4 ms·

The same story applies to Utah Valley, we are 60 years overdue

by cordite 13y ago

marknadal 13y ago
One of the biggest reasons I have avoided moving to SF is because of earthquake liquefaction dangers. I however just recently moved out to Provo Utah from LA temporarily - I didn't know this was a hotbed as well. How would you compare the threat levels here to SF? Comparable? Or severely less than SF?
- dalek2point3 13y ago
  You might fins some help here: http://geomaps.wr.usgs.gov/sfgeo/liquefaction/susceptibility.html http://geomaps.wr.usgs.gov/sfgeo/liquefaction/susceptibility...
- matrix 13y ago
  The risk is lower, but still high. The last estimate was a 16.5% chance of a magnitude 7.0 earthquake in 100 years[1]. I expect the probability will be revised to be higher in the seismic hazard assessments due to be released this year. 7.0 doesn't sound big, but keep in mind, the epicenter would be quite close to the surface and likely to be right in the middle of a highly populated area on the Wasatch Front (Salt Lake City, Cottonwood Heights, etc). Also, many buildings in the vulnerable areas pre-date construction codes that take seismic risk into account. For example, it wasn't until the late 70's that they started reinforcing masonry construction. [1] http://www.livescience.com/28782-utah-bigger-earthquakes-wasatch-fault.html http://www.livescience.com/28782-utah-bigger-earthquakes-was...
cossatot 13y ago
The Wasatch fault zone does pose a significant seismic hazard but earthquakes on the WFZ are not nearly as regular as on the San Andreas. On this page [1] (second picture, left side) is a figure showing the instances of known earthquakes along the fault through time; you can see that they're quite irregular, both in space and time. An updated compilation of this data [2, gated] has better accounting of the uncertainties and a fuller record, especially on the older end. In any case, it's clear that some of the fault zone seems to be 'overdue' but other segments, including the Weber and Provo segments, have had moderate earthquakes within the past ~500 years, which is a bit shorter than the median recurrence interval. The SLC segment on the other hand may have enough accumulated stress to cause a pretty significant earthquake. Note that it's really not clear to what extent faults rupture periodically; there are researchers who argue that the likelihood of an event is basically independent of the time since the last event (basically a Poisson distribution), those who argue for fairly regular events, and everything in between. It probably varies a lot from fault to fault and is more regular on big plate boundary faults like the San Andreas than on smaller (or slower-moving) intraplate faults like the Wasatch Fault. The real danger, both here and for the San Andreas system, is that an earthquake will break through the segment boundaries and cause a much, much larger earthquake. It's not been demonstrated that this is has happened on the Wasatch fault (there is one instance about 6000 years ago where this is possible within the data uncertainty) but this has happened in California in the 1906 S.F. and 1857 Fort Tejon earthquakes. Although this might not cause more displacement of the fault in any one location, it creates such a bigger swath of destruction that there are more opportunities for associated disasters (fires, floods from dams breaking, etc.) and make response much more difficult. It's also a bit outside my expertise but I think that for a long rupture, there is enough constructive interference of surface waves from the earthquake that ground shaking is much greater than is predicted from displacement at any point on the fault. [1]: http://michaelhaskinseportfolio.weebly.com/earthquake-hazards-along-the-wasatch-fault.html http://michaelhaskinseportfolio.weebly.com/earthquake-hazard... [2]: http://www.bssaonline.org/content/98/6/2918.short http://www.bssaonline.org/content/98/6/2918.short

The same story applies to Utah Valley, we are 60 years overdue

by cordite 13y ago

The same story applies to Utah Valley, we are 60 years overdue

One of the biggest reasons I have avoided moving to SF is because of earthquake liquefaction dangers. I however just recently moved out to Provo Utah from LA temporarily - I didn't know this was a hotbed as well. How would you compare the threat levels here to SF? Comparable? Or severely less than SF?

dalek2point3 13y ago
You might fins some help here: http://geomaps.wr.usgs.gov/sfgeo/liquefaction/susceptibility.html http://geomaps.wr.usgs.gov/sfgeo/liquefaction/susceptibility...
matrix 13y ago
The risk is lower, but still high. The last estimate was a 16.5% chance of a magnitude 7.0 earthquake in 100 years[1]. I expect the probability will be revised to be higher in the seismic hazard assessments due to be released this year. 7.0 doesn't sound big, but keep in mind, the epicenter would be quite close to the surface and likely to be right in the middle of a highly populated area on the Wasatch Front (Salt Lake City, Cottonwood Heights, etc). Also, many buildings in the vulnerable areas pre-date construction codes that take seismic risk into account. For example, it wasn't until the late 70's that they started reinforcing masonry construction. [1] http://www.livescience.com/28782-utah-bigger-earthquakes-wasatch-fault.html http://www.livescience.com/28782-utah-bigger-earthquakes-was...

cossatot 13y ago

The Wasatch fault zone does pose a significant seismic hazard but earthquakes on the WFZ are not nearly as regular as on the San Andreas. On this page [1] (second picture, left side) is a figure showing the instances of known earthquakes along the fault through time; you can see that they're quite irregular, both in space and time. An updated compilation of this data [2, gated] has better accounting of the uncertainties and a fuller record, especially on the older end. In any case, it's clear that some of the fault zone seems to be 'overdue' but other segments, including the Weber and Provo segments, have had moderate earthquakes within the past ~500 years, which is a bit shorter than the median recurrence interval. The SLC segment on the other hand may have enough accumulated stress to cause a pretty significant earthquake. Note that it's really not clear to what extent faults rupture periodically; there are researchers who argue that the likelihood of an event is basically independent of the time since the last event (basically a Poisson distribution), those who argue for fairly regular events, and everything in between. It probably varies a lot from fault to fault and is more regular on big plate boundary faults like the San Andreas than on smaller (or slower-moving) intraplate faults like the Wasatch Fault. The real danger, both here and for the San Andreas system, is that an earthquake will break through the segment boundaries and cause a much, much larger earthquake. It's not been demonstrated that this is has happened on the Wasatch fault (there is one instance about 6000 years ago where this is possible within the data uncertainty) but this has happened in California in the 1906 S.F. and 1857 Fort Tejon earthquakes. Although this might not cause more displacement of the fault in any one location, it creates such a bigger swath of destruction that there are more opportunities for associated disasters (fires, floods from dams breaking, etc.) and make response much more difficult. It's also a bit outside my expertise but I think that for a long rupture, there is enough constructive interference of surface waves from the earthquake that ground shaking is much greater than is predicted from displacement at any point on the fault. [1]: http://michaelhaskinseportfolio.weebly.com/earthquake-hazards-along-the-wasatch-fault.html http://michaelhaskinseportfolio.weebly.com/earthquake-hazard... [2]: http://www.bssaonline.org/content/98/6/2918.short http://www.bssaonline.org/content/98/6/2918.short