| “Earthquakes” are normally known as quakes that naturally occurred at shallow depth(<70 km). However, here are two other kinds of unique earthquakes that are different from these great numbers and highly studied shallow quakes. One called as “deep earthquakes” which focal depths are so deep that rocks cannot undergo brittle deformation. The other called as “induced earthquakes” which normally considered to be triggered by human activities.For this study, deep earthquakes are occurred in the northeast China where is also in the hinge zone of Northwestern Pacific plate(NPP). An intermediate depth(~250-350 km) seismic gap having dimensions of ~300 km ×500 km is found in this research. And an sub-gap cluster located in the northeast China, situated directly beneath the prominent seismic gap, is the most unusual cluster as it is characterized by a b-value(0.6) significantly smaller than observed for other isolated clusters in the NPP(b ~0.8), and events within it have a more complex pattern of earthquake focal mechanisms than other clusters. We speculate why these sub-gap earthquakes are isolated and suggest two possible models explaining the presence of the gap and the characteristics of the underlying cluster beneath northeast China. In the first model of model A, the seismic gap is attributable to aseismic deformation caused by abnormally high temperatures in the subducting lithosphere. And, the characteristics of the sub-gap clusters and its related slab structures are attributed to slab buckling caused by resistance to subduction. In the second model of model B, the seismic gap represents absent subducting lithosphere and the missing material has migrated to the bottom of subducting slab inducing different characteristics of sub-gap cluster. Both models are consistent with our knowledge of deep earthquakes and the results of recent studies near Northeast China. Moreover, such as they can provide a simpler answer for the discontinues in the mantle transition zones.In this research, induced earthquakes are studied on Cogdell oil field, TX, USA. Between 1957 and 1982, water-flooding was conducted to improve petroleum production in the Cogdell oil field and studies concluded this induced earthquakes during this period. After that, stations detected no further activity between 1983 and 2005, but between 2006 and 2011 reported 18 earthquakes having magnitudes 3 and greater. To investigate these earthquakes, data recorded by six temporary seismograph stations deployed by the USArray program are analyzed, and multiply methods are used to relocate 93 well-recorded earthquakes occurring between March 2009 and December 2010. The results indicated earthquakes occurred within several NE-SW-trending linear clusters, with trends corresponding to nodal planes of regional focal mechanisms, possibly indicating the presence of previously unidentified faults. Data concerning injection and extraction of oil, water, and gas in the Cogdell field are also evaluated. Water injection cannot explain the 2006-2011 earthquakes, especially as net volumes(injection minus extraction) are significantly less than in the 1957-1982 period. However, since 2004 significant volumes of gases have been injected into the Cogdell field. The timing of gas injection and the correlations between injection rates and earthquakes activities suggests gas may have contributed to triggering the recent seismic activity. If so, this is the first reported instance where gas injection has triggered earthquakes having magnitudes M3 and larger. This may helpful for discus assertions concerning risks associated with large-scale carbon capture and storage as a strategy for managing climate change. |
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