J. H. Jiang

The 2011 Magnitude 9.0 Tohoku-Oki Earthquake: Mosaicking the Megathrust from Seconds to Centuries

Detailed geophysical measurements reveal features of the 2011 Tohoku-Oki megathrust earthquake. Geophysical observations from the 2011 moment magnitude (Mw) 9.0 Tohoku-Oki, Japan earthquake allow exploration of a rare large event along a subduction megathrust. Models for this event indicate that the distribution of coseismic fault slip exceeded 50 meters in places. Sources of high-frequency seismic waves delineate the edges of the deepest portions of coseismic slip and do not simply correlate with the locations of peak slip. Relative to the Mw 8.8 2010 Maule, Chile earthquake, the Tohoku-Oki earthquake was deficient in high-frequency seismic radiation—a difference that we attribute to its relatively shallow depth. Estimates of total fault slip and surface secular strain accumulation on millennial time scales suggest the need to consider the potential for a future large earthquake just south of this event.

A detailed source model for the Mw9.0 Tohoku‐Oki earthquake reconciling geodesy, seismology, and tsunami records

The 11 March 2011 Mw9.0 Tohoku-Oki earthquake was recorded by an exceptionally large amount of diverse data offering a unique opportunity to investigate the details of this major megathrust rupture. Many studies have taken advantage of the very dense Japanese onland strong motion, broadband, and continuous GPS networks in this sense. But resolution tests and the variability in the proposed solutions have highlighted the difficulty to uniquely resolve the slip distribution from these networks, relatively distant from the source region, and with limited azimuthal coverage. In this context, we present a finite fault slip joint inversion including an extended amount of complementary data (teleseismic, strong motion, high-rate GPS, static GPS, seafloor geodesy, and tsunami records) in an attempt to reconcile them into a single better resolved model. The inversion reveals a patchy slip distribution with large slip (up to 64 m) mostly located updip of the hypocenter and near the trench. We observe that most slip is imaged in a region where almost no earthquake was recorded before the main shock and around which intense interplate seismicity is observed afterward. At a smaller scale, the largest slip pattern is imaged just updip of an important normal fault coseismically activated. This normal fault has been shown to be the mark of very low dynamic friction allowing extremely large slip to propagate up to the free surface. The spatial relationship between this normal fault and our slip distribution strengthens its key role in the rupture process of the Tohoku-Oki earthquake.

Deeper penetration of large earthquakes on seismically quiescent faults

A microseismic turn off Certain strike-slip faults do not have the expected number of microearthquakes between larger earthquakes. Jiang and Lapusta suggest that this behavior is down to what the last big earthquake looked like. They found that microseismicity turns off if an earthquakes rupture runs deeper than the faults locking depth. This appears to be the case along the famous San Andreas Fault and also along other strike-slip faults around the world. The discovery may allow for better estimates of historic earthquake magnitudes and improve hazard assessments. Science, this issue p. 1293 Reduced seismicity along certain strike-slip fault segments may be due to unexpectedly deep ruptures during big earthquakes. Why many major strike-slip faults known to have had large earthquakes are silent in the interseismic period is a long-standing enigma. One would expect small earthquakes to occur at least at the bottom of the seismogenic zone, where deeper aseismic deformation concentrates loading. We suggest that the absence of such concentrated microseismicity indicates deep rupture past the seismogenic zone in previous large earthquakes. We support this conclusion with numerical simulations of fault behavior and observations of recent major events. Our modeling implies that the 1857 Fort Tejon earthquake on the San Andreas Fault in Southern California penetrated below the seismogenic zone by at least 3 to 5 kilometers. Our findings suggest that such deeper ruptures may occur on other major fault segments, potentially increasing the associated seismic hazard.

Cirrus induced polarization in 122 GHz aura Microwave Limb Sounder radiances

[1] Previous simulation studies have outlined the possibility of significant polarization signals in microwave limb sounding due to horizontally aligned ice crystals in cirrus clouds. From the recently launched Aura MLS instrument, we present the first polarized microwave limb sounding observations of cirrus clouds. We also present polarized radiative transfer simulations, which show qualitative agreement with these observations, and indicate the limits to which aligned non-spherical particles are influencing bulk optical properties of cirrus clouds at microwave wavelengths. Although 122 GHz is not ideal for cloud measurements due to strong O2 absorption, data and simulations suggest that preferential crystal orientation is causing small, but noticeable, partial vertical polarization, which can be replicated in simulations by considering all particles as horizontally aligned oblate spheroids with aspect ratios of around 1.2 ± 0.15. Citation: Davis, C. P., D. L. Wu, C. Emde, J. H. Jiang, R. E. Cofield, and R. S. Harwood (2005), Cirrus induced polarization in 122 GHz aura Microwave Limb Sounder radiances, Geophys. Res. Lett., 32, L14806,

The Iquique earthquake sequence of April 2014: Bayesian modeling accounting for prediction uncertainty

The subduction zone in northern Chile is a well-identified seismic gap that last ruptured in 1877. On 1 April 2014, this region was struck by a large earthquake following a two week long series of foreshocks. This study combines a wide range of observations, including geodetic, tsunami, and seismic data, to produce a reliable kinematic slip model of the Mw=8.1 main shock and a static slip model of the Mw=7.7 aftershock. We use a novel Bayesian modeling approach that accounts for uncertainty in the Greens functions, both static and dynamic, while avoiding nonphysical regularization. The results reveal a sharp slip zone, more compact than previously thought, located downdip of the foreshock sequence and updip of high-frequency sources inferred by back-projection analysis. Both the main shock and the Mw=7.7 aftershock did not rupture to the trench and left most of the seismic gap unbroken, leaving the possibility of a future large earthquake in the region.

Reconciling seismicity and geodetic locking depths on the Anza section of the San Jacinto Fault

Observations from the Anza section of the San Jacinto Fault in Southern California reveal that microseismicity extends to depths of 15–18 km, while the geodetically-determined locking depth is less than ~10 km. This contrasts with observations from other major faults in the region, and also with predictions of fault models assuming a simple layered distribution of frictional properties with depth. We suggest that an anomalously shallow geodetic fault locking may result from a transition zone at the bottom of seismogenic layer with spatially heterogeneous frictional properties. Numerical models of faults that incorporate stochastic heterogeneity at transitional depths successfully reproduce the observed depth relation between seismicity and geodetic locking, as well as complex spatio-temporal patterns of microseismicity with relatively scarce repeating earthquakes. Our models predict propagation of large earthquakes to the bottom of the transition zone, and ubiquitous aseismic transients below the locked zone, potentially observable using high-precision geodetic techniques.

Pulse-Like Partial Ruptures and High-Frequency Radiation at Creeping-Locked Transition during Megathrust Earthquakes

Megathrust earthquakes tend to be confined to fault areas locked in the interseismic period and often rupture them only partially. For example, during the 2015 M7.8 Gorkha earthquake, Nepal, a slip pulse propagating along strike unzipped the bottom edge of the locked portion of the Main Himalayan Thrust (MHT). The lower edge of the rupture produced dominant high-frequency (>1 Hz) radiation of seismic waves. We show that similar partial ruptures occur spontaneously in a simple dynamic model of earthquake sequences. The fault is governed by standard laboratory-based rate-and-state friction with the aging law and contains one homogenous velocity-weakening (VW) region embedded in a velocity-strengthening (VS) area. Our simulations incorporate inertial wave-mediated effects during seismic ruptures (they are thus fully dynamic) and account for all phases of the seismic cycle in a self-consistent way. Earthquakes nucleate at the edge of the VW area and partial ruptures tend to stay confined within this zone of higher prestress, producing pulse-like ruptures that propagate along strike. The amplitude of the high-frequency sources is enhanced in the zone of higher, heterogeneous stress at the edge of the VW area.

Probabilistic imaging of tsunamigenic seafloor deformation during the 2011 Tohoku-oki Earthquake

Diverse observations from the 2011 M_w 9.0 Tohoku-oki earthquake pointed to large coseismic fault slip proximal to the Japan Trench. This seismic failure prompted a reevaluation of the conventional view that the outer forearc is generally aseismic. However, the nature of near-trench fault slip during this event remains debated, without consensus on whether slip peaked at the trench or at greater depths. Here we develop a probabilistic approach to image the spatiotemporal evolution of coseismic seafloor displacement from near-field tsunami observations. In a Bayesian framework, we sample ensembles of nonlinear source models parameterized to focus on near-trench features, incorporating the uncertainty in modeling dispersive tsunami waves in addition to nominal observational errors. Our models indicate that seafloor in the region of the earthquake was broadly uplifted and tilted seaward approaching the deep-ocean trench. Over length scales of ~40 km, seafloor uplift peaks at 5 ± 0.6 m near the inner-outer forearc transition and decreases to 2 m at the trench axis over a distance of 50 km, corresponding to a seafloor tilt of 0.06 ± 0.02 m/km. Over length scales of ~20 km, peak uplift reaches 7 ± 2 m at the similar location, but uplift at the trench is less constrained. Elastic modeling that reproduces the observed tilt requires a coseismic slip deficit at the trench. Such a deficit is effectively consistent with a metastable frictional model for the shallowest megathrust. While large shallow earthquakes in the region cannot be completely ruled out, aseismic deformation is the most likely mode for satisfying the long-term slip budget.

Connecting depth limits of interseismic locking, microseismicity, and large earthquakes in models of long-term fault slip: DEPTHS OF SEISMICITY AND FAULT LOCKING

Thickness of the seismogenic zone is commonly determined based on the depth of microseismicity or the fault locking depth inferred from geodetic observations. The relation between the two estimates and their connection to the depth limit of large earthquakes remain elusive. Here we explore the seismic and geodetic observables in models of faults governed by laboratory-based friction laws that combine quasi-static rate-and-state friction and enhanced dynamic weakening. Our models suggest that the transition between the locked and fully creeping regions can occur over a broad depth range. The effective locking depth, D_(elock), associated with concentrated loading and promoting microseismicity, is located at the top of this transition zone; the geodetic locking depth, D_(glock), inverted from surface geodetic observations, corresponds to the depth of fault creeping with approximately half of the long-term rate. Following large earthquakes, D_(elock) either stays unchanged or becomes shallower due to creep penetrating into the shallower locked areas, whereas D_(glock) deepens as the slip deficit region expands, compensating for the afterslip. As the result, the two locking depths diverge in the late interseismic period, consistent with available seismic and geodetic observations from several major fault segments in Southern California. We find that D_(glock) provides a bound on the depth limit of large earthquakes in our models. However, the assumed layered distribution of fault friction and simple depth estimates are insufficient to characterize more heterogeneous faults, e.g., ones with significant along-strike variations. Improved observations and models are needed to illuminate physical properties and seismic potential of fault zones.