Anomalous shear strain at Parkfield during 1993–94

[1] In 1993 several baselines of the two-color electronic distance meter (EDM) network at Parkfield, California, deviated from their long-term rates, coincident with anomalous observations from nearby strain meters and a creep meter, as well as an increase in microseismicity. Between October 1992 and December 1994, three M $ 4.5 earthquakes occurred beneath Middle Mountain, near the hypocenter of the 1934 and 1966 Parkfield M6 events. We analyzed the two-color EDM data using a Kalman-filtering based technique to image the spatiotemporal evolution of slip on the fault at Parkfield between the mid-1980s and 2003. This method accounts for localized random walk motion of the geodetic monuments and a prominent seasonal signal that affects many baselines. We find that a slip rate increase occurred between January 1993 and July 1996 on the upper 8 km of the fault near Middle Mountain. The peak estimated slip rate during this time was 49 mm/yr, which exceeds the long-term geologic rate of $35 mm/yr. The slip rate evolution appears episodic, with an initial modest increase after the M4.3 earthquake and a much larger jump following the shallower M4.7 event in December 1994. This temporal correlation between inferred slip and seismicity suggests that the moderate earthquakes triggered the aseismic fault slip. The EDM data cannot resolve whether transient slip propagated across the nucleation zone of the 1934 and 1966 M6 Parkfield earthquakes. However, transient slip and its associated stress release in the hypocentral area of previous Parkfield events is consistent with the nucleation of the 2004 M6 Parkfield earthquake elsewhere on the fault.

Section: Introductionmentioning

confidence: 99%

Spatiotemporal evolution of a transient slip event on the San Andreas fault near Parkfield, California

Murray

Segall

2005

“…A second important creep rate variation began in 1992-1993. Since that time, a change in the rate of strain accumulation at Parkfield has been detected by borehole tensor strain meters (BTSMs) [Gwyther et al, 1996], and the two-color elec- This decimated data set is convenient for assessing rate changes that take place over months to years but lacks sufficient time resolution to study the episodic creep events that occur at some of the instruments.…”

Section: Introductionmentioning

confidence: 99%

Creep rate changes at Parkfield, California 1966–1999: Seasonal, precipitation induced, and tectonic

Roeloffs¹

2001

Abstract. Measured fault creep at Parkfield contains information about the slip rate on the San Andreas fault but also is affected by rainfall and other seasonal effects. Seven creep meters record a repeatable seasonal variation that can be removed from the data. Four of the creep meters exhibit surges in creep rate during the rainy season. For these instruments a function of rainfall is developed that crosses a threshold within a few days of the onset of accelerated creep. In addition to the seasonal variations and surges, most Parkfield creep meters record higher creep rates more frequently during the wet season than during the dry season. Characterizing this behavior makes it possible to distinguish between rainfall-influenced creep and creep rate changes of tectonic origin. Accelerated creep beginning in January 1983 was probably induced by rainfall, rather than by tectonic sources related to the Coalinga earthquake in May 1983. Four creep meters in the central part of the Parkfield fault segment are recording higher creep rates since 1993. A strong case can be made that the increased creep rate at CRR1 is tectonic, but for the other three creep meters an influence of rainfall is difficult to rule out. Strain rate changes recorded on three-component borehole strain meters suggest that the rate of subsurface creep to the northwest of these instruments is even greater. The 1993 creep rate increase is more abrupt and farther south than the expected signal from accelerating slip near the hypocenter of the 1966 Parkfield earthquake but is in the same location as rapid fault slip reported prior to that event.

“…Because that study was based on only two sites from a single instrument type and because such a multiyear transient is at the long-period end of the band where borehole instruments are typically used, the existence of the shear strain anomaly proposed by Gwyther et al [1996] has become an issue of debate.…”

mentioning

confidence: 99%

Analysis of deformation data at Parkfield, California: Detection of a long‐term strain transient

Gao¹,

Silver²,

Linde³

2000