Simultaneous Strainmeter and Creepmeter Observations of Creep events on the San Andreas Fault near Parkfield: A Framework to Observe and Quantify Creep Events

Borehole strainmeters and creepmeters have been used to observe creep events for decades. Due to the low sample rate of these early measurements the utility of these observations was limited to quantifying the static strain accompanying creep events. Several studies have attempted to develop a dynamic quasi-plastic model of creep with some success but these studies were not undertaken within a comprehensive rheological model of the dislocation propagation process. Here we observe and catalog creep events observed on a single borehole strainmeter (B073) and three creepmeters (XMM1,XMD1,XVA1), located near Parkfield, between 2009-2017. We present a new method to examine the depth evolution of shallow creep events and show that a combination of creepmeters and strainmeters allows one to quantify the properties of an aseismic slip event using just two parameters - a time constant (ω) and a slip velocity (v_p). This combination allows for the characterization of static and dynamic spatial-temporal behavior of creep events from the far-field. We develop a quasi-dynamic model and code to exploit this and apply it successfully to observed creep events. We also validate the model against models that utilize the full 3D wave equation. Creep events with multiple subevents cannot be emulated by this simple model. It also appears that while most events propagate along the fault at 8 km/hr, several have much higher velocities, on the order of 45 km/hr. While this may be a result of the tradeoff between v_p and ω resulting from using limited observations, or to an upward component of creep propagation resulting a record of an apparent surface phase-velocity, the identification of an anomalous v_p or ω would deserve further investigation. The observed set of creep events show creep activity occurring from the surface, where they are recorded by creepmeters, to at least 5 km depth, where the strain associated with their propagation are monitored by strainmeters alone.