Dynamic and Kinematic Modeling of Branch Faulting in Strike-Slip Faults

Report for SCEC Award #16018

Report Due March 15, 2017

Investigators:

Jeff Bayless, Dr. Percy Galvez, and Dr. Paul Somerville

(AECOM)

A. Introduction .............................................................................................................................................. 1B. Objectives ................................................................................................................................................ 1C.Approach ................................................................................................................................................. 1D.Results ..................................................................................................................................................... 5E. Conclusions ............................................................................................................................................. 7F. References .............................................................................................................................................. 7

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A. Introduction Ground motion prediction equations are based on a single fault plane representation of the earthquake source, and therefore do not provide reliable estimates of ground motions from complex fault systems such as branching strike-slip faults. Kinematic ground motion simulation can be used to estimate the ground motions from such complex faults, but there are many uncertainties in the details of how the dif-ferent fault segments interact which can only be addressed using dynamic rupture modeling. For exam-ple, branched faulting can only occur under a limited range of conditions (Kame et al., 2003). We need to understand how rupture initiates on the branch fault, so that we can kinematically model the location and timing of the branch rupture. In the modeling of individual fault segments, we need to know which source parameters such as rise time, slip duration and final slip should be determined from the seismic moment of the branching segment rather than those of the overall event.

B. Objectives The objective of this project is to use dynamic rupture modeling to investigate the conditions under which branching strike-slip faulting can occur, and to use that information to guide kinematic broadband ground motion simulations of branching strike-slip faulting. We perform dynamic earthquake rupture modeling with an unstructured 3-D spectral element method to model the Hosgri – Shoreline fault system (Figure 1) and explore the conditions under which branch faulting occurs. We explore how rupture initiates on the branch fault, and use these results to kinematically model the location and timing of the branch rupture. We also evaluate the dynamic source parameters such as rise time, slip duration and final slip of the indi-vidual fault segments, to generate guidelines for multi-segment branching kinematic simulations.

Figure 1. Depiction of the idealized Hosgri – Shoreline branching fault system, along with the simulation site of interest.

C. Approach Dynamic Rupture Modeling

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We use SPECFEM3D (Galvez et. al., 2014, 2016) to perform dynamic rupture modeling for branching faults. The fault geometry and the mesh for Hosgri-Shoreline fault is shown in Figure 2. The rupture pro-cess is simulated by rate-and-state friction with aging law (Dietriech, 1979). To compute the initial param-eters, i.e (stresses, friction and state) prior to one event, we perform earthquake cycle modeling. We use a quasi-dynamic solver with adaptive time step to resolve the interseismic period and nucleate naturally the earthquake. Once the event nucleates, we import the stresses, friction and state to the dynamic solver to perform dynamic rupture modeling. In this way, we do not need to prescribed or guess the initial condi-tions needed for the dynamic modeling. Figure 3 shows the setup for the earthquake cycle modeling. The Dc (characteristic slip distance for rate and state friction) follows a lognormal distribution based upon Luo et al. (2017). The (a-b) and normal stress along depth is shown in Figure 3b and 3d respectively. Using this set up, we run the quasi-dynamic solver and nucleate events on Hosgri fault. We nucleate several earthquakes along the Hosgri fault and chose the event in Figure 3c. After the nucleation, we import stresses, state, slip velocity to the dynamic solver to perform branching ruptures. Figure 4 shows the se-quence of the branching rupture. Once the rupture nucleates on the Hosgri fault, the rupture propagates towards the junction and reaches it at about 13.5 seconds. At the junction, the rupture splays and breaks the Hosgri and Shoreline faults simultaneously. The rupture front that breaks the Shoreline fault dies out after few seconds, breaking about 30 km of the branching fault. The second rupture on the Hosgri fault continues and breaks the whole fault. Finally we take the rupture time, the slip functions, final slip to per-form kinematic rupture modeling, as described in the next section.

Hosgri Fault

3D Hosgri-Shoreline faultsHosgri Fault

Shorelinefault

Shoreline fault

Hosgri Fault

Shoreline fault

Hardebeck et al. (2013)

Hosgri Fault

Shoreline Fault

10 km

Figure 2. 3-D Hosgri-Shoreline fault geometry. Hosgri fault length is 128 km. The grid size of the fault nodes is 125 meters with a Semi-spherical coarsening, where the grid size grows along the radius. The mesh has been build using the software CUBIT.

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Figure 3. (a) Characteristic slip distance taken for the earthquake cycle modeling. Based on Luo et al. (2017), we imposed log normal distribution of Dc (b) along the Hosgri-Shoreline faults. (d) The normal stresses and (a/b) profiles chosen for this study. For the cycle modeling, a quasi-dynamic solver (QDYN, Luo et al., 2017) was used. Using this solver we nu-cleate the event as shown in (c).

(a)

(c)

(b)

(b)

(d)

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Kinematic Rupture Modeling

We use the Graves and Pitarka (2015; GP hereafter) broadband simulation method, and perform the broadband kinematic simulations on the SCEC Broadband Platform. Our first trial, “Trial A” was to use three pieces of information from the dynamic simulations: the fault dimensions, the final slip distributions, and the rupture initiation timing on both faults. With these in hand, we created a GP source description standard rupture format (SRF) file for the Hosgri and Shoreline segments separately. These SRFs use the GP recipe for the scaling and shape of the slip rate functions on each subfault (which is a modified version of the Liu et al. (2006) slip rate function.) The resulting Hosgri and Shoreline segments have M7.6 and M6.58, respectively. The final slip distributions on each fault, along with rupture front contours, are shown in Figure 5. We then performed a broadband (0-10 Hz) simulation to obtain the simulated ground motions at the site of interest. This procedure was repeated for two realizations of the dynamic simulation; results shown here are for the realization with the hypocenter located approximately 20 km along strike (northwest) of the branch point (as shown by rupture initiation time contours in Figure 5).

In our second trial, “Trial B”, we used the full slip velocity time histories on each subfault from our dynamic simulations in the kinematic simulation. This trial also used the fault dimensions and rupture initiation tim-ing on both faults from the dynamic simulations. In order to use the SCEC Broadband Platform, it was necessary to convert the slip velocities on each subfault into SRF format, and then proceed with the broadband simulations.

Figure 4. The snapshot sequence of the branching rupture for the Hosgri-Shoreline using dynamic rupture modeling.

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Figure 6 shows a comparison of the two types of slip velocity time histories at four locations on the Hosgri fault. The blue slip velocities are created using final slip from the dynamic simulations, but with the GP recipe for slip rate. The red slip velocities are taken directly from the dynamic simulations.

Figure 5. Final slip used in the kinematic modeling on the Hosgri (left) and branching Shoreline (left) faults. The branch

point corresponds to X=0 km on the Hosgri segment and at X=16 km on the Shoreline segment.

Figure 6. A comparison of slip velocity functions at various locations on the Hosgri fault. Red lines are those from the dy-namic simulations in this study and blue lines are from the Graves and Pitarka (2015) recipe using the final slip from the dynamic simulations as input. X and Z coordinates are along strike and down dip coordinates as shown in Figure 5.

D. Results Trial A used the fault dimensions, the final slip distributions, and the rupture initiation timing on both faults from the dynamic simulations in the kinematic simulation. Trial B used the full slip velocity time histories on each subfault from our dynamic simulations in the kinematic simulation. The resulting ground motions and response spectra of the Hosgri, Shoreline and combined contributions are shown in Figures 8 and 9.

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Figure 7. Trial A waveforms at the site of interest, resulting from the kinematic broadband simulations.

Figure 8. Trial B waveforms at the site of interest, resulting from the kinematic broadband simulations. The Dynamic ve-locity seismograms at the same site are superimposed on the combined waveforms (black lines, bottom panels).

Figure 9. Trial A (left) and Trial B (right) RotD50 response spectra for the Hosgri and Shoreline contributions, along with the Combined spectrum.

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At periods between 0.2 and 0.8 seconds, the contributions of the Hosgri and Shoreline faults to the com-bined ground motions are comparable, whereas the contributions from the Hosgri fault are larger at long-er periods. Use of the full dynamic slip velocities instead of the kinematic slip velocities results in much lower ground motions at intermediate and long periods.

E. Conclusions We used dynamic rupture modeling to investigate the conditions under which branching strike-slip faulting can occur, and to use that information to guide kinematic broadband ground motion simulations of branching strike-slip faulting. We performed multicycle dynamic earthquake rupture modeling with an un-structured 3-D spectral element method to model the Hosgri – Shoreline fault system and explored the conditions under which branch faulting occurs. We use information from the dynamic rupture simulations to kinematically model the location and timing of the branch rupture.

In Trial A we used the fault dimensions, the final slip distributions, and the rupture initiation timing on both faults from the dynamic simulations in the kinematic simulation. Trial B used the full slip velocity time his-tories on each subfault from our dynamic simulations in the kinematic simulation. At periods between 0.2 and 0.8 seconds, the contributions of the Hosgri and Shoreline faults to the combined ground motions are comparable, whereas the contributions from the Hosgri fault are larger at longer periods. Use of the full dynamic slip velocities instead of the kinematic slip velocities resulted in much lower ground motions at intermediate and long periods.

The rupture velocity on the branching rupture is between (0.8-0.9) times shear velocity (Vs). The fault mechanism is righ-lateral strike slip. The angle between Hosgri-Shoreline faults is about 30 degrees and the event nucleates close to the junction. Under these conditions we found more favorable branching rup-ture where both Hosgri and Shoreline faults break at the same time. Kame et al. (2003) predicts similar branching rupture with similar conditions where both main and branch fault rupture at the same time.

F. References Dieterich, J. H. (1979), Direct observation of frictional contacts: New insights for state-dependent proper-

ties, Pure Appl. Geophys.,143, 283– 302. Galvez, P., J-P. Ampuero, L. A. Dalguer, S.N. Somala and T. Nissen-Meyer (2014). Dynamic earthquake

ruptured modeled with an unstructured 3-D spectral element method applied to the 2011 M9 Tohoku earthquake, Geophys. J. Int., 198(2), page 1222-1240, doi 10.1093/gji/ggu203.

Galvez, P., L. A. Dalguer, J-P Ampuero and D. Giardini (2016). Slip reactivation during the 2011 Tohoku earthquake: Dynamic rupture and strong ground motion simulations, Bull. Seismol. Soc. Am. 106, No 3, doi: 10.1785/0120150153.

Graves, R., and A. Pitarka (2015). Refinements to the Graves and Pitarka (2010) broadband ground mo-tion simulation method, Seismol. Res. Lett. 86, no. 1, doi: 10.1785/0220140101.

Harris, R.A., M. Barall, R. Archuleta, B. Aagaard, J.-P. Ampuero, H. Bhat, V. Cruz-Atienza, L. Dalguer, P. Dawson, S. Day, B. Duan, E. Dunham, G. Ely, Y. Kaneko, Y. Kase, N. Lapusta, Y. Liu, S. Ma, D. Oglesby, K. Olsen, A. Pitarka, S. Song, and E. Templeton (2009), The SCEC/USGS Dynamic Earth-quake Rupture Code Verification Exercise, Seismological Research Letters, vol. 80, no. 1, pages 119-126, doi:10.1785/gssrl.80.1.119.

Kame, N., J.R. Rice and R. Dmowska (2003). Effects of prestress state and rupture velocity on dynamic fault branching. J. Geophys. Res. 108, ESE 13-1 – 13-21.

Liu, P., R. J. Archuleta, and S. H. Hartzell (2006). Prediction of broadband ground-motion time histories: Hybrid low/high-frequency method with correlated random source parameters, Bull. Seismol. Soc. Am. 96, no. 6, 2118–2130, doi: 10.1785/0120060036

Luo, Y., J. P. Ampuero, K. Miyakoshi and K. Irikura (2017) Surface rupture effects on earthquake mo-ment-area scaling relations, PAGEOPH, Topical Volume on "Best Practices in Physics-based Fault Rupture Models for Seismic Hazard Assessment of Nuclear Installations", doi:10.1007/s00024-017-1467-4