Understanding of Shallow Structure in Mataram, Lombok, Indonesia by using High Frequency of Ambient Seismic Noise

Abstract:

In 2018, sequential events hit Lombok Island and caused significant damage to infrastructure such as housing, schools and public facilities. Around 1,973 felt earthquakes (M > 3) were recorded during August 2018 with three earthquakes with large magnitudes of Mw 6.9, Mw 6.8 and Mw 6.2. We deployed 25 seismometers across Lombok Island for aftershock monitoring. 60 points were used for microtremor survey in Mataram city. Our purpose is to observe shallow structure beneath Lombok Island. We used seismic waveforms from 25 seismometers to identify the dominant frequencies and amplification index. We applied H/V ratio method to analyze the seismic risk in the Lombok region. The basic concept of this method is to do a comparison between the horizontal and vertical component spectra 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 hard ground both components (horizontal and vertical) will be similar. Resulting predominant frequency, amplification, and seismic vulnerability maps 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 concludes that the maximum ground acceleration and the construction of the building should be considered when one wants to investigate the effect of an earthquake to the damage beside the site effects. Then, we combined them with local microtremor survey in Mataram city. The results show that the S-wave velocity structure derived from the inversion process of the HHT-HVSR curves is in better agreement with the previous study of Mataram city than the conventional method. Furthermore, the resulting S-wave velocity structure is also interpreted based on geological reference, giving new insights into the subsurface structure beneath Mataram city.