1. 1 Automated Processing of Earthquake Swarm Records at Very Short Epicentral Distances José M. Cepeda Universidad Centroamericana, (UCA) San Salvador, El Salvador

2. Universidad Centroamericana (UCA) - El Salvador Automated Processing of Earthquake Swarm Records… J. Cepeda – Cosmos Workshop2 Outline 1) Introduction 2) Strong Motion Networks in El Salvador 3) Cases of Application 4) Procedure for Swarm Records Purpose General Procedure Calculation of Trace Envelope Illustration of Procedure 5) Routine processing procedure 6) Availability of data 7) Future activities.

3. Universidad Centroamericana (UCA) - El Salvador Automated Processing of Earthquake Swarm Records… J. Cepeda – Cosmos Workshop3 1. Introduction Pacific Coast in Northern Central America Subduction earthquakes (Cocos – Caribbean plate boundary) Volcanic chain earthquakes 21,000 km 2

4. Universidad Centroamericana (UCA) - El Salvador Automated Processing of Earthquake Swarm Records… J. Cepeda – Cosmos Workshop4 1. Introduction Subduction vrs Volcanic Chain Earthquakes Differences in duration Definition of PET in instruments Long pre-event durations in Volcanic Chain Earthquakes

5. Universidad Centroamericana (UCA) - El Salvador Automated Processing of Earthquake Swarm Records… J. Cepeda – Cosmos Workshop5 1. Introduction Swarms in Volcanic Chain Earthquakes Period 1985-1999 10 significant swarms Records of 14,000+ Earthquakes Long pre-event portions Multiple events in single record

6. Universidad Centroamericana (UCA) - El Salvador Automated Processing of Earthquake Swarm Records… J. Cepeda – Cosmos Workshop6 2. Networks in El Salvador Owner institutions: Universidad Centroamericana (UCA) Servicio Nacional de Estudios Territoriales (SNET) Hydroelectrical Power Company (CEL) Geothermal Power Company (LAGEO) 48 digital instruments (1 instrument / 440 km 2 ) SSA-2:25% QDR:21% Etna:35% K-2:19%

7. Universidad Centroamericana (UCA) - El Salvador Automated Processing of Earthquake Swarm Records… J. Cepeda – Cosmos Workshop7 2. Networks in El Salvador West (19%) – Central (57%) – East (24%) North (19%) – Volcanic chain (69%) – South (12%)

8. Universidad Centroamericana (UCA) - El Salvador Automated Processing of Earthquake Swarm Records… J. Cepeda – Cosmos Workshop8 Strong motion recording since 1951. First digital recordings: 1996. Networks fully digital since 2002. Pre-event memory: 10.24 s Post-event running time: 15 s Trigger levels: 0.01 g horizontal, 0.005 g vertical. 2. Networks in El Salvador

9. Universidad Centroamericana (UCA) - El Salvador Automated Processing of Earthquake Swarm Records… J. Cepeda – Cosmos Workshop9 Total of Strong Motion Records: 1323 12% from analogue instruments (Montana and SMA-1). 88% from digital instruments (SSA-2, Etna, K2 and QDR). Important earthquakes recorded: 19/Jun/1982 (M S 7.3 subduction) – 1 record. 10/Oct/1986 (M S 5.4 shallow upper crustal) – 6 records. 13/Jan/2001 (M S 7.8 subduction) – 26 records. 13/Feb/2001 (M S 6.5 shallow upper crustal) – 21 records. Largest PGA recorded: 1.1 g (13/Jan/2001, 61 km rupture distance). 2. Networks in El Salvador

10. Universidad Centroamericana (UCA) - El Salvador Automated Processing of Earthquake Swarm Records… J. Cepeda – Cosmos Workshop10 3. Cases of Application Attenuation equations. Backanalysis of liquefaction phenomena. Observation of rupture directivity. Monitoring of instrumented slopes. Seismic risk scenarios.

11. Universidad Centroamericana (UCA) - El Salvador Automated Processing of Earthquake Swarm Records… J. Cepeda – Cosmos Workshop11 4. Procedure for swarm records Purpose General procedure Calculation of trace envelope Illustration of procedure

12. Universidad Centroamericana (UCA) - El Salvador Automated Processing of Earthquake Swarm Records… J. Cepeda – Cosmos Workshop12 Purpose Routine for the preliminary batch processing of swarm earthquake records. Cut large pre-event portions. Extract traces of multiple events contained in a single record. 4. Procedure for swarm records

13. Universidad Centroamericana (UCA) - El Salvador Automated Processing of Earthquake Swarm Records… J. Cepeda – Cosmos Workshop13 General procedure 1) SSA or EVT file  ASCII file. 2) Select vertical component. 3) Obtain ‘smoothed’ envelope of record and amplitude of envelope. 4) Obtain ‘crests’ in amplitude of envelope. 5) Obtain ‘valleys’ and ‘plains’ in amplitude of envelope. 6) Find values of time for lowest ‘valleys’ and ‘plains’ preceding ‘crests’. 7) Values of time in step (6) are cutting points. 4. Procedure for swarm records

14. Universidad Centroamericana (UCA) - El Salvador Automated Processing of Earthquake Swarm Records… J. Cepeda – Cosmos Workshop14 Calculation of trace envelope 1) Baseline correction (mean of record). 2) Obtain First and second derivative of record. 3) Identify flat portions (1st derivative is zero), local peaks (change of sign in 1st derivative) and identify maximum and minimum (sign of 2nd derivative). 4. Procedure for swarm records

15. Universidad Centroamericana (UCA) - El Salvador Automated Processing of Earthquake Swarm Records… J. Cepeda – Cosmos Workshop15 Calculation of trace envelope 4)Create vectors of maximum and minimum values. 5) ‘Smoothing’ of envelope: divide record in segments of width ‘deltat’ and select peaks of vectors in (4) within each segment. 6) Interpolate envelope to a fixed and uniform time step. 7) Calculate amplitude of envelope. 4. Procedure for swarm records

16. Universidad Centroamericana (UCA) - El Salvador Automated Processing of Earthquake Swarm Records… J. Cepeda – Cosmos Workshop16 Date: 2001/May/8 Time: 18:04 Magnitude: 3.6 M L Depth: 8.2 km Epicentral distance: 9.3 km Station: HSGT (San Vicente, El Salvador) Illustration of procedure

17. Universidad Centroamericana (UCA) - El Salvador Automated Processing of Earthquake Swarm Records… J. Cepeda – Cosmos Workshop17 SWS by Kinemetrics (preliminary batch processing). 1) SSA to DAT. 2) DAT to V1. 3) V1 to V2 (0.12 – 45 Hz). 4) V2 to V3 (-std option for calculation of SA). 5. Routine processing (SSA-2).

18. Universidad Centroamericana (UCA) - El Salvador Automated Processing of Earthquake Swarm Records… J. Cepeda – Cosmos Workshop18 5. Routine processing (SSA-2). Strong Motion Analyst by Kinemetrics (Record by record) 1) Conversion SSA to EVT format. 2) Conversion EVT to V1 unprocessed format. 3) V1 to V2 processing: a) Baseline and instrument correction. b) High pass filtering of velocity and displacement. c) Option: Shakal and Ragsdale (1984) method. d) Corner frequencies: [0.12-0.20] Hz and 45 Hz (criteria: velocity & displacement time histories). 4) V2 to V3 (Fourier & response spectra).

19. Universidad Centroamericana (UCA) - El Salvador Automated Processing of Earthquake Swarm Records… J. Cepeda – Cosmos Workshop19 6. Availability of data. www.uca.edu.sv/investigacion/terremoto/indice.htm (M S 7.8 - 13/Jan/2001, M S 6.5 - 13/Feb/2001, M W 5.1 - 17/Feb/2001). www.uca.edu.sv/investigacion/terremoto/indice.htm db.cosmos-eq.org/scripts/earthquakes.plx#CAM (M S 7.8 - 13/Jan/2001, M S 6.5 - 13/Feb/2001, M W 5.1 - 17/Feb/2001). db.cosmos-eq.org/scripts/earthquakes.plx#CAM www.snet.gob.sv/Geologia/Sismologia/13%20de%20febr ero/13%20de%20febrero.htm (M S 6.5- 13/Feb/2001) www.snet.gob.sv/Geologia/Sismologia/13%20de%20febr ero/13%20de%20febrero.htm nsmp.wr.usgs.gov/data_sets/20010113_1.html (M S 7.8 - 13/Jan/2001) nsmp.wr.usgs.gov/data_sets/20010113_1.html nsmp.wr.usgs.gov/data2/cdrom_sm_1933- 1986/1986/1986.zip (M S 5.4 - 10/Oct/1986) nsmp.wr.usgs.gov/data2/cdrom_sm_1933- 1986/1986/1986.zip

20. Universidad Centroamericana (UCA) - El Salvador Automated Processing of Earthquake Swarm Records… J. Cepeda – Cosmos Workshop20 7. Future activities. More QDRs and Etnas. Modem connection to SSA-2. Installation of instruments in Guatemala (near the Motagua Fault) and Managua, Nicaragua. Availability of the full UCA data set in the Internet.

21. 21 Thank you