Temporal behavior of the background seismicity rate in central Japan, 1998 to mid-2003

We studied the temporal behavior of the background shallow seismicity rate in 700 circular areas across inland Japan. To search for and test the significance of the possible rate changes in background seismicity, we developed an efficient computational method that applies the space–time ETAS model proposed by Ogata in 1998 to the areas. Also, we conducted Monte Carlo tests using a simulated catalog to validate the model we applied. Our first finding was that the activation anomalies were found so frequently that the constant background seismicity hypothesis may not be appropriate and/or the triggered event model with constraints on the parameters may not adequately describe the observed seismicity. However, quiescence occasionally occurs merely by chance. Another outcome of our study was that we could automatically find several anomalous background seismicity rate changes associated with the occurrence of large earthquakes. Very significant seismic activation was found before the M6.1 Mt. Iwate earthquake of 1998. Also, possible seismic quiescence was found in an area 150 km southwest of the focal region of the M7.3 Western Tottori earthquake of 2000. The seismicity rate in the area recovered after the mainshock.     

An observational test of the stress accumulation model based on seismicity preceding the 1992 Landers, CA earthquake

We test the Bowman and King [Bowman, D.D., King, G.C.P., 2001a, Accelerating seismicity and stress accumulation before large earthquakes. Geophys. Res. Lett., 28 (21), 4039–4042, Bowman, D.D., King, G.C.P., 2001b. Stress transfer and seismicity changes before large earthquakes. C. R. Acad. Sci. Paris, 333, 591–599] Stress Accumulation model by examining the evolution of seismicity rates prior to the 1992 Landers, California earthquake. The Stress Accumulation (SA) model was developed to explain observations of accelerating seismicity preceding large earthquakes. The model proposes that accelerating seismicity sequences result from the tectonic loading of large fault structures through aseismic slip in the elasto-plastic lower crust. This loading progressively increases the stress on smaller faults within a critical region around the main structure, thereby causing the observed acceleration of precursory activity. A secondary prediction of the SA model is that the precursory seismicity rates should increase first at the edges of the critical region, with the rates gradually rising over time at closer distances to the main fault. We test this prediction by examining year-long seismicity rates between 1960 and 2004, as a function of distance from the Landers rupture. To quantify the significance of trends in the seismicity rates, we auto-correlate the data, using a range of spatial and temporal lags. We find weak evidence for increased seismicity rates propagating towards the Landers rupture, but cannot conclusively distinguish these results from those obtained for a random earthquake catalog. However, we find a strong indication of periodicity in the rate fluctuations, as well as high correlation between activity 130–170 km from Landers and seismicity rates within 50 km of the Landers rupture temporally offset 1.5–2 years. The implications of this spatio–temporal correlation will be addressed in future studies.     

The 1998 Hida Mountain, Central Honshu, Japan, earthquake swarm: Double-difference event relocation, frequency–magnitude distribution and Coulomb stress changes

By using the double-difference relocation technique, we have determined the fine structure of seismicity during the 1998 Hida Mountain earthquake swarm. The distribution of seismic activity defines two main directions (N–S and E–W) that probably correspond to the regional stress pattern. The detailed structure of seismicity reveals intense spatio-temporal clustering and earthquake lineations. Each cluster of events contains a mainshock and subsequent aftershock activity that decays according to the Omori law. The seismicity and the b-value temporal and spatial patterns reflect the evolution of the static stress changes during the earthquake swarm. About 80% of the swarm's best-relocated events occur in regions of increased ΔCFF. The smaller value of b found in the northern part of the swarm region and a larger b-value observed to the south, for the same period of time, could be well explained by the static stress changes caused by the larger events of the sequence. We argue that the state of stress in the crust is the main factor that controls the variation of b-value.     

Induced seismicity in the Castanhão reservoir, NE Brazil — Preliminary results

The Castanhão reservoir was built in the state of Ceará, a dry region in Northeastern Brazil, to regulate the flow of the Jaguaribe River, for irrigation, and for power generation. It is an earth-filled dam, 60 m high, with a water capacity of 4.5 × 10⁹ m³. The seismicity in the area has been monitored since 1998, with a few interruptions, using one analog or one digital station and, during a few periods, a three-station network. The first earthquakes likely to be induced events were detected in 2003, when the water level was about 20 m high. In early 2004 a very heavy rainfall season quickly filled the reservoir. Shortly after, an increase in the seismic activity occurred and many micro-earthquakes were recorded. We suggest that this activity resulted from an increase in pore pressure due to undrained response. Therefore, we may classify this cluster of micro-earthquakes as “initial seismicity.” We deployed a network with four analog stations in the area, following this activity, to determine the epicentral zone. At least three epicentral areas under the reservoir were detected. The spatio-temporal analysis of the available data revealed that the seismicity occurs in clusters and that these were activated at different periods. We identified four sets of faults (N–S-, E–W-, NW–SE-, and NE–SW-oriented), some of which moved in shallow crustal levels and as recently as the Quaternary (1.8 Ma). Under the present-day stress regime, the last two sets moved as strike-slip structures. We suggest a possible correlation between dormant faults and the observed induced seismicity.     

Seismotectonics of strike–slip earthquakes within the deep crust of southern Italy: Geometry, kinematics, stress field and crustal rheology of the Potenza 1990–1991 seismic sequences (Mmax 5.7)

We present a revision and a seismotectonic interpretation of deep crust strike–slip earthquake sequences that occurred in 1990–1991 in the Southern Apennines (Potenza area). The revision is motivated by: i) the striking similarity to a seismic sequence that occurred in 2002  140 km NNW, in an analogous tectonic context (Molise area), suggesting a common seismotectonic environment of regional importance; ii) the close proximity of such deep strike–slip seismicity with shallow extensional seismicity (Apennine area); and iii) the lack of knowledge about the mechanical properties of the crust that might justify the observed crustal seismicity. A comparison between the revised 1990–1991 earthquakes and the 2002 earthquakes, as well as the integration of seismological data with a rheological analysis offer new constraints on the regional seismotectonic context of crustal seismicity in the Southern Apennines. The seismological revision consists of a relocation of the aftershock sequences based on newly constrained velocity models. New focal mechanisms of the aftershocks are computed and the active state of stress is constrained via the use of a stress inversion technique. The relationships among the observed seismicity, the crustal structure of the Southern Apennines, and the rheological layering are analysed along a crustal section crossing southern Italy, by computing geotherms and two-mechanism (brittle frictional vs. ductile plastic strength) rheological profiles. The 1990–1991 seismicity is concentrated in a well-defined depth range (mostly between 15 and 23 km depths). This depth range corresponds to the upper pat of the middle crust underlying the Apulian sedimentary cover, in the footwall of the easternmost Apennine thrust system. The 3D distribution of the aftershocks, the fault kinematics, and the stress inversion indicate the activation of a right-lateral strike–slip fault striking N100°E under a stress field characterized by a sub-horizontal N142°-trending σ1 and a sub-horizontal N232°-trending σ3, very similar to the known stress field of the Gargano seismic zone in the Apulian foreland. The apparent anomalous depths of the earthquakes (> 15 km) and the confinement within a relatively narrow depth range are explained by the crustal rheology, which consists of a strong brittle layer at mid crustal depths sandwiched between two plastic horizons. This articulated rheological stratification is typical of the central part of the Southern Apennine crust, where the Apulian crust is overthrusted by Apennine units. Both the Potenza 1990–1991 and the Molise 2002 seismic sequences can be interpreted to be due to crustal E–W fault zones within the Apulian crust inherited from previous tectonic phases and overthrusted by Apennine units during the Late Pliocene–Middle Pleistocene. The present strike–slip tectonic regime reactivated these fault zones and caused them to move with an uneven mechanical behaviour; brittle seismogenic faulting is confined to the strong brittle part of the middle crust. This strong brittle layer might also act as a stress guide able to laterally transmit the deviatoric stresses responsible for the strike–slip regime in the Apulian crust and may explain the close proximity (nearly overlapping) of the strike–slip and normal faulting regimes in the Southern Apennines. From a methodological point of view, it seems that rather simple two-mechanism rheological profiles, though affected by uncertainties, are still a useful tool for estimating the rheological properties and likely seismogenic behaviour of the crust.     

Coseismic reactivation of the Samambaia fault, Brazil

Northeastern Brazil is, within the present knowledge of historical and instrumental seismicity, one the most seismic active areas in intraplate South America. Seismic activity in the region has occurred mainly around the Potiguar basin. This seismicity includes earthquake swarms characterized by instrumentally-recorded events ≤ 5.2 m_b and paleoseismic events ≥ 7.0. Our study concentrates in the João Câmara (JC) epicentral area, where an earthquake swarm composed of more than 40,000 aftershocks occurred mainly from 1986 to 1990 along the Samambaia fault; 14 of which had m_b > 4.0 and two of which had 5.1 and 5.0 m_b. We describe and compare this aftershock sequence with the present-day stress field and the tectonic fabric in an attempt to understand fault geometry and local control of seismogenic faulting. Earthquake data indicate that seismicity decreased steadily from 1986 to 1998. We selected 2,746 epicenters, which provided a high-quality and precise dataset. It indicates that the fault trends 37° azimuth, dips 76°–80° to NW, and forms an alignment  27 km long that cuts across the NNE–SSW-trending ductile Precambrian fabric. The depth of these events ranged from  1 km to  9 km. The fault forms an echelon array of three main left-bend segments: one in the northern and two in the southern part of the fault. A low-seismicity zone, which marks a contractional bend, occurs between the northern and southern segments. Focal mechanisms indicate that the area is under an E–W-oriented compression, which led to strike–slip shear along the Samambaia fault with a small normal component. The fault is at 53° to the maximum compression and is severely misoriented for reactivation under the present-day stress field. The seismicity, however, spatially coincides with a brittle fabric composed of quartz veins and silicified-fault zones. We conclude that the Samambaia fault is a discontinuous and reactivated structure marked at the surface by a well-defined brittle fabric, which is associated with silica-rich fluids.     

Variations of Seismicity in the Avachinsky Gulf (Kamchatka, Russia)

Variations of seismic mode in the region of the Avachinsky Gulf (Kamchatka, Russia) are considered. Observed anomalies (seismic quiescence, the ring seismicity, reduction of the slope of the earthquake recurrence diagram) provide a basis to consider this region as a place of strong earthquake preparation. The Kamchatka regional catalogues of earthquakes between 1962–1995 were used in the analysis. A reduced seismicity rate is observed during 10 years in an area of 150 km × 60 km in size. During the last five years, in the vicinity of the area considered, earthquakes with M > 5 occurred three times more often than the average over thirty years. It is interpreted as ring seismicity. The block of 220 km × 220~km in size, including the quiescence zone, is characterized by a continuous decrease of the recurrence diagram slope, which has reached a minimum value for the last 33 years in this region.     

Physical and stochastic models of earthquake clustering

The phenomenon of earthquake clustering, i.e., the increase of occurrence probability for seismic events close in space and time to other previous earthquakes, has been modeled both by statistical and physical processes.From a statistical viewpoint the so-called epidemic model (ETAS) introduced by Ogata in 1988 and its variations have become fairly well known in the seismological community. Tests on real seismicity and comparison with a plain time-independent Poissonian model through likelihood-based methods have reliably proved their validity.On the other hand, in the last decade many papers have been published on the so-called Coulomb stress change principle, based on the theory of elasticity, showing qualitatively that an increase of the Coulomb stress in a given area is usually associated with an increase of seismic activity. More specifically, the rate-and-state theory developed by Dieterich in the ′90s has been able to give a physical justification to the phenomenon known as Omori law. According to this law, a mainshock is followed by a series of aftershocks whose frequency decreases in time as an inverse power law.In this study we give an outline of the above-mentioned stochastic and physical models, and build up an approach by which these models can be merged in a single algorithm and statistically tested. The application to the seismicity of Japan from 1970 to 2003 shows that the new model incorporating the physical concept of the rate-and-state theory performs not worse than the purely stochastic model with two free parameters only. The numerical results obtained in these applications are related to physical characters of the model as the stress change produced by an earthquake close to its edges and to the A and σ parameters of the rate-and-state constitutive law.     

A new case of reservoir triggered seismicity: Govind Ballav Pant reservoir (Rihand dam), central India

We report here that seismicity near Govind Ballav Pant reservoir is strongly influenced by the reservoir operations. It is the second largest reservoir in India, which is built on Rihand river in the failed rift region of central India. Most of the earthquakes occurred during the high water stand in the reservoir with a time lag of about 1 month. We use the concept of coulomb stress change and use Green's function based approach to estimate stresses and pore pressure due to the reservoir load. We find that the reservoir increases coulomb stress on the nearby faults of the region that are favourably oriented for failure in predominantly reverse slip manner under the NNE–SSW compression and thus promotes failure. The above two factors make it an obvious, yet so far unreported case of reservoir triggered seismicity.     

Global-scale analysis of seismic activity prior to 2004 Sumatra-Andaman mega-earthquake

Following our experience in intermediate-term monitoring of seismicity before large earthquakes worldwide and in some regions we apply a similar approach to the analysis of seismicity on global scale several decades before the December 26, 2004 Sumatra–Andaman mega-earthquake. The Earth lithosphere is being considered as a single whole, representing the ultimate level of the complex Earth's hierarchy. The study brings up the following questions: Are there any anomalies of the global seismicity behaviour observed in advance of the mega-earthquake? Do these anomalies correspond to the global scale seismic patterns similar to those detected on the regional scale before great, major, and strong events? In other words, does the Earth lithosphere considered as a single whole show up an approach of the mega-earthquake in the way typical for events on the smaller levels of hierarchy? The results of our investigation favour the positive answer to the questions. Specifically we found that during the decade before the Sumatra–Andaman mega-earthquake the Earth lithosphere reveals, at least in intermediate-term scale, classical symptoms of instability, which can be depicted by known precursory seismic patterns. These are: (i) transformation of frequency–magnitude distribution, (ii) change in the rate of seismic activity, and (iii) depth redistribution of activity. Moreover changes of dependencies between magnitudes of different types are detected at the same time. The observed global scale patterns of collective behaviour of seismicity may indicate the state of criticality of the Earth lithosphere before the Sumatra–Andaman mega-earthquake.     

Characteristics of three recent earthquake sequences in the Taupo Volcanic Zone, New Zealand

We have analysed three recent earthquake sequences in the northern part of the Taupo Volcanic Zone. A 1998 sequence at Haroharo with a largest event of M_L 4.8, and a 2004 sequence near Lake Rotoehu (largest event M_L 5.4), had normal b-values, and displayed an aftershock decay pattern, with most of the activity within the first few days. In contrast, a 2005 sequence a few tens of kilometres away at Matata (largest event M_L 4.1), had very different characteristics, with a slow development and decay, no tendency for enhanced seismicity after the larger events, and a very high b-value.The focal mechanisms of the Rotoehu and Matata events are normal, and have stress patterns consistent with the geodetically observed extension of the Taupo Volcanic Zone in a northwest–southeast direction. The extensive recent volcanism in the Okataina Volcanic Centre does not seem to have affected the stress pattern in this area.The Rotoehu sequence showed a strong resemblance, particularly in the time distribution of events, to the well-known swarm activity in the Vogtland region on the German/Czech border, in which larger events were followed by a burst of seismicity, as in a normal aftershock sequence. Some of the arguments that have been advanced to explain the Vogtland swarm as seismicity induced by fluid injection apply to Rotoehu, but there is no direct evidence of fluid involvement. The Matata sequence appears to have a continuing trigger mechanism, either a slow injection of fluid, or a slow slip event, in an environment in which opening pore spaces prevent high overpressures developing. The Matata sequence occurred close to the area of the 1987 M_L 6.3 Edgecumbe Earthquake, so exhibiting two extremes of seismic temporal pattern, namely mainshock–aftershock and a swarm with many events of similar magnitude, within a small area.     

Local seismic array observations at north Evoikos, central Greece, delineate crustal deformation between the North Aegean Trough and Corinthiakos Rift

In order to better constrain and define the microseismic activity at the north Evoikos Gulf and its surrounding area we deployed an onshore/offshore seismic array consisting of 31 three-component seismic digital stations. The array was active from 30 June to 24 October 2003, and covered an area of 2500 km². We located more than 2000 seismic events ranging from 0.7 to 4.5 M_L by using six stations as a minimum in order to define the foci parameters. Recorded seismicity delineated three major zones of deformation: from south to north, the Eretria–Parnis–eastern Corinthiakos zone, the Psachna–Viotia zone, and the Northern Sporades–North Evia–Bralos zone. Alignments of the recorded seismicity follow the tectonic trends and their orientation in the above zones. The whole area accommodates the stress field between the North Aegean Trough and the Corinthiakos Gulf. Rate of deformation intensifies from north to south, as revealed also by historical and instrumental seismicity. The successive change of orientation between the two stress fields fragments the crust in relatively small units and the fault systems developed do not permit the generation of major earthquakes in the north Evoikos area and its immediate vicinity. This is also supported by the instrumental seismicity of the last century. Larger events reported in historical times are probably overestimated.Most seismic activity is crustal. Subcrustal events were recorded mainly below the Lichades area and are interpreted as the consequence of the subduction of the Ionian oceanic lithosphere below the Hellenides. The Lichades volcano is the most northern end of the Hellenic volcanic arc.At present the highest seismic activity is associated with the Psachna region of north Evia that has been continuously active since 2001. Considering, however, the development of the seismic activity during the last decade, there has been a sequence of large events, i.e., Parnis in 1999, Skyros in 2001 and Psachna in 2001–2003. This demonstrates the fact that the tectonic deformation in all this area is intense and important for the accommodation of the stress field of the North Aegean Trough to that of the Corinthiakos Rift.     

A new synthesis of Pyrenean seismicity and its tectonic implications

The Pyrenean range, which results from the convergence of the Iberian and Eurasian plates along the North Pyrenean fault, exhibits a permanent seismic activity with moderate magnitude events. From the end of the 1980s, seismic instrumentation has been deployed in the Pyrenees, making now possible the computation of improved seismicity maps. We have gathered all the arrival times published for the period 1989–1996 by the different Spanish and French institutions in charge of the seismic survey of the range, and reprocessed them in an homogeneous way, in order to obtain a coherent seismicity map over the whole range. Particular attention has been paid to the evaluation of the quality of the locations and to the focal depth determinations. The comparison with previous maps of the Pyrenean seismicity reveals significant improvements in both the quality of locations and the threshold of detection. The new seismicity map reveals that the North Pyrenean fault is active only in the western part of the range. In the central and eastern parts, the seismicity involves other tectonic units such as the Maladeta and Canigou granitic massifs, the North Pyrenean Frontal Thrust, the Tet fault and the volcanic units in Catalonia. Despite the short time interval considered, this new seismicity file may be a valuable tool for future tectonic studies.     

Seismic history and consistent seismicity: Evidence from southern Italy

In this paper a method is proposed to evaluate the seismicity level of an area in a given historical period, based on records of seismic events, source characteristics and intensity attenuation with distance. Also considered is the seismic activity recorded in southern Italy during the 10th and 11th centuries, seismic records being obtained from all available sources. To determine the level of seismicity, a key role is played by source characteristics, i.e. recording modalities and activity periods of recording centers. In addition, models of intensity attenuation with distance allow the assessment of the size of the area under investigation. This paper identifies the areas, in the 10th and 11th centuries, where major earthquakes (M 6.5) did not occur during periods of silence of sources, as well as those where such events cannot be excluded. For each area, different levels of probability were determined by applying the Cox linear logistic model to historical seismic data. The completeness analysis, in terms of area and time-span coverage, is a valuable tool to assess seismicity in seismogenetic areas. The reproducibility of the model for lower magnitude earthquakes (M < 6.5) is reliable.     

Background seismicity in the Central Apennines of Italy: The Abruzzo region case study

We investigate background seismic activity of the Abruzzo region, a 5000 km² area located within the Central Apennines of Italy, where in the past 600 years at least 5 large earthquakes (I = XI–X) have occurred.Between April 2003 and September 2004, a dense temporary seismic network composed of 30 digital three-component seismic stations recorded 850 earthquakes with 0.9 < M_L < 3.7. We present earthquake locations and focal mechanisms obtained by standard procedures and an optimized velocity model computed with a search technique based on genetic algorithms.The seismicity occurs at a low and constant rate of  2.6 e− 04 events/daykm² and is sparsely distributed within the first 15 km of the crust. Minor increases in the seismicity rate are related to the occurrence of small and localised seismic sequences that occur at the tip of major active normal faults along secondary structures.We observe that during the 16 months of study period, the Fucino fault system responsible for the 1915 Fucino earthquake (M_S = 7.0), and the major normal faults of the area, did not produce significant seismic activity.Fault plane solutions evaluated using P-wave polarity data show the predominance of normal faulting mechanisms ( 55%) with NE-trending direction of extension coherent with the regional stress field active in this sector of the Apennines. Around 27% of the focal solutions have pure strike–slip mechanisms and the rest shows transtensional faulting mechanisms that mainly characterise the kinematics of the secondary structures activated by the small sequences.We hypothesize that the largest known NW-trending normal faults are presently locked and we propose that in the case of activation, the secondary structures located at their tips may act as transfer faults accommodating a minor part of the extensional deformation with strike–slip motion.     

Forecasting rupture dimension using the pattern informatics technique

The Pattern Informatics (PI) technique [Tiampo, K.F., Rundle, J.B., McGinnis, S., Gross, S., Klein, W., 2002. Mean-field threshold systems and phase dynamics: An application to earthquake fault systems, Europhys. Lett., 60, 481–487] is founded on the premise that changes in the seismicity rate are a proxy for changes in the underlying stress. This new approach to the study of seismicity quantifies its local and regional space–time patterns and identifies regions of local quiescence or activation. Here we use a modification of the PI method to quantify localized changes surrounding the epicenters of large earthquakes in California in an attempt to objectively quantify the rupture zones of these upcoming events. We show that this method can be used to forecast the size and magnitude of future earthquakes.     

Crustal seismicity in central Chile

Both the genesis and rates of activity of shallow intraplate seismic activity in central Chile are poorly understood, mainly because of the lack of association of seismicity with recognizable fault features at the surface and a poor record of seismic activity. The goal of this work is to detail the characteristics of seismicity that takes place in the western flank of the Andes in central Chile. This region, located less than 100 km from Santiago, has been the site of earthquakes with magnitudes up to 6.9, including several 5+ magnitude shocks in recent years. Because most of the events lie outside the Central Chile Seismic Network, at distances up to 60 km to the east, it is essential to have adequate knowledge of the velocity structure in the Andean region to produce the highest possible quality of epicentral locations. For this, a N–S refraction line, using mining blasts of the Disputada de Las Condes open pit mine, has been acquired. These blasts were detected and recorded as far as 180 km south of the mine. Interpretation of the travel times indicates an upper crustal model consisting of three layers: 2.2-, 6.7-, and 6.1-km thick, overlying a half space; their associated P wave velocities are 4.75–5.0 (gradient), 5.8–6.0 (gradient), 6.2, and 6.6 km/s, respectively.Hypocentral relocation of earthquakes in 1986–2001, using the newly developed velocity model, reveals several regions of concentrated seismicity. One clearly delineates the fault zone and extensions of the strike-slip earthquake that took place in September 1987 at the source of the Cachapoal River. Other regions of activity are near the San José volcano, the source of the Maipo River, and two previously recognized lineaments that correspond to the southern extension of the Pocuro fault and Olivares River. A temporary array of seismographs, installed in the high Maipo River (1996) and San José volcano (1997) regions, established the hypocentral location of events with errors of less than 1 km. These events are clustered along no particular lineament approximately 25 km away from the San José and Maipo volcanoes. Recurrence intervals, based on a frequency magnitude relationship for lower magnitude events, indicate that earthquakes with magnitudes of 4.7 and 7 have a repeat time of 1 and 1200 years, respectively. Focal mechanisms of the two largest events indicate horizontal maximum and minimum compressive stresses with σ₁ varying from a NW–SE orientation in the north to E–W at the southern extreme.     

A database for worldwide seismicity quantification

The first step in a seismicity analysis usually consists of defining the seismogenic units, seismic zones or individual faults. The worldwide delimitation of these zones involves an enormous effort and is often rather subjective. Also, a complete recording of faults will not be available for a long time yet. The seismicity model presented in this paper therefore is not based on individually defined seismic zones but rather on the assumption that each point in a global 1/2° grid of coordinates represents a potential earthquake source. The corresponding seismogenic parameters are allocated to each of these points. The earthquake occurrence frequency, one of the most important parameters, is determined purely statistically by appropriately spreading out the positions of past occurrences. All the other significant seismicity characteristics, such as magnitude-frequency relations, maximum possible magnitude and attenuation laws including the dependence on focal depth are determined in a global 1/2° grid of co-ordinates. This method of interpreting seismicity data allows us to establish a transparent, sufficiently precise representation of seismic hazard which is ideally suited for computer-aided risk analyses.     

Earthquakes in the Eastern Great Lakes Basin from a regional perspective

This paper presents a summary of the seismicity and its relation to stress and geologic structures in the Eastern Great Lakes Basin (EGLB) and compares it with that of other regions in the central and eastern North America (CENA). The earthquakes scattered throughout the EGLB are occurring at a rate somewhat less than that of the Appalachians and along the Atlantic Seaboard. Paleoseismology studies suggest that the lower seismicity rate may be characteristic of the EGLB since the Late Wisconsin. North of the EGLB, earthquakes have primarily thrust mechanisms, while to the south of the EGLB, most earthquakes are strike-slip. Throughout the region, including the EGLB, the average P axes of the earthquakes are oriented NE–SW and are aligned with the direction of the current plate driving stress. On a regional basis, earthquakes are centered primarily in the Precambrian basement beneath the Paleozoic cover. Many of the earthquakes in the EGLB have occurred in areas of preexisting faults, at least some of which may have been active during past episodes of continental rifting. For individual faults that have been studied in some detail, however, it is not clear whether earthquakes represent reactivations of local preexisting structures or nucleation of new ruptures in or near the old fault zones.     

Seismicity and structural investigations of the Romanian Vrancea Region: Evidence for azimuthal variations of p-wave velocity and poisson's ratio

A study of the shallow and intermediate depth seismicity of the Romanian Vrancea region in the period 1964–1981 has been performed. The seismic events have been relocated by a standard location procedure using a regional velocity model. From the temporal and spatial distribution of the seismic activity, aspects of the seismicity before the large March 4, 1977 earthquake are treated, in particular the seismic gap in space and time prior to this event, found by Mârze (1979), which is critically discussed and revised. The concept of the precursor time/magnitude relationships of different authors is applied and its validity to the Vrancea region assessed. The hypocentral distribution shows that the intermediate depth seismic activity is confined to a small volume with dimensions of only some tens of kilometers. The results are interpreted in terms of the tectonics of the region. From an analysis of the travel-time residuals at different local stations, evidence for lateral velocity heterogeneities beneath the region is obtained e.g. a high velocity zone southeastwards of the Carpathian chain. Finally mean ratios, (i.e. Poisson's ratios), for various stations are calculated from P- and S-wave travel times. They show azimuthal variations of up to 6% for stations within the area where the intermediate seismic activity occurs in comparison with the station Focsani, situated eastwards in the Carpathian foredeeps. All these results are compatible with the plate tectonic concept for the Vrancea region, that is the subduction of an oceanic lithospheric slab under the Carpathian mountain arc, giving rise to such a highly active seismic zone.