Investigation of Ground Motion and Local Site Characteristics of the 2018 Lombok Earthquake Sequence

In 2018, Lombok Island was hit by a major earthquake sequence. The Indonesia Meteorological, Climatological, and Geophysics Agency (BMKG) reported that the Lombok Island earthquake sequence started with an M_w 6.4 foreshock, followed by an M_w 6.8 main shock, aftershocks of M_w 5.8 and M_w 6.2, and a second mainshock of M_w 6.9 in the eastern part of Lombok. This study presents an investigation of strong motion characteristics using the Indonesia National Strong Motion Network (INSMN) data from two accelerometer stations, the MASE station (at Praya Lombok International Airport, Lombok Island, V_S30 = 770 m/s, SB site class) and TWSI station (in Sumbawa Island, V_S30 = 1152 m/s, SB site class). Signal analysis techniques using a power spectrum via fast Fourier transform, wavelet transform and horizontal-to-vertical spectral ratio (HVSR) have been applied in this study. There are significant differences in the results (e.g., predominant frequencies, wavelets, H/V ratios, and frequencies at peak H/V ratio) for the MASE and TWSI stations, highlighting the importance of actual V_S30 profiles and the limitation of the site class system in providing necessary predictive information. The variation of the peak ground acceleration (PGA) values and the spectral amplitudes could only be explained by hypothesizing the effect of the volcanic structure of Mount Rinjani on the strong motion waveforms.