Using Low‐Frequency Earthquake Families on the San Andreas Fault as Deep Creepmeters

Abstract

The central section of the San Andreas Fault hosts tectonic tremor and low‐frequency earthquakes (LFEs) similar to subduction zone environments. LFEs are often interpreted as persistent regions that repeatedly fail during the aseismic shear of the surrounding fault allowing them to be used as creepmeters. We test this idea by using the recurrence intervals of individual LFEs within LFE families to estimate the timing, duration, recurrence interval, slip, and slip rate associated with inferred slow slip events. We formalize the definition of a creepmeter and determine whether this definition is consistent with our observations. We find that episodic families reflect surrounding creep over the interevent time, while the continuous families and the short time scale bursts that occur as part of the episodic families do not. However, when these families are evaluated on time scales longer than the interevent time these events can also be used to meter slip. A straightforward interpretation of episodic families is that they define sections of the fault where slip is distinctly episodic in well‐defined slow slip events that slip 16 times the long‐term rate. In contrast, the frequent short‐term bursts of the continuous and short time scale episodic families likely do not represent individual creep events but rather are persistent asperities that are driven to failure by quasi‐continuous creep on the surrounding fault. Finally, we find that the moment‐duration scaling of our inferred creep events are inconsistent with the proposed linear moment‐duration scaling. However, caution must be exercised when attempting to determine scaling with incomplete knowledge of scale.

Constraints on Friction, Dilatancy, Diffusivity, and Effective Stress From Low‐Frequency Earthquake Rates on the Deep San Andreas Fault

“…The catalog consists of the occurrence time of the first LFE in each episode, representing the episode onset time, for all the continuous families relative to the tidal stress ( Figure 5). Thomas et al (2017) conclude that for continuous families, during clusters, LFE occurrence rates are not proportional to the fault slip rate. As shown in Figure 5 there is little difference between the tidal correlation of the whole LFE catalog (2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012) and the post Parkfield subset (2005.5-2012).…”

“…We extend this idea to the behavior of all the continuous families by constructing a "declustered" catalog. The latter result has been discussed in a more general context by Thomas et al (2017), who consider in detail the assumption underlying our analysis, that LFE rates are proportional to the slip rate of the fault. To avoid possible influences of the 2004 Parkfield earthquake (e.g., Shelly, 2017) we undertake the analysis from 2005.5 (07/02/05) to the end of the catalog (1/4/2012).…”

“…These observations confirm that the LFE catalog is dominated by the behavior of continuous families, and on the time scale of the episodes, the correlation does not arise from modulation of slip by the tides. The latter result has been discussed in a more general context by Thomas et al (2017), who consider in detail the assumption underlying our analysis, that LFE rates are proportional to the slip rate of the fault. Thomas et al (2017) conclude that for continuous families, during clusters, LFE occurrence rates are not proportional to the fault slip rate.…”

“…The latter result has been discussed in a more general context by Thomas et al (2017), who consider in detail the assumption underlying our analysis, that LFE rates are proportional to the slip rate of the fault. Thomas et al (2017) conclude that for continuous families, during clusters, LFE occurrence rates are not proportional to the fault slip rate. Consequently, these short duration bursts cannot be interpreted as slow slip events but instead represent some kind of compound rupture driven to failure by surrounding slow slip.…”

“…Since the duration of episodes is long relative to earthquake risetimes, this would be a triggered creep model with each episode onset being used as a proxy for the onset of detectable slip (see Thomas et al (2017) for detailed discussion of the temporal resolution of LFE families). Since the duration of episodes is long relative to earthquake risetimes, this would be a triggered creep model with each episode onset being used as a proxy for the onset of detectable slip (see Thomas et al (2017) for detailed discussion of the temporal resolution of LFE families).…”