Kathmandu Basin as a local modulator of seismic waves: 2D modelling of nonlinear site response under obliquely incident waves

The 2015 Mw 7.8 Gorkha, Nepal earthquake is the largest event to have struck the capital city of Kathmandu in recent times. One of its surprising features was the frequency content of the recorded ground motion, exhibiting a notable amplification at low frequencies (< 2 Hz) and a contrasting depletion at higher frequencies. The latter has been partially attributed to the damper behaviour of the Kathmandu basin. While such weak high-frequency ground motion helped avoiding severe damage in the city, the catastrophic outcomes of earlier earthquakes in the region attest to a contrasting role of the Kathmandu basin as a broadband amplifier, in addition to possible source effects. Given the possibility of future strong events in the region, our main objective is to elucidate the seismic behaviour of the Kathmandu basin by focusing on site effects. We numerically model 2D P-SV wave propagation in a broad frequency band (up to 10 Hz), incorporating the most recent data for the Kathmandu basin geometry, soil stratigraphy and geotechnical soil properties, and accounting for the non-linear effect of multi-dimensional soil plasticity on wave propagation. We find that: 1) the Kathmandu basin generally amplifies low frequency ground motion (< 2 Hz); 2) waves with large incidence angles relative to vertical can dramatically amplify the high frequency ground motion with respect to bedrock despite the damping effect of soil nonlinearity; 3) the spatial distribution of peak ground motion amplitudes along the basin is highly sensitive to soil nonlinearity and wave incidence (angle and direction), favoring larger values near the basin edges located closer to the source, as observed during the 2015 event. Our modelling approach and findings can support the ongoing resilience practices in Nepal and can guide future seismic hazard assessment studies for other sites that feature similar complexities in basin geometry, soil stratigraphy and dynamic soil behaviour.

Testing Nonlinear Amplification Factors of Ground-Motion Models

ABSTRACT We explore nonlinear site effects in the new Japanese ground-motion dataset compiled by Bahrampouri et al. (2020). Following the approach of Seyhan and Stewart (2014), we evaluate the decrease of soil amplification according to the increasing and corresponding ground motion on surface rock (VS30=760 m/s). To better predict the rock ground motion associated with each record, we take into account the between-event variability of the ground motion, and to better evaluate the impact of nonlinearity, we correct observed ground motion on soil by the site-specific linear amplification. Instead of grouping the stations by site-response proxy, we focus on individual stations with several strong-motion records. We develop a framework to test recently published nonlinear site amplification models against a linear site amplification model and compare the results with recent building codes that include nonlinearity. The results show that the site response varies greatly from site to site, indicating that conventional site proxies, such as VS30, are not sufficient to characterize nonlinear site response. Out of all of the Kiban–Kyoshin network stations, 20 stations are selected as having recorded sufficient data to be used in the test. Out of these 20 stations, five stations show signs of nonlinearity, that is, the nonlinear models performed better than the linear-amplification model for all periods T. For most sites, however, the linear site amplification models get the best score. This suggest that, for the range of predicted rock motion considered in this study (peak ground acceleration &lt;0.2g), nonlinearity may not have a sufficiently large impact on soil ground motion to justify the use of nonlinear site terms in ground-motion functional forms and seismic building codes for such moderate-level shaking.

Nonlinear Site Effects from the 30 November 2018 Anchorage, Alaska, Earthquake

ABSTRACT Anchorage, Alaska, is a natural laboratory for recording strong ground motions from a variety of earthquake sources. The city is situated in a tectonic region that includes the interface and intraslab earthquakes related to the subducting Pacific plate and crustal earthquakes from the upper North American plate. The generalized inversion technique was used with a local rock reference station to develop site response at &gt;20 strong-motion stations in Anchorage. A database of 94 events recorded at these sites from 2005 to 2019 was also compiled and processed to compare their site response with those in the 2018 Mw 7.1 event (main event). The database is divided into three datasets, including 75 events prior to the main event, the main event, and 19 aftershocks. The stations were subdivided into the site classes defined in the National Earthquake Hazards Reduction Program based on estimated average shear-wave velocity in of the upper 30 m (VS30), and site-response results from the datasets were compared. Nonlinear site response was observed at class D and DE sites (VS30 of 215–300 and 150–215 m/s, respectively) but not at class CD and C sites (VS30 of 300–440 and 440–640 m/s, respectively). The relationship of peak ground acceleration versus peak ground velocity divided by VS30 (shear-strain proxy) was shown to further support the observation that sites with lower VS30 experienced nonlinear site response.

Bounding surface models in site response analysis: comparison with the equivalent linear approach

&lt;p&gt;A reliable prediction of the response of a soil column subjected to earthquake excitation is a basic although challenging achievement in geotechnical earthquake engineering problems.&lt;/p&gt;&lt;p&gt;A critical step is the analysis type selection. Nowadays, the equivalent linear approach is extremely widespread, mainly for its low computational demand and for its suitability to simulate soil behaviour up to the medium-strain level. However, this approach approximates the hysteresis loop exhibited by soils during a load cycle through an average shear modulus and damping ratio. Consequently, the nonlinear approach would more adequately describe the real soil behaviour, but it requires a large amount of data to be correctly calibrated that is not always available.&lt;/p&gt;&lt;p&gt;To address the differences by using these approaches, a comparison between the surficial acceleration response spectra of a single-layered 20 m-thick soil column of Messina Gravels (gravel and sand with occasionally silty levels) underlain by a rigid bedrock, subjected to strong-motion recordings, is presented.&lt;/p&gt;&lt;p&gt;The software STRATA was used for the equivalent linear analyses. The nonlinear site response analyses were performed with the OpenSees framework, while the bounding surface-based SANISAND constitutive model was selected to reproduce the soil nonlinear behaviour.&lt;/p&gt;&lt;p&gt;A single column in 3D space with periodic boundaries to simulate 1D conditions was considered. The input excitation was applied at the base nodes of the column and the parameters to be assigned to the model were obtained from Gorini (2019).&lt;/p&gt;&lt;p&gt;For both analysis methods, linear elastic analyses were performed by applying a 0.3g sine sweep with frequencies up to 30 Hz. The obtained results were interpreted in terms of acceleration transfer function and a satisfactory congruence was achieved.&lt;/p&gt;&lt;p&gt;The non-linear behaviour of the soil was triggered by applying three accelerograms from strong-motion events (Kobe, Kocaeli and Chi-Chi), downloaded from the PEER database. As results, for periods higher than 1.5 s neglectable amplification effects are observed, so a good accordance is highlighted between equivalent linear and nonlinear analyses. For the period range 0.3-1.5 s, amplification occurs but it is still correctly caught by both the approaches. Strong differences are, instead, observed in the lower periods range, up to 0.3 s, where the equivalent linear approach returns essentially similar spectral accelerations as those of the input motions, while nonlinear analysis highlights amplification and eventually deamplification effects.&lt;/p&gt;&lt;p&gt;In conclusion, it appears that the soil non-linearity should be carefully evaluated for high-seismicity areas because the equivalent linear method tends to underestimate the response, assuming a stiffer behaviour. This was clear for a single-layered soil column and it becomes certainly more complex for stratified soil deposits. To this end, the non-linear approach appears more appropriate to avoid underpredictions of the input motion to be applied for design purposes, but a high effort should be made to properly characterize the soil for the calibration of the selected nonlinear model as well.&lt;/p&gt;

Simulation-based site amplification model for shallow bedrock sites in Korea

A new simulation-based site amplification model for shallow sites with thickness less than 30 m in Korea is developed. The site amplification model consists of linear and nonlinear components that are developed from one-dimensional linear and nonlinear site response analyses. A suite of measured shear wave velocity profiles is used to develop corresponding randomized profiles. A VS30 scaled linear amplification model and a model dependent on both VS30 and site period are developed. The proposed linear models compare well with the amplification equations developed for the western United States (WUS) at short periods but show a distinct curved bump between 0.1 and 0.5 s that corresponds to the range of site natural periods of shallow sites. The response at periods longer than 0.5 s is demonstrated to be lower than those of the WUS models. The functional form widely used in both WUS and central and eastern North America (CENA), for the nonlinear component of the site amplification model, is employed in this study. The slope of the proposed nonlinear component with respect to the input motion intensity is demonstrated to be higher than those of both the WUS and CENA models, particularly for soft sites with VS30 < 300 m/s and at periods shorter than 0.2 s. The nonlinear component deviates from the models for generic sites even at low ground motion intensities. The comparisons highlight the uniqueness of the amplification characteristics of shallow sites that a generic site amplification model is unable to capture.