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08-15-2015, 07:47 PM
Hi all,
A paper will be published in Geophysical Research Letters any day now, that models giant tsunamis from local earthquakes, affecting Ventura and other places. This paper is really deeply flawed IMHO. I think it will get a lot of local press. I just a few minutes submitted my Southern California Earthquake Center abstract. Parts are technical: I'll be interested whether Brian has any trouble with it. Co-author Behl did not reply to my emails asking whether OK to list him as co-author: because many people cannot keep up with their email, I generally write "lack of a response will be taken as OK".
see below:
Chris
Christopher C. Sorlien, Craig Nicholson, Marc J. Kamerling, and Richard J. Behl
The Hosgri fault zone offshore south-central California and the North Channel-Pitas Point-Red Mountain fault system (NC-PP-RM) in Santa Barbara Channel are imaged with multiple grids of closely spaced 2D multichannel seismic reflection (MCS) profiles and seven 3D MCS surveys. From these data and precise stratigraphic age control, we constructed digital 3D fault and fold surfaces. Active strike-slip faults that dip moderately in their upper few km include the southern Hosgri fault. This fault is locally even flatter, dipping less than 30°E below 1 km depth. Ten km west of UCSB, two strands of the Red Mountain fault dip 60°N, with the southern strand exhibiting normal-separation of early Quaternary strata (late Quaternary is missing). There is a segment boundary in the NC-PP-RM fault system in this area, with a 25° bend in fault strike so that the faults to the west are not perpendicular to the shortening direction. West of this bend, the Red Mountain fault strands merge with the underlying blind faults dipping 25° N. The shallow northern strand terminates into a right-stepping en-echelon pattern on the sea floor, consistent with left-lateral shear. A component of left-lateral strike-slip motion is thus expected on the underlying gently-dipping blind faults.
Ten dated horizons ranging in age from 1,800 ka to 110 ka were interpreted across the 120 km-long offshore part of the NC-PP-RM fault-fold system. The horizons illustrate folding by progressive tilting, requiring continuously variable vertical motion perpendicular to strike. On scales of hundreds of kyr, or even during a single earthquake, the vertical motion will not be a step function across an active axial surface or an emergent fault. It will gradually increase from zero in the basin to a peak at the anticline crest. Some of the blind fault strands do not appear to have propagated updip during the last few hundred kyr. Wherever there is forelimb progressive tilt, if the faults propagate at all, they have to do so more slowly than the slip rate. For non-propagating (oblique) thrust faults, in any one earthquake or hundreds of quakes, the slip is gradually absorbed updip by folding across several km of the hanging wall. Thus, this folding should be incorporated in tsunami and strong ground motion modeling. The localized 6-to-8 m Holocene uplift events at Pitas Point are thus unlikely to be representative of the seafloor offset along the principal N-dipping fault system that extends offshore.
A paper will be published in Geophysical Research Letters any day now, that models giant tsunamis from local earthquakes, affecting Ventura and other places. This paper is really deeply flawed IMHO. I think it will get a lot of local press. I just a few minutes submitted my Southern California Earthquake Center abstract. Parts are technical: I'll be interested whether Brian has any trouble with it. Co-author Behl did not reply to my emails asking whether OK to list him as co-author: because many people cannot keep up with their email, I generally write "lack of a response will be taken as OK".
see below:
Chris
Christopher C. Sorlien, Craig Nicholson, Marc J. Kamerling, and Richard J. Behl
The Hosgri fault zone offshore south-central California and the North Channel-Pitas Point-Red Mountain fault system (NC-PP-RM) in Santa Barbara Channel are imaged with multiple grids of closely spaced 2D multichannel seismic reflection (MCS) profiles and seven 3D MCS surveys. From these data and precise stratigraphic age control, we constructed digital 3D fault and fold surfaces. Active strike-slip faults that dip moderately in their upper few km include the southern Hosgri fault. This fault is locally even flatter, dipping less than 30°E below 1 km depth. Ten km west of UCSB, two strands of the Red Mountain fault dip 60°N, with the southern strand exhibiting normal-separation of early Quaternary strata (late Quaternary is missing). There is a segment boundary in the NC-PP-RM fault system in this area, with a 25° bend in fault strike so that the faults to the west are not perpendicular to the shortening direction. West of this bend, the Red Mountain fault strands merge with the underlying blind faults dipping 25° N. The shallow northern strand terminates into a right-stepping en-echelon pattern on the sea floor, consistent with left-lateral shear. A component of left-lateral strike-slip motion is thus expected on the underlying gently-dipping blind faults.
Ten dated horizons ranging in age from 1,800 ka to 110 ka were interpreted across the 120 km-long offshore part of the NC-PP-RM fault-fold system. The horizons illustrate folding by progressive tilting, requiring continuously variable vertical motion perpendicular to strike. On scales of hundreds of kyr, or even during a single earthquake, the vertical motion will not be a step function across an active axial surface or an emergent fault. It will gradually increase from zero in the basin to a peak at the anticline crest. Some of the blind fault strands do not appear to have propagated updip during the last few hundred kyr. Wherever there is forelimb progressive tilt, if the faults propagate at all, they have to do so more slowly than the slip rate. For non-propagating (oblique) thrust faults, in any one earthquake or hundreds of quakes, the slip is gradually absorbed updip by folding across several km of the hanging wall. Thus, this folding should be incorporated in tsunami and strong ground motion modeling. The localized 6-to-8 m Holocene uplift events at Pitas Point are thus unlikely to be representative of the seafloor offset along the principal N-dipping fault system that extends offshore.