scholarly journals The Cephalonia, Ionian Sea (Greece), sequence of strong earthquakes of January-February 2014: a first report

2014 ◽  
Vol 3 (1) ◽  
Author(s):  
Gerassimos A. Papadopoulos ◽  
Vassilios K. Karastathis ◽  
Ioannis Koukouvelas ◽  
Maria Sachpazi ◽  
Ioannis Baskoutas ◽  
...  
Keyword(s):  
Fault Plane ◽  
Moment Tensor ◽  
Time Function ◽  
P Wave ◽  
Ionian Sea ◽  
Fault Rupture ◽  
Source Time Function ◽  
Ground Acceleration ◽  
Strong Earthquakes ◽  
Source Duration

On 26.1.2014 and 3.2.2014 two strong earthquakes of M<sub>w</sub>6.0 and M<sub>w</sub>5.9 ruptured the western Cephalonia Isl., Ionian Sea (Greece), at the SSW-wards continuation of the Lefkada segment of the Cephalonia Transform Fault Zone (CTFZ), causing considerable damage and a variety of ground failures. High-precision relocation of the aftershocks implies that the seismogenic layer was of 35 km in length (L) striking NNE-SSW, of 10 km maximum in width and 15 km in thickness. Two aftershock spatial clusters were revealed at north (<em>L<sub>1</sub></em>~10 km) and at south (<em>L<sub>2</sub></em>~25 km). However, no time correlation was found between the two clusters and the two strong earthquakes. Fitting the temporal evolution of aftershocks to the Omori-law showed slow aftershock decay. Fault plane solutions produced by moment tensor inversions indicated that the strong earthquakes as well as a plenty of aftershocks (M<sub>w</sub>≥4.0) were associated with dextral strikeslip faulting with some thrust component and preferred fault planes striking about NNE-SSW. Average fault plane parameters obtained for the three largest events are: strike 21(±2)<sup>0</sup>, dip 65.5(±3)<sup>0</sup>, slip 173(±3)<sup>0</sup>. Broadband P-wave teleseismic records were inverted for understanding the rupture histories. It was found that the earthquake of 26.1.2014 had a complex source time function with c. 62 cm maximum slip, source duration of ~12 s and downwards rupture. Most of the slip was concentrated on a 13x9 km fault rupture. The earthquake of 3.2.2014 had a relatively simple source time function related with one big patch of slip with maximum slip c. 45 cm, with 10 s source duration. The rupture was directed upwards which along with the shallow focus (~5 km) and the simple source time function may explain the significantly larger (0.77 g) PGA recorded with the second earthquake with respect to the one recorded (0.56 g) with the first earthquake. Most of the slip was concentrated on a 12x6 km fault rupture. Maximum seismic intensity (<em>I<sub>m</sub></em>) of level VII and VIII to VIII+ was felt in Lixouri town and the nearby villages from the first and the second earthquake, respectively. The rupture histories and the increased building vulnerability after the damage caused by the first shock may account for the larger <em>I<sub>m</sub></em> caused by the second shock. However, the ground failures area of the second earthquake was nearly half of that of the first earthquake, which is consistent with the faster attenuation of ground acceleration away from the meizoseismal area caused by the second earthquake with respect to the first one. From that the 2014 earthquakes ruptured on land western Cephalonia we suggested to revise the CTFZ geometry in the sense that the Lefkada CTFZ segment does not terminates offshore NW Cephalonia but extends towards SSW in western Cephalonia.

scholarly journals Fully probabilistic seismic source inversion – Part 1: Efficient parameterisation

2013 ◽  
Vol 5 (2) ◽  
pp. 1125-1162 ◽  
Author(s):  
S. C. Stähler ◽  
K. Sigloch
Keyword(s):  
Moment Tensor ◽  
Source Parameters ◽  
Practical Importance ◽  
Principal Component ◽  
Seismic Source ◽  
Empirical Orthogonal Functions ◽  
Time Function ◽  
Source Inversion ◽  
Source Time Function ◽  
Earthquake Parameters

Abstract. Seismic source inversion is a non-linear problem in seismology where not just the earthquake parameters themselves, but also estimates of their uncertainties are of great practical importance. Probabilistic source inversion (Bayesian inference) is very adapted to this challenge, provided that the parameter space can be chosen small enough to make Bayesian sampling computationally feasible. We propose a framework for PRobabilistic Inference of Source Mechanisms (PRISM) that parameterises and samples earthquake depth, moment tensor, and source time function efficiently by using information from previous non-Bayesian inversions. The source time function is expressed as a weighted sum of a small number of empirical orthogonal functions, which were derived from a catalogue of >1000 STFs by a principal component analysis. We use a likelihood model based on the cross-correlation misfit between observed and predicted waveforms. The resulting ensemble of solutions provides full uncertainty and covariance information for the source parameters, and permits to propagate these source uncertainties into travel time estimates used for seismic tomography. The computational effort is such that routine, global estimation of earthquake mechanisms and source time functions from teleseismic broadband waveforms is feasible.

Deriving source-time functions using principal component analysis

1989 ◽  
Vol 79 (3) ◽  
pp. 711-730
Author(s):  
D. W. Vasco
Keyword(s):  
Principal Component Analysis ◽  
Moment Tensor ◽  
Principal Component ◽  
Singular Values ◽  
Singular Value ◽  
Time Function ◽  
Basis Functions ◽  
Single Source ◽  
Source Time Function ◽  
The Moment

Abstract Factors such as source complexity, microseismic noise, and lateral heterogeneity all introduce nonuniqueness into the source-time function. The technique of principal component analysis is used to factor the moment tensor into a set of orthogonal source-time functions. This is accomplished through the singular value decomposition of the time-varying moment tensor. The adequacy of assuming a single source-time function may then be examined through the singular values of the decomposition. The F test can also be used to assess the significance of the various principal component basis functions. The set of significant basis functions can be used to test models of the source-time functions, including multiple sources. Application of this technique to the Harzer nuclear explosion indicated that a single source-time function was found to adequately explain the moment tensor. It consists of a single pulse appearing on the diagonal elements of the moment-rate tensor. The decomposition of the moment tensor for a deep teleseism in the Bonin Islands revealed three basis functions associated with relatively large singular values. The F test indicated that only two of the principal components were significant. The principal component associated with the largest singular value consists of a large pulse followed 16-sec later by a diminished pulse. The second principal component, a long-period oscillation, appears to be a manifestation of the poor resolution of the moment-rate tensor at low frequencies.

Source retrieval for deep local earthquakes with broadband records

1993 ◽  
Vol 83 (6) ◽  
pp. 1855-1870
Author(s):  
Masayuki Kikuchi ◽  
Mizuho Ishida
Keyword(s):  
Seismic Moment ◽  
Body Wave ◽  
Time Function ◽  
P Wave ◽  
Source Time Function ◽  
Initial Portion ◽  
Large Earthquakes ◽  
Vast Range ◽  
Multiple Shock ◽  
Source Retrieval

Abstract Body wave data recorded at a small network of broadband seismograph stations are analyzed to investigate local events with focal depths deeper than about 50 km. For these events the initial portion of P-wave displacement represents well the source time function with a scaler correction for the seismic moment. The magnitudes of the analyzed earthquakes range from MW = 3.1 to 6.5. It is shown that the seismic moment M0 and the pulse width τ are well correlated as M0/τ3 = constant, indicating that the stress drop is largely constant. This dynamic similarity seems to be valid for a vast range of earthquake sizes: MW = 1 ∼ 8. It is also shown that source complexity such as a multiple shock nature is not a characteristic of only large earthquakes but is often observed even for small earthquakes.

scholarly journals Fully probabilistic seismic source inversion – Part 1: Efficient parameterisation

Solid Earth ◽  
2014 ◽  
Vol 5 (2) ◽  
pp. 1055-1069 ◽  
Author(s):  
S. C. Stähler ◽  
K. Sigloch
Keyword(s):  
Moment Tensor ◽  
Source Parameters ◽  
Practical Importance ◽  
Principal Component ◽  
Seismic Source ◽  
Empirical Orthogonal Functions ◽  
Time Function ◽  
Source Inversion ◽  
Source Time Function ◽  
Earthquake Parameters

Abstract. Seismic source inversion is a non-linear problem in seismology where not just the earthquake parameters themselves but also estimates of their uncertainties are of great practical importance. Probabilistic source inversion (Bayesian inference) is very adapted to this challenge, provided that the parameter space can be chosen small enough to make Bayesian sampling computationally feasible. We propose a framework for PRobabilistic Inference of Seismic source Mechanisms (PRISM) that parameterises and samples earthquake depth, moment tensor, and source time function efficiently by using information from previous non-Bayesian inversions. The source time function is expressed as a weighted sum of a small number of empirical orthogonal functions, which were derived from a catalogue of >1000 source time functions (STFs) by a principal component analysis. We use a likelihood model based on the cross-correlation misfit between observed and predicted waveforms. The resulting ensemble of solutions provides full uncertainty and covariance information for the source parameters, and permits propagating these source uncertainties into travel time estimates used for seismic tomography. The computational effort is such that routine, global estimation of earthquake mechanisms and source time functions from teleseismic broadband waveforms is feasible.

scholarly journals Characteristics of Recent Aftershocks Sequences (2014, 2015, 2018) Derived from New Seismological and Geodetic Data on the Ionian Islands, Greece

Data ◽  
2021 ◽  
Vol 6 (2) ◽  
pp. 8
Author(s):  
Alexandra Moshou ◽  
Panagiotis Argyrakis ◽  
Antonios Konstantaras ◽  
Anna-Christina Daverona ◽  
Nikos C. Sagias
Keyword(s):  
Seismic Moment ◽  
Moment Tensor ◽  
Local Population ◽  
Aftershock Sequence ◽  
Geodetic Data ◽  
Seismic Moment Tensor ◽  
Ionian Sea ◽  
National Observatory ◽  
Strong Earthquakes ◽  
The Third

In 2014–2018, four strong earthquakes occurred in the Ionian Sea, Greece. After these events, a rich aftershock sequence followed. More analytically, according to the manual solutions of the National Observatory of Athens, the first event occurred on 26 January 2014 in Cephalonia Island with magnitude ML = 5.8, followed by another in the same region on 3 February 2014 with magnitude ML = 5.7. The third event occurred on 17 November 2015, ML = 6.0 in Lefkas Island and the last on 25 October 2018, ML = 6.6 in Zakynthos Island. The first three of these earthquakes caused moderate structural damages, mainly in houses and produced particular unrest to the local population. This work determines a seismic moment tensor for both large and intermediate magnitude earthquakes (M > 4.0). Geodetic data from permanent GPS stations were analyzed to investigate the displacement due to the earthquakes.

Seismic moment and long-period radiation of underground nuclear explosions

1973 ◽  
Vol 63 (3) ◽  
pp. 847-857
Author(s):  
Gerhard Müller
Keyword(s):  
Point Source ◽  
Seismic Moment ◽  
Time Function ◽  
Moment Function ◽  
P Wave ◽  
Underground Explosion ◽  
Source Time Function ◽  
Nuclear Explosions ◽  
Long Period ◽  
The Moment

abstract The moment function of an explosion is introduced, using the equivalence of an explosive point source and three mutually perpendicular linear dipoles. The seismic moment of an explosion is the final value, for large times, of the moment function. Its relation to source parameters is similar to that of the moment of an earthquake: M1 = (λ + 2μ)S1D1 (λ, μ = Lamé's parameters, S1 = surface area of a sphere surrounding the explosion in the elastic zone, D1 = static radial displacement on this sphere). From strain observations of other authors (Romig et al., 1969; Smith et al., 1969), the moment of the underground nuclear explosion BENHAM is estimated to be about 1024 dyne cm. This moment value supports the assumption that the source-time function for the long-period radiation from large nuclear explosions (periods greater than about 10 sec) is essentially a step-function. On the other hand, a quantitative estimate of the long-period P-wave spectrum of the explosions JORUM, HANDLEY and MILROW and a comparison with observed spectra of Molnar (1971) for JORUM and HANDLEY and Wyss et al. (1971) for MILROW support the assumption of an impulsive source-time function. This discrepancy, which is typical of current opinions among seismologists, is not resolved. It is concluded that an explosive point source is possibly not a sufficient model for the long-time radiation and the static displacement field of a nuclear underground explosion whose elastic radius is about equal to its depth and which is detonated in a prestressed medium.

Focal depth determination from the signal character of long-period P waves

1976 ◽  
Vol 66 (4) ◽  
pp. 1221-1232
Author(s):  
Robert B. Herrmann
Keyword(s):  
Epicentral Distance ◽  
Focal Depth ◽  
Time Function ◽  
P Wave ◽  
Source Time Function ◽  
P Waves ◽  
Long Period ◽  
The Earth ◽  
Resolution Capability ◽  
Earth Structures

abstract The shape of long-period teleseismic P-wave signals is a function of many factors, among which are focal depth, focal mechanism, the source time function, and the earth structures at both the source and receiver. The effect of focal depth is quite pronounced, so much so, that focal depths should be able to be determined to within 10 km on the basis of the long-period P-wave character. This resolution capability is demonstrated for events occurring in continental and oceanic crust as observed by seismographs in the 30° to 80° epicentral distance range.

Waveform Characteristics and Focal Mechanism Solutions of Five Aftershocks of the 1983 Goodnow, New York, Earthquake by Polarization Analysis and Waveform Modeling

1991 ◽  
Vol 62 (2) ◽  
pp. 123-133 ◽  
Author(s):  
Zuyuan Liu ◽  
Robert. B. Herrmarnn ◽  
Jiakang Xie ◽  
E. Cranswick
Keyword(s):  
New York ◽  
Focal Mechanism ◽  
Fault Plane ◽  
Body Wave ◽  
Moment Tensor ◽  
Moment Tensor Inversion ◽  
Systematic Search ◽  
P Wave ◽  
S Wave ◽  
Focal Mechanism Solutions

Abstract Waveforms of the direct P-, SV- and SH-waves of five 1983 Goodnow, New York, aftershocks (mb = 1.4–3.1), locally recorded at four hard-rock sites (epicentral distances=1.9–8.0 km) with GEOS systems, were studied to obtain their focal mechanism solutions by waveform fit using both systematic search and moment tensor inversion. Both synthetic and observed data were low-pass filtered at 10 Hz to reduce sensitivity to shallow earth structure. It was discovered that only the first cycle of P-wave and S-wave appear to have pure direct body wave characteristics. The strong P- and S-coda have no stable polarization. The five aftershocks have similar locations, identical P-first motions, but varying direct S-waveforms. A layered velocity model with a P-wave velocity of 4.4 km/s in the surface layer was derived. Fault plane solutions of four events indicate reverse faulting mechanisms that have a near horizontal P-axis with a strike of ENE. This is similar to the fault plane solution of the mainshock (October 7, 1983, mb = 5.1) and the composite focal mechanism of the aftershocks. Four aftershocks occurred on the fault planes with the strike NW-N and dip of 52°–64° toward NE-E. The fifth event studied has significant strike-slip motion with the P axis is also nearly horizontal and oriented NE. The results of systematic search technique agree well with those of moment tensor inversion. The first motion directions, pulse widths, amplitudes, amplitude ratios and arrival times of the direct P-, SV- and SH-phases of the synthetic seismograms are consistent with those of the observed seismograms. The results of the research demonstrated that the S-wave amplitude can provide important constraints on the focal mechanism.

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