For this year, we have studied the near-regional Lg-coda from the 6.0 Wells, Nevada earthquake of February 21, 2008 as well as a local 4.2 event from the San Francisco Bay area that exhibited a clear case of source directivty. The Wells event sequence was unique because the U.S. Array broadband seismic deployment had recorded this event along with its aftershocks, thus providing excellent station coverage and redundancy. Figure 2.26 below shows that the local to near-regional coda is very stable in comparison to the direct Lg. We have formed amplitude ratios, in this case 3.5 Hz, between the mainshock and a selected aftershock. The coda velocity and envelope shape functions were found to be virtually identical from station to station, in spite of significant geologic variation for paths traveling east versus those to the north and north-west. Though this event did not exhibit strong directivity (D. Dreger, pers. comm., 2009), we still see a radiation pattern in the direct Lg ratios, or perhaps random variation between the two sources. However, for the coda, we see significantly less variation, confirming that the coda is not sensitive to the source mechanism.
For our second example, we studied an 4.2 event that occurred along the Hayward fault in the San Francisco Bay area on July 20, 2007. This event was very interesting because there were seismic stations equidistant from the mainshock along the strike of the Hayward fault. The mainshock exhibited over a factor of 10 larger amplitude in the north-westerly direction (along strike) relative to the south-east direction. Figure 2.28 shows that station CVS to the north-west has much larger direct wave amplitudes than station MHC to the south-east; however, the coda envelopes are exactly the same after a few tens of seconds. In addition, an aftershock for the same stations and frequency bands does not show any directivity.
We have documented the coda's property of insensitivity to both the source radiation pattern and directivity. The examples shown in Figures 2.26, 2.27, and 2.28 are strong evidence that the coda's averaging properties also applies to the source heterogeneity, not just path heterogeneity. The recent 2008 5.8 Wells, Nevada earthquake was well recorded by the U.S. Array, and we show that the local-to-near-regional coda is virtually insensitive to the source radiation pattern and directivity effects. In addition, we demonstrate that the coda wavefield becomes homogenized a few tens of seconds past the expanding direct-wave front.
Mayeda, K., L. Malagnini, W.R. Walter, A new spectral ratio method using narrow band coda envelopes: Evidence for non-self-similarity in the Hector Mine sequence, Geophys. Res. Lett., doi:10.1029/2007GL030041, 2007.
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