Source processes of the 1981 Gulf of Corinth earthquake sequence from body-wave analysis

1984 ◽  
Vol 74 (2) ◽  
pp. 459-477
Author(s):  
Won-Young Kim ◽  
Ota Kulhánek ◽  
Klaus Meyer
Keyword(s):  
Main Shock ◽  
Body Wave ◽  
The Body ◽  
Body Waves ◽  
Earthquake Sequence ◽  
Wave Analysis ◽  
Normal Faulting ◽  
Source Characteristics ◽  
Gulf Of Corinth ◽  
Stress Drops

Abstract Teleseismic long-period body waves from the 24 February 1981 Gulf of Corinth earthquake and its two principal aftershocks of 25 February (02h35m) and 4 March (21h58m) 1981 are studied to determine source characteristics. Focal mechanisms, along with observed surface fault breaks, suggest that the Corinth earthquake sequence represents normal faulting due to the N-S trending extension. Depths of the three shocks, estimated by matching synthetic seismograms to observations, are found to lie between 4 and 12 km. The azimuthal variation of observed body-wave duration indicates that the main shock is a multiple event and that the main rupture occurred about 3 to 4 sec after a relatively small foreshock and propagated toward the W-NW. Seismic moments deduced from the body-wave synthetics are 8.1 ×1025, 2.7 ×1025, and 2.2 ×1025 dyne-cm for the main, 25 February and 4 March shocks, respectively. Average final displacements and stress drops are estimated to be 37 cm and 10 bars for the main shock (for a circular fault of radius 15 km); 22 cm and 8 bars for the 25 February shock, and 18 cm and 7 bars for the 4 March shock (for circular faults of radius 11 km). The striking features of the earthquake sequence are the low stress drops of the main shock and its two principal aftershocks, and the clear eastward migration of aftershock activities. The unusually long source-time function rise times (4 sec for the main shock, 2.5 sec for both aftershocks) and low stress drops suggest an overall slow energy release during the earthquake sequence.

scholarly journals Teleseismic analysis of the 1980 Mammoth Lakes earthquake sequence

1982 ◽  
Vol 72 (4) ◽  
pp. 1093-1109
Author(s):  
Jeffrey W. Given ◽  
Terry C. Wallace ◽  
Hiroo Kanamori
Keyword(s):  
Surface Wave ◽  
Sierra Nevada ◽  
Body Wave ◽  
The Body ◽  
Body Waves ◽  
Earthquake Sequence ◽  
Local Data ◽  
Fault Plane Solutions ◽  
Principal Axes ◽  
Mammoth Lakes

abstract The source mechanisms of the three largest events of the 1980 Mammoth Lakes earthquake sequence have been determined using surface waves recorded on the global digital seismograph network and the long-period body waves recorded on the WWSSN network. Although the fault-plane solutions from local data (Cramer and Toppozada, 1980; Ryall and Ryall, 1981) suggest nearly pure left-lateral strike-slip on north-south planes, the teleseismic waveforms require a mechanism with oblique slip. The first event (25 May 1980, 16h 33m 44s) has a mechanism with a strike of N12°E, dip of 50°E, and a rake of −35°. The second event (27 May 19h 44m 51s) has a mechanism with a strike of N15°E, dip of 50°, and a slip of −11°. The third event (27 May, 14h 50m 57s) has a mechanism with a strike of N22°E, dip of 50°, and a rake of −28°. The first event is the largest and has a moment of 2.9 × 1025 dyne-cm. The second and third events have moments of 1.3 and 1.1 × 1025 dyne-cm, respectively. The body- and surface-wave moments for the first and third events agree closely while for the second event the body-wave moment (approximately 0.6 × 1025 dyne-cm) is almost a factor of 3 smaller than the surface-wave moment. The principal axes of extension of all three events is in the approximate direction of N65°E which agrees with the structural trends apparent along the eastern front of the Sierra Nevada.

Source processes of the 1981 Gulf of Corinth earthquake sequence from body-wave analysis

1985 ◽  
Vol 118 (3-4) ◽  
pp. 253-255
Author(s):  
Won-Young Kim ◽  
Ota Kulhánek ◽  
Klaus Meyer
Keyword(s):  
Body Wave ◽  
Earthquake Sequence ◽  
Wave Analysis ◽  
Gulf Of Corinth

The foreshock sequence of the 1986 Chalfant, California, earthquake

1988 ◽  
Vol 78 (1) ◽  
pp. 172-187
Author(s):  
Kenneth D. Smith ◽  
Keith F. Priestley
Keyword(s):  
Main Shock ◽  
Slip Surface ◽  
Body Waves ◽  
Aftershock Distribution ◽  
Time Period ◽  
Main Shocks ◽  
Short Period ◽  
Local Shear ◽  
Stress Drops ◽  
Foreshock Activity

Abstract The ML 6.4 Chalfant, California, earthquake of 21 July 1986 was preceded by an extensive foreshock sequence. Foreshock activity is characterized by shallow clustering activity, including 7 events greater than ML 3, beginning 18 days before the earthquake, an ML 5.7 foreshock 24 hr before the main shock that ruptured only in the upper 10 km of the crust, and an “off-fault” cluster of activity perpendicular to the slip surface of the ML 5.7 foreshock associated with the hypocenter of the main shock. The Chalfant sequence occurred within the local short-period network, and the spatial and temporal development of the foreshock sequence can be observed in detail. Seismicity of the July 1986 time period is largely confined to two nearly conjugate planes; one striking N30°E and dipping 60° to the northwest associated with the ML 5.7 foreshock and the other striking N25°W and dipping 70° to the southwest associated with the main shock. Focal mechanisms for the foreshock period fall into two classes in agreement with these two planes. Shallow clustering of earthquakes in July and the ML 5.7 principal foreshock occur at the intersection of the two planes at a depth of approximately 7 km. The seismic moments determined from inversion of the teleseismic body waves are 4.2 × 1025 and 2.5 × 1025 dyne-cm for the principal foreshock and the main shock, respectively. Slip areas for these two events can be estimated from the aftershock distribution and result in stress drops of 63 bars for the principal foreshock and 16 bars for the main shock. The main shock occurred within an “off-fault” cluster of earthquakes associated with the principal foreshock. This cluster of activity occurs at a predicted local shear stress high in relation to the slip surface of the 20 July earthquake, and this appears to be the triggering mechanism of the main shock. The shallow rupture depth of the principal foreshock indicates that this event was anomalous with respect to the character of main shocks in the region.

Preliminary body-wave analysis of the St. Elias, Alaska earthquake of February 28, 1979

1980 ◽  
Vol 70 (2) ◽  
pp. 419-436
Author(s):  
John Boatwright
Keyword(s):  
New York ◽  
Body Wave ◽  
The Body ◽  
Rupture Velocity ◽  
Wave Analysis ◽  
S Waves ◽  
Initial Event ◽  
Whole Space ◽  
A New Technique ◽  
Alaska Earthquake

abstract Employing a new technique for the body-wave analysis of shallow-focus earthquakes, we have made a preliminary analysis of the St. Elias, Alaska earthquake of February 28, 1979, using five long-period P and S waves recorded at three WWSSN stations and at Palisades, New York. Using a well determined focal mechanism and an average source depth of ≈ 11 km, the interference of the depth phases (i.e., pP and sP, or sS) has been deconvolved from the recorded pulse shapes to obtain velocity and displacement pulse shapes as they would appear if the earthquake had occurred within an infinite medium. These “approximate whole space” pulse shapes indicate that the rupture contained three distinct subevents as well as a small initial event which preceded this subevent sequence by about 7 sec. From the pulse rise times of the subevents, their rupture lengths are estimated as 12, 27, and 17 km, assuming that the subevent rupture velocity was 3 km/sec. Overall, the earthquake ruptured ≈ 60 km to the southeast with an average rupture velocity of 2.2 km/sec. The cumulative body-wave moment for the whole event, 1.2 × 1027 dyne-cm, is substantially smaller than the surface-wave moments reported by Lahr et al. (1979) of 5 × 1027 dyne-cm. The moments of the subevents are estimated to be 0.6, 3.2, and 7.5 × 1026 dyne-cm, respectively.

scholarly journals Secondary Microseisms generated by typhoons in the Northwestern Pacific Ocean

2021 ◽  
Author(s):  
Nassima Benbelkacem ◽  
Eléonore Stutzmann ◽  
Martin Schimmel ◽  
Véronique Farra ◽  
Fabrice Ardhuin ◽  
...  
Keyword(s):  
Gravity Waves ◽  
Body Wave ◽  
Ocean Waves ◽  
Body Waves ◽  
Northwestern Pacific ◽  
Northwestern Pacific Ocean ◽  
Wave Reflections ◽  
Source Characteristics ◽  
Beamforming Technique ◽  
The Relationship

<p>Secondary Microseisms (SM) are recorded by seismometers in the period band 3-10 s. They are generated by the interaction of ocean gravity waves of similar frequencies and coming from nearly opposite directions. Typhoons create such ocean waves, and the purpose of this study is to investigate the relationship between typhoons and microseism source characteristics. We focused our study on the Northwestern Pacific and we analyzed seismic signals recorded by the Alaska array and the corresponding storm catalog. While P body waves enable to characterize the amplitude and the localization of the sources, secondary microseisms are dominated by surface waves. Therefore, we apply beamforming technique to the vertical components in order to highlight the weaker body wave signals. This analysis permits us to track the localization of SM sources every 6 hours. Our results show three cases: In the case of one active typhoon, the positions of SM sources are localized close to the typhoon position. In the case of two nearby typhoons acting simultaneously, the SM sources are localized in between the typhoons. Finally, when the typhoon arrives close to the coast, we observe sources generated by ocean wave reflections. In conclusion, the three mechanisms proposed by Ardhuin et al., (2011) are necessary to explain secondary microseisms generated by typhoons.</p>

Nonlinear traveltime inversion scheme for crosshole seismic tomography in tilted transversely isotropic media

Geophysics ◽  
2008 ◽  
Vol 73 (4) ◽  
pp. D17-D33 ◽  
Author(s):  
Bing Zhou ◽  
Stewart Greenhalgh ◽  
Alan Green
Keyword(s):  
Seismic Tomography ◽  
Velocity Structure ◽  
Body Wave ◽  
Transversely Isotropic ◽  
The Body ◽  
Body Waves ◽  
Inversion Method ◽  
Perturbation Equation ◽  
Transversely Isotropic Media ◽  
Isotropic Media

Crosshole seismic tomography often is applied to image the velocity structure of an interwell medium. If the rocks are anisotropic, the tomographic technique must be adapted to the complex situation; otherwise, it leads to a false interpretation. We propose a nonlinear kinematic inversion method for crosshole seismic tomography in composite transversely isotropic media with known dipping symmetry axes. This method is based on a new version of the first-order traveltime perturbation equation. It directly uses the derivative of the phase velocity rather than the eigenvectors of the body-wave modes to overcome the singularity problem for application to the two quasi-shear waves. We applied an iterative nonlinear solver incorporating our kinematic ray-tracing scheme and directly compute the Jacobian matrix in an arbitrary reference medium. This reconstructs the five elastic moduli or Thomsen parameters from the first-arrival traveltimes of the three seismic body waves (qP, qSV, qSH) in strongly and weakly anisotropic media. We conducted three synthetic experiments that involve determining anisotropic parameters for a homogeneous rock, reconstructing a fault embedded in a strongly anisotropic background, and imaging a complicated four-layer model containing a small channel and a buried dipping interface. We compared results of our nonlinear inversion method with isotropic tomography and the traditional linear anisotropic inversion scheme, which showed the capability and superiority of the new scheme for crosshole tomographic imaging.

scholarly journals Magnitude Scales in Central Greece

2018 ◽  
Vol 40 (3) ◽  
pp. 1114 ◽  
Author(s):  
G. Kaviris ◽  
P. Papadimitriou ◽  
K. Makropoulos
Keyword(s):  
Goodness Of Fit ◽  
Body Wave ◽  
Velocity Model ◽  
The Body ◽  
Moment Magnitude ◽  
Coefficient Of Determination ◽  
Gulf Of Corinth ◽  
Duration Magnitude ◽  
The Moment

The Gulf of Corinth is one of the most active tectonic rifts around the world. Data used in the present study are obtained by the four digital stations of the Cornet Network which was installed in 1995 around the Eastern Gulf of Corinth. A velocity model was calculated, while the majority of local events were located within the Gulf of Corinth. Main scope of the study is the determination of a reliable earthquake magnitude. Concerning the duration magnitude Mo, a multiple linear regression technique was developed for the determination of the constants α, β and γ with very satisfactory values of errors. The coefficient of determination (goodness of fit) R2 was found equal to 0.99. Following, the moment magnitude Mw, which is considered to be the most reliable magnitude scale, was determined. Spectral analysis was applied for the calculation of the seismic moment M0 and a seismic catalogue was created. After the determination of the moment magnitude Mw and of the duration magnitude MD for the same dataset, a relationship between them was obtained, according to which Mw is systematically larger than Mjy Relationships between these magnitudes, the local magnitude ML and the body wave magnitude mb  were also obtained.

scholarly journals Aquatic vertebrate locomotion: wakes from body waves

1999 ◽  
Vol 202 (23) ◽  
pp. 3423-3430 ◽  
Author(s):  
J.J. Videler ◽  
U.K. Muller ◽  
E.J. Stamhuis
Keyword(s):  
Vortex Shedding ◽  
Body Wave ◽  
Phase Relationship ◽  
The Body ◽  
Body Waves ◽  
First Approximation ◽  
Inflection Points ◽  
Aquatic Vertebrate ◽  
The Mean ◽  
Tail Tip

Vertebrates swimming with undulations of the body and tail have inflection points where the curvature of the body changes from concave to convex or vice versa. These inflection points travel down the body at the speed of the running wave of bending. In movements with increasing amplitudes, the body rotates around the inflection points, inducing semicircular flows in the adjacent water on both sides of the body that together form proto-vortices. Like the inflection points, the proto-vortices travel towards the end of the tail. In the experiments described here, the phase relationship between the tailbeat cycle and the inflection point cycle can be used as a first approximation of the phase between the proto-vortex and the tailbeat cycle. Proto-vortices are shed at the tail as body vortices at roughly the same time as the inflection points reach the tail tip. Thus, the phase between proto-vortex shedding and tailbeat cycle determines the interaction between body and tail vortices, which are shed when the tail changes direction. The shape of the body wave is under the control of the fish and determines the position of vortex shedding relative to the mean path of motion. This, in turn, determines whether and how the body vortex interacts with the tail vortex. The shape of the wake and the contribution of the body to thrust depend on this interaction between body vortex and tail vortex. So far, we have been able to describe two types of wake. One has two vortices per tailbeat where each vortex consists of a tail vortex enhanced by a body vortex. A second, more variable, type of wake has four vortices per tailbeat: two tail vortices and two body vortices shed from the tail tip while it is moving from one extreme position to the next. The function of the second type is still enigmatic.

scholarly journals Inversion of the body waves from the Borrego Mountain earthquake to the source mechanism

1976 ◽  
Vol 66 (5) ◽  
pp. 1485-1499 ◽  
Author(s):  
L. J. Burdick ◽  
George R. Mellman
Keyword(s):  
Body Wave ◽  
High Stress ◽  
Time Function ◽  
The Body ◽  
Body Waves ◽  
Polarization Angle ◽  
Long Period ◽  
Amplitude Data ◽  
Short Period ◽  
Wide Range

abstract The generalized linear inverse technique has been adapted to the problem of determining an earthquake source model from body-wave data. The technique has been successfully applied to the Borrego Mountain earthquake of April 9, 1968. Synthetic seismograms computed from the resulting model match in close detail the first 25 sec of long-period seismograms from a wide range of azimuths. The main shock source-time function has been determined by a new simultaneous short period-long period deconvolution technique as well as by the inversion technique. The duration and shape of this time function indicate that most of the body-wave energy was radiated from a surface with effective radius of only 8 km. This is much smaller than the total surface rupture length or the length of the aftershock zone. Along with the moment determination of Mo = 11.2 ×1025 dyne-cm, this radius implies a high stress drop of about 96 bars. Evidence in the amplitude data indicates that the polarization angle of shear waves is very sensitive to lateral structure.

scholarly journals Focal mechanism and depth of the 1956 Amorgos twin earthquakes from waveform matching of analogue seismograms

Solid Earth ◽  
2014 ◽  
Vol 5 (2) ◽  
pp. 1027-1044 ◽  
Author(s):  
A. Brüstle ◽  
W. Friederich ◽  
T. Meier ◽  
C. Gross
Keyword(s):  
Surface Wave ◽  
Focal Mechanism ◽  
Wave Amplitude ◽  
Body Wave ◽  
The Body ◽  
Grid Search ◽  
Source Depth ◽  
Normal Faulting ◽  
Intermediate Depth ◽  
First Motion

Abstract. Historic analogue seismograms of the large 1956 Amorgos twin earthquakes which occurred in the volcanic arc of the Hellenic subduction zone (HSZ) were collected, digitized and reanalyzed to obtain refined estimates of their depth and focal mechanism. In total, 80 records of the events from 29 European stations were collected and, if possible, digitized. In addition, bulletins were searched for instrument parameters required to calculate transfer functions for instrument correction. A grid search based on matching the digitized historic waveforms to complete synthetic seismograms was then carried out to infer optimal estimates for depth and focal mechanism. Owing to incomplete or unreliable information on instrument parameters and frequently occurring technical problems during recording, such as writing needles jumping off mechanical recording systems, much less seismograms than collected proved suitable for waveform matching. For the first earthquake, only seven seismograms from three different stations at Stuttgart (STU), Göttingen (GTT) and Copenhagen (COP) could be used. Nevertheless, the waveform matching grid search yields two stable misfit minima for source depths of 25 and 50 km. Compatible fault plane solutions are either of normal faulting or thrusting type. A separate analysis of 42 impulsive first-motion polarities taken from the International Seismological Summary (ISS bulletin) excludes the thrusting mechanism and clearly favors a normal faulting solution with at least one of the potential fault planes striking in SW–NE direction. This finding is consistent with the local structure and microseismic activity of the Santorini–Amorgos graben. Since crustal thickness in the Amorgos area is generally less than 30 km, a source depth of 25 km appears to be more realistic. The second earthquake exhibits a conspicuously high ratio of body wave to surface wave amplitudes suggesting an intermediate-depth event located in the Hellenic Wadati–Benioff zone. This hypothesis is supported by a focal mechanism analysis based on first-motion polarities, which indicates a mechanism very different from that of the first event. A waveform matching grid search done to support the intermediate-depth hypothesis proved not to be fruitful because the body wave phases are overlain by strong surface wave coda of the first event inhibiting a waveform match. However, body to surface wave amplitude ratios of a modern intermediate-depth event with an epicenter close to the island of Milos observed at stations of the German Regional Seismic Network (GRSN) exhibit a pattern similar to the one observed for the second event with high values in a frequency band between 0.05 Hz and 0.3 Hz. In contrast, a shallow event with an epicenter in western Crete and nearly identical source mechanism and magnitude, shows very low ratios of body and surface wave amplitude up to 0.17 Hz and higher ratios only beyond that frequency. Based on this comparison with a modern event, we estimate the source depth of the second event to be greater than 100 km. The proximity in time and space of the two events suggests a triggering of the second, potentially deep event by the shallow first one.

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