Erik Sevre

Dynamical causes for incipient magma chambers above slabs

Using a regional upper-mantle model with an unprecedented spatial resolution of ∼100 m, we have investigated at multiple resolutions the character of incipient magma chambers forming under oceanic arcs. The magma chambers are formed from wave-like structures propagating upward along descending slabs and consist of compositionally buoyant, hydrated, partially molten subducted crustal and mantle material. These wave structures are 300–500 °C colder than the mantle wedge and may have an upward velocity of >1 m/yr. Inverted temperature structures and transitory bimodal magmatism are plausible consequences of finger-like penetration of relatively cold, hydrated material of the incipient magma chambers into the hot mantle wedge. Apart from forming and periodic feeding of the magma chambers, “cold” waves may also transport upward thousands of cubic kilometers of subducted material and may cause the rapid exhumation of ultrahigh-pressure rocks along slabs.

Probabilistic Forecast of Tsunami Hazards along Chinese Coast

There is indeed a potential non-negligible threat for Chinese coast from tsunamogenic earthquakes originating at the neighboring subducting plate boundaries of Eurasian plate and Philippine sea plate: Manila trench and the Okinawa trough. This finding comes from our newly devised method for determining the probabilistic forecast of tsunami hazard (PFTH), which finds this probability distribution from direct numerical simulation of the waves excited by hypothetical earthquakes in these zones. There are significant differences in the bottom bathymetry between the South China Sea bordering the southern province of Guangdong and the East China Sea and Yellow Sea adjacent to the provinces of Zhejiang, Jiangsu, and Shandong. We have verified that the linear shallow-water equations can be employed to predict with sufficient accuracy the travel time of tsunami waves in the South China Sea, while the nonlinear shallow-water equations must be used for the shallower seas next to the northern Chinese provinces. Distribution for the possibility of tsunami waves with above 2.0 m hitting the coast has been shown in eastern China sea area, the delta region of the Yangzi River, the north-eastern coast of Zhejiang province, and northern Taiwan island. The distribution has also been displayed in South China Sea area, along the southeastern coast of mainland and Southwestern Taiwan. In this century the probability of a wave with a height of over 2.0 m to hit Hong Kong and Macau is about 10.0%, 0.5% for Shanghai, 3.2% for Wenzhou, and 7.2% for Keelung. Cities on eastern Chinese coast are less vulnerable than those on the southern Chinese coast. We also have discussed the prospects of tsunamis coming from large earthquakes along the Manila trench and the Ryukyu-Kyushu arc region to the north, as they can impact many countries in Southeast Asia, besides China.