SCEC Award Number 11062 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Imperial Valley and Path Calibration
Investigator(s)
Name Organization
Don Helmberger California Institute of Technology
Other Participants
SCEC Priorities C, B1, B3 SCEC Groups Seismology, FARM, CDM
Report Due Date 02/29/2012 Date Report Submitted N/A
Project Abstract
 The recent EMCE raises some fundamental questions about the complex-temporal rupture characteristics, Hauksson et al. (2010). The geometry of faults is usually thought to be more complicated at the surface than at depth and to control the initiation, propagation, and arrest of seismic ruptures. The fault system that runs from southern California into Mexico is a simple strike-slip boundary: the west side of California and Mexico moves northwards with respect to the east. However, the Mw7.2 2010 El Mayor-Cucapah earthquake on this fault system produced a pattern of seismic waves that indicates a far more complex source than slip on a planar strike-slip fault, Wei et al. (2011). Here we used geodetic, remote-sensing and seismological data to reconstruct the fault geometry and history of slip during this earthquake. We find that the earthquake produced a straight 120 km long fault trace that cut through the Cucapah mountain range and across the Colorado River delta. However, at depth, the fault is made up of two different segments connected by a small extensional fault. Both segments strike N130ºE, but dip in opposite directions. The earthquake was initiated on the connecting extensional fault and 15s later ruptured the two main segments with dominantly strike-slip motion, exemplary figure.
We are presently continuing the timing calibrations into Northern Baja and off-shore, using Ambient Seismic Noise (ASN) and well-located events as displayed in Fig. 2 (exemplary figure) for the EMCE aftershocks taking advantage of space geodetic methods. Applying these calibrations in conjunction with our teleseismic methods should help refine older events and rapidly assess future events near our SC station network.
We are presently continuing the timing calibrations into Northern Baja and off-shore, using Ambient Seismic Noise (ASN) and well-located events as displayed in Fig. 2 (exemplary figure) for the EMCE aftershocks taking advantage of space geodetic methods. Applying these calibrations in conjunction with our teleseismic methods should help refine older events and rapidly assess future events near our SC station network.
Intellectual Merit Our work has contributed significantly to SCEC in several ways; we have expanded the Cut-And-Paste (CAP) methodology to include (1-C) locating events with surface waves, Tan et al.(2010), Wei et al.(2011), (2-A9) short-period calibration and high resolution of rupture characterization of small events (Tan and Helmberger, 2007, 2010), Luo et al.(2010)(3-C) and the use of Ambient Seismic Noise (ASN) for path calibration, Zhan et al. (2011). We have also conducted a detailed study of the El Mayor-Cucapah Earthquake (EMCE) and some of the larger aftershocks.
Our work has also contributed to (B1) with the report on the 2010 El Mayor-Cucapah event. We have also initiated a joint study with Rob Graves on comparing 1D, 2D, & 3D synthetics from SCEC models (B3-B5) and expanding on the hybrid approach introduced by Graves involving the addition of the Kostrov-like square root singularity to longer period inversion models. We are testing this approach on the huge broadband dataset obtained from the Tohoku sequence against well-developed 3D models just submitted (Wei, Graves, and Helmberger).
Broader Impacts Our modeling group has developed some new tools for locating and obtaining rupture properties which will be passed on to the SCEC community. We are also developing a new code, Li and Helmberger (2012) which uses a GPU graphics card scheme. In short, we now have the computer power to address complex structures in interactive modeling which will be very useful in teaching and improving SCEC models.
Exemplary Figure FIGURE 2. (a) Mechanisms of aftershocks of the 2010 El Mayor - Cucapah earthquake and some earthquakes occurred
during the past decade. Mechanisms are determined by regional CAP inversion. A red star indicates the epicenter of the
mainshock. Event 14565620 happened on Dec 30, 2009 is studied in detail. It has a well-determined mechanism from
teleseismic modeling and is located by GPS. The color of beach balls represents depth. The black rectangles named F1,
F2, F3 and F4 are fault segments used in the finite fault inversion (Wei et al., 2010) projected onto the free surface.The
triangles are broadband stations nearby. The black triangle shows the station NE70 from the NARS narray (2003-2008).,
which is used in the noise cross-correlation study (Tian et al., 2011). (b) A 3D view of the Imperial Valley basin. The red
star indicates the epicenter of the main shock and the yellow star shows the master event (14565620) and two normal
events (green and blue dots) used in calibration. (c) The velocity models used in their study. (d) The best joint inversion
model of the El Mayor-Cucapah Earthquake.along with the static observation which is composed by optical image and
InSAR azimuthal offset image. The dots indicate the right lateral motion along strike, which are obtained from SPOT
observation (red) and azimuthal offsets of track 211 of ALOS PALSAR (blue). (e) Two representative teleseismic P-wave
displacements (black) and synthetic (red) generated by the preferred model with contributions from F1-F4. The station
names are indicated to the left of the traces along with the azimuths and epicentral distances in degrees. The peak amplitude
in micron of data are indicated above the end of each trace.