HAWKES BAY QUAKES

1921 – June 28  mag  7.0 M  (Guttenberg & Richter 1949)
                                         6.4.M  (Dowick & Smith 1990)
Epicentre 39.3 S  176.4E  (D Bullen 1937)
Depth:  80km

1931 – February 02 – Mag 7.8
Time 22.47UT
Epicentre 39.3S  177 E
Depth 30km
Occurring at 10.47am NZDS time on Tuesday Feb 3rd 1931 the Napier quake is recorded as having caused the largest lo of life and he most extensive damage ever recorded in New Zealand.  The effects of the earthquake were greatest in the towns of Napier and Hastings but other towns in the Hawke’s Bay area also suffered major damage .  The official death toll  was 256. 161 occurring in Napier, 93 in Hastings and 2 in Wairoa.

1931 – February 13 - aftershock
Time 01.27 UT
Magnitude 7.3
Epicentre 39.5S 177.5 E
Depth 30km

Information on the effect of this earthquake and other after shocks of the Napier earthquake is poor and would benefit from further studies.

1932- May 05
Time 08 23UT
Magnitude 5.9
Epicentre 39.5 S  177.5 E

This earthquake occurred eighteen months after the destructive Napier earthquake.  It as one of a number of larger magnitude events which followed the Napier earthquake and had locations in the Hawkes Bay area.  This event has been located 50 km to the northeast of the epicentre of the Napier earthquake.

As in the case with many of these early moderate magnitude earthquakes, there has been no recent reinterpretation of the source data.
Full summaries held at the Seismological Observatory have been converted from Rossi-Forel scale to Modified Mercalli and the isoseismal map has been drawn by Eiby.


WAIROA
1932 – September 15
Time 13.55 UT
Magnitude 6.9 (Dowick and Smith 1990)
Epicentre 38.9 S  177.6 E  (Bullen 1938)
Depth 30km

At 1.25am on Friday 16th September 1932 the earth was shaken so severely at Wairoa and Gisborne as to throw down chimneys, to crack weak buildings, to cause alluvial ground and fillings to open up and settle down , to fissure the surface soil on narrow ridges, and to detach
Steep country in slips.

….the biggest slip in the district occurred at McCardle’s three miles (5km) south west of Wairoa, where the sea-cliff 30 chains (600m) long slidseaward and the ground came down for a width of 1.5chains (300m)  leaving behind it a scar standing 500 ft. (150m) above the sea level (Ongley 1937).

Preliminary dislocation modelling (Hayes & Darby 1987) has indicated that: “The 1932 Wairoa Earthquake was possibly on a north eastward continuation of this rupture (ie that of the 1931 Napier earthquake) from 50km to 80 km long from a 5km to possibly 35km deep dipping about 68 deg norwest, with  1 m of either reverse or normal dip slip, and 1 m right lateral strike-slip”


There has been no recent revision of the seismological data. But a study of the original felt reports is in progress (Dowick pers. Comm.. ) . Preliminary results indicate an intensity MM8 in Wairoa.

Jenny who would know why one is marked as an aftershock which going by the readings there were many in the area as you would expect after the 1931 one and others were not.
In the book I have it is not marked as an aftershock for Wairoa but there is plenty of other information on the quakes that I have listed above. Not much help to you really but at east one thing has come out of it that on revision the 6.9 mag should maybe have been a mag 8 in Wairoa going by the felt reports.  It also mentions the bridge and destruction etc.  Will try and get the book away to you this week.

Info taken from the book
Atlas of Isoseismal Maps of New Zealand Earthquakes.
G&N Sciences 1995.