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Hi All. Have you ever wondered why accurately predicting earthquakes is so damn hard? It's bars of stress/pressure. No one knows precisely how much there is on any given fault, how much it takes to cause the fault to fail and how close we are to that point. A seismologist once told me that if they knew how much bars of stress there was on any given fault and the exact location of where that stress was they could almost predict to the hour when the quake would occur. Not all agree with that.
Right now all they can do is estimate and they don't really know how good those estimates are. If the fault is a strong fault it could take a lot of stress to cause it to fail, but then again if it is a weak fault it may not take very much stress to cause it to fail. There is also a problem with that. No one is really sure as to which faults are strong and which ones are weak.
The meaning of strong and weak faults is the resistance to move. The stronger the fault the less likely it is to move. The Loma Prieta is now thought to have moved because of two 5.0M+ quakes that occurred 15 months and 2 months prior to the quake. These two quakes didn't trigger the quake directly but indirectly. They allowed the fault the quake occurred on to loose up a bit, which in turn allowed it to move. It was thought that it was so tightly compressed that it couldn't move. The Hector Mine quake was another such quake. The 1992 Landers quake loosened the fault enough that it started moving. It took around 7 years for it to loosen enough for the quake to occur.
I read a report some years ago that stated the 7.5M Kern County quake of 1952 was the start of the current quake activity in Southern California. Whether it is or isn't is immaterial. When the next major quake quake occurs in the heart of Southern California it will be unlike any quake seen in California. This is due to the population density. The support system will be overwhelmed. Take Care...Don
Snip: Southern California Earthquake Center
“Phase I entitled "Future Seismic Hazards in Southern California: Implications of
the 1992 Landers Earthquake Sequence" was an in-house report developed jointly
with the U.S. Geological Survey, the California Office of Emergency Services and
the California Department of Conservation. It was the first comprehensive study to
evaluate how a large earthquake might influence future earthquakes along major
nearby faults – an important ingredient in seismic hazard analysis. The report noted
that: 1) portions of the southern San Andreas fault appear ready for failure, and
that where data are available, the time elapsed since the last large earthquake
exceeds the long-term average, 2) since 1985, earthquakes have occurred at a
higher rate than for the preceding four decades, 3) the M7.3 Landers earthquake is
estimated to have increased the stress toward failure on parts of the southern San
Andreas fault, and 4) Some aftershocks of the Landers earthquake sequence
occurred near the San Andreas fault, while a few appeared to be within the mapped
fault zone in areas where, typically, the seismicity has been relatively low. Major
findings are as follows:”
“The Landers earthquake increased the stress toward failure by up to 10 bars
for the San Bernardino segment of the San Andreas, and less than 1 bar for
the Coachella Valley segment, but decreased the stress toward failure by less
than 1 bar on the Mojave segment (Figure 8 in report).”
“Changes in failure stress up to a couple of bars also occurred on the
somewhat more distant San Jacinto and Garlock faults.”
“The increase in earthquake activity since 1985, including the Landers
sequence, resulted (as of 1992) in an increase in estimates of the annual
probability throughout southern California. The annual probability of a M7
or larger earthquake prior to 1985 was estimated to be about 4%. Following
Landers, estimates ranged from 5-12%, depending on the effects of stress
redistribution by the Landers earthquake and the ripeness for failure of the
southern San Andreas fault. (Note: the nearby M7.1 Hector Mine earthquake
occurred in 1999).”
http://geology.gsapubs.org/content/30/6/571.short
http://www.scec.org/aboutscec/documents/STCFinalReport9.24.2002.pdf
http://www.earthsciences.osu.edu/b4/Site/Welcome_files/Belle_B4.pdf
Right now all they can do is estimate and they don't really know how good those estimates are. If the fault is a strong fault it could take a lot of stress to cause it to fail, but then again if it is a weak fault it may not take very much stress to cause it to fail. There is also a problem with that. No one is really sure as to which faults are strong and which ones are weak.
The meaning of strong and weak faults is the resistance to move. The stronger the fault the less likely it is to move. The Loma Prieta is now thought to have moved because of two 5.0M+ quakes that occurred 15 months and 2 months prior to the quake. These two quakes didn't trigger the quake directly but indirectly. They allowed the fault the quake occurred on to loose up a bit, which in turn allowed it to move. It was thought that it was so tightly compressed that it couldn't move. The Hector Mine quake was another such quake. The 1992 Landers quake loosened the fault enough that it started moving. It took around 7 years for it to loosen enough for the quake to occur.
I read a report some years ago that stated the 7.5M Kern County quake of 1952 was the start of the current quake activity in Southern California. Whether it is or isn't is immaterial. When the next major quake quake occurs in the heart of Southern California it will be unlike any quake seen in California. This is due to the population density. The support system will be overwhelmed. Take Care...Don
Snip: Southern California Earthquake Center
“Phase I entitled "Future Seismic Hazards in Southern California: Implications of
the 1992 Landers Earthquake Sequence" was an in-house report developed jointly
with the U.S. Geological Survey, the California Office of Emergency Services and
the California Department of Conservation. It was the first comprehensive study to
evaluate how a large earthquake might influence future earthquakes along major
nearby faults – an important ingredient in seismic hazard analysis. The report noted
that: 1) portions of the southern San Andreas fault appear ready for failure, and
that where data are available, the time elapsed since the last large earthquake
exceeds the long-term average, 2) since 1985, earthquakes have occurred at a
higher rate than for the preceding four decades, 3) the M7.3 Landers earthquake is
estimated to have increased the stress toward failure on parts of the southern San
Andreas fault, and 4) Some aftershocks of the Landers earthquake sequence
occurred near the San Andreas fault, while a few appeared to be within the mapped
fault zone in areas where, typically, the seismicity has been relatively low. Major
findings are as follows:”
“The Landers earthquake increased the stress toward failure by up to 10 bars
for the San Bernardino segment of the San Andreas, and less than 1 bar for
the Coachella Valley segment, but decreased the stress toward failure by less
than 1 bar on the Mojave segment (Figure 8 in report).”
“Changes in failure stress up to a couple of bars also occurred on the
somewhat more distant San Jacinto and Garlock faults.”
“The increase in earthquake activity since 1985, including the Landers
sequence, resulted (as of 1992) in an increase in estimates of the annual
probability throughout southern California. The annual probability of a M7
or larger earthquake prior to 1985 was estimated to be about 4%. Following
Landers, estimates ranged from 5-12%, depending on the effects of stress
redistribution by the Landers earthquake and the ripeness for failure of the
southern San Andreas fault. (Note: the nearby M7.1 Hector Mine earthquake
occurred in 1999).”
http://geology.gsapubs.org/content/30/6/571.short
http://www.scec.org/aboutscec/documents/STCFinalReport9.24.2002.pdf
http://www.earthsciences.osu.edu/b4/Site/Welcome_files/Belle_B4.pdf