俯冲断裂带巨大地震的极高压气体成因与机理简述(英文)

Cause and mechanism of great earthquakes due to highly compressed methane gas mass trapped in subduction faults

Zhong-qi Quentin Yue

Department of Civil Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong, China,

Email: yueqzq@hku.hk

The frequent occurrences of many great damaging earthquakes at subduction faults in last decade allowed us to have acquired massive observational data and greatly improved our understanding about subduction zone process. In his Birch Lecture atthe 2015 AGU Fall Meeting, Professor Kelin Wang^[1，2] presented the findings that all subduction faults are extremely weak, usually represented by apparent friction coefficient  lower than 0.05 according to Byerlee’s law^[3]below,

                                                                      (1)

where  is the fault shear strength and  is the normal compressive stress acting on the fault.

The faults that produced giant earthquakes are the weakest, smooth and do not have any geometrical  irregularities such as subducting seamounts. The rupture-zone average stress drops in great earthquakes are as small as 2 to 5 MPa.

This article attempts to use the highly compressed methane gas mass model to geochemically, geophysically and geomechanically interpret and explain the occurrence of great earthquakes at subduction fault with long recurrenceintervals^[4]. The gas model can be described as follows with thereference to Figure 1.

The subduction faults between the continental and oceanic rock crusts are weak zones and can become a reservoir for collection and accumulation of methane gasgenerated in deeper rocks. With time t,the mass M(t), volume V(t), pressure P(t) and temperature T(t) of the reservoir gas can change and increase because of the high confining compressive tectonic stresses and the low permeability of the two crustal rocks.  The high the , the high the M(t), V(t), P(t) and the great the earthquake. At the faultzone, the Byerlee’s law can be expressed by

                                                                      (2)

where is the actual frictional coefficient of the fault rocks, usually equals 0.6 to 0.8.

Using

                                                      (3)

we have

                                                            (4)

At the critical moment of the fault rupturing of great earthquakes, so,

                                                                 (5)

If the in-situ stress  = 400 PMa, then P(0) = 360 ~ 380 MPa.

Rapidly,at the time t = 0, a huge amount of gas with the mass , volume ,  pressure P(0) and temperature T(0) is escaping the reservoir trap at the focus A and, flowing and expanding quickly into upper and adjacent rock masses along the weakest fault zones, which causes the seismic waves globally. The highly compressed gas mass is jetting into the seawater and expanding quickly, which induces seawaters and tsunami. Subsequently, it is rapidly jetting and flowing into the atmosphere, which causes temperature drop, pressure fluctuations, clouds, rains and/or snows over vast sky regions.

The gas also carries electrical charges, which can cause the disturbance ofelectromagnetic fields of the ground and the atmosphere.

The expanding energy released by the escaped gas mass  can be estimated using the following equation[4,5].

                                                                        (6)

Under the constraints of the confining tectonic stresses and crustal rocks, the arc crust is subsiding substantially due to the huge loss of the gas mass support in the inclined subduction fault zones. Immediately, the arc crust is moving largely seaward due to both the subsidence and the high compressive horizontal tectonic stress . The escaped gas volume  approximately equals the subsided volume of the crustal rocks. The release of the gravitational potential energy of the rock crust mass due to its subsidence can be estimated using the following equation.

                                                                    (7)

where is the average subsidence of the crustal rock mass , and g = 9.8 m/s².

The continued escaping and flowing and expanding of the highly compressed gas masses in various existing or new gas reservoirs induce numerous aftershocks.  

Because the escaped gas mass   and volume is only a few percentage of the total gas mass M(t) and volume V(t) in the subduction fault reservoirs, the rupture-zone average stress drops in great earthquakes are small and only be 2 to 5 MPa, which can be used to estimate the M(t) and volume V(t) at the rupturing time t= 0 as follows

                                                                 (8)

So,we have

                                               (9)

Or,

                                                             (10)

If the escaped gas mass = 1 km³, the total gas volume V(0) at the pressure of 360 to 380 MPa can be 75 to 180 km³. If the ruptured area of the great earthquake source is 200 km long by 50 km wide, the average thickness of the gas reservoir is about 7 m to 18 m.

With time, the aftershocks become less and less, which shows the gases in various reservoirs gains compatible and equilibrium with trap strengths and the confining tectonic stresses. In the meantime, the subduction fault reservoirs re-collect and re-accumulate new gas masses from deep and surrounding rocks. Therefore, it takes time to rebuild fault gas mass and pressure to the level of trap rock rupture, which is consistent with great earthquakes of long recurrence intervals.

This process of gas mass and pressure rebuilding is strongly affected by the chemical reactions and physical conditions of the hot mantle and core materials.  Because of lack of gas mass and pressure in the subduction fault zones, most of the arc rocks continues to relax and moves seaward following a great earthquake. But, they would gradually reverse direction to move landward as the gas mass in the subduction fault has collected and accumulated enough. The gas pressure becomes high enough and gasmass can transfer the landward movement of the oceanic crust to the continental crust, which may indicate the possible occurrence of the next great earthquakes. If it needs 100 years to rebuild the highly compressed gas mass of 1 km³ at a particular subduction zone for nest great earthquake, the accumulation rate of the highly compressed gas in the fault reservoir would be about 30,000 m³ per day.

References

[1] Wang, Kelin., 2015. Subduction Faults as We See Them in the 21st Century, Birch Lecture,Tectonophysics, December 16, 2015, the 2015 AGU Fall Meeting,  San Francisco, USA.

[2] 岳中琦，2016. 2015年AGU秋季会上聆听了克林师兄的Birch讲座, 科学网岳中琦博客http://blog.sciencenet.cn/blog-240687-950405.html

[3] Byerlee, James D. (July 1978). "Friction ofRocks". Pure and Applied Geophysics,116 (4-5): 615–626. doi:10.1007/BF00876528. ISSN 0033-4553.

[4] Yue, Z.Q., 2014. On cause hypotheses of earthquakes with external tectonic plate and/or internal dense gas loadings, Acta Mechnica 225(4), 1447–1469 (2014), doi10.1007/s00707-013-1072-2

[5] 岳中琦, 2014. 牛顿是如何站在巨人肩膀上？, 科学网岳中琦博客http://blog.sciencenet.cn/blog-240687-842660.html

2016年2月13日17:45写成于香港大学黄克兢楼办公室