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The Mw7.0 2016 Kumamoto earthquake occurred on the previously mapped Futagawa–Hinagu fault causing significant strong ground motions. A ~ 30-km-long dextral surface rupture appeared on the major fault zone and dextral slip was up to 2–3 m. However, the surface ruptures were also broadly and remotely distributed approximately 10 km away from the primary rupture zone. These numerous distributed secondary surface slips with vertical displacement of less than a few tens of centimeters were detected by the interferometric synthetic aperture radar (InSAR) technology in previous studies. Such displacements occurred not only on previously mapped faults but also on unknown traces. Here, we addressed the following fundamental issues: whether the broadly distributed faults were involved in the past major earthquakes in the neighborhood, and how the fault topography of such secondary faults develops, seismically or aseismically. To find clues for understanding these issues, we show the results of field measurements of surface slips and paleoseismic trenching on distributed secondary faults called the Miyaji faults inside the Aso caldera, 10 km away from the eastern end of the primary rupture zone. Field observations revealed small but well-defined dextral slip surface ruptures that were consistent with vertical and dextral offsets derived from InSAR. On the trench walls, the penultimate event with vertical displacements almost similar to the 2016 event was identified. The timing of the penultimate event was around 2 ka, which was consistent with that of the primary fault and archeological information of the caldera. Considering the paleo-slip event and fault models of the Miyaji faults, they were presumed not to be source faults, and slip on these faults have been triggered by large earthquakes along major adjacent active faults. The results provide important insights into the seismic hazard assessment of low-slip-rate active faults and fault topography development due to triggered displacement along secondary faults.

Studying bedrock rivers during their transient states helps understand the response of a fluvial system to the changing boundary conditions. Although theoretical studies predict how river form adjusts to changes in incision or rock uplift rates, field constraints on the timescale of this adjustment are limited. We investigated the transient behavior of channels and hillslopes and estimated the adjustment times of channel width and angle of valley‐side slopes to accelerated incision based on knickpoint travel time. We documented channel slopes, channel widths, and hillslope angles along six rivers around an active normal fault in Iwaki, Japan, and identified river sections in a transient state. Channel slopes and basin‐averaged erosion rates determined from 10Be concentrations are distinct between rivers near and distant from the fault, suggesting that past increases in fault throw rates triggered the knickpoint formation and the observed transient response. Adjustment time for width is at least 105 years and can take 106 years after the knickpoint passage. Adjustment time for hillslope angles is generally shorter than for channel width. However, the hillslope adjustment may take longer than previously reported if the effect of width adjustment on hillslope angles is significant or the complete adjustment of hillslope angles is considered. The fact that channel slope, channel width, and hillslope angle have distinct adjustment times underlines the importance of correctly identifying river sections that are fully adjusted to the new boundary conditions when inferring erosion or relative uplift rates for bedrock rivers.

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