Issue link: https://wardsworld.wardsci.com/i/1382081
Earthquake (continued) and magnitude of an earthquake, would save lives and billions of dollars in housing and infrastructure costs. Unfortunately, successful earthquake predictions are extremely rare. There are two basic categories of earthquake predictions: forecasts (months to years in advance) and short-term predictions (hours or days in advance). Forecasts are based a variety of research, including the history of earthquakes in a specific region, the identification of fault characteristics (including length, depth, and segmentation), and the identification of strain accumula- tion. Data from these studies are used to provide rough esti- mates of earthquake sizes and recurrence intervals. An example of an earthquake forecast is the identification of seismic gaps, portions of the plate boundaries that have not ruptured in a major earthquake for a long time. These regions are most likely to experience great earthquakes in the future. Earthquake probability estimates are another example of fore- casting. Geologic, geodetic, and seismic information are com- bined to estimate the frequencies of damaging earthquakes in a specific region. Regional earthquake probability estimate studies have resulted in forecasts of an 80–90% probability of a magnitude 7 or larger earthquake in the southern California re- gion before 2024, and a 70% probability of a magnitude 6.7 or larger earthquake in the San Francisco Bay region before 2030. Short-term earthquake prediction is still entirely in the realm of ongoing research, and no method is known to be reliable. Evidence is emerging that short-term prediction may be inher- ently impossible due to the complex and chaotic nature of the earthquake process. Deep earthquakes Most earthquakes occur at depths shallower than about 50 km (30 mi) and are usually found near plate boundaries. A few per- cent of all shocks occur at depths of 300–700 km (183–427 mi), depths that correspond to earth pressures of 100,000–250,00 atm (1–2.5 × 1010 Pa; Fig. 5). That the mantle can suddenly rup- ture rather than flow plastically at such conditions has elicited wonder since deep earthquakes were first discovered in the 1920s. Modern insight into these phenomena has come from scientific advances of plate tectonics, seismic tomography, and the mineral physics of the deep mantle based on very high- pressure experiments on mantle minerals. Most, if not all, deep earthquakes occur in inclined belts inside slabs, the cold, dense, and strong lithospheric plates that dive deeply into the Earth's mantle in places where plates are converging. Seismic waves have been used to image variations in the seismic wave speeds in the Earth. These anomalies in seismic tomographic images reflect differences in temperature, mineralogy, or composition. As expected, deep earthquakes + ward ' s science Fig. 5 Depth histogram of earthquakes compared to the mineralogical structure of the mantle. (a) Depth histogram of well-located earthquakes in relationship to seismic-wave speed discontinuities caused by mineralogical changes in normal mantle and in cold slabs. (b) Hypothetical mineralogical structure of very cold slabs and normal mantle, empha- sizing the phase changes associated with the olivine component [(Mg,Fe) 2 SiO 4 ] of the mantle [α (olivine), β (modified spinel) and γ (spinel)]; Mw + Pv are magnesiowüstite and perovskite, the higher pressure minerals dominating the lower mantle. Transformational faulting is a shear instability that can occur in metastable olivine under stress and can produce deep earthquakes.