Offshore Fault Research Articles

Holocene Paleoseismic History of Upper-Plate Faults in the Southern Hikurangi Subduction Margin, New Zealand, Deduced from Marine Terrace Records

Abstract The formation and uplift of Holocene marine terraces along a 160-km-long stretch of the Wairarapa coast in the southern Hikurangi subduction margin, North Island, New Zealand, are interpreted in terms of causative faults and associated large earthquakes during the past circa 7000 cal yr B.P. Distinctive stepped terrace morphology, the clustering of radiocarbon ages on each uplifted terrace, and the historic occurrence of coseismic uplift in other parts of eastern and southern North Island, support the contention that coastal uplift occurred suddenly and repeatedly during large-magnitude earthquakes. The rupture of five west-dipping reverse or reverse-oblique faults within the Australian plate, whose surface traces lie a short distance seaward of the present shoreline, are inferred to be responsible for coastal uplift. Individual structures range in length from 25 to 60 km. These lengths, together with uplift events in the range of 1–4 m, suggest earthquakes in the range of M w 7–7.5. Approximately 20% of the radiocarbon ages are anomalously young for their elevations, but many have ages indistinguishable from accepted ages for terrace uplift. We infer that many of the anomalous ages are from samples deposited by tsunami that occurred in association with offshore fault rupture on adjacent sections of the coast.

Influence of Neoproterozoic tectonic fabric on the origin of the Potiguar Basin, northeastern Brazil and its links with West Africa based on gravity and magnetic data

The Potiguar Basin is a ∼6,000 m thick aborted NE-trending rift that was formed during the Cretaceous in the continental margin of northeastern Brazil. Its ∼E–W-trending offshore faults form part of the successful continental margin rift that evolved into the South Atlantic Ocean. The region represents one of the most significant pre-Pangea breakup piercing points between eastern South America and West Africa. We used gravity, aeromagnetic, and geological data to assess the role of reactivated Precambrian shear zones and major terrain boundaries in the development of the Potiguar Basin from the Cretaceous to the Cenozoic. We also looked for possible links between these structures in northeastern Brazil and their continuation in West Africa. Our results indicate that the major fault systems of the Potiguar Basin were superimposed on the Precambrian fabric. Both gravity and magnetic maps show lineaments related to the shear zones and major terrain boundaries in the Precambrian crystalline basement, which also characterize the architecture of the rift. For example, the Carnaubais fault, the master fault of the rift system, represents the reactivation of the Portalegre shear zone, the major tectonic boundary between Precambrian terrains in the crystalline basement. In addition, part of the Moho topography is controlled by these shear zones and developed during the period of main rift extension in the Neocomian. The shear zones bounding the Potiguar rift system continue in West Africa around and underneath the Benue Basin, where fault reactivation also took place.

Using Active Fault Studies for Raising Public Awareness and Sensitisation on Seismic Hazard: A Case Study from Lesvos Petrified Forest Geopark, NE Aegean Sea, Greece

Seismic hazard is commonly assessed by using seismicity records and local geotechnical conditions. It is however important to accurately define the probable seismic sources of the broader study area and assess their seismic potential, as earthquake intensities are expected to increase in the close vicinity of active faults. Although onshore faults are considered more hazardous, due to their immediate proximity to inhabited areas, the offshore fault hazard is considerable too, due to their proximity to the islands. In this paper, the identified seismically active faults are used as main elements of an educational programme in the Lesvos Petrified Forest Geopark to raise public awareness and sensitivity on seismic hazard.

Assessing historical rate changes in global tsunami occurrence

SUMMARY The global catalogue of tsunami events is examined to determine if transient variations in tsunami rates are consistent with a Poisson process commonly assumed for tsunami hazard assessments. The primary data analyzed are tsunamis with maximum sizes >1m. The record of these tsunamis appears to be complete since approximately 1890. A secondary data set of tsunamis >0.1m is also analyzed that appears to be complete since approximately 1960. Various kernel density estimates used to determine the rate distribution with time indicate a prominent rate change in global tsunamis during the mid-1990s. Less prominent rate changes occur in the early- and mid-20th century. To determine whether these rate fluctuations are anomalous, the distribution of annual event numbers for the tsunami catalogue is compared to Poisson and negative binomial distributions, the latter of which includes the effects of temporal clustering. Compared to a Poisson distribution, the negative binomial distribution model provides a consistent fit to tsunami event numbers for the >1m data set, but the Poisson null hypothesis cannot be falsified for the shorter duration >0.1m data set. Temporal clustering of tsunami sources is also indicated by the distribution of interevent times for both data sets. Tsunami event clusters consist only of two to four events, in contrast to protracted sequences of earthquakes that make up foreshock–main shock–aftershock sequences. From past studies of seismicity, it is likely that there is a physical triggering mechanism responsible for events within the tsunami source ‘mini-clusters’. In conclusion, prominent transient rate increases in the occurrence of global tsunamis appear to be caused by temporal grouping of geographically distinct mini-clusters, in addition to the random preferential location of global M >7 earthquakes along offshore fault zones.

New GPS constraints on active deformation along the Africa–Iberia plate boundary

We use velocities from 65 continuous stations and 31 survey-mode GPS sites as well as kinematic modeling to investigate present day deformation along the Africa–Iberia plate boundary zone in the western Mediterranean region. The GPS velocity field shows southwestward motion of the central part of the Rif Mountains in northern Morocco with respect to Africa varying between 3.5 and 4.0 mm/yr, consistent with prior published results. Stations in the southwestern part of the Betic Mountains of southern Spain move west–southwest with respect to Eurasia (∼ 2–3 mm/yr). The western component of Betics motion is consistent with partial transfer of Nubia–Eurasia plate motion into the southern Betics. The southward component of Betics motion with respect to Iberia is kinematically consistent with south to southwest motion of the Rif Mountains with respect to Africa. We use block modeling, constrained by mapped surface faults and seismicity to estimate the geometry and rates of strain accumulation on plate boundary structures. Our preferred plate boundary geometry includes one block between Iberia and Africa including the SW Betics, Alboran Sea, and central Rif. This geometry provides a good fit to the observed motions, suggesting a wide transpressive boundary in the westernmost Mediterranean, with deformation mainly accommodated by the Gloria–Azores fault system to the West and the Rif–Tell lineament to the East. Block boundaries encompass aspects of earlier interpretations suggesting three main deformation styles: (i) extension along the NE–SW trending Trans-Alboran shear zone, (ii) dextral strike-slip in the Betics corresponding to a well defined E–W seismic lineament, and (iii) right lateral strike-slip motion extending West to the Azores and right-lateral motion with compression extending East along the Algerian Tell. We interpret differential motion in the Rif–Alboran–Betic system to be driven both by surface processes related the Africa–Eurasia oblique convergence and sub-crustal dynamic processes associated with the long history of subduction of the Neotethys ocean lithosphere. The dextral slip identified in the Betic Mountains in Southern Spain may be related to the offshore fault that produced the Great 1755 Lisbon Earthquake, and as such may represent a significant seismic hazard for the West Mediterranean region.

Misattributed tsunami: Chile, Sumatra and the subduction model

Tsunami intensity is poorly correlated with earthquake magnitude. The distribution of aftershocks that immediately followed the 2010 Maule (Chile), the 2004 Sumatra–Andaman and the 2005 Nias (Indonesia) events supports the view that faulting within an accretionary wedge or an outer rise can sometimes disrupt the seafloor more effectively than a megathrust even if the associated seismicity is minor. Monitoring offshore faults would thus seem an effective way to supplement modes of tsunami early warning which hinge on instrumental earthquake detection or wave height and period.

The structures, stratigraphy and evolution of the Gulf of Corinth rift, Greece

SUMMARY A multichannel seismic and bathymetry survey of the central and eastern Gulf of Corinth (GoC), Greece, reveals the offshore fault geometry, seismic stratigraphy and basin evolution of one of Earths most active continental rift systems. Active, right-stepping, en-echelon, north-dipping border faults trend ESE along the southern Gulf margin, significantly overlapping along strike. The basement offsets of three (Akrata-Derveni, Sithas and Xylocastro) are linked. The faults are biplanar to listric: typically intermediate angle (∼35° in the centre and 45–48° in the east) near the surface but decreasing in dip and/or intersecting a low- or shallow-angle (15–20° in the centre and 19–30° in the east) curvi-planar reflector in the basement. Major S-dipping border faults were active along the northern margin of the central Gulf early in the rift history, and remain active in the western Gulf and in the subsidiary Gulf of Lechaio, but unlike the southern border faults, are without major footwall uplift. Much of the eastern rift has a classic half-graben architecture whereas the central rift has a more symmetric w- or u-shape. The narrower and shallower western Gulf that transects the >40-km-thick crust of the Hellenides is associated with a wider distribution of overlapping high-angle normal faults that were formerly active on the Peloponnesus Peninsula. The easternmost sector includes the subsidiary Gulfs of Lechaio and Alkyonides, with major faults and basement structures trending NE, E–W and NW. The basement faults that control the rift architecture formed early in the rift history, with little evidence (other than the Vrachonisida fault along the northern margin) in the marine data for plan view evolution by subsequent fault linkage. Several have maximum offsets near one end. Crestal collapse graben formed where the hanging wall has pulled off the steeper onto the shallower downdip segment of the Derveni Fault. The dominant strikes of the Corinth rift faults gradually rotate from 090–120° in the basement and early rift to 090–100° in the latest rift, reflecting a ∼10° rotation of the opening direction to the 005° presently measured by GPS. The sediments include a (locally >1.5-km-) thick, early-rift section, and a late-rift section (also locally >1.5-km-thick) that we subdivide into three sequences and correlate with seven 100-ka glacio-eustatic cycles. The Gulf depocentre has deepened through time (currently >700 mbsl) as subsidence has outpaced sedimentation. We measure the minimum total horizontal extension across the central and eastern Gulf as varying along strike between 4 and 10 km, and estimate full values of 6–11 km. The rift evolution is strongly influenced by the inherited basement fabric. The regional NNW structural fabric of the Hellenic nappes changes orientation to ESE in the Parnassos terrane, facilitating the focused north-south extension observed offshore there. The basement-penetrating faults lose seismic reflectivity above the 4–14-km-deep seismogenic zone. Multiple generations and dips of normal faults, some cross-cutting, accommodate extension beneath the GoC, including low-angle (15–20°) interfaces in the basement nappes. The thermally cool forearc setting and cross-orogen structures unaccompanied by magmatism make this rift a poor analogue and unlikely precursor for metamorphic core complex formation.

Derivation of direct on‐fault submarine paleoearthquake records from high‐resolution seismic reflection profiles: Wairau Fault, New Zealand

Direct, on‐fault submarine paleoearthquake records can be derived from high‐resolution seismic reflection profiles of active fault growth sequences. Coseismic vertical increments of displacement are best preserved in the architecture of the growth sequence when the long‐term rate of sedimentation exceeds the rate of fault vertical displacement. Postseismic stratigraphic intervals can be recognized on vertical displacement history curves, from which estimates of the earthquake timing and vertical displacements can be made. The strike‐slip Wairau Fault is a major seismic hazard in central New Zealand and is partially submarine. Analysis of 10 high‐resolution seismic profiles spanning a 20 km section of the offshore fault trace reveals two types of postseismic growth sequences and a composite paleoearthquake record with up to eight surface‐rupturing earthquakes since 18 ka. The recurrence intervals range from ∼0.9 to 3.8 ka (mean is ∼2.2 ka), while the coseismic vertical displacements range from 0.5 to 5.3 m (mean is ∼2.5 m). These data conform to the variable slip model of earthquake behavior. The vertical coseismic displacement is not always predictable from the recurrence interval preceding the event. The data indicate that the seismic moment release has not been constant between earthquakes or that the ratio of horizontal to vertical coseismic displacement at a given site has varied over multiple earthquake cycles. The offshore data are consistent with onshore paleoearthquake records, long‐term dextral slip rate of >2 mm/yr (compared to 3–5 mm/yr, 40 km in land), and paleoearthquakes of >Mw7.5. The last earthquake occurred on the fault 2.0 ± 0.3 kyr ago, indicating that significant elastic strain has now accrued.

Rupture characteristics of the A.D. 1912 Mürefte (Ganos) earthquake segment of the North Anatolian fault (western Turkey)

Abstract: The Ganos fault is the westernmost segment of the North Anatolian Fault (NAF) that generated the 9 August 1912 Murefte (Ganos) earthquake (Mw=7.4). We study the 1912 earthquake characteristics using co-seismic fault slip and fault segmentation coupled with an analysis of historical seismic records. Surface ruptures with small releasing and restraining structures and 1.5 to 5.5 m right-lateral offsets have been measured at 45 sites of the onland ~45-km-long fault section. Similar structures are delineated by fresh fault scarps and prominent pull-apart basins in the Sea of Marmara and Saros Bay. A second shock with Mw=6.8 occurred on 13 September 1912 implying 20 to 40-km- long rupture; the damage distribution and analysis of seismic records suggest an epicenter located further west near Saros Bay, which indicates the location of western termination of the 9 August rupture. Our modeling of historical seismic records reveals a relative source time function between the two events and indicates a 40 second rupture duration, in agreement for a 120±30-km-long fault rupture for the 9 August shock. An estimated total rupture length of 150±30 km for the two earthquakes combined with onshore and offshore fault segmentation allow us to better constrain the western limit of the Marmara Sea seismic gap and related potential for producing a large earthquake that was sharply increased by the devastating 1999 Izmit earthquake in the east.

Coseismic strike slip at a point during the last four earthquakes on the Wellington fault near Wellington, New Zealand

We analyze progressively displaced late Quaternary (<12 ka) fluvial terraces along the Wellington fault, near Wellington, New Zealand. Optically stimulated luminescence dating indicates that degradational terraces were produced at a rate of about one terrace per 1000 years, similar to the rate of earthquake surface rupturing. Along the Hutt River near Te Marua, we measured the strike slip of 15 terrace risers and paleochannels on the lowest 8 of these terraces, of Holocene age. The river, after earthquakes, was generally capable of smoothing its faulted riverbanks. The dextral offsets appear to fall into several groupings that record slip accumulation during the last four earthquakes. We calculate a mean single‐event slip of 5.0 ± 0.24 m (95% confidence) with an RMS scatter (1σ) of slips about the mean of ±1.5 m. The coefficient of variation (CV) of single‐event slip is thus 0.30. This CV is slightly less than a recently compiled global average for point measurements on strike‐slip faults, suggesting that the southernmost Wellington fault has behaved in a more nearly characteristic way. We speculate that recent large earthquake ruptures have been bounded on their southern end by the Wellington fault's offshore fault termination and perhaps on their northern end by a ∼2 km wide releasing step over. Such persistent sources of rupture arrest might have led to a relative uniformity of rupture dimensions and slip amounts. We infer a late Holocene dextral slip rate of ≥4.5 ± 0.4 mm/yr (1σ) and <8.2 mm/yr, and a mean earthquake recurrence interval of ∼610–1100 years.

Insights into active tectonics of eastern Taiwan from analyses of geodetic and geologic data

About 50 mm/yr of convergence between the Philippine Sea and Eurasian plates is absorbed in eastern Taiwan, and it remains unclear how the convergence is partitioned among active faults. The Longitudinal Valley fault (LVF), the most seismically active fault in eastern Taiwan, creeps at the surface in the south and not in the north; however, it is unclear how much of the fault is locked or creeping at depth. To address these problems, we model Holocene and interseismic deformation of elastic lithospheric blocks moving over a viscoelastic asthenosphere in eastern Taiwan. Through a fully probabilistic scheme, we invert GPS, interferometric synthetic aperture radar, creepmeter, and Holocene marine terrace data for block motions, fault slip rates, and distribution of interseismic creep. The data are explained with four blocks separated by three faults, Central Range fault, LVF, and an offshore fault. The model explains the essential features of interseismic and Holocene deformation. We find that 35–55 mm/yr of slip on the offshore fault is necessary to fit marine terrace uplift rates, which is a larger fraction of the plate convergence than previously recognized. The LVF has a Holocene slip rate of 20–30 mm/yr with approximately equal magnitudes of reverse‐slip and left‐lateral strike‐slip components. Only about half of the surface area of the Longitudinal Valley fault appears to be locked. The southern segment of the LVF creeps at a rate of 5–28 mm/yr down to a depth of 15–20 km, while the northern segment is locked from the surface to a depth of 20 km.

In the Aftermath of Haiti's Earthquake: A Discussion of Lessons Learned

The 12 January 2010 earthquake in Haiti brought massive devastation to that country (see Figure 1). In this week's issue of Eos, three noted seismologists respond to questions from Eos senior writer Randy Showstack in a news roundtable format.Paul Mann, senior research scientist with the Institute for Geophysics at the University of Texas at Austin, has just returned from Haiti, where he and a colleague worked on a fault rupture survey; they plan to conduct an offshore fault survey soon. Glen Mattioli, professor of geosciences at the University of Arkansas, Fayetteville, has been part of a team conducting a Global Positioning System (GPS) survey of Haiti to measure ground deformation following the earthquake and to install a number of continuous GPS sites to examine after slip, viscoelastic relaxation, and the time return to interseismic deformation (see Figure 2). Work by Mann, Mattioli, and their colleagues has been supported through a U.S. National Science Foundation Rapid Response Research ( RAPID) proposal grant provided to Purdue University, with Eric Calais serving as principal investigator. Carol Prentice, a seismologist with the U.S. Geological Survey's Earthquake Hazards Team, has been conducting paleoseismic research on the active faults in the Caribbean region since 1991, including projects on Hispaniola, Puerto Rico, Trinidad, and Jamaica.

Cenozoic range-front faulting and development of the Transantarctic Mountains near Cape Surprise, Antarctica: Thermochronologic and geomorphologic constraints

[1] The Transantarctic Mountains (TAM) define the western flank of the West Antarctic rift system. Cape Surprise, near the Shackleton Glacier in the central TAM, is the only location along the range's >3500 km length where upper Paleozoic Beacon Supergroup strata are down-faulted to near sea level. Previous studies have inferred a range front master normal fault accommodates extension and rock uplift across the TAM front in this region. The history of rock uplift is debated, suggested as early as Mesozoic, typically Cenozoic, or even Pliocene or younger. Structural observations, apatite fission track (AFT) thermochronology, and geomorphologic mapping undertaken within the TAM front east of the Shackleton Glacier indicate extension, faulting, and denudation was mostly late Eocene–late Oligocene and likely into the early Miocene. An exhumed AFT partial annealing zone is found at the coast and traced >50 km inland. The base of that exhumed partial annealing zone indicates denudation accelerated at ∼40 Ma near the coast and at 30–35 Ma on the inland side of the TAM front. Vertically offset AFT isochrones across the TAM front reveal a step-faulted architecture rather than a single master fault. The cumulative vertical offset of the 55 Ma isochrone is 2.3–2.7 km, compared to 2.4–2.6 km offset of Beacon strata, indicating that all significant normal faulting is Cenozoic, and not related to Mesozoic extension within the West Antarctic rift system. Denudation from <26 Ma to ∼14 Ma produced a locally preserved erosion surface within the TAM front. Erosion surface remnants indicate the TAM front has undergone minimal internal deformation and only 290–790 m of surface uplift along offshore faults since the middle Miocene.

Neogene structural evolution of the Sierra San Felipe, Baja California: Evidence for proto-gulf transtension in the Gulf Extensional Province?

The Sierra San Felipe, located in the Gulf Extensional Province of northeastern Baja California, experienced a complex deformation history of integrated normal and strike-slip faulting, block rotations and extension-parallel folding as a result of Neogene transtensional plate margin shear between the Pacific and North American plates. The eastern range-front of the Sierra San Felipe is defined by three left-stepping, en-echelon detachment faults that are linked by dextral and sinistral transfer faults and accommodation zones. The Las Cuevitas, Santa Rosa and Huatamote detachments comprise NE- to SE-dipping, moderate- to low-angle normal faults that accommodate between ~ 1.5 and 9 km of broadly E- to SE-directed extension and show strong along-strike displacement gradients. Hanging wall half-grabens are infilled with northwest tilted Miocene–Pliocene volcanic and sedimentary rocks that have been deposited nonconformably onto the batholithic basement. Stratigraphic relationships indicate that faulting on the Las Cuevitas and Santa Rosa detachment faults initiated synchronously as a kinematically linked fault system before ~ 7 Ma (~ 9–8 Ma based on the timing of footwall exhumation), during the so-called ‘proto-gulf’ phase of rifting. Paleostress calculations suggest a transtensional stress regime with NE- to SE-directed extension and permutating vertical to N–S trending, subhorizontal contraction. Fault kinematics and paleostress orientations of the fault array do not vary through time, but reflect the spatial distribution of fault planes with respect to a transtensional stress regime that lasted throughout the entire slip history of the detachments. Our data indicate that clockwise vertical-axis block rotations and constrictional folding were an integral part of the deformation history in the Sierra San Felipe since rifting began in the late Miocene, and may have played an important role in facilitating the transfer of deformation between the Main Gulf Escarpment and offshore faults in the Gulf of California. These observations support the hypothesis that middle Miocene to present oblique-divergent plate motion was accommodated by a single phase of integrated transtensional shearing in the Gulf Extensional Province.

Seismicity of the Pejibaye-Matina, Costa Rica, region: a strike-slip tectonic boundary?

Este estudio provee ubicaciones de temblores en un esfuerzo por determinar orientaciones de fallas e investigar la existencia de un límite tectónico transcurrente en la costa Caribe de Costa Rica. Un conjunto de 121 temblores de la secuencia sísmica de Pejibaye de Turrialba de 1993 fueron relocalizados usando estaciones sísmicas cercanas a la fuente. La distribución epicentral muestra una sismicidad de fondo dispersa con dos agrupamientos, uno cerca de Pejibaye y otro junto a la falla Chirripó. En Pejibaye, la distribución de temblores tiende al noreste y es paralela a varias fallas pequeñas. Este alineamiento concuerda con el plano noreste de las soluciones focales para el temblor de Pejibaye de 1993. El grupo de temblores cercano a la falla Chirripó está asociado con una falla submarina de orientación noroeste que generó el terremoto de Limón de 1991. La sismicidad, el fallamiento y la deformación cortical no son consistentes con interpretaciones previas de un límite tectónico transcurrente de orientación noreste en el área.

The 8.5+ magnitude, AD365 earthquake in Crete: Coastal uplift, topography changes, archaeological and historical signature

An up to 9 m uplift of western Crete, a cluster of coastal uplifts in the East Mediterranean radiocarbon dated approximately ∼1500 BP, as well as historical and archaeological data are evidence for major, although poorly documented seismic destruction on a nearly-Mediterranean scale in the 4th to 5th c. AD, including the destruction of the Nile Delta in Egypt by a tsunami in AD365. These data represent parts of a puzzle for historians, archaeologists, geologists and seismologists. Detailed analysis of geological, historical and archaeological data, including precise numismatic evidence, permits recognition of a major earthquake in Crete responsible for coastal uplift at AD365. Elastic dislocation analysis of coastal uplift data reveals that this earthquake was associated with a reverse fault offshore of southwestern Crete, that its minimum magnitude was 8.5, and that this model is consistent with the available seismological and large-scale tectonic data. Despite its magnitude, this earthquake cannot have produced the necessary short-period, high-energy waves which are necessary to explain the seismic damage which occurred circa AD365 in a very broad region, from Sicily to Cyprus and Libya. Hence, the AD365 earthquake sequence included at least two other major events, with epicentres close to Cyprus and Sicily. Although significant changes in the coastal morphology, widespread destruction, and a high human death toll occurred, the AD365 Crete earthquake was not responsible for any major cultural change. It occurred during the transition from the Roman to the Christian period, characterized by vast, luxurious villas and modest dwellings on Crete, respectively.

Introduction to the special section for the 2007 Noto Hanto earthquake

A large shallow earthquake occurred in the Noto Peninsula on March 25, 2007. The Japan Meteorological Agency (JMA) estimated the hypocenter and origin time to be 136.685E, 37.220N and 09:41:57.9 JST, respectively, and the depth to be 11 km. The magnitude of the main shock is Mj (JMA scale) 6.9 and Mw 6.6, and the focal mechanism of the main shock is a reverse fault with right-lateral components. The strike, dip and rake are 58◦, 66◦ and 132◦, respectively, according to the National Research Institute for Earth Science and Disaster Prevention (NIED). The two largest aftershocks of Mj 5.4 occurred at the northeast and southwest edges of the aftershock area, at 18:11 on May 25 and at 07:16 on May 26, respectively. A maximum seismic intensity, 6 upper on the JMA scale, was observed at several stations situated in the source area. The earthquake caused severe damage on the Noto Peninsula and in the surrounding prefectures of Ishikawa, Toyama, Niigata and Fukui. The final report of the Fire and Disaster Management Agency reported one human fatality, 356 people injured and 29,349 houses completely and partially collapsed. Although several moderate-sized earthquakes had occurred in the northeast and southern parts of Noto Peninsula prior to this event, there is no record of large earthquakes with M > 7 occurring in this area during the last 1500 years. In addition, events of M 6.0 ∼ 6.5 have never occurred in the source region of the March 25, 2007 earthquake, and even the seismic activity of small and microearthquakes has been very low for more than 25 years, based on the high-gain seismic-observations by the Kamitakara Observatory, Kyoto University (Ito et al., 2007). Large active faults have not been identified on land in the source area. Nevertheless, some offshore faults in west off Noto Peninsula have nearly the same strike as that of the aftershock area, which may be related to the March 25, 2007 earthquake. Well-arranged high-density nation-wide networks have been constructed, and these provided abundant data for the sequence of the earthquake. In particular, strong-motion and high-gain earthquake records as well as the GPS data provided fundamental information on the earthquake sequence and related phenomena from immediately after the main shock. In addition, temporary dense seismic stations were deployed for aftershock observations

Evidence for Quaternary liquefaction-induced features in the epicentral area of the 21 May 2003 Zemmouri earthquake (Algeria, M w = 6.8)

Evidence of ancient liquefaction-in duced features is presented in the area of the 2003 Zemmouri earthquake (M w 6.8). This earthquake was related to an offshore unknown 50-km long fault. A 0.55-m coseismic coastal uplift was generated and extensive liquefaction has been induced in the most susceptible area which correspond to the seaside and along the hydrographic network, mainly the Sebaou and Isser valley rivers. Field investigations allowed us to identify past liquefaction traces in the Quaternary deposits. The observed features are represented by sand dikes, sills, and sand vents as well as well-preserved sand boiled volcanoes. In this work, we also describe the alluvial environment, the hosted localized stratigraphic layer, the morphology and the geometry of the observed features, as well as the observed deformation (settlement) of the hosted layers that are among characteristics of the seismically induced features as described in worldwide examples. Our observations represent a step towards paleoseismological studies in the region knowing that the May 21st 2003 Zemmouri earthquake is produced by an offshore fault where a direct study of the seismogenic fault is inaccessible.

Drilling the North Anatolian Fault

) indicate an accel-erated movement in the recent geological past. During the twentieth century, the NAFZ has ruptured over 900 km of its length. A series of large earthquakes starting in 1939 near Erzincan in eastern Anatolia propagated westward towards the Istanbul-Marmara region in northwestern Turkey. A large part of the Sea of Marmara today represents a seismic gap along a ≥100-km-long segment which did not rupture since 1766 and may have accumulated a slip deficit of 4–5 m. It is believed to be capable of generating two M≥7.1 earth-quakes within the next decades or could even rupture in a large single event. The most recent devastating earthquakes in the region occurred in 1999 near Izmit and Duzce with magnitudes greater than 7. Their western termination of rupture is located offshore below the eastern Sea of Marmara possibly extending to just south of the Princes Islands within ~20 km of Istanbul (Fig.1). Below the Sea of Marmara, the NAFZ branches in two prominent offshore fault segments (Fig. 1) that bound a pull-apart depocenter (Cinarcik Basin) in the NE and SE containing 16–20 mm yr

Seismic hazard of the Canterbury region, New Zealand

We present a new probabilistic seismic hazard model for the Canterbury region, the model superseding the earlier model of Stirling et al. (1999, 2001). The updated model incorporates new onshore and offshore fault data, new seismicity data, new methods for the earthquake source parameterisation of both datasets, and new methods for estimation of the expected levels of Modified Mercalli Intensity (MMI) across the region. While the overall regional pattern of estimated hazard has not changed since the earlier seismic hazard model, there have been slight reductions in hazard in some areas (western Canterbury Plains and eastern Southern Alps), coupled with significant increases in hazard in one area (immediately northeast of Kaikoura). The changes to estimated acceleration for the new versus older model serve to show the extent that major changes to a multidisciplinary source model may impact the final estimates of hazard, while the new MMI estimates show the added impact of a new methodology for calculating MMI hazard.