Correct Seattle Fault Zone (SFZ) rates and magnitudes

[pmpowers-usgs]

Reference issue: nshmp-haz-fortran #32

Table 1. The following table summarizes the SFZ GR parameterization spanning 3 model updates: 2002, 2008, and 2014.

Strand	M (2002)	M (2008)	L (2008)	a (2008)	M (2014)	L (2014)	a (2014)	M (WC94L)
North	7.2	7.2	71	1.15	7.23	71	1.15	7.23
Middle	7.1¹	7.2¹	64	1.11	7.2	64	1.15	7.18
South	7.2¹	7.1¹	56	1.16	7.2	56	1.15	7.10

¹ The 2002 OFR describes the magnitudes as presented, but this is inconsistent with their lengths and was likely a typo.

Table 2. Recurrence Intervals

Year	GR	CH	Total (50% CH, 50% GR)
2008	977	5000	1629
2014	982	5000	1636
2014x²	—	—	1408

² This the recurrence interval for the incorrectly updated 2014 model that needs to be reverted.

Background

1. For the 2014 model (in the USGS fortran codes), there was a lot of back and forth between members of the team that involved both the periodic automatic generation and manual editing of hazard input files. This introduced a variety of errors and inconsistencies that were corrected in late 2015, e.g. #28, #30, #31.
2. In computing fault slip rates, the geodetic modelers were supposed to only be supplied with the primary traces for faults. That is, for faults with alternate traces, for example the Southern Whidbey Island Fault, only the primary 'middle' trace was to be used when inverting for slip rates. This detail was agreed upon internally but not recorded in the 2014 OFR.
3. The primary description of the Seattle Fault Zone is on p. 79 of the 2014 OFR. Although the description includes updated references relative to the 2002 and 2008 models, the description states: "the recurrence parameters are the same as those used in the 2008 NSHMP maps". Only on p. 204, in the discussion of hazard changes in the WUS is it mentioned that: We have not modified this fault using the geodetic model. Although recurrence rates for the SFZ are difficult to constrain, this treatment is inconsistent with the treatment of WUS faults generally, and should be better justified and documented.
4. The implementation approach that was used to 'ignore' the geodetic models was to swap in the geologic rates so as to not have to change the weights on the geologic model. However, in the 2014 model, geologic magnitudes and rates for the SFZ changed, without apparent justification, further confusing things; more on this below.

Introduced Error

In making the corrections in item 1, above, item 3 was overlooked and therefore geodetic based slip rates were incorrectly added to the model. This issue serves as a marker that this change needs to be reverted.

GR Parameter Changes (2008 to 2014)

In 2002, Art Frankel revised the SFZ to be modeled as 3 distinct strands. The northern strand was modeled as a characteristic M7.2 rupture with a 5000yr recurrence receiving 50% weight, and all three strands were used to model GR behavior (also with 50% weight), with the total rate across all three strands approximating a M≥6.5 recurrence rate of 1100yrs. The actual MFDs for the 2008 GR branch give a recurrence interval closer to 1000 years (977 years in Table 2). The total return period considering both GR and CH branches is around 1630 years.

In 2008, the Seattle model was not modified in any way, and there is no discussion of it in the corresponding OFR. For some reason, however, despite citing consistency with 2008 in the 2014 OFR, both the geologic rates and magnitudes changed (values in bold in Table 1). While the rate changes affect have little effect on recurrence interval (1636 vs 1629 years), the magnitude changes certainly will affect hazard. Although the traces were modified slightly in 2014 to correct GIS projection errors, they only moved on the scale of meters and the lengths remained the same. Table 1 also shows the Wells & Coppersmith (1994) length based magnitudes to two decimal places. In the case of the northern strand, the magnitude change reflects an increase in precision consistent with the length of its trace. If a precision change was to be made, however, why was it then not applied to the middle and southern strands as well? In any case, the M7.2 for the southern strand is inconsistent with WC94L M7.1 and is likely in error.

The 2014 model also added CH branches with nominal rates of 1e-7 for the middle and southern strands. This has little effect on M≥6.5 rates and hazard, and provides consistency between GR and CH representations.

Pending further comment, the 2014 SFZ model will be reverted to match that used in 2008 while preserving the 1e-7 characteristic branches.

See also: #17