The search for hard-rock kappa (κ) in NGA-East: A semi-automated method for large, challenging datasets in stable continental regions

This article describes the work undertaken within the Next Generation Attenuation (NGA)-East project with the aim of estimating κ0 (the site-specific component of the high-frequency decay parameter, κ) for rock sites in Central and Eastern North America (CENA), using the project’s shallow crustal dataset. We introduce a methodology to address the numerous challenges in CENA: a large dataset in a low-seismicity stable continental region, with poor magnitude and distance coverage, undesirable recording sensor characteristics (low sampling rates leading to poor high-frequency resolution), high uncertainty in the regional stress drop, and lack of site-specific velocity characterization. We use two band-limited κ estimation approaches, the acceleration and displacement spectrum (AS and DS), applied above and below the source corner frequency ( fc), respectively. For band-limited approaches, the key requirement is an estimate of fc, which—apart from the event magnitude readily available in the flatfile—also heavily depends on the highly uncertain stress drop. By considering lower and upper bounds on regional stress drop, we propose a new method to quickly and automatically screen such very large datasets to identify all possible recordings for which band-limited κ approaches can be used. Combining them produces better-quantify estimates of κ and its epistemic uncertainties for this challenging dataset. The mean κ0 values combining the two methods are 13 ± 23 ms for horizontal ground motion.

Relations between MaxRotD50 and Some Horizontal Components of Ground-Motion Intensity Used in Practice

ABSTRACT The most commonly used intensity measure of ground motion in earthquake engineering is the 5% damped spectral ordinate, which varies in different directions. Several different measures have been proposed over the years to combine the intensity of the two horizontal recorded ground motions to derive ground-motion models as well as for design purposes. This study provides the relation to seven previously used measures of horizontal ground motion with respect to a recently proposed orientation-independent measure of horizontal ground-motion intensity referred to as MaxRotD50. This new measure of horizontal intensity is defined as the median value of the maximum spectral ordinate of two orthogonal directions computed for all possible nonredundant orientations. The relations are computed using 5065 pairs of horizontal ground motions taken from the database of ground motions recorded in shallow crustal earthquakes in active tectonic regions developed as part of the Pacific Earthquake Engineering Research Center’s Next Generation Attenuation-West2 project. Empirically derived period-dependent relations are presented for three quantities that permit transforming any of the seven other definitions of horizontal ground-motion intensity to MaxRotD50, namely, (1) geometric mean of the ratio of MaxRotD50 to any of the seven other measures of intensities, (2) standard deviation of the natural logarithm of the ratio of MaxRotD50 to any of the seven other measures of intensities, and (3) the correlation between the natural logarithm of the ratio of MaxRotD50 to the other measures of intensities and the natural logarithm of the other measure of intensity. In addition, the influence of site class at the recording station, earthquake magnitude, and distance to the horizontal projection of the rupture is examined on the geometric mean of the ratio of MaxRotD50 to the median intensity of all nonredundant orientations (i.e., RotD50), showing negligible influence of site class and only a relatively small influence of magnitude and distance.

Dynamic Response and Failure Characteristics of Slope with Weak Interlayer under Action of Near-Fault Ground Motion

Investigations into the Wenchuan earthquake (2008, China) demonstrated that landslides were concentrated in the near-fault areas, and numerous large-scale landslides occurred in slopes with weak interlayers. A mathematical model was established based on the shear beam theory, while a numerical model was developed based on the discrete element method which perfectly matched layer boundary theory. Through a theoretical analysis and numerical simulation, the dynamic response and failure modes of the slope with a weak interlayer under the near-fault ground motion were studied. It was found that a combined effect took place between the near-fault ground motion and the weak interlayer, causing the slope near a fault to be destroyed more easily. The coupling between the near-fault ground motion and the weak interlayer leads to a maximum amplification effect of the slope. The existence of a weak interlayer induces nonconforming vibration between the upper and the lower rock masses of the interlayer. The variation in the amplification effect along the slope elevation is related to the ratio of the input seismic period to the natural slope period. Under horizontal ground motion, weak interlayers will be subjected to impacting and shearing action. The failure mode of the slope with a weak interlayer under near-fault ground motion can be expressed as a trailing edge tension crack, as well as weak interlayer impacting and shearing failure.

Ground deformation related to slip and afterslip of the 29 December 2020 Mw 6.4 Petrijna earthquake (Croatia) imaged by InSAR

<p>On December 29, 2020, at 11:19 UTC, a strong (M6.4), shallow earthquake occurred in the central region of Croatia. The epicentre was located near the town of Petrinja, about 40 km to the south of the capital, Zagreb. Here we present a preliminary analysis of the geodetic data (differential InSAR & GNSS) and preliminary estimates of the slip that occurred on the fault during the earthquake and subsequent aftershocks. We picked InSAR data to invert for the seismic fault assuming linear rheology and Okada-type dislocation (rectangular) source with non-uniform slip. The interferograms show an asymmetric, four‐lobed pattern, centered on a NW‐SE oriented discontinuity that is in agreement with the right-lateral plane of the moment tensor solutions for the mainshock. We found that the Petrijna earthquake ruptured a segment of a strike-slip fault zone that is shorter (8 km) than average and with larger slip (~ 3 m). All parameters of the seismic fault are well constrained by InSAR modeling due to the full azimuthal coverage with both ascending and descending data of good quality. The fit to the fringes is better with a steep dip angle (76°) than with a purely vertical fault. The upper edge of the modeled fault is at a depth of ~1 km, this means that the slip drop from 3 to 0 m in the uppermost kilometer and our geodetic analysis cannot assess whether the fault reached the surface in some sections of the fault, however should this be the case, we expect ruptures at the surface in the range of 0.1 to 0 m for consistency with our model and the structure of the fringes pattern. In particular, preliminary modelling results with distributed fault-slip show that the slip reached a peak of more than 2.5 m at a depth of about 2 km. We also found that, differently from what reported in the European database of seismogenic sources (EDSF), the seismic fault dips westward instead of eastward. Additionally, the 2020 rupture and the InSAR mapped trace do not match the EDSF composite seismogenic fault surface trace. Kinematic analysis of GNSS waveforms at station BJEL (about 70-km east of the epicentre) revealed that horizontal ground motion reached 7-cm (peak-to-peak). The InSAR data revealed a 7 km of right-lateral afterslip on the NW-edge of the rupture, and 5 km to the south of the main fault rupture. In particular, the afterslip data on the NW edge of the rupture document the curved shape of the post-seismic deformation, that highlights the non-planarity of faults in nature and possibly indicating the existence of a ramp structure connecting to the neighboring segment towards north.</p>

Response spectral matching of horizontal ground motion components to an orientation-independent spectrum (RotDnn)

This article presents a methodology to spectrally match two horizontal ground motion components to an orientation-independent target spectrum (RotDnn). The algorithm is based on the continuous wavelet transform decomposition and iterative manipulation of the two horizontal components of a seed record. The numerical examples presented follow current ASCE/SEI 7 specifications and therefore maximum-direction spectra (RotD100) are used as target for the match. However, the proposed methodology can be used to match other RotDnn spectra, like the median spectrum (RotD50). It is shown that with the proposed methodology the resulting RotDnn from the modified horizontal components closely match the smooth target RotDnn spectrum, while the response spectrum for each horizontal component continue to exhibit a realistic jagged behavior. The response spectra variability at the component level within suites of spectrally matched motions was found to be of the same order than the variability measured in suites composed of amplitude scaled records. Moreover, the spectrally matched records generated preserved most of the characteristics of the seed records, including the nonlinear characteristics of the time history traces and the period-dependent major axis orientations.

Seismic assessment of a cable-stayed arch bridge under three-component orthotropic earthquake excitation

The occurred damages during the past significant earthquakes have proved that vertical seismic excitation has tremendous effect on bridges. Three-component earthquake excitations are preferred to resemble the earthquakes. In this article, a cable-stayed arch bridge, a new type of bridge with the hybrid system of half-through arch and stay-cables, was analyzed under a set of different earthquake excitations (more than 21 ground motion records). Both vertical and horizontal components of the ground motions were considered to act simultaneously at the bridge supports. By using different three-component earthquake excitations, the dynamic responses of the bridge, including the displacements and accelerations of the main parts of the bridge, were obtained. The effects of various parameters such as soil type, epicentral distance, spatial variation of the ground motions, and dimensional variation of the structure were investigated. The results of the numerical study indicate that the cable-stayed arch bridge subjected to both horizontal and vertical components of earthquakes are more vulnerable than those subjected to horizontal ground motion only.

Non-stability of the noise HVSR at sites near or on topographic heights

SUMMARY The horizontal to vertical spectral ratio (HVSR) of seismic noise is often used to investigate site effects, and it is usually assumed to be a stable feature of the site considered. Here we show that such an assumption is not always justified, and may lead to incorrect conclusions. The HVSR analysis was performed on ambient seismic noise recordings lasting from weeks to months at many sites in Calabria, Italy. Results show a variety of site effects, from the resonance of a shallow sedimentary layer to the polarized amplification of horizontal ground motion associated with topographic effects. We describe the results of seven sites whose HVSR is characterized by dual content: one that is persistent, and another appearing only occasionally. Two sites very near the coast of the Tyrrhenian sea and five sites in the Calabrian Arc mountains show the most remarkable results. The shape of the HVSR changes significantly at these sites when the amplitude of background noise increases in a broad frequency band during periods of bad weather. The occasional contribution to the HVSR consists of one or more peaks, depending on the site, that appear only when the amplitude of ambient noise is higher than usual. The seven sites where we observe the HVSR variability are all located in complex geological environments, on mountains, ridges or foothills. A variation of the HVSR correlated with the day–night cycle is also observed at some of these sites.