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.

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.

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.

The Chelmsford-Lowell, Massachusetts, earthquake of 23 November 1980: Depth control and fault plane solution

1981 ◽  
Vol 71 (4) ◽  
pp. 1369-1372
Author(s):  
Jay J. Pulli ◽  
Michael J. Guenette
Keyword(s):  
New England ◽  
Fault Plane ◽  
Seismic Station ◽  
Focal Depth ◽  
Strike Slip ◽  
Fault Plane Solution ◽  
Origin Time ◽  
Small Magnitude ◽  
Depth Control ◽  
Plane Solution

abstract On 23 November 1980, a small (magnitude 2.9) earthquake occurred on the Chelmsford-Lowell, Massachusetts, border, approximately 10 km northeast of the MIT seismic station at Westford, Massachusetts (WFM). Thus we were able to accurately determine the focal depth, which is generally not the case in New England. Our hypocentral solution was latitude 41.63, longitude −71.36, depth 1.5 km, at origin time 00:39:32.0 UTC. The fault plane solution shows either strike-slip or dip-slip faulting with a P axis trending NE-SW, which is in agreement with overcoring measurements in a nearby granite quarry.

Rampart seismic zone of central Alaska

1983 ◽  
Vol 73 (3) ◽  
pp. 813-829
Author(s):  
P. Yi-Fa Huang ◽  
N. N. Biswas
Keyword(s):  
Main Shock ◽  
Fault Plane ◽  
B Value ◽  
Aftershock Sequence ◽  
Strike Slip ◽  
Lithospheric Plate ◽  
Normal Faults ◽  
Seismic Zone ◽  
The West ◽  
Fault Plane Solutions

abstract This paper describes the characteristics of the Rampart seismic zone by means of the aftershock sequence of the Rampart earthquake (ML = 6.8) which occurred in central Alaska on 29 October 1968. The magnitudes of the aftershocks ranged from about 1.6 to 4.4 which yielded a b value of 0.96 ± 0.09. The locations of the aftershocks outline a NNE-SSW trending aftershock zone about 50 km long which coincides with the offset of the Kaltag fault from the Victoria Creek fault. The rupture zone dips steeply (≈80°) to the west and extends from the surface to a depth of about 10 km. Fault plane solutions for a group of selected aftershocks, which occurred over a period of 22 days after the main shock, show simultaneous occurrences of strike-slip and normal faults. A comparison of the trends in seismicity between the neighboring areas shows that the Rampart seismic zone lies outside the area of underthrusting of the lithospheric plate in southcentral and central Alaska. The seismic zone outlined by the aftershock sequence appears to represent the formation of an intraplate fracture caused by regional northwest compression.

Aftershocks of the Benham nuclear explosion

1969 ◽  
Vol 59 (6) ◽  
pp. 2271-2281
Author(s):  
R. M. Hamilton ◽  
J. H. Healy
Keyword(s):  
Fault Plane ◽  
Nuclear Explosion ◽  
Test Site ◽  
Strike Slip ◽  
Nevada Test Site ◽  
Ground Zero ◽  
Fault Plane Solutions ◽  
Fault Movement ◽  
Near Surface ◽  
Earthquake Distribution

abstract The Benham nuclear explosion, a 1.1 megaton test 1.4 km beneath Pahute Mesa at the Nevada Test Site, initiated a sequence of earthquakes lasting several months. The epicenters of these shocks were located within 13 km of ground zero in several linear zones that parallel the regional fault trends. Focal depths range from near surface to 6 km. The earthquakes are not located in the zone of the major ground breakage. The earthquake distribution and fault plane solutions together indicate that both right-lateral strike-slip fault movement and dip-slip fault movement occurred. The explosion apparently caused the release of natural tectonic strain.

Tectonics of the arctic seismic belt in the light of fault-plane solutions of earthquakes

1966 ◽  
Vol 56 (1) ◽  
pp. 241-245
Author(s):  
A. E. Scheidegger
Keyword(s):  
Statistical Method ◽  
Fault Plane ◽  
The Arctic ◽  
Seismic Belt ◽  
Fault Plane Solutions ◽  
Mid Atlantic Ridge ◽  
Tectonic Interpretation ◽  
Tectonic Pattern

abstract A statistical method devised earlier by the writer for the tectonic interpretation of fault plane solutions of earthquakes, is applied to fault plane solutions of Arctic earthquakes which were recently published in Russia. It is shown that the earthquakes on the Greenland side of the Arctic seismic belt represent the same tectonic pattern as seen on the Mid-Atlantic Ridge, whereas the earthquakes on the Siberian side are tectonically connected with the Verkhoyan range.

Seismicity in South Carolina

1988 ◽  
Vol 59 (4) ◽  
pp. 165-171
Author(s):  
Kaye M. Shedlock
Keyword(s):  
South Carolina ◽  
Compressive Stress ◽  
Coastal Plain ◽  
Fault Plane ◽  
Strike Slip ◽  
Fault Plane Solutions ◽  
Reservoir Induced Seismicity ◽  
Southeastern Us ◽  
Horizontal Compressive Stress ◽  
Shallow Crust

Abstract The largest historical earthquake in South Carolina, and in the southeastern US, occurred in the Coastal Plain province, probably northwest of Charleston, in 1886. Locations for aftershocks associated with this earthquake, estimated using intensities based on newspaper accounts, defined a northwest trending zone about 250 km long that was at least 100 km wide in the Coastal Plain but widened to a northeast trending zone in the Piedmont. The subsequent historical and instrumentally recorded seismicity in South Carolina images the 1886 aftershock zone. Except for a few scattered earthquakes and a swarm of shallow (≤ 4 km deep), small (ML ≤ 2.5), primarily reverse faulting earthquakes that occurred along the flanks of a granite pluton about 60 km northwest of Columbia, the seismicity in the Piedmont province has been associated with water level changes in reservoirs. Reservoir induced seismicity (RIS) is shallow (≤ 6 km deep), primarily strike-slip or thrust faulting corresponding to an inferred maximum horizontal compressive stress oriented approximately N 60° E. Instrumentally recorded seismicity in the Coastal Plain province occurs in 3 seismic zones or clusters: Middleton Place-Summerville (MPSSZ), Adams Run (ARC), and Bowman (BSZ). Approximately 68% of the Coastal Plain earthquakes occur in the MPSSZ, a north trending zone about 22 km long and 12 km wide, lying about 20 km northwest of Charleston. The hypocenters of MPSSZ earthquakes range in depth from near the surface to almost 12 km. Thrust, strike-slip, and some normal faulting are indicated by the fault plane solutions for Coastal Plain earthquakes. The maximum horizontal compressive stress, inferred from the P-axes of the fault plane solutions, is oriented NE-SW in the shallow crust (< 9 km deep) but appears to be diffusely E-W between 9 to 12 km deep. Although there is localized variability, the current seismicity and associated faulting in South Carolina probably represent a regional response to the NE-SW maximum horizontal compressive stress prevalent throughout eastern North America.

The Livermore Valley, California, sequence of January 1980

1981 ◽  
Vol 71 (2) ◽  
pp. 451-463
Author(s):  
B. A. Bolt ◽  
T. V. McEvilly ◽  
R. A. Uhrhammer
Keyword(s):  
Fault Plane ◽  
B Value ◽  
Strike Slip ◽  
Causal Connection ◽  
Fault Plane Solutions ◽  
Lateral Strike Slip ◽  
Marsh Creek ◽  
Long Duration ◽  
The University ◽  
Rapid Deployment

abstract At 19h00m09.46s UTC, on 24 January 1980, a strong earthquake (ML = 5.5) that caused a surprising amount of damage occurred north of Livermore Valley about 12 km to the southeast of Mt. Diablo, and was associated with surface rupture along the Greenville Fault. There was a foreshock (ML = 2.7) a minute and a half earlier and a sequence of 59 events (ML ≧ 2.5) in the ensuing 6 days. On 27 January at 02h33m35.96s, a larger magnitude earthquake occurred in the sequence (ML = 5.6). This second principal shock was located 14 km to the south of the first principal earthquake toward the southern end of the Greenville Fault. Preliminary estimates of the seismic moments of the two principal shocks are 5.3 × 1024 and 1.3 × 1024 dyne-cm, respectively. In addition to the lower seismic moment, the ML = 5.6 shock on 27 January exhibits a clearly focused radiation pattern, with large amplitudes toward the northeast. Field investigations after the first principal shock indicated surfaced rupture along the Greenville fault zone for at least 6 km, with both right-lateral strike-slip and some dip-slip motion with the northeast side up. Variable offsets on surface cracks suggested displacements of a few centimeters (with evidence of increases in some places after the second 27 January earthquake). There were eight earthquakes with ML ≧ 4.0 in the sequence up to 5 February 1980. No foreshocks near the Greenville Fault (ML ≧ 1.5) were observed by the University of California Seismographic Stations in the prior 3 months. Rapid deployment of field seismographs by a number of seismological organizations permitted precise locations and fault-plane solutions. Some results on seismicity are as follows. The rupture propagated over 15 km to the southeast along the Marsh Creek-Greenville faults on 24 January and stopped in the vicinity of Highway 580. This southern progression may have had some causal connection with the relatively high intensities reported near the southwest end of the Greenville Fault. The two principal shocks of the sequence have slight but significant differences in the fault-plane solutions; both are predominantly right-lateral strike-slip, but the strike of the northern one is N13°W, whereas the strike of the southern one is N39°W. This change in strike is not evident in the mapped strikes of the Marsh Creek and the Greenville faults. In contrast to the second principal earthquake, the first principal shock was followed by two others (ML &gt; 4.0) in rapid succession, one 53 sec and the other 97 sec after. This repetition gave a relatively long duration to the shaking on 24 January, and was commented on in felt reports. It may explain the greater intensity reported in many localities on 24 January compared to 27 January. The b value (0.64 ± 0.13) for the sequence is somewhat lower than the b = 0.70 ± 0.17 for the recent Coyote Lake earthquake sequence on the Calaveras Fault on 5 August 1979. There are fewer earthquakes than normal in the range 3.0 &lt; ML &lt; 4.0 in the Greenville sequence.

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.

The North Gower, Ontario, Earthquake of 11 October 1983: Focal Mechanism and Aftershocks

1987 ◽  
Vol 58 (3) ◽  
pp. 65-72 ◽  
Author(s):  
Rutger Wahlstreöm
Keyword(s):  
Focal Mechanism ◽  
Fault Plane ◽  
Focal Depth ◽  
P Wave ◽  
Focal Mechanism Solution ◽  
Nodal Plane ◽  
Fault Plane Solutions ◽  
Stress Regime ◽  
Motion Data ◽  
The North

Abstract An earthquake near Ottawa (45.20°N, 75.75°W, focal depth 12 km) of unusually large size for the region's seisrhicity (mb(Lg) = 4.1) provided good P-wave first-motion data for a focal-mechanism solution. The maximum intensity was V(MM) and the area of perceptibility in Canada about 80,000 km2. The first and largest recorded aftershock occurred nine minutes after the main shock with magnitude mb(Lg) = 1.7. Two further small aftershocks ware recorded by a field network. The mechanism is thrust faulting with a predominantly horizontal pressure axis trending 154°. Thrust mechanisms have been found for other earthquakes in southeastern Canada and the northeastern U.S., but orientations of their stress axes are different, and so the North Gower earthquake may reflect a local and not a regional stress field. The nodal planes have strike 71°, dip 75°, and strike 221°, dip 17°. The spatial distribution of aftershocks suggests the gently-dipping nodal plane could be the fault plane. There is uncertainty about the seismotectonics of the region, and the orientation of neither nodal plane correlates with known geological features. More earthquake fault-plane solutions are required to interpret the seismotectonics and the stress regime.

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