Isotropic Events Observed with a Borehole Array in the Chelungpu Fault Zone, Taiwan

Cracking Up Hydraulic fracturing by fluids at high pressure results in damage or breakage along cracks in deep rocks, a process that in some cases causes earthquakes. This process can occur naturally when the hydrologic setting is just right, or can be induced by human activity when fluids are pumped at high pressure into deep aquifers. By studying the fault along which the 1999 magnitude 7.6 Chi-Chi earthquake occurred in Taiwan, where there are currently low tectonic stresses following the large earthquake, Ma et al. (p. 459) observed an unusual type of earthquake-like event that they attribute to natural hydraulic fracturing. High-pressure fluids induce fracturing along preexisting cracks near the site of a recent earthquake. Shear failure is the dominant mode of earthquake-causing rock failure along faults. High fluid pressure can also potentially induce rock failure by opening cavities and cracks, but an active example of this process has not been directly observed in a fault zone. Using borehole array data collected along the low-stress Chelungpu fault zone, Taiwan, we observed several small seismic events (I-type events) in a fluid-rich permeable zone directly below the impermeable slip zone of the 1999 moment magnitude 7.6 Chi-Chi earthquake. Modeling of the events suggests an isotropic, nonshear source mechanism likely associated with natural hydraulic fractures. These seismic events may be associated with the formation of veins and other fluid features often observed in rocks surrounding fault zones and may be similar to artificially induced hydraulic fracturing.

[1]  J. Virieux,et al.  Stress and fluid transfer in a fault zone due to overpressures in the seismogenic crust , 2007 .

[2]  E. Brodsky,et al.  In situ measurement of the hydraulic diffusivity of the active Chelungpu Fault, Taiwan , 2006 .

[3]  Yi-Ben Tsai,et al.  Slip zone and energetics of a large earthquake from the Taiwan Chelungpu-fault Drilling Project , 2006, Nature.

[4]  Mark D. Zoback,et al.  Oscillation of fluid‐filled cracks triggered by degassing of CO2 due to leakage along wellbores , 2010 .

[5]  H. Kao,et al.  Active detachment of Taiwan illuminated by small earthquakes and its control of first-order topography , 2002 .

[6]  C. Wang,et al.  Initial Science Report of Shallow Drilling Penetrating into the Chelungpu Fault Zone, Taiwan , 2002 .

[7]  R. Davies,et al.  Hydraulic fractures: How far can they go? , 2012 .

[8]  Michael Roth,et al.  Automated seismic event location for hydrocarbon reservoirs , 2003 .

[9]  Jeen‐Hwa Wang.  Summary of Physical Properties Measured at Several Boreholes Penetrating through the Chelungpu Fault in Central Taiwan , 2010 .

[10]  A. McGarr,et al.  An implosive component in the seismic moment tensor of a mining‐Induced tremor , 1992 .

[11]  M. Kinoshita,et al.  Core description and characteristics of fault zones from Hole-A of the Taiwan Chelungpu-fault Drilling Project , 2007 .

[12]  V. Oye,et al.  Observation and scaling of microearthquakes from the Taiwan Chelungpu-fault borehole seismometers , 2012 .

[13]  Jia‐Jyun Dong,et al.  Seismic velocities, density, porosity, and permeability measured at a deep hole penetrating the Chelungpu fault in central Taiwan , 2009 .

[14]  K. Okamoto,et al.  Fossil fluid reservoir beneath a duplex fault structure within the Central Range of Taiwan: implications for fluid leakage and lubrication during earthquake rupturing process , 2005 .

[15]  Yuan-Hsi Lee,et al.  Surface Rupture of 1999 Chi-Chi Earthquake Yields Insights on Active Tectonics of Central Taiwan , 2004 .

[16]  R. Sibson.  Frictional Mechanics of Seismogenic Thrust Systems in the Upper Continental Crust—Implications for Fluid Overpressures and Redistribution , 2004 .

[17]  R. Sibson.  Implications of fault-valve behaviour for rupture nucleation and recurrence , 1992 .

[18]  Y. Guglielmi,et al.  Hydromechanical interactions in a fractured carbonate reservoir inferred from hydraulic and mechanical measurements , 2005 .

[19]  P. Segall,et al.  Temporal and spatial variations of post-seismic deformation following the 1999 Chi-Chi, Taiwan earthquake , 2007 .

[20]  Y. Ben‐Zion,et al.  Seismic radiation from regions sustaining material damage , 2009 .

[21]  Michael B. Smith,et al.  Hydraulic Fracturing: History of an Enduring Technology , 2010 .

[22]  K. Ma,et al.  Subsurface structure, physical properties, fault-zone characteristics and stress state in scientific drill holes of Taiwan Chelungpu Fault Drilling Project , 2009 .

[23]  Kuo-Fong Ma,et al.  Strong ground motion simulation of the 1999 Chi‐Chi, Taiwan earthquake from a realistic three‐dimensional source and crustal structure , 2007 .

[24]  M. Zoback,et al.  Stress orientations of Taiwan Chelungpu‐Fault Drilling Project (TCDP) hole‐A as observed from geophysical logs , 2007 .

[25]  H. Kanamori,et al.  Evidence for fault lubrication during the 1999 Chi‐Chi, Taiwan, earthquake (Mw7.6) , 2003 .

[26]  B. Julian,et al.  Earthquake mechanisms from linear-programming inversion of seismic-wave amplitude ratios , 1996, Bulletin of the Seismological Society of America.

[27]  W. Soh,et al.  Transport properties and dynamic processes in a fault zone from samples recovered from TCDP Hole B of the Taiwan Chelungpu Fault Drilling Project , 2009 .

[28]  Hiroki Sone,et al.  Stress-dependence of the permeability and porosity of sandstone and shale from TCDP Hole-A , 2010 .