Arias Intensity Attenuation Relationship in Sichuan-yunnan Region, China

Arias intensity is an essential ground motion measure correlating with the potential for earthquake-induced landslides. The Sichuan-Yunnan region, which is primarily mountainous, is a high incidence region of earthquake-induced landslides in China. However, there is no available attenuation relationship for this intensity measure due to the backward construction of the stations. In this study, we developed a region-specific Arias intensity attenuation relationship using the China Strong-Motion Networks Center (CSMNC) database which was established in 2008. We recommend this relationship be applied in the Sichuan-Yunnan region for moment magnitudes ranging between 4.2 and 7.9, distances ranging between 0 and 400 km and with Vs30 (the average shear-wave velocity in the upper 30 meters of a soil profile) ranging between 128 and 760 m/s. The current study finds that this relationship’s intra-event, inter-event, and total standard deviations are greater than for other regions. This is likely caused by the complicated seismotectonic activities, nonlinear site effects, error from inferring Vs30, basin effects, etc. However, this relationship has the best performance in fitting and predicting the data from the Sichuan-Yunnan region.

New Insights into Long-Period (>1 s) Seismic Amplification Effects in Deep Sedimentary Basins: A Case of the Po Plain Basin of Northern Italy

ABSTRACT This study investigates and quantifies the influence of the shallower deposits (down to few hundreds of meters) of the Po Plain sedimentary basin (northern Italy) on the long-period component (i.e., 1 s<T<3 s) of seismic ground motion, in which amplification effects due to the soft sediments above seismic bedrock were observed. A new seismostratigraphic model of the shallow deposits of the entire basin is provided with an unprecedented detail by taking advantage of recently acquired geophysical data. The seismostratigraphic model is used to simulate the ground motion amplification in the Po Plain by means of extensive 1D ground response analysis. Results are compared with seismic observations available at a number of sites equipped with borehole seismic stations, where earthquakes have been recorded both at the surface and at the seismic bedrock depth. Despite the general agreement with observations concerning the seismic resonance frequencies, our model may fail in capturing the amplitude of the actual seismic amplification of the basin in the long-period range. We observe that 3D basin effects related to surface waves generated at the edge of the basin may play a significant role in those zones where seismic hazard is controlled by distant sources. In these cases, 1D modeling leads to average underestimations of 30%, up to a maximum of 60%. The amplification functions need to be corrected for a basin-effects correction term, which in this case is provided by the ground-motion prediction equation of the study area. The corrected amplification functions agree with the empirical observations, overcoming the uneven distribution of the recording stations in strong-motion datasets. These results should be taken into account in future seismic microzonation studies in the Po Plain area, where the 1D approach is commonly adopted in ground response analyses, and in site-specific seismic hazard assessments aimed at the design of structures that are sensitive to the long-period component of seismic ground motion (e.g., long-span bridges and tall buildings).

Assessment of the Design Spectrum with Aggravation Factors by 2D Nonlinear Numerical Analyses: A Case Study in Gemlik Basin, Turkey

The response spectra of multidimensional analyses are compared with the one-dimensional (1D) local models to couple the irregular soil stratification effect in a site. In recent studies, the surface motion spectra ratios of 2D/1D or 3D/1D are defined as spectral aggravation factors for each region in a site. Particularly in alluvial basins, where the soil media is typically formed by fault ruptures or topographic depressions filled with sediments, the inclination of the rock outcrop in the edge of the basin has a considerable effect on the site response, and such effect has not yet been taken into consideration of recent seismic building codes and general engineering applications. In this study, the natural alluvial basin near the North Anatolian Fault in Gemlik, Maramara Region, Turkey, was investigated by 40 seismic site tests and 4 validation borings. The 2D and 1D nonlinear response history analyses in north-south and east-west directions of the Gemlik basin were performed by numerical model on finite difference scheme considering nonlinear elasto-plastic material behaviors and geometric discontinuities. 22 strong ground motions recorded on rock site are excited vertically as SH waves. The numerical results exhibited the narrow basin effects are derived not only by reflection, refraction, and shifting behavior but also by focusing and superposition of the seismic waves propagating from both opposite basin edges. As a result, the site-specific spectral aggravation factors, SAF2D/1D defined by the ratio between the 2D and 1D acceleration response spectra for each period and any location on the site, were proposed for the Gemlik basin. The variations of the aggravation factors were observed as increasing values to 1.2–2.2 on the near edge and basin center.

30 Ooctober 2020 Samos-Sigacik Earthquake: On Strong Ground Motion and Local Site Amplification

<p>The Mw 6.9 earthquake that took place offshore between the Greek island of Samos and Turkey’s İzmir province on 30 October 2020 came hardly as a surprise. Due to the extensional tectonic regime of the Aegean and high deformation rates, earthquakes of similar size frequently occur in the Aegean Sea on fault segments close to the shores of Turkey, affecting the settlements on mainland Turkey and on the Greek Islands. Samos-Sigacik earthquake had a normal faulting mechanism. It was recorded by the strong motion networks in Turkey and Greece. Although expected, the earthquake was an  outstanding event in the sense of  highly localized, significant levels of building damage as a result of amplified ground motion levels. This presentation is an overview of strong ground motion characteristics of this important event both regionally and locally. Mainshock records suggest that local site effects, enhanced by basin effects could be responsible for structural damage in central Izmir, the third largest city of Turkey located at 60-70 km epicentral distance. We installed a seven-station network in Bayraklı and Karşıyaka districts of İzmir within three days of the mainshock in search of site and basin effects.  Through analysis of recorded aftershocks we explore the amplification characeristics of soils in the two aforementioned districts  and try to understand the role basin effects might have played in the resulting ground motion levels and consequently damage. </p>

Basin effects and limitations of 1D site response analysis from 2D numerical models of the Thorndon basin

Plane strain (2D) finite element models are used to examine factors contributing to basin effects observed for multiple seismic events at sites in the Thorndon basin of Wellington, New Zealand. The models consider linear elastic soil and rock response when subjected to vertically-propagating shear waves. Depth-dependent shear wave velocities are considered in the soil layers, and the effects of random variations of soil velocity within layers are modelled. Various rock shear wave velocity configurations are considered to evaluate their effect on the modelled surficial response. It is shown that these simple 2D models are able to capture basin reverberations and compare more favourably to observations from strong motion recordings than conventional 1D site response models. It is also shown that consideration of a horizontal impedance contrast across the Wellington Fault affects spectral response and amplification at longer periods, suggesting the importance of this feature in future ground motion modelling studies in the Wellington region.