The development of most unconventional oil and gas resources relies upon subsurface injection of very large volumes of fluids, which can induce earthquakes by activating slip on a nearby fault. During the last 5 years, accelerated oilfield fluid injection has led to a sharp increase in the rate of earthquakes in some parts of North America. In the central United States, most induced seismicity is linked to deep disposal of coproduced wastewater from oil and gas extraction. In contrast, in western Canada most recent cases of induced seismicity are highly correlated in time and space with hydraulic fracturing, during which fluids are injected under high pressure during well completion to induce localized fracturing of rock. Furthermore, it appears that the maximum-observed magnitude of events associated with hydraulic fracturing may exceed the predictions of an often-cited relationship between the volume of injected fluid and the maximum expected magnitude. These findings have far-reaching implications for assessment of inducedseismicity hazards.

S U M M A R YWe introduce a new method, which we call the Source-Scanning Algorithm (SSA), for imaging the distribution of seismic sources in both time and space. Using trial locations and origin times, the method calculates the 'brightness' function by summing the absolute amplitudes observed at all stations at their respective predicted arrival times. The spatial and temporal distribution of sources is then identified by a systematic search throughout the model space and time for the maximum brightness. The greatest advantages of this method are that: (1) it exploits waveform information (both arrival times and relative amplitudes) without the need to calculate highfrequency synthetic seismograms; and (2) it requires neither pre-assembled phase-picking data nor any a priori assumptions about the source geometry. A series of tests using synthetic data have shown that this method is robust and can faithfully recover the input source configuration to within 1 grid interval. Finally, we demonstrate the value of the algorithm by locating a typical tremor event with emergent waveforms that occurred during the recent episodic tremor and slip (ETS) sequence in the northern Cascadia subduction zone.The precise determination of the origin time and hypocentre of a seismic source is probably the most fundamental issue in earthquake seismology. Traditionally, this is achieved by minimizing the difference between the observed and predicted arrival times of various phases (for examples, P and/or S) at a number of seismic stations. Despite significant advances in both theoretical seismology and computational facilities, most seismic networks still rely on phase-picking methods to locate seismic sources.There are at least two major drawbacks to phase-picking methods that use arrival times only. First, phase picks can be in error if the onset of the arrival is misidentified, especially when the signal-tonoise ratio is poor. Secondly, and probably even more seriously, the proper correlation of individual phases from the same source among different stations becomes extremely difficult when multiple events are closely spread in space and time. In general, phase-picking methods are most effective for individual events that are well separated in time and that generate clear arrivals at seismic stations with a relatively low level of background noise.There have been major efforts to improve the precision of relative locations within a cluster of seismic events using the traveltime differences between pairs of events (Waldhauser & Ellsworth 2000) or stations (Zhou 1994). However, these methods are subject to the same difficulties as mentioned above. Inversion of seismic waveforms, on the other hand, can provide better estimates of the source distribution in both time and space. The basic principle is that the complete source configuration can be recovered from the constructive and/or destructive interference observed at stations along different azimuths/distances (Hartzell & Heaton 1983). In reality, however, the waveform inversion met...

[1] Deep tremor under Shikoku, Japan, consists primarily, and perhaps entirely, of swarms of low-frequency earthquakes (LFEs) that occur as shear slip on the plate interface. Although tremor is observed at other plate boundaries, the lack of cataloged low-frequency earthquakes has precluded a similar conclusion about tremor in those locales. We use a network autocorrelation approach to detect and locate LFEs within tremor recorded at three subduction zones characterized by different thermal structures and levels of interplate seismicity: southwest Japan, northern Cascadia, and Costa Rica. In each case we find that LFEs are the primary constituent of tremor and that they locate on the deep continuation of the plate boundary. This suggests that tremor in these regions shares a common mechanism and that temperature is not the primary control on such activity.

We combined precise focal depths and fault plane solutions of more than 40 events from the 20 September 1999 Chi-Chi earthquake sequence with a synthesis of subsurface geology to show that the dominant structure for generating earthquakes in central Taiwan is a moderately dipping (20 degrees to 30 degrees ) thrust fault away from the deformation front. A second, subparallel seismic zone lies about 15 kilometers below the main thrust. These seismic zones differ from previous models, indicating that both the basal decollement and relic normal faults are aseismic.

The Cascadia subduction zone is thought to be capable of generating major earthquakes with moment magnitude as large as M(w) = 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.

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[1] We explore the physical conditions that enable triggering of nonvolcanic tremor and earthquakes by considering local seismic activity on Vancouver Island, British Columbia during and immediately after the arrival of large-amplitude seismic waves from 30 teleseismic and 17 regional or local earthquakes. We identify tremor triggered by four of the teleseismic earthquakes. The close temporal and spatial proximity of triggered tremor to ambient tremor and aseismic slip indicates that when a fault is close to or undergoing failure, it is particularly susceptible to triggering of further events. The amplitude of the triggering waves also influences the likelihood of triggering both tremor and earthquakes such that large amplitude waves triggered tremor in the absence of detectable aseismic slip or ambient tremor. Tremor and energy radiated from regional/local earthquakes share the same frequency passband so that tremor cannot be identified during these smaller, more frequent events. We confidently identify triggered local earthquakes following only one teleseism, that with the largest amplitude, and four regional or local events that generated vigorous aftershock sequences in their immediate vicinity. Earthquakes tend to be triggered in regions different from tremor and with high ambient seismicity rates. We also note an interesting possible correlation between large teleseismic events and episodic tremor and slip (ETS) episodes, whereby ETS events that are ''late'' and have built up more stress than normal are susceptible to triggering by the slight nudge of the shaking from a large, distant event, while ETS events that are ''early'' or ''on time'' are not.