Natural and Induced Seismicity in the Lake Erie-Lake Ontario Region: Reactivation of Ancient Faults with Little Neotectonic Displacement

The two most prominent seismic zones in the Lake Erie-Lake Ontario region are associated with the Akron magnetic lineament and with the Clarendon-Linden fault. Both these features are recognized from geophysical data as regional basement structures related to the Grenville collisional orogen. Neotectonic displacement is not geologically evident, although Paleozoic reactivation is manifested by the Clarendon-Linden fault. We have sharpened the definition of seismic zones in the region by introducing newly discovered events, improving constraints on locations and size for many others, and omitting unreliable ones. This seismicity tends to occur on old faults with minor neotectonic displacements. Related conclusions are: 1) neotectonic surface displacement is not necessary for fault capability, 2) seismogenic faults may have geological and/or geophysical expressions, and 3) a stationary moment release at the historic level requires more capable faults than the ones active during the historic period. Waste fluid injection, oil recovery, and salt-brine recovery have been implicated in cases of induced seismicity in the study area and might have contributed a significant portion of the known earthquakes. Fluid is being injected into the basal platform formation at a depth of 1.8 km near Ashtabula, Ohio. In July 1987, about a year after the onset of injection, a mbLg=3.8 main shock occurred within a 60 km wide area with no known prior seismicity. Aftershocks detected by a short-term local seismic network define a vertical left-lateral fault in the basement just below the platform rocks as close as 700 m from the injection well, probably a reactivated pre-existing fault. Subsequent seismicity suggests a westward migration by 5-10 km, possibly along the same fault.Les deux principales zones sismiques de la région sont associées au linéament magnétique d'Akron et à la faille de Clarendon-Linden. Selon les données géophysiques, ces deux éléments sont reconnus comme étant des structures régionales du socle reliés à l'orogenèse de Grenville. On a précisé la détermination des zones sismiques dans la région en ajoutant les séismes découverts récemment, en précisant les lieux et les dimensions d'autres séismes et en éliminant ceux qui sont mal connus. La sismicité tend à se manifester dans d'anciennes failles avec peu de décalage néotectonique. On en conclut que 1) le décalage néotectonique superficiel ne met pas en cause la compétence d'une faille; 2) les failles d'origine sismique peuvent avoir des manifestations géologiques ou géophysiques; 3) un relâchement momentané localisé survenant au cours de la période historique requiert plus de compétence qu'en ont les failles actives au cours de cette période. L'injection de liquide, la récupération de pétrole ou de sel ont été impliquées dans les cas de sismicité provoquée dans la région et ont probablement causé une bonne partie des séismes connus. Près d'Ashtabula, en Ohio, on a injecté des liquides à une profondeur de 1,8 km à la base de la plate-forme. En juillet 1987, environ un an après le début des injections, un séisme de mbLg= 3,8 s'est produit dans une aire de 60 km de superficie apparamment non sismique. Les répliques enregistrées par un réseau temporaire ont laissé voir une faille verticale à décrochement sénestre, immédiatement sous la plate-forme rocheuse, à près de 700 m du puits d'injection; il s'agit probablement d'une faille réactivée. Par la suite, la sismicité indique une migration de 5 à 10 km, probablement le long de la même faille.Die zwei hauptsàchlichen seismischen Zonen in der Eriesee-Ontariosee-Region werden mit dem magnetischen Lineament von Akron in Verbindung gebracht. Mittels geophysikalischer Daten erkennt man in diesen Bildungen régionale Untergrundstrukturen, die mit der Kollisions-Orogenese von Grenville verbunden sind. Wir haben die Definition der seismischen Zonen in dem Gebiet pràzisiert, indem wir kùrzlich entdeckte Erdbeben ergànzt und fur viele andere die Definition der Ort und GroRe betreffenden Zwànge verbessert haben, und die Unzuverlàssigen weggelassen haben. Dièse Erdbeben haben die Tendenz, auf alten Verwerfungen mit geringen neotektonischen Verstellungen aufzutreten. Hieraus kann man schlieBen: 1) neotektonische Oberflàchenverstellung ist keine Bedingung fur Verwerfungsfâhigkeit, 2) durch Erdbeben entstandene Verwerfungen kônnen sich geologisch und/oder geophysikalisch ausdrùcken und 3) eine ortsgebundene momentané Entlastung auf historischer Ebene erfor-dert mehr fàhige Verwerfungen als die, welche wàhrend der historischen Période aktiv waren, In den Fallen von induziertem Auftreten von Erdbeben im untersuchten Gebiet waren Zufùhrung von Flùssigkeit und ôl- und Salzgewinnung beteiligt, und sie haben wohl einen bedeutenden Anteil der bekannten Erdbeben hervorgerufen. Bei Ashtabula, Ohio, hat man in einer Tiefe von 1.8 km Flùssigkeit an der Basis der Plattformbildung eingespritzt. Im JuIi 1987, etwa ein Jahr nach Beginn der Ein-spritzungen, kam es zu einem neuen gewi-chtigen Beben von mbM) = 3.8 innerhalb eines 60 km breiten Gebiets, in dem vorher kein Erdbeben bekannt war. Nach beben, die von einem kurzfristigen ôrtlichen Erdbeben-Netzwerk registriert wurden, ergeben eine vertikale linksseitige Verwerfung im Untergrund unter der Felsenplattform, etwa 700 m vom Einspritzungsbohrloch entfernt; es handelt sich moglicherweise um eine reaktivierte, schon existierende Verwerfung. Darauffolgende Erdbeben weisen auf eine Wanderung westwàrts von 5-10 km, moglicherweise Iangs derselben Verwerfung

Arc-parallel strain in a short rollback-subduction system: The structural evolution of the Crotone basin (northeastern Calabria, southern Italy)

Calabria, located in southern Italy, is the exposed part of the forearc in the Ionian/Tyrrhenian subduction-rollback boundary. We present a tectonic model for the evolution of Calabria during the last 12 Ma that incorporates the structure and stratigraphy of the Crotone basin. At the re-initiation of rollback, we document listric normal faults that accommodated a deep-rooted extensional regime. Extension ceased in the Tortonian and did not continue throughout rollback. The middle Tortonian to early Messinian is characterized by distal sedimentation despite rapid rollback. Westward verging thrusts in Tortonian sediments and olistrostrome deposits within the Messinian section can be explained by instabilities in the accretionary wedge during the Messinian salinity crisis. Tectonic quiescence returned to the basin after the crisis and continued until a north-south shortening event in the middle Pliocene. Our kinematic data and new evidence of two basin inversions suggest arc-parallel shortening of the forearc. We propose that this shortening correlates with the passage of the forearc through the Apulia-Nubia narrow. This data also dispute previous interpretations of the Crotone basin as a pull-apart or transtensional basin. The identification of the main structures in the Crotone basin suggest a shift from quiescence to upheaval, beginning with Pliocene arc-parallel shortening associated with the passage of the forearc through the Apulia-Nubia narrow and continuing until today

The 1912 Iceland earthquake rupture: Growth and development of a nascent transform system

We have mapped in detail surface ruptures of the 1912 magnitude 7.0 strike-slip earthquake in south Iceland. This earthquake ruptured fresh basalt flows that had covered the pre-existing fault. The observed style of surface fracturing closely matches both theoretical predictions of the first stages of shear fracture development and microscopic-scale observations from laboratory experiments. The shear offset distributed across the zone of surface fractures produced by this earthquake is right-lateral and is in the range of 1 to 3 m. Total mapped rupture length is 9 km, but total rupture length is probably at least ∼ 20 km. This interplate earthquake had an exceptionally high ratio of slip to fault length and, by inference, stress drop. The north-south trending rupture of the 1912 earthquake is part of the “bookshelf” faulting in the east-west trending South Iceland Seismic Zone. We ascribe the “bookshelf” faulting in the South Iceland Seismic Zone to a combination of the early development stage of the transform and regional strength anisotropy of the crust.This research was supported by the National Science Foundation, the Icelandic National Power Authority (Landsvirkjun), and the Department of Geological Sciences of Columbia University. Lamont-Dohert Contribution 5036.Peer Reviewe

Seafloor fault ruptures along the North Anatolia Fault in the Marmara Sea, Turkey: Link with the adjacent basin turbidite record

The relation between seafloor fault ruptures and the generation of turbidity currents was investigated to better understand the structural growth of tectonic basins with direct implications for earthquake hazard assessment. This study focuses on the Holocene earthquake record of transtensional basins in the Marmara Sea, Turkey, that are associated with the North Anatolian Fault system. The physical and chemical composition of three 10 m-long cores recovered from the Central Basin was studied at high-resolution and turbidite–homogenite units were identified. Turbidite–homogenite units (T–H units) are complex deposits that consist of a sharp basal contact and multiple fining upward beds of sand to coarse silt, above. All are capped by a 25 cm to 75 cm thick layer of medium to fine silt. A chronology developed from radiocarbon and short-lived radioisotopes allowed the correlation of these T–H units to the historical record of earthquakes that in Turkey goes back 2000 years. We found that the best location to recover the most complete sedimentation record is in the deepest part of a basin or “depocenter” where T–H units constitute ~ 80% of the sediments. A very good correlation was established between T–H units in Central Basin and proximal inferred historic epicentres along the central Marmara segment of the North Anatolia Fault that occurred in 1343, 860, 740, and 557 AD, and two more distal earthquakes that occurred in 268 and 1963 (or possibly1964). These sedimentation events can then be referred to as “seismo-turbidites”. The results when compared to findings from other transform basins in Marmara Sea reveal a very good correlation between T–H units and historic ruptures. Most importantly, there is a strong correlation between the inferred locations of historical earthquakes and the preservation of turbidite–homogenite units in the basin adjacent to the inferred rupture. The 740 AD earthquake correlates with T–H units in Izmit Gulf and Central Basin and could represent a multi-segment rupture of the NAF. Generally, T–H units appear to be clustered through the Holocene sections, suggesting temporal earthquake clustering in the Marmara Sea region. Such clustering may account for the lack of T–H units and hence large ruptures through the Central Basin since 1343

Seafloor fault ruptures along the North Anatolia Fault in the Marmara Sea, Turkey: Link with the adjacent basin turbidite record

The relation between seafloor fault ruptures and the generation of turbidity currents was investigated to better understand the structural growth of tectonic basins with direct implications for earthquake hazard assessment. This study focuses on the Holocene earthquake record of transtensional basins in the Marmara Sea, Turkey, that are associated with the North Anatolian Fault system. The physical and chemical composition of three 10 m-long cores recovered from the Central Basin was studied at high-resolution and turbidite–homogenite units were identified. Turbidite–homogenite units (T–H units) are complex deposits that consist of a sharp basal contact and multiple fining upward beds of sand to coarse silt, above. All are capped by a 25 cm to 75 cm thick layer of medium to fine silt. A chronology developed from radiocarbon and short-lived radioisotopes allowed the correlation of these T–H units to the historical record of earthquakes that in Turkey goes back 2000 years. We found that the best location to recover the most complete sedimentation record is in the deepest part of a basin or “depocenter” where T–H units constitute ~ 80% of the sediments. A very good correlation was established between T–H units in Central Basin and proximal inferred historic epicentres along the central Marmara segment of the North Anatolia Fault that occurred in 1343, 860, 740, and 557 AD, and two more distal earthquakes that occurred in 268 and 1963 (or possibly1964). These sedimentation events can then be referred to as “seismo-turbidites”. The results when compared to findings from other transform basins in Marmara Sea reveal a very good correlation between T–H units and historic ruptures. Most importantly, there is a strong correlation between the inferred locations of historical earthquakes and the preservation of turbidite–homogenite units in the basin adjacent to the inferred rupture. The 740 AD earthquake correlates with T–H units in Izmit Gulf and Central Basin and could represent a multi-segment rupture of the NAF. Generally, T–H units appear to be clustered through the Holocene sections, suggesting temporal earthquake clustering in the Marmara Sea region. Such clustering may account for the lack of T–H units and hence large ruptures through the Central Basin since 1343

Seismicity and fault interaction, Southern San Jacinto Fault Zone and adjacent faults, southern California: Implications for seismic hazard

The southern San Jacinto fault zone is characterized by high seismicity and a complex fault pattern that offers an excellent setting for investigating interactions between distinct faults. This fault zone is roughly outlined by two subparallel master fault strands, the Coyote Creek and Clark-San Felipe Hills faults, that are located 2 to 10 km apart and are intersected by a series of secondary cross faults. Seismicity is intense on both master faults and secondary cross faults in the southern San Jacinto fault zone. The seismicity on the two master strands occurs primarily below 10 km; the upper 10 km of the master faults are now mostly quiescent and appear to rupture mainly or solely in large earthquakes. Our results also indicate that a considerable portion of recent background activity near the April 9, 1968, Borrego Mountain rupture zone (M_L=6.4) is located on secondary faults outside the fault zone. We name and describe the Palm Wash fault, a very active secondary structure located about 25 km northeast of Borrego Mountain that is oriented subparallel to the San Jacinto fault system, dips approximately 70° to the northeast, and accommodates right-lateral shear motion. The Vallecito Mountain cluster is another secondary feature delineated by the recent seismicity and is characterized by swarming activity prior to nearby large events on the master strand. The 1968 Borrego Mountain and the April 28, 1969, Coyote Mountain (M_L=5.8) events are examples of earthquakes with aftershocks and subevents on these secondary and master faults. Mechanisms from those earthquakes and recent seismic data for the period 1981 to 1986 are not simply restricted to strike-slip motion; dipslip motion is also indicated. Teleseismic body waves (long-period P and SH) of the 1968 and 1969 earthquakes were inverted simultaneously for source mechanism, seismic moment, rupture history, and centroid depth. The complicated waveforms of the 1968 event (M_o=1.2 × 10^(19) Nm) are interpreted in terms of two subevents; the first caused by right-lateral strike-slip motion in the mainshock along the Coyote Creek fault and the second by a rupture located about 25 km away from the master fault. Our waveform inversion of the 1969 event indicates that strike-slip motion predominated, releasing a seismic moment of 2.5 × 10^(17) Nm. Nevertheless, the right-lateral nodal plane of the focal mechanism is significantly misoriented (20°) with respect to the master fault, and hence the event is not likely to be associated with a rupture on that fault. From this and other examples in southern California, we conclude that cross faults may contribute significantly to seismic hazard and that interaction between faults has important implications for earthquake prediction

Back-arc extension in the Andaman Sea: Tectonic and magmatic processes imaged by high-precision teleseismic double-difference earthquake relocation

The geometry, kinematics, and mode of back-arc extension along the Andaman Sea plate boundary are refined using a new set of significantly improved hypocenters, global centroid moment tensor (CMT) solutions, and high-resolution bathymetry. By applying cross-correlation and double-difference (DD) algorithms to regional and teleseismic waveforms and arrival times from International Seismological Centre and National Earthquake Information Center bulletins (1964–2009), we resolve the fine-scale structure and spatiotemporal behavior of active faults in the Andaman Sea. The new data reveal that back-arc extension is primarily accommodated at the Andaman Back-Arc Spreading Center (ABSC) at ~10°, which hosted three major earthquake swarms in 1984, 2006, and 2009. Short-term spreading rates estimated from extensional moment tensors account for less than 10% of the long-term 3.0–3.8 cm/yr spreading rate, indicating that spreading by intrusion and the formation of new crust make up for the difference. A spatiotemporal analysis of the swarms and Coulomb-stress modeling show that dike intrusions are the primary driver for brittle failure in the ABSC. While spreading direction is close to ridge normal, it is oblique to the adjacent transforms. The resulting component of E-W extension across the transforms is expressed by deep basins on either side of the rift and a change to extensional faulting along the West Andaman fault system after the Mw = 9.2 Sumatra-Andaman earthquake of 2004. A possible skew in slip vectors of earthquakes in the eastern part of the ABSC indicates an en-echelon arrangement of extensional structures, suggesting that the present segment geometry is not in equilibrium with current plate-motion demands, and thus the ridge experiences ongoing re-adjustment

Evidence for widespread creep on the flanks of the Sea of Marmara transform basin from marine geophysical data

"Wave" fields have long been recognized in marine sediments on the flanks of basins and oceans in both tectonically active and inactive environments. The origin of "waves" (hereafter called undulations) is controversial; competing models ascribe them to depositional processes, gravity-driven downslope creep or collapse, and/or tectonic shortening. Here we analyze pervasive undulation fields identified in swath bathymetry and new high-resolution multichannel seismic (MCS) reflection data from the Sea of Marmara, Turkey. Although they exhibit some of the classical features of sediment waves, the following distinctive characteristics exclude a purely depositional origin: (1) parallelism between the crests of the undulations and bathymetric contours over a wide range of orientations, (2) steep flanks of the undulations (up to ∼40°), and (3) increases in undulations amplitude with depth. We argue that the undulations are folds formed by gravity-driven downslope creep that have been augmented by depositional processes. These creep folds develop over long time periods (≥0.5 m.y.) and stand in contrast to geologically instantaneous collapse. Stratigraphic growth on the upslope limbs indicates that deposition contributes to the formation and upslope migration of the folds. The temporal and spatial evolution of the creep folds is clearly related to rapid tilting in this tectonically active transform basin