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

Abstract

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.

Annual modulation of seismicity along the San Andreas Fault near Parkfield, CA

“…If runoff and groundwater recharge are focused such that change in the water table is locally enhanced, stress change at seismogenic depths may exceed 2 kPa. In addition, some creepmeters show increased creep rates during the rainy season, while others show accelerated creep following individual storms [Roeloffs, 2001], similar to the relationship we infer between precipitation and earthquakes in the locked and creeping sections of the fault, respectively.…”

“…Rainfall in Parkfield follows an annual cycle, with the onset of rainfall typically occurring in November and largest storms typically occurring from February through April [Roeloffs, 2001] (Figure 4). We compiled rainfall data from the Parkfield station [Roeloffs, 2001] and four stations near Parkfield: Paso Robles, Black Mountain, Bradley, and Santa Margarita (http://cdec.water.ca.gov/); where data were missing or erroneous, averages of the remaining stations were used. Between 1984 and 2004, 13 months had >15 cm of rainfall.…”

Thirty-Five-Year Creep Rates for the Creeping Segment of the San Andreas Fault and the Effects of the 2004 Parkfield Earthquake: Constraints from Alignment Arrays, Continuous Global Positioning System, and Creepmeters

“…Random walk of the anchor points (e.g., Langbein and Johnson, 1997), the effects of changes in precipitation and seasonal variations (e.g., Roeloffs, 2001), and tectonically induced variations in creep rates are ignored in our simplistic analysis and are assumed to average down to insignificant levels over the long time spans considered here. Creep rates for select creepmeters on 10-to 30-year timescales are reported in Table 2.…”

Behavior of Repeating Earthquake Sequences in Central California and the Implications for Subsurface Fault Creep

“…The geometry of the fault zone itself has also been suggested to influence aseismic creep (Moore and Byerlee, 1992). Furthermore, surface creep can be affected by non-tectonic environmental factors, such as rainfall and yearly seasonal variations (Roeloffs, 2001).…”