The Sanriku-Oki low-seismicity region on the northern margin of the great 2011 Tohoku-Oki earthquake rupture

We examine a region of the megathrust fault offshore of northeastern Honshu (38.75°–40.25°N, 141.5°–143.25°E) that we designate as the Sanriku-Oki low-seismicity region (SLSR). The SLSR, located near the northern termination of the 2011 Tohoku-Oki (M_w 9.0) rupture, lacks historical great earthquake ruptures and has relatively low levels of moderate-size (M_j ≥ 5.0) earthquakes, with subregions having many small events (M_j 2.5–5.0) in the Japan Meteorological Agency unified catalog. The SLSR is located downdip along the megathrust from the rupture zone of the great 1896 Sanriku tsunami earthquake and the great 1933 Sanriku outer trench slope normal-faulting event; weak seismic coupling of the SLSR had been deduced based on the occurrences of those unusual events. Relatively low slip deficit on the SLSR megathrust was estimated based on GPS deformations prior to 2011 compared with adjacent areas with strong inferred locking to the south and north. The southern portion of the SLSR appears to have had, at most, modest levels (<5 m) of coseismic slip during the 2011 event. Some thrust-faulting aftershocks did occur in the SLSR, primarily at depths near 40 km where there had previously been small (M_j ~5.0) repeating earthquakes (e.g., the Kamaishi repeater). An M_w ~ 7.4 underthrusting aftershock occurred near the northeastern edge of the SLSR ~22.5 min after the great 2011 event. Postseismic convergence along the megathrust is peaked in the SLSR. The collective observations indicate that the SLSR is primarily undergoing quasi-static aseismic convergence, and the lack of regional strain accumulation likely delimited the northern extent of the great 2011 rupture as well as the downdip extent of the 1896 rupture. The triggering of the M_w 7.4 aftershock and heightened activity in the downdip repeater regions suggest that high postseismic strain rates drove the region to have ephemerally increased seismic failure, but it appears unlikely that a great earthquake will nucleate or rupture through this region. Similar properties may exist on the megathrust near the southern end of the 2011 rupture

Ground Shaking and Seismic Source Spectra for Large Earthquakes around the Megathrust Fault Offshore of Northeastern Honshu, Japan

Large earthquake ruptures on or near the plate boundary megathrust fault offshore of northeastern Honshu, Japan, produce variable levels of regional high‐frequency ground shaking. Analyses of 0.1–10 Hz strong ground motion recordings from K‐NET and KiK‐net stations and 0.3–3.0 Hz short‐period recordings from Hi‐net stations establish that the shaking variations result from a combination of differences in seismic source spectra and path attenuation. Eleven earthquakes with M_w 6.0–7.6 are analyzed, including interplate events at different positions on the megathrust within the rupture zone of the 11 March 2011 Tohoku (M_w 9.0) event and nearby intraplate events within the underthrusting Pacific slab. The relative ground shaking for frequencies of 5–10 Hz is strongest for the 7 April 2011 (M_w 7.2) intraslab event near the coast, followed by intraplate events beneath the outer‐trench slope. Decreasing levels of high‐frequency shaking are produced by interplate megathrust events moving from the down‐dip edge of the seismogenic zone to the up‐dip region near the trench. Differential attenuation measurements from averaged spectral ratios of strong‐motion recordings indicate that average path attenuation is lower for events deeper on the megathrust or within the slab below the coast. Empirical Green’s function analysis isolates the source spectra for the passband 0.3–3.0 Hz, indicating higher corner frequencies for intraplate events and deep megathrust events than for shallow megathrust events. Similar differences in average source spectra are found for teleseismic P waves. Depth‐varying source radiation and path attenuation thus account for the high‐frequency shaking for the 2011 Tohoku mainshock originating from the down‐dip portion of the megathrust

Intraplate and interplate faulting interactions during the August 31, 2012, Philippine Trench earthquake (M_w 7.6) sequence

On August 31, 2012, a large (M_w 7.6) thrust earthquake occurred within the subducting Philippine Sea plate seaward of a low seismicity region of the plate boundary (9.5°N–11.5°N), possibly as a result of horizontal compressional stress accumulation offshore of a locked megathrust. The mainshock ruptured from ∼30–50 km depth, with high radiated-energy/seismic-moment ratio and enriched short-period P-wave radiation. The nine largest aftershocks with global centroid moment tensor solutions (M_w ∼5.2–5.6) were shallow (10–13 km) normal-faulting outer-rise events, and a waveform template analysis using regional broadband data indicates many (48/110) similar normal faulting events (m_b 4.0–5.5) and a few (8/110) likely shallow thrust faulting events on the megathrust with additional very small unidentified events. Coulomb stress perturbations may contribute to the mix of intraplate and interplate faulting. Geodetic assessment of any slip deficit on the megathrust is essential for quantifying the potential for a future large interplate rupture in this region

The 25 March 2020 M_w 7.5 Paramushir, northern Kuril Islands earthquake and major (M_w ≥ 7.0) near-trench intraplate compressional faulting

Large compressional-faulting earthquakes located relatively deep in oceanic lithosphere entering subduction zones are primarily caused by plate bending stress, but their timing, depth extent and size can be influenced by temporally-varying shear stress on the plate boundary. The 25 March 2020 M_W 7.5 event in the Pacific plate seaward of Paramushir Island (northern Kuril Islands), is among the largest recorded events of this type. Its rupture extends along a large-slip region in the southwestern portion of the 1952 Kamchatka M_W 9.0 rupture zone. This region has somewhat lower interplate coupling than the megathrust fault along Kamchatka to the northeast, but there could be 68 yrs of strain accumulation. The 2020 event is considered in the context of the 24 recorded major (M_W ≥ 7.0) near-trench intraplate compressional-faulting events. An updated compilation of temporally varying near-trench intraslab faulting relative to major interplate ruptures indicates that the stress cycles on the plate boundary influence both extensional and compressional near-trench faulting caused by plate bending. Particularly noteworthy are such events seaward of areas presumed to be in an advanced stage of their seismic cycle, including relatively shallow compressional events along the 1944 M_W 8.1 Tonankai, Japan rupture zone, along with activity along the 1952 Kamchatka and 1922 Chile rupture zones

The 23 June 2014 Mw 7.9 Rat Islands archipelago, Alaska, intermediate depth earthquake

On 23 June 2014, the largest intermediate depth earthquake (M_w 7.9) of the last 100 years ruptured within the subducting Pacific plate about 100 km below the Rat Islands archipelago of the Western Aleutian Islands, Alaska. The unusual faulting orientation, strike = 206°, dip = 24°, and rake = −14°, is possibly related to curvature of the underthrust slab and high obliquity of the relative plate motions. The first ~15 s of the rupture generated relatively weak seismic waves, followed by strong energy release for the next 25 s. The seismic moment is 1.0 × 10^(21) Nm, and slip of up to ~10 m is concentrated within a 50 km × 50 km region. The radiated energy is 1.1 to 2.7 × 10^(16) J, assuming attenuation t* of 0.4 to 0.7 s. This type of intraplate faulting can be very damaging for populated regions above subduction zones such as Japan, Taiwan, Chile, and Indonesia

Dynamically triggered slip on a splay fault in the M_w 7.8, 2016 Kaikoura (New Zealand) earthquake

We investigate the Mw 7.8, 2016 Kaikoura (New Zealand) earthquake using optical satellite imagery and seismology to reveal the main features of the rupture process. Correlation of Landsat8 images reveals a 30-40 km surface rupture on the Kekerengu Fault and Jordan Thrust, with up to 12 m of right-lateral slip. A previously unrecognized conjugate strike-slip fault, the Papatea Fault, also slipped co-seismically (3-4 m). The global CMT centroid indicates both thrust and right-lateral slip, and is located ~100 km NE of the mainshock epicenter. The significant non-double-couple component of the gCMT (25%) suggests the mainshock is not well represented by a single planar fault. Back-projection of teleseismic P waves reveals two main bursts of seismic radiation: (1) at 10-20 s, near the mainshock epicenter, and (2) at ~70 s, close to the observed surface ruptures. We determine a finite source kinematic model of the rupture from the inversion of seismic waveforms. We use two faults in our model, defined to match the observed slip on the Kekerengu Fault, and a deeper offshore fault with a lower dip angle to satisfy the long period seismological observations. We compute the equivalent moment tensor from our finite source model and find it to be remarkably consistent with the gCMT solution. Although little is known about the geometry of these faults at depth, if the Kekerengu fault splays from the deeper thrust, it provides a rare example where the contribution of slip on a splay fault can be clearly isolated in the seismological waveforms

Rupture characteristics of major and great (M_w ≥ 7.0) megathrust earthquakes from 1990 to 2015: 1. Source parameter scaling relationships

Source parameter scaling for major and great thrust-faulting events on circum-Pacific megathrusts is examined using uniformly processed finite-fault inversions and radiated energy estimates for 114 M_w ≥ 7.0 earthquakes. To address the limited resolution of source spatial extent and rupture expansion velocity (V_r) from teleseismic observations, the events are subdivided into either group 1 (18 events) having independent constraints on V_r from prior studies or group 2 (96 events) lacking independent V_r constraints. For group 2, finite-fault inversions with V_r = 2.0, 2.5, and 3.0 km/s are performed. The product V_r^3Δσ_E, with stress drop Δσ_E calculated for the slip distribution in the inverted finite-fault models, is very stable for each event across the suite of models considered. It has little trend with M_w, although there is a baseline shift to low values for large tsunami earthquakes. Source centroid time (T_c) and duration (T_d), measured from the finite-fault moment rate functions vary systematically with the cube root of seismic moment (M_0), independent of assumed V_r. There is no strong dependence on magnitude or Vr for moment-scaled radiated energy (E_R/M_0) or apparent stress (σ_a). Δσ_E averages ~4 MPa, with direct trade-off between V_r and estimated stress drop but little dependence on M_w. Similar behavior is found for radiation efficiency (η_R). We use V_r^3Δσ_E and T_c/M_0^(1/3) to explore variation of stress drop, V_r and radiation efficiency, along with finite-source geometrical factors. Radiation efficiency tends to decrease with average slip for these very large events, and fracture energy increases steadily with slip

Constraining the Dip of Shallow, Shallowly Dipping Thrust Events Using Long-Period Love Wave Radiation Patterns: Applications to the 25 October 2010 Mentawai, Indonesia, and 4 May 2018 Hawaii Island Earthquakes

Constraining precise faulting geometry for shallow, shallowly dipping thrust earthquakes is a common challenge. Plate boundary megathrust faults near the trench and décollement faults beneath volcanic islands and nappes may dip only a few degrees. Long‐period point source moment tensor waveform inversions provide limited resolution of shallow fault dip angle. The possibility of splay faulting requires precise dip estimation. High sensitivity to dip is provided by long‐period Love wave amplitude azimuthal radiation patterns, which undergo rapid change from four lobed to two lobed as dip decreases from 10° to 0°. Modeling variability in Love wave nodal amplitudes allows the dip to be determined to within a few degrees. This is demonstrated for the 25 October 2010 Mentawai (M_(WW) 7.8) tsunami earthquake, which ruptured the 2.0–5.0° dipping megathrust beneath the shallow sedimentary wedge offshore of Indonesia, and the 4 May 2018 Hawaii Island (M_(WW) 6.9) thrust earthquake, which ruptured the 2.5–7.5° dipping décollement under the island flank