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| Titel |
Correlation of pre-earthquake electromagnetic signals with laboratory and field rock experiments |
| VerfasserIn |
T. Bleier, C. Dunson, C. Alvarez, F. Freund, R. Dahlgren |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1561-8633
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| Digitales Dokument |
URL |
| Erschienen |
In: Natural Hazards and Earth System Science ; 10, no. 9 ; Nr. 10, no. 9 (2010-09-24), S.1965-1975 |
| Datensatznummer |
250008408
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| Publikation (Nr.) |
 copernicus.org/nhess-10-1965-2010.pdf |
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| Zusammenfassung |
Analysis of the 2007 M5.4 Alum Rock earthquake near San José California
showed that magnetic pulsations were present in large numbers and with
significant amplitudes during the 2 week period leading up the event. These
pulsations were 1–30 s in duration, had unusual polarities (many with only
positive or only negative polarities versus both polarities), and were
different than other pulsations observed over 2 years of data in that the
pulse sequence was sustained over a 2 week period prior to the quake, and
then disappeared shortly after the quake. A search for the underlying physics
process that might explain these pulses was was undertaken, and one theory
(Freund, 2002) demonstrated that charge carriers were released when various
types of rocks were stressed in a laboratory environment. It was also
significant that the observed charge carrier generation was transient, and
resulted in pulsating current patterns. In an attempt to determine if this
phenomenon occurred outside of the laboratory environment, the authors scaled
up the physics experiment from a relatively small rock sample in a dry
laboratory setting, to a large 7 metric tonne boulder comprised of Yosemite
granite. This boulder was located in a natural, humid (above ground) setting
at Bass Lake, Ca. The boulder was instrumented with two Zonge Engineering,
Model ANT4 induction type magnetometers, two Trifield Air Ion Counters, a
surface charge detector, a geophone, a Bruker Model EM27 Fourier Transform
Infra Red (FTIR) spectrometer with Sterling cycle cooler, and various
temperature sensors. The boulder was stressed over about 8 h using expanding
concrete (Bustartm), until it fractured into three major pieces. The
recorded data showed surface charge build up, magnetic pulsations, impulsive
air conductivity changes, and acoustical cues starting about 5 h before the
boulder actually broke. These magnetic and air conductivity pulse signatures
resembled both the laboratory rock stressing results and the 30 October 2007
M5.4 Alum Rock earthquake field data.
The second part of this paper examined other California earthquakes, prior to
the Alum Rock earthquake, to see if magnetic pulsations were also present
prior to those events. A search for field examples of medium earthquakes was
performed to identify earthquakes where functioning magnetometers were
present within 20 km, the expected detection range of the magnetometers. Two
earthquakes identified in the search included the 12 August 1998 M5.1 San
Juan Bautista (Hollister Ca.) earthquake and the 28 September 2004 M6.0
Parkfield Ca. earthquake. Both of these data sets were recorded using EMI
Corp. Model BF4 induction magnetometers, installed in equipment owned and
operated by UC Berkeley. Unfortunately, no air conductivity or IR data were
available for these earthquake examples. This new analysis of old data used
the raw time series data (40 samples per s), and examined the data for short
duration pulsations that exceeded the normal background noise levels at each
site, similar to the technique used at Alum Rock. Analysis of Hollister
magnetometer, positioned 2 km from the epicenter, showed a significant
increase in magnetic pulsations above quiescient threshold levels several
weeks prior, and especially 2 days prior to the quake. The pattern of
positive and negative pulsations observed at Hollister, were similar, but not
identical to Alum Rock in that the pattern of pulsations were interspersed
with Pc 1 pulsation trains, and did not start 2 weeks prior to the quake,
but rather 2 days prior. The Parkfield data (magnetometer positioned 19 km
from the epicenter) showed much smaller pre-earthquake pulsations, but the
area had significantly higher conductivity (which attenuates the signals).
More interesting was the fact that significant pulsations occurred between
the aftershock sequences of quakes as the crustal stress patterns were
migrating.
Comparing laboratory, field experiments with a boulder, and earthquake
events, striking similarities were noted in magnetic pulsations and air
conductivity changes, as well as IR signals (where instrumented). More
earthquake samples, taken with the appropriate detectors and within
10–15 km proximity to large (>M5) earthquakes, are still needed to
provide more evidence to understand the variability between earthquakes and
various electromagnetic signals detected prior to large earthquakes. |
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