Focal Mechanisms Of Shallow Earthquakes Research Articles

Late Cenozoic stress state distributions at the intersection of the Hellenic and Cyprus Arcs, SW Turkey

The history of the Late Cenozoic stress regime was determined for an area between the gulfs of Fethiye and Antalya. Fault kinematic analysis and inversion of focal mechanisms of shallow earthquakes reveal significant evolution of the regional stress regime in SW Anatolia, i.e., the area of interaction between the Hellenic and Cyprus arcs, from the Mio-Pliocene to the present time. Fault kinematic analysis yields two different normal faulting stress regimes along the southwestern part of Fethiye-Burdur Fault zone, e.g., in and around Çameli Basin (Zone A1) and two different strike-slip to normal faulting stress regimes characterized by a roughly orthogonal set of extensional axes between Fethiye and Demre (Zone B) with an older NW–SE σ3 axis for Mio-Pliocene and a younger NE–SW σ3 axis for Plio-Quaternary time. Inversion of focal mechanisms of the earthquakes occurring in Zone A1 provides an extensional stress state with approximately N-S σ3 axis. Inversion of those occurring in Zone B, south of Zone A1, yields a dominantly strike-slip stress state with a NE-SW σ3 axis and a NW-SE σ1 axis respectively. The inversion slip vectors from fault planes yield a consistent normal faulting stress regime in Burdur Basin and its surroundings (i.e., along the northeastern part of Fethiye-Burdur Fault Zone, (Zone A2)) during Plio-Quaternary, continuing into recent time as indicated by earthquake focal mechanism inversions. Both states have a consistent NW–SE σ3 axis. Fault kinematic analysis indicates NW-SE extension acting in Zone C (subarea between Demre and Antalya), south of Zone A2, during Mio-Pliocene time. The inversion of focal mechanisms yields normal faulting also characterized by a consistent NW-SE σ3 axis. The nearly orthogonal extensional stress regimes (NW-SE and NE-SW) obtained by inversion of both measured and seismic faults seem to have been acting contemporaneously with each other at different intensities from the Mio-Pliocene onwards in SW Turkey. This may be attributed to the geodynamic effects related to the subduction of the African plate beneath Anatolia diffusing along the Hellenic and Cyprus arcs and in the west-southwestward extrusion of Anatolia. The cause of the early NW–SE extension is the slab-pull force due to the subduction process along the Cyprus arc, considered to be dominant until the Plio-Quaternary in the western part of the study area in zones A1 and B. The dominant status of the Cyprus arc continues today in the eastern part of study area in zones A2 and C. The later NE–SW to present day approximately N–S extension, dominant since the Plio-Quaternary, is related to the combined forces of the Anatolian extrusion and the subduction process along the Hellenic arc.

Current states of stress in the northern Andes as indicated by focal mechanisms of earthquakes

Systematic inversion of double couple focal mechanisms of shallow earthquakes in the northern Andes reveals relatively homogeneous patterns of crustal stress in three main regions. The first region, presently under the influence of the Caribbean plate, includes the northern segment of the Eastern Cordillera of Colombia and the western flank of the Central Cordillera (north of 4°N). It is characterized by WNW–ESE compression of dominantly reverse type that deflects to NW–SE in the Merida Andes of Venezuela, where it becomes mainly strike–slip in type. A major bend of the Eastern thrust front of the Eastern Cordillera, near its junction with the Merida Andes, coincides with a local deflection of the stress regime (SW–NE compression), suggesting local accommodation of the thrust belt to a rigid indenter in this area. The second region includes the SW Pacific coast of Colombia and Ecuador, currently under the influence of the Nazca plate. In this area, approximately E–W compression is mainly reverse in type. It deflects to WSW–ENE in the northern Andes south of 4°N, where it is accommodated by right-lateral displacement of the Romeral fault complex and the Eastern front of the northern Andes. The third, and most complex, region is the area of the triple junction between the South American, Nazca and Caribbean plates. It reveals two major stress regimes, both mainly strike–slip in type. The first regime involves SW–NE compression related to the interaction between the Nazca and Caribbean plates and the Panama micro-plate, typically accommodated in an E–W left-lateral shear zone. The second regime involves NW–SE compression, mainly related to the interaction between the Caribbean plate and the North Andes block which induces left-lateral displacement on the Uramita and Romeral faults north of 4°N. Deep seismicity (about 150–170 km) concentrates in the Bucaramanga nest and Cauca Valley areas. The inversion reveals a rather homogeneous attitude of the minimum stress axis, which dips towards the E. This extension is consistent with the present plunge of the Nazca and Caribbean slabs, suggesting that a broken slab may be torn under gravitational stresses in the Bucaramanga nest. This model is compatible with current blocking of the subduction in the western northern Andes, inhibiting the eastward displacement of slabs, which are forced to break and sink in to the asthenosphere under their own weight.

A synthesis: Late Cenozoic stress field distribution at northeastern corner of the Eastern Mediterranean, SE Turkey

Abstract Fault kinematic analysis and inversion of focal mechanisms of shallow earthquakes reveal significant evolution of the regional stress regime in the northeastern most corner of the Eastern Mediterranean region since the Mio-Pliocene to the present time. This study was carried out in the interaction area between the Arabian/African plates and the Anatolian block. The evolution of stress regimes consists of a change from older transpression to younger transtension. Both strike-slip stress regimes having a NNW- to northwest-trending σ Hmax ( σ 1 ) and ENE- to northeast-trending σ Hmin ( σ 3 ) axes induce a sinistral component of displacement on the major intra-continental Karatas–Osmaniye and Misis–Ceyhan faults elongated with the northeast-trending Misis Range between Adana and Osmaniye provinces (sub-area i ) and by a NNE-trending plate boundary Amanos fault running along Amanos Range between Antakya and Kahramanmaras provinces (sub-area ii ). The inversion results show that the transtensional stress regime is dominantly strike-slip to extension, with an ENE- to northeast-trending σ Hmin ( σ 3 ) axis for sub-areas ( i ) and ( ii ), respectively. The inversions of earthquake focal mechanisms indicate that the transtensional stress regime is still active in the whole study area since probably recent Quaternary time. To cite this article: S. Over et al., C. R. Geoscience 336 (2004).

Focal mechanisms of shallow earthquakes in the Aegean Sea and the surrounding lands determined by waveform modelling: a new database

Source parameters for 40 earthquakes with epicentres in the Aegean Sea and the surrounding lands have been determined using P and SH body waveform inversion of digital waveforms from the Global Seismograph Network (GSN). These results are combined with 145 previous solutions, 41 of which were determined by first motion polarities and 104 by waveform modelling. Thus, a new updated database of source parameters for earthquakes for the period 1953–1999 is available. These fault plane solutions confirm our previous knowledge about the seismotectonics of the Aegean Sea and the surrounding lands. They also provide strong evidence that the western part of the Peloponnese (Gulf of Patras to Gulf of Kyparissia) is mainly deforming by strike-slip faulting, parallel in strike to the Cephalonia Fault and are in favour of the models that suggest a connection of the North Anatolian Fault (NAF) zone with the transform zone of the Ionian Islands. The sedimentary arc (inner part of the Hellenic trench) is deforming by normal faulting with the T-axes trending ∼E–W. We did not determine any fault plane solutions at the eastern part of the Hellenic arc indicating pure strike-slip motions as expected from the presence of the Pliny and Strabo trenches. However, we did determine solutions that indicate normal faulting combined with a considerable strike-slip component. In the central Aegean Sea the newly determined focal mechanisms support the conclusion that the area, extending from the eastern coasts of Evia up to the coasts of Turkey, is exhibiting a clear strike-slip character. Thus, the effect of the propagating tip of the North Anatolian fault into the Aegean is very clearly pronounced. The focal mechanisms in the back-arc Aegean area confirm the existence of normal faulting with T-axes trending NNE–SSW in western Anatolia and NNW–SSE in Greece. A zone showing NNE–SSW extension that runs parallel to Northern Aegean branch of the NAF zone up to the Corinth Gulf is depicted from the mechanisms of few earthquakes.

An integrated global model of present-day plate motions and plate boundary deformation

In this paper we present a global model (GSRM-1) of both horizontal velocities on the Earth's surface and horizontal strain rates for almost all deforming plate boundary zones. A model strain rate field is obtained jointly with a global velocity field in the process of solving for a global velocity gradient tensor field. In our model we perform a least-squares fit between model velocities and observed geodetic velocities, as well as between model strain rates and observed geological strain rates. Model velocities and strain rates are interpolated over a spherical Earth using bi-cubic Bessel splines. We include 3000 geodetic velocities from 50 different, mostly published, studies. Geological strain rates are obtained for central Asia only and they are inferred from Quaternary fault slip rates. For all areas where no geological information is included a priori constraints are placed on the style and direction (but not magnitude) of the model strain rate field. These constraints are taken from a seismic strain rate field inferred from (normalized) focal mechanisms of shallow earthquakes. We present a global solution of the second invariant of the model strain rate field as well as strain rate solutions for a few selected plate boundary zones. Generally, the strain rate tensor field is consistent with geological and seismological data. With the assumption of plate rigidity for all areas other than the plate boundary zones we also present relative angular velocities for most plate pairs. We find that in general there is a good agreement between the present-day plate motions we obtain and long-term plate motions, but a few significant differences exist. The rotation rates for the Indian, Arabian and Nubian plates relative to Eurasia are 30, 13 and 50 per cent slower than the NUVEL-1A estimate, respectively, and the rotation rate for the Nazca Plate relative to South America is 17 per cent slower. On the other hand, Caribbean–North America motion is 76 per cent faster than the NUVEL-1A estimate. While crustal blocks in the India–Eurasia collision zone move significantly and self-consistently with respect to bounding plates, only a very small motion is predicted between the Nubian and Somalian plates. By integrating plate boundary zone deformation with the traditional modelling of angular velocities of rigid plates we have obtained a model that has already been proven valuable in, for instance, redefining a no-net-rotation model of surface motions and by confirming a global correlation between seismicity rates and tectonic moment rates along subduction zones and within zones of continental deformation.

Determination of Euler pole for contemporary relative motion of Caribbean and North American plates using slip vectors of interplate earthquakes

A number of different Euler poles have been proposed by various authors during the past 20 years to describe the relative motion of Caribbean (CA) and North American (NA) plates. These different solutions are a consequence of the small number of focal mechanism solutions of earthquakes, the short length of the plate boundaries of the Caribbean that consists of an active spreading center, the complicated and multibranched fault systems of several parts of the plate boundaries, and biases introduced in using slip vectors along the Middle American Trench to close plate motion circuits in computing CA‐NA Euler poles. Slip vectors from focal mechanisms are now the strongest constraints on the direction of current plate motion. We use 66 slip vectors derived from 117 focal mechanisms of shallow earthquakes located on or near the boundaries between the Caribbean and either the NA or South American (SA) plates to calculate a best fitting, present‐day rotation pole for CA‐NA. It is located at 68.4°N, 126.3°W, near Barrow, Alaska. In making this calculation, we effectively extended the length of the plate boundary considered in the calculation by using data from the CA‐SA plate boundary and rotating them to the CA‐NA reference frame using the Nuvel 1 pole for NA‐SA motion and 20 mm/yr as the rate of CA‐SA plate motion along the southeastern Caribbean. The result is not very sensitive to CA‐SA rates. The azimuth of plate motion is about N70°E along most of the CA‐NA boundary. For example, along the Puerto Rico Trench, several focal mechanisms indicate highly oblique motion with a small component of plate convergence along shallow, south dipping thrust faults. The highly oblique plate motion along the inner wall of the Puerto Rico Trench is indicative of strong coupling of that forearc with the Caribbean plate and of the potential of that zone to generate great earthquakes. Slip vectors are very consistent with one another in areas in which at least one of the interacting plates is oceanic. Focal mechanisms and the distribution of earthquakes exhibit greater scatter, however, in areas such as Hispaniola where continental or thick island arc lithosphere is being deformed and is present on both sides of that complex plate boundary. In addition, other mechanisms indicate northerly subduction of the Caribbean beneath southeastern Hispaniola along the western Muertos Trough, normal faulting to the east of the Lesser Antilles associated with the bending of the downgoing plate, and normal faulting or reverse faulting in more localized areas near the plate boundary. As part of the analysis, we also calculated Euler poles for CA‐SA motion at 52°N, 81°W and Cocos‐Caribbean (CO‐CA) at 22°N, 120°W. These poles are consistent with recent models of transtensional development since about 10 m.y. ago along the plate boundary zone of the southeastern Caribbean and with CO‐CA motion that involves subduction along the Middle America Trench as well as right‐lateral, strike‐slip faulting and extension along the volcanic zone of Central America. The Euler pole for CO‐CA is also consistent with recent geodetic data on CO‐CA rates of motion.

Analysis of seismotectonic processes in subduction zones from the standpoint of a keyboard model of great earthquakes

The block-like structure of the frontal part of the overriding plate in the subduction zones of the Aleutian and Kurile trenches is described. Both block interaction with a subducted plate and reciprocal interblock actions are considered. These interactions are reflected: 1. (1) in the seismic regime; 2. (2) in the distribution of the sources of great earthquakes; 3. (3) in the focal mechanisms of shallow earthquakes. The mechanical keyboard model proposed incorporates these interactions. This model is the basis of a mathematical description of the subsequent slow accumulation of strain and fast seismic release in ajacent blocks in the course of their seismotectonic evolution. Some results of numerical simulations are given. They are in good agreement with the main features of distribution of great earthquakes in space and time for the Kuriles, Japan and Central and South America. “Keyboard” analysis of seismic cycles can be considered as a mechanical basis for long-term prediction of great earthquakes which allows the distinguishing of a number of new earthquake precursors.

Seismotectonics of the central segment of the Indonesian Arc

In this paper, a revision of seismicity affecting the central segment of the Indonesian island arc ranging between 110° and 126° E is presented. Using the areal and in-depth distribution of seismic activity, strain release maps and focal mechanisms, lateral changes in the Wadati-Benioff zone have been analyzed to determine possible boundaries between portions of lithosphere with different subduction geometries. The seismicity pattern indicates that the Sumbawa-Flores-Wetar sector shows different forms of behaviour with respect to the adjacent sectors. These include driving mechanism, inclination and continuity of the subducting slab and subduction features. This area therefore seems to be isolated from the Sunda and Banda arcs by two principal boundaries, these having a nearly N-S trend in the Bali region and with a nearly E-W trend in the region ranging between Wetar-Northern Timor and Tanibar. The first boundary, characterized by an absolute minimum of seismic activity at all ranges of depth, has been interpreted in terms of subduction of the Roo Rise aseismic bathymetric ridge. For the second boundary, dividing a northwardly steeply-dipping slab from an E-W subducting slab dipping with an angle of 30 ° –40 °, a tear in the upper part (depth less than 300 km) and a hinge fault system in the deepest part of the lithosphere, have been proposed. From the analysis of focal mechanisms of shallow earthquakes, it was inferred that the central part of the Indonesian Arc is subject to a vortex-shaped stress field centred on the Savu Basin. In this model, the compressive axes appear to rotate counterclockwise (from SW to NNE) in the Sumba-Sumbawa-Western Flores region and clockwise (from W to NNW) in the Timor-Eastern Flores zone. To interpret these features, on the basis of seismological evidence, a lateral discontinuity in the arc-trench system close to Sumba, a collision between Sumba and Sumbawa and a rotation towards the north-northeast of Sumba have been suggested. The proposed structural discontinuity, trending NW-SE, may represent a major transcurrent fault zone through which the Australian continental lithosphere comes into contact with the Indian oceanic lithosphere.

山陰, 近畿, 北陸に発生する浅発地震の発震機構

山陰, 近畿, 北陸に発生する浅発地震の発震機構

Accurate source depths and focal mechanisms of shallow earthquakes in western South America and in the New Hebrides Island Arc

Synthetic seismograms are matched to long‐period P waveforms in order to obtain accurate depths of shallow earthquakes with known focal mechanisms. Accurate depths are obtained for a large sample of moderate‐sized (6 < Ms < 7) events which produced relatively simple P waveforms and do not require intensive analysis. The estimated error in the determination of depth is about ±5 km or less for most of the events considered. The procedure is applied to two suites of events, one in South America (83 events) and one in the New Hebrides (61 events). In these two areas of contrasting styles of subduction, the accurate depths provide new information on the bending of descending plates near trenches, the seismicity and tectonics of the upper plate, and the geometry and structure of the zone of contact between the upper and descending plates. Depths of suboceanic earthquakes which occurred near the trench and which have either tensional and compressional horizontal stress axes agree well with Chapple and Forsyth's (1979) preferred model of a bending elastic‐perfectly plastic plate. However, an unusually deep event occurred beneath the Fiji plateau in an area of young ocean floor supposed to have been formed by sea floor spreading processes only since late Miocene time. The calculated depth of the event, 48 km, places it below the lithosphere‐asthenosphere boundary derived from thermal models. Vertical cross sections through the shallow parts of the subduction zones show that most of the earthquakes with thrust faulting focal mechanisms can be interpreted to be interplate events located in a thin (<10 km thick) curved zone of contact dipping arcward or landward from the trench axis. However, a few events with focal mechanisms similar to the interplate events do not fit on the inferred thin zone of contact. These events would be accommodated by an interplate zone 15 km thick. In both subduction zones, interplate events occur at depths between about 15 and 50 km. However, the downdip width of the South American interplate zone is 2 to 3 times larger than that in the New Hebrides. Most of the intraplate events near the interplate boundary are located in the descending plate. In the upper plate, seismicity is more pronounced in the “back arc” regions of the two subduction zones. Thus our results support the generality of a relatively aseismic zone in the upper plate located trenchward of the magmatic arc. Depths of shallow earthquakes in the eastern Andes and sub‐Andean regions of South America range between about 9 and 36 km, indicating that both the upper and middle crust are seismically active; about 75% of the events occurred in the upper 25 km. Furthermore, the focal mechanisms indicate that from Ecuador to Argentina the crust of the eastern side of the Andean Cordilleran is in a state of east‐west compression oriented nearly parallel to the direction of convergence between the Nazca and South American plates. The depths, the orientations of the earthquake nodal planes, and the close relationships to Quaternary tectonics imply basement‐involved crustal shortening in much of the Eastern Andes and sub‐Andean zones of Peru and Argentina.

千島弧・東北日本弧会合部付近の浅発地震活動と重ね合せ発震機構

千島弧・東北日本弧会合部付近の浅発地震活動と重ね合せ発震機構