Probabilistic Point Source Modelling at the ISC
The International Seismological Centre Probabilistic Point Source Model (ISC-PPSM) catalogue provides earthquake moment tensors, source time functions and depths for moderate magnitude earthquakes (Mw 5.8 – 7.2) from across the Globe. The inversion methodology, developed in collaboration between the ISC and the seismology group at the University of Oxford solves for the earthquake point source, and its uncertainty using a Bayesian direct search approach. The methodology addresses two key areas of uncertainty in the global seismic catalogues;
A third important feature of the ISC-PPSM catalogue is the parameterised source time function (STF). By solving directly for the earthquake STF, we are able to address the trade-offs between STF length and earthquake depth, as well as adding new constraints to the energy release rates of moderate magnitude earthquakes.
Methods
The probabilistic inversion utilises broadband body wave observations (i.e. P and SH waves), with synthetic seismograms calculated using instaseis (van Driel et al 2015), and the WKBJ methodology (Chapman, 1978). The earthquake point source model is described by 16 parameters, 1 designating the earthquake depth, 5 defining the symmetrical moment tensor using the parameterisation proposed by Tape & Tape (2012, 2015), and 10 discretising the STF as the sum of 4 Gaussian ‘spikes’.
This 16-dimension parameter space is then explored using the neighbourhood algorithm adaptive grid search of Sambridge (1999), with the resulting likelihood map being explored using MCMC resampling (e.g. Goodman & Weare, 20210), facilitated by the emcee toolbox (Foreman-Mackay et al., 2013). An example of the resulting probabilistic earthquake point source model is shown below.
The uncertainty in the moment tensor is expressed by the Bayesian beachball on the top left, with the preferred solution shown to the right of it in red. Below this, the probabilistic STF is shown in the black colours, with the preferred solution again shown in red. The bottom of the left-hand panels then shows the probability density function (PDF) of the depth, with the peak of the PDF representing the preferred reported depth. The right-hand panel show the waveform fits for this event. The observed waveforms are shown in black, with the synthetics shown in red. Where the channel is BHZ, BHH or HHH, the synthetics are calculated by Instaseis, and the observed waveforms are filtered to the frequency band of the synthetics. Where the channel is HHZ, the synthetics are calculated using the WKBJ methodology and the observed waveforms are unfiltered.
Uses of ISC-PPSM
ISC-PPSM adds depth resolution to shallow earthquakes, where the depth phases are subsumed into the STF. In remote settings, with no nearby seismic stations, this adds depth resolution to earthquakes that would otherwise be set to a default depth. In these cases, the ISC Bulletin depth is fixed to the integer of the most likely depth determined in by ISC-PPSM.
Data Access and Citation
The methodology used to calculate the point source models is described in full by Garth et al (in prep). Work is ongoing to calculate a full Global catalogue of PPSM solutions. So far, preliminary solutions are available through the ISC bulletin searches for earthquake that occurred between 2019 and 2022. Solutions from 2022 onwards, are added 2 years behind real time, and the catalogue will be extended back in time in the near future.
Please cite:
ISC-PPSM: Assessing moment tensor resolution and addressing shallow earthquake depth resolution. T. Garth1,
K. Sigloch2, D. Storchak1, IUGG 2023, IUGG23-2916.
For more information, please contact tom.garth
isc.ac.uk
