Shallow earthquakes: Shallower than expected?

Janský, Jaromír; Zahradnı́k, Jiřı́; Plicka, Vladimír

doi:10.1007/s11200-009-0016-8

Cited by 4 publications

(2 citation statements)

References 20 publications

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“…This issue has to be studied in more detail elsewhere, similarly to the disagreement between hypocenter and centroid depth found by Zahradnik et al (2008) and Janský et al (2009), but the obtained indication of a very shallow depth is rather strong. The thrust mechanism of Event 1 and the centroid depth similar to the hypocenter depth cannot be ruled out, but with the existing velocity model such a solution seems less likely due to a few polarity disagreements.…”

Section: Discussionmentioning

confidence: 65%

Waveform inversion and focal mechanisms of two weak earthquakes in Cordillera Principal (Argentina) between 35° and 35.5° S

Villegas

Zahradnı́k

Nacif

et al. 2016

Journal of South American Earth Sciences

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Only few (six) focal mechanism, in CMT Catalog, have been so far known for intraplate shallow events in the Andean chain close to Chile-Argentina state border at latitudes ~35° S. We add two more mechanisms, depths and moment magnitudes by carefully analyzing full waveforms of weak events recorded by broad-band stations of the Chile Argentina Geophysical Experiment (southern profile). The moment magnitudes of both events (Mw = 3.6 and 3.7) are lower than the duration magnitudes (Md = 4.0 and 4.29)reported by NEIC. The source depth, constrained by waveforms for one of the studied events (5.5 to 8.5 km) seems to be considerably shallower than the hypocenter depth located by means of arrival times (~20 km). The waveform analysis was complemented by first-motion polarities which resulted in an uncertainty assessment of the focal mechanism. Event 1 (2001-11-03) has a strike-slip mechanism with a small normal component and almost vertical nodal planes in the north-south and east-west directions. The north-south nodal plane could be related to the Calabozos faults system. Event 2 (2002-02-16) has a strike-slip mechanism with a small thrust component. The latter event (its subhorizontal nodal plane) could be associated with the El Diablo-El Fierro fault system.Dextral strike-slip solutions are consistent with recent studies in the area.

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Section: Discussionmentioning

confidence: 65%

Waveform inversion and focal mechanisms of two weak earthquakes in Cordillera Principal (Argentina) between 35° and 35.5° S

Villegas

Zahradnı́k

Nacif

et al. 2016

Journal of South American Earth Sciences

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“…This observation seems to suggest the MT inversion at the location (hypocenter) depth, especially for moderate-size earthquakes, whose hypocenter and centroid are close to each other. However, it has been shown that for shallow crustal events the depth determination is always difficult (Zahradnik, Janský, et al, 2008;Janský et al, 2009Janský et al, , 2012Sokos et al, 2012), hence the MT inversion at the hypocenter depth cannot be generally recommended, unless dense local networks and precise crustal models are available. It might also seem that a compromising solution is to perform an MT inversion only for the deviatoric part (constrain ISO component to zero), determine the centroid depth, and then, keeping the depth value fixed, rerun the inversion using the full MT decomposition.…”

Section: Discussionmentioning

confidence: 99%

Resolvability of Isotropic Component in Regional Seismic Moment Tensor Inversion

Křížová

Zahradnı́k

Kiratzi

2013

Bulletin of the Seismological Society of America

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We propose a new approach to resolve the isotropic component of the seismic moment tensor and its uncertainty. In linearized inversion problems, where the earthquake or explosive-source location and origin time are fixed (e.g., assumed to be known), the uncertainty of the moment tensor can be studied through the eigenvalues and the eigenvectors of the design matrix, which allows the representation of the theoretical misfit by means of a 6D error ellipsoid. Because the design matrix depends only on the structural model and receiver source geometry, the analysis can be performed using recorded seismic waveforms, or without. In the nonlinear inversion problems, where the free parameters are eight (e.g., the six elements of the moment tensor, depth, and origin time), we propose a waveform-inversion scheme in which the trace of the moment tensor varies systematically and the remaining seven free parameters are optimized for each specific value of the trace. In this way, a 1D experimental probability density function of the moment tensor trace is constructed. To demonstrate the applicability of the method, we apply it to two shallow earthquakes (M w 4.9 and 4.7) with epicenters close to the Columbo volcano, located 20 km northeast of the island of Santorini, Aegean Sea, Greece. We use 15 near-regional (60-310 km) records at frequencies below 0.1 Hz and two alternative crustal models. We conclude that the main uncertainties are attributed to the crustal model and to the trade-off between the isotropic component and the source depth.

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