Spatial and temporal patterns of nonvolcanic tremor along the southern Cascadia subduction zone

Abstract

[1] Episodic tremor and slip (ETS), the spatial and temporal correlation of slow slip events monitored via GPS surface displacements and nonvolcanic tremor (NVT) monitored via seismic signals, is a newly discovered mode of deformation thought to be occurring downdip from the seismogenic zone along several subduction zone megathrusts. To provide overall constraints on the distribution and migration behavior of NVT in southern Cascadia, we apply a semiautomated location algorithm to seismic data available during the EarthScope Transportable Array deployment to detect the most prominent pulses of NVT and invert analyst-refined relative arrival times for source locations. In the processing, we also detect distinct and isolated bursts of energy within the tremor similar to observations of low-frequency earthquakes in southwest Japan. We investigate in detail eight NVT episodes between November 2005 and August 2007 with source locations extending over a 650 km along-strike region from northern California to northern Oregon. We find complex tremor migration patterns with periods of steady migration (4-10 km/d), halting, and frequent along-strike jumps (30-400 km) in activity. The initiation and termination points of laterally continuous tremor activity appear to be repeatable features between NVT episodes which support the hypothesis of segmentation within the ETS zone. The overall distribution of NVT epicenters occur within a narrow band primarily confined by the surface projections of the 30 and 40 km contours of the subducting plate interface. We find as much as 50 km spatial offset from the updip edge of the tremor source zone to the downdip edge of the thermally and geodetically defined transition zone, which may inhibit ETS from triggering earthquakes further updip. Intriguingly, NVT activity is spatially anticorrelated with local seismicity, suggesting the two processes are mutually exclusive. We propose that the transition in frictional behavior coupled with high pore fluid pressures in the ETS zone favor tremor generation instead of regular interplate seismicity and frequent ETS produces a semicontinuous relaxation of strain within the overriding and subducting plates that further inhibit seismogenesis surrounding the ETS source region.Citation: Boyarko, D. C., and M. R. Brudzinski (2010), Spatial and temporal patterns of nonvolcanic tremor along the southern Cascadia subduction zone,

Tremor activity inhibited by well-drained conditions above a megathrust

“…2f) suggests that LFE activity and seismicity in the overlying plate are anti-correlated. A similar spatial pattern, in which tremor activity is inversely proportional to seismicity, has been reported in Cascadia2728, where this anti-correlation is interpreted in terms of a semi-continuous relaxation of strain in the overlying plate produced by frequent episodic tremor and slip28. Although strain relaxation by episodic tremor and slip may have some role in Nankai, we consider that the observed spatial anti-correlation of seismicity with LFE activity relates principally to the magnitude of fluid flux from the megathrust.…”

Characteristics and interactions between non-volcanic tremor and related slow earthquakes in the Nankai subduction zone, southwest Japan

“…In Cascadia, migration patterns have been described as steady, halting, and jumping Boyarko and Brudzinski, 2010). "Steady" migration shows continuous source propagation with constant or smoothly varying speed.…”

“…The greatest density of tremor epicenters is located slightly downdip from the area of maximum slip (Wech et al, 2009;Boyarko and Brudzinski, 2010;the Cascadia 2007 andBeyond Working Group, 2010). Two possibilities may explain the discrepancy between temporal correlation and spatial inhomengeneity.…”

Dynamics of interplate domain in subduction zones: influence of rheological parameters and subducting plate age

“…(a) The range of maximum depth of seismogenic rupture along the subduction interplate, D z , encountered in world-wide subduction zones compiled by Heuret et al (2011), is depicted by the blue dotted domain. Green boxes represent depth intervals of tectonic tremors observed in the vicinity of the subduction interplate, possibly located close to z BDT , after (1) Brown et al (2009), (2) Brown et al (2010), (3) Ide (2012), (4) Peterson and Christensen (2009), (5) Kao et al (2009), (6) Boyarko and Brudzinski (2010), and (7) Gomberg et al (2010). (b) Interplate decoupling depth inferred from seismic tomography (purple dots, depth of high lateral contrast in seismic wave propagation velocity and/or in attenuation in the subduction wedge tip), and z dec estimates from heat flow profiles from trench to back-arc (red boxes and diamonds).…”