Up to 2 cm of triggered slip on the SAF in the Mecca Hills reported by Ray Weldon who is currently in the field. So far this slip has been observed between Painted and Thermal Canyons.

Mike - I am just catching up on some of the postings.  We (CGS & USGS geologists) have mapped rupture northward from the border on the two principal splays of the Laguna Salada Fault.  We have also mapped significant rupture (up to 4.5 cm) on a previously unmapped northeast trending fault within the Yuha basin.

Eight seismometers were deployed on April 6 & 7 just North of the California-Mexico border by teams from UCSB, UCR, and SDSU. The seismometers include: 1) three SCEC PBIC stations: two Q330 recorders with L4 (weak motion sensor) and EpiSensors (strong motion sensor) sensors and cell modems and one of the old RefTek recorders also with an L4 and EpiSensor and 2) five UCR stations with new RefTek recorders with L4 and EpiSensors. The two stations with cell modems had unreliable connections, so it is not clear if data will be uploaded in real time. All stations are recording data continuously at 100 sps. Data is recorded on site and will be recovered in subsequent site visits. The attached map shows the station locations with SCEC PBIC stations shown in maroon, UCR stations shown in blue, and SCSN stations shown in light green.

Attached is the latest Google Earth KML for the Double-Difference
relocations for the period 2010-03-20 through 2010-04-10 22:12z.

Also attached is a Goggle Earth screen shot of the same.

As before,
M < 3 orange circles
3 <= M < 4 red circles
4 <= M < 7 red stars
M > 7 purple star

every event is clickable for parametric info

Expand tree on left for detailed time sequencing.

Google Earth graphics are done by Bob Dollar

Attached are a screen shot and KML file set, including the GPS track collected by Prof. John Fletcher of CICESE during the initial helicopter reconnaissance flight on 6 April 2010. The track log is also attached separately as a KML file.

Also, all of the full-resolution photos mentioned in the earlier posting have been uploaded to the anonymous ftp site.

Again, here is the URL for these photos:

     ftp://mavericks.gps.caltech.edu/pub/users/hudnut/aerialsurveys/

The NASA/JPL airborne repeat-pass L-band InSAR system, UAVSAR will start the first San Andreas 'lawn mowing' flight of 2010 on Monday, April 12. The plan also contains one line over the Salton Sea, making it a total of 14 lines repeating flight lines flown in 2009. These lines should capture all of the deformation from the April 4th earthquake that we can measure with UAVSAR. The attached map shows the coverage of the planned lines. Note that there is an FAA long-range radar station at Mt. Laguna that does not allow radar imaging within a radius of 10 nautical miles (20 km), which causes gaps in many of the lines. For more information please see flightplans at JPL.

Airborne Lidar: Please comment on the AOIs defined in the attached KML. The main AOI that covers the ruptures of the Borrego fault, Laguna Salada fault, and Pescaderos fault will be submitted Monday as part of the NSF RAPID proposal for airborne post-event lidar. The northern AOI, or other AOIs, would require funding beyond what will be proposed to NSF. However, once the aircraft is in the area the cost of adding coverage is relatively inexpensive.

Ground-based Lidar: Teams from UC Davis and University of Kansas / UNAVCO will converge on the Borrego fault rupture on Tuesday (assuming that customs goes smoothly). Two TLS instruments will be conducting high resolution scans of the fault rupture for the next week. We will be working in collaboration with the geology field teams led by John Fletcher and Tom Rockwell.

Air Photos: Tom Rockwell reports that air photos are being flown for free by Eric Frost. There are also reports that INEGI may fly post-event aerial photography soon.

Need a hand?  I will be in the affected area early next week looking primarily at liquefaction, faulting, and infrastructure response.  I can prolong my time in the field if anyone needs an extra hand.  I'm a geologist by training for those who don't know, and have experience with post-eq response, landslide analysis, and paleoseismology.

Please send me an email at hstenner@gmail.com or a call on my cell at 650-906-6180.

Heidi Stenner

We have created a preliminary finite-fault slip model for the Sierra El Mayor Earthquake.  We used 3-D coseismic vectors from continuous GPS, computed in post-processing at SOPAC using GAMIT/GLOBK.  The fault geometry is 65 km long, 20 km deep, striking at 320 degrees and dipping towards the west at 80 degrees.  The computed moment magnitude from our model is 7.15, mostly strike slip, with a magnitude between 1.2 and 1.7 meters, reaching a maximum near the hypocenter.

The figure sierramayor_slip.jpg is the slip on the eastern block with respect to the western block. 

Please check the GPSExplorer page for more information, including rapid high-rate results

(create your own account and go to "Earthquakes" tab)

Brendan Crowell and Yehuda Bock

Attached figure compares observed peak ground velocity (PGV) with values from obtained from my initial 3D ground motion simulation. There are a total of 114 sites (CI network only) extending from just north of the international border to beyond the San Fernando basin (region covered by the simulation can be seen in the animation posted earlier today).

The rupture model used in this simulation (800-v1.0) has a moment of 7.8e+26 dyne-cm which gives a magnitude of Mw 7.22.

In the figure, I plot the natural logarithm of the ratio of simulated to observed PGV as a function of observed PGV. The results are displayed for two bandwidths: f < 0.2 Hz (5 sec) and f < 0.5 Hz (2 sec). These peak values are measured from the observed and simulated three-component waveforms after low pass filtering.

In general, the simulated values tend to overpredict the observations by about 30% for both frequency bands. This suggests that the moment used in the simulation is too high. Scaling the moment down by 30% would give a value of 6.0e+26 dyne-cm and thus reduce the magnitude to Mw 7.15, which is entirely plausible.

There is also a clear trend of decreasing over-prediction for increasing observed PGV values (i.e., simulation has larger over-prediction for low observed PGV compared to the higher observed PGV values). This may represent a distance trend where the more distant sites (lower observed PGVs) have larger overprediction by the simulation compared to the closer sites (higher observed PGV). This would suggest the Q used in the simulation might be too high (I use Qs = 50 * Vs, Vs in km/s; Qp = 2 * Vs). However, since the trend is about the same for both frequency bands, anelastic attenuation may not be the only culprit. Other factors (e.g., basin amplification, wave guide effects, etc.) may certainly be coming into play as well, so this needs to be investigated further.

The attached PDF file from Nancy King summarizes the USGS activities for the Baja earthquakes as of 4/8/2010.

Brad

Attached  is a PDF of a scan of Barnard's cross sections.  These are E-W cross sections going from N to S at 1:1 scale.  He describes the Borrego fault as dipping 50 to 60 degrees to the NE and having primarily dip slip offset.  He describes the Pescadores fault as dipping 50 to 60 degrees to the NE

The full citation is "Structural Geology of the Sierra de Los Cucapas, Northeastern Baja California, Mexico, and Imperial County, California" by Frederick L Barnard, PhD thesis, Univ. of Colorado, 157 pages, 1968.