R. Bürgmann | USGS Science

USGS2021

Identification of low-frequency earthquakes on the San Andreas fault with deep learning

Low-frequency earthquakes are a seismic manifestation of slow fault slip. Their emergent onsets, low amplitudes, and unique frequency characteristics make these events difficult to detect in continuous seismic data. Here, we train a convolutional neural network to detect low-frequency earthquakes near Parkfield, CA using the catalog of Shelly (2017), https://doi.org/10.1002/2017jb014047 as training data. We explore how varying model size and targets influence the performance of the resulting network. Our preferred network has a peak accuracy of 85% and can reliably pick low-frequency earthquake (LFE) S-wave arrival times on single station records. We demonstrate the abilities of the network using data from permanent and temporary stations near Parkfield, and show that it detects new LFEs that are not part of the Shelly (2017), https://doi.org/10.1002/2017jb014047 catalog. Overall, machine-learning approaches show great promise for identifying additional low-frequency earthquake sources. The technique is fast, generalizable, and does not require sources to repeat.

Geophysical Research Letters↗

USGS2012

Aseismic slip on the San Andreas Fault south of Loma Prieta

Two digital creepmeters installed within the San Andreas fault zone after the 18 Oct 1989 Loma Prieta main shock show less than 1 cm of post seismic right-lateral slip in the four months following the earthquake. At Mt. Madonna road a 23 mm coseismic fracture slipped a further 3 mm after heavy rain, and at Nyland Ranch near San Juan Bautista the fault slipped approximately 9 mm starting 42 days after the main shock. If the current trend at Nyland Ranch persists, more than 2 cm of post seismic slip will develop by 1991. At both sites minor left-lateral displacements occurred which are attributed to near-surface soil effects. The abutments of the railroad bridge across the Pajaro River at Chittenden, which were extended by the 1906 earthquake, were not extended during the Loma Prieta event although they have evidently moved apart by more than 7 cm since bridge reconstruction in 1940. This corresponds to 10 cm of right-lateral slip which could be related to M>5 events in mid-century or could be due to aseismic slip at a mean rate of 2.1 mm/a. The absence of significant surface slip within the fault zone in the decades before and the months following the Loma Prieta event suggests either that near-surface deformation is distributed over a wide zone or that a slip deficit remains. Several authors have proposed this region as a future location for M≈5 events.

California↗