Embed Size (px)
Citation preview
Annual Report 2010
10194: Finite fault parameterization of moderate and large earthquakes in Southern
California using both strong motion and high rate GPS
Chen Ji (UCSB)
Finite fault parameterization of moderate and large earthquakes in Southern California using both strong motion and high rate GPS
1. Software development
This year we received $20,000 from SCEC to continue the development of realtime finite fault system using both strong motion and high rate GPS data. With this support, we have modified the system to include the high rate GPS time series into the realtime finite fault system. We also optimized algorithms to take advantage of multi-core CPUs using Open Multi-Processing (OpenMP) technique. Using a currently typical PC workstation powered by only one i7 2.66 GHz Quad-Core CPU and for a typical inversion scenario that using 50 seismograms to constrain the fault rupture on 200 subfaults, the data processing and subfault Green’s function (Ji et al., 2002a) now could be accomplished in a minute and a single finite fault inversion could be accomplished in 4 minutes, three times faster than the previous algorithm which relies on single computational thread. The computational time could be easily reduced a few more times with multiple 6-core CPUs before it reaches the barrier of parallelized computing. Although the total latency from the rupture initiation and quick result available online also includes the delays for earthquake locating, moment tensor inversion, fetching the strong motion and high rate GPS data, graphic and web page preparations etc, to be less than 15 minutes, the goal when this SCEC project was initiated in 2008, becomes quite feasible.
Figure 1. Screen capture of this Web-based Finite Fault Inversion System (WFFIS), using the 2010 El Mayor earthquake as an example. Note that now all surface information, such as fault trace, as well as fault geometry, aftershocks, could be viewed in both 2D (map view) and 3D.
We attempt to accelerate the upgrade processes of finite fault inversion when other geophysical and geological information becomes available and also reduce the time in training graduate students to perform such tasks. It leads us to be developing a web-based finite fault inversion system using Java and Perl Scripts. Figure 2 shows a screen capture of the main web page under testing, which uses the 2010 El Mayor-Cucapah earthquake as an example. It can be seen that now you can build fault planes with either an external file or dynamically filling the forms, review observations, and launch the finite fault inversions, through this web interface. It also allows you to import and view various KML/KMZ files with Google topography as the background, using Google Maps API. The modules of 3D visualizations to the fault slip and aftershock distribution are also added. Other modules, such as multiple point source inversions mentioned above could be included as well in a future effect. We plan to archive all results obtained during our previous kinematic studies in this system.
2. 2010 El Mayor-Cucapah earthquake
The El Major-Cucapha earthquake of 22:40 UTC, April 4th, 2010, occurred in northern Baja California, nearly 50 km south of the USA-Mexico border. The hypocenter determined by California Integrated Seismic Network (CISN) located at 32.259oN, 115.287oW at a depth of 10 km (http://www.scsn.org/). Prior to this event, in 1892 a magnitude 7.2 Laguna Salada earthquake had occurred in this region (Figure 1)[Hough and Elliot, 2004]. By May 1st, there had been more than 84 aftershocks with a magnitude ≥ 4.0 (Figure 1), extending for about 120 km from the coast of the Gulf of California to the USA-Mexico border.
The El Major-‐Cucapha earthquake is an unusual event because there is a large compensated linear-‐vector dipole (CLVD) component in its moment tensor solutions obtained by different research groups, besides the Mw 7.2 strike-‐slip dominant mechanism on a high angle fault orienting northwest. It was the reason why our realtime system which uses the nodal plane of the best double couple solution inferred from the moment tensor as fault plane failed to simultaneously match the long period surface waves and broadband body waves. If we describe the relative size of CLVD by parameter ε, defined as ),max(/ 312 λλλ− , here 321 ,, λλλ are diagonal element of the moment tensor in the principal axes coordinate system, ordered such that 321 λλλ >> , [e.g., Nettles and Ekstrom, 1998]. The ε is zero for pure double couple and ± 0.5 for pure CLVD. The estimated ε of the El Mayor-‐Cucapah mainshock varies from 0.28 to 0.38 in the moment tensor solutions of the GCMT, USGS and SCSN, respectively. In contrast, only 4% of shallower earthquakes in the GCMT catalog since 1976 deviate strongly from a double-‐couple geometry with a ε>0.3 [Nettles and Ekstrom, 1998]. The ε of the 1992 Landers and 1999 Hector Mines earthquakes were -‐0.01 and 0.05, respectively. The large CLVD might reflect the slip occurring on a curved fault or the existence of additional sub-‐events with a focal mechanism different significantly from the focal mechanism of the best double couple solution.
Figure 1. (a) Map of the fault geometry of the 2010 El Major-‐Cucapha earthquake. The boxes show the surface projection of the preferred three-‐segment fault model we inferred. Purple triangles and blue circles denote strong-‐motion stations and GPS sites with 5 Hz sample rate, respectively. Comparison (left top) of the horizontal component vectors of the observed (black arrow) and synthetic (red arrow) GPS displacements is showed. (b) Cross-sections of slip distributions. PF denotes the Pescadores Fault, BF marks the Borrego Fault. Yellow dots denote ML≥4 aftershocks. Slip motion directions of the hanging wall relative to the footwall are indicated by white arrows. Contours show the rupture initiation time in sec.
We have combined inverted four different datasets in our preliminary investigation (Zhao et al., 2010, in preparation). Our teleseismic dataset includes 39 teleseismic P, 40 SH, and 51 long period surface waves, providing very good far field azimuth coverage. Our local dataset composed of 1) horizontal components of 5-‐Hz sampling rate GPS waveforms at 14 PBO stations with epicentral distances less than 130 km (Courtesy of Dr. K. Larson). We have lowpass filtered these data to less than 0.2 Hz to reduce 3D basin effects. 2) The horizontal GPS displacement measurements with amplitudes larger than 1 cm. 3) waveforms of seven CICESE stations. We use a three-‐segment fault geometry to approximate this complex ruptures. The rupture initiated at Fault-‐1 near the hypocenter which has a strike of 11 degrees and dip 57
degrees to the east, the Fault-‐2 locates at northwest of epicenter with a strike of 317 degrees and dip 75 degrees to the northeast. Fault-‐3 locates at southeast of epicenter and dip to the southwest.
Our result reveals a complex rupture process of the 2010 event. It had a weak initiation. The energetic rupture started from 6 s to 12 s after initiation as a normal fault near the hypocenter with a magnitude of 6.1 to 6.2 and then ruptured bilaterally to both southeast and northwest directions. The rupture is very heterogeneous, changing from pure strike-‐slip motion to oblique motion from epicenter to the northwest, consistent with surface observations. The rupture stopped sharply 50 km northwest of epicenter. Our result suggests that this rupture extended as far as 50 km to the N133oE and had two large asperities. The first one is close to the epicenter and the second one is 40-‐45 km northwest, with a peak slip of 5 m. The inversion also requires a large normal motion (Mw 6.9) on a separating fault plane with a lower dip angle (65o) to match the long period surface waves. The total seismic moment is 1.3x1020 Nm, with most of slip occurring 18 to 40 s after the rupture initiation.
