Reelfoot Fault Research Articles

Drought-induced seismicity modulation in the New Madrid Seismic Zone, central United States

The New Madrid Seismic Zone (NMSZ) in the central United States is a seismically active intraplate region composed of two fault segments: the Reelfoot and Cottonwood Grove fault segments. It has witnessed several major earthquakes, including a devastating 1811–1812 earthquake sequence of three ~M7 events. Nearly 200 years later, earthquakes still continue today in this complex seismic zone. This seismic zone is located in a domain with higher hydrological load than surrounding regions, which may play a crucial role in seismicity modulation. However, the hydrological unloading or loading-induced seismicity modulation and the underlying earthquake dynamics on this stable plate interior remain equivocal. Our study demonstrates that increased climate variability and drought-induced hydrological unloading can be potential drivers for the crustal stress change in the upper Mississippi embayment and seismicity modulation in the NMSZ. The seismicity rate associated with the Reelfoot fault segment shows ~60% increase during drought-induced prolonged drier periods, linked to La Niña cycles, than during the relatively wetter periods. However, such a feature is lacking for the seismicity associated with the Cottonwood Grove fault segment. We argue that the near-lithostatic pore pressure condition explicitly on the Reelfoot fault segment leads to an increase in the amplitude of the velocity perturbation, making this fault segment more susceptible to seismicity modulation on a multi-annual or annual time scale by the resonance destabilization process.

Influence of Farallon Slab Loading on Intraplate Stress and Seismicity in Eastern North America in the Presence of Pre‐Existing Weak Zones

AbstractDespite the stability of the continental interior, eastern North America has hosted many significant historical earthquakes. Seismicity concentrates within tectonically inherited structures, which can act as weak zones where stress accumulates. Within these zones, systematic stress rotations may be explained by long‐wavelength sources. We test the hypothesis that mantle‐flow driven by the Farallon slab contributes to intraplate seismicity via the reactivation of pre‐existing faults. We model the stress field using seismically constrained global high‐resolution finite‐element flow models with CitcomS. To isolate the slab's effect, we vary its buoyancy between a case of neutrality and a case with full negative thermal buoyancy derived from tomography. Low‐viscosity lithospheric weak zones located at failed rifts, loaded by a mass anomaly at depth, transmit elevated stresses to the overlying crust. The sinking of the Farallon slab drives localized mantle flow beneath the central‐eastern US, generating a large stress amplification of 100–150 MPa peaking over the New Madrid Seismic Zone (NMSZ). This stress amplification exerts a continent‐wide clockwise rotation on the stress field, which in the presence of weak zones reproduces some observed deviations of the seismically inferred SHmax from the regional borehole SHmax, bringing optimally oriented faults, closer to failure, some of which are associated with major historical earthquakes, including the Reelfoot Fault in the NMSZ and the Timiskaming Fault in Western Quebec. However, stronger lithospheric viscosity gradients, shallower weak zones, or weaker faults are still needed to fully reproduce the observed stress field in some areas.

A New Madrid Seismic Zone Fault System Model From Relative Event Locations and Application of Optimal Anisotropic Dynamic Clustering

AbstractA refined model of fault structure in the active New Madrid Seismic Zone (NMSZ) is presented based on relocated hypocenters and application of a statistical clustering method to determine fault planes. Over 200 earthquakes are recorded in the NMSZ every year, but the three‐dimensional (3‐D) fault structure is difficult to determine because the zone is covered by thick, Mississippi Embayment sediment. The distribution of earthquakes in the NMSZ indicates four major arms of seismicity, suggesting the presence of a northeast‐southwest trending strike‐slip fault system with a major northwest trending, contractional stepover fault. Relocation of 4,131 earthquakes using HypoDD resulted in major improvement in the depiction of fault structure in the NMSZ including three‐dimensional structural variations along the along the Reelfoot fault (RF) and a well‐defined intersection of the Axial and RFs. Optimal Anisotropic Dynamic Clustering analysis of the relocated hypocenters produced a fault model consisting of 12, well resolved planes. The RF is continuous along strike from the northern end to the Ridgely fault, located south of the intersection with the Axial fault (AF). The crosscutting Ridgely fault may serve as a barrier to rupture propagation along the entire fault. The strike‐slip arms of seismicity are well resolved and correspond to near vertical planes. Three planes are resolved in the southern part of the AF and are associated with the Osceola igneous complex.

Quaternary Reelfoot Fault Deformation in the Obion River Valley, Tennessee, USA

AbstractBlind reverse faults are challenging to detect, and earthquake records can be elusive because deep fault slip does not break the surface along readily recognized scarps. The blind Reelfoot fault in the New Madrid seismic zone in the central United States has been the subject of extensive prior investigation; however, the extent of slip at the southern portion of the fault remains unconstrained. In this study, we use lidar to map terraces and lacustrine landforms in the Obion River valley and investigate apparent broad folding resulting from slip on the buried Reelfoot fault. We compare remote surface mapping results with three auger boreholes in the ∼24 ka Finley terrace and interpret apparent warping as due to tectonic folding and not stratigraphic thickening. We combine our results with historical records of coseismic lake formation that indicate surface deformation dammed the Obion River in the 1812 CE earthquake. Older terraces (deposited at least 35–55 ka) record progressive fold scarps ≥1, ≥2, and ≥8 m high indicating a long record of earthquakes predating the existing paleoseismic record. Broad, distributed folding above the Reelfoot fault into the Obion River valley is consistent with a deep active fault tip along the southern reaches of the fault. Our analyses indicate the entire length of the fault (≥70 km) is capable of rupture and is more consistent with longer rupture scenarios.

Four Major Holocene Earthquakes on the Reelfoot Fault Recorded by Sackungen in the New Madrid Seismic Zone, USA

AbstractThree sequences of well‐documented, major ~M7+ earthquakes (1811–1812, ~1450, and ~900 CE) in the New Madrid seismic zone, USA, contribute significantly to seismic hazard in the region. However, it is unknown whether this <550‐year recurrence interval has been constant throughout the Holocene given limited geomorphic evidence of prior earthquakes. We extend the record of paleoearthquakes along the Reelfoot fault via investigation of ridgetop gravitational failure features, interpreted as sackungen. The sackungen occur in bluffs along the eastern margin of the Mississippi River floodplain and are concentrated near (<15 km) the southwest dipping Reelfoot reverse fault. A paleoseismic trench excavated across sackungen at the Paw Paw site exposed four packages of colluvial sediment that postdate 30‐ to 11‐ka Peoria Loess. We interpret the colluvial packages to have been deposited following episodic failure of the sackungen as a result of strong ground motions from the following sequence of earthquakes: event 4, 1640 ± 1730 BCE; event 3, 270 ± 670 CE; event 2, 1430 ± 380 CE; and event 1, 1810 ± 50 CE (2 sigma). Event timing corresponds to previously documented earthquakes and represents the longest archive of paleoearthquakes on the Reelfoot fault. If the trenched sackungen record all major Reelfoot fault earthquakes, our observations in combination with prior investigations indicate a period of quiescence from at least 11 to 4.4 ka, followed by four major seismic events culminating in the 1811–1812 CE sequence. This clustered earthquake recurrence pattern helps place bounds on seismic hazard and geodynamic models in the New Madrid seismic zone.

Coseismic Sackungen in the New Madrid Seismic Zone, USA

AbstractHigh‐resolution lidar reveals newly recognized evidence of strong shaking in the New Madrid seismic zone in the central United States. We mapped concentrations of sackungen (ridgetop spreading features) on bluffs along the eastern Mississippi River valley in northwestern Tennessee that likely form or are reactivated during large earthquakes. These sackungen are concentrated on the hanging wall of the Reelfoot reverse fault and show a preferential orientation indicating ground failure normal to fault strike. These observations suggest that the sackungen record one or more earthquakes on the southern Reelfoot fault since the deposition of the ~30‐ to 11‐ka Peoria Loess and potentially constrain the minimum intensity of near‐fault ground motion. This study demonstrates that sackungen can be used to infer fault source and mechanism and, in combination with field‐based techniques, improve paleoseismic records and seismic hazard models.

Continuity of the Reelfoot Fault across the Cottonwood Grove and Ridgely Faults of the New Madrid Seismic Zone

Previous investigators have argued that the northwest‐striking Reelfoot fault of northwest Tennessee and southeastern Missouri is segmented. One segment boundary is at the intersection of the northeast‐striking Cottonwood Grove and Ridgely strike‐slip faults with the Reelfoot fault. We use seismic reflection and geologic mapping to locate and determine the history of the Reelfoot South fault across this boundary zone. One reflection profile revealed a southwest‐dipping (81°) Reelfoot South reverse fault that displaces the top of the Paleozoic 65 m, Cretaceous 40 m, Paleocene 31 m, Eocene Wilcox Group 20 m, and Eocene Memphis Sand 16 m. A second reflection profile reveals a north‐dipping (84°) reverse fault 4.3 km south of the Reelfoot South fault, which defines the southwest margin of the Tiptonville dome. A geologic profile of the base of the ∼3.1 Ma Upland complex (Mississippi River terrace alluvium) within the Mississippi River bluffs reveals ∼6 m of displacement across the Reelfoot South fault. Similarly, Quaternary stream terrace distribution suggests ∼6 m of Reelfoot South hanging‐wall (Tiptonville dome) uplift that is probably Holocene. Fault strike trends show the Reelfoot fault and its hanging‐wall Tiptonville dome are not laterally offset across the Cottonwood Grove and Ridgely faults. The Reelfoot South fault northwest and southeast of the Cottonwood Grove and Ridgely faults has very similar vertical displacement on common stratigraphic marker horizons in the upper 900 m. These data indicate the Reelfoot fault/Tiptonville dome has acted as one continuous fault zone across the Cottonwood Grove and Ridgely faults since Late Cretaceous.

Crustal deformation in the New Madrid seismic zone and the role of postseismic processes

AbstractGlobal Navigation Satellite System data across the New Madrid seismic zone (NMSZ) in the central United States over the period from 2000 through 2014 are analyzed and modeled with several deformation mechanisms including the following: (1) creep on subsurface dislocations, (2) postseismic frictional afterslip and viscoelastic relaxation from the 1811–1812 and 1450 earthquakes in the NMSZ, and (3) regional strain. In agreement with previous studies, a dislocation creeping at about 4 mm/yr between 12 and 20 km depth along the downdip extension of the Reelfoot fault reproduces the observations well. We find that a dynamic model of postseismic frictional afterslip from the 1450 and February 1812 Reelfoot fault events can explain this creep. Kinematic and dynamic models involving the Cottonwood Grove fault provide minimal predictive power. This is likely due to the smaller size of the December 1811 event on the Cottonwood Grove fault and a distribution of stations better suited to constrain localized strain across the Reelfoot fault. Regional compressive strain across the NMSZ is found to be less than 3 × 10−9/yr. If much of the present‐day surface deformation results from afterslip, it is likely that many of the earthquakes we see today in the NMSZ are aftershocks from the 1811–1812 New Madrid earthquakes. Despite this conclusion, our results are consistent with observations and models of intraplate earthquake clustering. Given this and the recent paleoseismic history of the region, we suggest that seismic hazard is likely to remain significant.

Ground‐Motion Simulations of 1811–1812 New Madrid Earthquakes, Central United States

We performed a suite of numerical simulations based on the 1811–1812 New Madrid seismic zone (NMSZ) earthquakes, which demonstrate the importance of 3D geologic structure and rupture directivity on the ground‐motion response throughout a broad region of the central United States (CUS) for these events. Our simulation set consists of 20 hypothetical earthquakes located along two faults associated with the current seismicity trends in the NMSZ. The hypothetical scenarios range in magnitude from M 7.0 to 7.7 and consider various epicenters, slip distributions, and rupture characterization approaches. The low‐frequency component of our simulations was computed deterministically up to a frequency of 1 Hz using a regional 3D seismic velocity model and was combined with higher‐frequency motions calculated for a 1D medium to generate broadband synthetics (0–40 Hz in some cases). For strike‐slip earthquakes located on the southwest–northeast‐striking NMSZ axial arm of seismicity, our simulations show 2–10 s period energy channeling along the trend of the Reelfoot rift and focusing strong shaking northeast toward Paducah, Kentucky, and Evansville, Indiana, and southwest toward Little Rock, Arkansas. These waveguide effects are further accentuated by rupture directivity such that an event with a western epicenter creates strong amplification toward the northeast, whereas an eastern epicenter creates strong amplification toward the southwest. These effects are not as prevalent for simulations on the reverse‐mechanism Reelfoot fault, and large peak ground velocities (>40 cm/s) are typically confined to the near‐source region along the up‐dip projection of the fault. Nonetheless, these basin response and rupture directivity effects have a significant impact on the pattern and level of the estimated intensities, which leads to additional uncertainty not previously considered in magnitude estimates of the 1811–1812 sequence based only on historical reports. The region covered by our simulation domain encompasses a large portion of the CUS centered on the NMSZ, including several major metropolitan areas. Based on our simulations, more than eight million people living and working near the NMSZ would experience potentially damaging ground motion and modified Mercalli intensities ranging from VI to VIII if a repeat of the 1811–1812 earthquakes occurred today. Moreover, the duration of strong ground shaking in the greater Memphis metropolitan area could last from 30 to more than 60 s, depending on the magnitude and epicenter. Online Material: Tables of 1D velocity models used to generate the high‐frequency synthetics, and figures of source models and peak ground motion synthetics.

Relocation of Earthquakes in the New Madrid Seismic Zone: Estimation of 1D Velocity Structure and Geometry of a Seismogenic Fault

Abstract Determination of reliable hypocenters of earthquakes is crucial to earthquake seismology and to evaluate hazards associated with earthquakes. There are many associated computer codes for this purpose; however, most of the location algorithms are designed to determine hypocentral parameters based on previously determined velocity models. In contrast, we employed a location method that is independent of the initial velocity model, using a genetic algorithm (GA) to determine an optimal 1D velocity model and the locations of earthquakes. Using this GA, we relocated earthquakes that occurred in the New Madrid Seismic Zone (NMSZ) in the central United States between October 1989 and August 1992. The goal of this work was to delineate the possible fault planes by reliable relocation of those earthquakes and to determine a 1D velocity structure for the NMSZ. A total of 502 earthquakes recorded by 37 Portable Array for Numerical Data Acquisition (PANDA) stations were used in the relocation study. In the relocation process, the root mean square travel‐time residuals were reduced by ∼35%, corresponding to an average of 2.3 km deeper in depth, 0.7 km shift in latitude, and 0.8 km shift in longitude compared with those in the initial catalog locations. The hypocenters of the earthquakes can be subdivided into four groups based on their spatial distributions. The group that corresponds to the Cottonwood Grove fault (CGF) in the southwestern NMSZ represents a very steep plane, whereas the other three groups fall into Reelfoot fault (RF). We inverted P ‐ and S ‐wave travel times from the new hypocentral parameters to determine 1D velocity models. The resulting eight‐layered velocity models consist of a 2 km thick surface layer followed by seven 2 km thick layers, with V P ranges from 5.36 to 6.74 km/s and V S ranges from 2.83 to 3.90 km/s for both CGF and RF regions. Online Material: Interactive visualizations of hypocentral distributions.

Mysterious Tremor-Like Signals Seen on the Reelfoot Fault, Northern Tennessee

Abstract A phased array of 19 broadband seismometers was deployed from November 2009 to September 2011 in an effort to detect nonvolcanic tremor or tectonic tremor associated with the Reelfoot fault, northern Tennessee. An autodetection algorithm using broadband frequency–wavenumber analysis was used to search for the recurrence of signals first reported during an active source experiment in 2006. The original signals appeared as short duration, impulsive arrivals with a high phase velocity ranging from 3 to 25 km/s. We have identified thousands of similar signals on the 2‐year long array data. Two distinct detection peaks are observed with event azimuths from the west and northeast. The detections are most similar to the events seen in 2006 and are inferred to come from very small ( M L ∼−1) microearthquakes that occur in the shallow basement on faults adjacent to the Reelfoot fault. These include detections with coherent S ‐wave energy that reinforce the interpretation of very small local and regional events. Other signals detected show distinct changes in slowness and azimuth as a function of time. These events were interpreted as atmospheric acoustic sources. The high‐frequency content and impulsive arrivals of the nonacoustic arrivals are not consistent with tectonic tremor as seen in other parts of the world but do indicate seismic activity in the crust near the Reelfoot thrust fault that was previously unknown.

Northeast-Oriented Transpression Structure in the Northern New Madrid Seismic Zone: Extension of a Shear Zone across the Reelfoot Fault Stepover Arm

High‐resolution seismic‐reflection profiles recently acquired 12 km northeast of the New Madrid seismic zone’s Reelfoot thrust and along the central axis of the Reelfoot rift, imaged steeply dipping N30°E striking faults that have uplifted and arched post‐Paleozoic sediments in a manner consistent with a dextral strike‐slip component of displacement. The subparallel fault strands have been traced 1.4 km between reflection profiles. In order to evaluate the structure’s potential regional scale, the strike was projected northeast 22 km to its intersection with a nearby industry profile. At the intersection, this lower‐resolution profile exhibits a discrete 0.75 km wide structure with style and offset similar to the high‐resolution lines. The high‐resolution images indicate the deformation extends above Paleozoic bedrock, affecting the Late Cretaceous and Eocene Mississippi embayment sediments, as well as the base of the Quaternary. The Paleozoic and Cretaceous horizons show as much as 75 and 50 m of relief, respectively, with the middle Eocene and basal Quaternary disrupted 25 and 15 m, respectively. Geologic and geophysical logs from a borehole adjacent to the seismic lines constrain the depth, velocity, and stratigraphic interpretations. We interpret the faults as a minimum 34 km northeast extension of the Axial fault zone from a throughgoing intersection with the left‐stepover Reelfoot thrust.

Shear-Wave Splitting Study of Crustal Anisotropy in the New Madrid Seismic Zone

Abstract This study investigates crustal anisotropy in the New Madrid Seismic Zone (NMSZ) by analyzing shear‐wave splitting measurements from local earthquake data. In addition to the waveforms provided by the Center for Earthquake Research and Information (CERI) for over 3000 events, seismograms recorded by the portable array for numerical data acquisition (PANDA) network were obtained for over 800 events. Data reduction led to a final data set of 168 and 43 events from the CERI and PANDA data, respectively. One‐hundred and eighty‐six pairs of measurements were produced from the CERI data set by means of the automated shear‐wave splitting measurement program mfast and 49 from the PANDA data set. Two dominant directions, respectively striking northeast–southwest and west‐northwest–east‐southeast, are identified and interpreted to be due to stress‐aligned microcracks. The northeast–southwest polarization direction is consistent with the maximum horizontal stress orientation of the region and has previously been observed in the NMSZ, whereas the west‐northwest–east‐southeast polarization direction has not. Path‐normalized time delays range from 1 to 33 ms/km for the CERI network data and 2 to 31 ms/km for the PANDA data. These results produce a range of estimated differential shear‐wave anisotropy between 1% and 8%. These values are higher than those previously determined in the region. The majority of large path‐normalized time delays (>20 ms/km) are located along the Reelfoot fault segment. These high values are believed to be indicative of high crack densities and high pore fluid pressures, which agrees with previous results from local earthquake tomography and microseismic swarm analysis. Online Material: Tables of stations, events, and associated shear‐wave splitting measurements.

High‐resolution 3‐D P wave attenuation structure of the New Madrid Seismic Zone using local earthquake tomography

AbstractA three‐dimensional (3‐D), high‐resolution P wave seismic attenuation model for the New Madrid Seismic Zone (NMSZ) is determined using P wave path attenuation (t*) values of small‐magnitude earthquakes (MD < 3.9). Events were recorded at 89 broadband and short‐period seismometers of the Cooperative New Madrid Seismic Zone Network and 40 short‐period seismometers of the Portable Array for Numerical Data Acquisition experiment. The amplitude spectra of all the earthquakes are simultaneously inverted for source, path (t*), and site parameters. The t* values are inverted for QP using local earthquake tomography methods and a known 3‐D P wave velocity model for the region. The four major seismicity arms of the NMSZ exhibit reduced QP (higher attenuation) than the surrounding crust. The highest attenuation anomalies coincide with areas of previously reported high swarm activity attributed to fluid‐rich fractures along the southeast extension of the Reelfoot fault. The QP results are consistent with previous attenuation studies in the region, which showed that active fault zones and fractured crust in the NMSZ are highly attenuating.

Variation of Seismic b-Value in the New Madrid Seismic Zone: Evidence that the Northern Reelfoot Fault is Creeping

ABSTRACT The spatial variation of seismic b ‐value is mapped for the New Madrid seismic zone using local earthquakes occurring between 1995 and 2013. A region of high b ‐value at about 1.8 is found in the northern part of the Reelfoot fault. By performing probability tests and following different procedures to obtain the result, we show the anomalously high b ‐value is not a processing artifact and is statistically significant. We attribute the b ‐value anomaly to creep behavior on the northern segment of the Reelfoot fault. Creep behavior is suggested by the recently discovered possible tremor and by the presence of quartz‐rich rock as indicated by low V P / V S ratios detected in a local earthquake tomography study. Because quartz is a weak mineral, ductile, creeping behavior could be facilitated at depth resulting in generation of earthquakes in the shallower, brittle crust. To the south of the Reelfoot fault, the b ‐value is about 1.2. The character of seismic activity along the Reelfoot fault clearly changes from north to south indicating a change in the physical state of the fault zone.

Northwestern Extension of the Reelfoot North Fault Near New Madrid, Missouri

Abstract The New Madrid seismic zone had at least three M >7 earthquakes during the winter of 1811–1812, but its faults are poorly mapped. Interpretation of 140, 91.4 m (300 ft) deep well logs west of New Madrid, Missouri, reveals a 9.7 km by 6.4 km subsurface high with 30 m of relief on Pleistocene‐age Mississippi River gravels. Seismic soundings also reveal an average of 55 m of relief on the Paleocene Wilcox Group, 56 m on the top of the Cretaceous, and 83 m on the top of the Paleozoic section. We interpret this as predominantly structural uplift, herein called the Lilbourn uplift. The uplift appears to be a northwestern extension of the Reelfoot North fault and its hanging wall deformation (the Tiptonville dome). It is not known whether the full length of the Reelfoot fault (70 km) or just the Reelfoot North fault (32 km) ruptured during the 7 February 1812 earthquake. However, an extension of 14.5 km on the Reelfoot North fault would increase its length to 47.5 km and thus permit an M ∼7 earthquake, whereas a 14.5 km increase on the combined Reelfoot North and Reelfoot South faults would increase its length to 84.5 km and thus permit an M ∼7.2 earthquake.

Imaging the New Madrid Seismic Zone using double‐difference tomography

AbstractThree‐dimensional P and S wave velocity (VP and VS) models and high‐resolution earthquake relocations are determined for the New Madrid Seismic Zone using double‐difference local earthquake tomography. The data set consists of arrival times and differential times recorded by the Cooperative New Madrid Seismic Network (CNMSN) from 2000 to 2007 and the 1989–1992 Portable Array Network and Data Acquisition deployment. Waveform cross correlation–derived differential times for the CNMSN data are also incorporated. The velocity solutions are compatible with previous solutions centered on the active arms of seismicity and cover a broader area, including mafic intrusions along the margin of the Reelfoot rift. Major features include elevated VP and VS associated with the mafic plutons and reduced VP and VS along and southeast of the Axial fault (AF), a major arm of seismicity trending along the rift axis. Low VP extends to a depth of at least 20 km along the portion of the AF that extends south of the Missouri bootheel. A locally high VP/VS anomaly imaged along the central portion of the Reelfoot fault is spatially correlated with a significant change in fault trend and is interpreted as a region containing high pore pressure and/or water‐filled microcracks.

Significant Motions between GPS Sites in the New Madrid Region: Implications for Seismic Hazard

Position time series from Global Positioning System (GPS) stations in the New Madrid region were differenced to determine the relative motions between sta- tions. Uncertainties in rates were estimated using a three-component noise model con- sisting of white, flicker, and random walk noise, following the methodology of Langbein, 2004. Significant motions of 0:37 0:07 (one standard error) mm/yr were found between sites PTGVand STLE, for which the baseline crosses the inferred deep portion of the Reelfoot fault. Baselines between STLE and three other sites also show significant motion. Site MCTY (adjacent to STLE) also exhibits significant motion with respect to PTGV. These motions are consistent with a model of interseismic slip of about 4 mm=yr on the Reelfoot fault at depths between 12 and 20 km. If constant over time, this rate of slip produces sufficient slip for an M 7.3 earthquake on the shallow portion of the Reelfoot fault, using the geologically derived recurrence time of 500 years. This model assumes that the shallow portion of the fault has been pre- viously loaded by the intraplate stress. A GPS site near Little Rock, Arkansas, shows significant southward motion of 0:3-0:4 mm=yr (0:08 mm=yr) relative to three sites to the north, indicating strain consistent with focal mechanisms of earthquake swarms in northern Arkansas.

A New Analysis of the Magnitude of the February 1663 Earthquake at Charlevoix, Quebec

Abstract This paper presents a new and comprehensive analysis of the magnitude of the 1663 Charlevoix, Quebec, earthquake. Based on a modified Mercalli intensity scale (MMI) of about VI from reports of damage to chimneys and a masonry wall in Roxbury and Boston, Massachusetts, the best estimate of the moment magnitude of this earthquake is M 7.3 to 7.9 from MMI attenuation relations. Using ground-motion attenuation relations and the threshold for chimney and masonry damage from fragility curves for seventeenth- and eighteenth-century dwellings in Boston, the magnitude of the 1663 earthquake is at least M 6.9 if the damage occurred on very soft soil conditions and is M 7.3 to M 7.7 if the damage took place on firm soil or bedrock. Both the best estimate of a length of 73 km for the most active section of the Charlevoix Seismic Zone and an estimated fault area of 73 km×25 km for the 1663 earthquake are consistent with an earthquake of M 7.3 to 7.6 based on scaling relationships. Also, in the 1811–1812 New Madrid earthquake sequence only the 7 February 1812 shock, the largest of the sequence, caused chimney damage beyond about 600 km. The 7 February New Madrid event is thought to have taken place on the Reelfoot fault, for which the length of the recent earthquake activity and of the suspected 1812 rupture is less than that of the active seismicity zone at Charlevoix. These observations suggest that the 1663 Charlevoix earthquake was approximately comparable in magnitude to or even larger than the largest of the 1811–1812 New Madrid earthquakes and therefore was at least M 6.8. When put together, the several lines of analysis in this study indicate that the best estimate of the size of the 1663 earthquake is M 7.5±0.45.

The Reelfoot Magnetic Anomaly and Its Relationship to the Pascola Arch and the Reelfoot Fault

Most contemporary New Madrid seismic zone earthquakes correlate geographically with the location of a basement uplift, the Pascola arch, and a distinct magnetic anomaly called the Reelfoot magnetic anomaly (RMA). We invert magnetic data for several profiles crossing the RMA and demonstrate that the anomaly is produced by an igneous intrusion related to the formation of the Pascola arch. The models require approximately 2 km of vertical uplift on crystalline basement rock, indicating that structure associated with the Pascola arch extends below Paleozoic rocks. The modeled uplift corresponds to estimates of Paleozoic sediment erosion from the top of the arch. We suggest that faulting accompanied emplacement of the RMA intrusion, establishing a zone of weakness in the upper crust that manifests today as the Reelfoot fault. Faulting accompanying intrusion provides an explanation for why earthquakes occur within the RMA intrusion rather than surrounding it, as is the case with all other intrusions emplaced along the margins and axis of the Reelfoot graben.