| Abstract | During October of 2006, the 20-km long Superstition Hills fault (SHF) in the Salton Trough, Southern California, slipped aseismically producing a maximum offset of 27 mm as recorded by a creepmeter. We investigate this creep event as well as the spatial and temporal variations in slip history since 1992 using ERS and ENVISAT satellite data. During a 15-year period, steady creep is punctuated by at least three events. The first two events were dynamically triggered by the 1992 Landers and 1999 Hector Mine earthquakes. In contrast, there is no obvious triggering mechanism for the October 2006 event. Field measurements of fault offset after the 1999 and 2006 events are in good agreement with the InSAR data indicating that creep occurred along the 20 km-long fault above 4 km depth with most of the slip occurring at the surface. The moment released during this event is equivalent to a Mw 4.7 earthquake. This event produced no detectable aftershocks and was not recorded by the continuous GPS stations that were 9 km away. Modeling of the long-term creep from 1992 to 2007 creep using stacked ERS interferograms also shows a maximum creep depth of 2-4 km with slip tapering with depth. Considering that the sediment thickness varies between 3 km and 5 km along the SHF, our results are consistent with pervious studies suggesting that shallow creep is controlled by sediment depth, perhaps due to high pore pressures in the unconsolidated sediments. |
| InSAR Data | ERS1/2 InSAR data covering a time period of more than 15 years constrain the long-term creep rate of the Superstition Hills fault, and 2 years of ENVISAT data constrain the displacement during the October 2006 event. Both ERS and ENVISAT data along track 356 were collected by the European Space Agency and obtained through the WINSAR archive (Figure 4). For the ERS data, we processed two frames, 2925 and 2943, together to better estimate the long wavelength error. ENVISAT data are used to image the 2006 creep event (ascending track 77, frame 657; descending track 356, frame 2943). The InSAR data was processed using SIOSAR software, and SRTM data were used to remove the topographic effect. Deformation along the southern-most end of the SHF was not fully recovered because all interferograms were decorrelated in the agricultural areas of the Imperial Valley. |
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(a) Research area in southern California. The square box is the location of figure 1b. (b) Stacked 13 interferograms of 15 years ERS1/2 data (Track 356 Frame 2925/2943). The square box on the mid-bottom is the area of figure 1c and 1d. (c) Stacked 7 descending interferograms of ENVISAT data (Track 356 Frame 2943) that span the 2006 creep event. The black dots trace the SHF fault, the Elmore Ranch fault and the Superstition Mountain fault. The black triangle is the location of a creepmeter. (d) Stacked 2 ascending interferograms of ENVISAT data (Track 77 Frame 657) that span the 2006 creep event. Faults names are abbreviated as follows: SAF, San Andreas fault; SJF, San Jacinto fault; EF, Elsinore fault; SHF, Superstition Hills fault; SMF, Superstition Mountains fault; IF, Imperial fault; ERF, Elmore Ranch fault; BSZ, Brawley Seismic Zone; CCF, Coyote Creek fault. |
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| Three dimensional inversion result. Consistent with field measurements, the fault is divided into two patches and the southern segment sliped more then the northern one. |
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