Determination of earthquake prone zones at university of tadulako based on dominant periods and peak ground acceleration (PGA)

University of Tadulako is the largest State University in Palu City. When an earthquake with a magnitude of 7.4 Mw on September 28, 2018, occurred, many buildings were damaged and even collapsed at that time, even causing casualties due to the rubble. Research on the Local Site Effect is essential for the assessment of seismic hazard. In this study, the local site effect was analyzed using the HVSR method based on microtremor data. The predominant Period (To) ranges between 1.709 s to 3.816 s, indicates that this area consists of alluvium and has a very thick sediment layer. Another parameter calculated in this paper is the peak ground acceleration (PGA) with values from 0.914 g to 0.924 g. This value is the first indicated soil damage level due to ground motions. The results of this study can be used as a consideration in the development of regional spatial planning and building structures based on earthquake analysis.

Vulnerability Analysis of Earthquake Hazards in Tasikmalaya City Using Horizontal to Vertical Spectral Ratio (HVSR) Method

Tasikmalaya City is one of the regions in West Java Province that is often hit by earthquakes due to its location near the Indo-Australian Plate subduction zone towards the Eurasian Plate. The surface deposits in this city are alluvium and weakly consolidated step deposits which can cause wave amplification during an earthquake. As a mitigation effort, seismic zoning needs to be carried out to map the areas that will experience heavy damage when an earthquake occurs. This study uses the Horizontal to Vertical Spectral Ratio (HVSR) method which is applied to the microtremor recording data to obtain spatial variations in the predominant frequency and amplification values that can explain the characteristics of the geological layer beneath the surface. Based on the obtained results, the predominant frequency ranging from 0.7 to 9.5 Hz with the lowest frequency distribution in the eastern and northwestern parts, which indicates a thicker sediment layer. Amplification ranging from 1.2 to 12.6 with the distribution of higher values in the eastern, southeastern, and northwestern parts. The inversion of the HVSR curves was carried out to determine the value of shear wave velocity (V s ) in order to obtain a more detailed subsurface geological structure that can be used to determine the level of vulnerability of earthquake hazards. The Neighborhood Algorithm is used to find an optimum model. Based on the results of the inversion process, the V s ranging from 150 - 3054 m/s with lower V s values in the eastern, southeastern, and northwestern parts at depth of about 25 meters. The average value of shear wave velocity at a depth of 30 meters (V s 30) can also be used to determine the type of soil for geotechnical study. From the obtained V s 30data, the types of soil in the research area are classified into moderate soil, hard soil, and rocks.

Microzonation of Cisarua District Using Horizontal Vertical Spectral Ratio

The Bandung region is part of the framework of the Indonesian tectonic system, namely the tectonic plate meeting zone, where the Indo Autralia plate is infiltrated under the Eurasian plate in a convergent manner. The subduction process produces an effect in the form of an active fault geological structure in the Bandung area. One of these active faults is the Lembang Fault, which has a length of ± 29 kilometers and a shear acceleration of 3 to 5.5 millimeters per year. The microtremor measurement method is a passive geophysical method that utilizes natural subsurface vibrations so that it can provide dominant frequency data and amplification factors for soil layers. Based on the results of seismic susceptibility research using microtremor measurements using the HVSR method in the Lembang Fault zone in Cisarua Sub-District, it can be seen that the distribution of the dominant frequency values tends to be influenced by lithology and topography. In the research area, it is known to have a dominant frequency value that varies due to the different types of lithological units. In general, the dominant frequency ranges from 1-3 Hz because it is dominated by tuff sand and tuff pumice, and areas composed of volcanic breccias have a dominant frequency value between 3-6 Hz. Meanwhile, the amplification factor value will be influenced by rock deformation and weathering. The area that has a very high amplification factor value is in the southeast of the study area with an A0 value greater than 5. This indicates that the area is composed of a layer of thick and not dense tuff sand

Characterizing Fundamental Resonance Peaks on Flat-Lying Sediments Using Multiple Spectral Ratio Methods: An Example from the Atlantic Coastal Plain, Eastern United States

ABSTRACT Damaging ground motions from the 2011 Mw 5.8 Virginia earthquake were likely increased due to site amplification from the unconsolidated sediments of the Atlantic Coastal Plain (ACP), highlighting the need to understand site response on these widespread strata along the coastal regions of the eastern United States. The horizontal-to-vertical spectral ratio (HVSR) method, using either earthquake signals or ambient noise as input, offers an appealing method for measuring site response on laterally extensive sediments, because it requires a single seismometer rather than requiring a nearby bedrock site to compute a horizontal sediment-to-bedrock spectral ratio (SBSR). Although previous studies show mixed results when comparing the two methods, the majority of these studies investigated site responses in confined sedimentary basins that can generate substantial 3D effects or have relatively small reflection coefficients at their base. In contrast, the flat-lying ACP strata and the underlying bedrock reflector should cause 1D resonance effects to dominate site response, with amplification of the fundamental resonance peaks controlled by the strong impedance contrast between the base of the sediments and the underlying bedrock. We compare site-response estimates on the ACP strata derived using the HVSR and SBSR methods from teleseismic signals recorded by regional arrays and observe a close match in the frequencies of the fundamental resonance peak (f0) determined by both methods. We find that correcting the HVSR amplitude using source term information from a bedrock site and multiplying the peak by a factor of 1.2 results in amplitude peaks that, on average, match SBSR results within a factor of 2. We therefore conclude that the HVSR method may successfully estimate regional linear weak-motion site-response amplifications from the ACP, or similar geologic environments, when appropriate region-specific corrections to the amplitude ratios are used.

The Soft Layer Thickness Estimation using Microtremor Measurement to Identify Landside Potential in Watukumpul, Central Java, Indonesia.

Watukumpul is an area that is prone to landslides, so determining the soft layer thickness is very important to identify the landslide potential. The soft layer thickness can be estimated using microtremor signal measurements which analyzed using the Horizontal to Vertical Spectral Ratio (HVSR). In this study,we measured microtremor signal of 33location around Watukumpul, Pemalang, Central Java area to determine soft layer thickness. Micretremor signal was analyzed based on theHVSR method using Geopsy software and follow the standard of the Sesame Europan Project. The results of the HVSR method are the HVSR curve that fulfills the reliable curve standard. HVSR curve shows that the dominant frequency of soft layer ranges from 1.36 – 7.62 Hz and the amplification values ranges from 9.00 – 41.45. The soft layer thickness value in the study area ranges from 17.58 - 103.60 meters. The high landslide potential area are located at W7, W8, W18, W30 and W32 where has thin soft layer and high soil slope.

Seismic Vulnerability Indices for Ground in Derince-Kocaeli (Turkey)

Derince town is one of the most densely industrialized (oil refineries, ports, etc.) and populated urban areas which is located in one of the highest seismically active regions in Turkey. The area was damaged by a severe earthquake on 17 August 1999 in Kocaeli (Mw = 7.4). The geotechnical properties of layers play the most important role in the formation of damage. One of the weakness indicators for the soil structure in a region is the ground vulnerability indices (Kg). The reduction of damage from earthquakes is related to the knowledge of these ground properties. Therefore, the microtremor horizontal-to-vertical spectral ratio (HVSR) method was applied to single site measurements at 43 stations over an area of 40 km2 to evaluate local site effects in terms of ground vulnerability indices (Kg), fundamental frequency and amplification factor. Structural damage occurring during the August 17, 1999 Kocaeli (Mw=7.4) earthquake is compared with the estimated ground vulnerability indices. The results indicate that the Kg values are in good agreement with damage distribution. Large Kg values indicate weak points in the study area. According to results, the areas with Kg values greater than 14 seem to be the most vulnerable locations in the study. The Kg and soil types overwhelmingly comply with each other very well. Poor ground conditions are seen in areas with high vulnerability. Ground conditions should be taken into account during the planning and design of urban areas. The results obtained by considering ground conditions can be used as a quick method to identify risky areas.

Seasonal variations in amplitudes and resonance frequencies of the HVSR amplification peaks linked to groundwater

SUMMARY Following the installation of a temporary seismological network in western Greece north of the Gulf of Patras, we determined the quality of the sites of each of the 10 stations in the network. For this, we used the horizontal-to-vertical spectral ratio (HVSR) method and calculated an average curve over randomly selected days between 0 and 10 Hz. The daily HVSR curve is determined by the HVSR 12-hr calculation (1 hr every two) without distinction between seismic ambient noise and earthquake signal. The HVSR curves obtained can be classified in three categories: flat curves without amplification, curves with a amplification peaks covering a large frequency range, and curves with one or more narrow peaks. In this third category C3, one station has one peak, two have two and one has three. On the contrary of what it is commonly assumed, the amplitudes and the resonance frequencies of these narrow peaks are not stable over time in C3. We determined the maximum of the amplitude of each peak with the corresponding central frequency for each day during 2.5 yr. Except for the station with three peaks, which finally appears stable within the uncertainties, the principal peak exhibits a seasonal variation, with a maximum in winter and a minimum in summer, the observations being more dispersed during winter. The second peak, when it exists, varies in the same way except at one station where it varies oppositely. These variations are clearly correlated with the loading and unloading cycle of the underlying aquifers as shown by the comparison with water level and yield measurements from wells located close to the stations. Moreover, they are also correlated with the vertical surface displacements observed at continuously recording GPS stations. The dispersion of the observed maximum amplitude in winter is probably related to the rainfall and the soil moisture modifying the S-wave velocity as revealed by other studies. From this study, we would like to emphasize that the use the HVSR method to constrain the S-wave velocity and the thickness of the sediment layer over the bedrock in the basin, has to be done with caution. Upon further confirmation of its robustness, the HVSR methodology presented here could be a good and easy-to-use tool for a qualitative survey of the aquifer backdrop and its seasonal behaviour, and of the soil moisture conditions.