Han Yue

Depth‐varying rupture properties of subduction zone megathrust faults

Subduction zone plate boundary megathrust faults accommodate relative plate motions with spatially varying sliding behavior. The 2004 Sumatra-Andaman (M_w 9.2), 2010 Chile (Mw 8.8), and 2011 Tohoku (M_w 9.0) great earthquakes had similar depth variations in seismic wave radiation across their wide rupture zones – coherent teleseismic short-period radiation preferentially emanated from the deeper portion of the megathrusts whereas the largest fault displacements occurred at shallower depths but produced relatively little coherent short-period radiation. We represent these and other depth-varying seismic characteristics with four distinct failure domains extending along the megathrust from the trench to the downdip edge of the seismogenic zone. We designate the portion of the megathrust less than 15 km below the ocean surface as domain A, the region of tsunami earthquakes. From 15 to ∼35 km deep, large earthquake displacements occur over large-scale regions with only modest coherent short-period radiation, in what we designate as domain B. Rupture of smaller isolated megathrust patches dominate in domain C, which extends from ∼35 to 55 km deep. These isolated patches produce bursts of coherent short-period energy both in great ruptures and in smaller, sometimes repeating, moderate-size events. For the 2011 Tohoku earthquake, the sites of coherent teleseismic short-period radiation are close to areas where local strong ground motions originated. Domain D, found at depths of 30–45 km in subduction zones where relatively young oceanic lithosphere is being underthrust with shallow plate dip, is represented by the occurrence of low-frequency earthquakes, seismic tremor, and slow slip events in a transition zone to stable sliding or ductile flow below the seismogenic zone.

En échelon and orthogonal fault ruptures of the 11 April 2012 great intraplate earthquakes

The Indo-Australian plate is undergoing distributed internal deformation caused by the lateral transition along its northern boundary—from an environment of continental collision to an island arc subduction zone. On 11 April 2012, one of the largest strike-slip earthquakes ever recorded (seismic moment magnitude Mw 8.7) occurred about 100–200 kilometres southwest of the Sumatra subduction zone. Occurrence of great intraplate strike-slip faulting located seaward of a subduction zone is unusual. It results from northwest–southeast compression within the plate caused by the India–Eurasia continental collision to the northwest, together with northeast–southwest extension associated with slab pull stresses as the plate underthrusts Sumatra to the northeast. Here we use seismic wave analyses to reveal that the 11 April 2012 event had an extraordinarily complex four-fault rupture lasting about 160 seconds, and was followed approximately two hours later by a great (Mw 8.2) aftershock. The mainshock rupture initially expanded bilaterally with large slip (20–30 metres) on a right-lateral strike-slip fault trending west-northwest to east-southeast (WNW–ESE), and then bilateral rupture was triggered on an orthogonal left-lateral strike-slip fault trending north-northeast to south-southwest (NNE–SSW) that crosses the first fault. This was followed by westward rupture on a second WNW–ESE strike-slip fault offset about 150 kilometres towards the southwest from the first fault. Finally, rupture was triggered on another en échelon WNW–ESE fault about 330 kilometres west of the epicentre crossing the Ninetyeast ridge. The great aftershock, with an epicentre located 185 kilometres to the SSW of the mainshock epicentre, ruptured bilaterally on a NNE–SSW fault. The complex faulting limits our resolution of the slip distribution. These great ruptures on a lattice of strike-slip faults that extend through the crust and a further 30–40 kilometres into the upper mantle represent large lithospheric deformation that may eventually lead to a localized boundary between the Indian and Australian plates.

Modeling near‐field tsunami observations to improve finite‐fault slip models for the 11 March 2011 Tohoku earthquake

The massive tsunami generated by the 11 March 2011 Tohoku earthquake (M_w 9.0) was widely recorded by GPS buoys, wave gauges, and ocean bottom pressure sensors around the source. Numerous inversions for finite-fault slip time histories have been performed using seismic and/or geodetic observations, yielding generally consistent patterns of large co-seismic slip offshore near the hypocenter and/or up-dip near the trench, where estimated peak slip is ~60 m. Modeling the tsunami generation and near-field wave processes using two detailed rupture models obtained from either teleseismic P waves or high-rate GPS recordings in Japan allows evaluation of how well the finite-fault models account for the regional tsunami data. By determining sensitivity of the tsunami calculations to rupture model features, we determine model modifications that improve the fit to the diverse tsunami data while retaining the fit to the seismic and geodetic observations.

The 1 April 2014 Iquique, Chile, Mw 8.1 earthquake rupture sequence

PUBLICATIONS Geophysical Research Letters RESEARCH LETTER 10.1002/2014GL060238 Key Points: • The 1 April 2014 Mw 8.1 earthquake ruptured about 20% of the 1877 seismic gap • The rupture was very localized and did not rupture to the trench • The northern and southern ends of the 1877 gap have now had similar ruptures Supporting Information: • Readme • Figure S1 • Figure S2 • Figure S3 • Figure S4 • Figure S5 • Figure S6 • Figure S7 The 1 April 2014 Iquique, Chile, M w 8.1 earthquake rupture sequence Thorne Lay 1 , Han Yue 1 , Emily E. Brodsky 1 , and Chao An 2 Department of Earth and Planetary Sciences, University of California, Santa Cruz, California, USA, 2 School of Civil and Environmental Engineering, Cornell University, Ithaca, New York, USA Abstract On 1 April 2014, a great (M w 8.1) interplate thrust earthquake ruptured in the northern portion of the 1877 earthquake seismic gap in northern Chile. The sequence commenced on 16 March 2014 with a magnitude 6.7 thrust event, followed by thrust-faulting aftershocks that migrated northward ~40 km over 2 weeks to near the main shock hypocenter. Guided by short-period teleseismic P wave backprojections and inversion of deepwater tsunami wave recordings, a finite-fault inversion of teleseismic P and SH waves using a geometry consistent with long-period seismic waves resolves a spatially compact large-slip (~2–6.7 m) zone located ~30 km downdip and ~30 km along-strike south of the hypocenter, downdip of the foreshock sequence. The main shock seismic moment is 1.7 × 10 21 N m with a fault dip of 18°, radiated seismic energy of 4.5–8.4 × 10 16 J, and static stress drop of ~2.5 MPa. Most of the 1877 gap remains unbroken and hazardous. Correspondence to: T. Lay, tlay@ucsc.edu 1. Introduction Citation: Lay, T., H. Yue, E. E. Brodsky, and C. An (2014), The 1 April 2014 Iquique, Chile, M w 8.1 earthquake rupture sequence, Geophys. Res. Lett., 41, 3818–3825, doi:10.1002/2014GL060238. Received 15 APR 2014 Accepted 21 MAY 2014 Accepted article online 24 MAY 2014 Published online 6 JUN 2014 Northern Chile experienced a great subduction zone megathrust earthquake on 9 May 1877 with an estimated seismic magnitude of 8.7–8.9 [Comte and Pardo, 1991] and a tsunami magnitude M t of 9.0. Recent geodetic measurements of eastward deformation of the upper plate indicate that most of the 1877 rupture zone (Figure 1) from 19°S to 23°S has a high coupling coefficient, albeit with some patchiness along strike and along dip [e.g., Bejar-Pizarro et al., 2013; Metois et al., 2013]. This region has been identified as the north Chilean seismic gap [Kelleher, 1972; Nishenko, 1985] based on the lack of large earthquakes for the 137 years over which the Nazca plate has been underthrusting South America at about 65 mm/yr [DeMets et al., 2010]. On the order of 6 to 9 m of slip deficit may have accumulated since 1877. The earthquake history prior to 1877 is uncertain [Nishenko, 1985; Comte and Pardo, 1991], so it is unclear whether the region regularly fails in huge single ruptures or intermittently in larger ruptures then sequences of smaller ruptures, as is the case along the Ecuador-Colombia coastline [Kanamori and McNally, 1982]. The rupture zone of the 1868 Peru earthquake, with an estimated seismic magnitude of 8.5–8.8, partly reruptured in the 23 June 2011 M w 8.4 Peru earthquake (Figure 1), leaving an ~100 km long region offshore of southeastern Peru just north of the 1877 gap that may also have large slip deficit [e.g., Loveless et al., 2010]. On 16 March 2014, a M w 6.7 thrust event occurred on or near the megathrust about 60 km north- northwest of Iquique, Chile, and was followed by two weeks of thrust aftershocks that slowly migrated (~20 km/week) northward along the megathrust from 20.2°S to 19.6°S. The location of this sequence in the northern portion of the 1877 seismic gap focused attention on the region, and on 1 April 2014, a M w 8.1 interplate thrust earthquake initiated at the northern end of the foreshock sequence (19.642°S, 70.817°W, 23:46:46 UTC [U.S. Geological Survey (USGS) National Earthquake Information Center (NEIC): http://earthquake.usgs.gov/regional/neic/]). The global centroid moment tensor (gCMT) solution for this event [http://www.globalcmt.org/CMTsearch.html] indicates an almost purely double-couple faulting geometry with strike 357°, dip 18°, and rake 109° at a centroid depth of 21.9 km and centroid location south of the hypocenter (19.77°S, 70.98°W), with a centroid time shift of 42.5 s and seismic moment of 1.69 × 10 21 N m (M w 8.1) (Figure 1). A substantial aftershock sequence ensued, the largest of which occurred on 3 April 2014 with M w 7.7 (02:43:14 UTC, 20.518°S, 70.498°W, centroid depth 31.3 km), 49 km southwest of Iquique. The region extending from updip of the 3 April event southward to ~23°S, updip of the 2007 M w 7.7 Tocopilla earthquake rupture zone [e.g., Bejar-Pizarro et al., 2010; Schurr et al., 2012], remains strained and has potential for either an ~M w 8.5 event or several smaller great events (Figure 2). LAY ET AL. ©2014. American Geophysical Union. All Rights Reserved.

Localized fault slip to the trench in the 2010 Maule, Chile Mw = 8.8 earthquake from joint inversion of high-rate GPS, teleseismic body waves, InSAR, campaign GPS, and tsunami observations

The 27 February 2010, Mw 8.8 Maule earthquake ruptured ~500 km along the plate boundary offshore central Chile between 34°S and 38.5°S. Establishing whether coseismic fault offset extended to the trench is important for interpreting both shallow frictional behavior and potential for tsunami earthquakes in the region. Joint inversion of high-rate GPS, teleseismic body waves, interferometric synthetic aperture radar (InSAR), campaign GPS, and tsunami observations yields a kinematic rupture model with improved resolution of slip near the trench. Bilateral rupture expansion is resolved in our model with relatively uniform slip of 5–10 m downdip beneath the coast and two near-trench high-slip patches with >12 m displacements. The peak slip is ~17 m at a depth of ~15 km on the central megathrust, located ~200 km north from the hypocenter and overlapping the rupture zone of the 1928 M ~8 event. The updip slip is ~16 m near the trench. Another shallow near-trench patch is located ~150 km southwest of the hypocenter, with a peak slip of 12 m. Checkerboard resolution tests demonstrate that correctly modeled tsunami data are critical to resolution of slip near the trench, with other data sets allowing, but not requiring slip far offshore. Large interplate aftershocks have a complementary distribution to the coseismic slip pattern, filling in gaps or outlining edges of large-slip zones. Two clusters of normal faulting events locate seaward along the plate motion direction from the localized regions of large near-trench slip, suggesting that proximity of slip to the trench enhanced extensional faulting in the underthrusting plate.

Rupture process of the 2010 Mw 7.8 Mentawai tsunami earthquake from joint inversion of near-field hr-GPS and teleseismic body wave recordings constrained by tsunami observations

The 25 October 2010 Mentawai tsunami earthquake (Mw 7.8) ruptured the shallow portion of the Sunda megathrust seaward of the Mentawai Islands, offshore of Sumatra, Indonesia, generating a strong tsunami that took 509 lives. The rupture zone was updip of those of the 12 September 2007 Mw 8.5 and 7.9 underthrusting earthquakes. High-rate (1 s sampling) GPS instruments of the Sumatra GPS Array network deployed on the Mentawai Islands and Sumatra mainland recorded time-varying and static ground displacements at epicentral distances from 49 to 322 km. Azimuthally distributed tsunami recordings from two deepwater sensors and two tide gauges that have local high-resolution bathymetric information provide additional constraints on the source process. Finite-fault rupture models, obtained by joint inversion of the high-rate (hr)-GPS time series and numerous teleseismic broadband P and S wave seismograms together with iterative forward modeling of the tsunami recordings, indicate rupture propagation ~50 km up dip and ~100 km northwest along strike from the hypocenter, with a rupture velocity of ~1.8 km/s. Subregions with large slip extend from 7 to 10 km depth ~80 km northwest from the hypocenter with a maximum slip of 8 m and from ~5 km depth to beneath thin horizontal sedimentary layers beyond the prism deformation front for ~100 km along strike, with a localized region having >15 m of slip. The seismic moment is 7.2 × 1020 N m. The rupture model indicates that local heterogeneities in the shallow megathrust can accumulate strain that allows some regions near the toe of accretionary prisms to fail in tsunami earthquakes.

Source Rupture Models for the Mw 9.0 2011 Tohoku Earthquake from Joint Inversions of High‐Rate Geodetic and Seismic Data

The space-time history of fault slip during the 11 March 2011 Tohoku earthquake (Mw 9.0) is determined using high sample rate three-component Global Po- sitioning System (GPS) recordings from regional stations across Japan, teleseismic broad- band Pwaves, global R1 source time functions determined by empirical Greens function deconvolutions of short-arc Rayleigh waves, and ocean-bottom deformation observa- tions. Least-squares inversions are performed for models with prescribed rupture-front expansion velocity. Joint inversion yields improved resolution of slip compared with inversions using any single data type for both checkerboard rupture simulations and the actual data. Joint inversion stabilizes inversions with respect to some key parameters, such as the rupture expansion velocity and subfault total rupture durations, due to lower dependence on these parameters of some datasets (mainly the high sample rate (1 sample=s) three-component GPS recordings (hr-GPS) data). The preferred joint inver- sion model has a seismic moment estimate of 4:2 × 10 22 N·m( Mw 9.0), with a primary large-slip patch with maximum slip of ∼50-60 m located up-dip of the hypocenter on the shallow megathrust and distributed slip of 20-30 m near the hypocenter. A down-dip low-slip extension to the south is also resolved, with centroid source time later than 110 s. Online Material: Figures of inverted slip distributions and associated source time functions and centroid times, and waveform fits.

The 2012 Mw 8.6 Wharton Basin sequence: A cascade of great earthquakes generated by near-orthogonal, young, oceanic mantle faults

We improve constraints on the slip distribution and geometry of faults involved in the complex, multisegment, Mw 8.6 April 2012 Wharton Basin earthquake sequence by joint inversion of high-rate GPS data from the Sumatran GPS Array (SuGAr), teleseismic observations, source time functions from broadband surface waves, and far-field static GPS displacements. This sequence occurred under the Indian Ocean, ∼400 km offshore Sumatra. The events are extraordinary for their unprecedented rupture of multiple cross faults, deep slip, large strike-slip magnitude, and potential role in the formation of a discrete plate boundary between the Indian and Australian plates. The SuGAr recorded static displacements of up to ∼22 cm, along with time-varying arrivals from the complex faulting, which indicate that the majority of moment release was on young, WNW trending, right-lateral faults, counter to initial expectations that an old, lithospheric, NNE trending fracture zone played the primary role. The new faults are optimally oriented to accommodate the present-day stress field. Not only was the greatest moment released on the younger faults, but it was these that sustained very deep slip and high stress drop (>20 MPa). The rupture may have extended to depths of up to 60 km, suggesting that the oceanic lithosphere in the northern Wharton Basin may be cold and strong enough to sustain brittle failure at such depths. Alternatively, the rupture may have occurred with an alternative weakening mechanism, such as thermal runaway.

Supershear rupture of the 5 January 2013 Craig, Alaska (Mw 7.5) earthquake

Supershear rupture, in which a fractures crack tip expansion velocity exceeds the elastic shear wave velocity, has been extensively investigated theoretically and experimentally and previously inferred from seismic wave observations for six continental strike-slip earthquakes. We find extensive evidence of supershear rupture expansion of an oceanic interplate earthquake, the 5 January 2013 Mw = 7.5 Craig, Alaska earthquake. This asymmetric bilateral strike-slip rupture occurred on the Queen Charlotte Fault, offshore of southeastern Alaska. Observations of first-arriving Sn and Sg shear waves originating from positions on the fault closer than the hypocenter for several regional seismic stations, with path calibrations provided by an empirical Greens function approach, indicate a supershear rupture process. Several waveform inversion and modeling techniques were further applied to determine the rupture velocity and space-time distribution of slip using regional seismic and geodetic observations. Both theoretical and empirical Greens functions were used in the analyses, with all results being consistent with a rupture velocity of 5.5 to 6 km/s, exceeding the crustal and upper mantle S wave velocity and approaching the crustal P wave velocity. Supershear rupture occurred along ~100 km of the northern portion of the rupture zone but not along the shorter southern rupture extension. The direction in which supershear rupture developed may be related to the strong material contrast across the continental-oceanic plate boundary, as predicted theoretically and experimentally. The shear and surface wave Mach waves involve strongly enhanced ground motions at azimuths oblique to the rupture direction, emphasizing the enhanced hazard posed by supershear rupture of large strike-slip earthquakes.

Validation of linearity assumptions for using tsunami waveforms in joint inversion of kinematic rupture models: Application to the 2010 Mentawai Mw 7.8 tsunami earthquake

Tsunami observations have particular importance for resolving shallow offshore slip in finite-fault rupture model inversions for large subduction zone earthquakes. However, validations of amplitude linearity and choice of subfault discretization of tsunami Greens functions are essential when inverting tsunami waveforms. We explore such validations using four tsunami recordings of the 25 October 2010 Mentawai M_w 7.8 tsunami earthquake, jointly inverted with teleseismic body waves and 1 Hz GPS (high-rate GPS) observations. The tsunami observations include near-field and far-field deep water recordings, as well as coastal and island tide gauge recordings. A nonlinear, dispersive modeling code, NEOWAVE, is used to construct tsunami Greens functions from seafloor excitation for the linear inversions, along with performing full-scale calculations of the tsunami for the inverted models. We explore linearity and finiteness effects with respect to slip magnitude, variable rake determination, and subfault dimensions. The linearity assumption is generally robust for the deep water recordings, and wave dispersion from seafloor excitation is important for accurate description of near-field Greens functions. Breakdown of linearity produces substantial misfits for short-wavelength signals in tide gauge recordings with large wave heights. Including the tsunami observations in joint inversions provides improved resolution of near-trench slip compared with inversions of only seismic and geodetic data. Two rupture models, with fine-grid (15 km) and coarse-grid (30 km) spacing, are inverted for the Mentawai event. Stronger regularization is required for the fine model representation. Both models indicate a shallow concentration of large slip near the trench with peak slip of ~15 m. Fully nonlinear forward modeling of tsunami waveforms confirms the validity of these two models for matching the tsunami recordings along with the other data.