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Tom Rockwell is posting info and photos from the field reconnaissance at
ftp://roswell.sdsu.edu/incoming/Rockwell/Borrego%20Earthquake/
Brad
The attached PDF file from Nancy King summarizes the USGS activities for the Baja earthquakes as of 4/7/2010.
Brad
I have just completed a low frequency (f<0.5 Hz), finite fault ground motion simulation of the Sierra El Mayor event. The rupture model consists of 3 segments striking roughly to the NW and dipping at 80 degrees to the east. The rupture model was calculated using the pseudo-dynamic approach of Graves and Pitarka (2010). The slip distribution is stochastic and constrained to have a wavenumber squared falloff with the standard deviation set to 85% of the mean slip (mean slip = 1.5 meters). The average rake is set to -160 degrees (normal oblique). See attached figure for more details. Note that no information regarding observed displacements were used in generating this slip distribution. As more observations become available these can be incorporated into revised versions of the rupture model.
The simulation uses Version 4 of the SCEC 3D Community Velocity model discretized at 200 meter spacing and with a minimum shear velocity threshold set at 500 m/s. The simulation was run at USC’s HPCC Linux cluster under a SCEC CME project grant.
The simulation clearly shows the strong concentration of wave energy toward the NW due to rupture directivity effects. Strong basin amplification and extended shaking durations are also clearly evident in the Laguna Salada (just west of fault), Imperial, Coachella and Los Angeles basins.
- Rob Graves
A joint USGS/CGS team has been mapping fault rupture north of the border in the Yuha
Desert. As of April 7 we have mapped ground fractures on five faults. We found
right-lateral displacement of up to 2 cm along the northern extension of the Laguna Salada
Fault (south from Sunrise Butte/ Highway 98 to Pinto Wash) and similar displacement along an unnamed
north-northwest trending fault to the west. We also mapped left-lateral displacement (up to 4.5
cm) along a previously unmapped northeast-trending fault that crosses Highway 98 to the
east of Sunrise Butte. This fault has surface expression indicating a history of past
displacement. It lies within the broad zone of aftershocks north of the border.
Additionally, we mapped fractures along two small previously mapped scarps, west of the Laguna
Salada Fault. One fracture set along a northeast-trending scarp may be triggered slip
while another set, on a north-northwest trending scarp, appears to be just trace
fracturing.
Katherine Kendrick
Michael Rymer
Jerry Treiman
I've converted the lidar planning polygons shown in the map by Ramon to KMZ format so that they can be viewed in Google Earth. See attached.
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Caltech and USGS seismologists have analyzed data collected during the April 4, 2010, M7.2 Sierra El Mayor, Mexico earthquake. They have been able to identify two separate subevents that occurred within approximately 15 seconds of each other.
The first subevent began at the epicenter that was originally reported, 11 miles west-south-west of Guadalupe Victoria, Baja California, Mexico. The second subevent occurred about 15 seconds after the first just north of the original epicenter. The second subevent was larger than the first and the magnitude of the two events combined still stands at M7.2. The seismograms show a complex over print of these two events, and more research needs to be done to sort out additional details.
This new result helps explain several observations. First, that the aftershock distribution forms two clusters that correspond to these two subevents. Similarly, preliminary field mapping of the fault rupture at the surface is consistent with slip on two faults. The first subevent was most likely on the Pescadores fault while the second subevent appears to have broken the Borrego fault in Baja California.
Complex earthquakes comprised of subevents have been identified in the past, notably the M7.3 Landers, California event of 1992 and the M7.9 Denali event in Alaska in 2002.
Subevents are "bursts of energy" that sometimes can be identified as separate events in seismograms and assigned a location within the faults that ruptured in the earthquake.
At 4 pm (Pacific) on Wednesday afternoon (April 7th), the Earthquake Notification System sent out a notification of a M6.6 event that had occurred Sunday on April 4th . This notification was mistaken by some as a new event on Wednesday but in fact, it was one of the subevents being processed. The M6.6 estimate is not considered to be a reliable magnitude for this subevent.
Report from John Tinsley and Robert Sickler.
This is a brief report of findings of the USGS liquefaction reconnaissance team on 4/6/2010.
The weathered plutonic rocks along our way in the Peninsular Ranges were observed to exhibit of many precarious rocks, until we were 5 to 6 miles west of Ocotillo, when small rockfalls and dented pavement, guard rails, and road verges (CALTRANS cleans up quickly) showed we had entered an area where recent ground motions were strong enough to dislodge rocks from road-cuts. This was along I-8 where the growing array of aftershocks is extending to the NW; we may have been seeing some degree of directivity? None of these rockfalls was more than a few cubic meters for volume, but you would not have cared to be there when they occurred.
We also phoned the GEER folks. Jonathan Bray was in his office and got through to Jon Stewart and Scott Brandenberg. Cell coverage turns out to be spotty in distant parts of the valley, but is adequate in most areas. Scott was still in the field, but was slated to return to UCLA that evening; Jon Stewart already had returned to UCLA, but we talked to one another, and then arranged to meet Scott after lunch to see what they'd been doing the previous day or so.
Scott generously gave us copies of their GPS track lines, so we could see where they had and had not been. This enabled us to pick targets that enhanced and added to the field recon, so all hands should understand that total collaboration and coordination is obviously well underway; it makes it easier for all concerned when these arrangements go so smoothly. The GPS satellite cluster was favorable, and our locations should all be well within 4-5 m horizontal positional accuracy. We will be exchanging location data etc when we return to Menlo. Something is squirrely with my Garmin software, which is quite annoying. I thought this issue was fixed, but I'll have to deal with it upon my return to Menlo. My downloads abort mysteriously.
We GPS-located some 48 waypoints and snapped more than 70 photos documenting ground failures along our way from El Centro to Calexico, thence west to the terminus of the All American Canal and thence north along the Westside Main Canal towards Interstate 8. Clearly effects were more intense the further west (towards source) we proceeded. I'll wager that there are some really fine liquefaction features closer to the epicenter in Mexico; hopefully someone will be working to locate and describe those effects.
The initial feature of interest for our follow up was the distribution of liquefaction-related effects commencing at the western terminus of the All American Canal, where it feeds into the Westside Main Canal, which in turn supplies irrigation water to the entire west side of the Imperial Valley. The flow control gate area suffered widespread liquefaction, vented sand, and lateral spreading with attendant ground cracking and bank collapse involving fill materials associated with the levee embankments and natural deposits adjacent to the canal channels. Proceeding north and west along the Westside canal banks, we observed nearly continuous zones of lateral spreading, extensional cracking, becoming less intense the farther we ranged to the north and west. There was liquefaction of both approaches to the bridge where Drew Road crosses New River; GEER documented this site thoroughly, so we simply took photos, chatted with the locals who were amused by scientists at work.
Vented sand and silt are mainly associated with deposits and fills along major canals and drainages such as New River. Except near its terminus, we've seen no vented sand (sand blows or sand boils) in agricultural fields; depth to ground water is probably too great. Only along the drainages containing perennial flows is liquefaction well expressed. Even at that, extensional cracking with cracks opening toward the free face represented by the stream channel or canal is the most common expression of ground failure, with or without differential settlement. Locally we have observed differential settlement of levees, where the mass of the levee apparently crossed a liquefied substrate, and sank differentially into the liquefied zone at depth. We didn't take the time to document the details of these ground failures, leaving that for another day, but we located them and photographed them, along with making a few measurements of apparent surface displacement.
The All American Canal was wholly still functional, although some repairs are going to be needed to the earthworks here and there. The Imperial Irrigation District was busily grading and compacting one area of damage; repairs were quite complete, so photos were not terribly illuminating as to what had happened. I'll suppose that liquefaction caused levee settlement, as they had re-leveled and re-graded and filled a 150-meter long reach of levee on the north side of the canal. We found a second area where lateral spreading had damaged the northern embankment, and seepage had occurred through part of the levee, creating soft wet ground that would not support one's weight. The Irrigation District had been there, but I'll guess that they will be back before too long.
The Border Patrol is quite watchful and supportive; they came by about every 15 or 20 minutes and were always friendly and attentive, and often paused to relate assorted tales of the their earthquake experiences. The border fence is locally damaged and the Border Patrol has staked out these breaches, and presumably repairs are pending, but have yet to occur.
On Wednesday (I guess that is later today) we will continue our recon of the Westside Main Canal, and along the New River north of Interstate 8. I'd really like to try to nail down the northern limit of liquefaction if possible.
Raul Castro of CICESE is coordinating the deployment of seismometers in Mexico. They are in the process of deploying 30 seismometers they have borrowed from IRIS-PASSCAL. Attached is a map that Raul sent, showing the locations of the first 14 installed. He said the remaining IRIS-PASSCAL stations will be deployed east of Laguna Salada.
Octavio Lazaro-Mancilla of UABC-Mexicali has deployed 2 of the 3 Caltech seismometers and is deploying the third one today. One is deployed in Mexicali, and the other at the Mexicali airport (east of the city). The location of the third one will be updated here when we have that information.
Paper proposing active structure on east side of Sierra el Mayor is attached. Epicenter of the mainshock is just east of the "inferred fault" on figure 3.
I digitized (using Google Earth and published papers) a set of the faults in the Sierra El Mayor and Sierra Cucapa so here are *.kml and *.txt versions in case
anybody wants to use them for figures with Google Earth or GMT . These are not very exact nor complete. I am sure John Fletcher has better ones but
these are probably OK for the purpose of making regional maps.
I have not heard of any InSAR results yet but if they become available (particularly if they help to pinpoint any deformation east of the Sierra
Mayor or Sierra Cucapah) then you may want some flexibility for LiDAR over there too.
John-
Phil Armstrong and I are looking at the fabulous photos you took from the helicopter. We're wondering if you flew along the Rio Hardy region on the eastern side of the range? We saw a map with pins for locations, but didn't know if these were sites you visited. As we spoke about in the field, both the initial epicenter and a bunch of the aftershocks line up with the eastern edge of the range. This would be consistent with some rupture on the structure that Armstrong proposed. Of course, if the first sub-event happened on that structure it may have been too small for surface rupture, in which case it'll only be seen seismologically... In an earlier post, Ken Hudnut mentioned the abstract of the Armstrong paper - I'll attach a pdf of the paper as soon as I have it.
Expanding the survey across the Yuha basin to link up with the southern end of the Elsinore fault is a great idea. It hooks the survey into geoearthscope and gives us a great new data set to more closely examine a fault stepover / rupture termination region. We will incorperate this suggestion into the survey plan.
I generally agree with Mike's assessment that something like version 3 would be best. But let me second Duncan's comments regarding the need for coverage of a broader area at the northern end of the rupture. This event appears to have the potential for significant distributed deformation, especially (but not limited to) its northern extent, and we need the broadest possible coverage in these areas (within reason [aka budget constraints - what are they, by the way?]. So at the northern end of the rupture, perhaps use something between the limits shown as option 4 and option 1 (if we can afford the latter). Along those same lines, it would be great if we could at least get the northern part of the Laguna Salada during this acquisition, again contingent upon budget constraints. Thanks to Mike and Ramon for organizing the LiDAR acquisition. Nice job!
In reply to Duncan:
(1) If there are reports of rupture north of the border then we can include those in the proposed footprint. There may be special issues with flying over (or even turning around over) the border that need to be considered and could restrict imaging there.
(2) We will gather air photos simultaneously if at all possible. NCALM is working to borrow an air photo unit to use on the proposed survey.
Two thoughts on the LIDAR footprint:
1. It would seem important to get good coverage of the border-crossing Sierra-Centinela/Yuha-Desert region; this is where the rupture seems to have become most complicated (and so will be hardest to map) and eventually stopped. Understanding stopping is at least as important as understanding starting.
2. I hope the enthusiasm for LIDAR will not cause us to neglect the need for high-quality imagery, for which Avouac has shown how to get sub-pixel displacements, even if the older imagery is less than ideal.
There is a tar file of high sample broadband waveforms from the CI network available on ftp://www.data.scec.org/pub/CI14607652.tar.gz
It contains all CI stations with a time window of 100 s before the event origin time and 5 minutes afterwards.
Here is 'Laguna_Salada_1.1' to replace 'Laguna_Salada_1.0' that we submitted two days ago.
Here is also a new calculation, 'Laguna_Salada_1.1_stress_on_faults'
Please post both of these figures on the SCEC and UNAVCO sites.
We will update and revise these as more data becomes available.
Here is 'Laguna_Salada_1.1' to replace 'Laguna_Salada_1.0' that we submitted two days ago.
Here is also a new calculation, 'Laguna_Salada_1.1_stress_on_faults'
Please post both of these figures on the SCEC and UNAVCO sites.
We will update and revise these as more data becomes available.
The attached map, produced by Ramon Arrowsmith, shows various ideas for a lidar survey of the fault rupture. I am in favor of zone 3, which is 300 sq. km of continuous data covering the two ruptures detected (Borrego fault and Pescadero fault) as well as the nearby Laguna Salada dextral-normal fault that ruptured in 1892. I would also like to allocate an additional 100 sq. km for other targets that may be detected in coming days, or for imaging the Canon David detachment fault. Please comment on this plan or various alternatives. Bear in mind that airborne lidar is cheaper per sq. km if the survey is continuous. A large part of the cost is airplane air time.
We have added data for the two days after the earthquake generating the coseismic offsets as the difference between two days before and after. The height signal center south of the Salton Sea on site P497 and P498 are interesting. It looks like the region may have risen on average 10 mm during the earthquake.
The attached files have text and pdf figure.
Apparently it was not activated? Not sure what happened.
Those photos are really spectacular and now this rupture location becomes more interesting.
I wanted to understand better where things were, so I made the attached kmz of the reported locations in the recon report and also I rectified the Fletcher and Spelz 2009 map so you can see where the Borrego and Pescadoros faults are and that they are in the range a bit relative to the range-front-bounding Laguna Salada Fault.
Hopefully the kmzs work ok for you.
thanks!
Field Report Update from Reconnaissance Flight on Tuesday, 6 April 2010
(report updated as of 1:30 am on 7 April)
John Fletcher, Prof. CICESE, Tom Rockwell Prof. SDSU, Javier
Gonzalez-Garcia, Prof. CICESE, Ken Hudnut, Geophysicist, USGS
Anthony Guarino, Seismic Analyst, Caltech Seismological Lab
The magnitude 7.2 Sierra El Mayor earthquake caused extensive surface rupture on a 28 km portion of the Borrego fault in northern Baja California Norte, Mexico. The approximate endpoints of the rupture are as follows; the southeast end is located at 32° 25.6'N; -115° 34.3'W. The rupture continues on a N36°W orientation to the northwest end, located at 32°37.1'N; -115°44.6'W. At the northwest end of the fault rupture on the Borrego fault, many splay faults wrap northward along the west side of El Centinela mountain.
The approximate location of maximum slip is 32° 29.1'N; -115° 37.5'W, but was only observed from air, because this area will be easily reached by ground vehicles on Wed. April 7. At the maximum slip area, the group consensus based on observations from the air is that the total slip vector length is 2.5m +-0.75 m of oblique slip, with dominantly east-down motion, but also with a significant component of right-lateral motion.
To the southeast of the maximum slip section, and in an area that will be difficult to reach by vehicle, we landed and measured displacements at 32°27.7' -115° 36.6' as follows along a zone oriented N 36° W (see several photos below):
East-down: 1.10 +/- 0.1 meters
Right-lateral: 0.78 +/-0.1 meters
Total length of slip vector: 1.35 +/-0.15 meters
Fracturing along the Laguna Salada fault may be tectonic, at least in places, but the breaks may be due to compaction of the lake sediments lying west of the fault. The documentation of these fractures and discrimination between tectonic and ground failure cause will require extensive follow-up work.
An extensive search for faulting was conducted in the epicentral area of the Sierra El Mayor. Despite thorough searching in the epicentral area on the previously mapped fault traces, only a small amount of slip was observed along a short (few km) section of the Pescadores fault. The Pescadores fault, oriented N40°W, 55°E (Barnard, 1968), did not have nearly as much surface slip overall as the Borrego fault in this event.
The total slip on the Pescadores fault was relatively minor and localized. There was a small break on the Pescadores fault at 32.313°N ; -115.396°. There was ~20-40 centimeters of slip on the Pescadores fault along this short section (see photo below). This location was observed only during air reconnaissance, and more field work will be required, despite difficult vehicle access to this remote area.
Initial interpretation of today's observations is that the rupture process likely began on Pescadores fault, which is aligned N40°W from the epicenter. Possibly rupture proceeded from the epicenter for 18 km to the northwest along the Pesdadores fault. Based on seismological source-time function studies, we think there may then have been pause, possibly 6-12 secs, and then triggering of a much larger rupture along the Borrego fault after rupture had slowed and then skipped over to the Borrego fault.
Extensive liquefaction-related features were observed along canal levees, which also experienced widespread shattering along the levee crests. Other phenomena were also observed or reported, such as minor flooding, ground failure/cracking (not related to tectonic faulting), and sand blows.
Well over 1500 air photos were taken, containing embedded GPS coordinates; these will be made available to the public. Thanks to Jim McCrory, helicopter pilot and to USGS for funding today's flight.
[ in what I e-mailed out, captions were associated with each photo & map; sorry, but I don't see how to readily do these on this response page ]
University of Kansas (Doug Walker) can provide a Riegl LMS-Z620 scanner and an experienced field operator. The Riegl 620 is capable of longer range measurements than most other scanners (up to ~2km). Logistics and PI contacts would be facilitated by UNAVCO.