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An Mw 7.5 earthquake struck Palu in the northern coast of Sulawesi island, Indonesia, on 28 September 2018. Its focal mechanism was determined to be a left‐lateral strike‐slip fault, which is generally expected to not produce a tsunami. However, a large tsunami with runup heights of more than 6 m was observed along the coast of Palu city. Here, we show a complex triggering supershear source model as determined by teleseismic waveform inversion. Three asperities with different slip characteristics were found on the 120‐kilometer‐long rupture zone. Significant triggering rupture with a supershear speed was observed south of the epicenter, which was just beneath Palu city. This special rupture process can cause a strong directivity effect that produced anomalously large ground shaking with nonlinear effects in Palu area. The coseismic deformation determined from the inverted source model showed large horizontal displacements. These horizontal movements combined with complex bathymetry and topography could have pushed seawater to generate a tsunami even though the Palu earthquake was a strike‐slip event.

On 6 February 2016, an M L 6.6 earthquake occurred in the Meinong area of southern Taiwan, causing anomalously large ground shaking. In this study, a joint source inversion was performed to understand the rupture process of this event and to identify how the anomalous strong motion was produced. The results show that the rupture process of this event was complex; at least two asperities were identified on the fault plane. The rupture mainly developed in the down‐dip direction and propagated toward the northwest. Results from a 3D wave propagation simulation further indicate that the strong ground shaking observed in southwest Taiwan was caused by a combination of three effects: (1) rupture directivity, (2) source radiation pattern, and (3) sedimentary amplification. The results demonstrate that constructive source rupture and wave propagation effects due to a moderate earthquake can cause anomalously strong ground motion and serious seismic hazards in nearby densely populated areas.