Synchronous low frequency earthquakes and implications for deep San Andreas Fault slip

Abstract

Low Frequency Earthquakes (LFEs) are slip events that occur repeatedly at source locations within the lower crust. LFEs, and the associated seismic broadcast known as tremor, have been observed in a diverse array of tectonic environments. Here we develop a suite of statistical tools to conduct a systematic study of the spatial and temporal correlations of the event occurrence patterns of the 88 LFE sources beneath the greater Parkfield section of the San Andreas Fault. We first examine correlations in the occurrence patterns on long time scales to show that the regions to the north and south of Parkfield behave independently. We next use the cumulative event signatures of each source to characterize the individual occurrence patterns on shorter time scales. Through application of a statistical clustering algorithm, we demonstrate that individual LFE sources form spatially coherent clusters that may represent localized elastic structures or asperities on the deep fault interface. We conclude by examining the fine-scale features of the event rates within the LFE occurrence patterns. Through quantitative comparison to analogous laboratory shear experiments on granular, fault gouge-like materials, we infer that the distinctive features of LFE occurrence patterns reflect variations in the insitu stress and frictional conditions at the individual LFE source locations. These observations provide a framework to understand the spatial and temporal diversity of fault slip that occurs within the lower crust beneath Parkfield and that may influence seismic hazard in the region.

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

“…Assuming a constant stress drop results in a scaling of 0.19, inconsistent with either of the previously proposed scalings. Increasing C by a factor of 5 for the episodic families (which corresponds to an effective stress of 5 MPa) results in maximum length scales on the order of tens of kilometers, which is consistent with the spatial extents inferred from other studies (Shelly, 2015;Trugman et al, 2015;Wu et al, 2015). We determine length scales of creep events by taking the values shown in Table 1, C = 0.047 GPa/m (assuming an effective stress of 1 MPa), and = 30 GPa.…”

“…Using equation (10), = 0.98, the median value for the episodic families, and taking the shear modulus in the source region to be 30 GPa results in a creep patch dimension of ≈30 km, consistent with the estimates of Shelly (2015) and Trugman et al (2015) deduced from the spatiotemporally coherent occurrence of the LFE families. Using equation (10), = 0.98, the median value for the episodic families, and taking the shear modulus in the source region to be 30 GPa results in a creep patch dimension of ≈30 km, consistent with the estimates of Shelly (2015) and Trugman et al (2015) deduced from the spatiotemporally coherent occurrence of the LFE families.…”

“…First, while the continuous families generally have fewer LFEs per episode than episodic families (see Table 1), they still have multiple LFEs per episode (typically 3-8 LFEs). Finally, previous studies have used analyses similar to that presented here to explore the spatial extent of interactions among families (Shelly, 2015;Trugman et al, 2015;Wu et al, 2015). Second, the continuous families have a larger fraction of interepisode LFEs than the episodic families many of which occur in isolation.…”

“…Hypocenters of earthquakes that occurred in the last decade (i.e., 2006-2016) Shelly and Johnson (2011) also note a first-order decrease of episodicity with depth. Additionally, some LFE families have slip histories that closely resemble those of neighboring families suggesting that these distinct LFE families take part in the same underlying slip episode (Shelly, 2015;Trugman et al, 2015). Additionally, some LFE families have slip histories that closely resemble those of neighboring families suggesting that these distinct LFE families take part in the same underlying slip episode (Shelly, 2015;Trugman et al, 2015).…”

A 15 year catalog of more than 1 million low‐frequency earthquakes: Tracking tremor and slip along the deep San Andreas Fault

“…These include characterizations of the effects of the nearby 2003 M 6.5 San Simeon earthquake and the 2004 M 6.0 Parkfield earthquake [ Shelly and Johnson , ; Johnson et al , ], triggering by dynamic stresses of teleseismic and occasionally regional earthquakes [ Peng et al , ; Shelly et al , ; Peng et al , ; Hill et al , ; Peng et al , ], and modulation by tidal forces [ Thomas et al , ; van der Elst et al , ]. Additionally, multiple studies have examined the interactions and observed migration among LFE sources [ Wu et al , , ; Shelly , ; Trugman et al , ]. This LFE catalog has also been used to constrain physical models of tremor occurrence [ Daub et al , ] and to infer fault rheological properties [ Beeler et al , ].…”