Earthquake fossils reveal multiple ancient earthquakes in southeastern California
Location
Bent Corridor, Science Center
Document Type
Poster - Open Access
Start Date
5-1-2026 12:00 PM
End Date
5-1-2026 2:00 PM
Abstract
Pseudotachylyte (p-tach) is frictional melt produced during a seismic event. Preserved as veins of glass and broken host rock, they aid in characterizing the conditions and magnitude of paleoearthquakes. We analyze p-tach veins preserved in granitic gneiss rocks from the Riverside Mountains in southeastern California, a region that underwent high-magnitude extension of the crust during the Miocene epoch (ca. 23 to 8 Myrs ago). We investigate the character and relative timing of pre-earthquake ductile deformation and earthquake-related brittle, vein-forming events using a sample from below the Riverside detachment fault, which was active between ca. 18 and 14 Myrs ago. The host rock comprises layered quartz, feldspar, and greenschist-facies minerals (epidote, chlorite, and biotite). We observed a possible temperature gradient, where quartz demonstrates grain boundary migration (GBM) concentrated at the bottom of the sample and gradually turning to subgrain rotation (SGR) and bulging (BLG) towards the top. In general, older, pre-seismic fabrics are defined by quartz with high-temperature (ca. 500°C) ductile texture (GBM) overprinted by medium (450°C) and lower-temperature (350°C) ductile textures (SGR and BLG). In the brittle regime, at least 3 generations of p-tach veins are distinguished by thickness, orientation, and composition. Vein thickness provides an estimate of seismic displacement. Approximate displacements generated by the main p-tach forming event range from ca. 0.4 to 0.7 m, corresponding to a Moment Magnitude of ca. 2.9. Investigating the magnitude of ancient earthquakes is paramount to understanding modern seismic hazard in regions undergoing high-magnitude extension of the crust today.
Keywords:
Paleoearthquakes, Pseudotachylyte, Minerals, Southeastern California
Recommended Citation
Rodríguez Arroyo, Kimberly, "Earthquake fossils reveal multiple ancient earthquakes in southeastern California" (2026). Research Symposium. 13.
https://digitalcommons.oberlin.edu/researchsymp/2026/posters/13
Major
Geosciences; Latin American Studies
Project Mentor(s)
Megan E. Flansburg, Geosciences
2026
Earthquake fossils reveal multiple ancient earthquakes in southeastern California
Bent Corridor, Science Center
Pseudotachylyte (p-tach) is frictional melt produced during a seismic event. Preserved as veins of glass and broken host rock, they aid in characterizing the conditions and magnitude of paleoearthquakes. We analyze p-tach veins preserved in granitic gneiss rocks from the Riverside Mountains in southeastern California, a region that underwent high-magnitude extension of the crust during the Miocene epoch (ca. 23 to 8 Myrs ago). We investigate the character and relative timing of pre-earthquake ductile deformation and earthquake-related brittle, vein-forming events using a sample from below the Riverside detachment fault, which was active between ca. 18 and 14 Myrs ago. The host rock comprises layered quartz, feldspar, and greenschist-facies minerals (epidote, chlorite, and biotite). We observed a possible temperature gradient, where quartz demonstrates grain boundary migration (GBM) concentrated at the bottom of the sample and gradually turning to subgrain rotation (SGR) and bulging (BLG) towards the top. In general, older, pre-seismic fabrics are defined by quartz with high-temperature (ca. 500°C) ductile texture (GBM) overprinted by medium (450°C) and lower-temperature (350°C) ductile textures (SGR and BLG). In the brittle regime, at least 3 generations of p-tach veins are distinguished by thickness, orientation, and composition. Vein thickness provides an estimate of seismic displacement. Approximate displacements generated by the main p-tach forming event range from ca. 0.4 to 0.7 m, corresponding to a Moment Magnitude of ca. 2.9. Investigating the magnitude of ancient earthquakes is paramount to understanding modern seismic hazard in regions undergoing high-magnitude extension of the crust today.
