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Temperature-Related Earthquake Mechanism Supported by Researchers

10 Jan, 2014 | Labroots

Most people associate earthquakes with damage on the Earth's surface, and are unaware of other rumblings deep within the Earth that don't propagate to the surface. However, seismometers (and seismologists) regularly take note of these deep tremors in order to study propagation mechanisms and improve earthquake prediction and warning capabilities. The journal Geophysical Research Letters recently published a study where researchers from Stanford and MIT analyzed earthquakes in a geologically interesting section of Colombia known as the Bucaramanga nest (a region around the city of Bucaramanga in the northeastern part of Colombia). This area has a rich seismic history—it's well known for earthquakes that are generated deep in the Earth—and consequently it provides an outstanding environment for collecting seismic data. A typical year produces a magnitude-4 earthquake on a roughly monthly basis, as well as one significantly larger quake. The data acquired in this study suggests a thermal propagation mechanism for these tremors. Seismometers were strategically placed on the surface to gather two important sets of data regarding earthquakes deep in the lithosphere (the outer surface of the Earth including the crust and upper mantle, varying in depth to around 60 miles). The stress drop (the total energy release caused by an earthquake) was compared to the radiated seismic energy (the component of the released energy that causes surface tremors). The research team discovered that of the overall energy release, only 2 percent makes its way to the surface. The obvious question becomes: what happens to the rest of that energy? A more subtle form of the question is: how is the energy dissipated, absorbed, or further propagated? Generation of heat is thought to be a dominant mechanism. Rock layers sliding over one another at these depths and pressures generate intense heat spikes. An earthquake of intermediate magnitude (in the range of 4-5) is expected to send the local temperature soaring to 1,000° C (1,800° F) within a second. The temperature increase heats up the surface of nearby rock, effectively providing a lubricating surface. This leads to further movement of rock, and starts a spiraling, runaway process that causes earthquakes and simultaneously spreads the energy over a much larger area. This proposed mechanism of deep earthquake formation is challenged by a competing theory known as dehydration embrittlement. In this scenario, the lubricating effect is caused by the release of water under the high heat and pressure conditions deep in the lithosphere. The water release is thought to cause fractures leading to earthquakes. The results of this study suggest that the thermal runaway mechanism is more likely, but there is no definitive conclusion at this point. Approximately 25% of earthquakes originate in the lithosphere, so research in this field can have important implications. Deeper (no pun intended) understanding of the propagation mechanisms between deep-layer quakes and surface tremors may allow for better prediction mechanisms and risk assessment—with respect to after-shocks as well as the primary earthquake. This may result in saving of lives and increased protection of structures in earthquake-prone areas of the world.