Large Prehistoric Earthquakes Research Articles

Differentiating between artificial and natural sources of electromagnetic radiation at a seismogenic fault

Ultralow frequency (ULF) to low frequency (LF) electromagnetic radiation represents one of the most promising effects of brittle rock strain and microcracking that might be potentially helpful for short term earthquake forecasting. In this study the results of a six month monitoring campaign are presented from Obir Cave in the eastern Alps. Direct experimental observations of electromagnetic radiation have been made using a customised broadband data logger installed next to the Obir Fault - this seismogenic fault near the Periadriatic Lineament is known to be related to at least three large prehistoric earthquakes. On the basis of these measurements it has been possible to characterise a number of distinct signals: artificial constant narrowband signals at discrete frequencies; short serial broadband impulses; high energy broadband impulses; and low energy broadband impulses. The narrowband artificial signals were removed from the electromagnetic radiation time series analysis so that the natural signals were enhanced and could be compared more easily to meteorological parameters and rock strain indicators. Critically, the high energy broadband impulses show a strong correlation with lightning activity across much of central Europe and the eastern Mediterranean while the low energy broadband impulses appear to be associated with local rock strain in and around Obir Cave. Unfortunately, it seems certain that some of the essential strain related impulses are likely to have been overprinted by the larger lightning related impulses.

Mass wasting along the Cascadia subduction zone: Implications for abyssal turbidite sources and the earthquake record

The only submarine records of large (>Mw7) prehistoric earthquakes along the Cascadia subduction zone are derived from sequences of deep sea turbidites interpreted to represent synchronous, shaking-induced failures along the continental slope. However, the spatial correlation of these deposits along the margin is complicated and the chronological constraints involve significant uncertainties, raising questions about how these deposits were generated. Here we present the most comprehensive spatial database of seafloor failure scarps across the Cascadia margin to date, with more than 8700 features mapped. We observe pervasive mass wasting of the steep lower slope along the ∼800 km length of the margin. This portion of the margin is heavily deformed, uplifted and oversteepened, such that it may be optimally preconditioned to fail during intense shaking events. Our results suggest disintegration of the steep lower slope is likely the primary source of turbidity flows triggered by earthquake shaking in Cascadia. This more proximal source for seismoturbidite generation may be the principal process along subduction margins with steep lower slope topography and may explain why seismoturbidites are found in areas containing few submarine canyons. Furthermore, the extensive mass wasting present on the steep lower slope of central Cascadia, a critical 250 km long section where no records currently exist, suggests additional turbidite records may be found along this portion of the margin by selecting optimal sample sites based on failure patterns. The strong clustering of seafloor failures along the lower slope also suggests the most earthquake intense shaking may be spatially restricted to a narrow zone along the deformation front, which could have important implications for hazard predictions across the region.

Three large prehistoric earthquakes in the Eastern Alps evidenced by cave rupture and speleothem damage

Spectacular geomorphic evidence of recent tectonic activity and three potentially strong prehistoric earthquakes were discovered in Obir Caves in the Karawanken Mountains. This paper presents active fault structures, geomorphic features, and the age constraints attributed to particular seismotectonic events. The 1976 Mw 6.7 Friuli earthquake about 100 km from Obir Caves caused only local speleothem damage. The 100 km hypocentral distance enabled attenuation of the seismic waves so speleothem damage due to stalagmite resonance frequency was dampened. The documented dripstone column damage was rather caused by its sudden shortening due to passing elastic S-waves with estimated 3.5 (+8.2/−1.5) mm PGD amplitudes. At least three distinct seismotectonic events in the Late Pleistocene and Early to Middle Holocene were constrained by radiometric dating. The 40.6 cm sinistral fault slip occurred between 41.8 ± 1.3 ka and 18.7 ± 0.4 ka ago, the 2.6 cm dextral oblique reverse fault slip happened between 10.73 ± 0.23 and 8.61 ± 0.15 ka ago, and another speleothem damage event took place between 6.28 ± 0.24 ka and 5.7 ± 1.2 ka ago. These events were most probably accompanied by distinct destructive to very destructive paleoearthquakes with local intensities ranging from VIII to X on the ESI 2007 scale, significant rock weakening, and forming the large deep-seated gravitational slope deformation on the southeastern mountain slopes adjacent to Obir Caves.

A database of potential paleoseismic evidence in Switzerland

Paleoseismological evidence is indispensable for the identification of large prehistoric earthquakes and the extension of the temporal coverage of historical and instrumental earthquake catalogues. In the geological record, diverse traces of past earthquakes are found. This study presents a database of potential primary and secondary evidence for seismic activity in the past 20,000 years in Switzerland. The database includes data from sedimentological, archaeological, speleological and geomorphological research. This unique dataset allows identifying periods during which the geologic record reveals enhanced occurrence of evidence that are further discussed as potentially earthquake-triggered. For the most recent 700 years, an increased occurrence of evidence features was found. This clustering is an effect of the historical earthquakes. Furthermore, periods with enhanced occurrence are identified at six phases during the past 20,000 years. Even though dating uncertainties for the geological record are large (e.g. 14C calibration range) and an unequivocal attribution of earthquakes as trigger mechanism for secondary evidence is not possible, the database reflects the natural-hazard potential of a region and represents valuable information for seismic hazard assessment. Furthermore, despite the uncertainty regarding the definition of the trigger mechanism, we propose that the database can be used to validate and improve earthquake-hazard models.

A semi-automated algorithm to quantify scarp morphology (SPARTA): application to normal faults in southern Malawi

Abstract. Along-strike variation in scarp morphology reflects differences in a fault's geomorphic and structural development and can thus indicate fault rupture history and mechanical segmentation. Parameters that define scarp morphology (height, width, slope) are typically measured or calculated manually. The time-consuming manual approach reduces the density and objectivity of measurements and can lead to oversight of small-scale morphological variations that occur at a resolution impractical to capture. Furthermore, inconsistencies in the manual approach may also lead to unknown discrepancies and uncertainties between, and also within, individual fault scarp studies. Here, we aim to improve the efficiency, transparency and uniformity of calculating scarp morphological parameters by developing a semi-automated Scarp PARameTer Algorithm (SPARTA). We compare our findings against a traditional, manual analysis and assess the performance of the algorithm using a range of digital elevation model (DEM) resolutions. We then apply our new algorithm to a 12 m resolution TanDEM-X DEM for four southern Malawi fault scarps, located at the southern end of the East African Rift system: the Bilila–Mtakataka fault (BMF) and three previously unreported scarps – Thyolo, Muona and Malombe. All but Muona exhibit first-order structural segmentation at their surface. By using a 5 m resolution DEM derived from high-resolution (50 cm pixel−1) Pleiades stereo-satellite imagery for the Bilila–Mtakataka fault scarp, we quantify secondary structural segmentation. Our scarp height calculations from all four fault scarps suggest that if each scarp was formed by a single, complete rupture, the slip–length ratio for each earthquake exceeds the maximum typical value observed in historical normal faulting earthquakes around the world. The high slip–length ratios therefore imply that the Malawi fault scarps likely formed in multiple earthquakes. The scarp height distribution implies the structural segments of both the BMF and Thyolo fault have merged via rupture of discrete faults (hard links) through several earthquake cycles, and the segments of the Malombe fault have connected via distributed deformation zones (soft links). For all faults studied here, the length of earthquake ruptures may therefore exceed the length of each segment. Thus, our findings shed new light on the seismic hazard in southern Malawi, indicating evidence for a number of large (Mw 7–8) prehistoric earthquakes, as well as providing a new semi-automated methodology (SPARTA) for calculating scarp morphological parameters, which can be used on other fault scarps to infer structural development.

Slip Rate and Rare Large Prehistoric Earthquakes of the Red River Fault, Southwestern China

AbstractThe Red River fault is an important plate‐boundary fault that has played a significant role in the tectonic evolution of northern Southeast Asia. Nonetheless, its millennial slip rate and earthquake recurrence behavior are poorly constrained. Analysis of 5‐ and 30‐m‐resolution topography reveals right‐lateral offsets that range from 60 m to 24 km along its “mid‐valley” trace but none along its “range‐front” trace. This strongly implies that the range‐front fault has experienced very little lateral slip for millions of years, even though it is the geologically more significant fault. Stratigraphic and geomorphologic investigation of the mid‐valley fault within a small channel near Gasa yields a14C‐based slip rate of 1.1 ± 0.4 mm/year, averaged over the last 30,000–50,000 years. Three‐dimensional paleoseismic excavation of colluvial wedges produced by collapse of shutter ridges into the channel shows that sudden dextral ruptures of the fault have occurred every 6,000 ± 1,000 years over the past 30,000 years. Two and possibly three large surface ruptures occurred in the past 13,500 years, and two previous ones occurred at 18,500 and 24,500 calendar years before present (cal yr BP). The oldest one in the section likely occurred at or a few 1,000 years before 29,800 ± 2,000 cal yr BP. The 3‐D extent of the colluvial wedges implies dextral offsets of ≥4.5 m, amounts that are consistent with the slip rate and recurrence interval. The evidence for low slip rate and rare large seismic events is consistent with the lack of large historical earthquakes along the fault and the low Global Positioning System (GPS)‐derived slip rate, but is much lower than widely cited geological slip rates.

Mineral, Virginia, earthquake illustrates seismicity of a passive‐aggressive margin

The August 2011 M 5.8 Mineral, Virginia, earthquake that shook much of the northeastern U.S. dramatically demonstrated that passive continental margins sometimes have large earthquakes. We illustrate some general aspects of such earthquakes and outline some of the many unresolved questions about them. They occur both offshore and onshore, reaching magnitude 7, and are thought to reflect reactivation of favorably‐oriented, generally margin‐parallel, faults created during one or more Wilson cycles by the modern stress field. They pose both tsunami and shaking hazards. However, their specific geologic setting and causes are unclear because large magnitude events occur infrequently, microseismicity is not well recorded, and there is little, if any, surface expression of repeated ruptures. Thus presently active seismic zones may be areas associated with higher seismicity over the long term, the present loci of activity that migrates, or aftershock zones of large prehistoric earthquakes. The stresses causing the earthquakes may result from platewide driving forces, glacial isostatic adjustment, localized margin stresses, and/or dynamic topography. The resulting uncertainties make developing cost‐effective mitigation strategies a major challenge. Progress on these issues requires integrating seismic, geodetic, and geological techniques.

Probabilistic Seismic-Hazard Assessment Including Site Effects for Evansville, Indiana, and the Surrounding Region

Abstract Evansville, Indiana, is one of the closest large urban areas to both the New Madrid Seismic Zone, where large earthquakes occurred in 1811–1812, and the Wabash Valley Seismic Zone, where there is evidence of several large prehistoric earthquakes in the last 14,000 yr. For this reason, Evansville has been targeted as a priority region for urban seismic-hazard assessment. The probabilistic seismic-hazard methodology used for the Evansville region incorporates new information from recent surficial geologic mapping efforts, as well as information on the depth and properties of near-surface soils and their associated uncertainties. The probabilistic seismic-hazard calculation applied here follows the method used for the 2008 United States Geological Survey (USGS) national seismic-hazard maps, with modifications to incorporate estimates of local site conditions and their uncertainties, in a completely probabilistic manner. The resulting analysis shows strong local variations of acceleration with 2% probability of exceedance in 50 yr, which are clearly correlated with variations in the thickness of unconsolidated soils above bedrock. Spectral accelerations at 0.2-s period range from 0.6 to 1.5 g , values that are much greater than those of the USGS national seismic-hazard map, which assume B/C site conditions with an average shear-wave velocity of 760 m/s in the top 30 m. The presence of an ancient bedrock valley underlying the current Ohio River flood plain strongly affects the spatial pattern of accelerations. For 1.0-s spectral acceleration, ground motions are significantly amplified due to deeper soils within this structure, to a level comparable to that predicted by the national seismic-hazard maps with D site conditions assumed. For PGA and 0.2-s spectral acceleration, ground motions are significantly amplified outside this structure, above the levels predicted by the national seismic-hazard maps with uniform D site conditions assumed.

Quaternary tectonic faulting in the Eastern United States

Paleoseismological study of geologic features thought to result from Quaternary tectonic faulting can characterize the frequencies and sizes of large prehistoric and historical earthquakes, thereby improving the accuracy and precision of seismic-hazard assessments. Greater accuracy and precision can reduce the likelihood of both underprotection and unnecessary design and construction costs. Published studies proposed Quaternary tectonic faulting at 31 faults, folds, seismic zones, and fields of earthquake-induced liquefaction phenomena in the Appalachian Mountains and Coastal Plain. Of the 31 features, seven are of known origin. Four of the seven have nontectonic origins and the other three features are liquefaction fields caused by moderate to large historical and Holocene earthquakes in coastal South Carolina, including Charleston; the Central Virginia Seismic Zone; and the Newbury, Massachusetts, area. However, the causal faults of the three liquefaction fields remain unclear. Charleston has the highest hazard because of large Holocene earthquakes in that area, but the hazard is highly uncertain because the earthquakes are uncertainly located. Of the 31 features, the remaining 24 are of uncertain origin. They require additional work before they can be clearly attributed either to Quaternary tectonic faulting or to nontectonic causes. Of these 24, 14 features, most of them faults, have little or no published geologic evidence of Quaternary tectonic faulting that could indicate the likely occurrence of earthquakes larger than those observed historically. Three more features of the 24 were suggested to have had Quaternary tectonic faulting, but paleoseismological and other studies of them found no evidence of large prehistoric earthquakes. The final seven features of uncertain origin require further examination because all seven are in or near urban areas. They are the Moodus Seismic Zone (Hartford, Connecticut), Dobbs Ferry fault zone and Mosholu fault (New York City), Lancaster Seismic Zone and the epicenter of the shallow Cacoosing Valley earthquake (Lancaster and Reading, Pennsylvania), Kingston fault (central New Jersey between New York and Philadelphia), and Everona fault–Mountain Run fault zone (Washington, D.C., and Arlington and Alexandria, Virginia).

The Commerce Geophysical Lineament and Its Possible Relation to Mesoproterozoic Igneous Complexes and Large Earthquakes in the Central Illinois Basin

New inversions of existing magnetic and gravity data lead to insights on upper crustal structures in the central Illinois basin. The results of 2D and 3D inversion techniques suggest that the source of the Commerce geophysical lineament follows the southeast boundary of a dense and magnetic, northeast-trending igneous center, named the Vincennes igneous center. Thus the Commerce geophysical lineament, defining the 5- to 10-km-wide Commerce deformation zone, appears to have influenced the structural development of the Vincennes igneous center. Overlying this igneous center is the Centralia seismic-reflection sequence, expressed as highly coherent reflectors (McBride and Kolata, 1999). We suggest that the buried Vincennes igneous center is the source of inferred volcanic units of the Centralia sequence and is related to a rifted margin or a Proterozoic plate boundary. A comparison of gravity and magnetic fields over the Vincennes igneous center and the St. Francois Mountains igneous center in southeastern Missouri suggests that the associated sources in these regions are similar in composition and perhaps origin. Such a conclusion, although speculative, suggests that ∼1.47 Ga-year-old volcanic fields of high-silica rocks, with more abundant mafic components at depth, characterize basement in the regions of the Vincennes igneous center and the St. Francois Mountains. Further, we conclude that the Commerce deformation zone evolved in the Mesoproterozoic (1.1 to 1.5 Ga) as a major cratonic rheological boundary and was the focus of episodic reactivation related to varying stress regimes throughout its history. Spatial relations of the Commerce deformation zone with large Pleistocene and Holocene earthquakes suggest that the zone represents a major rheological boundary intimately related to both surface and deep structures and to the seismic hazard of the Illinois basin region. Assuming recent right-lateral slip along the Commerce deformation zone, we propose that a jog or left step in the Vincennes area leads to thrusting or a restraining bend, where associated stress accumulations may have resulted in nearby large prehistoric earthquakes and may also be the nucleation sites of future large earthquakes.

Evidence for a buried fault system in the Coastal Plain of the Carolinas and Virginia--Implications for neotectonics in the southeastern United States

Research Article| February 01, 2000 Evidence for a buried fault system in the Coastal Plain of the Carolinas and Virginia—Implications for neotectonics in the southeastern United States Ronald T. Marple; Ronald T. Marple 1Geosignal, 11111 Katy Freeway, Houston, Texas 77082 Search for other works by this author on: GSW Google Scholar Pradeep Talwani Pradeep Talwani 2Department of Geological Sciences, University of South Carolina, Columbia, South Carolina 29208 Search for other works by this author on: GSW Google Scholar Author and Article Information Ronald T. Marple 1Geosignal, 11111 Katy Freeway, Houston, Texas 77082 Pradeep Talwani 2Department of Geological Sciences, University of South Carolina, Columbia, South Carolina 29208 Publisher: Geological Society of America Received: 08 Aug 1998 Revision Received: 01 Jun 1999 Accepted: 15 Jun 1999 First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (2000) 112 (2): 200–220. https://doi.org/10.1130/0016-7606(2000)112<200:EFABFS>2.0.CO;2 Article history Received: 08 Aug 1998 Revision Received: 01 Jun 1999 Accepted: 15 Jun 1999 First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Ronald T. Marple, Pradeep Talwani; Evidence for a buried fault system in the Coastal Plain of the Carolinas and Virginia—Implications for neotectonics in the southeastern United States. GSA Bulletin 2000;; 112 (2): 200–220. doi: https://doi.org/10.1130/0016-7606(2000)112<200:EFABFS>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Geomorphic, geologic, and geophysical data suggest the presence of an ∼600-km-long, north-northeast–trending buried fault system in the Coastal Plain of the Carolinas and Virginia, herein named the East Coast fault system. The East Coast fault system is expressed by anomalous changes in fluvial geomorphology that locally coincide with one or more of the following features: linear aeromagnetic anomalies, buried faults interpreted from seismic reflection data, surface faults offsetting Pliocene–Pleistocene surficial units, locally brecciated phyllites and argillites, gently upwarped sediments, topographic highs, and seismicity near Summerville, South Carolina.River anomalies where the rivers traverse unconsolidated upper Pleistocene–Holocene flood-plain sediments are evidence of deformation along the East Coast fault system during the past 130 to 10 k.y. which may be ongoing. The fault system traverses the epicentral area of the 1886 Charleston, South Carolina, earthquake and lies west of paleoliquefaction sites along the outer South Carolina Coastal Plain; thus, the fault system could be the source of the Charleston earthquake and other large prehistoric earthquakes. Therefore, confirmation and demarcation of the East Coast fault system and elucidation of its history will help better assess seismic hazards in the southeastern United States. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Taphonomy of tidal marsh foraminifera: implications of surface sample thickness for high-resolution sea-level studies

Previous research has shown that intertidal foraminiferal faunas can be used to document Holocene relative sea-level change and large prehistoric earthquakes. Applications like these, however, require an understanding of the impact of infaunal habitat and taphonomic processes on foraminiferal assemblages. To evaluate these effects, we analyzed surface sediment samples collected along a transect across a tidal marsh at Zeballos on Vancouver Island, British Columbia. Samples of the uppermost 10 cm of sediment in the marsh contain foraminiferal assemblages that permit recognition of a greater number of elevation-controlled marsh assemblages than samples of the top centimeter, which are generally used in sea-level studies. This is because the upper 10 cm contain most infaunal foraminifera species, whereas the top centimeter commonly lacks some of these species. A 10-cm thickness is somewhat arbitrary, but most foraminiferal taphonomic biasing occurs in the top 10 cm of the marsh.

Paleoliquefaction evidence and periodicity of large prehistoric earthquakes in Shillong Plateau, India

The tectonic setting and the occurrence of the great Assam earthquake ( M=8.7) of 1897 in the Shillong Plateau succeeded by three great earthquakes (1905, 1934 and 1950) in the adjoining Himalayan frontal arc, indicates the vulnerability of the Shillong Plateau to large earthquakes. The lack of seismicity records of the region earlier than 100 years and data on the recurrence of damaging earthquakes led us to investigate the paleoseismicity of the Shillong Plateau. Our paleoseismic investigations in the meizoseismal area of the 1897 earthquake revealed well-preserved liquefaction and deformed syndepositional features at 10 selected sites in the alluvial deposits along two north flowing tributaries of the Brahmaputra river. These features are 14C dated using associated organic samples. As the liquefaction of sediments was an important feature of the 1897 earthquake, we identified this seismic event, and other large/major prehistoric earthquakes through paleoliquefaction and other coseismic structural deformation at the investigated sites. In addition to the 1897 event, we provide geological evidence for at least three large seismic events. Two of them occurred during 1450–1650 and 700–1050 AD, the third predates 600 AD. The analysis of the 14C data suggests a return period of about 400–600 yr for the large earthquakes in the Shillong Plateau. This finding is the first of its kind from the Himalaya and adjoining region.

Evidence for Large Prehistoric Earthquakes in the Northern New Madrid Seismic Zone, Central United States

We surveyed the area north of New Madrid, Missouri, for prehistoric liquefaction deposits and uncovered two new sites with evidence of pre-1811 earthquakes. At one site, located about 20 km northeast of New Madrid, Missouri, radiocarbon dating indicates that an upper sand blow was probably deposited after A.D. 1510 and a lower sand blow was deposited prior to A.D. 1040. A sand blow at another site about 45 km northeast of New Madrid, Missouri, is dated as likely being deposited between A.D. 55 and A.D. 1620 and represents the northernmost recognized expression of prehistoric liquefaction likely related to the New Madrid seismic zone. This study, taken together with other data, supports the occurrence of at least two earthquakes strong enough to induce liquefaction or faulting before A.D. 1811 and after A.D. 400. One earthquake probably occurred around A.D. 900 and a second earthquake occurred around A.D. 1350. The data are not yet sufficient to estimate the magnitudes of the causative earthquakes for these liquefaction deposits although we conclude that all of the earthquakes are at least moment magnitude M ∼ 6.8, the size of the 1895 Charleston, Missouri, earthquake. A more rigorous estimate of the number and sizes of prehistoric earthquakes in the New Madrid seismic zone awaits evaluation of additional sites.

Estimated Magnitudes and Accelerations Associated with Prehistoric Earthquakes in the Wabash Valley Region of the Central United States

Seismic hazards in the central United States are typically based on occurrences of earthquakes in the New Madrid Seismic Zone. However, paleoliquefaction evidence shows that large prehistoric earthquakes also occurred in the Wabash Valley region of Indiana and Illinois. A geotechnical study of the soil conditions at paleoliquefaction sites there is used to estimate both the magnitudes and accelerations of the prehistoric earthquakes. This study covers an area of the Wabash River drainage approximately 250 km north to south and 180 km east to west, in southern Indiana and Illinois. In situ soil strength parameters were measured at 22 sites. The measured strength parameters are used in conjunction with liquefaction susceptibility analyses to estimate moment magnitude ( M ) and peak surface accelerations of four separate paleoearthquakes. In addition, site response studies based on the Atkinson and Boore (1995) model of bedrock motions for eastern North American earthquakes are used to develop attenuation relations. This allows a comparison of the results of the geotechnical study with a seismological prediction of peak surface accelerations due to the paleoearthquakes. Using a regionally appropriate relationship between magnitude and maximum distance to liquefaction effects (Obermeier et al. , 1993) leads to preliminary magnitude estimates of M 6.8, M 6.9, M 7.2, and M 7.8 for the four paleoearthquakes. Also, a method we developed, an energy-stress approach, is used to estimate the peak surface accelerations at these magnitudes that would be required to induce the observed liquefaction effects. Next, the acceleration estimates are compared with peak surface motions predicted by a seismological attenuation model for various magnitudes in order to determine the best-fitting magnitude. For the four paleoearthquakes studied, the geotechnical and seismological estimates of surface accelerations most closely agree for M 6.9, M 7.1, M 7.3, and M 7.8. Thus, two different methods yield basically the same paleomagnitudes, and therefore provide our best estimates.

Archeological and pedological evidence for large prehistoric earthquakes in the New Madrid seismic zone, central United States

Prehistoric liquefaction features have been identified by careful observation of their structural and stratigraphic relations to Native American occupation horizons and their subtle soil characteristics. The ages of these liquefaction features have been estimated from radiocarbon dating of wood associated with the features and Native American artifacts found within bounding occupation horizons. At three sites near Blytheville, Arkansas, in the central part of the New Madrid seismic zone, one sand-blow crater formed between A.D. 800 and 1400, two sand-blow deposits formed between A.D. 800 and 1670, and three, possibly four, sand dikes formed since 4035 B.C. Where not found in association with Native American occupation horizons and artifacts, prehistoric liquefaction features can be difficult to distinguish from features that formed during the great New Madrid earthquakes of A.D. 1811 and 1812. This raises the possibility that prehistoric liquefaction features may have been misinterpreted during previous studies in the area. Nevertheless, a paleoearthquake chronology is beginning to emerge for the New Madrid seismic zone. Our findings are consistent with paleoseismological studies in the northern part of the seismic zone and suggest a recurrence interval of hundreds of years for earthquakes large enough to induce liquefaction in this region (M ≥ 6.4). By mapping the age distribution of liquefaction features, a more accurate assessment of the long-term earthquake potential of the region will be possible.

Detection of Large Prehistoric Earthquakes in the Pacific Northwest by Microfossil Analysis

Geologic and palynological evidence for rapid sea level change approximately 3400 and approximately 2000 carbon-14 years ago (3600 and 1900 calendar years ago) has been found at sites up to 110 kilometers apart in southwestern British Columbia. Submergence on southern Vancouver Island and slight emergence on the mainland during the older event are consistent with a great (magnitude M >/= 8) earthquake on the Cascadia subduction zone. The younger event is characterized by submergence throughout the region and may also record a plate-boundary earthquake or a very large crustal or intraplate earthquake. Microfossil analysis can detect small amounts of coseismic uplift and subsidence that leave little or no lithostratigraphic signature.

Paleoliquefaction studies along the Atlantic seaboard: Implications for long-term seismic hazard

The search for paleoliquefaction evidence of large prehistoric earthquakes is presently underway both within the epicentral area of the 1886 Charleston earthquake and elsewhere along the Atlantic seaboard. To date, no conclusive evidence of large prehistoric earthquakes originating outside of South Carolina has been found. However, in addition to liquefaction resulting from the 1886 earthquake, paleoliquefaction data indicate that perhaps as many as five prehistoric earthquakes greater than about M b 5.5 to 6.2 occurred near Charleston during Holocene times. Furthermore, the data suggest that past large earthquakes within coastal S.C. may have not been limited exclusively to the established Charleston epicentral area. The mean return period for large liquefaction associated earthquakes is about 600 years for the past four earthquake cycles. Since only about 100 years have elapsed since the 1886 event, the probability of a similar earthquake occurring within the next several decades is judged to be low. However, the hazard presented by smaller earthquakes should not be overlooked.

The Search for Evidence of Large Prehistoric Earthquakes Along the Atlantic Seaboard

The spacial distribution of seismically induced liquefaction features discovered along the Atlantic seaboard suggest that during the last 2000 to 5000 years, large earthquakes (body wave magnitude, m(b) >/= 5.8 +/- 0.4) in this region may have been restricted exclusively to South Carolina. Paleoliquefaction evidence for six large prehistoric earthquakes was discovered there. At least five of these past events originated in the Charleston, South Carolina, area, the locale of a magnitude 7+ event in 1886. During the past two millennia, large events may have occurred about every 500 to 600 years.

Characteristics of Seismically Induced Liquefaction Sites and Features Located in the Vicinity of the 1886 Charleston, South Carolina Earthquake

Abstract The ages of seismically induced paleoliquefaction features located in the Charleston, S. C. area suggest that the return period between large events similar to the 1886 earthquake is much longer than the historic record. If large prehistoric earthquakes have occurred elsewhere along the Atlantic Seaboard, then evidence of liquefaction features associated with them should be present in unconsolidated Pleistocene and Holocene deposits. To establish a comprehensive control data base for a regional analysis, liquefaction sites and features located in the Charleston area have been evaluated. Over 100 liquefaction sites were identified on the basis of a detailed review of historical accounts of the 1886 earthquake, and results of recent field studies. These studies then centered on characterizing the geologic, stratigraphic, and hydrologic setting of these sites and identifying criteria by which similar locales could be recognized elsewhere in the Atlantic Coastal Plain. This investigation also included the development of recognition criteria to distinguish seismically induced liquefaction features from pseudoliquefaction features (other features which look similar but are not seismic in origin). Guided by these findings, a systematic search for paleoliquefaction features outside the epicentral area of the 1886 Charleston earthquake is now underway.