scholarly journals Stress tensor inversion in Western Greece using earthquake focal mechanisms from the Kozani-Grevena 1995 seismic sequence

1999 ◽  
Vol 42 (4) ◽  
Author(s):  
A. A. Kiratzi
Keyword(s):  
Stress Tensor ◽  
Fault Plane ◽  
Strike Slip ◽  
Focal Mechanisms ◽  
Normal Faulting ◽  
Stress Tensor Inversion ◽  
Fault Plane Solutions ◽  
The North ◽  
Western Greece ◽  
Slip Motion

Stress tensor inversion has been applied to estimate principal stress axes orientations in Western Greece, from 178 earthquake fault plane solutions from the Kozani-Grevena May 13, 1995 sequence. All focal mechanisms were previously determined through the deployment of a dense portable array. The magnitude range is 2.7-6.5 and the depth range is 4.0-15 km. A single stress tensor with an average misfit of 6.5°, small enough to support the assumption of stress homogeneity, can describe the stress field. The maximum compressive stress, s1, has a NNE-SSW trend (N26°E) and a nearly vertical plunge (80°) while the minimum compressive stress, s3, has a NNW-SSE orientation (N159°E) and a shallow plunge (7°) southwards. The scalar quantity, R (stress ratio) was found equal to 0.4 suggesting a transtensional regime (normal faulting with strike-slip motions) in which s2 is compressional. The identification of the fault plane from the auxiliary plane was achieved for 99 fault plane solutions out of 178 in total (56%). Vertical cross sections support previous results concerning the north dipping main fault segments and the south dipping antithetic faulting. The strike-slip motion is mainly dextral, along NNE-SSW structures, which follow the direction of the main neotectonic faults while the scarce sinistral strike-slip motion is connected to NW-SE trending zones of weakness pre-existing the old phase of compression in the Aegean. The strong strike slip motion that supports the transtensional regime probably reflects the effect of the motions of the North Anatolian Fault, taken up by normal faulting in the area of Western Greece.

scholarly journals The 1991 Sierra Madre earthquake sequence in southern California: Seismological and tectonic analysis

1994 ◽  
Vol 84 (4) ◽  
pp. 1058-1074 ◽  
Author(s):  
Egill Hauksson
Keyword(s):  
Los Angeles ◽  
Fault Plane ◽  
B Value ◽  
Tectonic Stress ◽  
Aftershock Sequence ◽  
Strike Slip ◽  
Focal Mechanisms ◽  
Stress Axis ◽  
The North ◽  
Sierra Madre

Abstract The (ML 5.8) Sierra Madre earthquake of 28 June 1991 occurred at a depth of 12 km under the San Gabriel Mountains of the central Transverse Ranges. Since at least 1932 this region had been quiescent for M ≧ 3. The mainshock focal mechanism derived from first-motion polarities exhibited almost pure thrust faulting, with a rake of 82° on a plane striking N62°E and dipping 50° to the north. The event appears to have occurred on the Clamshell-Sawpit fault, a splay of the Sierra Madre fault zone. The aftershock sequence following the mainshock occurred at a depth of 9 to 14 km and was deficient in small earthquakes, having a b value of 0.6. Twenty nine single-event focal mechanisms were determined for aftershocks of M > 1.5. The 4-km-long segment of the Clamshell-Sawpit fault that may have ruptured in the mainshock is outlined by several thrust focal mechanisms with an east-northeast-striking fault plane dipping to the north. To the west, several thrust aftershocks with east-striking nodal planes suggest some complexity in the aftershock faulting, such as a curved rupture surface. In addition, several strike-slip and normal faulting events occurred along the edges of the mainshock fault plane, indicating secondary tear faulting. The tectonic stress field driving the coexisting left-lateral strike-slip and thrust faults in the northern Los Angeles basin is north-south horizontal compression with vertical intermediate or minimum principal stress axis.

scholarly journals Distributed earthquake focal mechanisms in the Aegean Sea

2018 ◽  
Vol 40 (3) ◽  
pp. 1125 ◽  
Author(s):  
A. Kiratzi ◽  
C. Benetatos ◽  
Z. Roumelioti
Keyword(s):  
Aegean Sea ◽  
Strike Slip ◽  
Focal Mechanisms ◽  
Normal Faulting ◽  
Small Magnitude ◽  
The North ◽  
Earthquake Focal Mechanisms ◽  
Reverse Faulting ◽  
Different Types ◽  
Transform Fault Zone

Nearly 2,000 earthquake focal mechanisms in the Aegean Sea and the surroundings for the period 1912- 2006, for 1.5 <M<7.5, and depths from 0 to 170 km, indicate a uniform distribution and smooth variation in orientation over wide regions, even for the very small magnitude earthquakes. ~ 60% of the focal mechanisms show normal faulting, that mainly strikes ~E-W. However, a zone ofN-S normal faulting runs the backbone of Albanides-Hellenides. Low-angle thrust and reverse faulting is confined in western Greece (Adria-Eurasia convergence) and along the Hellenic trench (Africa-Eurasia). In the central Aegean Sea the effect of the propagating tip of the North Anatolian Fault into the Aegean Sea is pronounced and strike-slip motions are widely distributed. Shearing does not cross central Greece. Strike-slip motions reappear in the Cephalonia-Lefkada Transform Fault zone and in western Péloponnèse, which shows very complex tectonics, with different types of faulting being oriented favourably and operating under the present stress-field. Moreover, in western Péloponnèse the sense of the observed shearing is not yet clear, whether it is dextral or sinistral, and this lack of data has significant implications for the orientation of the earthquake slip vectors compared to the GPS obtained velocity vectors.

scholarly journals RECENT SEISMIC ACTIVITY IN CENTRAL GREECE REVEALING LOCAL SEISMOTECTONIC PROPERTIES

2017 ◽  
Vol 43 (4) ◽  
pp. 2075
Author(s):  
Ch. K. Karamanos ◽  
V. G. Karakostas ◽  
L. Seeber ◽  
E. E. Papadimitriou ◽  
A.A. Kilias
Keyword(s):  
Fault Zone ◽  
Fault Plane ◽  
Focal Mechanisms ◽  
Normal Faulting ◽  
Seismic Excitations ◽  
Lateral Component ◽  
Fault Plane Solutions ◽  
Central Greece ◽  
Source Characteristics ◽  
The Town

The December 2008, M=5.2 earthquake occurred in the Voiotikos–Kifissos basin near the town of Amfikleia in Central Greece and was followed by an intense sequence with hundreds of earthquakes. Mainshock source characteristics derived from the recordings of the Greek National Seismological Network are consistent with previous known earthquakes as well as with the current nearly N–S extensional regime. The adequate azimuthal coverage and the calculated time residuals at each seismological station ensure high location accuracy, whereas the stations operated close to the seismic excitations constrained 80% of the focal depths between 8 and 12km. Distances from the mainshock epicenter to the 10 closest seismological stations vary from 15 to 75 km. Hypoinverse and HypoDD were used for locations, and FPFIT was used for fault plane solutions of events with an adequate number of clear first arrivals. The hypocenters and focal mechanisms illuminate a ≈10km–long fault zone striking nearly E–W with oblique normal faulting and a small left lateral component. The Voiotikos–Kifissos basin is bordered in the south by two left–stepping en echelon segments known as the Pavliani fault zone and the Parnassos detachment, which strike NW and dip NE. In our preferred interpretation, the Amfikleia mainshock ruptured a previously recognized south–dipping fault antithetic to the basin border faults. This fault may be associated with the left step on the border fault, which would be releasing if that fault had a sinistral component.

Aftershocks of the Managua, Nicaragua, earthquake of December 23, 1972

1974 ◽  
Vol 64 (4) ◽  
pp. 1005-1016
Author(s):  
C. J. Langer ◽  
M. G. Hopper ◽  
S. T. Algermissen ◽  
J. W. Dewey
Keyword(s):  
Fault Plane ◽  
Strike Slip ◽  
Fault System ◽  
Fault Plane Solutions ◽  
Lateral Strike Slip ◽  
Seismic Zones ◽  
The North ◽  
Primary Zone ◽  
Slip Displacement ◽  
Strain Release

abstract Epicenters determined from 164 of the Managua aftershocks define two seismic zones. The primary zone, which is 15 to 20 km in length and strikes northeast along the Tiscapa-Ciudad Jardin fault system, contains 80 per cent of the aftershock locations. A subsidiary zone, northwest of Managua, suggests strain release possibly related to the north-south striking San Judas fault. Depth of foci are principally in the upper 7 km for both zones. Composite fault-plane solutions indicate a predominate left-lateral strike-slip displacement; the preferred planes for each zone agree with the strike of surface fractures or previously mapped faults.

Relocation of Koyna earthquakes

1980 ◽  
Vol 70 (5) ◽  
pp. 1849-1868
Author(s):  
B. K. Rastogi ◽  
P. Talwani
Keyword(s):  
Fault Plane ◽  
Seismic Velocity ◽  
Landsat Imagery ◽  
Velocity Model ◽  
Strike Slip ◽  
Normal Faulting ◽  
Large Area ◽  
Fault Plane Solutions ◽  
Lateral Strike Slip ◽  
Induced Earthquake

abstract The Koyna earthquake of December 10, 1967 was the most damaging reservoir-induced earthquake. It was followed by a long sequence of earthquakes which is still continuing. Precise locations of the Koyna earthquakes have been very much disputed as different locations of the main earthquake and stronger aftershocks were obtained by various workers. Over 1,500 epicenters of Koyna earthquakes through 1973 were obtained by Guha et al. (1974). They cover a large area in a diffused pattern. In view of the continuing seismicity and a recently obtained seismic velocity model, the larger events (ML ≧ 4.0) and about 300 selected smaller events (ML &lt; 4.0) were relocated. The relocated epicenters show some concentration and suggest the possibility of two trends in the NNE and NW directions. There is a NNE trend of epicenters near the dam and another about 20 km west of the reservoir. The NW trend cuts through these NNE trends. The events were grouped to obtain their composite fault-plane solutions which indicate left-lateral strike-slip faulting along the NNE faults and normal faulting in the NW direction. Faults observed in the LANDSAT imagery match with these trends.

Focal parameters of four Sverdrup Basin, Arctic Canada, earthquakes in November and December of 1972

1977 ◽  
Vol 14 (11) ◽  
pp. 2481-2494 ◽  
Author(s):  
H. S. Hasegawa
Keyword(s):  
Fault Plane ◽  
Source Region ◽  
Earthquake Swarm ◽  
Focal Depth ◽  
Unknown Origin ◽  
Strike Slip ◽  
The Arctic ◽  
Fault Plane Solutions ◽  
Sverdrup Basin ◽  
Slip Motion

Fault plane solutions are derived for the four largest magnitude (Ms 5.1–5.7) earthquakes which occurred during the months of November and December, 1972, and which are part of the Byam Martin Channel earthquake swarm. The preferred fault plane solutions of all four have dextral strike-slip motion in a northeasterly direction on steeply dipping (approximately 80°) fault planes. For the December 27 earthquake, which has the largest Ms (5.7), the fault motion is almost pure strike slip and the focal depth is shallow, being just below the base of the Sverdrup Basin. Forthe November 19, 21, and December 28 earthquakes which have smaller Ms (5.1–5.2), the fault plane solutions have an appreciable dip-slip component (normal faulting) and the foci are at intermediate crustal depths. The two known active tectonic processes in the Arctic do not correlate with the fault plane solutions: active spreading at the Arctic mid-oceanic ridge (Gakkel or Nansen Ridge) cannot account for the orientation of the stress vectors from fault plane solutions; postglacial uplift predicts opposite dip-slip motion to what is derived from fault plane solutions. The proposed triggering mechanism for the earthquake swarm in the Byam Martin Channel is attributed to remanent stresses (predominantly horizontal and of unknown origin) acting upon a (heterogeneous) source region that is weakened by reactivated localized intrusions.

The 25 October, 2018 Zakynthos (Greece) earthquake: seismic activity at the transition between a transform fault and a subduction zone.

2020 ◽  
Author(s):  
P Papadimitriou ◽  
V Kapetanidis ◽  
A Karakonstantis ◽  
I Spingos ◽  
K Pavlou ◽  
...  
Keyword(s):  
Spatial Clustering ◽  
Accretionary Prism ◽  
Velocity Model ◽  
Strike Slip ◽  
Double Difference ◽  
Strain Partitioning ◽  
Transform Fault ◽  
Fault Plane Solutions ◽  
The North ◽  
Waveform Modelling

Summary The properties of the Mw = 6.7 earthquake that took place on 25 October 2018, 22:54:51 UTC, ∼50 km SW of the Zakynthos Island, Greece, are thoroughly examined. The main rupture occurred on a dextral strike-slip, low-angle, east-dipping fault at a depth of 12 km, as determined by teleseismic waveform modelling. Over 4000 aftershocks were manually analysed for a period of 158 days. The events were initially located with an optimal 1D velocity model and then relocated with the double-difference method to reveal details of their spatial distribution. The latter spreads in an area spanning 80 km NNW-SSE and ∼55 km WSW-ENE. Certain parts of the aftershock zone present strong spatial clustering, mainly to the north, close to Zakynthos Island, and at the southernmost edge of the sequence. Focal mechanisms were determined for 61 significant aftershocks using regional waveform modelling. The results revealed characteristics similar to the mainshock, with few aftershocks exhibiting strike-slip faulting at steeper dip angles, possibly related to splay faults on the accretionary prism. The slip vectors that correspond to the east-dipping planes are compatible with the long-term plate convergence and with the direction of coseismic displacement on the Zakynthos Island. Fault-plane solutions in the broader study area were inverted for the determination of the regional stress-field. The results revealed a nearly horizontal, SW-NE to E-W-trending S1 and a more variable S3 axis, favouring transpressional tectonics. Spatial clusters at the northern and southern ends of the aftershock zone coincide with the SW extension of sub-vertical along-dip faults of the segmented subducting slab. The mainshock occurred in an area where strike-slip tectonics, related to the Cephalonia Transform Fault and the NW Peloponnese region, gradually converts into reverse faulting at the western edge of the Hellenic subduction. Plausible scenarios for the 2018 Zakynthos earthquake sequence include a rupture on the subduction interface, provided the slab is tilted eastwards in that area, or the reactivation of an older east-dipping thrust as a low-angle strike-slip fault that contributes to strain partitioning.

Microearthquake seismicity and tectonics of Coastal Northern California

1970 ◽  
Vol 60 (5) ◽  
pp. 1669-1699 ◽  
Author(s):  
Leonardo Seeber ◽  
Muawia Barazangi ◽  
Ali Nowroozi
Keyword(s):  
High Frequency ◽  
Fault Plane ◽  
San Andreas Fault ◽  
High Gain ◽  
Strike Slip ◽  
Fault System ◽  
Northern California ◽  
Fault Plane Solutions ◽  
Sharp Bend ◽  
San Andreas

Abstract This paper demonstrates that high-gain, high-frequency portable seismographs operated for short intervals can provide unique data on the details of the current tectonic activity in a very small area. Five high-frequency, high-gain seismographs were operated at 25 sites along the coast of northern California during the summer of 1968. Eighty per cent of 160 microearthquakes located in the Cape Mendocino area occurred at depths between 15 and 35 km in a well-defined, horizontal seismic layer. These depths are significantly greater than those reported for other areas along the San Andreas fault system in California. Many of the earthquakes of the Cape Mendocino area occurred in sequences that have approximately the same magnitude versus length of faulting characteristics as other California earthquakes. Consistent first-motion directions are recorded from microearthquakes located within suitably chosen subdivisions of the active area. Composite fault plane solutions indicate that right-lateral movement prevails on strike-slip faults that radiate from Cape Mendocino northwest toward the Gorda basin. This is evidence that the Gorda basin is undergoing internal deformation. Inland, east of Cape Mendocino, a significant component of thrust faulting prevails for all the composite fault plane solutions. Thrusting is predominant in the fault plane solution of the June 26 1968 earthquake located along the Gorda escarpement. In general, the pattern of slip is consistent with a north-south crustal shortening. The Gorda escarpment, the Mattole River Valley, and the 1906 fault break northwest of Shelter Cove define a sharp bend that forms a possible connection between the Mendocino escarpment and the San Andreas fault. The distribution of hypocenters, relative travel times of P waves, and focal mechanisms strongly indicate that the above three features are surface expressions of an important structural boundary. The sharp bend in this boundary, which is concave toward the southwest, would tend to lock the dextral slip along the San Andreas fault and thus cause the regional north-south compression observed at Cape Mendocino. The above conclusions support the hypothesis that dextral strike-slip motion along the San Andreas fault is currently being taken up by slip along the Mendocino escarpment as well as by slip along northwest trending faults in the Gorda basin.

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