A model for the generation of intermediate and silicic igneous rocks is presented, based on experimental data and numerical modelling. The model is directed at subduction-related magmatism, but has general applicability to magmas generated in other plate tectonic settings, including continental rift zones. In the model mantlederived hydrous basalts emplaced as a succession of sills into the lower crust generate a deep crustal hot zone. Numerical modelling of the hot zone shows that melts are generated from two distinct sources; partial crystallization of basalt sills to produce residual H2O-rich melts; and partial melting of pre-existing crustal rocks. Incubation times between the injection of the first sill and generation of residual melts from basalt crystallization are controlled by the initial geotherm, the magma input rate and the emplacement depth. After this incubation period, the melt fraction and composition of residual melts are controlled by the temperature of the crust into which the basalt is intruded. Heat and H2O transfer from the crystallizing basalt promote partial melting of the surrounding crust, which can include meta-sedimentary and meta-igneous basement rocks and earlier basalt intrusions. Mixing of residual and crustal partial melts leads to diversity in isotope and trace element chemistry. Hot zone melts are H2O-rich. Consequently, they have low viscosity and density, and can readily detach from their source and ascend rapidly. In the case of adiabatic ascent the magma attains a super-liquidus state, because of the relative slopes of the adiabat and the liquidus. This leads to resorption of any entrained crystals or country rock xenoliths. Crystallization begins only when the ascending magma intersects its H2O-saturated liquidus at shallow depths. Decompression and degassing are the driving forces behind crystallization, which takes place at shallow depth on timescales of decades or less. Degassing and crystallization at shallow depth lead to large increases in viscosity and stalling of the magma to form volcano-feeding magma chambers and shallow plutons. It is proposed that chemical diversity in arc magmas is largely acquired in the lower crust, whereas textural diversity is related to shallow-level crystallization.

/pdf/the-genesis-of-intermediate-and-silicic-magmas-in-deep-3g95p0zv3p.pdf

The Genesis of Intermediate and Silicic Magmas in Deep Crustal Hot Zones

We found that repeated slow slip events observed on the deeper interface of the northern Cascadia subduction zone, which were at first thought to be silent, have unique nonearthquake seismic signatures. Tremorlike seismic signals were found to correlate temporally and spatially with slip events identified from crustal motion data spanning the past 6 years. During the period between slips, tremor activity is minor or nonexistent. We call this associated tremor and slip phenomenon episodic tremor and slip (ETS) and propose that ETS activity can be used as a real-time indicator of stress loading of the Cascadia megathrust earthquake zone.

/pdf/episodic-tremor-and-slip-on-the-cascadia-subduction-zone-the-14g5w8f5tg.pdf

Episodic Tremor and Slip on the Cascadia Subduction Zone: The Chatter of Silent Slip

Changes in the global water cycle can cause major environmental and socioeconomic impacts. As the average global temperature increases, it is generally expected that the air will become drier and that evaporation from terrestrial water bodies will increase. Paradoxically, terrestrial observations over the past 50 years show the reverse. Here, we show that the decrease in evaporation is consistent with what one would expect from the observed large and widespread decreases in sunlight resulting from increasing cloud coverage and aerosol concentration.

/pdf/the-cause-of-decreased-pan-evaporation-over-the-past-50-ajk7dbk8n5.pdf

The cause of decreased pan evaporation over the past 50 years.

Extended-duration seismic signals occur episodically on some major faults, often in conjunction with aseismic or 'slow-slip' earthquake events. The mechanism underlying this tremor and its relationship to the aseismic slip are as yet unresolved. David Shelley et al. demonstrate that tremor beneath Shikoku, Japan can be explained as a swarm of small, low-frequency earthquakes, each of which occurs as shear faulting on the subduction zone plate interface. This suggests that tremor and slow slip are different manifestations of a single process. Tremor beneath Shikoku, Japan can be explained as a swarm of small, low-frequency earthquakes, each of which occurs as shear faulting on the subduction zone plate interface. This suggests that tremor and slow slip are different manifestations of a single process. Non-volcanic tremor is a weak, extended duration seismic signal observed episodically on some major faults, often in conjunction with slow slip events1,2,3,4. Such tremor may hold the key to understanding fundamental processes at the deep roots of faults, and could signal times of accelerated slip and hence increased seismic hazard. The mechanism underlying the generation of tremor and its relationship to aseismic slip are, however, as yet unresolved. Here we demonstrate that tremor beneath Shikoku, Japan, can be explained as a swarm of small, low-frequency earthquakes, each of which occurs as shear faulting on the subduction-zone plate interface. This suggests that tremor and slow slip are different manifestations of a single process.

Non-volcanic tremor and low-frequency earthquake swarms

Non-volcanic seismic tremor was discovered in the Nankai trough subduction zone in southwest Japan and subsequently identified in the Cascadia subduction zone. In both locations, tremor is observed to coincide temporally with large, slow slip events on the plate interface downdip of the seismogenic zone. The relationship between tremor and aseismic slip remains uncertain, however, largely owing to difficulty in constraining the source depth of tremor. In southwest Japan, a high quality borehole seismic network allows identification of coherent S-wave (and sometimes P-wave) arrivals within the tremor, whose sources are classified as low-frequency earthquakes. As low-frequency earthquakes comprise at least a portion of tremor, understanding their mechanism is critical to understanding tremor as a whole. Here, we provide strong evidence that these earthquakes occur on the plate interface, coincident with the inferred zone of slow slip. The locations and characteristics of these events suggest that they are generated by shear slip during otherwise aseismic transients, rather than by fluid flow. High pore-fluid pressure in the immediate vicinity, as implied by our estimates of seismic P- and S-wave speeds, may act to promote this transient mode of failure. Low-frequency earthquakes could potentially contribute to seismic hazard forecasting by providing a new means to monitor slow slip at depth.

https://courses.washington.edu/ess502/ShellyBerozaEtal_Nature_2006.pdf

Low-frequency earthquakes in Shikoku, Japan, and their relationship to episodic tremor and slip

Deep long-period tremors were recognized and located in a nonvolcanic region in southwest Japan. Epicenters of the tremors were distributed along the strike of the subducting Philippine Sea plate over a length of 600 kilometers. The depth of the tremors averaged about 30 kilometers, near the Mohorovic discontinuity. Each tremor lasted for at most a few weeks. The location of the tremors within the subduction zone indicates that the tremors may have been caused by fluid generated by dehydration processes from the slab.

/pdf/nonvolcanic-deep-tremor-associated-with-subduction-in-1bqv7lmrof.pdf

Nonvolcanic deep tremor associated with subduction in southwest Japan.

We report the repeating occurrence of short- and long-term slow slip events (SSE) which are accompanied by deep tremor activity around the Bungo channel region, southwest Japan. Both of these activities are detected by NIED Hi-net, which is composed of densely distributed observatories equipped with a set of tiltmeter and a high-sensitivity seismograph. Since the short-term SSE is small in magnitude, GPS can detect only the long-term SSE. Some of these episodes have nearly the same surface deformation pattern. This shows the existence of ‘slow slip patches’ on a plate interface, where the episodic slow slip is the characteristic slip behavior. We observe a change in periodicity and size of the short-term episode after the onset of the long-term SSE. Moreover, the long-term slow slip accelerates when the short-term activity takes place. This suggests that there is an interaction between these two types of SSEs.

/pdf/repeating-short-and-long-term-slow-slip-events-with-deep-4b6qxvps0u.pdf

Repeating short- and long-term slow slip events with deep tremor activity around the Bungo channel region, southwest Japan

http://epsc.wustl.edu/~ggeuler/reading/07-spring/seismology/week11/obara_and_hirose_2006_tectonophysics_non-volcanic_deep_low-frequency_tremors_accompanying_slow_slips_in_the_southwest_japan_subduction_zone.pdf

Non-volcanic deep low-frequency tremors accompanying slow slips in the southwest Japan subduction zone

[1] We report characteristic P wave spectra and stress drops of very low frequency (VLF) earthquakes that occurred within the accretionary prism along the Nankai Trough, southwestern Japan. Although many VLF earthquakes showed no distinct phases on a raw seismogram, a few showed slightly distinct phases. The arrival times of some phases were consistent with the calculated P wave arrival times. The corner frequencies of the VLF earthquakes were calculated using the stacked P wave spectra assuming an omega-square spectrum. Stress drops were very low, i.e., in the range of 0.1–10 kPa, which corresponds to 0.1%–1% of those of ordinary earthquakes. Such extremely low stress drops of the VLF earthquakes suggest that the fault strength within the accretionary prism may have weakened due to the existence of a fluid within the thrust fault system of the accretionary prism.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2006GL025883

Very low frequency earthquakes within accretionary prisms are very low stress‐drop earthquakes

Anomalous seismic events were observed after the occurrence of the foreshock (Mw=7.2) and the main shock (Mw=7.5) of the 2004 off the Kii peninsula earthquakes. These anomalous events are characterized by very low-frequency energy of around 10 seconds with almost no higher-frequency energy and are considered the same as the very low-frequency (VLF) earthquakes discovered by Ishihara (2003) in some places along the Nankai trough, southwest Japan. The VLF seismic activity is mainly coincident with the aftershock area of the 2004 off the Kii peninsula earthquakes; however a minor activity was also excited in the southern Kii channel area. The VLF seismograms sometimes include higher-frequency wave trains with amplitudes much smaller than that of regular aftershocks. This indicates that VLF earthquakes have different source properties from the regular earthquakes. The centroid moment tensor analysis for one of the larger events suggests that the source depth is very shallow and the focal mechanism is the reverse faulting. These features suggest that the event occurs on the well-developed reverse fault system in the large accretionary prism near the Nankai trough. The swarm activity of VLF earthquakes might be considered as a chain-like occurrence of slips on the reverse fault system and thus the signature of a dynamic deformation process in the accretionary prism.

/pdf/very-low-frequency-earthquakes-excited-by-the-2004-off-the-4jdfodsi6u.pdf

Kazushige Obara

Papers

Nonvolcanic deep tremor associated with subduction in southwest Japan.

Repeating short- and long-term slow slip events with deep tremor activity around the Bungo channel region, southwest Japan

Non-volcanic deep low-frequency tremors accompanying slow slips in the southwest Japan subduction zone

Very low frequency earthquakes within accretionary prisms are very low stress‐drop earthquakes

Very low frequency earthquakes excited by the 2004 off the Kii peninsula earthquakes : A dynamic deformation process in the large accretionary prism