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Low-Frequency Earthquakes and Pore Pressure Transients In Geophysical Research Letters RESEARCH LETTER Low-Frequency Earthquakes and Pore Pressure Transients 10.1029/2018GL079893 in Subduction Zones Key Points: 1,2 3 1 1 • Radiation of low-frequency Nikolai M. Shapiro , Michel Campillo , Edouard Kaminski , Jean-Pierre Vilotte , earthquakes can be explained by a and Claude Jaupart1 single-force mechanism • Rapid fluid pressure changes occur 1Institut de Physique du Globe de Paris, Univ. Paris Diderot, Paris, France, 2Schmidt Institute of Physics of the Earth, Russian within the fault zone 3 • The fluid flux is provided by slab Academy of Sciences, Moscow, Russia, Institut des Sciences de la Terre, Université Grenoble Alpes, Grenoble Cedex, France dehydration Abstract Low-frequency earthquakes (LFEs) have been observed in subduction zones and some major Correspondence to: tectonic faults and may well be the most important constituents of tectonic tremors. In subduction zones, N. M. Shapiro, [email protected] they were initially attributed to fluids released by dehydration reactions in downgoing slabs. Their seismic radiation pattern, however, is consistent with shear slip on the subduction interface, and this rapidly became the favored model. Recent studies indicate that the source duration of LFEs does not scale with magnitude, Citation: Shapiro, N. M., Campillo, M., which can hardly be explained by shear rupture. We revisit the characteristics of LFE events in subduction Kaminski, E., Vilotte, J.-P., & zones as retrieved from local seismic arrays. We demonstrate that they can be explained equally well Jaupart, C. (2018). Low-frequency by forces acting in the direction of fluid motion. Such forces may be generated by a fast local pressure earthquakes and pore pressure transients in subduction zones. variations associated with unsteady fluid motion. The amount of fluid required for LFE activity is consistent Geophysical Research Letters, 45. with dehydration reaction rates. https://doi.org/10.1029/2018GL079893 Plain Language Summary We discuss possible mechanisms of the low-frequency earthquakes, Received 6 AUG 2018 a component of the slow earthquake phenomena observed in fault zones, and suggest that they can be Accepted 9 OCT 2018 generated by very rapid fluid transients releasing the strong pressures gradients built during nonstationary Accepted article online 12 OCT 2018 fluid transport in the fault zones. 1. Introduction Since their discovery in Japan by Obara (2002), tectonic tremors have been observed in many subduction and fault zones (e.g., Husker et al., 2012; Kao et al., 2005; Kostoglodov et al., 2010; Nadeau & Dolenc, 2005; Payero et al., 2008; Walter et al., 2011). Drawing on analogies with volcanic tremors (e.g., Chouet, 1996; Fehler, 1983), early authors attributed them to fluid-driven seismogenic processes and named them “nonvolcanic tremors.” More detailed observations have motivated other hypotheses. First, it was established that, in sub- duction zones, these tremors are closely associated with slow slip events (e.g., Obara & Hirose, 2006; Rogers & Dragert, 2003; Schwartz & Rokosky, 2007). They are in large part composed of low-frequency earthquakes (LFEs) characterized by small coherent impulsive arrivals recorded at several stations (Shelly et al., 2006). LFE hypocenters are aligned with the subduction interface and their radiation patterns can be explained by shear slip along that interface (e.g., Bostock et al., 2012; Frank et al., 2013; Ide et al., 2007; Shelly et al., 2007). This has led to models that consider slow slip events, tremors and LFEs as part of the same “slow earthquake” process, such that LFEs and tremors are generated by small stick-slip asperities in a larger slowly sliding fault. The stick-slip asperity model is attractive but does not account for several observations. LFEs occur as clusters of many hundreds or thousands of highly repetitive multiplets, implying that their sources are very close to one another and are due to the same physical mechanism. According to recent studies, LFE source duration scales poorly with magnitude (e.g., Bostock et al., 2015; Farge et al., 2017), casting doubt on the validity of the self-similar rupture model that has been so successful for “regular” earthquakes. The nearly constant source duration of LFEs implies the predominance of asperities that have a well-defined characteristic size and yet slip by variable amounts. Processes that could account for a nearly constant asperity size have not been iden- tified, however. We shall show that several LFE features, such as their radiation pattern, may be explained by mechanisms other than shear slip. ©2018. American Geophysical Union. In this paper, we reexamine the possibility that LFEs are related to fluid transients in subduction zones. All Rights Reserved. Our study is motivated by analogies between tectonic LFEs and volcanic long-period earthquakes, and SHAPIRO ET AL. 1 Geophysical Research Letters 10.1029/2018GL079893 particularly those that occur at similar depths in subduction zones and active volcanic areas (Shapiro et al., 2017). We first show that the LFE seismic radiation pattern observed over a local seismic array can be explained by a single force mechanism as well as by a double-couple one. We propose that such forces arise from rapid pressure perturbations generated by fluid flow transients along the subduction interface. Finally, we estimate the amounts of fluids required to generate the observed LFE activity and find that they are consistent with petrologically-derived values for dehydration reactions in downgoing slabs (e.g., van Keken et al., 2011). 2. The Radiation Pattern of LFEs Determining LFE focal mechanisms in subduction zones is fraught with many difficulties because radiation patterns must be extracted out of low signal-to-noise and relatively high-frequency waveforms. Ide et al. (2007) compared stacked LFE waveforms in Shikoku, Japan, to those of earthquakes with known mechanisms in the neighbouring subducting Philippine Sea Plate. They derived focal mechanisms from P wave polarities and moment tensors from S waveforms and concluded that both characteristics are consistent with shear slip on the plate interface. A similar approach was applied to LFEs in Cascadia and Mexico by Bostock et al. (2012), Royer and Bostock (2014), and Frank et al. (2013). In another approach the polarization of tectonic tremors has been measured and found approximately parallel to the direction of relative plate motion, which is consistent with S waves generated by the shear slip on the plate interface (e.g., Wech & Creager, 2007; Imanishi et al., 2016). These observations have provided strong support for a shear-slip origin for LFEs and tectonic tremors (Shelly et al., 2007). The weak LFE signals are recorded in local seismic arrays directly above source regions. In subduction zones with relatively gently dipping slabs, these arrays are located in the vicinity of the P wave nodal planes of poten- tial pure shear slip events. The P wave radiation pattern of a pure shear slip event is mainly controlled by the sin(2) term, where is the angle with respect to the nodal plane (Aki & Richards, 2002). For a single impul- sive force acting in the shear direction, the radiation pattern is ∝ sin() instead, but this is almost identical to the previous one over the limited aperture of a local seismic array, as illustrated in Figure 1a. In such an array, S waves are mainly observed in transverse motion components with radiation patterns ∝ cos(2) and cos() for shear slip and single force events, respectively, which are again very close to each other for small val- ues. S wave radiation patterns are distinctive at angles close to 45∘, where S nodal planes are expected for shear slip mechanisms (Figure 1b). LFE recordings are rare at such high angles, however, and those that are available are plagued by lower signal to noise ratios due to larger source-to-receiver distances. S wave polari- sation direction from a single force mechanism is parallel to the force vector. In a case of upward fluid motion along the plate interface, this force would be nearly parallel to the inter-plate slip vector. In such configura- tion, at stations just above the source a double couple and a single force mechanisms will generate S waves with nearly parallel polarization directions and, therefore, cannot be distinguished base on simple polariza- tion analysis of tremors. Thus, the very configuration of local seismic arrays in subduction zones does not allow discrimination between shear slip and single force mechanisms. We therefore conclude that the radiation pattern of subduction zone LFEs may well be due to a single impul- sive force and look for potential physical mechanisms. Single-force mechanisms are generated by volcanic blasts and landslides (e.g., Dahlen, 1993; Ekström et al., 2003; Favreau et al., 2010; Kanamori & Given, 1982; Kanamori et al., 1984; Kawakatsu, 1989). They can also result from deep mass transport processes (Takei & Kumazawa, 1994), for example, in active volcanic conduits (e.g., Chouet & Matoza, 2013; Waite et al., 2008). Fluids that are released at large depths, due, for example, to metamorphic reactions and magma degassing, are bound to rise through intricate pathways and involve transient viscous drag forces and pressure gradients. 3. Fluid Flow in Permeable Fault Zones Seismic activity is highly sensitive to the presence of fluids in fault zones (Rice, 1992; Sibson, 1992b; Sleep & Blanpied, 1994). The basic principle is that fluid pressure acts to lower the fault strength by decreasing the effective normal stress and reducing the threshold value of shear stress required for slippage. Accordingly, intermittent seismic activity may be due to fluctuations of fluid pressure. Mechanisms that have been pro- posed include shear-induced compaction in sealed fault segments leading to high pore pressure (Sleep & Blanpied, 1992) and the influx of high-pressure fluid from below the fault (Rice, 1992). In contrast to upper crustal environments which involve mostly meteoric water, subduction zones are fed by fluids generated by dehydration metamorphic reactions at large depths and pressures (e.g., van Keken SHAPIRO ET AL.
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