'T-Phase' seismic events recorded at EPSO, Coonabarabran, NSW.

This page records T-phase observations logged at EPSO.  Overseas, Australian, Local, Mining related and few curious seismic events are logged on separate pages.

Date
Time (UTC)
Details
Summary
Seismogram
PSN data
file
2018-11-24
23:42:39
A ML=5.8 event SW of the South Island of New Zealand generated a T-phase which arrived 24m39s following the origin time of the event, implying a mean propagation speed of 1.56km/s.  The frequency spectrum of the three axes of motion peaked at 1.6-1.8Hz.
S6000 triaxial
BB-13 (Z)
psn
psn
2018-10-06
09:37:15
A ML=5.2 event in the ocean nearby Milford Sound generated a T-phase which arrived 22m16s following the origin time of the event, implying a mean propagation speed of 1.64km/s.  The frequency spectrum of the three axes of motion peaked at 1.6-2.7Hz. S6000 triaxial
EPSO Array
psn
psn
2017-09-20
01:43:24
A ML=6.1 event in the ocean nearby the Auckland Islands generated a T-phase which arrived 25m15s following the origin time of the event, implying a mean propagation speed of 1.61km/s.  The frequency spectrum of the three axes of motion peaked at 1.6-1.7Hz. S6000 triaxial

psn

2017-07-11
07:00:08 A ML=6.5 event in the ocean nearby the Auckland Islands generated a T-phase which arrived 23m49s following the origin time of the event, implying a mean propagation speed of 1.64km/s.  EPSO's recorded peak horizontal ground velocity was around 1.4/s, which is more than twice as large as any T-phase motion recorded thus far in this log.  The frequency spectrum of the horizontal motion peaked at 1.6-2.0Hz, whereas the frequency spectrum of the vertical component appeared to peak at a slightly higher frequency of around 2.3Hz.
VM hor velocity
S6000 triaxial
EPSO Array
psn
psn
psn
2016-11-13





2016-11-14


2016-11-15
2016-11-18
11:32:09
11:52:47
13:21:13
13:31:30
18:59:07
22:19:32
00:34:23
06:47:54
07:21:05
17:30:33
14:22:57
Following the M7.9 quake below, there were numerous large aftershocks located in the north-eastern region of the South Island of New Zealand, and many of these quakes transmitted hydroacoustic T-phase waves into the Tasman Sea.  T-phase producing quakes varied in size between M5.2 to M6.2, and what this earthquake sequence has demonstrated is that any land based earthquake occurring in New Zealand's South Island with a magnitude of ≈5.2 or greater, is likely to generate a T-phase into the Tasman Sea, which then arrives in south eastern Australia around 20-25 minutes later.  The psn files shown to the right are all of 30 minutes duration and have been band-pass filtered between 1-4Hz.


S6000 triaxial
S6000 triaxial
S6000 triaxial
S6000 triaxial
S6000 triaxial
S6000 triaxial
S6000 triaxial
S6000 triaxial
S6000 triaxial
S6000 triaxial
S6000 triaxial
psn
psn
psn
psn
psn
psn
psn
psn
psn
psn
psn
2016-11-13
11:03:01 A Mwp=7.9 earthquake located on the east coast of the South Island of New Zealand generated a T-phase which arrived 24m56s following the origin time of the quake, giving mean propagation speed of 1.63km/s.  Unusually, this event occurred on land on the eastern side of New Zealand (≈140km from the nearest western coast) yet still generated a large ocean T-phase signal which crossed the Tasman Sea.  The peak spectral amplitude was around 2.0Hz with a FWHM of ≈2Hz, which is fairly typical for T-phase events originating on the ocean floor near New Zealand.  The peak T-phase amplitude was around 0.6m/s (as recorded by EPSO Array's SS-1 Ranger geophones) which is more that twice the amplitude of any ocean-floor-generated T-phase yet recorded at EPSO.
VM hor acceln
VM hor velocity
EPSO Array
psn
psn
psn
2016-07-25
19:42:24
A ML=5.0 event in the ocean off the West Coast of New Zealand generated a T-phase which arrived 22m43s following the origin time of the event, giving mean propagation speed of 1.61km/s.  The arrival of this T-Phase was masked by an earlier M6.5 event located in the Bismark Sea, but the signal was clear after suitable filtering.  The arrival of the P-Phase at EPSO was buried in the noise.
S6000 triaxial
Willmore (Z)
EPSO Array
psn
psn
psn
2016-07-16
14:42:40
A ML=4.6 event in the ocean NW of the Snares Island generated a T-phase which arrived 22m26s following the origin time of the event.  A initial P-phase arrived at EPSO 4m33s following the event.  The mean propagation speed of this T-phase was 1.67km/s, which is typical for events originating in this region.
S6000 triaxial
Willmore (Z)
EPSO Array
psn
psn
psn
2016-06-07
02:55:21
A Mb=5.5 event offshore New Zealand's Fiordland generated a T-phase which arrived 22m11s following* the origin time of the event (*EPSO peak amplitude time for the T-phase was recorded at 03:17:32).   Thus the mean propagation speed of this T-phase was 1.63km/s.  The peak of the T-phase amplitude was recorded at DNSO*, Sydney with a provisional arrival time of 03:16:3110s (*range 331.6km @ 148.9 from EPSO).  Since EPSO's bearing to DNSO (148.9) is similar to that of the event (139.9) then the difference in arrival time of the T-phase waves between the two stations gives an indication of the propagation speed of the T-phase signal over land.  In this case ΔEPSO-DNSO is ≈61s implying a speed over land of ≈5.4km/s.
Assuming the transition between hydro-acoustical T-phase waves to seismic waves occurs nearby the coast, and that ocean T-phase waves propagate along straight lines,  then the DNSO T-phase arrival time implies a wave propagation speed in ocean of 1852km/1270s = 1.46km/s.  1.5km/s is approximately the speed of sound in seawater.
S6000 triaxial
Willmore (Z)
EPSO Array
psn
psn
psn
2016‑01‑14
12:15:29
A M5.6 event in the Macquarie Island region generated a T-phase which arrived 22m37s following* the first P-phase arrival at EPSO (*peak amplitude time for the T-phase, at 12:41:15).  This event occurred around 100km southward of a very similar event that occurred on 2015-07-01, making this one the most southerly T-phase yet recorded at EPSO.  The mean propagation speed of this T-phase was 1.66km/s.
S6000_triaxial
EPSO Array
psn
psn
2015-10-28
13:33:41
A M4.6 event off West Coast of South Island, New Zealand, generated a T-phase with no detectable regular phases recorded at EPSO.  Implied mean propagation speed = 1.6km/s.
S6000 triaxial
EPSO Array
psn
psn
2015-07-02
20:25:09
A M5.1 event off West Coast of South Island, New Zealand, generated a T-phase with no detectable regular phases recorded at EPSO.  Implied mean propagation speed = 1.6km/s. S6000 triaxial
EPSO Array
psn
psn
2015-07-01
14:30:19
The most southerly event to generate a T-phase yet recorded at EPSO.  An M6.0 event in the Macquarie Island region generated a T-phase which arrived 20m46s following* the first P-phase arrival (*peak amplitude time for the T-phase, at 14:56:07).
S6000 triaxial
psn
2015-05-06
16:23:54
T-phase from a M5.0 earthquake in New Zealand, with the Geonet epicentre location in the ocean around 50 km NW of Milford Sound.  The P-phase arrival at EPSO was 16:28:23 with first T-phase first arrival at about 16:44:45, reaching peak amplitude at 16:46:27.  First arrival of the T-phase was around 20m51s following the quake, implying an average propagation velocity from the epicentre of around 1.73km/s.
S6000 triaxial
EPSO Array
psn
psn
2015-05-04
02:29:10
T-phase from a M5.8 earthquake in New Zealand, with the Geonet epicentre location around 30 km north-west of Wanaka.  The P-phase arrival at EPSO was 02:33:51 with first T-phase first arrival at about 02:50:30, which was 21m20s following the quake, implying an average propagation velocity from the epicentre of around 1.76km/s.  Of particular note with this event, is that the epicentre was located inland around 70km from the nearest coastline, at a supposed depth of 4km.
VM hor velocity
S6000 triaxial
EPSO Array
psn
psn
psn
2015-02-25 12:29:55 Another seismic T-phase from a M4.8 earthquake in New Zealand, with the Geonet epicentre location around 75km west of Te Anau.  In this case the P-phase arrival at EPSO was too weak to be detected above the noise.  The T-phase first arrival at EPSO was about 12:51:10, which was 21m15s following the quake, implying an average propagation velocity from the epicentre of around 1.7km/s.
S6000 triaxial
EPSO Array
psn
psn
2014-01-13 11:40:01
Another seismic T-phase from a M5.5 earthquake in southwest New Zealand, with the Geonet's epicentre location placed around 25km inland from the open ocean.   P-arrival at EPSO commenced at 11:44:28 with T-arrival following around 16.8 minutes later at around 12:01:18.  As measured by the EPSO Array SS-1 Ranger sensors (vertical) the peak frequency amplitude of the P-waves was ≈1.0Hz and for the T-waves ≈1.6Hz.  As is the usual case for T-waves waves arriving from the south-west New Zealand region, the T-waves arrived with greater amplitude than the P-waves, by a factor of around 1.6 in this case.  Analysis of the arrival directions of the T- & P-waves by EPSO Array again indicate (within experimental error) identical arrival directions. EPSO Array psn
2014-01-05
17:48:41 Yet another seismic T-phase associated with an earthquake originating from the southern New Zealand region, but this time from an event located inland about 40km from the nearest coast.  As with earlier events listed below, analysis of EPSO Array data indicate that the P-phase and T-phase waves arrived at EPSO from almost identical directions.  P-arrival at EPSO commenced at 17:53:29 with T-arrival following around 17.6 minutes later at around 18:11:10.  As measured by the SS-1 Ranger sensors (vertical) the peak frequency amplitude of the P-waves was 1.0Hz and for the T-waves, 2.4Hz. VM hor acceln
VM hor velocity
S6000 triaxial
EPSO Array
psn
psn
psn
psn
2014-12-12
08:02:07
Yet another seismic T-phase has arrived from our local T-phase 'nursery' located SW of the South Island of New Zealand.  USGS reported an event had occurred at 08:02:07, located at -46.963, 165.832 and although no usual seismic phases were detected at EPSO, around 22 minutes after the event (≈08:24:05) an apparent T-wave arrived.  A quick check of the relevant EPSO Array data showed the arrival direction of these waves was almost identical to the event recorded on 2014-12-01 described immediately below.
S6000 triaxial
EPSO Array
psn
psn
2014-12-01 13:04:27 A seismic T-phase from an undersea earthquake located in the Auckland Islands Region of New Zealand.  This T-phase event was very similar to another which occurred in the same area on 2013-12-16 (see below), but this latest event was recorded by EPSO Array and apparent arrival angles of various earthquake waves may now be estimated.  There has been some speculation that the T-waves may preferentially originate from particularly steep sections of the Australian continental shelf, where the ocean sound waves are more efficiently converted to a seismic P-wave in the crust.  Thus T-waves may arrive in somewhat different directions from the usual P and S phases.  However, for this particular event the P-phase and T-phase had identical apparent arrival directions, suggesting that the conversion from ocean-acoustic to seismic-P occurred at the coast somewhere along the great circle track between EPSO and the earthquake.  Inspection of an undersea map of this coastal area shows that the continental shelf is relatively steep and oriented approximately perpendicular to the earthquake's great circle track, so it is perhaps not so surprising that the P and T waves recorded by EPSO Array had very similar arrival directions.
VM hor acceln
VM hor velocity
S6000 triaxial
EPSO Array
psn
psn
psn
psn
2013-12-16
12:07:27
A Mb=5.3 earthquake located off the West Coast of the South Island of New Zealand generated a conspicuous T-wave, with the T-wave arriving at EPSO around 16.3 minutes following the initial P-Wave (a very similar event occurred at 2012-03-14 19:07:44UTC - see below for details).  For the first time such an event was also captured by the EPSO Array and served as a good system test.  The initial deep Earth P-waves from this event passed the EPSO Array with an apparent horizontal velocity of ≈13km/s, implying that the true wave fronts impacted the array from the deep Earth with an inclination angle of ≈60 with respect to the EPSO Array ground surface.  The later T-wave originated somewhere near the Australian continental shelf, at a distance of ≈400km.  The T-wave passed over the EPSO Array with an apparent horizontal velocity of ≈8km/s, implying that the true wave front had an inclination angle of around 40, as one might expect from a much closer event.
VM hor acceln
S6000 triaxial
SS-1 vertical
EPSO Array
psn
psn
psn
psn
2012-03-14
19:07:44
A relatively small quake off the Off W Coast of South Island, New Zealand, appears to have generated a conspicuous T-wave, where acoustical water waves have been generated on the sea floor by the earthquake, and propagated until they encountered Australia's continental shelf, where some of the acoustical energy was converted back to seismic waves.  The Great Circle distance from EPSO to this earthquake's epicentre is ≈2174km.  The distance from EPSO to the edge of the Australian continental shelf (on the direct EPSO-earthquake track) is about 380km.  From this location, a seismic P-wave takes around 54 seconds to travel to EPSO.  By analysing the seismic traces (see right), the delay between the earthquake's origin time (19:07:44) and shaking commencing at EPSO (19:28:44) resulting from the T-wave arrival, is around 1260 seconds.  Assuming this shaking was due to T-waves traveling via the direct Great Circle track and impacting the Australian continental shelf offshore from Sydney, these waves were propagating in the ocean for 1260-54=1206 seconds and covered 2174-380=1794km.  In this case the derived speed of the T-waves is 1206/926=1.3km/s in the ocean.  But if the T-waves impacted the Australian continental shelf at the closest point to the epicentre (i.e. eastern Tasmania, 1480km distant from the epicentre, and 1280km from EPSO), from this location a seismic P-wave would take around 167 seconds to travel to EPSO.  So via this route, the T-waves would have propagated for 1480km for 1206-167=813 seconds, suggesting a T-wave propagation velocity of 1.8km/s.  Therefore the ocean T-wave propagation speed is in the range 1.3-1.8km/s.  The average of these two limits is around 1.5km/s which is approximately the speed of sound in seawater, and is consistent with the propagation of the T-waves waves via the so called SOFAR channel.  Most of the spectral energy in the T-wave induced shaking observed at EPSO, is contained within the 1.5-2.6Hz spectral band.
S6000
SS-1 vertical
psn
psn

Sensors used for these measurements include the Volksmeter (VM), Kinemetrics SS-1 Ranger (SS-1), Marks Products L-15B (L15B) and Sprengnether S-6000 (S6000)