Searching for Spot-Fire Earthquakes Triggered During the 2004 Parkfield Mainshock

Norman H. Sleep

Published July 22, 2018, SCEC Contribution #8186, 2018 SCEC Annual Meeting Poster #212

Strong P waves have the potential to trigger earthquakes ahead of the main rupture front. The triggered daughter event may grow large and merge with the parent event as a supershear rupture. Alternatively, a daughter nucleation may die quickly, just as most ordinary earthquakes do. The kinematics of failed daughter events are similar to those of spot fires set by wind-blown firebrands ahead of the main fire line. We seek to determine whether such spot fire earthquakes are common by analyzing recordings on the SAFOD Pilot Hole array of the 2004 Parkfield mainshock. The array is situated just off the north end of the rupture in the forward directivity direction for this earthquake. The interval between initial P wave and direct S wave on the array could contain both P and S waves from suitably located spot-fire earthquakes. For the Parkfield mainshock, almost all of the waves in this interval are P waves. We detected two transverse S wave arrivals 2 s before the direct S arrival that are plausibly from spot fire earthquakes of Mw ~3.5. However, data from small aftershocks near the mainshock hypocenter show that strong mainshock coda arrived between initial P and direct S on both the radial and transverse horizontal components. Transverse coda would tend to mimic S waves from spot fire earthquakes. In addition, the short 240 m length of the array at the time of the mainshock was too small to migrate arrivals back to apparent sources. That is, we could not clearly distinguish converted transverse phases derived from the mainshock P wave from those of true spot fire earthquakes. In general, detecting modest earthquakes triggered by mainshock P waves would help quantify the reality of stress concentrations that may accommodate supershear rupture and rupture jumps between fault segments.

Key Words
Supershear, triggering, induced earthquakes, rupture jumps

Citation
Sleep, N. H. (2018, 07). Searching for Spot-Fire Earthquakes Triggered During the 2004 Parkfield Mainshock. Poster Presentation at 2018 SCEC Annual Meeting.

Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)