ALBERT

Description: Empirical relationships between the macroseismic intensity and the ground-motion parameters for the Himalayan region are derived in this study. A strong-motion database from 21 moderate-to-large earthquakes, along with their corresponding macroseismic intensity, is considered. All the intensity values are inferred from isoseismal maps and earthquake damage reports and then converted to the modified Mercalli intensity (MMI) scale. An orthogonal regression analysis is used to find the best correlation between MMI and peak ground acceleration (PGA), peak ground velocity (PGV), and pseudospectral acceleration (PSA) at 0.3, 1.0, 2.0, and 3.0 s to accommodate the uncertainty in the regression coefficients. In addition to the ground-motion parameters, the MMI is related to other potential independent variables, including the moment magnitude, the hypocentral distance, and the 30-m average shear-wave velocity ( V S 30 ). The study shows that site effect is noticed predominantly in the MMI–PGA relationship for a hypocentral distance of more than 200 km. When relating MMI with PGV, PSA 0.3 s , PSA 1.0 s , PSA 2.0 s , and PSA 3.0 s , however, the contribution of site effects to the correlation is negligible. To eradicate the site effect in the PGA–MMI relationship, the derived empirical relationship is modified, based on the statistical analysis of MMI observed and MMI predicted, with or without including V S 30 as a potential independent variable. Furthermore, the developed regression models are verified using several statistical tests, the F -test, the t -test, the Durbin–Watson test, and the Breusch–Pagan test. Additionally, the Euclidean distance concept is evaluated in the study. It was concluded that the PGA is a good indicator for deriving the MMI value in the Himalayan region, but that one should use site-specific MMI versus PGV and MMI versus PSA relationships to predict reliable parameters.

Description: Back-azimuth analysis is a classical method used in seismological studies for the determination of the direction (azimuth) of an otherwise-unknown seismic source. This is accomplished through the analysis of three-component (3C) data according to a procedure based on the evaluation of Rayleigh-wave polarity and is typically accomplished by assuming a retrograde motion. The same principles are here considered in the framework of the analyses performed while considering active data collected by a 3C geophone, thus enabling us to easily and unambiguously compute the Rayleigh-wave particle motion (RPM) frequency curve that describes the motion of Rayleigh waves as a function of the frequency. The analyses performed for three test sites characterized by different stratigraphic conditions show that, contrary to the common assumption, in the considered frequency range (about 2–40 Hz), prograde motion is actually quite common. Although for two of the three presented case studies we consider a single-offset acquisition, in order to evaluate the variations of the RPM frequency curve as a function of the offset, we also considered two multi-offset datasets (one synthetic and one from a field acquisition). Results seem to indicate that, although some dependency on the offset necessarily exists, the overall trend is a characteristic of the site. Potential applications of the described approach are discussed in particular with respect to seismic-hazard studies, as well as in the light of the exploitation of the RPM frequency curve for better constraining the subsurface model. Electronic Supplement: Animations of particle motion.