An Empirical Correlation between the Residual Gravity Anomaly and the H/V Predominant Period in Urban Areas and Its Dependence on Geology in Andean Forearc Basins

The study of site amplification effects is crucial to assess earthquake hazards that can produce great damage in urban structures. In this context, the gravity and the ambient noise horizontal-to-vertical spectral ratio (H/V) are two of the most used geophysical methods to study the properties of the subsoil, which are essential to estimate seismic amplification. Even though these methods have been used complementarily, a correlation between them has not been thoroughly studied. Understanding this correlation and how it depends on geology could be important to use one method as an estimator of the other and to make a distinction between the seismic and gravimetric basement. In this research, a comparison between the residual gravity anomaly and the H/V predominant period is performed using a long dataset from different projects on sedimentary basins in a group of the most important cities in Chile. To simplify the geological information, a seismic classification is used for soils, which considers the Vs30 and the predominant period of vibration (T0). The results of this comparison show a direct correlation between both parameters, the higher the negative residual gravity anomaly the higher the H/V predominant period. This correlation improves when only soft soils are considered, increasing the R2 value in more than a 50% in all the individual cities with respect to the overall correlation. When all the cities are considered, the R2 value for soft soils increases up to 0.87. These results suggest that the ideal geological background for this correlation is when a soft soil layer overlies a homogeneous bedrock. Heterogeneities in the bedrock and in the soil column add dispersion to the correlation. Additionally, the comparison between the depth to basement inferred by both methods show differences of less than 15% in soft sites; in denser sites, the difference increases up to 30% and the definition of a clear H/V peak is more difficult. In general, the gravimetric basement is deeper than the seismic one. However, gravimetric depths to basement can be under/over-estimated in zones with a heterogeneous soil column.

Issues related to velocity structure estimation in small coastal sedimentary plains - Case of Tottori plain facing the Sea of Japan

Issues of predominant period of ground motion and derived underground velocity structure model were investigated in the coastal plains affected by the soft sedimentary layer after the last ice age. Specifically, it is found that two predominant periods due to the shallow soft sediments and deep sedimentary layers over the seismic bedrock created by the tectonic movement after the quaternary period are close in a small plain such as the Tottori Plain, Japan as an example. It was shown by the analysis of underground velocity structure derived from H/V spectrum ratio of earthquake ground motions with the diffuse wave field theory. It is feared that the interaction of close predominant periods due to the different layer boundaries with high contrast may amplify the seismic motion in the period range that affects building structures in the small plains in coastal area.

Estimation of Ground Profiles Based on Microtremor Survey in the Bangkok Basin

Multiple earthquakes have been felt in high-rise buildings in Bangkok despite the epicenters being far away. Seismic wave recordings in the Bangkok basin show a low-frequency peak. This study uses horizontal-to-vertical spectral ratio (HVSR) analysis and array analysis of ambient vibration data to find the predominant period of the ground and the shear wave velocity profiles at five sites in Bangkok. The accuracy of the accelerometer used for the ambient data recording was verified by comparing results with velocity-meter results. The estimated predominant period was within 0.68–0.86 s. From the array records, dispersion curves of the Rayleigh-wave phase velocity were extracted and inverted for the deep layers. The results show that the shear-wave velocity of the top clay layer is low (82–120 m/s) at depths of 11–14.3 m. The low-frequency peak in the HVSR of the earthquake data, and the sediment layer with low shear-wave velocity implies that Bangkok is at risk of amplification of long-period earthquake waves.

Energy- and Predominant-Period-Dependent P-Wave Onset Picker (EDP-Picker)

An energy- and predominant-period-dependent (EDP) P-wave onset automatic picking (EDP-picker) algorithm is proposed to deal with the problem of inaccurate P-wave onset picking in cases in which the P-wave onset is hidden in high-amplitude ambient noise or the energy difference between the seismic P-wave and ambient noise is indistinguishable. The algorithm evaluates the energy change using a characteristic variable ΔE, which describes the energy increment of the P wave above ambient noise. The period change is evaluated using two variables with respect to the predominant period, namely Tpd as proposed by Hildyard et al. (2008) and ΔTpd as the gradient of Tpd. The EDP-picker algorithm has two steps: (1) threshold-based cursory P-wave onset picking and (2) precise P-wave onset picking using an Akaike information criterion function, in which both energy information and period information are considered. All three parameters are determined in a 1 s sliding window. The proposed algorithm is verified on a large dataset comprising 13,481 vertical strong ground motion records for 570 events selected from K-NET (Japan) and China Strong Motion Networks Center data. For all records with an epicentral distance of less than 150 km, 93.5% of residuals of manual picks and auto picks are within ±0.5 s. The results demonstrate that EDP-picker is robust and suitable for real-time systems.

A new structural health monitoring system for real-time evaluation of building damage

The author has developed a new method for evaluating the seismic performance of existing structures from measured accelerations based on the capacity spectrum method. This involves comparing the performance curve, which is the equivalent nonlinear behavior of a simplified single-degree-of-freedom system, and the demand curve, which is the relationship between the response acceleration and displacement spectra. Two telecommunication towers in Japan were instrumented in 2016, and their responses during several earthquakes have been recorded. This paper discusses the evaluation of damage during the two earthquakes. Moreover, parameters such as the predominant period and the required performance are discussed. The proposed system evaluated both towers as being “elastic.” The damping ratios of the towers are very low, which caused the oscillations to continue for more than 5 min after the mainshock of each earthquake because of long-period components of the seismic motion

INFLUENCE OF THE VONGFONG 2014 HURRICANE ON THE IONOSPHERE AND GEOMAGNETIC FIELD AS DETECTED BY SWARM SATELLITES: 2. GEOMAGNETIC DISTURBANCES

Strong meteorological disturbances in the atmosphere, accompanied by the generation of waves and turbulence, can affect ionospheric plasma and geomagnetic field. To search for these effects, we have analyzed electromagnetic measurement data from low-orbit Swarm satellites during flights over the typhoon Vongfong 2014. We have found that there are “magnetic ripples” in the upper ionosphere that are transverse to the main geomagnetic field fluctuations of small amplitude (0.5–1.5 nT) with a predominant period of about 10 s caused by small-scale longitudinal currents. Presumably, these quasiperiodic fluctuations are produced by the satellite’s passage through the quasiperiodic ionospheric structure with a characteristic scale of ~70 km induced by the interaction of acoustic waves excited by the typhoon with the E-layer of the ionosphere. In one of the flights over the typhoon, a burst of high-frequency noise (~0.3 Hz) was observed, which can be associated with the excitation of the ionospheric Alfven resonator by atmospheric turbulence.

INFLUENCE OF THE VONGFONG 2014 HURRICANE ON THE IONOSPHERE AND GEOMAGNETIC FIELD AS DETECTED BY SWARM SATELLITES: 2. GEOMAGNETIC DISTURBANCES

Strong meteorological disturbances in the atmosphere, accompanied by the generation of waves and turbulence, can affect ionospheric plasma and geomagnetic field. To search for these effects, we have analyzed electromagnetic measurement data from low-orbit Swarm satellites during flights over the typhoon Vongfong 2014. We have found that there are “magnetic ripples” in the upper ionosphere that are transverse to the main geomagnetic field fluctuations of small amplitude (0.5–1.5 nT) with a predominant period of about 10 s caused by small-scale longitudinal currents. Presumably, these quasiperiodic fluctuations are produced by the satellite’s passage through the quasiperiodic ionospheric structure with a characteristic scale of ~70 km induced by the interaction of acoustic waves excited by the typhoon with the E-layer of the ionosphere. In one of the flights over the typhoon, a burst of high-frequency noise (~0.3 Hz) was observed, which can be associated with the excitation of the ionospheric Alfven resonator by atmospheric turbulence.

Peak Ground Acceleration and Earthquake Intensity Microzonation in Watukumpul, Pemalang Regency

Watukumpul is located in Pemalang District, Central Java, which is adjacent to the fault seismotectonic line of Baribis fault in the north and subduction area of the Eurasian and Indies-Australian plates in the south. It makes Watukumpul often experiences an earthquake. This study aimed to map the peak ground acceleration calculated using the Kanai equation and earthquake intensity calculated using Wald equations in Watukumpul. This study used historical earthquake data occurred in 1988-April 2018 from the International Seismological Center and microtremor measurements of 33 points. Microtremor data were processed using the Horizontal to Vertical Spectral Ratio method and resulted the predominant period of study area ranged from 0.13 to 0.74 s. The results showed that the study area had a PGA value of 26.93 - 63.25 gal. The intensity calculation showed that the study area has the potential for earthquake damage with an III-IV MMI scale. Keywords: Kanai, Watukumpul, Intensity, Earthquake

Benefits from using two receivers for interpretation of low-strain integrity tests on pipe piles

The double-velocity symmetrical superposition method (DVSSM), which consists of superimposing and averaging two synchronization signals measured at two symmetrical points of the neutral plane of flexural vibration (in an angle of 90°), is proposed to eliminate the high-frequency interference at the pipe pile head without increasing the predominant period of impact pulse. An analytical solution is derived using the transfer matrix method. The calculated responses from the developed solution are compared with the experimental results for different receiving radius angles to evaluate the effectiveness of the developed solution. A parametric study is also conducted to investigate the suitability of the DVSSM. The findings demonstrate that the high-frequency interference is caused by a combination of the flexural behavior of the pile cross section and the wave propagation along the pipe pile head. The flexural vibration mode comprises the primary component of the high-frequency interference, which can be eliminated through the DVSSM without increasing the predominant period of impact pulse. The DVSSM can serve as an efficient method to detect defects near the pile head with much higher detection accuracy than the conventional method.