Multiple Point Source Inversions on Teleseismic Waveform Data Reveal the Complex Super-Shear Rupture Process of the 2018 Mw 7.5 Palu Earthquake

Date: 4/25/2019

Time: 06:00 PM

Room: Fifth Avenue

Multiple Point Source(MPS) model, which is a time-space sequence of double-couple sources, fills in the earthquake source representations between single point source and the finite fault model. Here we generalized our Markov Chain Monte Carlo based MPS inversion scheme to cover teleseismic waveform and applied it to the 2018 Mw7.5 Palu earthquake. With path calibration from the 2012 (Mw6.3) and 2017 (Mw6.6) events, we selected 62 teleseismic P waves and 35 SH waves filtered to the station-dependent frequency ranges (~3-100s) for the inversion. We tested the number of sources and found that five sources can fit the observations adequately. Our results show the five subevents (S1-5) align in near N-S direction over a distance of ~140km and occurred subsequently from N to S, with a total duration of ~40s and a total Mw of 7.4. All S1-5 have similar sub-vertical strike-slip focal mechanism with slight differences in strike and dip, where S4 and S5 are featured with stronger normal slip. Their Mw, however, shows more dramatic differences with S2 (Mw7.2) and S3 (Mw7.1) dominating the moment release. S1 is located ~10km to the south of the epicentre and the distances between the neighbouring subevents (from N to S) are 17, 38, 33 and 40km, with corresponding timing separations of 7, 11, 8 and 8s. The source durations are 5~7s.The apparent rupture speed, defined by the centroid location and timing of the neighbouring events, gradually increased from 2.4km/s (S1 to S2) to 4.7km/s (S4 to S5), which agree well with the back-projection results from the Australian array. As revealed by the geodetic data, the rupture did not reach to the surface near S1 and S2 where topography feature of fault trace is not clear, although two compressional step-overs could be identified in the geodetic data. In contrast, the satellite images show clear and smooth surface rupture around S3 and S4, where clear fault-related topography can be identified . The gradual increasing rupture speed can be explained as the rupture propagation from the less active faults to the more mature Palu fault.

Presenting Author: Qibin Shi

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