Yefei Bai

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.

Two regions of seafloor deformation generated the tsunami for the 13 November 2016, Kaikoura, New Zealand earthquake

The 13 November 2016 Kaikoura, New Zealand, M_w 7.8 earthquake ruptured multiple crustal faults in the transpressional Marlborough and North Canterbury tectonic domains of northeastern South Island. The Hikurangi trench and underthrust Pacific slab terminate in the region south of Kaikoura, as the subdution zone transitions to the Alpine fault strike-slip regime. It is difficult to establish whether any coseismic slip occurred on the megathrust from on-land observations. The rupture generated a tsunami well recorded at tide gauges along the eastern coasts and in Chatham Islands, including a ~4 m crest-to-trough signal at Kaikoura where coastal uplift was about 1 m, and at multiple gauges in Wellington Harbor. Iterative modeling of teleseismic body waves and the regional water-level recordings establishes that two regions of seafloor motion produced the tsunami, including an M_w ~7.6 rupture on the megathrust below Kaikoura and comparable size transpressional crustal faulting extending offshore near Cook Strait.

Tsunami surges around the Hawaiian Islands from the 1 April 2014 North Chile Mw 8.1 earthquake

The 1 April 2014 Iquique Mw 8.1 earthquake ruptured a segment of the megathrust fault offshore of northern Chile and generated a moderate-size tsunami across the Pacific. Tide gauges in Hawaii recorded over 1 m of wave height despite the long distance from the source and position away from the main radiated energy lobe. Inversion of global teleseismic body waves combined with forward modeling of the tsunami at four near-field DART stations arrives iteratively at a self-consistent finite-fault model with very compact dimensions. The slip distribution produces a NNE-SSW trending seafloor uplift patch that enhances the tsunami directionality in the WNW, resulting in good matches to observed DART and tide gauge records around the Hawaiian Islands. The relatively large waves at selected locations in Hawaii can be attributed to a combination of the spatial slip distribution and the resulting short-period waves that triggered localized resonance over the insular shelves. This event highlights the importance of characterizing detailed slip distributions in analysis or forecasting of tsunamis even for a compact source.

The 2017 Mw 8.2 Chiapas, Mexico, Earthquake: Energetic Slab Detachment

On 8 September 2017, a great (M_w 8.2) normal faulting earthquake ruptured within the subducting Cocos Plate ~70 km landward from the Middle American Trench beneath the Tehuantepec gap. Iterative inversion and modeling of teleseismic and tsunami data and prediction of GPS displacements indicate that the steeply dipping rupture extended ~180 km to the northwest along strike toward the Oaxaca coast and from ~30 to 70 km in depth, with peak slip of ~13 m. The rupture likely broke through the entire lithosphere of the young subducted slab in response to downdip slab pull. The plate boundary region between the trench and the fault intersection with the megathrust appears to be frictionally coupled, influencing location of the detachment. Comparisons of the broadband body wave magnitude (m_B) and moment-scaled radiated energy (E_R/M_0) establish that intraslab earthquakes tend to be more energetic than interplate events, accounting for strong ground shaking observed for the 2017 event.

Effects of dispersion in tsunami Green's functions and implications for joint inversion with seismic and geodetic data: a case study of the 2010 Mentawai MW 7.8 earthquake

Tsunami observations play an important role in resolving offshore earthquake slip distributions. Nondispersive shallow-water models are often used with a static initial sea surface pulse derived from seafloor deformation in computation of tsunami Greens functions. We compare this conventional approach with more advanced techniques based on a dispersive model with a static initial sea surface pulse and with the surface waves generated from kinematic seafloor deformation. These three sets of tsunami Greens functions are implemented in finite-fault inversions with and without seismic and geodetic data for the 2010 Mentawai M_w 7.8 tsunami earthquake. Seafloor excitation and wave dispersion produce more spread-out waveforms in the Greens functions leading to larger slip with more compact distribution through the inversions. The fit to the recorded tsunami and the deduced seismic moment, which reflects the displaced water volume, are relatively insensitive to the approach used for computing Greens functions.

Rupture Along 400 km of the Bering Fracture Zone in the Komandorsky Islands Earthquake (M W 7.8) of 17 July 2017: 2017 Mw 7.8 Komandorsky Islands Earthquake

The 17 July 2017 Komandorsky Islands M_W 7.8 earthquake involved arc-parallel right-lateral patchy strike-slip faulting along ~400 km of the Bering Fracture Zone (BFZ) in the westernmost Aleutian Islands back arc. The large size of the earthquake indicates that the BFZ serves regionally as the primary plate boundary extending from the Near Islands to Kamchatka, with the fore-arc Komandorsky Sliver translating rapidly parallel to the Aleutian Trench. The slip distribution is determined by analysis of seismic, tsunami, and geodetic observations. Fault displacements of 4 to 8.5 m, mostly in the upper 15 km, but with localized extension to 20 to 30 km depth along a ~100 km long segment of the BFZ, are comparable to the possible slip deficit since the last major earthquakes in this region in 1849 and 1858, given an estimated 5.1 cm/yr rate between the Komandorsky Sliver and the Bering Plate.