From Dan Ponti (USGS) 8/26/14 01:36 PDT:

All -

Tom Holzer, Tim Dawson and I observed well-expressed right-lateral and transpressive faulting in natural ground just west of Buhman Ave between Twin Oaks Drive and Congress Valley Rd. Right lateral displacement within the ~0.4 km long zone that we were able to access is the largest observed to date, and is on the order of 40 cm, with the maximum displacement measured at 46 cm. Over the region we examined, strike of the fault rupture changed from ~345 deg. northward to nearly 0 deg. As the strike changes to the northerly trend, displacement sense changes from nearly pure strike-slip to a more transpressive rupture with notable vertical offset on the order of 15-20 cm (down to the east) and E-W shortening estimated at 10-15 cm. This observation would indicate a clockwise rotation in the slip vector to a N-NE orientation in association with the change in fault strike.

Although we were unable to walk out the zone today farther to the north and south due to time and access constraints, the zone of fault rupture projects to the north to where UC Davis investigators reported offsets at Oak Rock and Leaning Oak Drives, and to the White Cliff Circle area where the deformation zone trends N-NE and several investigators have reported dominantly compressive deformation, evidenced by pavement buckling and tented sidewalks. To the south these large ruptures likely connect with cracking and small displacements noted at Henry Road near the intersection of Buhman Ave. In the vineyards to the south of the zone we mapped, the fault zone appears to become broader, and more diffuse although this needs to be verified with additional mapping. Some afterslip may be occurring, based on anecdotal comments from residents who described several cracks as growing in width on the order of 1 inch since the earthquake. Based on these same accounts, however, we would infer that most of the observed displacement is coseismic.

The region of large displacement occurs approximately 8.4 km NW of the epicenter of the mainshock, which is in the region where the preliminary finite fault model from UC Berkeley shows high slip within a few km of the surface. It also appears to be coincident with a bedrock fault shown by Graymer that juxtaposes Cretaceous and Paleogene sediment.

I've attached an image showing the location of rupture zone we documented in red, along with a couple of photographs to give you the flavor of the rupture character. Tom and I have a lot more photos and measurements that we will upload onto the Google Drive Tuesday.

Dan

Doug,

Chad and I were in the field again today to check some measurements and confirm observations from yesterday. We are planning to compile data for distribution tomorrow morning. We will include a summary with lat, lon, and slip in cm.

Alex

 Very cool! I will show all of this great information to my class tomorrow on Day 1 of 'Living with Earthquakes in California'. 

All,

Posting coseismic GPS offsets (and figure showing horizontal vectors). The pre-event positions are based on 7 days of daily positions. The post-event positions are based on a static solution using ~13.5 hours of data after the earthquake.

We will monitor subdaily positions to see if GPS sites show additional postseismic movement, especially given reports that surface ruptures have increased significantly since they were first observed. 

Thanks,

Sue Owen, Angie Moore, Zhen Liu & ARIA team. 

 We've derived a fault slip model from regional GPS. Slip distribution and location map/information are attached below. Please email wbarnhart@usgs.gov to request further details on the fault source model. We will continue to update on this forum as the GPS positions improve.

 Locations in kmz are in utm meters and accurate to a few meters. 

The READI group has been analyzing 1 Hz GPS data collected in real time from stations within 75 km of the epicenter. At SOPAC we have produced 1 Hz displacements using PPP-AR analysis (Geng et al., G-cubed, 2013). Attached is a slide with 1 Hz displacements from 6 stations. The complete set of plots and displacements will be posted on GPS Explorer (http://geoapp03.ucsd.edu/gridsphere/gridsphere?cid=Earthquakes) later this afternoon, or contact jgeng@ucsd.edu for a tar file. We've heard from UNAVCO that they are downloading 5 Hz GPS data from PBO stations within 300 km of the epicenter.

Attached is a list of stations and the slide.

From Doug Dreger (UC Berkeley) 8/25 11:25am:

This is a preliminary finite-source model. It uses BDM, BKS, HOPS, CVS, MNRC, MCCM, FARB, and JRSC. The model is unilateral to the NW and updip. I used the UCB TDMT focal mechanism (155,82,-172). Single time window, rise time 0.6 seconds. Data and GFs were filtered between 0.02 to 1 Hz.

I am working on filling in the Green's function data base that was used, and will then better explore the moment minimization and smoothing constraints, and will send you an update.

Jack, how does this compare with your results?

I have asked Ingrid for static GPS, which I can incorporate, and will send her a follow up email in a minute.

Best
Doug

An EERI Virtual Clearinghouse has been established at: http://www.eqclearinghouse.org/2014-08-24-south-napa/. It has interactive ArcGIS data map.