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arXiv · 2402.11149

Indications and implications of a borehole seismic monitoring result of the San Andreas Fault foresee future earthquake prediction

Ruiqing He·Bjorn Paulsson
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Abstract

The Parkfield M6 earthquake predicted from 1985 by the USGS to happen by 1993 happened 11 years later in 2004 instead. Till today, satisfactory answers to why this earthquake was mis-predicted have not been found. Seven months after the earthquake, we deployed a seismic array in the SAFOD main hole to monitor the San Andreas Fault. During a 13 days period, we recorded 220 earthquake events. By analyzing the projected hypocenters of 100 earthquake events, we found three main active sub-faults, namely SAF1, SAF2, and SAF3, with SAF1 being the normally regarded SAF. During the monitoring period, there was an earthquake migration trend that shows the move of SAF1 drags SAF2, which in turn drags SAF3, to slip and dip downwards. This could be reflected by the result of a geological trenching across the SAF. On SAF1, the smaller earthquakes tended to occur earlier in southeast and the larger earthquakes tended to occur later in northwest, which could indicate the existence of strong asperities. However, the new earthquakes on SAF2 were overall remarkably smaller, which could indicate that during the 2004 M6 earthquake the main underground rupture took place on SAF2, which could extend with a bend near Parkfield town to the Southwest Fracture Zone, and further to include the seemingly isolated 2004 M6 hypocenter. The possible prevailing of this triple active sub-faults model in the Parkfield area may account for 12, 24 or 36 (instead of a single 22 years as previously believed) recurring years of M6 earthquakes, depending on how many of these active sub-faults have basically absorbed the tectonic stress. In less than 0.1% of the earthquake's last intermission time, and at just one location, our borehole seismic monitoring has revealed so much insightful information that earthquake prediction may be feasible if the fault is monitored in boreholes at more locations and for longer periods of time.

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Ruiqing He, Bjorn Paulsson. 2024-08-19. Indications and implications of a borehole seismic monitoring result of the San Andreas Fault foresee future earthquake prediction. https://arxiv.org/abs/2402.11149

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