S U M M A R YWe relocate precisely micro-earthquakes induced by the Açu reservoir in Brazil and observe seismicity migration consistent with pore-pressure diffusion on a single fault zone. Fluids are believed to play a major role in triggering tectonic earthquakes; reservoir induced seismicity provides a natural laboratory in which to investigate the spatio-temporal evolution and triggering of earthquakes caused by fluid and pore-pressure diffusion.Between 1994 and 1997, 267 earthquakes (M L ≤ 2.1) were recorded and located beneath the Açu reservoir. The seismicity increased several months following annual water level peaks, implying that pore-pressure diffusion is the principal triggering mechanism. The small station spacing and very low-attenuation, Precambrian basement, rock enabled starting earthquake locations with uncertainties of only a few hundred metres. We relocate 155 earthquakes from the largest cluster at Açu using waveform cross-correlation to obtain groups of similar events. We use these groups to improve the pick accuracy (to subsample accuracy in 200 sample per second data), and then invert for more accurate hypocentral locations. Our uncertainties are on the order of 10 m, and our locations are more tightly clustered. We observe temporal migration of the earthquakes, both along strike, and to increasing depth. We observe a seismicity migration rate between 15 and 58 m d -1 . The rate is highest during the time of peak seismicity rate, and there is some suggestion that the rate decreases with increasing depth. Peak depth in seismicity is reached 175 d after the water peak, that is 192 d after the water low, and the maximum depth then decreases at a similar rate to the rate of increase. Our observations are consistent with triggering by pore-pressure diffusion within a heterogeneous fault zone with an average hydraulic diffusivity of ∼0.06 m 2 s -1 and fracture permeability of ∼6 × 10 −16 m 2 .

[1] Various models for the generation of tsunami earthquakes have been proposed, including shallow earthquake slip through low strength materials. Because these physical fault conditions would likely affect other earthquakes in the same rupture zone, source properties of other events may provide a guide to locations of tsunami earthquakes. The 25 October 2010 M w = 7.8 Mentawai tsunami earthquake and surrounding events provide a test of this hypothesis. We determine slip patterns for the mainshock and relocate aftershocks, with the majority occurring in the near trench region. The two largest magnitude aftershocks occurred within the downdip end of the mainshock rupture area and have long momentnormalized rupture duration, likely related to fault zone conditions. Several older relocated earthquakes at the northern edge of the 2010 rupture area also have long duration character, suggesting both spatial and temporal consistency in the conditions needed to produce slow seismic processes along this margin.

[1] Subduction zone earthquakes exhibit a wide spectrum of rupture times that reflect conditions on the megathrust fault. Tsunami earthquakes are examples of slower than expected ruptures that produce anomalously large tsunamis relative to the surface-wave magnitude. One model explaining tsunami earthquakes suggests slip within patches of low rigidity material at shallow depths. Heterogeneous fault conditions, such as having patches of low rigidity material surrounded by higher strength material, should produce heterogeneous earthquake rupture parameters. Here we investigate along-strike variation in rupture duration for 427 shallow thrust earthquakes (M w = 5.0-7.0) in the Peru, Chile, Alaska, Tonga, Kuril, Izu, and Java-Sumatra subduction zones to explore how heterogeneous seismic and tectonic characteristics, such as differences in sediment type, thickness, and roughness of subducting bathymetry, affect earthquake properties. Earthquake source parameters, including rupture durations, are estimated using multi-station deconvolution of teleseismic P and SH waves to solve for earthquake source time functions, and all events are relocated using additional depth phase information. We classify events into shallow (≤26 km) and deep (>26 km and ≤61 km) groups based on the overall mean depth and focus on the longest duration events with moment normalized rupture durations of >1 standard deviation above the mean duration for each group. We find long-duration events at all depths within the study regions except Peru and Chile. We find no correlation with incoming sediment thickness or type, and limited spatial correlation with regions of past tsunami earthquakes, regions of observed afterslip, and subducting bathymetric features.Citation: El Hariri, M., S. L. Bilek, H. R. DeShon, E. R. Engdahl, and S. Bisrat (2013), Along-strike variability of rupture duration in subduction zone earthquakes,

[1] The M w = 7.8 1994 and the M w = 7.7 2006 interplate thrust mechanism earthquakes that occurred in the Java subduction zone produced dominantly normal-faulting aftershocks, unusual for large megathrust main shocks. Various models proposed for these earthquake sequences invoke main shock rupture on an isolated portion of a decoupled plate boundary fault, with updip and outer-rise extension leading to the normal faulting. Other models suggest that these aftershocks occurred in a zone of the subduction zone where usually earthquakes cannot propagate or initiate, leading to the occurrence of normal-faulting aftershocks in the outer rise, overriding and subducting plates. Here we examine a simpler possibility, one in which Coulomb stress changes (DCFS) imparted by slip during the two large subduction events led to normal-faulting events on favorably oriented planes within the slab and near trench region of the subduction zone. We compute stress changes resulting from both events and subsequent large aftershocks using both uniform and variable slip models for main shock slip, resolved onto both aftershock nodal planes. We find that there is not a clear pattern of aftershock occurrence in areas of stress increase due to main shock slip. This implies that these aftershocks are not simply triggered by the static stress changes from the main shock and additional complexity should be considered to explain these unusual earthquake sequences.