2-D kinematic modeling of reverse fault offset and related folding. Total shortening is 7.2 km or 98%. Laramide reverse faults are interpreted as thick-skinned basement-cored uplifts, because they restore to moderate angles, have related fault-propagation folds, and involve significant crystalline basement rock. The Telegraph Canyon reverse fault has at least 5.3 km of offset, and the Walnut Canyon reverse fault has 3.2 km. The preferred estimate of the total vertical uplift for the fault system is 5.2 km but could be several kilometers greater. The restored strike direction of these faults combined with mid-Cenozoic erosion surfaces throughout the region suggests that this fault system may be responsible for the Laramide uplift of the Tortilla Mountains and Black Hills. In addition, most major porphyry centers appear to have been intruded into the footwall of this large uplift, with local examples including Ray and Resolution, suggesting that topography generated from this uplift may have been critical to preservation of these ore systems. Though definitive crosscutting relationships do not exist in the immediate map area, geologic relationships in a broader area suggest that shortening here began after 74 Ma and, in the Ray area, had ended by ~69 Ma and that porphyry formation postdated reverse faulting by as much as 5 m.y. to as little as <1 m.y.

scholarly journals Extension and slip rate partitioning in NW Iran constrained by GPS measurements

2011 ◽  
Vol 1 (4) ◽  
pp. 286-304 ◽  
Author(s):  
A. Rastbood ◽  
B. Voosoghi
Keyword(s):  
Large Scale ◽  
Slip Rate ◽  
Strike Slip ◽  
Fault System ◽  
Arabian Plate ◽  
Normal Faults ◽  
Gps Data ◽  
Gps Measurements ◽  
Nw Iran ◽  
Himalayan Mountain

Extension and slip rate partitioning in NW Iran constrained by GPS measurementsConvergence of 22±2 mm yr-1 between the northward motion of the Arabian Plate relative to Eurasia at N8° ±5° E is accommodated by a combination of thrust and strike-slip faults in different parts of Iran. Dislocation modeling is used to examine the GPS data for this part of the Alpine-Himalayan mountain belt with more concentration in NW Iran. First, the vectors due to known Arabia-Eurasia rotation are reproduced by introducing structures that approximate the large-scale tectonics of the Middle East. Observed features of the smaller scale fault system are then progressively included in the model. Slip rate amplitudes and directions adjusted to fit available GPS data. Geological evidences show strike-slip and reverse-slip faulting in NW Iran, but GPS data show normal faults in this region too. By slip partitioning we propose four locations for normal faults based on extensions observed by GPS data. Slip rate values were estimated between 2 ~ 5 mm/yr for proposed normal faults. Our modeling results prove that the NW Iran is not only affected by Arabia-Eurasia collision but also contributes in the subduction motion of the South Caspian and Kura basins basement beneath the Apsheron-Balkhan sill and the Great Caucasus respectively.

Assessment of the Style and Timing of Surficial Deformation Along the Central Reelfoot Scarp, Lake County, Tennessee

1992 ◽  
Vol 63 (3) ◽  
pp. 349-356 ◽  
Author(s):  
Keith I. Kelson ◽  
Roy B. VanArsdale ◽  
Gary D. Simpson ◽  
William R. Lettis
Keyword(s):  
Late Holocene ◽  
Surface Deformation ◽  
Ground Surface ◽  
Normal Faults ◽  
Reverse Fault ◽  
Seismic Zone ◽  
Vertical Separation ◽  
Near Surface ◽  
Recent Earthquakes ◽  
Marker Beds

Abstract Stratigraphic and structural relations exposed in a 90-m-long trench across the Reelfoot scarp in the central New Madrid Seismic Zone (NMSZ) provide data to assess the style and timing of late Holocene tectonic surficial deformation in the central NMSZ. Near-surface deposits exposed in the trench include natural levee, overbank, colluvial, and liquefaction-related deposits. The levee deposits consist of fine-grained, cross-bedded sands and silty sands and are overlain by clayey overbank and scarp-derived colluvial deposits. Liquefaction-related features include sand dikes and sills that intrude into the levee and overbank deposits, and a possible older extrusive sand deposit. Charcoal and archaeological artifacts from a deposit inset into and overlying the levee deposits suggest that the levee deposits are older than about A.D. 800 to 900. Charcoal from the overbank deposits yielded an age estimate of cal. A.D. 1310 ± 90; charcoal from the overlying colluvial deposits yielded an age estimate of cal. A.D. 1540 ± 90. Distinct marker beds within the levee deposits define a broad monocline that parallels the ground surface and exhibits more than 5 m of down-to-the-east vertical separation. This fold consists, in part, of four smaller-scale flexures each having amplitudes of about 1 m. Associated with these flexures are numerous west-dipping normal faults that have a total net vertical separation of about 0.4 m in a down-to-the-west sense, which is opposite in sense to that exhibited by the scarp and the silty marker beds. We interpret that these faults are related to extension in the crest of the monocline, and that the monocline represents deformation above a west-dipping reverse fault that reaches or approaches the ground surface east of the trench and the base of the scarp. At the trench site, this interpretation places the surface projection of the fault near the western margin of Reelfoot Lake. Stratigraphic relations exposed in the trench and several shallow boreholes permit identification of at least one and possibly three late Holocene earthquakes. Stratigraphic relations and age-estimates best support the interpretation that the unweathered liquefaction related features exposed in the trench are a result of the 1811–12 earthquakes, and that the scarp-derived colluvial deposits are a result of a prior event. Radiocarbon analyses show that this penultimate event occurred between about A.D. 1310 and A.D. 1540. Stratigraphic evidence of a third event prior to about A.D. 900 is present but equivocal. Given that the most-recent event occurred in A.D. 1812, we estimate that the time between the two most-recent earthquakes large enough to produce liquefaction and/or surface deformation was about 200 to 600 years.

Sign in / Sign up

Export Citation Format

Share Document