1 Automated Processing of Earthquake Swarm Records at Very Short Epicentral Distances José M....

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Automated Processing of Earthquake Swarm Records at Very Short Epicentral Distances

José M. Cepeda

Universidad Centroamericana, (UCA)

San Salvador, El Salvador

Automated Processing of Earthquake Swarm Records… J. Cepeda – Cosmos Workshop 2

Universidad Centroamericana (UCA) - El Salvador

Outline1) Introduction2) Strong Motion Networks in El Salvador3) Cases of Application4) Procedure for Swarm Records

Purpose General Procedure Calculation of Trace Envelope Illustration of Procedure

5) Routine processing procedure6) Availability of data7) Future activities.



1. Introduction

Pacific Coast in Northern Central America

Subduction earthquakes (Cocos – Caribbean plate boundary)

Volcanic chain earthquakes

21,000 km2



1. Introduction

Subduction vrs Volcanic Chain Earthquakes

Differences in duration

Definition of PET in instruments

Long pre-event durations in Volcanic Chain Earthquakes



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 record0 10 20 30 40 50 60 70

-25

-20

-15

-10

-5

0

5

10

15

20

Time (s)

Acc

eler

atio

n (c

m/s

2)



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 km2)SSA-2: 25%QDR: 21%Etna: 35%K-2: 19%




West (19%) – Central (57%) – East (24%)

North (19%) – Volcanic chain (69%) – South (12%)



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.




Total of Strong Motion Records: 132312% from analogue instruments (Montana and SMA-1).

88% from digital instruments (SSA-2, Etna, K2 and QDR).

Important earthquakes recorded:19/Jun/1982 (MS 7.3 subduction) – 1 record.

10/Oct/1986 (MS 5.4 shallow upper crustal) – 6 records.

13/Jan/2001 (MS 7.8 subduction) – 26 records.

13/Feb/2001 (MS 6.5 shallow upper crustal) – 21 records.

Largest PGA recorded: 1.1 g (13/Jan/2001, 61 km rupture distance).




3. Cases of Application

Attenuation equations.

Backanalysis of liquefaction phenomena.

Observation of rupture directivity.

Monitoring of instrumented slopes.

Seismic risk scenarios.



4. Procedure for swarm records

Purpose

General procedure

Calculation of trace envelope

Illustration of procedure



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.




General procedure1) 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.




Calculation of trace envelope1) 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).




Calculation of trace envelope4) 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.




0 10 20 30 40 50 60 70-25

-20

-15

-10

-5

0

5

10

15

20

Time (s)

Acc

eler

atio

n (c

m/s

2)

RecordEnvelope

0 10 20 30 40 50 60 70-20

-10

0

10

20

30

40

Time (s)

Amplitude of envelope (cm/s2)

Slope (cm/s3)

Date: 2001/May/8Time: 18:04Magnitude: 3.6 ML

Depth: 8.2 kmEpicentral distance: 9.3 kmStation:HSGT (San Vicente, El Salvador)

Illustration ofprocedure



• 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).



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).



6. Availability of data.• www.uca.edu.sv/investigacion/terremoto/indice.htm (MS 7.8 -

13/Jan/2001, MS 6.5 - 13/Feb/2001, MW 5.1 - 17/Feb/2001).• db.cosmos-eq.org/scripts/earthquakes.plx#CAM (MS 7.8 -

13/Jan/2001, MS 6.5 - 13/Feb/2001, MW 5.1 - 17/Feb/2001).• www.snet.gob.sv/Geologia/Sismologia/13%20de%20febrero/13%

20de%20febrero.htm (MS 6.5- 13/Feb/2001)

• nsmp.wr.usgs.gov/data_sets/20010113_1.html (MS 7.8 - 13/Jan/2001)

• nsmp.wr.usgs.gov/data2/cdrom_sm_1933-1986/1986/1986.zip (MS 5.4 - 10/Oct/1986)



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.

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Thank you

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