ALBERT

Description: We provide a complete description of the characteristics of excitation and attenuation of the ground motion in the Lake Van region (eastern Turkey) using a data set that includes three-component seismograms from the 23 October 2011 Mw 7.1 Van earthquake, as well as its aftershocks. Regional attenuation and source scaling are parameterized to describe the observed ground motions as a function of distance, frequency, and magnitude. Peak ground velocities are measured in selected narrow frequency bands from 0.25 to 12.5 Hz; observed peaks are regressed to define a piecewise linear regional attenu- ation function, a set of excitation terms, and a set of site response terms. Results are modeled through random vibration theory (see Cartwright and Longuet-Higgins, 1956). In the log–log space, the regional crustal attenuation is modeled with a bilinear geo- metrical spreading g r characterized by a crossover distance at 40 km: g r ∝ r^−1 fits our results at short distances (r 〈 40 km), whereas g r ∝ r^−0.3 is better at larger distances (40 〈 r 〈 200 km). A frequency-dependent quality factor, Q f =100( f/fref)^ 0:43 (in which fref 1.0 Hz), is coupled to the geometrical spreading. Because of the inherent trade-off of the excitation/attenuation parameters (Δσ and κ), their specific values strongly depend on the choice made for the stress drop of the smaller earthquakes. After choosing a Brune stress drop ΔσBrune 4 MPa at Mw 3:5, we were able to define (1) an effective high frequency, distance- and mag- nitude-independent roll-off spectral parameter, κeff = 0:03 s and (2) a size-dependent stress-drop parameter, which increases with moment magnitude, from ΔσBrune 4 MPa at Mw 3.5 to ΔσBrune 20 MPa at Mw 7.1. The set of parameters mentioned here may be used in order to predict the earthquake-induced ground motions expected from future earthquakes in the region surrounding Lake Van.

Description: Published

Description: 4T. Fisica dei terremoti e scenari cosismici

Description: JCR Journal

Description: open

Description: Inadequate seismic design codes can be dangerous, particularly when they underestimate the true hazard. In this study we use data from a sequence of moderate-sized earthquakes in northeast Italy to validate and test a regional wave propagation model which, in turn, is used to under- stand some weaknesses of the current design spectra. Our velocity model, while regionalized and somewhat ad hoc, is consistent with geophysical observations and the local geology. In the 0.02–0.1 Hz band, this model is validated by using it to calculate moment tensor solutions of 20 earth- quakes (5.6 MW 3.2) in the 2012 Ferrara, Italy, seismic sequence. The seismic spectra observed for the relatively small main shock significantly exceeded the design spectra to be used in the area for critical structures. Observations and synthetics reveal that the ground motions are dominated by long-duration surface waves, which, apparently, the design codes do not adequately anticipate. In light of our results, the present seismic hazard assessment in the entire Pianura Padana, including the city of Milan, needs to be re-evaluated. Citation: Malagnini, L., R. B. Herrmann, I. Munafò, M. Buttinelli, M. Anselmi, A. Akinci, and E. Boschi (2012), The 2012 Ferrara seismic sequence: Regional crustal structure, earthquake sources, and seismic hazard, Geophys. Res. Lett., 39, L19302, doi:10.1029/ 2012GL053214.

Description: Published

Description: L19302

Description: 4T. Fisica dei terremoti e scenari cosismici

Description: JCR Journal

Description: open

Description: Modern tectonic studies often use regional moment tensors (RMTs) to interpret the seismotectonic framework of an earthquake or earthquake sequence; however, despite extensive use, little existing work addresses RMT parameter uncertainty. Here, we quantify how network geometry and faulting style affect RMT sensitivity. We examine how data-model fits change with fault plane geometry (strike and dip) for varying station configurations. We calculate the relative data fit for incrementally varying geometries about a best-fitting solution, applying our workflow to real and synthetic seismograms for both real and hypothetical station distributions and earthquakes. Initially, we conduct purely observational tests, computing RMTs from synthetic seismograms for hypothetical earthquakes and a series of well-behaved network geometries. We then incorporate real data and station distributions from the International Maule Aftershock Deployment (IMAD), which recorded aftershocks of the 2010 M W 8.8 Maule earthquake, and a set of regional stations capturing the ongoing earthquake sequence in Oklahoma and southern Kansas. We consider RMTs computed under three scenarios: (1) real seismic records selected for high data quality; (2) synthetic seismic records with noise computed for the observed source-station pairings and (3) synthetic seismic records with noise computed for all possible station-source pairings. To assess RMT sensitivity for each test, we observe the ‘fit falloff’, which portrays how relative fit changes when strike or dip varies incrementally; we then derive the ranges of acceptable strikes and dips by identifying the span of solutions with relative fits larger than 90 per cent of the best fit. For the azimuthally incomplete IMAD network, Scenario 3 best constrains fault geometry, with average ranges of 45° and 31° for strike and dip, respectively. In Oklahoma, Scenario 3 best constrains fault dip with an average range of 46°; however, strike is best constrained by Scenario 1, with a range of 26°. We draw two main conclusions from this study. (1) Station distribution impacts our ability to constrain RMTs using waveform time-series; however, in some tectonic settings, faulting style also plays a significant role and (2) increasing station density and data quantity (both the number of stations and the number of individual channels) does not necessarily improve RMT constraint. These results may be useful when organizing future seismic deployments (e.g. by concentrating stations in alignment with anticipated nodal planes), and in computing RMTs, either by guiding a more rigorous data selection process for input data or informing variable weighting among the selected data (e.g. by eliminating the transverse component when strike-slip mechanisms are expected).

Description: The 2016 M5.8 Pawnee, Oklahoma, earthquake is the largest earthquake to have been induced by wastewater disposal. We infer the coseismic slip history from analysis of apparent source time functions and inversion of regional and teleseismic P waveforms, using aftershocks as empirical Green's functions. The earthquake nucleated on the shallow part of the fault, initially rupturing toward the surface, followed shortly thereafter by slip deeper on the fault. Deeper slip occurred below the aftershocks and at greater depths than most induced seismicity in the region, suggesting that small- to moderate-sized earthquakes may not occur on deeper parts of faults in Oklahoma because they are further from failure than shallower fault sections. Comparisons with models of pore pressure perturbations further suggest that the earthquake may have initiated within a region of higher pore pressure perturbation but was not confined to this zone. These observations inform source physics and understanding of maximum magnitudes. ©2019. American Geophysical Union. All Rights Reserved.

Description: A search technique is applied to determine focal mechanisms from the surface-wave spectral amplitudes of recent earthquakes in the eastern U. S. and Canada. The technique provides the five source parameters of dip, slip, strike, depth, and seismic moment through a combination of criteria requiring best correlation coefficients, least residuals between theoretical and observed spectral-amplitudes and equality between independent seismic moment estimates from Love and Rayleigh wave data. This technique is applied to eight earthquakes of mb ≈ 5 that occurred in the North American continent in recent years. The focal mechanism results, constrained by P-wave first motions, indicate that near horizontal pressure axes are in the ENE-WSW for the 1982 Miramichi, New Brunswick (Canada) mainshock and one large aftershock (another large aftershock has ESE-WNW P-axis), the 1982 Gaza, New Hampshire mainshock, the 1982 Arkansas mainshock, the 1983 Goodnow, New York mainshock, and the 1986 Perry, Ohio mainshock. On the other hand a near horizontal tension axis in the direction NNE-SSW is found for the 1984 Wyoming mainshock in the western part of North American continent. The results obtained are consistent with the regional stress patterns and generally agree with the solutions of other investigators who used other aspects of the seismic wavefield.

Description: Seismic hazard is calculated for a grid of sites on the Mississippi embayment and adjacent areas. The seismic zonations, seismicity parameters, and maximum magnitudes for these calculations were synthesized from the EPRI (Electric Power Research Institute) and LLNL (Lawrence Livermore National Laboratory) interpretations, augmented by information from recent relevant studies. Attenuation functions developed for this study include the effects of source size and crustal structure. The soil amplification factors include the effects of surficial geology, soil-column thickness, and soil nonlinearity. Seismic hazard maps, including the effect of site conditions, are presented for spectral accelerations at 1 and 10 Hz and for peak acceleration, for average return periods of 100, 500, and 1000 years. These maps show significantly lower ground-motion amplitudes than those reported by Algermissen et al. (1991). Because of these large differences, it is suggested that code- development decisions be made after consideration of both studies.

Description: In order to illustrate wave propagation concepts of ray theory, breakdown of ray theory at caustics, the formation of caustics, free oscillations, and the mode-ray duality, elastic wave propagation in a homogeneous, isotropic fluid sphere is considered. The analytical solution is simply expressed in terms of spherical Bessel functions, but their correct evaluation is an interesting numerical exercise. The solution shows some very interesting features, including near-field terms at short distances, and apparent polarity reversals due to the effect of double caustics.

Description: The analysis of point sources in homogeneous media is presented in an introductory but rigorous manner. Stokes solution to the elastic wave equation with a concentrated body force is extended to include couples and moment tensor sources. The single couple and the double couple without moment are discussed in detail because of their importance in earthquake source studies. To simplify the treatment, the transformation properties of tensors under rotations are used throughout. This guide is intended to complement that written by Jost and Herrmann (1989) on moment tensors. A FORTRAN 77 program is provided to plot P, SV, SH radiation patterns as well as S-wave polarizations on a lower hemisphere focal sphere for an arbitrary symmetric moment tensor.