Analysis of H/V Ratio Curve to Estimate Seismic Hazard Vulnerability in Lombok Island, West Nusa Tenggara, Indonesia

Abstract


 
 
 In 2018, Lombok Island was hit by a series of destructive earthquakes. According to Indonesian Meteo- rological, Climatological, and Geophysical Agency data, about 1,973 felt earthquakes (M > 3) which shaken Lombok were recorded during August 2018 with three earthquakes with the largest magnitude of 6.9 Mw, 6.8 Mw, and 6.2 Mw. National Board for Disaster Management reported about 555 deaths, 1,833 people injured, and 186,010 houses damaged as a result of Lombok earthquake on August 5th, 2018. A number of seismometers were placed on Lombok Island from August 3rd, 2018 to October 19th, 2018 to monitor the aftershock events. There are 17 stations that record seismicity in Lombok consisting of 10 broadband sensors and 7 short period sensors. In this study, we used the Horizontal-to-Vertical Spectral Ratio (HVSR) method to analyze the risk of earthquake in the Lombok region. The basic concept of this method is to do a comparison between the horizontal component spectrum and the vertical component spectrum of a wave, where theoretically the particle movement of the horizontal component is greater than the particle movement of the vertical component on soft ground, whereas on the hard ground both components (horizontal and vertical) will be similar. H/V curve obtained from earthquake record- ings (Earthquake Horizontal-to-Vertical Ratio/EHVR) and H/V curve obtained from microtremor recordings (Microtremor Horizontal-to-Vertical Ratio/MHVR) shows good agreement and high correlation. Empirical correction of EMR (Earthquake-to-Microtremor Ratio) managed to decrease the difference of estimation of predominant frequency and amplification factor between EHVR and MHVR. Predominant frequency, amplification, and seismic vulnerability map agree with the geological condition of Lombok Island, where high value of amplification and seismic vulnerability was found on soft and thick ground. This study conclude that the maximum ground acceleration and the construction of the building should also considered when one wants to investigate the effect of an earthquake to the damage occurred, beside the site effects. 
 
 

Review of the 2018 Lombok Earthquake, Indonesia, and Its Impact from Previous Studies

“…The result obtained from [16] study shows high values in the western region of Lombok that can be interpreted as the alluvium deposits (Fig. 5).…”

“…There are several studies that examine about 2018 Lombok Earthquake series such as [7,10,13,16], and [17]. Reference [7] from InSAR measurements revealed that earthquake sequences occurred in Lombok on 2018 was triggered from three segments in the north side of Lombok.…”

“…The vulnerability map produced by [16] indicates there is a zone with a high potential for earthquake damage located on the west of Lombok associated with a large thickness of alluvium unit. However, there are discrepancies between the earthquake damage map provided by National Disaster Mitigation Agency and vulnerability map.…”

“…The data that discussed in this study are from the previous research that conducted by [10,13,16], and [17]. The seismic data interval of the deployment was one month from 3 August to 9 September 2018 by 20 seismographs from Bandung Institute of Technology, the Earth Observatory of Singapore/Nanyang Technological University, and the Center of Volcanology and Hazard Mitigation.…”

“…For the seismic vulnerability index, the previous study from [16] calculated the value after the predominant frequency and amplification factors were obtained according to the equation that proposed by [18].…”