Searching for Supershear Rupture at Parkfield

Session: What Can We Infer About the Earthquake Source Through Analyses of Strong Ground Motion?
Type: Oral
Date: 4/29/2020
Time: 05:30 PM
Room: 115
Description: 

When the Parkfield earthquake experiment was designed in the mid-1980s, it was recognized that capturing the strong motion of the M 6 event by an array of strong motion sensors would provide unprecedented opportunity to unravel the dynamics of rupture propagation. In planning the experiment, Paul Spudich strongly advocated for the inclusion of dense seismic array observations as an essential tool for understanding the ground motions and rupture kinematics (Spudich and Oppenheimer, 1986). His vision became a reality in UPSAR, an array of 14 irregularly spaced accelerometers positioned near the middle of the rupture. UPSAR provided clear evidence that the main rupture was subshear while also producing remarkable observations of the dynamic strains, tilts and torsions caused by the 2004 Parkfield earthquake (Spudich and Fletcher, 2008). We revisit the question of supershear rupture using the SAFOD pilot hole array, located about 24 km northwest of the 2004 epicenter in the forward rupture propagation and just beyond the extent of rupture. The 7-levels of the array operational at that time are ideally situated to detect triggering of daughter ruptures or “spot-fire” events ahead of the main rupture front by strong P waves. The kinematics of spot-fire events are similar to those of spot fires set by wind-blown firebrands ahead of the main fire line. Detection of earthquakes triggered by mainshock P waves would help quantify stress concentrations that launch both supershear ruptures and rupture jumps between fault segments. Our results confirm the subshear rupture velocity for the main rupture front. They also contain evidence for transient S-wave arrivals before direct S that are plausibly from spot-fire events, failed daughter ruptures. However, the short 240 m length of the SAFOD pilot hole array limits our ability to discriminate between direct S waves from spot-fire events and transverse coda waves.

Presenting Author: William Ellsworth

Authors