Observations from the EPSO Temporary Christchurch Seismic Monitor.

Between March 14-19 I accompanied the 2nd AEES Earthquake Reconnaissance Mission to Christchurch.  Team member Gary Gibson brought with him a ES&S EchoPro Accelerograph, and I brought along a PSN 16-bit seismic logger with a 4.5Hz triaxial geophone sensor.  Given the expectation of relatively large ground motion, a 100:1 attenuator was additionally installed between my geophone sensor and amplifier, giving the logger-amplifier an effective voltage gain of 10×.

The EPSO Seismic briefcase portable temporary station was installed at a private residence at the Christchurch Port Hills suburb of Westmoreland.  This location is 6km from the epicentre of the Magnitude 6.3 event of February 22nd, 2011, and 34km from the M7.1 event of September 4th, 2010.  Gary set up his accelerograph station at the Central Park Motor Lodge (designated 'CPML'), 6.1km due north of my station, which was designated 'EPSO-B'.  EPSO-B ran continuously between 2011-03-13 18:40 and 2011-03-19  00:50 (UTC), periodically updating a website with 24h summary traces.  Dozens of small tremors were recorded, as displayed in the following table of daily 24h summary traces.  Click on the following thumbnails to view a full sized image.

Day (UTC)	N-S Velocity	E-W Velocity	Z Velocity
2011-03-13
2011-03-14
2011-03-15
2011-03-16
2011-03-17
2011-03-18

Due to the large number of seismic events potentially available for analysis, I decided to consider just those with a peak component velocity (PCV) greater than 10µm/s, which yielded 243 events.  These were recorded over 5.3 days of continuous logging, giving an average event occurrence of once every 39 minutes.  The beginning of the recording period was 38 days from the Magnitude 6.3 event of February 22nd, 2011.

Data from these 243 events recorded are presented in a linked table.  One of the first observations that one can make from this table, is that the direction of the peak velocity for each of the 243 events.  Clearly Eastward-Westward oriented peak velocities exceed those of Northward-Southward.  In fact Upwards-Downwards oriented peaks exceed those of North-South, and with the vertically oriented peak velocities, the strong tendency was to downward orientation.  These differences could simply reflect difference sensitivity of the respective N-S, E-W and Z geophone channels, and until such time as the geophone and logger are precisely calibrated, this possibility cannot be ruled out.  But assuming that the geophone channels are of equal sensitivity, then the ground at Westmoreland has a preferential tendency to shake along an East-West axis, with individual seismic events having PCVs oriented E-W around 80% of the time.  A high proportion of the events show a relatively strong initial downward motion, and in about 1/8 of the individual events, the velocity of this motion exceeds that of horizontal motions.  An example of this effect is shown here.

During our visit the strongest event measured had a PCV of 7.5mm/s, enough to rattle household crockery.  This occurred at 2011-03-18  07:59:06.4, which GNS logged as a M3.8 event at a depth of 6.7km, and with a epicentre of -43.58674, 172.63899.  This position is at horizontal distance of 3.1km from EPSO-B, and implies an true underground distance to the quake of around 7.4km, which is in agreement with the rough estimate of 9.3km produced by the Winquake analysis program and displayed the attached expanded trace (pdf).  This trace also demonstrates just how short and sharp most of the Christchurch aftershocks are, with most of the action being over within a couple of seconds.

There remains much scope for further analysis of this data set, which will have to wait until a rainy day.

2011-04-13