ALBERT

Description: We investigate coseismic deformation of the 24 March 2011 M w  6.8 Tarlay, Myanmar, earthquake using ALOS PALSAR data from both descending and ascending passes. Using high-quality synthetic aperture radar interferograms and amplitude-offset images, the nearly linear surface rupture is well traced along the western end of the Nam Ma fault and strikes ~69°. From both descending and ascending pass Interferometric Synthetic Aperture Radar data and a rigorous maximum a posteriori probabilistic inversion method, we infer that the event involved mostly a pure left-lateral strike-slip rupture with near-vertical geometry. Our one-segment model shows that the maximum slip of ~4.1 m occurred at ~4 km depth, much larger than the slip at the surface. Both interferograms also reveal a small segment to the east of the main rupture, in a densely populated farming area. Our inversion of a two-segment model shows a similar slip distribution on the main fault, in addition to ~0.1–0.3 m left-lateral slip with normal component on a 58° north-dipping segment. The total seismic moment from the two-segment model is 1.95 x 10 19 N·m, equivalent to an M w  6.79 earthquake, which is comparable to the U.S. Geological Survey seismic inversion estimate of 2.10 x 10 19 N·m ( M w  6.84). The earthquake occurred within a group of east-northeast-striking left-lateral strike-slip faults near the Myanmar–Laos border, which are seismically active and reflect a system of actively clockwise rotating blocks. Online Material: Figures of InSAR observations, amplitude-offset maps, InSAR deformation decomposition, one segment model inversion residuals, single data set inversion results, and tectonic setting.

Description: We compare the spatiotemporal progression, geometry, and earthquake source characteristics of a zone of anomalous swarm seismicity between the Maacama and Bartlett Springs faults (California, United States) within the northern San Andreas fault system to both laboratory studies of fracture initiation and structural field observations of fault formation. The similarities between laboratory and field studies of incipient faulting and the earthquake swarms suggest that the seismic lineament on which the swarms occur is a fault in an early stage of development. Kinematic descriptions of faulting and models of fault system development suggest that the ability of existing faults to accommodate deformation across plate boundaries is governed by the length scales over which they accommodate stress. Many of the characteristics of juvenile fault zones, such as segmentation, geometric complexity, and the depth extent of faulting, act to reduce this length scale; this requires the reactivation of existing faults or production of accessory faults to accommodate deformation across the plate boundary.

Description: Several retrospective analyses have proposed that significant increases in moment release occurred prior to many large earthquakes of recent times. However, the finding of accelerating moment release (AMR) strongly depends on the choice of three parameters: (1) magnitude range, (2) area being considered surrounding the events and (3) the time period prior to the large earthquakes. Consequently, the AMR analysis has been criticized as being a posteriori data-fitting exercise with no new predictive power. As AMR has been hypothesized to relate to changes in the state of stress around the eventual epicentre, we compare here AMR results to models of stress accumulation in California. Instead of assuming a complete stress drop on all surrounding fault segments implied by a back-slip stress lobe method, we consider that stress evolves dynamically, punctuated by the occurrence of earthquakes, and governed by the elastic and viscous properties of the lithosphere. We study the seismicity of southern California and extract events for AMR calculations following the systematic approach employed in previous studies. We present several sensitivity tests of the method, as well as grid-search analyses over the region between 1955 and 2005 using fixed magnitude range, radius of the search area and period of time. The results are compared to the occurrence of large events and to maps of Coulomb stress changes. The Coulomb stress maps are compiled using the coseismic stress from all M  〉 7.0 earthquakes since 1812, their subsequent post-seismic relaxation, and the interseismic strain accumulation. We find no convincing correlation of seismicity rate changes in recent decades with areas of high stress that would support the AMR hypothesis. Furthermore, this indicates limited utility for practical earthquake hazard analysis in southern California, and possibly other regions.

Description: The 11 April 2012 M  8.6 Indian Ocean earthquake was an unusually large intraoceanic strike-slip event. For several days, the global M ≥4.5 and M ≥6.5 seismicity rate at remote distances (i.e., thousands of kilometers from the mainshock) was elevated. The strike-slip mainshock appears through its Love waves to have triggered a global burst of strike-slip aftershocks over several days. But the M ≥6.5 rate subsequently dropped to zero for the succeeding 95 days, although the M ≤6.0 global rate was close to background during this period. Such an extended period without an M ≥6.5 event has happened rarely over the past century, and never after a large mainshock. Quiescent periods following previous large ( M ≥8) mainshocks over the past century are either much shorter or begin so long after a given mainshock that no physical interpretation is warranted. The 2012 mainshock is unique in terms of both the short-lived global increase and subsequent long quiescent period. We believe that the two components are linked and interpret this pattern as the product of dynamic stressing of a global system of faults. Transient dynamic stresses can encourage short-term triggering, but, paradoxically, it can also inhibit rupture temporarily until background tectonic loading restores the system to its premainshock stress levels.

Description: 〈p〉Episodic tremor and accompanying slow slip are observed at the down-dip edge of subduction seismogenic zones. While tremors are the seismic signature of this phenomenon, they correspond to a small fraction of the moment released; thus, the associated fault slip can be quantified only by geodetic observations. On continental strike-slip faults, tremors have been observed in the roots of the Parkfield segment of the San Andreas fault. However, associated transient aseismic slip has never been detected. By making use of the timing of transient tremor activity and the dense Parkfield-area global positioning system network, we can detect deep slow slip events (SSEs) at 16-km depth on the Parkfield segment with an average moment equivalent to 〈i〉M〈/i〉〈sub〉w〈/sub〉 4.90 ± 0.08. Characterization of transient SSEs below the Parkfield locked asperity, at the transition with the creeping section of the San Andreas fault, provides new constraints on the seismic cycle in this region.〈/p〉

Description: The M w 7.9 2008 Wenchuan earthquake ruptured about 280 km of faults in the Longmen Shan of Sichuan province, China, at the eastern edge of the Tibetan Plateau. We use teleseismic waveforms with geodetic data from Global Positioning System, synthetic aperture radar interferometry and image amplitude correlation to produce a source model of this earthquake. The model describes evolution of fault slip during the earthquake. The geodetic data constrains the spatial distribution of fault slip and the seismic waveforms constrain mostly the time evolution of slip. We find that the earthquake started with largely thrust motion on an imbricate system of faults beneath the central Longmen Shan, including the Beichuan Fault and Pengguan Fault, with fault slip at depth extending up to 50 km northwest of the mountain front. The fault ruptures continued northeast along the Beichuan Fault with more oblique slip (right-lateral and thrust) and the proportion of lateral motion increasing in the northern Longmen Shan. The northernmost fault segment has a much steeper dip, consistent with nearly pure strike-slip motion. The kinematic source model shows that the rupture propagated to the northeast at about 2.5–3.0 km s –1 , producing a cascade of subevents with a total duration of about 110 s. The complex fault ruptures caused shortening and uplift of the extremely steep central Longmen Shan, which supports models where the steep edge of the plateau is formed by thrusting over the strong crust of the Sichuan Basin.

Description: The 2010 March 4, Jia-Shian ( M w 6.3) earthquake in SW Taiwan caused moderate damage and no surface rupture was observed, reflecting a deep source that is relatively rare in western Taiwan. We develop finite-source models using a combination of seismic waveform (strong motion and broadband), Global Positioning System (GPS) and synthetic aperture radar interferometry (InSAR) data to understand the rupture process and slip distribution of this event. The rupture centroid source depth is 19 km based on a series of moment tensor solution tests with improved 1-D Green's functions. The preferred fault model strikes 322° and dips 27° to the NE and the mainshock is a thrust event with a small left-lateral component. The finite-source model shows a primary slip asperity that is about 20 km in diameter at a depth range from 22 to 13 km, with peak slip of 42.5 cm, a total scalar seismic moment of 3.25  x 10 18 N m ( M w 6.34) and with an average static stress drop of 0.24 MPa. The rupture velocity of this event is faster than the mid-crustal shear wave velocity in Taiwan, which suggests the possibility of a supershear event which has not been previously observed in Taiwan. Systematic resolution and sensitivity tests are performed to confirm the slip distribution, rupture velocity, the choice of weighting and smoothing for the joint inversions, and the consistency of the slip distribution. The first 24 hours of aftershocks appeared along the upper periphery of the main coseismic slip asperity. Both the mainshock and aftershocks are located in a transition zone where the depth of seismicity and an inferred regional basal décollement increases from central to southern Taiwan. The difference between the current orientation of plate convergence in Taiwan (120º) and the P axis of this event (052º) and nearby measurements of recent crustal strain directions (050° to 080°), as well as the relatively low static stress drop, suggest that the Jia-Shian event involves the reactivation of a deep and weak pre-existing NW–SE geological structure.

Description: New geochronologic and geomorphic constraints on the Little Lake fault in the Eastern California shear zone reveal steady, modest rates of dextral slip during and since the mid-to-late Pleistocene. We focus on a suite of offset fluvial landforms in the Pleistocene Owens River channel that formed in response to periodic interaction with nearby basalt flows, thereby recording displacement over multiple time intervals. Overlap between 40 Ar/ 39 Ar ages for the youngest intracanyon basalt flow and 10 Be surface exposure dating of downstream terrace surfaces suggests widespread channel incision during a prominent outburst flood through the Little Lake channel at ca. 64 ka. Older basalt flows flanking the upper and lower canyon margins indicate localization of the Owens River in its current position between 212 ± 14 and 197 ± 11 ka. Coupled with terrestrial light detection and ranging (lidar) and digital topographic measurements of dextral offset, the revised Little Lake chronology indicates average dextral slip rates of at least ~0.6–0.7 mm/yr and 〈1.3 mm/yr over intervals ranging from ~10 4 to 10 5 yr. Despite previous geodetic observations of relatively rapid interseismic strain along the Little Lake fault, we find no evidence for sustained temporal fluctuations in slip rates over multiple earthquake cycles. Instead, our results indicate that accelerated fault loading may be transient over much shorter periods (~10 1 yr) and perhaps indicative of time-dependent seismic hazard associated with Eastern California shear zone faults.

Description: The 2008 M w 6.3 Damxung earthquake on the Tibetan Plateau is investigated to (i) derive a coseismic slip model in a layered elastic Earth; (ii) reveal the relationship between coseismic slip, afterslip and aftershocks and (iii) place a lower bound on mid/lower crustal viscosity. The fault parameters and coseismic slip model were derived by inversion of Envisat InSAR data. We developed an improved non-linear inversion scheme to find an optimal rupture geometry and slip distribution on a fault in a layered elastic crust. Although the InSAR data for this event cannot distinguish between homogeneous and layered crustal models, the maximum slip of the latter model is smaller and deeper, while the moment release calculated from both models are similar. A ~1.6 yr post-seismic deformation time-series starting 20 d after the main shock reveals localized deformation at the southern part of the fault. Inversions for afterslip indicate three localized slip patches, and the cumulative afterslip moment after 615 d is at least ~11 per cent of the coseismic moment. The afterslip patches are distributed at different depths along the fault, showing no obvious systematic depth-dependence. The deeper of the three patches, however, shows a slight tendency to migrate to greater depth over time. No linear correlation is found for the temporal evolution of afterslip and aftershocks. Finally, modelling of viscoelastic relaxation in a Maxwell half-space yields a lower bound of 1 10 18 Pa s on the viscosity of the mid/lower crust. This is consistent with viscosity estimates in other studies of post-seismic deformation across the Tibetan Plateau.

Description: The central section of the San Andreas Fault hosts tectonic tremor and low-frequency earthquakes (LFEs) similar to subduction zone environments. LFEs are often interpreted as persistent regions that repeatedly fail during the aseismic shear of the surrounding fault allowing them to be used as creepmeters. We test this idea by using the recurrence intervals of individual LFEs within LFE families to estimate the timing, duration, recurrence interval, slip, and slip rate associated with inferred slow slip events. We formalize the definition of a creepmeter and determine whether this definition is consistent with our observations. We find that episodic families reflect surrounding creep over the interevent time, while the continuous families and the short time scale bursts that occur as part of the episodic families do not. However, when these families are evaluated on time scales longer than the interevent time these events can also be used to meter slip. A straightforward interpretation of episodic families is that they define sections of the fault where slip is distinctly episodic in well-defined slow slip events that slip 16 times the long-term rate. In contrast, the frequent short-term bursts of the continuous and short time scale episodic families likely do not represent individual creep events but rather are persistent asperities that are driven to failure by quasi-continuous creep on the surrounding fault. Finally, we find that the moment-duration scaling of our inferred creep events are inconsistent with the proposed linear moment-duration scaling. However, caution must be exercised when attempting to determine scaling with incomplete knowledge of scale. ©2017. American Geophysical Union. All Rights Reserved.