Towards a Local-Regional Coda Source Calibration for Puerto Rico: Apparent Stress Estimation and Magnitude Scaling from the 2019–2020 Earthquake Sequence and Application of LLNL's Coda Calibration Tool (CCT)

Session: Early Results from the 2020 M6.4 Indios, Puerto Rico Earthquake Sequence [Poster]
Type: Poster
Date: 4/30/2020
Time: 08:00 AM
Room: Ballroom
Description: 

Following the Mw 5.0 earthquake of December 29th, 2019 located offshore and south-west of Guánica, Puerto Rico (PR), we began collecting waveforms for an initial moment-tensor estimate. Uncharacteristically, the initial events were superseded by an Mw 5.7 on Jan 6th and an Mw 6.4 earthquake on Jan 7th, which included 78 M≥4 within 3 weeks of the onset of this sequence. We use this rich dataset for developing local-regional S-coda source calibration for PR, with the potential of integration into the PR seismic network routine processing workflow.

Our preliminary results show that a 1D velocity model based on LITHO1.0 (Pasyanos et al., 2014) in conjunction with upper crustal structure from Huerfano and Bataille (1994), provide very good fits to the data (15-50s period) for crustal events in the main island. The majority of the moment-tensor solutions for moderate-sized aftershocks (Mw<5) support fairly shallow (~4-to-12 km) oblique east-west trending left-lateral strike-slip mechanisms, while the Mw 5 and larger events seem to be predominantly shallow, normal faulting events. Next, we focused on the apparent stress and scaling using the coda spectral ratio method outlined in Mayeda et al. (2007) which provides stable, averaged apparent stress estimates that are free of path, site and source mechanism effects. This sequence is characterized by low apparent stress (~0.5 to 10 bars), falling on the lower range of other large magnitude crustal sequences (Malagnini et al. 2014), but also increases with magnitude for this sequence. Finally, we used CCT, a Java-based open source tool that provides fast and efficient coda envelope calibration, as well as routine network processing for stable estimates of Mw and source spectra. We used several well-constrained Mw’s and source spectra from this study to calibrate coda-derived source spectra for the entirety of Puerto Rico and then validated the results against independent scalar moments (Herrmann, 2020).

Presenting Author: Jorge I. Roman-Nieves

Authors