Dominant role of temperature on deep earthquake mechanics for the Tonga slab near the bottom of the upper mantle

Abstract

The effects of temperature on the mechanics of deep earthquakes are investigated based on detailed seismic images of a horizontal portion of the Tonga slab (19°S to 22°S). The hypocenter distribution of deep earthquakes and tomographic models have shown that the Tonga slab in this region stagnates laterally around the upper-and lower-mantle boundary. We analyze data from seismic networks in the United States and Japan from the deepest earthquakes that occurred in the stagnating part of the slab (focal depths 658 to 678 km). We observe a clear arrival approximately 8 s after the direct P wave and find that it is an S-to-P converted wave emitted upward from the deepest earthquake foci and reflected downward by a horizontal interface located nearly 30 km above the foci. The S wave speed drops upward across the interface by a few percent, so the interface is best interpreted as the former oceanic Moho in the stagnating Tonga slab. A numerical model of the thermal structure of the Tonga slab is constructed based upon the slab geometry, with the dynamics of the slab motion being incorporated. We find that if the deepest earthquakes occur in the coldest part of the slab, the vertical distance between the foci and the Moho should be approximately 30 km, which agrees well with the observations. The temperature around the foci is determined to be 750°C. We suggest that deep earthquakes are stringently restricted below this temperature, near the bottom of the upper mantle.

Evolution of grain sizes and orientations during phase transitions in hydrous Mg2SiO4

“…Indeed, at high stresses, i.e. , 300 MPa, which are expected for the deep Tonga slab [ Kaneshima et al , ; Kaneshima and Yoshioka, ], the boundary between these regimes is located at fine grain sizes of 0.5 µm for both minerals [ Shimojuku et al , ].…”

Temperature Dependence of H2O Solubility in Al‐Free Stishovite

“…It should be noted that some slabs, such as the Tonga and Java slabs, may have extremely low geotherms, less than 1000°C at 660 km depth (Kaneshima & Yoshioka, 2014; King et al., 2015). The positive temperature dependence of water solubility obtained in this study suggests that stishovite contains less than 50 wt.…”

Continental collisions and the origin of subcrustal continental earthquakes

“…When the loss of heat both upwards and downwards is taken into account, he found no evidence of earthquakes in material whose potential temperature is above ϳ600°C. Kaneshima and Yoshioka (2014) carried out a detailed study of the earthquake locations and temperature at the base of the seismic zone beneath Tonga. As well as direct seismic arrivals, they recognised reflections that they argued come from the Moho and from the top of the crust above the deep earthquakes.…”