HTDP in California (read as: Earthquake Land)
Here's a situation I haven't dealt with before:
We're working a GNSS project where a few of the local CORS were disturbed by the earthquakes in Ridgecrest (6.4 on 7-4-2019, and 7.1 on 7-6-2019 [sidebar: we got a pretty good shake from them 120+ miles away]). Thankfully SOPAC/CSRC has updated values for many of the affected stations.
My question is: am I still safe using HTDP's calculated velocities for these CORS? I understand that I have to be careful as to which controlling values I use before and after the seismic event, but other than that I should be okay... I think. Famous last words?
Which CORS do you plan to use? The time series plots I have viewed for the stations closest to the Ridgecrest events have mostly returned to their pre-event linear velocity rates. You should look at the time series plots for the stations you plan to use. If the pre and post velocity rates look consistent, you should be able to use the CSRC epoch 2019.55 positions and apply HTDP to move them to the date of your observations. Even better, why not use SECTOR to compute true-of-date positions for the CORS you plan to use for control?
Those are all UNAVCO stations. The time series plots for their stations can be accessed here:
An example for P566:
The portal to time series data through SOPAC can be accessed here:
Just so others understand the magnitude, one of the closest Continuous GNSS stations to the epicenters for the Ridgecrest events, P595, moved approximately 25 cm south by 63 cm east (about 2.2 feet total) from where it was before the events. Time series can be viewed here:
Those events, and the shifts associated with them, and several other earthquakes, are not modeled in HTDP because it hasn't been updated to include these events for many years.
You are correct that SECTOR only transforms positions over time for the CGPS stations SOPAC processes data from. HTDP will calculate a modeled velocity for any user defined location that falls within its grids. However, there are roughly 850 CGPS stations in California (included in the California Spatial Reference Network), of which only about 250 are NGS CORS.
SOPAC/CSRC has developed velocities for all of the stations and is working on a utility that will work much like HTDP (user defined locations) only be much more refined because more velocity models (nodes) will be used. It will also be capable of being updated in near real-time similar to how the 2019.55 epoch positions were established after the Ridgecrest seismic events.
This utility will also include a robust geophysical model to improve velocity interpolation in complex faulting areas between where the CGPS stations exist.
It would be possible to use the velocities of the SECTOR CGPS stations surrounding a project area and use a weighted interpolation to compute velocities for passive marks within the project. However, unless the velocities of the CGPS stations vary significantly in direction and rate, there would be little value gained in doing this manual interpolation. There are a few areas in California, such as on either side of the San Andreas or Hayward faults, where the velocity rates/direction differ significantly over relatively short distances, but those are the exception. Here is an example. The velocities shown are from HTDP. I use this slide in courses I teach.