Spectral Ratio Method Research Articles

Distribusi Kerentanan Seismik di Wilayah Pusat Kota Surabaya

Surabaya is the second major city of Indonesia and the economic capital of eastern Indonesia. The city’s central area is the governmental center of East Java Province. This area is traversed by the Surabaya section of the Kendeng Fault which could potentially generate a maximum M6.5 earthquake. The East Java megathrust zone also threatens this area with a potential maximum magnitude of M8.9 earthquake. The rock geology of this region is dominated by soft alluvial soil which could amplify earthquake shaking. This study aims to identify the distribution of seismic vulnerability index in Surabaya’s central area. Therefore, microtremor measurements were carried out at 61 points in this area. The results were then analyzed using the Horizontal to Vertical Spectral Ratio (HVSR) method to determine the amplification factor values and seismic vulnerability index. The results of the HVSR analysis show that the amplification factor value and seismic vulnerability index are in the low to medium category ranging from 0.8370 - 3.8298 and 0.6041 - 14.6268, respectively. The distribution of the results shows that the northern area is more vulnerable than the southern part. This is verified by the geological conditions of the northern part which is dominated by alluvial soil.

Seismic microzonation and soil-structure resonance analysis in Suryabinayak Municipality, Bhaktapur, Nepal: insights from ambient vibration measurements

Bhaktapur, lying in the Kathmandu Basin, suffered damages during the 2015 Gorkha Earthquake, potentially exacerbated by local site effects. This research addresses the lack of site response study on Suryabinayak Municipality, located in the southern part of Bhaktapur district. Horizontal to Vertical Spectral Ratio (HVSR) and Floor Spectral Ratio (FSR) methods were employed to determine the fundamental frequencies of soil deposits at 241 free field stations and 20 Reinforced Concrete (RC) isolated buildings respectively. The fundamental frequency of soil deposits varies from 0.27 Hz to 10.00 Hz. Higher frequencies were noted near the basin edges, attributed to shallow sediment deposits, whereas a lower frequencies prevailed towards the basin centre due to an increase in sediment thickness. Out of 20 buildings studied, 7 are highly susceptible to soil-structure resonance as the frequency disparity between building and free-field is less than 15%. This research not only quantified the frequency distribution and soil-structure resonance likelihood but also established a correlation between building height and its fundamental frequency. A significant correlation is observed with a coefficient of determination (R2) value of 60.64% and 83.36% in the longitudinal and transverse directions respectively. The study’s results can be endorsed to mitigate seismic hazards, build seismic-resilient structures, and maintain historical monuments.

Non-destructive detection of single corn seed vigor based on visible/near-infrared spatially resolved spectroscopy combined with chemometrics

Seed vigor is an essential quality evaluation index for seed selection. However, accurately detecting the vigor of a single corn seed is challenging. In this study, we constructed a single-fiber spatially resolved detection device using visible/near-infrared spectroscopy to investigate the patterns and correlations between spatially resolved spectroscopy (SRS) at 500–1000 nm and seed vigor. The device collected spectral data at a light source–detector distance of 5–6.6 mm on the embryo side (S1) and endosperm side (S2) of the corn seeds. The proposed spectral ratio method based on SRS and spectral combination analysis achieved an improvement in the detection accuracy of different corn seed vigor. Modeling by SG-CARS-PLSDA using the ratio method showed further improvement in the prediction ability. The highest accuracy for both S1 and S2 in the Zhengdan 958 variety was 91.67 %, while those of S1 and S2 for the Shaandan 650 variety were 86.67 % and 88.33 %, respectively. In addition, SRS was found to be more advantageous in S2 acquisition, verifying the potential of SRS in the non-destructive testing of seed vigor. This provides a favorable reference for the comprehensive evaluation of other internal quality indices of seeds.

A Seismic Monitoring Tool for Tidal-Forced Aquifer Level Changes in the Río de la Plata Coastal Plain, Argentina

Ambient seismic noise has gained extensive applications in seismology and plays a pivotal role in environmental seismic studies. This study focuses on the Río de la Plata Coastal Plain, employing the horizontal-to-vertical spectral ratio (HVSR) method on ambient seismic noise records to analyze subsurface dynamics. The region’s hydrogeology is complex, featuring partially interconnected coastal aquifers. The HVSR analysis reveals two peaks, with P0 associated with the sediment-basement interface and P1 linked to a shallower stratigraphic discontinuity. Temporal analysis of P1 highlights cyclical patterns correlated with estuarine levels, suggesting a relationship between variations in seismic velocities and tidal dynamics. Comparisons with aquifer data support the hypothesis that tidal variations influence subsurface mechanical properties, impacting the HVSR function. The study hints at the potential of ambient seismic noise analysis as a non-invasive and cost-effective method for studying coastal aquifers and understanding groundwater dynamics. Ongoing research aims to further explore these relationships for enhanced groundwater resource management.

Estimation of Seismic Attenuation and Gas Hydrate Concentration From Surface Seismic Data at Hydrate Ridge, Cascadia Margin

AbstractAn improved understanding of the effects of gas hydrate presence on seismic attenuation is important for accurate hydrate characterization and quantification. Based on a rock‐physics model recently presented for gas hydrate‐bearing fine‐grained clay‐dominated sediments, here we establish an integrated workflow for surface seismic data from extracting seismic attenuation to estimating gas hydrate concentration (Ch) in the sediment. We apply this workflow to the high‐resolution seismic data acquired at southern Hydrate Ridge, offshore Oregon, to reveal the hydrate distribution and clarify the controlling factor of hydrate formation. We first present an adaptive‐bandwidth spectral ratio method to robustly measure attenuation. The attenuation measurements show that the presence of hydrate suppresses the attenuation of the host sediment. We then calculate Ch by applying the rock‐physics model to the attenuation measurements. The estimated Ch are mostly low (<5%) in Hydrate Ridge and agrees well with the in‐situ Ch measured from core‐ or well log‐based data. Our result also suggests that the lithology and stratigraphic structures together control the distribution of gas hydrate at Hydrate Ridge, where relatively high Ch is found in the region where a gas‐charged conduit exists and in an anticlinal structure overlying a strong bottom simulating reflection. Adjacent to the anticline, however, a low amount of gas hydrate appears present, possibly due to the gas migration blocked by the anticlinal structure or the lack of gas conduits. Our study offers an effective strategy for detecting and quantifying gas hydrate in fine‐grained clayey sediments through surface seismic data.

Source Parameter Scaling Relations for Shallow Crustal Earthquakes: Exploration With the Single Asperity Model

AbstractUnderstanding the scaling laws of source parameters is a fundamental subject in seismology. This study conducts spectral ratio analysis to estimate the source parameters for 409 shallow crustal earthquakes with Mw 3.2–6.0 in Japan. Subsequently, the source parameter scaling relations are investigated for a wide magnitude range by combining the results of this and previous studies. The spectral ratio method applied in this study provides the finite source properties of the localized area with large slip. The single asperity model, a simple heterogeneous source model with a single localized area with large slip, is used to estimate the stress drop since it can be more suitable to characterize earthquake sources than the standard homogeneous circular source. This investigation calibrates the constant to link the corner frequency and the source radius by comparing the corner frequency and source area estimated by the spectral ratio analyses. This calibrated constant is used to re‐calculate the stress drop of small earthquakes from the corner frequency estimated in previous studies. The stress drop and apparent stress increase with increasing magnitude up to Mw ∼ 5 and become magnitude‐independent for larger earthquakes. The radiation efficiency ranges typically from 0.1 to 1.0 and is independent of magnitude. The slip dependences of the stress drop, apparent stress, radiation efficiency, and fracture energy observed in this study are explained by the ones predicted from the slip‐weakening model incorporating the thermal pressurization effect. Thermal pressurization is one of the possible mechanisms explaining the observed source parameter scaling relations.

MICROZONATION ANALYSIS IN MANNA CITY & PASAR MANNA SUBDISTRICTS UTILIZING MICROTREMOR DATA, SOUTH BENGKULU REGENCY

South Bengkulu is prone to earthquakes because it is located on the Great Sumatran Fault. Apart from that, it is also influenced by the activity of the Musi and Manna segments, so that besides being influenced by the two active tectonic plates it is also influenced by the local segment. This research is located in Manna City and Manna Market, which is mostly inhabited by people and is the capital of South Bengkulu. In connection with infrastructure development, it is necessary to know the condition of the soil layers to minimize the risk of building damage due to earthquakes. This research aims to analyze earthquake-prone areas based on dominant frequency parameters, soil amplification, seismic vulnerability index and sediment layer thickness (h). Research data comes from microtremor measurements at 60 measuring station points in Manna City and Manna Market. The tool used is a Short Period Portable Seismometer Model Gemini 2 SN-1405. This microtremor data is processed using the Horizontal to Vertical Spectral Ratio (HVSR) method. The results of data processing show that the research area has the lowest dominant frequency of 3.95 Hz and the highest of 49.71 Hz. The ground vibration amplification value is classified as small to medium, the lowest is 1.3 and the highest is 5.19. The lowest earthquake vulnerability index was 0.18 cm2/s and the highest was 2.24 cm2/s. The smallest sediment thickness was 3.77 meters and the thickest was 47.48 meters. From the results of the calculation of the seismic vulnerability index in Kota Manna and Pasar Manna Districts, the seismic vulnerability index value is relatively small, in accordance with the amplification factor and the thickness of the sediment layer. Based on this research, although Manna City and Manna Market are often shaken by earthquakes, the earthquake vibrations experience small amplification.

An effective Q extraction method via deep learning

Abstract The quality factor (Q) is a parameter reflecting the physical properties of reservoirs. Accurate estimation of the quality factor plays an important role in improving the resolution of seismic data. Spectral-ratio method is a widely used traditional method based on the linear least-squares fitting to extract the quality factor, but is sensitive to noise. This is the main reason preventing this method from being widely used. Some supervised deep-learning methods are proposed to extract Q in which the construction of training labels is a key link. The proposed method is based on the spectral-ratio method to create training labels, avoiding errors in generating them. In contrast to the least-squares method, this paper proposes to use a nonlinear regression algorithm based on a fully connected network to fit the spectral logarithmic ratio and frequency. Meanwhile, the empirical equation is applied to constrain prediction results. The proposed method can effectively overcome the influence of noise and improve the accuracy of prediction results. Tests on the synthesized data of vertical seismic profile and common middle profile show that the proposed method has better generalization ability than the spectral-ratio method. Applying the method to the field vertical seismic profile data successfully extracts the quality factor, which can provide effective information for dividing stratigraphic layers.

DETERMINATION OF SAND MINING PROSPECTIVE ZONES BASED ON SEDIMENT THICKNESS ANALYSIS USING HVSR MICROTREMOR AND GRAVITY METHODS: A CASE STUDY IN CANGKRINGAN, SLEMAN, DAERAH ISTIMEWA YOGYAKARTA

Mount Merapi is the most active type A volcano, especially on the island of Java. It produces sediment products totaling 140 million m3 located at the peak of Mount Merapi, descending through the mountain slopes to rivers originating from Mount Merapi. This sediment is subsequently utilized as a sand mining area. Therefore, it is necessary to determine prospect zones for sand mining that align with environmental, economic, and mitigation aspects in the Cangkringan area, Sleman, Yogyakarta Special Region. Seventeen microtremor measurement points were used with the Horizontal to Vertical Spectral Ratio (HVSR) method to determine sediment thickness, constituent lithology, and mining zones suitable for safety and mitigation aspects. Additionally, the gravity method was employed with 247 measurement stations obtained from GGM Plus to determine rock density and estimated thickness based on 2.5D modeling. The study resulted in the distribution of dominant soil frequencies ranging from 0.5 Hz to 7.5 Hz. High frequencies indicate old rocks, while medium frequencies suggest alluvial rocks with a thickness of ± 5 m. Low frequencies indicate alluvial rocks with a thickness exceeding ± 30 meters. Based on gravity results, the average depth is ±90-200 m with a density of 2.6 g/cm3, reflecting the depth of the bedrock in the study area, composed of andesitic volcanic rock. Soil vulnerability analysis in the study area classified into three categories, with values of 4.2-5.4 being highly vulnerable to surface deformation, values of 1.8-3.8 indicating moderate vulnerability, and values of -0.2-1.4 representing low vulnerability. Furthermore, the Peak Ground Acceleration (PGA) ranges from 500-1200 gal with intensity X. Ground Shaking Spectrum (GSS) data indicates that the study area experiences vibration phenomena with elastic soil dynamics. Therefore, the exploitation of sand resources in the Cangkringan area can be conducted in the southern region of Mount Merapi, approximately 2 km away from the mountain center. This is because the southern part of the research area has suitable mitigation measures. Moreover, based on the analysis, sediment thickness in this area ranges from ± 5-30 meters.

Investigation of site fundamental frequency using H/V method guided by geomorphic features derived from old aerial photos

The fundamental frequency of soil (non-lithified cover on the bedrock) is one of the most important factors in seismic hazard evaluations. On the other hand, this frequency expresses several factors such as the clast size, sorting, and the degree of compaction and cementation of soil, which is in close relationship with both the geomorphic setting and processes responsible for the soil formation. We present a rapid and cost-effective multi-disciplinary approach to study site effect in developing urban areas using the spatial correlation between geomorphic setting of soil units and their fundamental frequencies. As a case study, the city of Zanjan was selected because like many other cities of developing countries has been expanded drastically during the last half-century. We use old aerial photos (produced in 1966) of Zanjan and its surroundings to reconstruct a unified orthophoto-mosaic of the study area. This orthophoto-mosaic exhibits pristine geomorphic landforms and features, which predate the current urban coverage of the city, and provides an excellent possibility for mapping Quaternary geological units with distinct geomorphic settings and soil characteristics. Besides, the site effect is measured through the H/V spectral ratio method at 91 points along the profiles pre-designed based on our geomorphic mapping. Subsequently, the spatial pattern described in the geomorphic map was compared with the site effect values. This comparison shows a general correlation between the geomorphic setting of alluvia and the corresponding fundamental frequencies of the soil such that the closer to the mountain front, the higher the fundamental frequency. The results indicate that the peak frequency is observed in the northern regions of the city where a relatively thin layer of unsorted coarse-grained alluvium overlies the bedrock. In the southern geomorphic depressions close to the Zanjanrood river and behind the East-dipping North Zanjan reverse fault (east of the city), where a pile of fine-grained deposits is found, the fundamental frequency reaches its lowest values, namely less than 1 Hz. A comparison of the soil frequency map with the map of shear-wave velocities provided by geotechnical borehole data also indicates a meaningful correlation. Our approach provides a twofold benefit by (1) reducing both the time consumption and expenses in site effect studies through an optimal design of H/V measurement points and borehole drillings based on a precise mapping of distinct geomorphic units, and (2) proposing a unique possibility to produce a reliable site effect map in the urban areas developed during the last half-century.

A Comprehensive Stress Drop Map From Trench to Depth in the Northern Chilean Subduction Zone

AbstractWe compute stress drops for earthquakes in Northern Chile recorded between 2007 and 2021. By applying two analysis techniques, (a) the spectral ratio (SR) method and (b) the spectral decomposition (SDC) method, a stress drop map for the subduction zone consisting of 51,510 stress drop values is produced. We build an extended set of empirical Green’s functions (EGF) for the SR method by systematic template matching. Outputs are used to compare with results from the SDC approach, where we apply cell‐wise obtained global EGF's to compensate for the structural heterogeneity of the subduction zone. We find a good consistency of results of the two methods. The increased spatial coverage and quantity of stress drop estimates from the SDC method facilitate a consistent stress drop mapping of the different seismotectonic domains. Albeit only small differences of median stress drop, strike‐perpendicular depth sections clearly reveal systematic variations, with earthquakes at different seismotectonic locations exhibiting distinct values. In particular, interface seismicity is characterized by the lowest observed median value, whereas upper plate earthquakes show noticeably higher stress drop values. Intermediate depth earthquakes show comparatively high average stress drop and a rather strong depth‐dependent increase of median stress drop. Additionally, we observe spatio‐temporal variability of stress drops related to the occurrence of the two megathrust earthquakes in the study region. The presented study is the first coherent large scale 3D stress drop mapping of the Northern Chilean subduction zone. It provides an important component for further detailed analysis of the physics of earthquake ruptures.

ZONATION OF SEISMIC VULNERABILITY LEVELS IN SOUTH BENGKULU REGENCY, INDONESIA FOR DISASTER-BASED REGIONAL PLANNING

The accurate prediction and prevention of earthquakes remains challenging. Consequently, the primary approach to mitigate the impact of earthquakes is through disaster risk reduction efforts. One significant strategy involves conducting seismic vulnerability analyses based on disaster scenarios. This study aims to identify and map areas with varying levels of seismic vulnerability, analyzing the factors contributing to vulnerability in the South Bengkulu Regency. Secondary data, including peak ground acceleration (PGA) values, were collected, along with microtremor data obtained through the Horizontal to Vertical Spectral Ratio (HVSR) method. The recorded microtremor data serve as input parameters for PGA, Modified Mercalli Intensity (MMI), Seismic Vulnerability Index (Kg), shear wave velocity (Vs), and the time-averaged shear wave velocity for the first 30 m depths (Vs30) values. The findings reveal that, overall, seismic vulnerability in the South Bengkulu Regency can be categorized as low. However, specific areas, particularly in the southwestern and northeastern zones, exhibit relatively higher levels of vulnerability. The heightened vulnerability in these areas is attributed to elevated PGA values, despite the region's generally high soil density, which acts as a mitigating factor against earthquake threats.

Determining the quality factor of sandstone using acoustic laboratory measurements

The paper presents the velocity-pressure and quality factor-pressure dependences of a sandstone sample, using the Fourier spectra of the complete waveforms of the P waves measured in laboratory. Measurements were performed on sandstone and (a reference) aluminum sample at 40 different pressures. The professional literature uses approximate methods to determine the value of the quality factor. The spectral ratio method was used to evaluate our measurements. The measured velocity and quality factor data were processed using inversion methods. The results showed that both the velocity-pressure and the quality factor pressure dependence can be well-described utilizing the double relaxation model in forward modeling.

Mapping of Soil Dynamic Response of Palu City Based on Horizontal to Vertical Spectral Ratio (HVSR) Method

Palu is one of the earthquake prone coastal cities in Indonesia. The 28 September 2018 earthquake (Mw 7.5) destroyed buildings in Palu due to ground shaking amplification, liquefaction, and tsunami. This phenomenon implies the importance of soil dynamic properties knowledge as a basis for proper building design to reduce such an earthquake. This paper presents the results of microtremor measurements conducted at 110 locations in Palu to estimate the dynamic properties of alluvial and marine deposits for earthquake mitigation. The microtremor data were processed using the HVSR method to estimate dominant frequency and amplification factors. Based on these two parameters, the seismic vulnerability index value can be used to estimate the dynamic characteristic variations of the area. The results show that alluvial and marine sediments in Palu have dominant frequency values from 0.54 to 11.72 Hz, amplification from 1.64 to 6.15, and seismic vulnerability index of 1.49 to 56. Sediments with low dominant frequency and high amplification factors are mostly found in the western and eastern Palu and Mantikulore districts, which are associated with thick sediment layers. The areas with the highest amplification factor and seismic vulnerability index are Petobo and Balaroa, which experienced severe damage during the 2018 earthquake.

Earthquake Hazards Vulnerability Analysis of The Nankou-Sunhe Fault in Changping (Beijing) Using HVSR Method

During the Holocene period, three paleoseismic events (Mw=7.7-7.9) occurred on the Nankou-Sunhe Fault in Beijing. For disaster prevention and mitigation, earthquake zoning is needed to be carried out to map the areas that will experience severe damage when an earthquake occurs. This study uses the Horizontal to Vertical Spectral Ratio (HVSR) method to obtain not only the sedimentary layer thickness and fault structure according to empirical formulas, but also the spatial variations in the peak frequency and amplification values that can explain the characteristics of the geological layer beneath the surface. Based on the calculated results, the peak frequency(f) ranges from 0.2 to 4 Hz with the lowest frequency distribution in the southwest parts, which indicates a thicker sediment layer. Amplification(A) ranges from 2 to 14 with the distribution of higher values in the southeastern and northeastern parts. The K-value (A2/f) is in the range of 0-180 and the highest K-value in the southwest of the fracture indicates that this region is at high risk for damage. Our results provide a better understanding of the earthquake hazards vulnerability of the Nankou-Sunhe Fault in Changping, Beijing, and can be used as a basis for urban disaster prevention and reduction.

Preliminary Result: Analysis of Dominant Frequency in Western Part of Java, Indonesia using Hilbert-Huang Transform-HVSR Method

The western part of Java is situated near various geological formations stemming from the collision between the Indo-Australian and Eurasian tectonic plates. These formations include the Java Trench, Sunda Strait, and several active onshore faults, rendering the area susceptible to seismic activity. The dynamic response of the soil to seismic shaking in this region warrants thorough investigation, particularly due to the prevalence of alluvium deposits in certain areas, which have the potential to amplify ground motion. In this research endeavor, seismic data recorded by 81 temporary seismic stations in West Java were utilized. The analysis employed the Hilbert-Huang Transform technique in conjunction with the Horizontal to Vertical Spectral Ratio (HVSR) method to derive the Horizontal-to-Vertical (H/V) spectral response curve for each measurement point. The implementation of the Hilbert-Huang Transform served the purpose of mitigating non-stationary transient noise commonly encountered in urban measurements, thereby enhancing the accuracy of the HVSR curve in portraying the dominant frequency characteristics of the measurement area. The findings of this study reveal that within the western part of Java, the dominant frequency values fall within the range of 0.9 to 7 Hz. Areas exhibiting lower dominant frequencies are indicative of a greater sediment thickness, predominantly observed in the northern region of the western part of Java, the Bandung Basin, some areas surrounding active volcanoes as well as various locales within the province of Banten.

Soil dynamic response mapping and vulnerability analysis of earthquake hazard in Banda Aceh using HVSR method

Banda Aceh is a city with a high level of seismicity and earthquake risk. The seismicity level is mainly caused by two Sumatran Fault segments, i.e., the Aceh and Seulimeum Segments. Soft alluvium deposits aggravate the earthquake hazard in Banda Aceh. The presence of soft alluvium layers above the more rigid soil layer will cause earthquake wave amplification. This amplification phenomenon can increase the amplitude and energy of earthquake waves, making the earthquake more destructive.Therefore, dynamic response mapping and subsurface structure modeling are required to determine the soil characteristics of the research area. Such information is needed for the infrastructure and planning development of Banda Aceh City. This study obtained the subsurface model from microtremor data and a sequence of processing and modeling techniques. The process uses the Horizontal to Vertical Spectral Ratio method (HVSR). The resonance frequency and amplification values are identified from the ratio between the horizontal and vertical spectrum components depicted in the HVSR curves. Based on the obtained results, Banda Aceh has a predominant frequency values range of 0.54-11.72 Hz and an amplification range of 1.22-6.26. Low predominant frequency is mainly located in the study area’s southeast and center, which is associated with large sediment thickness. Meanwhile, high amplification values are mainly located on the study area’s southeast, middle, and northwest sides, which are associated with increased sediment depth and geological features in the form of swamp sediment. Then the value of seismic vulnerability (Kg ) can be obtained based on the predominant frequency and amplification value. In this study, the value of seismic vulnerability ranges between 0.72-31.77 area with relatively high vulnerability values in the middle, southeast, south, and southwest of the study area.

Measuring seismic attenuation in polar firn: method and application to Korff Ice Rise, West Antarctica

Abstract We present seismic measurements of the firn column at Korff Ice Rise, West Antarctica, including measurements of compressional-wave velocity and attenuation. We describe a modified spectral-ratio method of measuring the seismic quality factor (Q) based on analysis of diving waves, which, combined with a stochastic method of error propagation, enables us to characterise the attenuative structure of firn in greater detail than has previously been possible. Q increases from 56 ± 23 in the uppermost 12 m to 570 ± 450 between 55 and 77 m depth. We corroborate our method with consistent measurements obtained via primary reflection, multiple, source ghost, and critically refracted waves. Using the primary reflection and its ghost, we find Q = 53 ± 20 in the uppermost 20 m of firn. From the critical refraction, we find Q = 640 ± 400 at 90 m depth. Our method aids the understanding of the seismic structure of firn and benefits characterisation of deeper glaciological targets, providing an alternative means of correcting seismic reflection amplitudes in cases where conventional methods of Q correction may be impossible.

Regional spectral characteristics, quality factor and site responses in western-central Sichuan, China (I): Application of parametric generalized inversion technique

To study the source, site amplification and seismic wave attenuation characteristics in the Sichuan area, 1242 ground motion recordings corresponding to 62 earthquake events with Ms 4.3–6.4 recorded by 55 observation stations in the study area were selected. The source spectra, site effects and frequency-related quality factors in the western Sichuan area were inverted by the generalized inversion technique. Then, the source parameters, such as corner frequency, seismic moment and stress drop, were estimated by a grid-searching tool. In the generalized inversion method, a bedrock station was used to remove the trade-off between the site and source components. The inverted quality factor of 0.25–20 Hz frequency band is characterized as Q(f) = 219.4f0.7383. The results indicate that the inverted source spectrum is well matched with the ω2 source model, in which the stress parameter values range between 0.2 MPa and 3.5 MPa, with an average value of 1.2 MPa and source radii between 0.3 and 4 km. In addition, a comparison between the inversion results obtained from the horizontal-to-vertical spectral ratio and the generalized inversion methods indicates that these two approaches actually describe the same level of site effect, while the averaged site effect obtained by the generalized inversion method is always greater than the result obtained by the horizontal-to-vertical spectral ratio method. Finally, a residual defined as a function of hypocentral distance and moment magnitude was used to investigate whether there was any bias in the inversion results. The results indicate that the geometric spreading and source model used in this study resulted in an underestimation of the Fourier amplitude level in the far field and for large earthquakes. The results of this study can be applied in ground motion simulation and prediction to help the study of seismic disasters and risk assessment in the study area.

Regional spectral characteristics, quality factor and site responses in western-central Sichuan, China (II): Application to stochastic ground motion simulation

The stochastic finite-fault approach was used to model the 2022 Luding earthquake with a magnitude of Mw 6.6 in China at 16 selected near-field stations. To investigate the impact of the site effect on the synthetic results, two models were taken into account: One model was obtained from the generalized inversion technique, and the other model was obtained from the horizontal-to-vertical spectral ratio method. The high-frequency attenuation parameter was estimated to be 0.040 s and the value of 4.0 MPa for the stress parameter was adopted. Comparisons of the recorded and simulated Fourier amplitude spectrum, pseudospectral acceleration, peak ground acceleration, and peak ground velocity were performed to investigate the capability of the selected input parameters. In addition, the model bias, simulated peak ground acceleration and peak ground velocity were calculated by the local site amplification estimated by the two methods. The results show that the site effects estimated by the generalized inversion technique can well simulate the high-frequency response spectra and Fourier amplitude spectrum and can provide peak ground acceleration and peak ground velocity values consistent with the recorded values, while the local site amplification roughly calculated by the horizontal-to-vertical ratio method underestimates the main ground motion characteristics. Furthermore, strong-motion records with the same magnitude but different hypocentral distance ranges in the generalized inversion technique are used to estimate near-surface site effects, which shows that the distance range has a small impact on the estimation of local site amplification. Model parameters that performed well in this study provide confidence in understanding and quantifying seismic hazards in the Luding area from earthquake scenarios with different magnitudes.