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Quantifying the magnitude of an earthquake is very important for long-term and medium-term earthquake prediction, post-earthquake emergency rescue and seismic hazard assessment. Paleo-seismology is the investigation of past earthquakes in the geological record, in particular their location, timing and size. Uncertainties remain in the paleo-earthquake magnitudes determined by traditional surface rupture parameters, especially because most seismic events do not result in surface ruptures or are of less than 0.3 m (M = ~ 6–6.8). To address the problem of magnitude evaluation of earthquakes that did not reveal major dislocations, this paper deals with the methods used to determine the seismic shaking intensity based on the types and forms of soft-sediment deformation structures, including maximum liquefaction distance, thickness of disturbed layer, empirical formulae, and thickness of rapidly deposited sand layer. Then we discuss and analyze these methods in terms of their theoretical basis, advantages and disadvantages, accuracy, applicability and problems. We chose two case studies: first, a typical seismics-related deposit (liquefied layer and disrupted layer) represented by a seismite in the late-Pleistocene Lake Lisan section near Masada in the Dead Sea Basin; and second, the liquefied diapir triggered by an earthquake in the late-Quaternary lacustrine sediments at Luobozhai in the upper reaches of the Minjiang River, East Tibet. The five methods listed above are employed to determine earthquake magnitudes associated with the seismics-related deposit and liquefied diapir, yielding magnitudes of 5.5–6.5 and 6.0–7.0, respectively. The combination of the five methods, provided a new and relatively convenient method for determining seismic shaking, especially in lacustrine sediments. This study can serve as a valid reference for comparing methods of calculating the magnitude of a paleo-earthquake based on surface rupture parameters, and provides a better understanding of the long-term seismic activity and risk in tectonically active regions. [ABSTRACT FROM AUTHOR]

On 7th of August 2020, a Mw = 4.9 earthquake occurred in Mila province (Northeast Algeria). Consequently, more than 2000 houses were indirectly affected (partially damaged or collapsed) within 10 km from the earthquake's epicenter, in the locality of El Kherba. According to field surveys conducted 3 days after the earthquake, damages and casualties were directly linked to an earthquake-induced landslide that occurred after the main shock. This was evidenced by numerous cracks in the site's soil mainly perpendicular to the landslide direction. Local characteristics (i.e., geology, geomorphology) and anthropogenic activities expose this area to landslide hazard. This study shows the reliability of the ambient noise horizontal to vertical spectral ratio (HVSR) method in the characterization of an active landslide. Ambient noise records were processed through spectral ratio techniques to identify the landslide's geometry based on spectral responses analysis. The obtained results allowed the landslide's slip surface mapping and boundary delineation, which correlates well with the field observations. After the rotation of the signal's horizontal components, large variations in the experimental spectral ratios, in both amplitude and direction, revealed site effects (soil remoulding, crack network) on the soil dynamic behavior. [ABSTRACT FROM AUTHOR]

A high steep rock hill with two-side slopes near National Road 213 is used as a prototype in this paper. The full process from initial deformation to sliding of the slope during ground shaking is simulated by a new discrete element method-continuum-based discrete element method. Then, the seismic responses of a high steep rock hill with two-side slopes are researched from the base of time, frequency and joint time-frequency domain using Hilbert-Huang transform and Fourier Transform. The findings are: first, the stress concentration phenomenon occurs at the top of the sliding mass, and then some tension and shear failure points appear, which expand from the top toward the toe of the sliding mass along the structural plane. At the same time, the number of tension failure points gradually increases. Then the toe of the sliding mass fails, and shears out from its toe which results in the landslide. If the material parameters are under the same conditions, the landslide in the middle of the slope occurs before that at the foot of slope, and the starting time of landslide and the arrival time of the peak ground acceleration are synchronous or the former slightly lags behind the latter. The difference of distribution and dissipation of earthquake energy in the sliding body and sliding bed is the major influence factor to induce the landslide. When the accelerations are small, the instantaneous frequency of accelerations between sliding bed and sliding body is generally consistent, the energy transmittance coefficients of the sliding structural plane and the controlled frequency band of the energy all range in a limitation; with the increase of the seismic intensity, the instantaneous frequency and the energy transmittance coefficients gradually decrease, and then they are steady within the lower limitation. At the same time, the controlled frequency band also shifts gradually from high frequency band to the lower one. Based on the input seismic wave, the peak acceleration amplifies as the increase of elevation, regardless of the monitoring points on the steep slope, gentle slope side or inside of the slope. Generally speaking, amplification of the vertical peak acceleration is stronger than that of the horizontal peak acceleration, and amplification of the peak acceleration on a steep slope is stronger than that on a gentle slope, and that of inside of the slope is the weakest amplification. [ABSTRACT FROM AUTHOR]

Large-scale landslides along the Kubusu and Besso rivers in Toyama Prefecture are developed in the Miocene Iwaine Formation, which is composed of andesitic lava, tuff, and tuff breccia. In the middle member of this formation, the tuff is easily altered to montmorillonite-bearing rock, and subsequently plays an important role in the development of landslides events, which tend to be large-scale events, as the massive lava of the upper member forms a cap rock over the tuff. The Kiritani and Koinami basins, which are flat intermontane basins located along the Kubusu and Besso rivers, respectively, are interpreted as landslide-dammed lakes, later filled with sediment. Accelerator mass spectrometry C ages show that the landslides forming each dam occurred simultaneously, at approximately 2500 BP. These ages were measured from wood fragments embedded in the landslide material of Kiritani, and from an in situ stump drowned during the impoundment of Koinami. If the trigger of these landslides was an earthquake, it is most likely to have been the penultimate event along the Atotsugawa fault zone. [ABSTRACT FROM AUTHOR]

Seismic rockfall is one of the prevalent geohazards that cause huge losses in the earthquake-stricken areas. In the present research, a model is developed to map susceptibility (occurrence probability) of seismic rockfalls in a regional scale using Logistic Regression (LR) and Adaptive Neuro-Fuzzy Inference System (ANFIS) techniques. In this research, Firooz Abad-Kojour earthquake of 2004 was introduced as the benchmark and the model base. The susceptible zones predicted by LR and ANFIS methods were compared with the database (distribution map) of seismic rockfalls, by which the results revealed a good overlapping between the susceptible zones predicted by the ANFIS and the field observation of rockfalls triggered by this earthquake. Besides, for the statistical evaluation of results obtained by LR and ANFIS models, the verification parameters with high accuracy such as density ratio (Dr), quality sum (Qs), and receiver-operating characteristic curve (ROC) were used. By analyzing the susceptibility maps and considering the Qs index obtained by LR (21.04184) and ANFIS (26.75592), it could be found that the Qs of ANFIS is higher than that of LR. Moreover, based on the obtained value of the area under the curve (AUC) from LR (0.972) and ANFIS (0.984) methods, ANFIS provided a higher accuracy in zonation and susceptibility mapping of rockfalls triggered by Firooz Abad-Kojour earthquake of 2004 compared to the LR method. [ABSTRACT FROM AUTHOR]