The short‐period seismometer‐based magnitude saturation problem, especially for events with magnitude > 8, can be improved by a real‐time Global Navigation Satellite System (GNSS) positioning technique, which has enabled rapid estimation of a finite fault model for a large earthquake without any saturation. A new real‐time fault modeling system based on the GNSS Earth Observation Network (GEONET) is developed and is under experimental operation in Japan. In this paper, we present the newly developed system REGARD (the Real‐time GEONET Analysis system for Rapid Deformation monitoring), which consists of real‐time GNSS positioning, automatic detection of coseismic displacement by the event, and quasi real‐time finite fault model inversion routines. The performance of the automatic event detection system is tested through experimental real‐time operation based on GEONET data for 2 months. Furthermore, we also test the reliability of the finite fault model inversion routines using real raw GNSS data observed for past large earthquakes: the 2003 Tokachi‐oki earthquake (moment magnitude (Mw) 8.3), the 2011 Tohoku earthquake (Mw 9.0), and the 2011 off‐Ibaraki earthquake (Mw 7.7). A simulated 1707 Hoei‐type Nankai Trough earthquake (Mw 8.7) is also tested. The real‐time experimental operation shows that real‐time GNSS positioning is precise enough to detect all the tested earthquakes, and the inversion results demonstrate that the REGARD can reliably estimate the earthquake size and its extent within 3 min after the origin time. These results suggest that the REGARD system will complement the seismometer‐based magnitude determination system.
Beginning in April 2016, a series of shallow, moderate to large earthquakes with associated strong aftershocks struck the Kumamoto area of Kyushu, SW Japan. An Mj 7.3 mainshock occurred on 16 April 2016, close to the epicenter of an Mj 6.5 foreshock that occurred about 28 hours earlier. The intense seismicity released the accumulated elastic energy by right-lateral strike slip, mainly along two known, active faults. The mainshock rupture propagated along multiple fault segments with different geometries. The faulting style is reasonably consistent with regional deformation observed on geologic timescales and with the stress field estimated from seismic observations. One striking feature of this sequence is intense seismic activity, including a dynamically triggered earthquake in the Oita region. Following the mainshock rupture, postseismic deformation has been observed, as well as expansion of the seismicity front toward the southwest and northwest.
We present the initial results of rapid fault estimations for the 2016 Kumamoto earthquake on April 16 (M j 7.3), and coseismic displacements caused by the two large foreshocks that occurred on April 14 (M j 6.5) and April 15 (M j 6.4) from the GEONET real-time analysis system (REGARD), which is based on a Global Navigation Satellite System (GNSS) kinematic positioning technique. The real-time finite-fault estimate (M w 6.85) was obtained within 1 min and converged to M w 6.96 within 5 min of the origin time of the mainshock (M j 7.3). The finite-fault estimate shows rightlateral strike-slip fault along the Futagawa fault segment, which is consistent with the finite-fault model inferred from post-processed GNSS and InSAR analysis. Furthermore, significant coseismic displacements were observed due to the April 14 and April 15 foreshocks at nearby sites, though these earthquakes were smaller than the pre-assigned system threshold. Our results also demonstrate the potential for the GNSS-based earthquake early warning system for inland earthquakes.
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