the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
05 Feb 2021
05 Feb 2021
Very early identification of a bimodal frictional behavior during the post-seismic phase of the 2015 Mw8.3 Illapel, Chile, earthquake
- 1Université Côte d’Azur, CNRS, Observatoire de la Côte d’Azur, IRD, Geoazur, UMR 7329, Valbonne, France
- anow at: Institut de Physique du Globe de Strasbourg, UMR 7516, Université de Strasbourg, EOST, CNRS, Strasbourg, France
- bnow at: University of Surrey, International Study Centre, Guildford, United Kingdom
- 1Université Côte d’Azur, CNRS, Observatoire de la Côte d’Azur, IRD, Geoazur, UMR 7329, Valbonne, France
- anow at: Institut de Physique du Globe de Strasbourg, UMR 7516, Université de Strasbourg, EOST, CNRS, Strasbourg, France
- bnow at: University of Surrey, International Study Centre, Guildford, United Kingdom
Abstract. It is well-established that the post-seismic slip results from the combined contribution of seismic slip and aseismic slip. However, the partitioning between these two modes of slip remains unclear due to the difficulty to infer detailed and robust descriptions of how both evolve in space and time. This is particularly true just after a mainshock when both processes are expected to be the strongest. Using state-of-the-art sub-daily processing of GNSS data, along with dense catalogs of aftershocks obtained from template-matching techniques, we unravel the spatiotemporal evolution of post-seismic slip and aftershocks over the first 12 hours following the 2015 Mw8.3 Illapel, Chile, earthquake. We show that the very early post-seismic activity occurs over two regions with distinct behaviors. To the north, post-seismic slip appears to be purely aseismic and precedes the occurrence of late aftershocks. To the south, aftershocks are the primary cause of the post-seismic slip. We suggest that this difference in behavior could be inferred only few hours after the mainshock, and thus could contribute to a more data-driven forecasts of long-term aftershocks.
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Cedric Twardzik et al.
Status: final response (author comments only)
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CC1: 'Comment on se-2021-6', Sylvain Barbot, 11 Feb 2021
Dear Dr. Twardzik,
Your study "Very early identification of a bimodal frictional behavior during the post-seismic phase of the 2015 Mw8.3 Illapel, Chile, earthquake" unravels the evolution of early afterslip during the first 12 hours that follow the 2015 Illapel earthquake. You find two separate regions of afterslip, north and south of the mainshock rupture, that accumulate slip with different mechanisms. The southern region is almost entirely seismic, as shown by the tight correlation with the moment of aftershocks. Indeed, an Mw 7.1 aftershock occurred there in the first 20 minutes following the mainshock and it was followed by an Mw 6.8 aftershock some 5 hours later. When the coseismic offsets of these aftershocks are removed, the geodetic inversion does not resolve significant slip at that location. In the northern patch, there is spatial overlap between afterslip and aftershocks, but no correlation between the evolution of afterslip and the evolution of aftershocks (either considering the number or moment). Hence, these observations provide clear evidence for the spatial heterogeneity of the constitutive behavior. The study provides observational evidence for afterslip to be virtually entirely seismic at some locations. These results are important, providing clear evidence against the common assumption that stable or strengthening frictional regimes are dominant during the postseismic period. In hindsight, this is not surprising because Bath Law indicates that Mw 7+ earthquakes are likely after an Mw 8+ earthquake, but this study sheds nice new light on the phenomenon.
The study is clear and well-targeted. I just have a few minor comments.
Abstract lines 1-3: a lot of "slip" in just a few sentences.
Line 34: unclear what "potentially reducing the propagation of errors" means.
Line 46-48: detection of early aftershocks after the Gorkha earthquake was discussed in the study
Wang, X., Wei, S. and Wu, W., 2017. Double-ramp on the Main Himalayan Thrust revealed by broadband waveform modeling of the 2015 Gorkha earthquake sequence. Earth and Planetary Science Letters, 473, pp.83-93.
Line 64: 15 GNSS stations within 350 km does not sound like much. Discussion of resolution and sensitivity is in order.
Lines 104-113: Not sure why a Monte Carlo sampling method is used here as the problem is entirely linear and can be solved by least squares with Laplacian regularization. It would be useful to document the resolution of the inverse problem or to characterize it with a checkerboard test.
Line 137: I can't recall an example of the opposite. Do we have examples of afterslip distributions that are firmly not time/space separable?
Line 143-144: Note the work of
Salman, R., Hill, E.M., Feng, L., Lindsey, E.O., Mele Veedu, D., Barbot, S., Banerjee, P., Hermawan, I. and Natawidjaja, D.H., 2017. Piecemeal rupture of the Mentawai Patch, Sumatra: the 2008 mw 7.2 North Pagai earthquake sequence. Journal of Geophysical Research: Solid Earth, 122(11), pp.9404-9419.
that indicates afterslip in the area that ruptures coseismically. The study provides a numerical model based on rate-and-state friction that explains the phenomenon by the fact that the coseismic rupture extended significantly into a velocity-strengthening region.
Lines 156-158: Is it possible that this deep slip patch may in fact represent strain on crustal faults above the megathrust?
Line 181: Wasn't the geodetic moment 8E19 Nm after the deep patch is removed? So the seismic moment is actually greater than the geodetic moment? Also, shouldn't the comparison be with the geodetic moment at the time of the Mw 7.1 earthquake instead of at the end of the 12 hours?
Figure 2: It would be useful to show the "time since mainshock" as a second x-axis. Please also indicate the moment magnitude of the two large aftershocks next to their dashed blue lines.
Figures 3 & 5: the repetitive degrees around every subplots are redundant. Consider showing only the left and bottom ones. Consider better showing the trench with the usual chevrons. Indicate the meaning of the blue area in the legend. Add the moment magnitude of the aftershock next to the respective star.
Figure 4: Remove the title "postseismic 12 hours" as it shows afterslip distribution for longer periods.
Figure 7: This should be replaced by a composite with Figures S7.1 and S7.2. The corresponding discussion of the number of aftershocks and the logarithm of the same in the main text is not particularly useful. Instead, focus on the obvious difference between Figures S7.1 and S7.2.
Finally, please consider commenting the phenomenology shown in Figure S7.1. Why is the cumulative moment of aftershocks increasing so much in the northern segment around 6-7 hours? How does that translate in terms of fault slip? It does not seem clear from the various figures. It is hard to tell if the moment is significant because the plots use "normalized" time dependence. Since the geodetic and afterslip moment are so similar to the south, why not using moment (Nm) as the y-axis?
Best wishes,
Sylvain Barbot
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RC1: 'Comment on se-2021-6', Bernd Schurr, 04 Mar 2021
Twardzik and co-authors investigate the early, i.e. first 12 h of co-seismic deformation following the 2015 M8.3 Illapel Chile earthquake. For this purpose they use 2 already published data sets: 1) 30 sec sampled position time series from 15 cont. GNSS sites earlier published by the authors (Twardzik et al. 2019) and 2 earthquake catalogs based on template matching to obtain high completeness even during the early phase of the aftershock sequence, published by others (Huang et al. and Frank et al. both 2017). The authors derive hourly displacements from the position time series and invert them for hourly afterslip maps. These are compared to aftershock catalogs to distinguish between seismic and aseismic deformation during the early post-seismic phase. They find that the cumulative afterslip during the first 12 h skirts around the lower end of co-seismic slip, a pattern that persists also during the later and longer post-seismic period. There are two main lobes north and south of the co-seismic slip region that behave differently. The northern patch apparently slips mostly aseismically, whereas for the southern patch moment release is dominated by aftershocks (incl. an M7.1 and M6.8 aftershock).
The manuscript is well written and well illustrated. I have only few minor points I comment in the following.
l.58 “seismic noise” – I think this is not really seismic noise but overwhelming signal, i.e. numerous often simultaneous aftershocks, that is causing problems to most detectors.
l.28: change “activity” to “deformation”. I think there is a “itself” missing after “express”.
l.49: change “highly” to “more”
l.89ff: “The cumulative surface displacements are calculated at every hour since the mainshock origin time by computing the average positions over a 1-hour time window centered on the time of interest.” Does cumulative refer here for cumulative during the one hour processed or cumulative since the mainsock. I assume the earlier, but please clarify.
l.126: “yellow circles” should be “purple stars”?
l.129: There is a word missing after “second”. Maybe “patch”.
l.140ff: “When we look more closely, we see that some of the post-seismic slip might have penetrated inside the co-seismic rupture area (Figure 3).”
The general fuzziness of both co-seismic and post-seismic slip models makes this assertion difficult to maintain (and the authors actually relativize it later in the paragraph). In particular, different fault model used (simple plain slab like the authors or varying slab dip based on e.g. slab2.0) in the modeling will shift location of slip. To start interpreting this, at least the modeling set-up of co- and post-seismic slip should be the same.
l.186ff: “Our models show that post-seismic slip is now only observed north of the co-seismic rupture area and that the patch to the south has completely vanished.” And also Figure 3 and Figure 6.
First of all, slip in Fig. 3 and 6 I assume is the slip during the respective hour, not the cumulative slip added up also from the previous hours (must be based on the amplitudes and the fact that some patches vanish). I don’t understand why, if only the offsets of the 2 largest aftershocks are corrected (hour 1 and hour 5), all slip vanishes in the southern patch also during the other hours. Please explain.
Fig. S7.1: The second M6.8 aftershock occurs during hour 5 and clearly shows up as a step in the graphed moment. However a step in slip seems to occur mainly in hour 6? Is this an averaging effect?
l.197ff: Please elaborate in one or two sentences what this predicted accelation phase signifies.
l.201ff: Please mention where the cited studies were based (Ecuador and Japan).
l.209: Change “rate-and-state law” to “rate-and-state friction law” here and everywhere else.
l.257: I wonder, are aftershocks anywhere actually operationally forecasted based on some models (maybe a citation would be good)? If so, I would assume that models have to be simple and robust. Here e.g. CFS would naturally predict aftershocks around the co-seismic rupture area, where they do occur, for the Illapel eqk and also for many other subduction zone earthquakes. I wonder, how realistic and it is to actually do the hindsight analysis outlined here in near real time and if it really adds value. Of course, this could be tested.
l.271ff: “Our additional finding is that the slip patterns that we observe after 12 hours persists over the first 2 months. When that is the case, information about very-early post-seismic slip can help to characterize longer-lasting post-seismic slip, which can prove to be useful to include for the forecast of aftershocks locations.”
But can this really be generalized?
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CC2: 'Comment on se-2021-6', Dietrich Lange, 22 Mar 2021
Twardzik and co-authors investigate the first 12 h of post-seismic deformation following the 2015 M8.3 Illapel Chile earthquake. For the geodetic data, they use GNSS positions of 15 stations hourly displacements are inverted for hourly afterslip on the plate interface. For the seismicity, they and earthquake catalogs based on template matching based on previous studies. Then the seismicity is compared with the aftershocks. The northern patch is essentially aseismic slip, while in the southern patch, slip is essentially seismic slip related to aftershocks. Furthermore, the authors show that the ratio of seismic to aseismic slip might change over time at some locations.
The manuscript is well written and well-illustrated. I have only a few points related to the ratio of seismic to aseismic slip. Since for subduction zones, there are just limited observations on the relation of aftershocks to afterslip (in particular for the very first hours after the co-seismic), the topic is sound and relevant, and the paper contributed new and interesting aspects on this topic. The comments and suggestions below should all be feasible to be incorporated.
In the manuscript, you sometimes refer to “south/north of the rupture area,” but it seems that the two patches you classify as south and north of the rupture are still located in the co-seismic region (e.g., between 30-32°S). I suggest clarifying the two patches exact position and how they are related to the co-seismic slip.
In particular, previous authors observed a widening of the aftershock zone of the Illapel earthquake (e.g., Lange et al., 2016, GJI, doi: 10.1093/gji/ggw218). It would be interesting to state if this is observed with the catalog based on template matching and how this relates to the inverted afterslip. I assume that the possible expansion outside the co-seismic region is too small to be robustly inverted by the hourly evolving afterslip model. I suggest adding some words to this.
In particular, the authors find that the partitioning in seismic and aseismic slip changes in time (As mentioned in Line 207). I could not feasible see this in Figure 7 and S7.1, and suggest to simply plot the displacement versus the number of events for the patches to show the relationship between both processes. For example, Lange et al. (2014, GJI, doi: 10.1093/gji/ggu292) mapped for the late postseismic (e.g. >1d) of the Maule 2010 earthquake the partitioning of seismic to aseismic slip, which was relatively stable in time.
There is a similar observation for strike-slip faulting for early observation of afterslip and aftershocks, and their relation (Savage, 2007, GRL, doi: 10.1029/2010GL042872) shows that aftershock seismicity rate is not proportional to the stress relaxation rate for the San-Andreas fault.
Formal issues:
I suggest adding a caption to Fig. S7.1 and S7.2.
Figure S7, S7.1, and S7.2 (right panels) might need additional labeling for the number of aftershocks. Currently, only the slip is labeled.
Table 1 and 2 do not contribute and might belong to the supplementary.
I suggest showing Figure S7.1 (temporal development southern patch) in the primary material since parts of the findings are difficult to understand without this figure.
Line 9: …. and thus could contribute to a more data-drivenforecasts of long-term aftershocks.
I cannot necessarily follow the argument in Line 9. Since the partitioning of seismic slip in some places changes in time prediction might be very difficult if this process remains enigmatic. Does the suggested forecast suggested here use Omori-laws and constant b-values, such as modelled by (Jonsdottir et al., 2006, Tectonophysics 424, https://doi.org/10.1016/j.tecto.2006.03.036.)?
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RC2: 'Comment on se-2021-6', Mathilde Radiguet, 11 May 2021
Twardzik and co-author investigate the early postseismic signal of the Mw 8/3 Illapel earthquake in Chile. The author use subdaily GNSS processing to extract the postseismic signal of the first 12 hours after the earthquake. The GNSS displacements are inverted to provide an image of the slip distribution on the plate interface every hour. Two main slip patches are identified, the one located South of the rupture being mostly seismic (the cumulative moment is mostly due to aftershocks), whereas the Northern patch is mostly aseismic. They observe that the patches identified in the first 12 hours are consistent over longuer time period.The author then discuss the implications of their results in terms of frictionnel behavior and driving mechanism for aftershock occurence.
The paper is well written in general, and the figures are clear. I have a few suggestions, mostly minor, which are detailed below.section 3:
The discussion on the relative location of the co-seismic and postseismic slip is difficult to follow since the co-seismic slip distribution is not shown. It would be useful to add on Figure 3 the coseismic slip distribution estimated from previous studies (for example the one inferred by Melgar et al. 2016, as the authors refer to it), or inverted by the authors. Of course uncertainties exist on this slip distribution based on the data, inversion scheme and fault geometry used by previous authors, but the authors could show several models if needed.
The authors could also perform their own inversion of the co-seismic slip (using only GNSS data). Even if this inversion would be constrained only by geodetic data, it would be interesting because fully consistent with the post-seismic study in terms of fault geometry, Green’s function and data with the post-seismic study. it could be added to the supplementary material.l. 104: "We search for the spatial distribution of slip amplitude and rake angle independently for each time step": the slip amplitudes obtained are shown but not the rake angles. Do they vary from one time step to another ? The optimal rake for each time step should be given in the supplementary materials.
Section 4:
The discussion on the relationship between early aftershocks and afterslip could be clarified by a better description of the areas involved when the seismic or geodetic moments are calculated: l.181"a seismic moment of 9.5x 10^19 Nm." what is the region considered for the calculation ? Is it the same as the one shown in Fig. S6.2 ?
l. 189-190: same question, what is the area considered ?The times series from figure S7 could be a nice illustration of the seismic/aseismic ratio on both regions, if the time series showed the seismic moment in both regions (instead of an arbitrary normalised unit). I understand the interest of the normalised unit when comparing the number of aftershock with the cumulative slip, but this is already shown in Fig 7 and S6 for both regions. In Fig. S7, the cumulative slip could be converted to seismic moment so that one can see where the values given in the text for the seismic/aseismic rations come from.
The time series from Fig.7 could also be included in the main text, as they are really relevant for the discussion of the paper.On FigureS7.2: what is happening between 6h and 7h (strong increase in cumulative moment): is there a large aftershock at this time ?
Formal issues:
Figure 1 (legend):
- "yellow circles" should be "pink stars"Several problems with figure/table numbers (they appear with "??"): Line 85, 136, 165...
Cedric Twardzik et al.
Supplement
Data sets
30s Position Time Series for the 2015 Illapel earthquake Cedric Twardzik https://doi.org/10.5281/zenodo.4498200
Model code and software
Python Code for sidereal filtering Cedric Twardzik https://doi.org/10.5281/zenodo.4498160
Cedric Twardzik et al.
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