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Towards a Local-Regional Coda Source Calibration for Puerto Rico: Modeling Moment Tensors, Apparent Stress and Scaled Energy From the 2019-2020 Earthquake Sequence

Session: The 7 January 2020 South of Indios (M6.4) Earthquake in Puerto Rico, Response and Lessons [Poster]

Type: Poster

Date: 4/22/2021

Presentation Time: 11:30 AM Pacific

Description: 

Following an increase in offshore seismicity in southwest Puerto Rico (PR) in late December 2019, which included two Mw 5.0 & 4.7 events on December 29, we began collecting local and near-regional broadband waveforms for the determination of moment tensors and apparent stress. Rather than following conventional mainshock-aftershock behavior, the initial events were followed by an Mw 5.7 (Jan. 6), and the Mw 6.4 January 7, 2020 ‘Indios’ earthquake. The high-level activity included hundreds of offshore and onshore events, including 78 Mw≥ 4.0 within 3 weeks from the sequence onset. We use this rich dataset to obtain stable estimates of Mw, apparent stress, and scaled energy, and for the development of a preliminary local-regional coda source calibration for PR.

Our preliminary results show that a 1D velocity model based on LITHO 1.0 in conjunction with upper crustal structure from Huerfano and Bataille (1994), provide excellent fits to the data in the 18-50 s period range for tens of events located within 25 km from the Mw 6.4 event. Most moment tensor solutions support fairly shallow oblique, east-west trending, left-lateral strike-slip mechanisms (consistent with the regional tectonics) while Mw ≥ 5 events appear as shallow, normal faulting earthquakes. For apparent stress calculation, we use the coda spectral ratio method outlined in Mayeda et al. (2007) which provides very 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 8 bars) but fall within the range of other large magnitude strike-slip crustal sequences worldwide (Malagnini et al., 2014). Using several well-constrained source spectra calculated from this sequence and other events around PR, we obtain a preliminary local-regional coda source calibration using the open source Coda Calibration Tool that can be used for routine magnitude processing by the Puerto Rico Seismic Network

Presenting Author: Jorge I. Roman-Nieves

Student Presenter: No

Authors