UC Berkeley

The dynamic transition from slow to rapid sliding along a frictional interface is of interest to geophysicists, engineers and scientists alike. In our direct shear experiment, we simulated a pre-existing frictional fault similar to those occurring naturally in the Earth. Laboratory experiments have been used in the past to successfully link the fields of rock mechanics and seismology and have been able to produce better estimates of seismic hazard. These laboratory studies found that prior to large earthquakes a nucleation region grows outwardly. Within this region shear stress is overcome through the accumulation of slip across the fault. This phenomena is referred to as slip-weakening. Once the region grows large enough an earthquake ensues. The growth phase of this nucleation zone is called the premonitory phase. Newer seismological instruments are showing that this phase actually carries seismic signatures (originally thought to be an aseismic process). This precursory seismicity is observed as foreshocks and migrating swarm-like foreshocks occur at the fringe of this expanding nucleation front. The previous laboratory studies, used to develop the major phenomenological earthquake models, did not observe this precursory seismicity, possibly due to the lack of sensors capable of measuring this high-frequency phenomenon. The laboratory study reported here has incorporated appropriate sensors that can detect foreshock events on the fringe of a nucleation zone prior to a gross fault rupture (main shock).

\par \ident During loading we observed foreshocks sequences as slip transitioned from slow to rapid sliding. These laboratory-induced foreshocks showed similar acoustic characteristics and spatio-temporal evolution as those detected in nature. Through direct observation (video camera), foreshocks were found to be the rapid, localized (millimeter length scale) failure of highly stresses asperities formed along the interface. The interface was created by the meshing of two rough polymethyl methacrylate (PMMA) bodies in a direct shear configuration. A carefully calibrated pressure sensitive film was used to map the contact junctions (asperities) throughout the interface at a range of applied normal loads $F_{n}$. Foreshocks were found to coalesce in a region of the fault that exhibited a more dense distribution of asperities (referred to as the \emph{seismogenic} region).

\par \indent Microscopy of the interface in the \emph{seismogenic} region displayed a variety of surface roughness at various length scales. This may have been introduced from the surface preparation techniques use to create a mature interface. The mature interface consisted of `flat-topped' asperity regions with separating sharp valleys. The `flat-topped' sections spanned millimetric length scales and were considerably flatter (nanometric roughness) that the roughness exhibited at longer length scales (tens of millimeters). We believe that the smoother, `flat-topped' sections were responsible for the individual asperity formation (determining their size and strength), whereas the longer length scale roughness influenced the asperity-asperity interaction during the nucleation phase. Asperities in the \emph{seismogenic} region where shown to exist close enough to each other so that elastic communication (through the off-fault material) could not be neglected.

\par \indent Prior to gross fault rupture (i.e. mainshock), we measured the propagation of a slow nucleating rupture into the relatively `locked', \emph{seimsogenic} region of the fault. Slow slip dynamics were captured using slip sensors placed along the fault that measured a non-uniform slip profile leading up to failure. We found that the propagation of the slow rupture into the locked region was dependent on the normal force $F_{n}$. Higher $F_{n}$ was found to slow the propagation of shear rupture into the locked region. Within the relatively `locked' region, a noticeable increase in size and a more compact spatial-temporal distribution of foreshocks were measured when $F_{n}$ was increased.

\par \indent In order to develop an understanding of the relationship between $F_{n}$ and the resistance of the fault to slow rupture, a quasi-static finite element (FE) model was developed. The model used distributions of asperities measured directly from the pressure sensitive film in a small section of the interface where foreshocks coalesced; specifically, the region where the slowly propagating slip front encountered the more dense distribution of asperities. A single asperity was modeled and followed the Cattaneo partial slip asperity solution. As the shear force increased along the fault, the asperities in this model were able to accommodate tangential slip by entering a partial sliding regime; the central contact of the asperities remained adhered while sliding accumulated along its periphery. Partial slip on the asperity propagated inwards as the shear force was incrementally increased. A further increase in the shear force caused the asperity to enter a full sliding condition. Increasing confining loads caused increased stiffness and increased capacity to store potential shear strain energy -- a possible measure of the `degree of coupling' between the fault surfaces. Physics from the numerical model followed the qualitative observations made using photometry of asperities along the interface, which visualized asperities in the `locked' region -- larger asperities remained stuck throughout the loading cycle and the light transmitted through individual asperities decreased from the periphery as shear loads increased.

\par \indent The numerical partial slip, quantified by the potential energy stored by the asperity, increased relative to the normal pressure $p$. Asperity-asperity interactions were modeled along the interface using a quasi-static analysis. Progression of slip into the asperity field was increasingly inhibited as the normal confining force $F_{n}$ was increased. The computational model provided an explanation as to why an increased confining force $F_{n}$ could result in an increased resistance to slow rupture as well as an increased potential for larger foreshocks within the resistive, relatively `locked' section of a fault. The experiments and modeling presented in this study lay the foundation for more innovative laboratory work that could potentially improve the phenomenological models currently used to estimate earthquake hazard.