Amplification Analysis and Seismic Vulnerability Index of Air Tawar Barat Village of Padang City Using HVSR Method

Padang City is an area that has a very high level of seismic activity in Indonesia. The city of Padang is also an area severely affected by the earthquake in the Subduction Zone. Considerable earthquakes impact building damage and infrastructure, one of the factors that can see the vulnerability of an area with a seismic vulnerability index. Therefore, a study was conducted to determine and analyze the amplification and seismic vulnerability index in the Air Tawar Barat Village of Padang City and create a map of the natural frequency distribution, amplification, and seismic vulnerability index. Microtremor data retrieval was carried out in February-May 2022 using the Seismograph Sysmatrack MAE and S3S Sensors at Air Tawar Barat Village, Padang City. The method used in this study is the HVSR (Horizontal to Vertical Spectral Ratio) method using geopsy software to remove noise in the data and see the natural frequency value and the value of the amplification factor. Furthermore, from the data, contour maps were made using Surfer 13, where the type of research carried out was in the form of descriptive research. Based on the results of the study found that the natural frequency value (f0) in the study area ranged from 0.59141 Hz – 12.9172 Hz, the amplification (A0) ranged from 2.39825 to 13.7759, and the seismic susceptibility index (kg) was 0.4802 – 320.885. Based on the data processing results, it can be concluded that fresh western water is an area with a high seismic vulnerability index, and the location is very vulnerable when shaken by an earthquake.


Introduction
One of the provinces on the west coast of central Sumatra Island with four active fault segments is West Sumatra.This area is a section of the Sumatra Fault zone [1]- [3].Sumatra Island has a relatively unique tectonic setting, so many geologists and geographers make this island an object of seismic research [4]- [7].Padang City geographically stretches from "00°44'00-01°08'35" South Latitude and "100°05'05-100°34'09" East Longitude.Tectonically, Padang City is also one of the areas that have high levels of seismic activity, which is very high in Indonesia [8].
A quick release of seismic wave energy causes an earthquake.The deformation of tectonic plates that takes place in the Earth's crust is what results in this energy release.This energy is released through a mechanism known as elastic waves, namely seismic waves or earthquakes that travel to the Earth's surface and cause vibrations that harm nearby structures or things [9], [10].Damage to the Earth's crust suddenly causes vibrations that struggle in all directions, propagating to the ground surface.Ground vibration is known as tectonic earthquakes [11].Weak ground vibrations, known as microtremors, can be brought on by both natural and artificial disturbances, including wind, ocean waves, traffic, and industrial gear.Using microtremor data and the HVSR (Horizontal to Vertical Spectral Ratio) technique, determine the amplification value and seismic susceptibility index that describe the dynamic qualities of the soil [12], [13].
Padang City is one of the coastal areas that could experience a big earthquake as a result of tectonic activity in the subduction zone to the west of this region.The epicenter of one of the 7.6-magnitude earthquakes that occurred on September 30, 2009, was at a depth of 71 km, with coordinates 99.65° East Longitude and 0.84° South Latitude (57 km southwest of Pariaman -West Sumatra) [14], an earthquake that caused light to heavy damage to residential buildings and high-rise buildings owned by the government and private sector in the city of Padang.
The geological conditions of a location have an impact on earthquake damage in addition to the earthquake strength's magnitude [15].Areas with thick, brittle Sediment on top of solid bedrock are those that are most vulnerable to earthquake damage [16].An area's vulnerability to earthquakes increases with the amount of brittle (smooth) rocks present.This is due to the fact that rocks in dangerous regions typically aren't compact, which causes them to decay.In the event of an earthquake, the damage it causes will be far more severe [17], [18].Due to the public nature and frequent use of the Freshwater area, seismic vulnerability index research was conducted to determine the area's susceptibility to earthquake-related calamities.Sediment layers' natural frequency and amplification factor can be determined using the HVSR method's seismic susceptibility index calculation.
HVSR is a method that uses the idea that the ratio of horizontal to vertical vibration spectra on a surface defines a displacement function.It also demonstrates that if the microwave waveform is detected on three components, two horizontal components and one vertical component, one may roughly measure the dynamic properties of the surface layer at the observed position [19], [20].The HVSR method compares the spectral ratio of the microtremor signal's vertical and horizontal components.The HVSR study shows a spectrum peak at the natural frequency (fo), and the amplification factor (Ao), which characterizes the dynamic qualities of the soil, may be constructed from these data [21].
The frequency value that regularly occurs and has been determined to be the frequency of the nearby rock layers is known as the natural frequency.This frequency value can be used to infer the composition of the rock.The results of the simulation test conducted by [22], which makes use of six straightforward geological structure models and a combination of shear wave velocity contrast fluctuations and soft layer thickness, demonstrate that the peak frequency values vary depending on the specific geological conditions.According to studies, the dominant frequency value found there can be used to infer the characteristics of the rock beneath the surface.Based on the prevailing frequency value, this is depicted in the soil categorization table [23].
Amplification is the maximum amplitude value obtained from data processing.The amplification value is influenced by geological formations, thickness, and physical properties of soil and rock layers, such as rocks undergoing weathering, folding, and shifting (deformation) that change the physical properties of rocks.The classification of amplification factors Pursuant to Ratdomopurbo in [24], [25].
The seismic vulnerability index (Kg) is an index that describes the degree of susceptibility of the topsoil layer to deformation during an earthquake.According to Nakamura [26], the seismic susceptibility index is calculated as the peak value of the microtremor spectrum divided by the resonance frequency.
The natural frequency is F0, the amplification factor is A0, and the seismic vulnerability index is Kg.Previous research has demonstrated a relationship between earthquake damage and kilogram value.A high seismic vulnerability index is associated with significant earthquake damage, according to the study's assessment of the seismic vulnerability index values [27], [28].
When the surface soil layer is deformed by the shear-strain boundary, destructive earthquakes happen [29].The vulnerability index can give details on the potential intensity of shock brought on by an earthquake in a location and is also used to predict weak zones during earthquakes.The local effect that causes damage during earthquakes collaborates with the microtremor HVSR parameter, which is characterized by a low soil dominant frequency (f0) (high period) and a high soil amplification factor (A0).The susceptibility index (Kg) shows the relationship between the amplification of the soil spectrum (A0) and the natural frequency (f0).

Research Methods
In the Air Tawar Barat Village region of Padang City, microtremor measurements were made.From February-May 2022, there were 9 points with a distance between points of 300-500 meters.Measurements are made using the Seismograph Sysmatrack MAE and S3S Sensor with a duration of ± 40 minutes.Data processing can be described through a flow chart as shown in Figure 1: In order to calculate the seismic vulnerability index, it is crucial to first understand the value of the natural frequency and the amplification factor at each data collection station (see Figure 1) (kg).The values for the natural frequency and amplification factor are obtained by using the HVSR data processing method.We can examine the Air Tawar Barat Village and determine if the region is vulnerable to earthquake shocks after acquiring the seismic vulnerability index result.
Geopsy software is used to process microtremor data using the HVSR approach.The HVSR approach contrasts the horizontal and vertical components of the spectrum [30].It is filtered to reduce noise.The next step is to do windowing manually to get the expected signal.Manual selection is conducted to select a signal within a certain time window that is stationary and has little noise.The stationary signal is a signal with the same amplitude and does not change with time [31].After the windowing process had been completed, the signals consisting of (North-South), (East-West), and Top-Bottom, were analyzed using HVSR.It is possible to calculate the values of natural frequency (f0) and amplification (A0).Equation 1 can be used to determine the seismic vulnerability index, and a map can be generated in Surfer to show the natural frequency distribution, the amplification, and the seismic vulnerability index.

Result and Discussion
Research in Air Tawar Barat Village with a geographical location of 0°52'21.38"S-0°53'58.88"Sand 100°20'41.35"E-100°20'59.69"E was carried out from February to May 2022.By detecting the microtremor signal, the dominant frequency parameter (f0) and the amplification value (A0), which are utilized to examine the soil properties in the research region, are produced.The distribution map of these values, as well as the seismic susceptibility index, is then formed (Kg).
The study's findings are displayed at Air Tawar Barat Village, Padang City, in the form of a natural frequency distribution map, amplification, and seismic risk score.The microtremor data consists of three parts: a horizontal (east-west), a vertical (north-south), and a horizontal (vertical) component.Microtremor data were analyzed by sensory geopsy, namely by windowing and cutting for signal selection without noise.Figure 2 shows the microtremor data at measurement point 5.For other measurement points, see the Appendix.Choosing a stationary window on each Fourier spectrum component (FTT) component to transfer the signal into the frequency domain in each window is the first step in the HVSR method for processing microtremor data.A Konno Ohmachi smoothing filter is used to smooth the FFT results with a bandwidth coefficient of 40.Horizontal component data (N-S and E-W) be amalgamated by calculating the average spectrum of each component so that the H component is obtained.In the frequency domain, the horizontal component data is divided by the vertical component to provide the H value.For each window, H/V. Figure 3 shows the H/V curve calculated by averaging the H/V measurements of each component.4 (e).5 (f).6 (g).7 (h).8 (i).9.
The HVSR curve from measurement points 1 to 9 is depicted in Figure 3. Natural frequency and amplification can be determined with the use of the aforementioned curve.Figure 4 depicts the magnitude of the enhancement.According to Figure 4, the Air Tawar Barat Village's amplification ranges from 2.39825 to 13.7759.The measurement point 5 area, which is in front of Basko Mall, has the highest amplification value.In contrast, measuring point 8 near the Faculty of Sports Science Field had the lowest amplification value.Areas that are prone to building damage due to earthquake vibrations are areas whose surface is composed of soft Sediment (peat, sand, silt) with hard bedrock.Because in geological conditions like this, the impedance contrast (difference between sedimentary layers and bedrock) is significant.Table 1 displays the findings for all of the measurements taken.According to Table 1, there are zones with relatively low, medium, and very high soil amplification values in the research area.Areas with a low soil spectrum amplification value, namely with an A0 value of less than 3.While the zone area has a relatively moderate soil spectrum amplification value, the amplification value ranges from 3 to 6. Furthermore, the amplification value is significant for relatively very high areas at 9.
In research conducted by [32], the amplification value obtained ranges from 0 to 15.For plains composed of alluvial, alluvial deposits in coastal areas tend to amplify between 3 and 6.Similar to what was observed in the Air Tawar Barat Village study, the amplification found along the coas ranges from 3 to 6.The obtained amplification ranges from 2.39825 to 13.7759.The results obtained in the research area conclude that it is close to the value in previous research.Under the soil surface, a thick layer of Sediment typically dominates alluvial deposits.In contrast, a medium to a dense layer of sand dominates the soil layer in coastal areas.
The amplification value indicates a change in the impedance of the layer and the possibility of a change in rock density caused by changes in rock cohesiveness.This makes areas with high soil amplification factor values vulnerable to earthquake shocks.Amplification and the proximity of bedrock to the ground surface are not the only factors that determine the extent of structural damage caused by an earthquake.
The peak value of the HVSR curve and a reflection of the hard and soft physical characteristics of the Sediment is the amplification factor of the soil spectrum (A0).The sediment layer should be more complicated if the soil spectrum's amplification factor is low.Larger areas will be affected by earthquake shocks, and hence there is a greater chance that buildings in the area will be damaged if the value of the soil spectrum's amplification factor is high.
The results of the distribution of the amplification value in Air Tawar Barat Village are also depicted in Figure 5. Based on Figure 5, the microtremor measurement data ranged from 0.59141 Hz to 12.9172 Hz.The value of the seismic vulnerability index (kg) can also be calculated using equation (5), which is displayed in Figure 6 as a map from the amplification value (A0) and the natural frequency value (f0).According to Figure 6 and the analysis's findings, Air Tawar Barat Village's seismic vulnerability index value ranges from 0.4802 s2/cm -320.885s2/cm.
The Air Tawar Barat Village's seismic vulnerability index was visualized using surfing software.The natural frequency and amplification values must first be obtained before the seismic vulnerability index can be calculated by dividing the fair value of the amplification against the natural frequency.By combining the natural frequency values with the Classification Table Soil pursuant to the Dominant Frequency Value of Microtremor Pursuant to Kanai and Omete-Nakajima, it is possible to locate the majority of the places of Air Tawar Barat Village.
Rock types II and IV make up the majority of the rocks in Air Tawar Barat Village, but there are also a few tertiary rocks, including rock type I at measurement points 9, 3, and 4. With tertiary or older rocks, the natural frequency runs from 6.7 to 20 and is made up of hand-sanded rocks, gravel, etc.With the complex soil makeup and a thin sediment layer dominated by rocky outcrops.
In sedimentary regions with the same hardness and soil type, the natural frequency is inversely related to the thickness of the Sediment.A very high natural frequency value is correlated with an area with a low sediment thickness, and vice versa for areas with a low natural frequency value are correlated with a high sediment thickness.Table 2 shows that the highest natural frequency is at point 8, 11.9769, while the lowest natural frequency is at point 5, which is 0.59141.Measurement point 1 and measurement point 6 indicate the type of soil II with a frequency range of 4 Hz to 6.7 Hz composed of sandy gravel, sandy hand day, and metal with surface sediment thickness ranging from 5 to 10 meters.
For measuring points 5, 7, and 2, soil type IV and a frequency range of less than 2.5 Hz are often generated from topsoil, muck, and deltaic sediments at a depth of 30 m or more.Additionally, the thickness of the surface silt at this measurement site is abnormally thick in comparison to previous observations.The Table of Soil Classification also displays the main frequency value of microtremors according to Kanai and Omete-Nakajima.(1983), Pursuant to Lacet and Brad (1994), who have carried out simulations with results showing that from the natural frequency values measured on the surface, the characteristics of the rock below can be known, and these results are in the table and map of the natural frequency distribution, which can be seen in Fig. 4.After obtaining the frequency value for each measurement point, a seismic vulnerability index analysis is carried out.
According to research done in Padang City by [33], natural frequencies in the area ranged from 0.42 to 12.12 Hz.While this was going on, the study at Air Tawar Barat Village was able to determine the natural frequency by identifying the HVSR spectrum peak that was produced as a result of processing a three-component microtremor, with a value of f0 ranging from 0.59141 to 12.9172 Hz.The results obtained in the research area conclude that it is close to the matter in previous research.
Due to alluvium deposits near the coast, the f0 value obtained is getting lower towards the coast.After receiving the frequency value of each measurement point, a seismic vulnerability index analysis was carried out.
Equation 1 is used to determine the seismic vulnerability index value for each measurement point in Table 3's measurement table.3. The value for the seismic vulnerability index ranges from 0.4802 to 320.885.Figure 6 depicts the distribution of the seismic vulnerability index for Air Tawar Barat Village based on Table 3 data.The Seismic Vulnerability Index Distribution Map of Air Tawar Barat Village is studied in accordance with the Seismic Vulnerability Index Classification Table in Figure 6 [34].
At measurement points seven around Parakeet Road, 8, 2 around Enggang Road, and five around Basko Mall Front Road, the seismic vulnerability index score is comparatively high.Meanwhile, the seismic susceptibility index value is relatively low, namely at eight measurement points around the Field of the Faculty of Sports Science, four measurement points around the Tourism and Hospitality Faculty, three measurement points around the Social Sciences Faculty, and nine measurement points around the Chemistry Laboratory.Therefore, the Padang State University area includes a low level of vulnerability.The amplification value and natural frequency have an impact on the value of the seismic vulnerability index.
According to the study, Padang City's seismic risk index rating, which ranges from 0.58 to 170.61, is evaluated as high [35].Meanwhile, the seismic vulnerability index value derived from the study in the Air Tawar Barat Village ranged from 0.4802 to 320.885.The relatively high seismic susceptibility index values are typically found in regions with a predominance of alluvium deposits covering the underlying geological conditions.At the same time, the Air Tawar Barat Village is located in a region where alluvium deposits predominate in terms of geological conditions.The seismic vulnerability index will be higher in regions with relatively low natural frequency values and higher amplification values.
Areas with high amplification and low natural frequency values have high seismic susceptibility index values.High amplification values are associated with high impedance contrast, and small natural frequency values are associated with areas of high thickness and compactness.Low seismic susceptibility index values are obtained in areas with low amplification and thin sediment layer thickness.These areas are generally located in hilly areas.The seismic susceptibility index value may be low for the research area without hills because of the low amplification value and high natural frequency value, and vice versa.Pursuant to [36], areas with a low seismic vulnerability index will be more resistant to earthquake hazards, so the probability of damage is lower when an earthquake occurs.Pursuant to [37], rugged rocks have a high vulnerability index value, while coastal locations have a higher vulnerability index because they have more alluvium deposits.
The soil amplification factor (A0) value is more influenced by impedance contrast, resulting in a high vulnerability index value in this region.A place is more earthquake-prone if the seismic vulnerability index score is higher.According to [38], the high seismic vulnerability index score suggests that some of these sites would be considered hazardous in the case of an earthquake shock.

Conclusion
The value of the amplification factor in the Air Tawar Barat Village ranges from 2.39 to 13.7, according to the findings of the study and talks that have been conducted in the village.Measurement point 5 has the largest amplification, whereas measurement point 8, the faculty's field area, has the lowest amplification.The seismic vulnerability index in the Air Tawar Barat Village ranges from 0.48x10-6 s 2 /cm -320 x10-6 s 2 /cm.The front room of the basin had the highest seismic vulnerability index score, which was apparent around point 5.If an earthquake shock occurs, the point 5 area is the most vulnerable compared to other regions of the Air Tawar Barat Village of Padang City.

Figure 2 .
below is an example of microtremor data from measurements in the field.

Figure 4 .
Figure 4. Map of the distribution of amplification factors in Air Tawar Barat Village.

Figure 5 .
Figure 5. Map of a natural frequency distribution of Air Tawar Barat Village.

Figure 6 .
Figure 6.Map showing the seismic vulnerability index distribution in Air Tawar Barat Village.

Table 2
also displays the natural frequency values for each measurement location.

Table 3 .
Seismic Vulnerability Index Value Data.