The central segment of a SE-striking right-lateral fault that passes west of Petrinja ruptured at approximately 12:20 PM CET (11:20 UTC) on 29 December 2020 (Figure 1). In this study, we investigate the coseismic slip distribution of the event, which sheds light on its kinematic rupture process.
1. Slip Inversion
To derive the slip distribution, we used the teleseismic body-waveform slip inversion method of Kikuchi et al. (1993). P-recordings of 148 seismic stations in the epicentral distance of 20° to 95° were used. The recordings were filtered in the frequency range of 0.015 to 0.5 Hz to capture details of the rupture process.
Figure 2 shows the synthetic and observed seismograms. The inverted coseismic slip reveals two slip patches at depths less than 15 km (Figure 3), a major one above the hypocenter and a weaker one in the southeast. The main patch, MSP, has 15 km along-strike and 15 km along-dip dimensions, and its maximum slip is about 1 meter. The second patch, SSP, has a dimension of about 3 km by 3 km with a maximum slip of about 0.4 m. This secondary deeper patch is observed in other frequency bands and appears to be not an artifact of the modeling.
The observed SH seismograms showed a relatively poor match with the synthetic waveforms and therefore were removed from the inversion process.
2. Source parameters of the earthquake
The computed fault parameters for the event are consistent with those found by other studies, notably with the Global CMT solution, which reported a moment magnitude (Mw) of 6.4. Using the body-waveform slip inversion, we determined a focal mechanism of strike 132⁰, dip 82⁰, and rake 173⁰, which runs parallel to an existing fault (Figure 4). The spatial distribution of aftershocks confirms the SW-striking nodal plane as the causative fault plane (Figure 7). A depth of 11 km resulted in a minimum misfit solution, consistent with the 10 km depth reported by different seismological observatories. Source time function for the event shows a total rupture time of 13 s with the significant energy release during seconds 1 to 6 (Figure 2, top-right).
3. Implications of the slip distribution
Transpressional fault systems cause positive topography along the associated fault (Fossen et al., 1993; Dewey et al., 1998). The minor reverse component of the 2020/12/29 earthquake, as inferred from the rake angle of
Transpressional fault systems cause positive topography along the associated fault (Fossen et al., 1993; Dewey et al., 1998). The minor reverse component of the 2020/12/29 earthquake, as inferred from the rake angle of 173⁰, causes positive topography in the long-term (Figures 4, 5, and 6). A fault-normal compressive component increases the coupling on the fault plane; therefore, it may develop a barrier against rupture propagation. The following points can be stated on the connection of seismicity, slip distribution, and the geomorphological features in the macro-seismic area:
- The main slip patch, MSP, is delimited in the NW by a river R, and in the SE by a valley, V (Figure 3). This portion of the fault correlates with a hill (Figure 6).
- The secondary slip patch, SSP, partially ruptured the down-dip of the fault segment to the southeast of the valley V. Its shallow counterpart did not rupture due to its higher shear strength.
- The area between V and the northwestern end of SSP did not rupture during the earthquake, most likely due to high shear strength. This portion corresponds with a prominent positive topography, which asserts the strength of the associated fault segment.
- The hill corresponding to SSP has a smaller elevation compared to its immediate northwestern neighbor. It may indicate a weaker fault segment and explain the rupture of SSP (Figures 4 and 6).
- The majority of the aftershocks fall in the hanging block, to the southeast of the fault plane (Figures 5 and 7).
- Some of the aftershocks cover the northwestern continuation of the causative fault for a length of about 45 km. Provided that the whole 45 km fails in a single earthquake, a magnitude of about 6.9 is expected.
- Aftershocks located to the southeast of valley V, as well as the rupture of SSP, indicate that the area to the southeast of valley V is not in the early stages of its earthquake cycle; otherwise, the SSP should not have ruptured. An earthquake cycle starts right after a characteristic earthquake and ends with the next characteristic earthquake at the same location. An earthquake cycle may take centuries.
- Continuity of the fault-parallel topography between valley V and the border river W may infer a single characteristic event for the VW segment (Figure 6).
- Assuming the area under a fault-associated topography profile (Figure 6) corresponds with the causative seismic moment, the future earthquake confined within V and W will be 0.3 units larger (i.e. Mw 6.7) than the 2020/12/29 earthquake. The area under the VW profile is three times larger than that under the RV profile.
Some of the figures are created using GMT (Wessel, et al., 2019). Figures 5 and 6 are created using Google Earth. We appreciate Hadi Ghofrani for peer-reviewing the note.
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