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Exploring the underlying mechanism of LURR theory. Can Yin Supervisor: Prof. Peter Mora QUAKES, Department of Earth Sciences University of Queensland. Load/Unload Response Ratio – promising but controversial.
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Exploring the underlying mechanism of LURR theory Can Yin Supervisor: Prof. Peter Mora QUAKES, Department of Earth Sciences University of Queensland
Load/Unload Response Ratio – promising but controversial • As a phenomenon, high LURR prior to major earthquake has been found in many earthquakes by retrospective examination. It is also observed in several real time predictions. • Since accelerating seismic moment release is also observed in many earthquakes, it is attractive to find a common underlying mechanism for these two phenomena. If they share a common root, does the critical region size in AMR apply to LURR?
Definition of LURR LURR is originally defined in differential form, making it a state variable depending only on the state of the system at that moment. In current implementation, we define the loading duration instead of loading increment itself as P and introduce the integral form of response rate as follow: Accordingly, LURR is defined as If the averaging time is long enough, T+ is almost equal to T-.
However, the sacrifice of compromise to the integral definition of LURR is it now involves many other factors, whose relations with LURR are yet to be found. The new LURR is related to the history of system evolution and thus dependent of many extrinsic parameters, such as magnitude range of selected earthquakes, the spatial range of earthquakes used, fault parameters, etc. For example, when defining effective Coulomb Failure Stress for criteria of load/unload, we simply define it either as that on the fault plane of ensuing main shock or that on an optimal fault plane determined by the tectonic stress field. In the former case, not all the earthquakes occurred on the main fault plane; while in the latter case, it is proved that the tectonic stress field changes direction prior and after a major earthquake. In either case, the definition of Coulomb Effective Stress is kind of ambiguous.
Re-examine critical region sizes by LURR • We examine our study on last 5 major earthquakes in California with magnitudes greater than 6.5 between 32° N and 40°N latitude since 1980. The catalog used in this study is the Council of the National Seismic System (CNSS) Worldwide Earthquake Catalog, which is accessible via the World Wide Web at the Northern California Earthquake Data Center (http://quake.geo.berkeley.edu/cnss). According to the relation between critical region radius and ensuing mainshock magnitude in AMR, radii of the critical region of all 5 earthquakes are larger than 100km. • As in Australia, due to limited data, we only examined the last two well-recorded earthquakes with magnitude greater than 5.0.
Burakin, West Australia (a series of M>5.0 earthquakes since late 2001 till early 2002 )
Discussion Note: All plots here are plotted with incorporation of disturbance of random process, which is expressed by Y90, i.e. LURR= Y/Y90. LURR>1.0 means it can be considered as abnormality at 90% confidence level. LURR=0 indicates that data is insufficient to make significant calculation (the minimum sample size is 20 events in a group).And upper limit of magnitude of earthquake taken into calculation is 4.0. From the figures above, it is obvious that smaller the region, the more prominent the LURR abnormality will be. How to explain it? Two possible answers: (1) Either the method used in AMR to correspond the region size where LURR has peak value to critical region size does not apply to LURR; or (2) there is simply no critical region.More close to the future epicenter, more critical. This is a very intuitive explanation.
Look at the epicenters of these earthquakes. If the seismogenic time of an earthquake is decades long, given the proximity of time and space between two earthquakes, say Landers(7) and Northridge(8), how could you distinguish which precursory events belongs to which major earthquake? If they are not distinguishable, the critical regions of each earthquake will be intermingled. Then how large is each region?
Re-orientation of small faults as a possible mechanism of current LURR implementation • Some researchers found that the tectonic stress field rotate before and after the major earthquakes in California. • In order to find a clue for the relation between stress rotation and LURR abnormality, we calculate LURR under different fault planes, which is uniquely determined by tectonic pressure axis (P-axis). Below are some examples of LURR change with optimal P axis direction. P axis change from –90 deg(West) clockwise to 0 (North) until +90 deg(East).
Discussion • LURR abnormality pattern in California is obviously different from that in Australia. In Northridge, for example, LURR has peaks in many directions at different time. In Burakin, however, LURR has peaks only in direction along North-South. It is likely that the tectonic stress field fixes in this direction for a long time. This may be the result of the relatively stable tectonic stress field in Australian plate. • When the crust is far away from criticality, small faults tend to be randomly orientated, LURR calculated for any fixed direction will surely fluctuate around unity. But when the crust is close to failure, the number of fractures along a particular direction determined by the built-up tectonic stress will get dominant, causing calculated LURR along this direction high up. Such rotation mechanism partly solves the dilemma brought by the ambiguous definition of Coulomb Effective Stress in current implementation of LURR.
Summary • The method to identify critical region size in AMR doesn’t apply to LURR. Failure to find better power-law curve fitting at smaller region in AMR may be due to observational limit rather than physical meaning, becausethere is not enough seismic events to obtain a statistically significant curve-fit within smaller region. • LURR may not share a common underlying mechanism with AMR, but may be caused by critical sensitivity which is reached by the “phase up” of previously randomly oriented small fractures re-orientating towards a certain direction determined by tectonic stress prior to main shock, resulting in higher LURR in this direction. Such re-orientation may be a kind of underlying mechanisms of LURR phenomenon for current implementation.
