Shao-Ju Shan

A wide depth distribution of seismic tremors along the northern Cascadia margin

The Cascadia subduction zone is thought to be capable of generating major earthquakes with moment magnitude as large as Mw = 9 at an interval of several hundred years. The seismogenic portion of the plate interface is mostly offshore and is currently locked, as inferred from geodetic data. However, episodic surface displacements—in the direction opposite to the long-term deformation motions caused by relative plate convergence across a locked interface—are observed about every 14 months with an unusual tremor-like seismic signature. Here we show that these tremors are distributed over a depth range exceeding 40 km within a limited horizontal band. Many occurred within or close to the strong seismic reflectors above the plate interface where local earthquakes are absent, suggesting that the seismogenic process for tremors is fluid-related. The observed depth range implies that tremors could be associated with the variation of stress field induced by a transient slip along the deeper portion of the Cascadia interface or, alternatively, that episodic slip is more diffuse than originally suggested.

Spatial‐temporal patterns of seismic tremors in northern Cascadia

[1] We study in detail the two consecutive episodic tremor-and-slip (ETS) events that occurred in the northern Cascadia subduction zone during 2003 and 2004. For both sequences, the newly developed Source-Scanning Algorithm (SSA) is applied to seismic waveform data from a dense regional seismograph array to determine the precise locations and origin times of seismic tremors. In map view, the majority of the tremors occurred in a limited band bounded approximately by the surface projections of the 30-km and 50-km depth contours of the plate interface. The horizontal migration of tremor occurrence is from southeast to northwest with an average speed of 5 km/d. In cross section, tremors in both sequences span a depth range of over 40 km across the interface, with the majority occurring in the overriding continental crust. In particular, 50-55% of them are located within 2.5 km from the strong seismic reflector bands above the plate interface. The lack of vertical migration implies that a slow diffusion process in the vertical direction cannot be responsible for tremor occurrences. The source spectra of tremors clearly lack high-frequency content (>5 Hz) relative to local earthquakes. We propose two possible models to explain the relationship between slip and tremors. The first one regards ETS tremors as the manifestation of hydroseismogenic processes in response to the temporal strain variation associated with the episodic slip along the lower portion of the plate interface downdip from the locked zone. In the second model, tremors and slip are associated with the same process along the same structure in a distributed deformation zone across the plate interface. Neither model can be dismissed conclusively at this stage.

Source Characteristics of the 2012 Haida Gwaii Earthquake Sequence

Abstract All the significant ( M L ≥4) events in the 2012 Haida Gwaii earthquake sequence are systematically relocated, and their moment tensor solutions are determined from waveform inversion. The focal mechanism of the mainshock shows low‐angle thrust faulting along a shallowly dipping plane with a strike parallel to the Queen Charlotte fault (QCF), consistent with the inference of Pacific plate underthrusting beneath the overriding North American plate. The epicenter of the mainshock is located ∼5  km landward (northeast) of the surface trace of the QCF, suggesting the nucleation of the rupture was near the bottom of the seismogenic (locked) interface. Significant aftershocks appear to cluster on the periphery of the main rupture zone with most events located immediately seaward of the deformation front. The majority of these events show normal‐faulting mechanisms that are probably associated with the bending stress within the Pacific plate near the deformation front. Several normal and strike‐slip events at greater depths within the subducted Pacific slab show a consistent pattern of T ‐axis in the down‐dip direction, implying the subducted plate is under a stress regime of down‐dip extension. Only a few strike‐slip events were observed along or near the QCF. The limited size and distribution of these events suggest that most of the elastic strain accumulated along the QCF was not released during the 2012 Haida Gwaii sequence. Major strike‐slip earthquakes are likely to occur along the southernmost part of the QCF system in the future.

Tremor Activity Monitoring in Northern Cascadia

The Cascadia margin is characterized by a young (<8-million-year-old) subducting slab stretching from British Columbia, Canada, to northern California, marking the convergent boundary between the North America plate and the oceanic Juan de Fuca and Explorer plates (Figure 1). Geodetic measurements over the past two decades have established that the shallow portion (depth <15 kilometers) of the interface between the subducting Juan de Fuca plate and the overriding North America plate is strongly coupled. The inevitable “unlocking” process can and will generate a megathrust earthquake with a magnitude as large as 9, as confirmed by the studies of paleoearthquakes in the region. Paleoseismic and tsunami data have indicated that the last Cascadia megathrust event (magnitude ∼9) occurred in 1700 [e.g., Satake et al., 2003].