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Time (sec) after April 04, 2010 22:40:52 UTC
Comparing Magnitude Estimates for the
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Figure 6.5. Comparing the best-fitting complex earthquake cataloged envelopes (in red) and the incoming observed envelopes (in black) for the 2010 El Mayor-Cucapah mainshock.
Each row represents a station (labeled in the y-axis), and each column represents a component (labeled at the top). Acceleration, velocity, and displacement are also labeled accordingly.
Figure 6.6. Station locations with respect to the epicenter of the mainshock, epicenter of past events, and ruptured fault for the 2010 El Mayor-Cucapah mainshock. This particular rupture propagation is bilateral in the northwest and southeast direction.
6.3.3 2016 Kaikoura
The M7.8 Kaikoura earthquake is also known for its high complexity, as the rupture
propagates northwards across multiple faults. Therefore, modeling the earthquake as a point source may not be appropriate in this case as well. In this analysis, the stations to consider are WTMC (R~8 km), CULC, (R~30 km), and HSES (R~30 km). Additional stations that are near the northern end of the fault are KIKS (R~60 km), KEKS (R>100 km), and WDFS (R>100 km). These additional stations experience strong shaking, even though they are located far from the epicenter. The earthquake history at these stations, however, is not sufficient to ensure accuracy in waveform envelope fits and parameter estimates. In other words, the GeoNet catalog does not have available waveforms for the regions near the ruptured faults. Therefore, the templates cannot be generated in the same way as before, where existing waveforms from the past would be scaled to cover enough earthquake magnitudes. This particular earthquake is a case where the extended catalog search has insufficient database to produce accurate parameter estimates. An algorithm that is suited well for this case is FinDer, which is recommended to have running in parallel with the extended catalog search as a form of confirmation.
Another method that may have the potential to solve this problem is a multi-source model using Cua-Heaton ground motion envelopes (Yamada 2007). Here, the templates are created using the attenuation relationships developed by Cua, the ones used in the Virtual Seismologist (VS) method. However, instead of characterizing the earthquake as a point source model as the VS method does, the multi-source model creates the templates by combining the Cua-Heaton ground motion envelopes at different time delays, like in Eq. 6.1.
In her thesis, Yamada divides the fault surface into “sub-sources”, with each sub-source representing a single point source. Templates for complex earthquakes are simply combinations of Cua-Heaton waveform envelopes at each sub-source. The use of Cua- Heaton envelopes is at a disadvantage in comparison to the extended catalog search because they do not consider the path effects at the specific station and channel.
6.3.4 2019 Ridgecrest sequence
As mentioned in Chapter 4, the 2019 Ridgecrest mainshock also exhibits complex behavior.
However, majority of the stations that are considered in the extended catalog search have epicentral distances greater than the ruptured portion of the fault. Therefore, assuming point source produces error bands similar to those considering higher complexity (see Fig. 6.7).
Here, the templates for complexity are the combinations of two single events at different time delays.
Examining each individual station and channel shows that the two stations that have the poorest envelope fits relative to the other stations are CLC and CCC. CCC, in particular, is located in the direction of the rupture, where amplification of the ground motions is likely to occur. It is theorized that the Ridgecrest mainshock consists of four subevents (Ross et al.
2019). Therefore, the templates that would provide more accurate envelope fits are combinations of four single events, not two, spaced at different time delays. As shown in Fig. 6.8 in purple, these additional envelopes that represent higher complexity, larger motions that occur later in time are captured. Envelopes assuming point source, shown in orange in Fig. 6.8, fail to do so. Table 6.3 quantitatively exemplifies this trend. The ability to capture the larger motions means it has enhanced ability to alert regions of stronger shaking to come.
As shown in Fig. 6.9, the stations located closest to the fault are CLC and CCC. This explains the large amplifications that occur later in time in which finite fault approximation is needed. For the remaining stations farther away from the fault, point source approximation produces relatively small, satisfactory error bands. The results suggest the 2019 Ridgecrest mainshock is a complex event, with a M7.1 followed by a M6.9 at a delay of 5 seconds, followed by a M6.4 at a delay of an additional 10 seconds, and followed by a M6.0 at a delay of an additional 6 seconds. These envelope fits improve by a factor 2.24 and 1.51 for stations CLC and CCC, respectively. As shown in Table 6.3, for majority of the envelope fits, the error band and sum of squared residuals (SSR) are reduced when the templates for complex sequences are used (purple). Point source characterization (orange) produces poorer
envelope fits (larger error band and SSR).
The additional templates considered in this study use waveforms from the M6.4 foreshock. Of course, the search can include even more complexity. However, this study
constrains the model to a double event for consistent comparison to the solutions from Ross et al. 2019.
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