arXiv subjects

Bjorn Paulsson

Publications and source records attributed to Bjorn Paulsson.

3 recordsLinked to original sources

arXiv2024

High-resolution borehole earthquake monitoring at San Andreas Fault Observatory at Depth, Parkfield, California

Downhole earthquake monitoring, without the complex effects from the near surface, can record more and better seismic data than monitoring on surface. The San Andreas Fault Observatory at Depth (SAFOD) is a borehole observatory equipped with different instruments inside to study the earthquake mechanism of the San Andreas fault at Parkfield, California. During April to May in 2005, Paulsson deployed an 80-level 3-component geophone array in the SAFOD main hole, and continuously recorded seismic data for about 13 days. We located 125 local earthquakes from the borehole earthquake monitoring data using a homogeneous velocity model and compared it with 35 earthquakes' locations from surface earthquake monitoring by the United State Geological Survey (USGS) during the same monitoring time. The borehole earthquake locating is assumably more accurate in the borehole's vicinity. We also compared the result with 1,074 earthquakes' locations from the surface earthquake monitoring in the last 9 years from 2015 to 2024. The hypocenters from our nearly 2 weeks' borehole earthquake monitoring form similar structures as that from the 9 years' surface earthquake monitoring by the USGS.

physics.geo-ph↗

arXiv2024

Paraxial micro earthquake: a natural effective multi-purpose check shot for downhole earthquake monitoring

Downhole earthquake monitoring, without the effects from the overburden, can record better seismic data than monitoring on surface. However, in order to reasonably use the downhole vector seismic data, a constant challenge is how to accurately orient the downhole radial-component seismometers. A common practice is to use offset check shots on or near the surface. However, in areas with complex geologies, this routine may result in significant orientation errors. A ParAxial Micro Earthquake (PAME) is a micro earthquake at a close distance to the seismometers and near the extended path of the borehole's trajectory. It is rarely recorded during downhole earthquake monitoring unless designed for. If it is recorded, it can be a real treasure not only for P-wave and S-wave velocities' profiling, but for the downhole seismometers' orientation. As an example, during April to May in 2005, Paulsson installed an 80-level 3-component VSP (Vertical Seismic Profiling) array in the SAFOD (San Andreas Fault Observatory at Depth) main hole at Parkfield, California, and continuously recorded seismic data for about 13 days. Large charge offset check shots at 13 different locations near the surface were detonated in order to orient the downhole geophones; the orientation results were unsatisfactory but went unnoticed or unsolved. Besides this, a small charge "zero-offset" check shot was detonated near the wellhead in order to get the P-wave and S-wave velocity profiles, but only the P-wave velocity profiling was successful. Fortunately, we recorded a few PAMEs, through which we not only obtained better P-wave and S-wave velocity profiles, but satisfactorily oriented the downhole geophones.

physics.geo-ph↗

arXiv2024

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

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.

physics.geo-ph↗