<table><caption>**Table 1** Key to events noted in Fig. 4</caption><thead><tr><td>Number</td><td>Date</td><td>Event</td></tr></thead><tbody><tr><td>1</td><td>4 Oct. 1978</td><td>Wheeler Crest M 5.8 earthquake</td></tr><tr><td>2</td><td>25–27 May 1980</td><td>Three M 6 earthquakes on 25th. USGS releases Hazard Watch for possible additional M ~6 earthquakes on the 27th; a fourth M 6 earthquake four hours later (a “successful” forecast)</td></tr><tr><td>3</td><td>1980–1981</td><td>Savage and Clark (1982) re-level Hwy 395 to discover a 25-cm uplift of the resurgent dome. Ongoing swarm activity in the caldera</td></tr><tr><td>4</td><td>25–26 May 1982</td><td>L.A. Times article announcing planned release of USGS Notice of Potential Volcanic Hazards on 25th. Official release of USGS Notice of Potential Volcanic Hazard on the 26th</td></tr><tr><td>5</td><td>December 1982</td><td>D. Hill appointed USGS Scientist-in Charge (SIC) of the Long Valley project (LVO). CDMG established comparable role</td></tr><tr><td>6</td><td>7–14 Jan. 1983</td><td>Strong south moat swarm including two M 5 earthquakes</td></tr><tr><td>7</td><td>31 Aug. 1983</td><td>Hill and Filson attend “1000-year lunch” with business leaders</td></tr><tr><td>8</td><td>Oct 1983</td><td>Dedication of the “Mammoth Scenic Loop”, a 2nd road into town</td></tr><tr><td>9</td><td>Sept. 1983</td><td>USGS Director replaces Notice-Watch-Warning alert system by a single-level Hazard-Warning system</td></tr><tr><td>10</td><td>Aug. 1984<br>Oct. 1984</td><td>The Town of Mammoth Lakes incorporates S. McNutt joins CDMG as volcano seismologist</td></tr><tr><td>11</td><td>23 Nov. 1984</td><td>A M 6.1 earthquake in Round Valley</td></tr><tr><td>12</td><td>21 July 1986</td><td>A M 5.9 foreshock followed 12 h later by the M 6.4 Chalfant Valley earthquake</td></tr><tr><td>13</td><td>Aug. 1987</td><td>Hill and Bailey LVC field trip for Dr. Graham and Andrea Lawrence</td></tr><tr><td>14</td><td>May 1989 to<br>March 1990</td><td>11-month long earthquake swarm beneath Mammoth Mtn. Onset of deep LP (volcanic) earthquakes and magmatic CO<sub>2</sub> emissions from flanks of Mammoth Mtn</td></tr><tr><td>15</td><td>17–19 May 1990</td><td>Town and County officials attend USGS workshop on the 10th anniversary of the May 1990 Mount St. Helens eruption</td></tr><tr><td>16</td><td>1991</td><td>LVO adopts alphabetic A-E, alert-level system</td></tr><tr><td>17</td><td>1991–1997</td><td>Recurring earthquake swarms and continued resurgent dome inflation</td></tr><tr><td>18</td><td>June 1997</td><td>LVO replaces alphabetic alert-level system with a 5-level color code</td></tr><tr><td>19</td><td>Nov. 1997–Jan. 1998</td><td>Earthquake swarm activity including M 4.8 and 4.9 earthquakes accompanied by elevated resurgent dome inflation</td></tr><tr><td>20</td><td>28 June–4 July 1998</td><td>Mammoth Lakes City Manager accompanies Hill to 1st IAVCEI Cities on Volcanoes meeting in Rome and Naples, Italy.</td></tr><tr><td>21</td><td>1998–1999</td><td>M 5.1 earthquakes on 8/6/98 and 14/7/98 near the south-east margin of the caldera and a M 5.6 earthquake on 15/5/99 in Sierra Nevada block 5 km S. of caldera</td></tr><tr><td>22</td><td>Oct. 2006</td><td>USGS adopts a uniform ground/aircraft hazard warning system for all five volcano observatories</td></tr><tr><td>23</td><td>March 2008</td><td>Mammoth Mountain Ski Area opens the “Top of the Mountain” display illustrating the geologic and volcanic history of the area</td></tr><tr><td>24</td><td>Jan. 2009</td><td>M. Mangan becomes LVO SIC</td></tr><tr><td>25</td><td>2011–2014</td><td>Inflation of the resurgent dome resumes and continues at a rate of 2 cm/year through 2014 accompanied by low-level swarm activity under Mammoth Mountain, the caldera, and the Sierra Nevada block to the south</td></tr><tr><td>26</td><td>Feb. 2012</td><td>The California Volcano Observatory (CalVO) is established with responsibility for all California volcanoes including Long Valley</td></tr><tr><td>27</td><td>July 2014</td><td>Joint USGS-CGS Earthquake Hazard Scenarios report released (Chen et al. 2014)</td></tr></tbody></table>
<figure><img src="image_68.png" /><figcaption>**Fig. 4** Time history of resurgent dome uplift (solid line) and the cumulative number of M ≥ 3 earthquakes in Long Valley Caldera (dot-dashed line) and the Sierra Nevada block (dashed line) from 1978 through May 2016. Solid line represents uplift of the center of the resurgent dome with numbers tied to numbered events in Table 1. Heavy vertical bars mark occurrences of M ≥ 6 earthquakes (including their M ≤ 5 aftershocks) with</figcaption></figure>

length proportional to magnitude. Horizontal brackets
indicate periods of earthquake swarm activity. Dashed
arrows point to associated rate changes in cumulative
number of earthquakes The grey bar is the M = 6.4
Chalfant Valley earthquake and its aftershocks (see
Fig. 1), which do not contribute to the cumulative number
count for Sierra Nevada earthquakes. Bold italic numbers
are linked to numbered events in Table 1

(ICS) and Unified Command (UCS) systems as
well as emergency response plans including
“Plan Caldera” of the California OES and the
“Basic Emergency Plan” of Mono County
(Mader et al. 1987).

As low-level earthquake swarm activity con-
tinued through the summer and fall of 1982,
however, a series of public meetings did little to
mitigate the simmering anger of much of the
public and business community. Then, on the
afternoon January 7, 1983, activity abruptly
resumed with an intense earthquake swarm in the
south moat of the caldera that included two
M = 5.3 earthquakes accompanied by nearly
constant felt shaking from frequent M3 to 4
earthquakes over the next several weeks (Fig. 4).
This was an El Nino winter and the snow was
piled high along the roads within Mammoth
Lakes and along the only paved road connecting
the town to Highway 395. At his own initiative,

the Chairman of the Mono County Board of
Supervisors, ordered second (dirt) road plowed.
He also initiated steps to have this road widened
and paved to provide an alternate way out of
town. This was not a popular decision, in part,
because it carried an implicit acknowledgment
that there might actually be a volcanic hazard.
Setting some sort of record from inception to
completion, the newly paved “escape route” was
formally dedicated in October of 1983 as the
“Mammoth Scenic Loop”. The Chairman and a
second member of the board who had been
pro-active in support of the new road and other
mitigation issues (including monthly public
updates by the USGS on the evolving caldera
unrest) were voted out of office in a special recall
election over the following summer.

Re-leveling of the deformation network along
Highway 395 during the summer of 1983
showed that the resurgent dome had been
uplifted by an additional 7 cm over the winter. Following the intense January 1983 swarm, both the earthquake activity and deformation rates within the caldera activity began a gradual decline that persisted through the remainder of the 1980s.

Resentment of the media attention attracted by the “Notice of Potential Volcanic Hazards” was most acutely expressed by the real estate and business communities. Other agencies with responsibilities in the area, including the U.S. Forest Service, the Bureau of Land Management, National Park Service, and Mammoth Mountain Ski Area together with a number of residents were supportive of efforts to monitor the activity and communicate its significance. The Ski Area, for example, instigated a series of day-long geology field trips for the public and local officials led by Hill and Bailey that proved to be an especially effective means of explaining the active geologic setting of Long Valley caldera and Mono Craters area, and Hill provided a series of invited public lectures on the significance of the ongoing activity in the early 1980s.

On August 31, 1983, John Filson (then Chief of the USGS Office of Earthquakes, Volcanoes, and Engineering) and Hill attended a luncheon meeting in Mammoth Lakes with local business leaders that was organized by the president of the Chamber of Commerce. John Filson recalls that occasion as the “1000-year lunch.” One of the people at the 1000-year lunch was an influential resident rumored to have drafted several critical letters to the USGS Director on behalf of the local business community. He had earned BS and Ph.D. degrees in electrical engineering from prestigious universities, and at the time was working as a consultant for the aerospace industry from his home in Mammoth Lakes. As it turns out, this same individual, Dr. William Graham, became President Ronald Reagan’s Science Advisor from 1986 to 1989. Needless to say, scientists monitoring Long Valley caldera developed a bit of angst when they learned that the President’s science advisor and the resident from the 1000-year-lunch were one and the same person! In August 1987, Bailey and Hill had the opportunity to take the Science Advisor and

Andrea Mead Lawrence, an influential member of the Mono County Board of Supervisors (also a 1952 Olympic gold medalist in Alpine skiing), on a field trip around the caldera. In the end, both were instrumental in easing relations between scientists and the local business community as they came to understand the nature of hazard.

The continued existence of the “Notice of Potential Volcanic Hazards” was a thorn in the side of the Mammoth Lakes business community. The Federal Register (v. 24, no. 7, 1977) defined this notice as “*Information on the location and possible magnitude of a potentially hazardous geologic condition. However, available evidence is insufficient to suggest that a hazardous event is imminent or evidence has not been developed to determine the time of occurrence*”. Given this definition and the geologic history of the caldera and its unrest, USGS scientists had no basis for rescinding the notice. In September, 1983, the Director of the USGS announced that the three-level Notice/Watch/Warning hazard notification system was being replaced by a one-level “hazard warning” system, which would be used only when the situation required a “near-term” (hours to days) public response (Table 2a). This change in hazard communication terminology became official on 24 January 1984 (Federal Register, v. 49, no. 21, pp. 3838–3839, January 31, 1984). By default, this change removed the “Notice of Potential Volcanic Hazard” for Long Valley caldera. In August 1984, the town of Mammoth Lakes was formally incorporated, thereby requiring the scientific community to interact with a new civil authority. While activity remained low within the caldera following the January 1983 swarm, this was not the case for seismic activity outside the caldera. On November 23, 1984, the $M_L = 6.1$ Round Valley earthquake and its many aftershocks (located midway between the southeastern caldera boundary and Bishop) shook the region (Figs. 1 and 4). The area again was repeatedly shaken from late July through mid August, 1986, by the rich foreshock and aftershock sequence associated with the $M_W = 6.4$ Chalfant Valley mainshock of July 21, 1986 located 20 km southeast of the caldera. Each of these earthquake sequences generated a
<table><caption>**Table 2** Evolution of USGS hazard warning systems (HWS) in abbreviated form</caption><caption>(a) USGS-wide systems for hazard statements released by the Director<br>1977–1983, Federal Register, v. 42, no. 70, 1977</caption><thead><tr><th>Terminology</th><th>Significance<sup>a</sup></th></tr></thead><tbody><tr><td>NOTICE OF POTENTIAL HAZARD</td><td>Info on potential hazard but insufficient data to time of occurrence</td></tr><tr><td>HAZARD WATCH</td><td>Info on potentially catastrophic event within months to years</td></tr><tr><td>HAZARD WARNING</td><td>Info on time, location, and magnitude of a potentially disastrous geologic event</td></tr><tr><td>11 October 1983, Federal Register, v. 48, n. 197, 1983</td><td></td></tr><tr><td>HAZARD WARNING</td><td>Info on a potential geologic hazard posing a significant threat requiring a timely response</td></tr></tbody></table>

<table><caption>(b) Local hazard level systems for the Long Valley Caldera volcanic field with authority for releasing hazard statements delegated to the LVO Scientist-in-Charge</caption><caption>17 April 1991 Alphabetic system<sup>b</sup> (Hill et al. 1991)</caption><thead><tr><th>STATUS</th><th>USGS RESPONSE</th><th>ACTIVITY LEVEL</th><th>RECURRENCE</th></tr></thead><tbody><tr><td>N</td><td>Normal monitoring</td><td>Background</td><td>—</td></tr><tr><td>E STATUS</td><td>Notify responsible personal as appropriate</td><td>Weak unrest</td><td>Weeks</td></tr><tr><td>D STATUS</td><td>Notify mid-level USGS personnel, OES, CDMG, USFS</td><td>Moderate unrest</td><td>Weeks–months</td></tr><tr><td>C STATUS</td><td>Notify USGS Office Chief, OES headquarters, State Geologist</td><td>Strong unrest</td><td>Months–years</td></tr><tr><td>B ALERT</td><td>Alert USGS Director, trigger an EVENT RESPONSE</td><td>Intense unrest</td><td>Years–decades</td></tr><tr><td>A ALERT</td><td>Issue HAZARD WARNING</td><td>Eruption likely</td><td>Decades–centuries</td></tr></tbody></table>

<table><caption>(c) June 1997 LVO Color-code system<sup>c</sup> (Hill et al. 2002)</caption><tbody><tr><td>GREEN<br>(NORMAL)</td><td>Normal operations with information calls as appropriate</td><td>Background to<br>Strong unrest</td><td>Most of the time<br>Months to years</td></tr><tr><td>YELLOW<br>(WATCH)</td><td>Full call-down<br>Event response</td><td>Intense unrest</td><td>Years to<br>decades</td></tr><tr><td>ORANGE<br>(WARNING)</td><td>Full call-down<br>Event response (if not in place)</td><td>Accelerating unrest eruption likely</td><td>Decades to<br>centuries</td></tr><tr><td>RED<br>(ERUPTION)</td><td>Full call-down<br>Event response (if not in place)</td><td>Minor to moderate eruption with possible increase in intensity</td><td>Centuries to<br>millennia</td></tr></tbody></table>

<figure><img src="image_69.png" /></figure>

<figure><img src="image_70.png" /></figure>

<figure><img src="image_71.png" /></figure>

<figure><img src="image_72.png" /></figure>
<table><caption>**Table 2** (continued)</caption><thead><tr><th colspan="2">Ground-based system for USGS Volcano Observatories<sup>d</sup></th></tr><tr><th>Terminology</th><th>Description</th></tr></thead><tbody><tr><td>NORMAL</td><td>Non-eruptive, background activity levels</td></tr><tr><td>ADVISORY</td><td>Activity levels above background, or ongoing eruptive activity declining</td></tr><tr><td>WATCH</td><td>Heightened or escalating unrest, or ongoing eruption poses limited hazard</td></tr><tr><td>WARNING</td><td>Hazardous eruption imminent or underway</td></tr><tr><th colspan="2">Aviation color code used by USGS Volcano Observatories<sup>d</sup></th></tr><tr><th>Color</th><th>Description</th></tr><tr><td>GREEN</td><td>Volcano is in a non-eruptive state</td></tr><tr><td>YELLOW</td><td>Volcano is showing elevated unrest levels</td></tr><tr><td>ORANGE</td><td>Volcano is exhibiting escalating unrest: potential eruption time-frame uncertain, or minor eruption underway with no or minor ash emissions</td></tr><tr><td>RED</td><td>Eruption is imminent or underway with significant ash emissions high into the atmosphere</td></tr></tbody></table>

flurry of news stories on seismic activity and volcanic unrest in the Mammoth Lakes area, further aggravating the local business leaders over negative publicity.

Activity returned to the vicinity of the caldera with the onset of a persistent earthquake swarm beneath Mammoth Mountain that began in early May and continued into March 1990 (Hill et al. 1990; Langbein et al. 1993; Cramer and McNutt 1997). Only a handful of earthquakes in this swarm had magnitudes as large as M = 3, but the activity included numerous spasmodic bursts, which are commonly associated with active volcanic systems. The swarm was accompanied by the onset of magmatic CO₂ emissions around the flank of the mountain and a marked increase of magmatic Helium ratios to values as high as R/Rₐ ~ 7, where R = (³He/⁴He) is the ratio of light to heavy Helium isotopes measured in

fumarole gasses and Rₐ is the isotope ratio in the atmosphere. Taken together, these observations suggest an intrusion of magmatic fluids into the shallow crust beneath Mammoth Mountain (Hill and Prejean 2005). In an effort to keep local civil authorities apprised of the situation, Hill called the City Manager for Mammoth Lakes several times a week with updates on evolving activity. At one point in a conversation with Steve McNutt, the City Manager asked in apparent exasperation if we couldn't provide him with some sort of written criteria for how seriously he should regard the varying levels of activity. His request led to development a response plan that included an alphabetic scheme of five "alert levels" (E through A in ascending order of concern) modeled after that used for the Parkfield earthquake prediction experiment (Table 2b; Hill et al. 1991). This alphabetic system included

<footnote><sup>d</sup>No stand-down criteria specified</footnote>

<footnote><sup>e</sup>Stand-down criteria based on current ACTIVITY LEVEL</footnote>

<footnote><sup>f</sup>Stand-down criteria based on current ACTIVITY LEVEL</footnote>

<footnote><sup>g</sup>Stand-down criteria based on current activity levels noted under DESCRIPTION</footnote>
criteria for stepping down from an elevated alert level—an element that was absent in the old “Notice-Watch-Warning” system. With an important addition, the USGS Director ceded local authority to the Chief Scientist of the Long Valley monitoring project to communicate changes in alert level directly to local officials thus avoiding delays associated with high-level, inter-agency discussions.

In an effort to help local civil authorities better appreciate the issues involved with volcanic hazards, the USGS invited Mono County and Mammoth Lakes civil authorities to attend a conference commemorating the tenth anniversary of the May 1980 Mount St. Helens eruption. This conference, which was held adjacent to Mount St. Helens in Kelso Washington on May 17–19, 1990, included presentations by local civil authorities as well as scientists who were directly involved in responding to this catastrophic eruption. The occasion turned out to be enormously informative for all concerned.

Meanwhile, frequent trilateration measurements with the 2-color geodimeter showed that deformation rates across the resurgent dome began to increase substantially in October of 1989 (Langbein et al. 1993). Three months later, earthquake swarm activity resumed in the south moat of caldera and continued to wax and wane through mid-1997, accompanied by relatively steady uplift of the resurgent dome at a rate of 2-to 3-cm/year (Fig. 4; Langbein et al. 1993). Other developments that persisted through the 1990s involved high concentrations of magmatic CO₂ in the soil around Mammoth Mountain and growing number of long-period (LP) volcanic earthquakes occurring at depths of 10–20 km beneath the southwest flank of Mammoth Mountain (Pitt and Hill 1994). In May 28, 1998, a cross-country skier died of CO₂ asphyxiation after falling into a collapsed snow cave filled with 70% CO₂ at the base of Mammoth Mountain (Hill 2000). Both the CO₂ emissions and LP earthquakes were coincident with the 1989 Mammoth Mountain swarm, and together they serve as a reminder that this 11,000-foot high volcano, which last erupted 55,000 years ago, is not extinct.

The increased activity within the caldera provided ample opportunity to exercise the “alert-level” system. For a while, it seemed to work just fine as we moved back and forth between the lower “status levels” from E through C. The new system included a matrix, rather than top-down, system for information flow in which alerts are passed to State, County, and City authorities simultaneously. Flaws in this alphabetic scheme began to emerge, however, as the media added their own twist on the status levels. On a slow news day, for example, a swarm including a couple of M ~ 3, locally felt earthquakes corresponding to a “D-status” under this scheme would get reported in headlines as a “D-level volcano alert” (the latter two words added by the reporter or headline writer). Of course, most of the public didn’t understand what a “D-level alert” meant—except that is sounded serious. The result was exaggerated concern in the public (should I cancel my planned vacation to Mammoth?), and renewed frustration in the business community over negative “volcano” publicity (see Mader et al. 1987).

The USGS in consultation with Mono County, Mammoth Lakes, the California State Geologist and head of CDMG, the California Earthquake Prediction Evaluation Council (CEPEC), and the California Office of Emergency Services (CalOES), began discussing ways to improve this system by making it less susceptible to misinterpretation by both the media and the public at large. The result was a four-level color scheme, with the three lower status levels (E, D, and C) grouped under “condition GREEN” (no immediate risk), replaced level B by “condition YELLOW” (watch), and replaced level A by “condition ORANGE” (warning). A final color, “condition RED” was added to indicate an eruption was actually under way. Mono County suggested that we incorporate shapes with this scheme. The result associated GREEN with a circle, YELLOW with a square, and ORANGE with a diamond (skiers will recognize these as the shapes indicating beginner through advanced slopes). A triangle (the shape of a volcano) was chosen to represent RED. This new system was formally adopted in
June 1997, shortly before the onset of a sustained period of escalating unrest in the caldera (Table 2c; Fig. 4; Hill et al. 2002).

In April-May of 1997, the two-color geodimeter data again showed hints of an increasing horizontal deformation rate across the resurgent dome (Langbein 2003). By early July, earthquake swarm activity in the caldera picked up with an increasing frequency and intensity that persisted through the remainder of the year and into January 1998 (Fig. 4). The peak in this activity from mid-November through early January 1998 included nine earthquakes with magnitudes of M = 4.0 or greater accompanied by thousands of smaller events. The three largest earthquakes had magnitudes of M = 4.8-4.9. Resurgent dome deformation escalated through the second half of 1997, reaching a peak uplift rate of over 2 cm/month in mid-November. By the end of the year, the center of the resurgent dome was approximately 10 cm higher than in early May. Mammoth Mountain earthquake swarm activity also increased in September through December 1997. Under the new color-code notification system, the condition remained GREEN through this extended period of elevated activity. During particularly strong swarm sequences on November 22 and 30, however, activity came extremely close to meeting the guidelines for a condition YELLOW (it would have been level C under the old system—see Table 2). The Long Valley caldera web page, on which most of the monitoring data are available in real-time together with frequent written updates on the current condition and its significance, was receiving tens of thousands of “hits” a day. It also attracted lots of email with messages ranging from “why aren’t we at YELLOW yet?”, “you’re a pawn of the Realtors”, “what are you covering up?”, “I’m moving to Maine!” to “thanks for an excellent job of keeping us informed and keeping things in perspective”. Both the seismicity and deformation rates within the caldera gradually slowed through the first half of 1998. Six months later, however, Mammoth Lakes and vicinity was again shaken by three M 5 earthquakes in the Sierra Nevada just south of the caldera—a pair of M 5.1 events

on 8 June and 14 July 1998, respectively, and a M 5.6 event on 15 May 1999. The abundant aftershocks to these earthquakes included a number of M > 3 earthquakes, which were felt locally. On August 6, 1999, the Long Valley monitoring project was officially established as the Long Valley Observatory (LVO) with Hill as Scientist-in-Charge (SIC).

After nearly two decades of intense, episodic unrest, the Mammoth Lakes Town Council agreed to support travel for the Town Manager to attend the first Cities on Volcanoes (COV) meeting held in Naples and Rome from 28 June to 4 July 1998. This was the first of an ongoing series of international meetings under the IAVCEI (International Association of Volcanology and Chemistry of the Earth's Interior) umbrella bringing together scientists and civil authorities for discussion on volcanic hazards and mitigation. Having the Mammoth Lakes Town Manager at the COV marked an important milestone in the efforts of scientists to grow community awareness.

### 3 Maintaining Community Awareness and Preparedness During Low-Level Volcanic Unrest (2001-May 2016)

Since the turn of the century, unrest has diminished in Long Valley Caldera (Fig. 4; Wilkinson et al. 2014). The communication challenge has thus become one of maintaining public awareness and preparedness when there are few signs of volcanic unrest recognized by the public. Two notable exceptions during the relative quiescence from 2000 to 2011 included: (1) the tragic death of three skiers on April 6, 2006, as they fell into a CO₂-filled snow cave that had developed over the fumarole on the upper flank of Mammoth Mountain, and (2) the temporary onset of geyser activity in Hot Creek (a popular bathing area) in 2006. Instrumentally detected low-level unrest between 2000 and 2011 included additional inflation of the resurgent dome by 3-4 cm in 2002 accompanied by minor seismicity (Feng and Newman 2009). In 2006, 2008, and 2009
short-lived swarms of earthquakes, all too small to be felt, occurred at depths at 25–30 km below Mammoth Mountain (Shelly and Hill 2011). An increase in shallower earthquakes, again too small to be felt, followed within a few months of each swarm and magmatic CO₂ emission increased in tandem at the surface (Lewicki et al. 2014). Scientists relate these phenomena to deep intrusion and degassing of basaltic magma in the lower crust followed by upward migration of CO₂-rich fluids through fracture development. To the non-scientist, however, the significance of “stealth” magma supply events is hard to appreciate. Few earthquakes were felt, and the uptick in magmatic gas could be measured, but not seen. Years of low-level volcanic unrest require a sustained effort to educate a growing population base and a changing cast of elected officials, land managers, and civil authorities.

Effective hazard education requires persistence and strategic networking within at-risk communities. From the beginning, USGS scientists have demonstrated their commitment to hazard mitigation in the Long Valley region by attending quarterly meetings of the Mono and Inyo County Office of Emergency Services, Unified Command System (UCS). Through participation in the UCS scientists have learned to speak the language of emergency response teams and have become integral members. The standalone “Plan Caldera” developed in the 1980s during the height of caldera unrest has evolved into the Mono and Inyo county Emergency Operations Plan, which includes regularly updated contributions written by scientists. USGS scientists have provided face-to-face volcano hazard training to citizens in the local Community Emergency Response Team (CERT), a volunteer disaster response organization sponsored by the Mammoth Lakes Police Department, Mammoth Fire District and Mono County Public Health, as well as, specialized trainings for civil authorities through a partnership with the Federal Emergency Management Agency—National Disaster Preparedness Training Center and the University of Hawaii. In collaboration with USGS scientists, the USFS Ranger Districts in Mammoth Lakes and Mono Lake visitors centers

established an information display on the active geology of the region, and Mammoth Mountain Ski Area developed a “Top of the Mountain” display taking advantage of a spectacular overview of regional geology.

Sustaining a proactive hazard education program in the Long Valley region is time-consuming and costly, but necessary. In 2009 this responsibility fell to Margaret Mangan as the newly appointed LVO SIC as Hill stepped aside to become Scientist Emeritus. The necessity of continuing education is underscored by renewal of caldera inflation, which began in early 2011, and has continued through early 2017 (Montgomery-Brown et al. 2015). The time-averaged uplift rate from 2011 through 2014 was about 2 cm/year, comparable to that of the mid-1990s. Seismicity within the caldera increased as well with modest earthquake swarms in the caldera in June and July 2014 followed by the most energetic swarm since 1999 in September 2014 with eight earthquakes of magnitudes M ~ 3.0–3.8 that were felt in the town of Mammoth Lakes. Just prior to this swarm, USGS and CGS scientists had briefed the Mono and Inyo County Unified Commands on earthquake hazards in the region in preparation for release of a joint report describing scenario earthquake hazards for the Long Valley—Mono Lake area (Chen et al. 2014). Meanwhile, Mammoth Mountain has continued to produce episodic, low-level swarm activity (Shelly et al. 2015).

Periodic evaluation of the effectiveness of hazard communication is also necessary, and new methodologies must be developed to meet modern needs. Several years after the introduction of Long Valley’s four-level color-code warning system, for example, the USGS Volcano Hazards Program recognized the need to establish a national alert-notification system, one that covers all US volcanoes and that distinguishes between ground-based and atmospheric hazards (Gardner and Guffanti 2006; Fearnley et al. 2012). By 2006, two new communication vehicles where in use—Volcano Alert Notifications (VANs) and Volcano Observatory Notices for Aviation (VONAs), both of which use four-tiered threat level systems (Table 2d).
VANs use Normal-Advisory-Watch-Warning to specify increasing threat on the ground (e.g., lahars, lava flows) and VONAs use Green-Yellow-Orange-Red to specify increasing threat to aviation (ash clouds and volcanic aerosols). The dual ground-level and aircraft-ash systems recognize that a low-level effusive eruption might pose a serious local hazard (Warning) without posing a serious ash hazard for aircraft at 30,000 feet. In keeping with communications in the digital age, an email-based Volcano Notification Service (VNS) automatically delivers VANs, VONAs, and other volcano information to all subscribers who register online at http://volcanoes.usgs.gov/vns (Accessed 13 February 2017).

In 2012, the USGS Director announced establishment of the California Volcano Observatory (CalVO), giving the former LVO the responsibility to monitor Long Valley caldera, as well as the other young and restless volcanoes in the state (Stovall et al. 2014). As the director explained "By uniting the research, monitoring, and hazard assessment for all of the volcanoes that pose a threat to the residents of California, CalVO will provide improved hazard information products to the public and decision makers alike."

## 4 Conclusions

Applying scientific research to understand the processes driving magmatic unrest in a large caldera and communicating the significance of this research in terms of volcanic hazards represents a microcosm of the challenges that much of science faces today as we grapple with the societal relevance of scientific research. Long Valley caldera unrest has provided a rich experience in terms of both scientific return and the challenges in trying to effectively communicate socially useful information on natural hazards to the public.

We have come a long way since the highly charged relations between geologists and the business community of the early 1980s. Those involved on the scientific side learned to be much more effective at presenting messages on activity

levels and their significance in terms that can be understood by the general public, and the residents of Mono County and eastern California have gained a much better appreciation for the geologically active environment in which they live. The following three points are important in this regard:

1. We must continue to do the best science possible as we track the ongoing activity. Understanding the processes driving the unrest is key to understanding the nature of the hazards posed by the unrest and their likely evolution in terms of long-term forecasts and short-term predictions. This is similar to medicine, for which understanding the processes that cause disease provides the best guidance for treatment. Ultimately, our credibility with the public rests on a foundation of sound science.

2. Establishing and maintaining an effective and credible working relation with civil authorities and the public requires constant attention and a major commitment in time. Long-term continuity with scientific personnel and a stable policy with clearly defined lines of responsibility are important elements in this process. The tenures of Hill as LVO SIC from 1982 to 2009, Mangan as LVO SIC from 2009 to 2011 and CalVO SIC from 2012 to present have provided continuity of scientific and hazard information spanning 4 Mammoth Lakes Town Managers, 3 Mono County Sheriffs, 5 State Geologists, and 6 California Office of Emergency Services Directors.

3. Finally, although the "geologists not welcome" signs have long since disappeared from motel and restaurant windows in Mammoth Lakes, it's important to keep in mind that a protracted crisis and an evolving resort population will severely test the good will and trust scientists worked so hard to establish since the early 1980s. The threat of an impending volcanic eruption inevitably presents a narrow path and an element of uncertainty to a successful response. On one side, an over-anxious response will exacerbate the "false alarm" problem (one, or at most
two, “needless” evacuations, for example, may well destroy our credibility and effectiveness as scientists). On the other side, an overly conservative response (no need to worry yet) may lead to serious casualties and fatalities if an explosive eruption does develop. The 1991 eruption of Mount Pinatubo in the Philippines stands as an example of the narrow path to success in this business with timely evacuations implemented less than a week in advance of the most hazardous, explosive phase of the eruption. This places a premium on good science, experience “under fire”, cool heads, and, yes, a little luck.

## Additional information sources

Those not familiar with Long Valley caldera and its activity can find maps, background information, diagrams, and plots summarizing current activity under Long Valley caldera on the California Volcano Observatory web site at: http://volcanoes.usgs.gov/volcanoes/long_valley/ (Accessed on 15 January 2015)

**Acknowledgements** We are grateful to J. Lewicki and J. W. Cole for constructive reviews and to C. Jones for helping resolve inconsistencies of the dates of several events in the early 1980s.

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# Volcanic Hazard Communication at Pinatubo from 1991 to 2015

Chris Newhall and Renato U. Solidum

## Abstract

When Pinatubo re-awakened in early 1991, very few people within the vicinity were familiar with volcanic hazards, and even fewer believed that Pinatubo could impact them. Scientists knew more, but were still struggling to answer:

* How often and how explosively did Pinatubo erupt, and when was its most recent eruption?

* What precursors could be expected in advance of a very large (VEI $\ge$ 6) explosive eruption?

* What was happening beneath Pinatubo that was driving 1991 unrest?

To reach an exceptionally diverse audience and to counter widespread scepticism, scientists tried a whole package of communication measures, including simplified alert levels; a “worst case” hazard map; a probability tree; personalized briefings for local and national government officials, military and civil defense officials, nuns, and the news media; use of a IAVCEI video on volcanic hazards on broadcast TV and in briefings; volcanology tutorials for school teachers; talks on the mountain with villagers and anti-government guerrillas; and beer and hotdogs too. Forecasts were just-in-time and generally correct about what areas would be at risk. Overall, pre-eruption communication achieved its goal of getting people out of harm’s way. Three lessons stand out: use simple, multipronged communications, especially video; include worst case scenarios in your warnings, together with estimated probabilities thereof; and be willing, as scientists

<footnote>C. Newhall (✉)
Mirisbiris Garden and Nature Center, Santo Domingo, Albay, Philippines
e-mail: cgnewhall@gmail.com</footnote>

<footnote>R.U. Solidum
Philippine Institute of Volcanology and Seismology
(PHIVOLCS), Quezon City, Philippines</footnote>
and decision makers, to recommend evacuations even if uncertainty is still high and there is still a chance of false alarm. For more than a decade after the 1991 eruption, rain-induced lahars threatened even more people and more infrastructure than the eruption itself. Several groups of scientists and engineers worked on the lahar threat, each coming up with slightly different long-term assessments that appeared to the public as bickering or incompetence. Scientists' credibility was seriously diminished. Decisions of what lahar-mitigation projects to build—including a succession of inadequate ones—were influenced less by science and more by public pressure, pragmatism, back-room politics, and profit. Short-term or immediate lahar warnings were communicated by scientists and by police-manned watch points. The scientific warnings were technically superior but the police warnings had greater credibility, as they were from familiar sources and easily understood. Communication of hazard information at Pinatubo saved many lives, and we are proud and privileged to have been part of preventing a much worse disaster. However, margins of safety were narrow and some deaths that did occur could have been prevented by better communication.

# 1 Introduction

The 1991–92 eruptions of Mount Pinatubo, Philippines, would affect a population in Central Luzon that was unfamiliar with and initially sceptical about volcanic hazards. That population was exceptionally diverse, including indigenous Aeta people on the volcano itself, helped by trusted nuns and pastors, and sharing the volcano with a small but influential band of guerrillas of the New Peoples’ Army. Around the volcano were nearly a million lowland Filipinos in several large cities and towns of three provinces, and two large American military bases that were more like America than the Philippines. And in the skies above and around Pinatubo, commercial as well as military aviation had to be alerted.

Following small phreatic explosions on April 2, 1991, a team of Filipino and American scientists were trying urgently to decipher the history of the volcano and the unrest. What little we knew of Pinatubo's geologic history indicated that a major explosive eruption was possible, and geophysical and geochemical monitoring—just started—was indicating continuing but (as-yet) not escalating unrest. It was not clear that the

volcano WOULD erupt, but it was clear that IF IT DID, the eruption was likely to be large and explosive.

Facing strong scepticism from officials and the public unfamiliar with volcanic threats, and with no indication of how much time remained before an eruption could occur, we had to quickly find ways to teach about volcanoes and overcome that scepticism. If unrest escalated, officials would have to be convinced to evacuate large numbers of people to safety. Unrest did escalate, and most of those at high risk were successfully evacuated.

After the large eruption of June 15, 1991, focus shifted to the hazard of rain- (and lake-) related lahars. Though there was more time for a lahar information campaign, the long (>10 year) duration complicated the effort. Again, large numbers of people were at risk, and nearly all were moved to safety.

Although communications and mitigation were broadly successful, we stress that the successes were often “just in time” and “just barely enough.” Most of the deaths that did occur could have been prevented, and many more deaths could have occurred had the eruption or the lahars been slightly sooner or larger. We tell this
story in the hope that it will help colleagues in similar situations in the future add margins of safety.

## 2 Audiences: Who Needed to Learn About Volcanic and Lahar Hazard?

Audiences for our hazard communications were primarily decision-makers, community opinion leaders, and the news media. They in turn conveyed hazard messages and suggested actions to the public. Given that the total population at risk was roughly 1 million, the task was large and messages would need to be disseminated through many levels and in many ways. Scientists communicated directly with hundreds of officials at various levels, and they in turn reached out to the million.

In the Philippines, there is a well-established hierarchy of government and NGO bodies charged with disaster risk mitigation. At the top is the National Disaster Risk Reduction and Management Council (NDRRMC), known at that time as the National Disaster Coordinating Council, (NDCC), which includes top civilian, military, NGO officials, and scientists. The executive arm of the NDRRMC is the national Office of Civil Defense (OCD), led at the time by Engineer Fortunato Dejoras. Similar crisis coordinating functions are replicated at the regional (RDRRMC), provincial (PDRRMC), city or municipal (MDRRMC), and barangay (village) level, and execution is by local offices of OCD and corresponding offices of other government agencies and NGOs. Needs are assessed from the bottom or middle up; decisions are made mostly at the regional or provincial level, or higher if necessary, and passed down to local levels. In the case of volcanic hazards, assessments and recommendations are made by the Director of the Philippine Institute of Volcanology and Seismology (PHIVOLCS), or his/her designated representative.

At and around Pinatubo, virtually no one in this hierarchy had experience with volcanic hazards, so all needed urgent education. The sole

exceptions were geoscientists from PHIVOLCS, joined by colleagues from the US Geological Survey (USGS) and a few university-based geoscientists.

News media had perhaps slightly more experience with volcanic hazards, but not much. There are relatively few science reporters in the Philippines, and some of the best reporters turned out to be generalist regional reporters who familiarized themselves with the issues better than national reporters who were pulled in many more directions.

Before the climactic eruption of June 15, 1991, nearly all communications with the preceding groups were led by the late Dr. Raymond Punongbayan, then-director of PHIVOLCS. Ray was a talented communicator who quickly earned the trust of those he briefed. Briefings were held for the NDCC (in a meeting led by then-Defense Secretary and later President Fidel Ramos), for the Governors and staff of the three provinces that adjoin at the summit of Pinatubo (Zambales, Pampanga, and Tarlac), and for the Mayor of Angeles City. The Mayor of Olongapo City had the benefit of personal advice from several university-based scientists, including Dr. Kelvin Rodolfo and a young PHIVOLCS scientist, here the 2nd author. Other mayors joined the provincial level meetings. Dr. Punongbayan also developed a good rapport with news media, and he tapped them to help disseminate information. Other PHIVOLCS and USGS scientists were largely free to concentrate on field work and data interpretation, undistacted by media because Dr. Punongbayan handled their requests.

The American military bases were self-contained, with their own command structure, logistics, hospital, schools, businesses, and the like. Although the primary mission of the USGS scientists was to work with long-time PHIVOLCS colleagues in support of the PHIVOLCS mission, the USGS scientists were also granted access to logistical resources of the US military and, in return, kept the US military informed of developments in parallel with communications to the NDCC. Because the US Air Force was preparing a contingency plan in case evacuation became
necessary, it sought scientific advice not only for commanders but also for enlisted personnel on whom base operations depended, for teachers, and for hospital staff. An initial attempt to also teach schoolchildren about the hazard had to be abandoned—for lack of time—in favour of a single-day briefing for all science teachers of Clark Air Base.

## 3 Pre-eruption Messages

One might liken the scientific effort—and the communication effort—to running a race against an unknown competitor, Pinatubo. At the time of the first phreatic explosions on April 2, 1991, no instruments monitored the volcano and there were no background data of monitoring. Fortunately, there was some prior geologic knowledge of Pinatubo, from a site safety study for a nearby nuclear power plant and from exploration for geothermal power, which told of what HAD happened before and COULD happen again, but what officials wanted was information about what WOULD happen. Furthermore, even worldwide, there was no information about the expectable precursors of a VEI 6 eruption, which we could see from the geology was a strong possibility.

Scientists, by training, are conservative in what they say. Data must be ample and convincing, and uncertainties should be low before a paper is published or advice is given. However, that conservatism must sometimes be overridden in times of volcanic crises.

In chronological order, pre-eruption messages evolved through the following sequence:

* Pinatubo is a volcano, and is restless, so here is a primer on volcanic hazards (April-May).

* Near the volcano, the only way to protect yourself is to evacuate before an eruption. We may recommend evacuations from within 10, 15, 20 km radius (later, adding 30 and 40 km radius). (With radii adjusted for each specific volcano, this is a standard PHI-VOLCS message in the face of any volcanic crisis) (April-June).

* We are not yet sure whether Pinatubo WILL erupt, but if it does, the eruption will be big and bad (late April).

* Evidence (as of May 13) shows that rising magma is causing the unrest.

* Even though this volcano is new to us too, we will try to raise alerts progressively if an eruption is approaching, giving you days up to two weeks of advance warning and, later, final notice hours or a day before an eruption (May 13).

* In return, we ask you (officials and the public) to understand that there might be a false alarm or two along the way. Please bear with us.

* Many areas have been swept by lethal volcanic flows in the past, or affected by ashfall, and could be similarly affected by a new eruption (hazard map, May 23).

* The volcano MAY erupt within 2 weeks (June 5).

* The volcano MAY erupt within 24 h (June 7). All within a 20 km radius of the summit should evacuate.

* The volcano has started to erupt, though only with a lava dome (June 7).

* The volcano has started explosive eruptions (June 9).

* Even though what you saw yesterday and today was impressive, the big one is still to come (June 12, 13, 14). On June 14, all those within 30 km radius of the summit should evacuate.

* A typhoon will arrive on June 15 (message from the weather bureau PAGASA).

* A massive eruption is in progress (June 15). Those within 40 km radius of the summit should evacuate, a recommendation easier made than followed.

Our message about possible false alarms was given in briefings to Governors and other decision-makers. Volcanologists have been notably averse to false alarms ever since the 1976 crisis at Soufrière Guadeloupe (Fiske 1984) and initially, we worried about this at Pinatubo as well. But we were reassured in our briefing in Zambales province when one of the attendees, a nun working with indigenous Aetas, told us they
would happily accept premature warnings. They were more concerned that warnings might be too late. Not all would have been so forgiving, but we took the nun's words to heart. Social science research on earthquake and weather hazards recognizes some so-called "cry-wolf" reduction in scientific credibility (e.g., Atwood and Major 1998), but less than often presumed (Dow and Cutter 1998; Barnes et al. 2007 and references therein).

One communication that was, in retrospect, underemphasized, warned of ash in areas far from the volcano. Although warning was technically made, it was not emphasized sufficiently, especially in light of the typhoon rain that almost doubled the weight of ash on roofs and led to most of the eruption-related deaths. Another result of this under-emphasis was failure to warn aircraft outside the Philippine Flight Information Region (FIR), mostly over Indochina. We thought we had the aviation hazard covered through the Philippine FIR, and didn't even think of FIR's beyond the Philippines.

Additional details of eruption warnings, including dates and text of alert levels and recommendations for evacuation radii, may be found in Punongbayan et al. (1996) and Tayag et al. (1996).

# 4 How Were the Warnings Prepared and Presented?

## 4.1 Briefings and Video

By mid-May, as soon as we were reasonably sure that magma was rising and we had drafts of warning materials, the Office of Civil Defense arranged for the Director of PHIVOLCS (Dr. Punongbayan) to brief the Governors of each affected province (Pampanga, Zambales, and Tarlac), and the Mayor of Angeles City. The Governors, in turn, arranged for attendance of town mayors, captains of barangays closest to the volcano, and representatives of NGOs and news media. The general format of each meeting included an introduction by the Governor or Vice-Governor, followed by presentation of what

scientists knew about Pinatubo in general and about the hazard facing each province, and showing and discussion of a rough cut of the IAVCEI video "Understanding Volcanic Hazards" prepared by Maurice Krafft and others. Finally, there would be open discussion of general mitigation steps, though most discussion of specific mitigation steps came after these briefings. In most of these briefings, attention levels were high and many good questions were asked; in only one, that for the Mayor of Angeles City, was there official disinterest or hostility.

The IAVCEI video deserves special mention. Shortly after the terrible mudflow (lahar) disaster at Nevado del Ruiz Volcano (Colombia) in 1985, volcanologists agonized over how to prevent such disasters in the future. Because people of Armero and other towns at risk seemed not to have understood the seriousness of warnings they received, they did not walk to safety even though they could have done so. Apparently, they did not understand that a "flujo de lodo" (literally, a flow of mud) could in fact be a huge wall of mud, sand, boulders, trees, and more. Accordingly, Maurice Krafft and others set out to make a video that showed, in starkly graphic ways, the nature of each volcanic hazard, how far and fast it travels, and what it does when it hits houses and people. The ad-hoc steering committee for this video debated whether to include dead bodies—lest audiences find it too hard to watch—but in the end decided for inclusion to shock audiences into attention. Pinatubo was the next big event to threaten a population unfamiliar with volcanic hazards and, fortunately, scientists had by then a rough cut of Maurice's video, sufficient for public screening. This video was highly effective. Many who simply hadn't grasped the threat before seeing this video became converts soon after. The old saying, "A picture is worth 1000 words" might be re-written to say a "A video is worth 1,000,000 words!" Interestingly, we found no great revulsion from scenes of dead bodies—perhaps TV programming had already made this an everyday sight. But a short clip of a young girl trapped and shivering in the Armero lahar deposit drew audible gasps of horror in every briefing. It was VERY effective.
Unaware yet of its powerful reception at Pinatubo, Maurice Krafft and his wife Katia were dissatisfied with the video's footage of pyroclastic flows (much better footage is available now), so enroute to Pinatubo they stopped at Unzen to get better footage. They misjudged the threat and were themselves killed by a pyroclastic flow. That sad irony further increased the impact of the video as it was shown at Pinatubo.

Although technically the video was copyrighted by IAVCEI, we decided in the interest of time and the spirit in which the video was made to show it on broadcast TV and to distribute copies freely at each briefing. No doubt many copies of copies were also made. Today, one might post a video on YouTube; at that time, none of today's social media were available. Readers of this paper wishing to order a re-mastered DVD of this video, and a sequel, may order it at http://www.volcanovideo.com/p1IAVCEI.html.

In addition to briefings at the provincial level, Dr. Punongbayan also gave briefings to key national leaders. In one, he briefed then-President Corazon Aquino. In another, led by then-Secretary of National Defense Fidel Ramos and held at Camp Aguinaldo, he briefed assembled Cabinet members and other key officials. We recall a multi-tasking, busy Gen. Ramos calling a timeout after this briefing, and remarking to Dr. Punongbayan that he heard the message loud and clear, and half-joked that the Philippine government had better hurry up the renegotiation of the RP-US military bases agreement before there was nothing left to negotiate about.

While Dr. Punongbayan handled briefings at the national and provincial levels and for the media, several other members of the joint PHIVOLCS-USGS team provided briefings at lower levels. One PHIVOLCS team was based in villages on the NW flank of Pinatubo and quickly developed trust and provided information on the hazard. Field parties often encountered and stopped to talk with other villagers. One time, a group approached our helicopter while we were installing a seismic station. Other times, we stopped in roadside villages on our way to study

outcrops, and the requisite courtesy call on a village captain would inevitably and fruitfully turn into quite a long discussion. In one village, we met with both indigenous Aetas and rebel guerrillas of the New People's Army. In another village from which the volcano could not even be seen, we didn't find the village captain so we spoke instead with his wife, who seemed not to understand at all, yet we know that residents of this village did ultimately evacuate to safety. These encounters and outreach were by no means comprehensive. A few PHIVOLCS scientists started systematic outreach in towns around the foot of the volcano, but owing to limits in time and transport, most of our outreach was bootlegged onto our field work.

Those of us on Clark Air Base at the East foot of Pinatubo also provided near-daily briefings for military officers and officials from nearby towns. We spoke to several classes of schoolchildren but quickly realized that the only way we could reach larger numbers would be to teach the teachers, especially science teachers. Inside Clark Air Base, a session for science teachers was quickly arranged; outside the base, we connected with only a few teachers, far from all.

Also within Clark Air Base, we gave briefings to individual units including the hospital and, at the suggestion of the Deputy Base Commander, to a level of staff called “chief master sergeants.” The “chiefs” were eminently practical—the real “doers” of the base. Our makeshift Pinatubo Volcano Observatory (PVO), in a crowded apartment for enlisted personnel, became a hub for curious and concerned daily visitors, including the base meteorologists, officers, and even their wives. On one occasion when activity ramped up, we summoned top officers of Clark and Subic to PVO. Their arrival by helicopter and thence in cars bearing the flags of admirals and generals drew quite a lot of attention and gossip throughout the base. Because we relied initially on the base weather office for access to a fax machine, US Air Force weathermen (and by extension, their superior officers) were also privy to fax communications between scientific team at Clark and Director Punongbayan in Manila. One fax from Director Punongbayan, about alert
levels if we recall, arrived at PVO with yellow highlights! Even though the Base Commander wanted to keep our work quiet, there was really no way to do so, and occasional breaches were actually quite useful! Finally, in late May, the US Air Force command realized it needed to run an interview with a scientist on the base TV station, but the interview was tightly scripted and the pre-scripted wrap-up by the interviewer was much more reassuring than the interview itself.

## 4.2 Alert Levels

We soon recognized that, given the large, diverse audience and widespread unfamiliarity with the threat, all of our warnings would need to be very simple. One such simplification was definition of numbered alert levels from 0 (no unrest) to 5 (large explosive eruption in progress) (for details, see Punongbayan et al. 1996; Tayag et al. 1996). These were patterned, from vague and stressed memory, on alert levels first introduced at Rabaul (Papua New Guinea) and later adapted for eruptions in Alaska (USA) (see brief histories of alert levels in Fearnley et al. 2012; Fearnley 2013; Winson et al. 2014). Levels 3 and 4 of the Pinatubo scheme anticipated forecast time windows (2 weeks and 24 h) within which an eruption might occur, and each level had an interpretation of activity. We intended that Civil Defense could design and key their mitigation actions to these alert levels. Probably, they would have done so had not events developed so rapidly in early June that —in effect—recommended evacuation radii from the Director of PHIVOLCS pre-empted plans that Civil Defense was still preparing. In recent years there has been much re-examination of alert schemes and debate of whether they should include forecasts and formal linkage to responses, making scientists de-facto decision makers. Given the extreme urgency at Pinatubo, we think both the forecast and the de-facto decision-making role of PHIVOLCS were necessary, though we acknowledge that these matters should be discussed and agreed elsewhere on a country-by-country basis.

PHIVOLCS and civil defense leaders still link alert levels and responses, but in recognition of uncertainties and differences between volcanoes, PHIVOLCS has made the forecast windows less precise.

The general upward progression of levels resembled a familiar 3-level alert used for typhoons in the Philippines, but had the opposite sense to a countdown of alerts used by the US military. Fortunately, the US military agreed to use our scheme rather than their own.

We did note one misunderstanding of the wording on the alert levels. For alert levels 3 and 4, the wording stated that “an eruption was possible within (a specified timeframe, 2 weeks or 24 h).” Strictly speaking, we meant that we could no longer guarantee that an eruption would not occur within that period; however, the simpler reading of “could occur within that timeframe” would have been an acceptable simplification. In Pilipino, that would have been stated as “ma-aaring mangyari sa loob ng (dala-wang lingo o 24 oras).” Perhaps since we wrote it only in English, many misread it to mean that “an eruption WILL occur within 2 weeks or 24 h” or, worse yet, “an eruption WILL occur in exactly 2 weeks or 24 h.” If differences between the terms “could occur” versus “will occur”, and between “in” versus “within” might be misunderstood, use the local language to clarify! No serious harm was done, as those making final evacuation recommendations understood the terms as intended.

Our alert level scheme also included guidance for step down, with built-in delays to guard against premature lowering of alert levels. We think such guidance for step-downs is helpful, partly to guard against sudden decreases in activity that are “calm before the storm,” and partly to allow orderly stepdown, without embarrassment, should unrest truly stop. Many magma intrusions fail to reach the surface and moveable alert levels (up and down) are designed as an alternative to forecasts that may prove wrong. In the case of Pinatubo, we didn’t have occasion to use the stepdown until well after the climactic eruption.
## 4.3 Pre-eruption Hazard Map

Hazard maps are the geoscientist's standard response to the question, "What areas are at risk?" Our pre-eruption map was based mainly on quick reconnaissance of the maximum extent of pyroclastic flows from past eruptions of Pinatubo. In most areas this was immediately obvious both on the ground and on aerial photos. Our geological team doing field work compared notes with Director Punongbayan who was interpreting aerial photos from his office in Quezon City. In most cases, the field exposures (in canyon walls) extended slightly beyond the distinctive, dissected topography of old pyroclastic flows; in a few cases, e.g., near Barangay Pasbul, of Floridablanca town, suspect topography was found in aerial photos and later confirmed by ground visit. The aerial photos we had were well out of date, and we note with pleasure how much better it is to have modern satellite coverage with current roads and quarries.

We sketched the outline of prehistoric pyroclastic flows onto a single sheet of paper on which we had also traced main highways and towns. For safety, we added 0.5-1.0 km of buffer zone around the known pyroclastic flow extent. In our original sketch we showed that ash could fall anywhere on the area of the map; in a cleaned-up sketch, the most likely directions of ashfall were shown. Potential lahars were shown only as hash marks down the main river channels—a simplification that would be greatly expanded after the eruption. Without calling it as such, we intended this map to reflect our "worst-case" scenario, VEI 6 eruptions from Pinatubo. Fortunately, the actual reach of pyroclastic flows almost perfectly matched the hazard zones on our map, and did not reach as far as what we later discovered was the reach of an even larger prehistoric eruption. These sketched maps were shown, copied and distributed at all briefings after May 23 and also reproduced in one or more national newspapers.

In retrospect, we can see that our hazard map —while useful—was difficult for some audiences to understand. Many otherwise well-educated people are unable to read maps, and maps in standard plan view were an even more difficult

abstraction for those less educated. Haynes et al. (2007) and Leone and Lesales (2009) offer excellent suggestions on how to make hazard maps more readily understandable, e.g., by use of 3D visualization.

## 4.4 A Probability Tree

Newhall and Hoblitt (2002) and Newhall and Pallister (2015) describe relatively simple ways to estimate probabilities of volcanic events and their consequences, and thereby help officials and those at risk to decide on what risks to take. At Mount St. Helens, calculations were carried all the way to annual risk of death, which allowed loggers and others to compare volcanic risk to more familiar occupational and lifestyle risks.

The first probability tree at Pinatubo (May 17) considered just one scenario, of pyroclastic flows to the East and onto Clark Air Base. We carefully did not exaggerate any hazard or risk, but we wanted to make sure that officials understood that hazards and risk were, in fact, unacceptably high. Our estimate of a 3% chance of pyroclastic flows reaching Clark and killing thousands within the coming months was immediately understood by US Air Force officers to be unacceptable. The Director of the national Office of Civil Defense, Engr. Dejoras, saw similarly unacceptable risk for the civilian population all around Pinatubo. Social science research has reaffirmed the pitfalls of using ambiguous adjectives like "high" or "low" to describe hazard and risk, and the advantages of quantifying those hazards (Doyle et al. 2014 and references therein). Once quantified, volcanic risk can be compared to more familiar risks and to levels of risk judged acceptable under the circumstances.

Public discussion of the probability tree at Pinatubo might have helped in our general education campaign, but probability is a difficult concept for many non-technical persons so for want of time we discussed it only with those who already understood probabilities. When we did discuss probabilities, we usually spoke in terms of "percent chance" or "odds" rather than strict decimal probability numbers, as "odds" and
frequency expressions are more easily under-
stood (Gigerenzer and Edwards 2003; Leclerc
and Joslyn 2012; Henrich et al. 2015). We did
not carry estimation of hazard all the way on to
estimation of risk, so there was no quantification
of how much risk could be reduced by various
mitigation options. Pyroclastic flows are so lethal
that vulnerabilities would have been nearly 100%
and without evacuation, exposure would have
been 100% too. Discussion moved quickly to
plans for evacuations should an eruption become
imminent.

We did prepare one update to this tree, on
June 10, but by that time evacuation of Clark Air
Base and nearby areas was already well under-
way and the new probabilities—much higher of
course—were effectively moot.

In neither tree did we estimate uncertainty of
our probability estimates, but in briefings about
the first tree, we did indicate that uncertainty was
at least plus or minus one order of magnitude.
Simply using probabilities already indicated
uncertainty about what would transpire. Indeed,
in every episode of volcanic unrest there is a
range of possible outcomes, including the null
event of “no eruption.” Use of probabilities,
frequencies, or odds (especially, when combined
with alert levels) allows scientists to bypass the
as-yet unreachable goal of making very specific
deterministic predictions of what will occur. Yes,
officials and the news media will ask for such
predictions, but we believe that it is scientifically
more correct and educationally more useful to
indicate the range of possible scenarios and to
discuss how the probabilities of each scenario
can be estimated and can change. Interestingly,
recent research by Leclerc and Joslyn (2015)
found that including probabilities in frost fore-
casts increased recipients’ willingness to accept
false alarms.

4.5 Recommendations for Evacuation

In the Philippines, PHIVOLCS typically recom-
mends that a certain radius around a volcano be
declared a “permanent danger zone” and that
residence in this zone be forbidden. Outside that

zone, restrictions or measures such as evacua-
tions are at the discretion of local government
officials. As Pinatubo had not erupted in histor-
ical time, no permanent danger zone had been
designated, nor had there been any resolutions
within provincial or municipal governments
about when and where evacuations might be
ordered. The Director of PHIVOLCS, with
blessing from the national Office of Civil
Defense, took on the responsibility for recom-
mending radii of evacuation, and the Philippine
Army together with the Departments of Public
Works and Social Welfare and Development
implemented those recommendations. Although
an early, limited evacuation (in early April)
proved to be porous, later evacuations in June
had the advantage of visible eruptions and were
relatively smooth and effective.

Although the boundaries of hazard zones are
naturally irregular, based on topography, Direc-
tor Punongbayan and Director Dejoras decided
that recommended evacuations would be circles
with radii of 10 and 20 km (later, including 30
and 40 km). Circles drawn around the outer
boundaries of the mapped hazard inevitably
include some areas of relative high ground and
safety, but it was judged to be simpler and more
effective to base evacuations on simple circles
rather than on the boundaries of hazard maps that
some might not understand. Details are given in
Tayag et al. (1996).

4.6 Personal Communications

Most of our communications with officials were
of the formal types listed above—briefings, alert
levels, hazard map, a probability tree, and rec-
ommended radii of evacuation. However, we
found that informal, personal communications
were sometimes just as effective as the formal
ones, if not more effective. We already men-
tioned the gut-level emotional impact elicited by
film clips of the young girl trapped and shivering
in lahar deposit in Armero, Colombia.

A different and very effective form of personal
communication was movement of scientists
themselves to safer, fall-back positions—in the
northwest, from Sitios (hamlets) of Tarao and Yamot to Barangay Poonbato, and on the east, from the center of Clark Air Base to the far eastern edge of the air base. The move was for our own safety, but we realized as we planned it that our action would be a strong message for others. As we hoped, local officials and residents took note and took our warnings more seriously thereafter.

Yet another form of personal communication was developing simple friendships and trust with those at risk. Several members of the PHI-VOLCS team on the northwest side of Pinatubo, led by Julio Sabit, developed quick rapport with local residents. Eating together, drinking together, and sharing family histories and aspirations are wonderful ways to build the trust that becomes so essential when urgent warnings must be issued. A similar experience on the east side was generated accidentally on May 18. The scientific team was exhausted and stressed, so we called a time-out for a BBQ, inviting Air Force officers to join. Over hot dogs and beer, the officers discovered to their surprise that we scientists were just normal people, with families of our own just like them. Probably, scientists had the same revelation about the military officers. Up until that time we had regarded each other with some puzzlement and caution; after the BBQ and beer, things lightened up and more trust was evident.

## 5 Post-eruption Lahar Messages

As soon as the climactic eruption occurred, it was obvious to scientists that the big threat in coming years would be from rain-induced lahars (Janda et al. 1996). We didn't know exactly what percentage of the fresh deposit would be washed into the lowlands in lahars (and in normal muddy streamflow)—estimates ranged from around 15% to around 50%. Even 15% of the new deposit would be enormous and would more than fill river channels; 50% would bury huge areas of farmland and towns under several meters of sediment. Even on the back of an envelope,

scientists could see that these lahars would be bigger than anything they had ever seen or imagined. In contrast, most people at risk in the lowlands were blissfully oblivious to the lahar threat, grateful that they had survived the eruption. Even engineers and officials had a hard time envisioning the scale of the impending lahars.

Our messages for long-range and short-range lahar hazard included:

* There is an enormous amount of loose sand and pumice on the volcano that will be carried into the lowlands in coming months and years. (unquantified, no maps yet)

* Depending on assumptions, large areas including many towns may be buried, and here (on hazard maps) are the areas at high and lesser risk.

* If you build dikes to contain the sediment, most of these will fill and can breach if you don't build them big enough and strong enough.

* Some towns are not going to survive unscathed and may need to be sacrificed. People from those towns will need to be resettled in other places for the foreseeable future.

* Immediate warnings of lahars, e.g., "A (small/large) lahar has formed in the Sacobia River and will reach populated areas by (specified time)".

* Because lahars can go overbank and even breach protective dikes, populations still remaining at risk from these lahars should evacuate immediately.

In the late 1990s and in 2000–2001, two more messages were added:

* As the caldera lake fills, it will eventually overtop and may pose a severe lahar threat to Botolan (1998–2001).

* The threat of a breakout lahar still remains, because we didn't succeed to induce a rapid scouring/ breach of the loose material that forms a dam at the head of the Maraunot/Balin Baquero/Bucao river system (late 2001).
# 6 How Were Lahar Warnings Prepared and Presented?

## 6.1 Briefings and Video

Briefings followed more or less the same pattern as during pre-eruption time, though more were held at the regional level than before the eruption. The Regional Disaster Coordinating Council (RDCC 3) assumed a greater role for lahars than it had before the eruption. More players were involved as well—with notably increased involvement by the Department of Public Works and Highways (DPWH); the Department of Social Welfare and Development (DSWD) (for emergency relief and more permanent resettlement of those displaced by lahars), and the new Mount Pinatubo Commission (a mechanism to coordinate funding and response, but relying heavily on DPWH, DSWD, and their contractors).

The Krafft video was still used occasionally, but was increasingly supplanted by live and taped coverage of actual Pinatubo lahars, beamed over broadcast TV. Scientists and at least two television networks also prepared their own video documentaries on Pinatubo lahars.

## 6.2 Hazard Maps

Everyone wanted to know if his or her town would be hit by lahars. Engineers and planners also needed to know the likely volumes of sediment that would move into the lowlands, for planning engineering structures and for debating the relative merits of trying to control the sediment versus simply relocating communities and letting the sediment flow.

The earliest hazard maps were prepared by PHIVOLCS (Punongbayan et al. 1991) by the Pinatubo Lahar Hazards Taskforce (PLHT, a cooperative effort of the Mines and Geosciences Bureau, Univ. of the Philippines, Univ. of Illinois at Chicago, and PHIVOLCS) (1991a, b), and by the Bureau of Soils and Water Management (1991). Revised maps were prepared by Pierson et al. (1992), PHIVOLCS (1992, 1994); and the Zambales Lahar Scientific Monitoring Group

(ZLSMG) (1993, 1994). ZLSMG was the successor to PLHT, comprised of university-based scientists including some PHIVOLCS scientists on study leave. By mutual agreement with PHIVOLCS, the ZLSMG handled most of the lahar study on the west side of Pinatubo (1992, 1994).

Although there were minor differences between the various maps, most of the maps were broadly similar and confusion between maps seemed not to be a serious problem. Greater confusion may have arisen over the use of lines rather than gradations. Lines are satisfying, but they inevitably give a sense of more certainty than actually exists. Lahar hazard in lowland areas is gradational, without sharp boundaries.

Later, PHIVOLCS was given an additional duty of certifying whether specific land parcels (e.g., those for new construction and bank loans) were “safe” or “unsafe” from lahar. In general, that meant simply locating the parcel of land on the published hazard map and certifying that it was inside or outside hazard zones. Though unstated, delineation of “safe” zones implies a choice of how low a hazard must be in order to call the zone safe. Everything was changing too fast for us to identify an “X-year floodplain” (one event in X years) but qualitatively, areas outside the hazard zones were judged to have “very or extremely low” probability of being inundated.

In general, the hazard maps served dual purposes of letting communities know their (qualitative) chances of being buried in the coming years, and letting engineers and planners design appropriate responses. To be sure, some mitigation measures were technically inadequate or even foolish (e.g., construction in 1992 of a new school in Sta. Barbara, Bacolor, which would soon be buried by more lahars), but these instances were not for want of good scientific information. More likely, they were driven by inattention, bureaucracy, politics, or profit.

## 6.3 Short-Term, Immediate Lahar Warnings

Four systems were used for lahar warnings. One, installed by DPWH with information fed by
radio telemetry directly to the RDCC at Camp Olivas, San Fernando, used trip wires and rain gauges, placed near the foot of a pyroclastic fan and the head of the corresponding alluvial apron. They didn't last long, with the trip wires almost immediately tripped or stolen. A second, installed by PHIVOLCS and the USGS, used rain gauges high in the watershed and acoustic flow sensors (inexpensive, high-frequency exploration seismometers) lower on the pyroclastic fans but still above the alluvial fans. Data were telemeasured to PVO where they were interpreted 24/7, and warnings were relayed to RDCC by telephone. Sometimes, PHIVOLCS observers would also report from watchpoints but these were not an essential part of PHIVOLCS' warnings. A third was direct observations of lahars by scientists from the ZLSMG, from watchpoints they manned in Dalanaoan, San Marcelino, and Malumboy, Botolan. Dalanaoan was midway down the alluvial fan of the Marella/Sto. Tomas River but still upstream from populated areas; Malumboy was well down the Bucao River but still 11 km upstream from Botolan. Results were sent in real-time to authorities. The fourth system also used direct observations of lahars, by policemen posted near the heads of several alluvial fans who then radioed reports to the RDCC.

Of the four systems, the PHIVOLCS and ZLSMG systems were scientifically superior, giving early and semi-quantitative information about both the scale and the travel speed of lahars, and the benefit of scientific interpretation. In retrospect, the PHIVOLCS system would have been better accepted if one of its scientists with good communication skills had spent more time at the RDCC. The police reports were less accurate than those from scientists but had the advantage of being simple and from familiar sources.

Because everyday thundershowers at Pinatubo are very localized, and neither the national meteorological service (PAGASA) nor the military had modern Doppler radar, meteorologists did not play as great a role at Pinatubo as they would today. However, they did make major contributions by warning of incoming typhoons

that invariably generated lahars across the entire volcano.

In general, short-term lahar warnings did reach those at risk and saved many hundreds or even thousands of lives. Regrettably, they didn't manage to save all. We recall one instance in which PHIVOLCS had strong evidence for a lahar headed for Dolores, Mabalacat, and relayed its warning, but local officials deferred to the police system which did not sound the same alarm. Approximately 100 died as a result. In another instance, all systems warned of a major lahar headed for Bacolor town (by then, largely evacuated) and its satellite barangay, Cabalantian (not evacuated). Unfortunately, a large dike gave a false sense of security until it breached, and as many as 400 of those who did not evacuate perished.

## 6.4 Probabilities of Lahars

No effort was made to estimate either long-term or short-term probabilities of lahars. Most of the effort toward quantification was focused on estimating rates of sediment transport, both in lahars and in normal muddy streamflow.

## 6.5 Personal Factors

Ironically, the extra time afforded by lahars (relative to the pre-eruption period) created extra difficulties in communication. There were more end-users to be informed, and more scientists providing the information. In addition to the main players mentioned above, there were also engineering consultants from many countries. Scientists operating on limited (sometimes, shoestring) budgets anticipated the magnitude of the lahar hazard while engineers turned that information into lucrative contracts for sediment control and reconstruction.

There were, we must admit, some unfortunate clashes between scientists which contributed to the loss of scientific credibility. One area of seeming disagreement was on the efficacy of engineering works. Without going into more
details, suffice it to say that both real and imagined differences got translated, on front pages and lead stories, into a personalized competition of scientific expertise. Scientists' credibility would have been higher, and the public served better, had these differences been resolved behind closed doors, and officials and the public been given consensus statements.

## 7 Lessons to Remember

Pinatubo was a pressure-cooker for scientists, demanding warnings well before full data sets could be collected. Many times, we had to suppress our scientific instinct to say, “Wait, let me collect more data.” Information had to be given immediately. Timely communication of often-uncertain hazards information, pre- and post-eruption, saved thousands, perhaps even tens of thousands of lives.

Below, we list some lessons that we ourselves learned, and that we commend to readers:

* Start your communications immediately, preferably long before unrest begins. If that is not possible, then begin teaching about the volcano at least start as soon as unrest is noticed. Do NOT wait until you are sure about what the volcano will produce. Communicating well with all parties takes time, and volcanic crises can develop so quickly that there might not be enough time for communications if you wait until you know more.

* Make a checklist of what data must be gathered and analysed, and what information must be communicated to whom. It would be easy, in the rush and stress of unrest, to forget one thing or another. Do not think of this as an affront to your professional experience; rather, think of it as a pilot thinks of his or her checklist—as an extra safety measure.

* Even if the most immediate pre-eruption concern is for pyroclastic flows, don't forget to warn thoroughly of ash, in the air and on roofs. It can be life-threatening, even far from the volcano.

* Expect scepticism, especially where a volcano has long been dormant. This will make your communication job more difficult, but also more essential. Consider using a variety of approaches and tools (e.g., briefings, video, hazard maps, alert levels, probability trees, and personal touches) to reach both sceptics and converts. Different tools will be required for different audiences. Video is especially effective.

* Help officials to understand that every volcanic crisis has several possible outcomes, and that their relative likelihoods may change with time as new geologic and monitoring data are collected. Response plans should be flexible enough to account for several different scenarios.

* Don't be afraid to give warnings based on your best current data, even if you know those data are woefully inadequate. Some will argue that giving interpretations and warnings based on incomplete data may be worse than no warnings at all. We respectfully disagree, as we think it is scientists' societal duty to give the best warnings possible AT ALL STAGES of a crisis, even early on while uncertainties remain high. If Director Punongbayan had not been willing to risk his reputation and perhaps even his job to give warnings when he did, even though uncertainties remained high, we doubt that the evacuations would have been as successful as they were.

* Don't be afraid to include a “worst case scenario” among various scenarios, and put it in context by estimating relative probabilities of various scenarios. In the case of Pinatubo, we saw so much geologic evidence for VEI 6 eruptions and so little evidence for smaller eruptions that we put a high probability on the “worst-case” VEI 6 scenario.

* Similarly, don't be overly afraid of a false alarm. To be sure, officials and the public do have limited tolerance for false alarms (e.g., Atwood and Major 1998), but it is not zero, especially if you explain all of the possible scenarios and uncertainties. Use of alert levels and probability trees reduces the likelihood of
false alarms. Consider also a social contract with officials and those at risk: If they want to be sure of warning, they must accept some risk of a false alarm.

* After an eruption that produces a large volume of ash and other pyroclastic debris, expect lahars over an extended period. With more time and players, communications will get more complicated, and may require new, proactive communication strategies to keep scientific advice in the forefront of further planning.

* Both before and after an eruption, strive for scientific coordination of all messages BEFORE they go to officials and the public.

* Recognize the importance of trust and personal connections between scientists, officials, and the news media. Sometimes, these are as important or more important than the formal warnings.

We close this chapter, proud of the successes but also conscious of the near-misses and failures. Communication of hazards information at Pinatubo was complicated, but also absolutely essential to mitigation of what would have otherwise been a much worse disaster.

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Pinatubo Lahar Hazards Taskforce (PLHT; compilers,
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the Bucao River system, Pinatubo Volcano, Luzon,
Philippines (as of October 11, 1991). Scale 1:50,000

Pinatubo Lahar Hazards Taskforce (PLHT; compilers,
Umbal JV, Rodolfo KS) (1991b) Lahar hazard map of
the Sto. Tomas River system, Pinatubo Volcano,
Luzon, Philippines (as of August 23, 1991). Scale
1:50,000

Punongbayan RS, Besana GM, Daligdig JA, Torres RC, Daag AS, Rimando RE (1991) Mudflow Hazard Map, as of 01 October 1991. PHIVOLCS, printed and published by the National Mapping and Resource Inventory Authority, NAMRIA. Scale 1:100,000

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Zambales Lahar Scientific Monitoring Group (ZLSMG; compilers Alonso RA, Rodolfo KS, Remotigue CT) (1993) Lahar Hazard Map of the Santo Tomas-Pamatawan Plain, Southern Zambales Province, as of September 1, 1993). Scale 1:50,000

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# Instrumental Volcano Surveillance and Community Awareness in the Lead-Up to the 1994 Eruptions at Rabaul, Papua New Guinea

Chris McKee, Ima Itikarai and Hugh Davies

## Abstract

Instrumental volcano surveillance and community awareness played key roles in preparing for the outbreak of the 1994 VEI 4 volcanic eruptions at Rabaul (pop. 17,000). The eruptions were preceded by 23 years of fluctuating unrest involving swarms of caldera earthquakes (max $M_L$ 5.2) and co-seismic uplift of parts of the floor of Rabaul Caldera. Eruption contingency planning was formally driven by government authorities and involved all sections of the community. Community awareness of the volcanic threat was enhanced by the dissemination of relevant information by the Public Information Unit of the East New Britain Provincial Government and reached a peak in the mid-1980s at the time of a large increase in the strength and frequency of earthquake activity (between August 1983 and July 1985). However, the intensity of the unrest declined after July 1985 and another 9 years elapsed before a new and dramatically stronger phase of unrest took place. The strong and sustained earthquake activity on 18 September 1994, together with marked co-seismic uplift that took place that night, was the final episode of volcanic unrest prior to the outbreak of eruptions on the morning of 19 September 1994. Memories and stories of the seismic prelude to the previous eruptions, in 1937, are reported to have been a major influence on community response

C. McKee (✉)
Port Moresby Geophysical Observatory,
P.O. Box 323, Port Moresby, NCD,
Papua New Guinea
e-mail: chris_mckee@mineral.gov.pg

I. Itikarai
Rabaul Volcanological Observatory, P.O. Box 386,
Rabaul, ENBP, Papua New Guinea

H. Davies
Earth Sciences, University of Papua New Guinea,
P.O. Box 414 University NCD, Port Moresby, Papua
New Guinea
to the seismicity on 18 September 1994. The evacuation of all areas within the caldera proceeded efficiently from late afternoon of 18 September until the early hours of 19 September. These areas were almost deserted when the eruptions started at two vents, Tavurvur and Vulcan, on opposite sides of the caldera at 0606 and 0717 LT respectively on the morning of 19 September 1994. Ten deaths in the first six weeks of eruptive activity were volcano-related. Damage inflicted by the eruptions was severe. About 70% of Rabaul Town was destroyed by tephra fall from Tavurvur, and several villages were obliterated by pyroclastic flows and heavy tephra fall from Vulcan. The 23 years of precursory activity and the events around the start of the 1994 eruptions delivered a number of important lessons in the fields of volcano surveillance, communications and disaster management. Perhaps the most important lessons of all are that co-existence with active and potentially active volcanoes requires (i) open and effective lines of communication between volcano scientists, government officials, town authorities and the general public, facilitated by designated public information officers, and (ii) the establishment and frequent exercising of eruption contingency plans.

# 1 Introduction

Rabaul is the name of both a complex volcanic system and a town at the northeastern tip of the Gazelle Peninsula, New Britain Island, Papua New Guinea (Fig. 1). Since the latest caldera-forming eruption at Rabaul, at about 1400 BP (Heming 1974; Walker et al. 1981), small cones have grown at five vent areas within Rabaul Caldera. The recorded eruption history of the Rabaul system spans a period of about 250 years and includes six eruptive episodes that took place at intervals of 24–59 years: 1767, 1791, 1850s, 1878, 1937 and 1994 (McKee et al. 2016). The most active vents during the historical period were those in the Vulcan area and at Tavurvur, on the western and eastern sides of Rabaul Caldera respectively (Fig. 1). These centres were simultaneously active in 1878, 1937 and 1994.

The town of Rabaul is one of very few towns or cities worldwide that has been built inside the caldera of an active volcanic system. The town was established within Rabaul Caldera by the German Administration of New Guinea in the period 1904–1910 (Johnson and Threlfall 1985), about 30 years after the 1878 eruptions (Brown 1878;

Johnson et al. 1981). The early German capital had been at Kokopo about 20 km southeast of Rabaul (Fig. 1), outside the caldera, but the seat of government was shifted to Rabaul in 1910. The attraction of the new site was the sheltered deep-water harbour. The tenure of Rabaul as the capital of German New Guinea was short-lived. In 1914, at the beginning of the First World War, Germany lost its New Guinea territory to an invasion of Australian forces. After World War I, administration of Rabaul, and indeed of New Guinea, became the responsibility of Australia.

The disastrous eruptions of May 1937, during which more than 500 lives were lost (Fisher 1939; Johnson and Threlfall 1985), led to broad-ranging investigations by the Australian Administration concerning the suitability of Rabaul as a capital city. There was disagreement about the future of the town. However, it was proposed that Rabaul could remain as the capital provided that a volcanological observatory was established to monitor the volcanoes and to warn of future eruptions. Routine volcanological observations at Rabaul commenced in 1938, and in 1940 the first Rabaul Volcanological Observatory (RVO) was established, at a site on the
<figure><img src="image_73.png" /><figcaption>**Fig. 1** The northeastern tip of the Gazelle Peninsula, New Britain Island, showing the major stratovolcanoes, post-1400 BP vents within Rabaul Caldera, the towns Rabaul and Kokopo, RVO, villages mentioned in the text, emergency command posts, hospitals, airports and the emergency assembly area, Queen Elizabeth Park (QEP).</figcaption></figure>

Surveying benchmarks on the northwestern caldera rim (BM 21) and at Matupit Island mark the ends of the main levelling line. Inset map shows the islands of New Britain and New Ireland, and the capital city Port Moresby on mainland Papua New Guinea

northern rim of the caldera. The same site is occupied by the current RVO (Fig. 1). Following a resurgence of eruptive activity in June 1941 (Fisher 1976), a decision was made by the Administration in September 1941 to shift the capital to Lae on the New Guinea mainland, thus ending a period of 31 years in which Rabaul was a colonial capital.

Apart from suffering the effects of volcanic eruptions in the period 1937–1941, the town was totally destroyed by bombing during World War II, and substained about 70% destruction by volcanic activity in 1994 (Blong and McKee 1995; Davies 1995). Intermittent eruptions between 1994 and 2014 occasionally made life at Rabaul unpleasant.

Before the outbreak of the 1994 eruptions, the population of the town was about 17,000. A much larger population of about 100,000 lived in the satellite towns, villages and plantations of the Rabaul-Kokopo area. The urban and, to a lesser extent, plantation communities contained a mixture of short- and long-term residents, while residents of the villages could be regarded as constituting traditional communities. Apart from Rabaul Town the most-threatened population centres within the caldera are at the villages Tavana (near Vulcan), Talwat (near Tavurvur) and Matupit Island (between Vulcan and Tavurvur), as shown in Fig. 1. These villages, and indeed all areas within Rabaul Caldera, are frighteningly close to sources of volcanic threat.
The frequency of eruptions at Rabaul in the historical period, about 2 per century, has con- ditioned the traditional communities to the local volcanic threats. Local knowledge, particularly from the experience of the 1937 eruptions, cou- pled with the results of instrumental volcano monitoring and the long period of precursory activity (starting in 1971) resulted in a high level of volcano awareness prior to the 1994 eruptive outbreak. This awareness was an important ele- ment in the response to the volcanic unrest that preceded the 1994 eruptions.

The eruptive period that started at Rabaul in 1994 is important both as:

(i) a destructive event at a densely populated caldera volcano with a recent history of eruptions and caldera unrest—a case com- monly identified as being a particular chal- lenge for both monitoring and for warning communication, as in the context of Campi Flegrei, Italy, and Masaya, Nicaragua (e.g. Newhall and Dzurisin 1988).

(ii) an unusual case in which self-warning and evacuation of communities occurred in parallel with (and in some instances, in advance of) the formal volcano alert levels.

## 2 Precursory Activity (I): 1971–1985

### 2.1 The Nature of Volcanic Unrest at Rabaul—Characteristics of Rabaul Volcanic Crises

Historical records suggest that unrest between volcanic eruptions at Rabaul may be common, manifest as ground deformation and earthquakes. Uplift was reported to have occurred in the Sul- phur Creek to Matupit Island area (see Fig. 1) in the lead-up to an eruption at Sulphur Creek in about 1850 (Brown 1878; Boegershauser 1937; Fisher 1939). Immediately prior to the 1878 eruptions that created Vulcan Island and formed a new crater at Tavurvur, small rocky islets near the site of the Vulcan Island eruption were raised

about 1 m and massive uplift of ≈6 m occurred along the foreshore at the southern foot of Tavurvur (Brown 1878; Johnson et al. 1981). Frequent local earthquakes preceded the erup- tions, and tsunami activity was reported but its timing is unclear (Brown 1878; Fisher 1939). An account of earthquake activity at Matupit Island in 1891 by Edward Hernsheim (Sack and Clark 1983) refers to continual earth tremors and sub- terranean rumblings heard by residents of the island (including Hernsheim) that became so pronounced towards the end of 1891 that an eruption was considered imminent and partial evacuation of the island took place. Another phase of unrest, involving strong local seismicity and marked ground deformation in the Matupit Island area, occurred between 1916 and 1919 (Fisher 1939). The next eruptive activity even- tuated in 1937, 46 years after Hernsheim’s report and 21 years after the start of the 1916–1919 unrest, but not before a brief period of intense unrest, involving strong and sustained seismicity and emergence of near-shore areas, that com- menced 27 h before the onset of the eruptions at Vulcan and Tavurvur (Fisher 1939; Johnson and Threlfall 1985).

Seismic unrest within the Rabaul volcanic system primarily takes the form of volcano-tectonic (VT) earthquakes. VT earth- quakes are high frequency events similar to tec- tonic earthquakes but are associated with a volcano and are therefore assumed to represent failure (fault movements) resulting from strain induced by volcanic processes (Lahr et al. 1994; Chouet et al. 1994). The VT earthquakes at Rabaul occur as discrete events and in clusters or swarms. The largest non-eruptive unrest events are termed “volcanic crises”.

Volcanic crises at Rabaul are characterized by swarms of ≥ 150 to thousands of VT earth- quakes that occur within periods of a few hours (Mori et al. 1986, 1989). The threshold of 150 was based, somewhat arbitrarily, on the experi- ence of the 1983–1985 “Crisis Period” (see below). The strongest earthquakes usually take place at or near the beginning of a crisis. The largest recorded event in any crisis was a M$_L$ 5.2
<figure><img src="image_74.png" /><figcaption>**Fig. 2** Regions within the Rabaul Caldera seismic zone. BH Beehives; GH Greet Harbour; V Vulcan; BB Blanche Bay; KB Karavia Bay</figcaption></figure>

earthquake in October 1980. Seismicity of individual crises is usually confined to specific regions within the caldera seismic zone (Fig. 2), which is believed to include the fault system that bounds the youngest caldera (Mori et al. 1986, 1989). Caldera earthquakes take place at depths of 0–5 km, although most events are shallow, 1–2 km. In contrast, tectonic earthquakes of the northern Gazelle Peninsula region have a depth range of between about 10 and 280 km (Ripper et al. 1996; Ghasemi et al. 2016).

The ground deformation that takes place during volcanic crises is typically, but not always, inflationary, involving shallow-focussed uplift and tilting (McKee et al. 1984, 1989). The deformation occurs rapidly in direct association with the strongest earthquakes. Measured uplift at Matupit Island relative to a bench mark on the northwestern rim of the caldera (BM21, see

Fig. 1) has amounted to as much as 100 mm for an individual crisis. However, the greatest deformation is usually off-shore and is not readily measurable (Greene et al. 1986; McKee et al. 1989). The focus of the ground deformation is usually in the central part of the caldera, within the region bounded by the zone of caldera seismicity. However, the deformation may extend beyond the zone of seismicity.

## 2.2 Volcanic Crises in the Period 1971–1985

Following the eruptions of 1937–1943, volcanic unrest at Rabaul was at low levels until 1971. As far as is known, seismicity within the caldera from 1943 to 1971 was weak and consisted of occasional, isolated VT earthquakes of relatively small magnitude. Rates of ground deformation were also low as indicated by minimal elevation changes of a benchmark installed at Matupit Island in 1949 (Lauer, unpublished data). This benchmark is different to the one occupied in levelling surveys starting in 1973.

### 1. 1971–1983

The relative calm of the period following the eruptions of 1937–1943 was disturbed in November 1971 by the first recorded swarm of caldera earthquakes (Cooke 1977). Compared with many of the seismic swarms and crises in the following two decades, the crisis of November 1971 was unremarkable. During the period November 1971–July 1983, there was a trend of increasing numbers of earthquakes in the seismic swarms and crises, from <200 in November 1971 to ≈1200 in January 1982 (Fig. 3). Earthquake magnitudes also increased —the strongest earthquake was a $M_L$ 5.2 event in the swarm of October 1980, and a slightly weaker event, $M_L$ 5.1, was recorded in the swarm of March 1982 (Table 1). $M_L$ determinations were made at RVO using an electronically-simulated Wood-Anderson seismograph. For
<figure><img src="image_75.png" /><figcaption>**Fig. 3** Monthly caldera earthquake totals 1968–1985 and levelling results 1973–1985. Note increased frequency of levelling surveys starting from late-1983, during the early part of the Crisis Period</figcaption></figure>

comparison the $M_L$ 5.2 event of 28 October 1980 was assigned $M_L$ 5.0 by Port Moresby Geo-physical Observatory and $m_b$ 4.9 by the National Earthquake Information Centre's PDE catalogue.

The spatial distribution of seismicity at Rabaul was discerned for the first time during this period due to improvements to both the local seismic network and to earthquake location techniques. The pattern of surface-projected seismicity was first represented as a “D”-shaped zone (Cooke 1977), and subsequently as two inward-facing arcuate zones, on the eastern and western sides of the caldera (Almond and McKee 1982). Later analysis suggested that the seismicity was related to a ring fault system (Mori and McKee 1987; Itikarai 2008) elongated north–south. The distribution of caldera seismicity for this period (1971–1983) is shown in Fig. 4.

Ground deformation monitoring during this period comprised levelling surveys, tiltmeter and dry tilt measurements, and strandline measurements. The results of these surveys indicated a generally steady rate of uplift of about

110 mm/year in the central part of the caldera (Fig. 3). Gravity measurements reflected this uplift (McKee et al. 1989).

## 2. 1983–85: Crisis Period

The period September 1983–July 1985 was one of sustained intensified activity at Rabaul. At the time it was generally considered that the increased activity could be the prelude to an eruption (McKee et al. 1984). The period was characterized by frequent major crises (many hundreds of VT earthquakes) and a large number of minor crises (a few hundred VT earthquakes), and therefore was termed the “Crisis Period” by Mori et al. (1986). This period was immediately heralded by a generally steady build-up in background seismicity from a “normal” level of 5–10 caldera earthquakes per day in August 1983 to 50–80 per day before the first crisis in September 1983 (Mori et al. 1986).

The first crisis of this period took place on 19 September 1983, in the eastern part of the caldera seismic zone (Blanche Bay, Fig. 2) and included
<table><caption>**Table 1** Seismic crises: October 1980–May 1985 (adapted from Mori et al. 1989)</caption><thead><tr><th>Date</th><th>Location</th><th>No. Events<br>(4 or more stations)</th><th>Largest<br>event (M<sub>L</sub>)</th></tr></thead><tbody><tr><td><strong>28 Oct. 80</strong></td><td><strong>Vulcan</strong></td><td><strong>__*<sup>1</sup></strong></td><td><strong>5.2</strong></td></tr><tr><td><strong>4 Mar. 82</strong></td><td><strong>Blanche Bay</strong></td><td><strong>__*<sup>1</sup></strong></td><td><strong>5.1</strong></td></tr><tr><td>19 Sep. 83</td><td>Blanche Bay</td><td>210</td><td>4.2</td></tr><tr><td>15 Oct. 83</td><td>Greet Harbour/Beehives</td><td>334</td><td>3.6</td></tr><tr><td>28 Oct. 83</td><td>Greet Harbour</td><td>195</td><td>4.0</td></tr><tr><td>26 Nov. 83</td><td>Beehives/Vulcan</td><td>212</td><td>2.9</td></tr><tr><td>15 Jan. 84</td><td>Greet Harbour/Beehives</td><td>342</td><td>4.6</td></tr><tr><td>13 Feb. 84</td><td>Greet Harbour</td><td>164</td><td>2.6</td></tr><tr><td>18 Feb. 84</td><td>Karavia Bay/Vulcan</td><td>212</td><td>4.3</td></tr><tr><td>27 Feb. 84</td><td>Blanche Bay</td><td>159</td><td>3.0</td></tr><tr><td><strong>3 Mar. 84</strong></td><td><strong>Blanche Bay</strong></td><td><strong>484</strong></td><td><strong>5.1</strong></td></tr><tr><td>17 Mar. 84</td><td>Vulcan</td><td>219</td><td>2.1</td></tr><tr><td>25 Mar. 84</td><td>Beehives/Vulcan</td><td>398</td><td>3.5</td></tr><tr><td>11 Apr. 84</td><td>Greet Harbour</td><td>217</td><td>2.0</td></tr><tr><td>13 Apr. 84</td><td>Greet Harbour</td><td>212</td><td>2.2</td></tr><tr><td>20 Apr. 84</td><td>Greet Harbour</td><td>302</td><td>3.0</td></tr><tr><td>21 Apr. 84</td><td>Greet Harbour</td><td>287*<sup>2</sup></td><td>3.7</td></tr><tr><td><strong>22 Apr. 84</strong></td><td><strong>Greet Harbour</strong></td><td><strong>485</strong></td><td><strong>4.8</strong></td></tr><tr><td>4 May 84</td><td>Beehives/Vulcan</td><td>168</td><td>3.5</td></tr><tr><td>29 May 84</td><td>Beehives/Vulcan</td><td>323</td><td>3.6</td></tr><tr><td>13 Jul. 84</td><td>Vulcan</td><td>151</td><td>3.5</td></tr><tr><td>2 Aug. 84</td><td>Beehives</td><td>242</td><td>3.4</td></tr><tr><td><strong>17 Oct. 84</strong></td><td><strong>Greet Harbour</strong></td><td><strong>233*<sup>2</sup></strong></td><td><strong>4.9</strong></td></tr><tr><td>18 Oct. 84</td><td>Blanche Bay</td><td>158</td><td>3.8</td></tr><tr><td>26 Oct. 84</td><td>Vulcan</td><td>209</td><td>3.2</td></tr><tr><td>3 Mar. 85</td><td>Beehives</td><td>333</td><td>3.6</td></tr><tr><td>10 May 85</td><td>Vulcan</td><td>__*<sup>3</sup></td><td>3.7</td></tr></tbody></table>

Bold signifies stronger activity

M<sub>L</sub> determined at RVO using electronically-simulated Wood-Anderson seismograph

The event counts are for a 24 h period following the start of the crisis

*<sup>1</sup>Event count not available

a M<sub>L</sub> 4.2 earthquake. During the following few months the seismicity increased and by January 1984 the monthly total of caldera earthquakes had climbed to about 8300. In February and March 1984 the monthly total numbers of earthquakes remained about the same as in January. The crisis on 3 March 1984 was one of the strongest of the entire Crisis Period and included the largest

earthquake, a M<sub>L</sub> 5.1 event in the Greet Harbour area (Fig. 2). Re-occupation of the level line indicated that the southern tip of Matupit Island was rising at about 35 mm/month.

The activity reached a peak in April 1984 (Fig. 3). There were 5 crises in that month, all of which took place in the northeastern part of the caldera seismic zone (Greet Harbour, Fig. 2).
<caption>**Fig. 4** Caldera earthquake locations 01 November 1971–4.15 31 August 1983</caption>

<figure><img src="image_76.png" /></figure>

Crises took place on 3 consecutive days, 20th, 21st and 22nd of April, and the crisis on 22 April included a $M_L$ 4.8 earthquake. The total number of earthquakes in April 1984 was about 14,000. Ground deformation was greatest in the central-northeastern part of the caldera. As much as 80 mm of uplift was recorded.

From May 1984 the activity declined. However, crises continued to take place occasionally, including one on 17 October 1984 during which the second strongest earthquake of the Crisis Period, a $M_L$ 4.9 event, was recorded from the Greet Harbour area (Fig. 2). By the end of July 1985, seismic activity had returned to pre-Crisis levels. Also, there was a generally steady decline in the rate of uplift of the southern tip of Matupit Island, dropping to only a few mm/month by July 1985.

Altogether, there were 25 major crises during the period September 1983–May 1985 (Mori et al. 1986; Table 1). The intervals between the major crises ranged between 1 and 128 days. The seismicity shifted from one region to another of the caldera seismic zone during the Crisis Period but without showing any systematic patterns. The most active regions were those in the east to northeast (Blanche Bay, Greet Harbour; Fig. 2) and those in the west to northwest (Vulcan, Beehives; Fig. 2). Earthquake locations for the entire Crisis Period, September 1983–July 1985, are shown in Fig. 5. The greatest measured uplift for the 23 months of the Crisis Period was 767 mm, at a benchmark at the southeastern end of Matupit Island (McKee et al. 1989).
<caption>**Fig. 5** Caldera earthquake locations for the Crisis Period, 01 September 1983–31 July 1985</caption>

<figure><img src="image_77.png" /></figure>

# 3 Impact of Precursory Activity (I): Volcano Monitoring, Contingency Planning and Public Awareness, 1983–1985

## 3.1 Volcano Monitoring

Prior to the 1983–85 Crisis Period the volcano monitoring system at Rabaul was unable to provide the necessary monitoring information promptly. Seismic data acquisition relied on manual timing of earthquakes from a microfilm recorder (Develocorder) and off-site data processing using the computing services of an accounting company. Thus, the determination of earthquake locations could take hours to days. Ground deformation monitoring relied mostly on levelling which was out-sourced to surveying staff from Department of Lands. Surveys were

conducted at intervals ranging from 3 to 27 months.

Levelling surveys were more frequent from November 1983, at intervals of less than 1 month to about 3 months depending on events, and the surveys continued to be conducted by Lands Department staff. During a visit to Rabaul by Norman Banks of the United States Geological Survey (USGS) in December 1983, an electronic distance measuring (EDM) network was established and EDM monitoring commenced. In recognition of the need for a range of in-house ground deformation monitoring capabilities, a new position at RVO, Principal Surveyor, was created in 1985. The position was occupied from July 1985 and allowed RVO much greater control over the deformation monitoring program. With assistance from the Volcano Disaster Assistance Program (VDAP) of the USGS, telemetering electronic tiltmeters and tide gauges
were installed at Rabaul in the wake of the Crisis Period.

On-site seismic data processing commenced in 1985 following the recruitment of new seis- mological staff equipped with personal comput- ers. Earthquake location processing time was reduced to minutes, although the same microfilm recorder continued in service.

## 3.2 Contingency Planning

Concern about the growing volcanic unrest in the early 1980s prompted the development of a volcano contingency plan (VCP). Instigated by RVO, the first VCP for Rabaul was developed in early 1983 by provincial and national govern- ment authorities guided by UN-sponsored dis- aster planning expert, Brian Ward. The VCP was part of a Provincial Disaster Plan (PDP) for East New Britain, revised in 1985 by Captain Dwayne Hunt, RANR, and further revised in 1987 and in

early 1994 by the Provincial Disaster Committee (PDC). The PDP set out the organizational structure of the PDC which was to include Sub-Committees for Rescue, Transport and Requisition, Evacuation and Welfare, and Com- mand, Control and Communications. The com- position and responsibilities of each of the Sub-Committees were specified in the PDP. Volcanic hazards assessments (McKee 1981; McKee et al. 1985) were the basis for definition of areas of danger and of relative safety (Fig. 6), and for the development of a 4-stage alert system:

Stage 1—Eruption in years to months. Normal preparedness.

Stage 2—Eruption within months. Prepare safe areas.

Stage 3—Eruption within weeks or days. Vol- untary evacuation.

Stage 4—Eruption within days or hours. Evac- uation.

<figure><img src="image_78.png" /><figcaption>**Fig. 6** Part of a poster showing hazard/danger zones and (relatively) safe areas; developed in 1983–1984 during the early part of the Crisis Period</figcaption></figure>
The main objective of the VCP was the re-location of about 60,000 people from the danger areas to areas of relative safety. Duplication of principal services, including alternative airport, hospital and communications, would support the planned re-location. In considerable detail, the plan defined the roles of National and Provincial Disaster Organizations and the actions of 12 Government Departments and other organizations, including: Civil Aviation, Education, Electricity, Health, Harbours and Marine, Police, Communications, Works and Supply and Red Cross. It is remarkable that the major churches were not involved initially. Several exercises of the contingency plan were conducted over the following decade. Typically, the exercises concentrated on communications between government agencies, and did not involve businesses or the general community of Rabaul.

Wide acceptance that Rabaul was heading towards an eruption prompted the development of eight new Acts of disaster-related legislation, which were passed by the National Parliament in the first two weeks of March 1984 (Davies 1995). The eight Acts were:

### Disaster Management

- Emergency General Powers

- Emergency (Defence Force)

- Emergency (Register Evacuees)

- Emergency (Requisition)

- Emergency (Requisition Compensation)

- Emergency (Third Party)

- Emergency (Workers Compensation)

These actions demonstrated that concerns about an impending eruption at Rabaul had reached to the highest levels of public service in PNG.

Significant progress was made as a result of the Crisis Period in preparing for duplication of services and infrastructure in safe areas, notably:

1. Re-opening of a former airstrip (Vunakanau, a WW2 airstrip).

2. Roads to and water supplies in the safe areas were improved.

3. A safe area headquarters command post was established (at Vunadidir).

4. Equipment and medical supplies were moved from Nonga Hospital, near Rabaul, to Vunapope Hospital, near Kokopo.

5. Construction began on a wharf near Kokopo.

6. Construction started on a new airport (Tokua, a WW2 airstrip).

It was considered that logistical arrangements for RVO staff and civil defence officials must be established well in advance of any volcanic emergency for the following:

- Food

- Water

- Accommodation

- Care of families

- Transport

- Security

- Emergency electricity supplies

- Rostering of staff

- Protection of property

- Emergency communications

## 3.3 Establishment of Public Information Unit

The marked increase in volcanic unrest in August–September 1983 led to the establishment of a Public Information Unit (PIU) to disseminate information about the unrest. The declaration of Stage 2 volcanic alert in late-October 1983 accentuated the public need for volcano information, but because of increased workloads in the volcano monitoring program volcanologists were unable to maintain the flow of information to the public. Some public distrust and disquiet developed with fears that important information was being withheld. These fears were fanned by irresponsible and over-dramatic stories in the press, some of which stressed the possibility of a catastrophic eruption.

The PIU was established by the East New Britain Provincial Government in early-February
1984 (Davies 1995). At the suggestion of RVO the PDC requested the services of an Australian government geologist (Davies) to lead the unit. Davies had served in PNG for some years and was familiar with Rabaul, and was known to the RVO staff.

## 3.4 Three Initiatives of PIU

### 1. Make all information available to the public

As a first step towards confronting the problem of community distrust it was agreed among the volcanologists that all information should be made publicly available. Information released in the first week of March 1984 included plots showing the dramatic increases in seismic activity and in rates of ground deformation and explained the logic behind the declaration of a high-risk zone in the central-eastern part of the caldera, defined by ground deformation. A process was established for producing regular press releases: volcanologists presented information, PIU asked questions, and the resulting draft press release was vetted by volcanologists before release. The press releases were made available to all sections of media. Emphasis was placed on radio broadcasting as this was deemed the most effective method of reaching the largest audience.

### 2. Town meetings and meetings with special interest groups

Meetings involved:

* Chamber of Commerce

* School Principals

* Staff and Students of the most-affected schools

* Church Officials

* Insurance Company Representatives

* Defence Force Officers

* Harbours Board Management

The meetings provided valuable opportunities for the PIU to deliver information directly and, at the same time, to learn the concerns of the community.

The most-frequently-asked questions were:

* Would there be adequate warning?

* Might the situation jump from Stage 2 to Stage 4, without an intervening time in Stage 3?

* What was the best estimate of the size of the eruption?

Davies recalls that he was tempted to respond to questioners with reassuring words but realised that the success of his mission depended on retaining the trust of the people and for this reason it was necessary to be absolutely honest. He was pleased to find that the public response was not one of alarm but of calm, albeit concerned, acceptance. As had been observed by the celebrated volcanologist Gordon A. Macdonald: "People show a great ability to face dangerous situations with equanimity if they understand the situation. It is the unknown or the not understood danger that terrifies" (United Nations 1977).

### 3. Local newspaper

The national daily newspaper Post-Courier, printed in Port Moresby, proved to be an unsatisfactory conduit for conveying volcano information to the public. This problem was met face-on when local businessman, Joe Speccatori, established and produced Rabaul's own newspaper, the Rabaul Gourier (a pun based on the Melanesian Pidgin word "guria", meaning earthquake, and the name of the national newspaper). This provided up to date accurate information and was immensely popular during the worst of the crisis.

## 4 Precursory Activity (II): 1985–1994

### 4.1 Fluctuating Activity: August 1985 to Mid September 1994

For about 3 years following the 1983–85 Crisis Period the level of unrest at Rabaul was low. Seismicity was generally weak, interrupted occasionally by earthquake swarms. Subsidence
<figure><img src="image_79.png" /><figcaption>**Fig. 7** Monthly caldera earthquake totals 1968–1994 and levelling results 1973–1994</figcaption></figure>

was recorded near the margin of the youngest caldera during this interval, but the central part of the caldera floor continued to rise, albeit slowly (Fig. 7).

A $M_L$ 2.8 caldera earthquake on 10 April 1988 and associated inflation of the caldera floor marked the beginning of a period of 4 years of moderate activity. Swarms of caldera earthquakes became more frequent with the strongest earthquake being a $M_L$ 3.5 event in July 1990. Measured uplift at the Matupit Island benchmark during this interval amounted to about 150 mm, giving a rate of about 40 mm/year. This rate is less than half the uplift rate of the pre-Crisis Period.

An unusual swarm of earthquakes that started on 2 May 1992 marked the beginning of a prolonged period of increased activity that continued until December 1993 (Fig. 7). The seismicity in early May 1992 took place about 2 km outside the northern part of the caldera seismic zone

(Fig. 8), under the eastern fringe of Rabaul Town, and consisted of more than 300 high frequency earthquakes, the largest of which was $M_L$ 4.2 (Table 2). The locations of these earthquakes appeared to define a northeast-trending zone and led to the term “Northeast (NE) earthquakes” (Itikarai 2008). The significance of this seismicity remains uncertain, although a connection between NE earthquakes and phases of eruptive activity in the post-1994 period has been suggested (Itikarai 2008).

Caldera seismicity and rates of ground deformation began to increase after May 1992. A succession of caldera earthquake swarms was recorded, and one crisis took place, in May 1993 (Table 2). The crisis of 20 May 1993 was the first since the end of the 1983–85 Crisis Period, and included four $M_L$ 3.8 earthquakes in the Greet Harbour area (Fig. 2). Data from the tide gauge network indicated uplift of 50–60 mm in the area of inferred maximum deformation, in the
<figure><img src="image_80.png" /><figcaption>**Table 2** Larger Rabaul earthquakes: 2 May 1992–18 September 1994 (adapted from Stewart and Itilkarai, unpublished data)</figcaption></figure>

<table><thead><tr><th>Date</th><th>Time (LT)</th><th>Location</th><th>Magnitude (M<sub>L</sub>)</th></tr></thead><tbody><tr><td>02 May 92</td><td>0714</td><td>Namanula</td><td>4.2</td></tr><tr><td>20 May 93</td><td>0643</td><td>Greet Harbour</td><td>3.8</td></tr><tr><td>25 May 94</td><td>1043</td><td>Beehives</td><td>3.3</td></tr><tr><td>25 May 94</td><td>1058</td><td>Vulcan</td><td>3.0</td></tr><tr><td><strong>18 Sep. 94</strong></td><td><strong>0251</strong></td><td><strong>Greet Harbour</strong></td><td><strong>4.9</strong></td></tr><tr><td><strong>18 Sep. 94</strong></td><td><strong>0251</strong></td><td><strong>Vulcan</strong></td><td><strong>5.1</strong></td></tr><tr><td>18 Sep. 94</td><td>1203</td><td>Vulcan</td><td>4.3</td></tr><tr><td>18 Sep. 94</td><td>1709</td><td>Vulcan</td><td>4.4</td></tr><tr><td>18 Sep. 94</td><td>2042</td><td>Vulcan</td><td>4.4</td></tr><tr><td>18 Sep. 94</td><td>2054</td><td>Vulcan</td><td>4.5</td></tr><tr><td>18 Sep. 94</td><td>2307</td><td>Vulcan</td><td>4.1</td></tr><tr><td>18 Sep. 94</td><td>2323</td><td>Vulcan</td><td>4.1</td></tr></tbody></table>

Bold signifies stronger activity
eastern-central part of the caldera. Tide gauge data confirmed that the deformation coincided with the seismic crisis.

Through most of 1994 until the day preceding the outbreak of eruptions a trend of generally declining seismicity was recorded (Fig. 7). There were many small earthquake swarms, but two stronger swarms took place on 25 May 1994 involving $M_L$ 3.3 and 3.0 earthquakes in the Beehives and Vulcan areas respectively (Fig. 2, Table 2). Unusual “hybrid” earthquakes having high frequency onset and low frequency codas were recorded in the aftermath of the swarms of 25 May. The significance of those events was uncertain at that time, but it is now known that hybrid earthquakes of this type are common at other volcanoes with active hydrothermal systems and can be interpreted as episodes of fracture creating permeability that allows movement of hydrothermal fluids, or alternatively as episodes of fracture associated with propagation of magma-filled dykes (Faria and Fonseca 2014). Seismic swarms ceased after a relatively small swarm on 19 July 1994. In late August 1994, several hundred discrete earthquakes were recorded from near Tavurvur, and one hybrid earthquake was recorded. Earthquake locations for the period 01 August 1985–17 September 1994 are shown in Fig. 8.

Elevation changes within the caldera were moderate in the period January to mid-September 1994. A rise of about 100 mm was recorded at benchmarks at the southern end of Matupit Island during this period. The implied uplift rate is slightly greater than the average long-term rate of the pre-Crisis Period. However, in detail uplift rates declined at mid-year and there appeared to be slight subsidence in July. The decline in uplift rates may have coincided with the generation of the hybrid earthquakes, and may reflect depres- surization of the hydrothermal system at Tavurvur.

Although the events of the period May 1992 to mid-September 1994 were reported to authorities, the significance of the NE earthquakes, the hybrid earthquakes, and the decline in uplift rates in mid-1994 was not appreciated at that time. In addition, maintenance and

equipment problems led to a lack of telemetered data from tiltmeters and tide gauges by late 1993 which rendered the reliable interpretation of available ground deformation data more difficult. These deficiencies conspired to impede the gen- eration of warning messages from RVO.

## 4.2 The Ultimate Crisis of 18–19 September, 1994

### 1. Seismicity

The 1994 eruptive outbreak at Rabaul was immediately preceded by the “Ultimate Crisis”, 27 h of vigorous and fluctuating seismicity and remarkable ground deformation (Blong and McKee 1995). The timeframe and nature of these events in 1994 are eerily similar to the events that immediately preceded the 1937 eruptions (Fisher 1939). The seismicity of the Ultimate Crisis was initiated by two strong caldera earthquakes at 0251 on 18 September (Fig. 9, Table 2). The first earthquake was a $M_L$ 4.9 event located near Tavurvur. The second earthquake was larger ($M_L$ 5.1), and is believed to have originated from beneath the southern part of the Vulcan headland (Stewart and Itikarai, unpublished data). A small tsunami was generated by this earthquake and had greatest impact in the southern part of the caldera (Nishimura et al. 2005; Stewart and Iti- karai, unpublished data).

The early part of this intense seismicity resembled a typical crisis. However, by early afternoon of the 18th the sustained nature of this activity caused concern at RVO and in some communities, notably those on the western side of the caldera and at Matupit Island. After about 14 h of seismic swarm activity, at about 1700 on the 18th, the seismicity intensified and it became clear to many people that conditions were unu- sual. Numerous felt earthquakes, many with magnitudes equal to or greater than $M_L$ 3.5, occurred between 1700 on the 18th and 0300 on the 19th. The felt seismicity tapered off towards dawn on the 19th.

Most of the earthquakes were located in the west-northwestern part of the caldera seismic
<figure><img src="image_81.png" /><figcaption>**Fig. 9** Caldera earthquake locations 18–19 September 1994</figcaption></figure>

zone, i.e. the area between the Vulcan headland and Matupit Island, although the initial focus of the seismicity, following the main (M_L 4.9 and 5.1) events, was under the Vulcan headland (Fig. 9).

## 2. Ground Deformation

The new telemetering electric tiltmeters and tide gauges that had been deployed as a result of the 1983–85 Crisis Period had become unopera- tional by 1994 because of maintenance and funding difficulties. Remarkable ground defor- mation started on the night of the 18th or early next morning, but was not detected until dawn, at about 0515 on the 19th. Uplift had occurred in the Vulcan area where a tide gauge pylon was almost fully exposed, indicating uplift of about 6 m (Fig. 10). The western and southern coasts of Matupit Island had also been raised and the southern shoreline had migrated about 70 m

south. The estimated uplift of the western and southern parts of Matupit Island was about 2 m. There was little or no evidence of elevation changes at Tavurvur. The overall pattern of the ground deformation appeared to be east-hingedup-tilting of the caldera block within the caldera seismic zone.

## 4.3 Outbreak of Twin Eruptions: 19 September 1994

New eruptions at Rabaul began just after 0600 on 19 September 1994, ending a repose period of about 51 years. Following the pattern of the previous two eruptive episodes, in 1878 (Brown 1878; Johnson et al. 1981) and 1937–43 (Fisher 1939; Johnson and Threlfall 1985), there were almost simultaneous outbursts on opposite sides of the caldera as Tavurvur and Vulcan began erupting at 0606 and 0717, respectively.
<figure><img src="image_82.png" /><figcaption>**Fig. 10** Newly emerged sea floor at the eastern foot of Vulcan as seen at 6:15 a.m. on 19 September 1994. The near total exposure of the tide gauge pylon shown here in</figcaption></figure>

the small embayment indicates about 6 m of uplift in this area. Photo courtesy of N. Lauer, formerly of RVO

# 5 Responses to the Ultimate Crisis and Outbreak of Eruptions

## 5.1 Timeline of Events and Responses, 18–19 September 1994

Responses to the physical (volcano-related) events in the immediate pre-eruption period were reported by Davies (1994, 1995) and are summarized in Table 3. The responses include the actions by RVO, by the PDC and by communities resident within the caldera. The responses are discussed in the following sections.

## 5.2 Response of Government Authorities

As earthquake activity intensified during the afternoon of 18 September it became apparent that this

activity was more than a typical crisis. A recommendation to raise the alert to Stage 2 was made by RVO to the PDC at about 6 p.m. on 18 September. The raising of the alert level was broadcast by local radio and spread by word of mouth.

An exodus of people from villages near Vulcan started in late afternoon. By dusk people started to evacuate Matupit Island and the southern town area. Those who did not have transport were advised to congregate at the Queen Elizabeth Park (QEP) sports field (Fig. 1) in the Rabaul Town area (as stipulated in the VCP).

At around 11 p.m. the PDC began the evacuation of about 5000 people who had gathered at QEP. All available transport worked through the night until the early hours of the next day to ferry people out of the caldera.

At about midnight RVO advised the PDC that “Rabaul Volcano was on an irreversible course towards an eruption” and that a recommendation for declaration of a Stage 3 Alert was imminent. By 2 a.m. on 19 September that recommendation
<table><caption>**Table 3** Events and responses timeline, 18–19 September 1994</caption><thead><tr><th>Time</th><th>Event</th><th>RVO</th><th>PDC (NDES)</th><th>Communities</th></tr></thead><tbody><tr><td colspan="5"><strong>18 Sept</strong></td></tr><tr><td>0251</td><td>M<sub>L</sub> 4.9, 5.1 earthquakes, tsunami</td><td>Processed and analysed eq. data</td><td></td><td>Most people awakened by eq. shaking</td></tr><tr><td>0800–1200</td><td>Sustained moderate-strong seismicity</td><td>Processed eq. data Investigated eq., tsunami impact</td><td></td><td></td></tr><tr><td>1200–1700</td><td>Sustained strong seismicity</td><td>Processed eq. data</td><td></td><td>Growing concern with continuing seismicity</td></tr><tr><td>1700</td><td>Seismicity intensifies</td><td>Processed eq. data</td><td></td><td>Mobilization of communities on W side of caldera (Tavana, Valaur)</td></tr><tr><td>1800</td><td>Sustained intensified seismicity</td><td>Communication to PDC to increase alert to Stage 2</td><td>Stage 2 declared, broadcast on local radio</td><td>Evacuation of Matupit Is., southern part of Rabaul Town Assembly of residents at QEP</td></tr><tr><td>2300</td><td>"</td><td></td><td>Directed evacuation of people gathered at QEP</td><td>Evacuation of most communities resident within caldera</td></tr><tr><td>2400</td><td>"</td><td>Communication to PDC that eruption anticipated</td><td>Conveyed message that eruption anticipated</td><td>Continued evacuation</td></tr><tr><td colspan="5"><strong>19 Sept</strong></td></tr><tr><td>0200</td><td>"</td><td>Communication to PDC and NDES to increase alert to Stage 3 Arranged for aerial inspection at 0600</td><td>Continued to convey message that eruption anticipated</td><td>Evacuation of all communities resident within caldera completed</td></tr><tr><td>0300</td><td></td><td></td><td>Stage 3 declared (NDES), broadcast on Radio Australia</td><td></td></tr><tr><td>0515</td><td>Emergence of seafloor around Vulcan and Matupit Is first observed</td><td>Notified PDC that eruption imminent</td><td></td><td></td></tr><tr><td>0606</td><td>Outbreak of eruption at Tavurvur</td><td>Observed outbreak during aerial inspection</td><td></td><td></td></tr><tr><td>0700</td><td></td><td></td><td>Stages 3 and 4 declared locally, and request for declaration of State of Emergency (subsequently declared)</td><td></td></tr><tr><td>0717</td><td>Outbreak of eruption at Vulcan</td><td>Observed outbreak from RVO</td><td></td><td></td></tr></tbody></table>

had been faxed from RVO to the PDC and to the National Disaster and Emergency Services (NDES) headquarters in Port Moresby. The Director of NDES (Leith Anderson) subsequently notified the Port Moresby-based representative of

Radio Australia, Sean Dorney, of this development which was then broadcast (at about 3 a.m.).

Following the advice from RVO that an eruption was inevitable members of the PDC advised clubs, hotels, boarding schools and
others that people should move out from the caldera. This was done by phone calls and by Police and Red Cross officials moving through the streets advising evacuation.

The local radio station remained on air until 12:30 a.m. on 19 September, and resumed broadcasting at 5:30 a.m. until the eruption began. Through this time the station continued to broadcast the message that Rabaul was on Stage 2 Alert and that people should remain in their homes until further notice. This anomaly arose because the PDC had decided to delay declaring Stage 3 Alert until first light. The PDC reasoned that the evacuation was proceeding smoothly during the night and it was felt that announcement of a heightened state of alert might cause panic and unnecessary havoc on the roads.

Rabaul Town and all villages within the caldera were evacuated by the early hours of 19 September. The evacuation was conducted smoothly and the entire process was completed within 12 h. However, some residents of Talwat village, just outside the eastern margin of the

caldera, did not join the mass exodus from Rabaul on 18–19 September. Likewise, most residents of villages east and northeast of Rabaul Town (Baii, Nodup, Matalau, Korere etc., see Fig. 1) did not evacuate at this time.

At 7 a.m. on 19 September the PDC declared simultaneously Stage 3 and Stage 4 Alerts, and requested the declaration of a State of Emergency. By that time Tavurvur's eruption had already started and within about 1.5 h a dense emission plume had engulfed Rabaul Town (Fig. 11). The State of Emergency was approved by the Prime Minister and declared by the Director NDES (Leith Anderson). Anderson was appointed Controller of the Emergency and the Chairman of PDC, Ellison Kaivovo, was appointed Deputy Controller.

A command post for emergency operations was established at Ralum, near Kokopo, on the second morning of the eruptions. The original plan for command post operations to be headquartered at Vunadidir was abandoned when that area received tephra fall from the Vulcan

<figure><img src="image_83.png" /><figcaption>**Fig. 11** Tavurvur emission plume engulfing Rabaul Town at about 7.25 a.m. on 19 September 1994, as viewed from RVO. Photo courtesy of N. Lauer, formerly of RVO</figcaption></figure>
emissions. The Ralum location for the command post probably was a better option, being more conveniently-located for a range of services.

Telephone communications in the region were seriously disrupted during the first few days of the eruption. Lightning strikes in the mid-late afternoon of 19 September damaged the telephone system at RVO. The loss of the Rabaul Telephone Exchange on the morning of 20 September, due to collapse of the roof of the building under a heavy load of wet volcanic ash, caused a temporary break in communications in the Rabaul area. Earlier on the same day operation of the Kokopo Telephone Exchange was halted by an electrical power outage.

Loss of volcano-monitoring capability at RVO began on the first day of the eruptions. Seismic monitoring equipment and telemetry links were damaged by heavy falls of tephra and by lightning strikes. On the morning of 20 September heavy rainfall overwhelmed drainage from the roof of the main RVO building and incursion of muddy water into the roof space resulted in water damage inside the building and distraction from eruption monitoring activities.

The PIU played an important part in conveying volcano-related information from RVO to the PDC, to the displaced people in care centres (Fig. 12) and to the news media. Led by H. Davies, who travelled from Port Moresby to

<figure><img src="image_84.png" /><figcaption>**Fig. 12** Locations of care centres for evacuees</figcaption></figure>
Kokopo on 23 September, the PIU was assisted by local resident geologist David Lindley (Lindley 1995) and occasionally by volcanologists from RVO.

## 5.3 Community Response

The responses of the traditional communities to the volcanic events of 1994 were shaped by experience and knowledge. Memories and stories of the events preceding the 1937 eruptions and of the approximately 500 fatalities near Vulcan during that eruption are still strong within these communities. The 1937 eruptions were preceded by 27 h of intense local earthquake activity, accompanied by dramatic late-stage emergence of the seafloor in the Vulcan and Matupit Island areas (Fisher 1939; Johnson and Threlfall 1985). The timeframe and the nature of the geological activity on 18–19 September 1994 were very similar to the events of 1937. However, the general community response to the pre-eruption geological activity in 1994 probably was based only on the earthquake activity, as also suggested by Neumann (1996) from anthropological research, as the massive uplift in the Vulcan and Matupit Island areas took place at night and was not observed until first light on 19 September. By that time nearly all of the people normally resident within and near the caldera had evacuated.

Another element of the general community response in 1994 was the conditioning through 23 years of precursory activity. The attention of the community to the volcanic threat became highly focussed during the mid-1980s Crisis Period (McKee et al. 1985). Information provided by the PIU and other agencies prompted individuals, family units and larger groups to make their own volcano contingency plans in the light of the broader official volcano contingency plan. However, the long delay (10 years) between the peak of the Crisis Period and outbreak of eruptions may have caused some complacency within the community about the lingering volcanic threat. Additionally, changes to the migrant and expatriate communities would have weakened the focus on preparedness for eruptive activity.

It was reported by Neumann (1996) that a group of people from Valaur Village (near Tavana village) visited RVO on the morning of 18 September anxious about the seismic activity. The village group was allegedly told by RVO staff “not to worry” about the earthquakes. The details of that exchange cannot be verified, however at that early stage of the Ultimate Crisis a judgement on the significance of the seismicity would have been difficult to make. Similar activity had been recorded previously (see Table 1), notably September–October 1980 (including an $M_L$ 5.2 earthquake), March 1982 (including an $M_L$ 5.1 earthquake), March 1984 (including an $M_L$ 5.1 earthquake), April 1984 (three consecutive days of crisis activity including an $M_L$ 4.8 earthquake) and October 1984 (including an $M_L$ 4.9 earthquake).

Differences between the activity of 18–19 September 1994 and crises of the 1980s became clearer through the afternoon of the 18th when felt earthquake activity continued and appeared to be strengthening. A series of magnitude 4 earthquakes from the Vulcan area began at about midday on the 18th, with more frequent such earthquakes occurring after about 2000 LT (Table 2).

The rapid evacuation of areas near Vulcan in the afternoon of the 18th appears to have been a reasoned and conscious response to a situation perceived to be different to those experienced during the crises of the 1980s, and bearing similarities to the earthquake activity prior to the 1937 eruptions as remembered by village elders. A similar response may have occurred at Matupit Island. As the spontaneous evacuation gathered pace some people may have been influenced by the actions of those around them to join the exodus. Such behaviour would have been further fuelled by the sight and sound of aircraft taking off from Rabaul airport (Lakunai; Fig. 1) during the hours of darkness on 18–19 September. Those actions may have provided powerful communications to observers, as a case of “actions speaking louder than words”. This phenomenon was evident from interviews with survivors of the 2011 Tohoku tsunami (Ando et al. 2013) which showed that many people
began to evacuate coastal towns before the tsunami arrived, simply because they saw other people evacuating.

There was a sharp contrast between the actions and reactions of village communities east and northeast of Rabaul Town and those resident within the caldera. The village communities east and northeast of Rabaul Town may have heeded the local radio broadcast messages to stay put. Also, the lower intensity of felt earthquake activity in these areas may have been a factor in prompting these communities to not evacuate during the afternoon or night of the 18th. Evacuation of these communities and the remaining residents of Talwat took place on 20–21 September, assisted by a number of ships (Fig. 13).

<figure><img src="image_85.png" /><figcaption>**Fig. 13** Evacuation of some residents of Talwat village on 20 September 1994, assisted by MV *Madang Coast*. A dense, convoluting emission plume rises from Tavurvur in the background. Further in the background is the stratovolcano Kabiu. Photo courtesy of South Pacific Post Courier</figcaption></figure>

One very disturbing response to the Ultimate Crisis and outbreak of eruptions in 1994 was looting. The looting started in Rabaul Town on the night of 18 September as the evacuation proceeded. During the early part of the eruption looting became widespread as residents of villages outside of the caldera took advantage of unoccupied houses and the lack of strong action on the part of police to secure the town. Check points were established at the main roads but there was little effort to stop people from going into the town area or to stop trucks laden with looted goods from leaving.

## 5.4 Outcomes

The eruption at Vulcan was a powerful, short-lived plinian event (Fig. 14), producing about $260 \times 10^6 \text{ m}^3$ of tephra mostly in the first few days of activity, and concluding on 2 October 1994 (Blong and McKee 1995). Tavurvur's eruption was less powerful (vulcanian) but persistent, continuing for years. The output rate at Tavurvur peaked during the first days of activity when about $40 \times 10^6 \text{ m}^3$ of tephra was erupted.

The death toll from the 1994 eruptions was relatively small, officially 10. The 10 eruption-related deaths comprised 3 due to tephra fall and associated asphyxia, 1 due to lightning, 3 from road trauma, 2 from desertion (ill and incapacitated persons), and 1 from drowning in a flash flood 6 weeks after the start of the eruptions (Dent et al. 1995). This small death toll is in stark contrast to 1937 when as many as 500 people perished, mostly caught in pyroclastic flows and heavy tephra fall in the Vulcan area (Fisher 1939). As mentioned, the events of 1937 and the recognition of the on-going volcanic threat led to the establishment of RVO in 1940. The instrumental volcano surveillance that ensued helped to better inform the residents of the Rabaul area of the local volcanic threat and led to the detailed planning that helped to manage the long precursor period and the inevitable eruptions in 1994.
<figure><img src="image_86.png" /><figcaption>**Fig. 14** Vulcan's eruption column and plume as seen from the Space Shuttle at about 7 a.m. on 20 September 1994. At this time the eruption column was rising to at least 18 km (astronaut's estimate) and feeding a broad, wedge-shaped emission plume the axis of which was oriented southwest. In this view from the east-northeast, the eastern coast of New Ireland can be seen beneath the extensive white cloud cover in the foreground, and part of the eastern coast of the Gazelle Peninsula is visible in the left middleground. Photo courtesy of NASA</figcaption></figure>

A UNDHA mission to Rabaul in February 1995 (Tomblin and Chung 1995) reported that “the Rabaul Emergency Plan had in most respects worked very well”. Analysis of the events and responses of the precursory period and of the first few months of the eruptive period identified problems with telecommunications as one of 22 issues “which could be improved for the future”. This issue is discussed further in the next Sect. 6.

While responses to the events of 18–19 September 1994 were not ideal in some instances, the general recognition of the impending eruptive threat, the implementation of emergency plans, the willingness of most people to evacuate and the successful evacuation of areas of greatest danger resulted in avoidance of a repeat of the outcome of Vulcan's 1937 eruption and of the sort of tragedy that befell the town of Armero during the 1985 Nevado del Ruiz volcano catastrophe. At Armero, the loss of >22,000 lives was attributed mainly to “cumulative human

error—misjudgement, indecision and bureaucratic shortsightedness” (Voight 1990, p. 383).

An irony of the 1994 eruption is the extent of damage to Rabaul Town (70% destroyed) from the relatively small volume of Tavurvur tephra emissions (40 × 10⁶ m³). The output from Vulcan was much greater (260 × 10⁶ m³) but its impact was mostly in the vicinity of Vulcan itself. Several (evacuated) villages were obliterated by the pyroclastic flows and heavy tephra fall from Vulcan. The damage from Tavurvur tephra was maximized by the close proximity of Rabaul Town to Tavurvur and by the prevailing winds which carried the tephra directly towards the town. Such seemingly disproportionate impact was also evident at (the snow and ice covered) Nevado del Ruiz Volcano in 1985 where <5 × 10⁶ m³ of magma ejected as pyroclastic flows was able to generate about 60 × 10⁶ m³ of lahars from 10 to 20 × 10⁶ m³ of melt water (Calvache 1990; Pierson et al. 1990).
Real-time monitoring capability for both earthquake locations and seismic amplitudes (RSAM-Murray and Endo 1989) was deployed at Rabaul in October 1994 by a team from VDAP-USGS, replacing the damaged and seriously weakened existing seismic network. At the same time new electronic tiltmeters were also installed. High resolution GPS monitoring commenced in 1999 with sensors deployed at Matupit Island and at RVO. All of the immediate efforts to restore and upgrade volcano-monitoring facilities at Rabaul focussed on data transmission to RVO. Subsequently, plans were developed for a re-location of the volcano-monitoring hub to Kokopo, and in 2016 the first element (accommodation block) of a re-located volcano observatory had been completed.

In general, the scientists who visited Rabaul during the eruptions followed the principles of not engaging in discussions with local media and individuals, and releasing information only through official channels i.e. RVO and PIU. However, in one case a visiting scientist caused significant anxiety through public discussions of a personal prediction of increased volcanic activity during the second week of the eruptions.

Negative outcomes of the eruptions include: losses of lives, homes, property, businesses and livelihoods. Many people suffered significant financial loss. Some of these losses were a result of the inability to get adequate insurance because premiums had been raised due to the 1983–85 Crisis Period. The displacement of 105,000 people during the first weeks of the eruptions created a huge logistical problem of providing food, water and shelter in care centres. The number of displaced people was swelled by the unnecessary movement of some people from safe area locations to care centres. Probably this was driven by fear of the unknown while care centres offered the perception of “safety in numbers”, the provision of food and water, and access to information about the unfolding drama. For some, the displacement and disruption continued for many months. The short- and long-term psychological effects of the trauma of displacement and disruption were largely not addressed officially.

# 6 Lessons

The 1994 eruptions at Rabaul, which destroyed much of the town and several villages (Fig. 15), were remarkable for the very small loss of life. This outcome resulted from timely evacuations from areas most impacted by volcanic activity. Nevertheless, the rapid development of the late-stage precursory activity caught most residents by surprise, allowing insufficient time to pack up properly, with the result that many small businesses are no more and many people lost all possessions.

Experiences during the long period of precursory activity and during the 1994 eruptions, and comparisons with experiences during the 1937 eruptions and earlier activity, have framed the following lessons:

1. Volcanic crises and unrest are common at Rabaul

Historical records suggest that volcanic crises and unrest are common between eruptions at Rabaul. Most, if not all, of the volcanic crises at Rabaul follow a similar pattern. The earthquakes of crises are of high frequency type and the strongest events occur near the beginning of a crisis to be followed by a sequence of smaller earthquakes. Typically, uplift of parts of the caldera floor occurs during crises in association with the earthquake activity. Greater and more rapid uplift accompanies the sustained strong seismicity that occurs immediately before an eruption outbreak. Tsunami are generated by the stronger crises, particularly those immediately preceding eruptions.

2. An individual crisis may or may not lead directly to an eruption.

In terms of maximum earthquake magnitude, there was nothing to distinguish the crisis that immediately preceded the 1994 eruption from earlier crises. Caldera earthquakes of magnitude ($M_L$) 4.9 or greater occurred on at least 4 occasions (Mori et al. 1989) before the magnitude ($M_L$) 4.9 and 5.1 events of the Ultimate Crisis in
<figure><img src="image_87.png" /><figcaption>**Fig. 15** Aerial view of Rabaul Town from the northwest in early January 1996 showing destruction of the southern and central parts of the town area. A small eruption cloud stands over Tavurvur. The neighbouring stratovolcanoes</figcaption></figure>

Turagunan, Kabiu and Palangiangia are looming threats—all were active in the Middle-Late Holocene. Photo courtesy of Gazelle Restoration Authority, Kokopo, East New Britain

1994 (Table 1). These events were: October 1980—ML 5.2, March 1982—ML 5.1, March 1984—ML 5.1, October 1984—ML 4.9. However, the sustained high rate of felt earthquake occurrence in 27 h periods preceding the 1937 and 1994 eruptions was distinctive compared with other crises.

### 3. Late stage precursory activity can develop rapidly

Late stage precursory activity developed rapidly in 1994: only 27 h separated the start of the final crisis and the outbreak of eruptions. There was a re-intensification of seismicity 14–16 h after the initial strong earthquakes. The resurgence of seismicity halfway through the immediate pre-eruption crisis was associated

with the rapid uplift of parts of the caldera floor that began less than 12 h before the start of the 1994 eruptions. Memories and stories of a similar pattern of late-stage precursory activity that preceded the 1937 outbreak influenced the responses of traditional communities to the events of 1994.

### 4. Eruptions can occur at more than one vent

In the three most recent eruptive periods, 1878, 1937 and 1994, two vents on opposite sides of Rabaul Caldera erupted simultaneously. This pattern of activity has implications concerning the stability of Rabaul Volcano. Simultaneous activation of different parts of the volcanic system could lead to major de-stabilization, culminating in events of significantly larger scale.
5. Seismic data acquisition needs to be automated

A microfilm and paper chart seismic recording system was deployed through most of the 23 years of precursory activity and for about 10 days into the 1994 eruptions. Processing of data in this system involved manual timing of earthquakes and manual entry of data into a computer for locating the earthquakes. This system provided no facility for monitoring seismic amplitudes. The system was adequate for routine operations in non-eruptive periods when the most intense activity was isolated crises, but could not cope with a sustained high-intensity crisis or eruptions outbreak. Real-time monitoring capability for both earthquake locations and seismic amplitudes is essential.

6. Continuous real-time telemetered ground deformation monitoring is required

Remarkable ground deformation occurred on the night before the 1994 eruptive outbreak but was not observed. Telemetering electronic tiltmeters and tide gauges had been deployed at Rabaul as a result of the 1983–85 Crisis Period but had become unoperational by 1994 because of maintenance and funding difficulties. If operational, these instruments may have provided an earlier warning that an eruption was imminent.

7. Clear established links with media are needed

The experience of the Crisis Period in 1983–85 demonstrated the advantages of having a Public Information Unit to disseminate information from scientists and civil defence officials and to handle all media enquiries about volcanic activity. This allowed the scientists to concentrate on volcano monitoring. Nevertheless, it is desirable for scientists to communicate directly with media, if the circumstances permit.

8. Strong communication links between scientists and civil defence agencies are needed

Serious communications difficulties between scientists and civil defence officials were encountered during the first dew days of the 1994 eruptions because of the loss of telephone services. In order to ensure effective and reliable official communications it is necessary to have multiple communications links, including radios and both mobile and landline telephones. Protection and duplication of communications infrastructure, such as telephone exchanges, is essential.

9. Logistical arrangements for volcano monitoring teams and civil defence personnel need careful planning

At Rabaul in 1994 volcanologists and disaster co-ordinators were also victims of the eruptions and had to deal with a number of logistical issues, including care of families, accommodation, security, protection of property and emergency communications. This had a heavy impact on the ability of these officials to function effectively during the early days of the 1994 eruptions.

10. Protection of Property

One disturbing aspect of both the 1937 and 1994 eruptions was looting. In 1994 looting started on the night before the eruptions began, when there was a mass evacuation of Rabaul. In 1937 vigilante groups patrolled the town and severely punished looters. This did not happen in 1994 and during the first few days and weeks of the 1994 eruptions looting took place on a large scale, essentially unchallenged. The looting was a serious distraction for RVO staff and emergency officials who were residents of Rabaul Town and whose homes and possessions were targeted by looters.
# 7 Concluding Remarks

1. Living near active and potentially-active volcanoes requires open and effective lines of communication between volcano specialists, national and provincial government officials, town authorities and the general public. Communication between all of these sectors is vital at all stages of volcano surveillance, eruption contingency planning, town planning and disaster management. The creation of the PIU by the East New Britain Provincial Government in 1984 assisted greatly in the dissemination of volcano-related information.

2. The availability of volcano contingency plans provides great support during crisis periods. Involvement of all sectors of threatened communities in the creation of these plans is desirable. Frequent rehearsals and reviews of the plans maintains a high level of alertness.

3. A majority of the various communities of the Rabaul area coped well with the prospect of impending eruption once they were provided with relevant information. This information coupled with local knowledge and lessons from previous eruptions, particularly that of 1937, resulted in a high level of volcano awareness that led to decisive actions to evacuate the most-threatened areas during the immediate pre-eruption period. For some, “actions may have spoken louder than words”, as some residents may have been influenced to evacuate by the movements of others around them, resulting in a mass exodus of people from within the caldera.

**Acknowledgements** The manuscript benefitted greatly from the detailed reviews by Professor Emeritus Russell Blong and Dr. Simon Day. Marissa Sari Egara of the Port Moresby Geophysical Observatory helped with word processing of the manuscript. Sonick Taguse of Papua New Guinea's Mineral Resources Authority helped with preparation of some of the line diagrams. COM and II publish with the permission of the Secretary, Mr. Shadrach Himata, Department of Mineral Policy and Geo-hazards Management, Papua New Guinea.

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<figure><img src="image_88.png" /></figure>
Challenges in Responding
to a Sustained, Continuing Volcanic
Crisis: The Case of Popocatépetl
Volcano, Mexico, 1994-Present

Servando De la Cruz-Reyna, Robert I. Tilling
and Carlos Valdés-González

Abstract

Popocatépetl Volcano, located in the central Trans-Mexican Volcanic Belt, is surrounded by a densely populated region with more than 20 million people. During the past 23,000 years, this volcano has produced eruptions ranging widely in size and style, including Plinian events and massive sector collapses. However, the historical activity of Popocatépetl, recorded in detail since 1500, consists of only nineteen small to moderate eruptions, several similar in style to the current eruptive episode (1994-present). After nearly 70 years of quiescence since its eruptions in the mid-1920s, Popocatépetl reawakened in December 21, 1994. This eruptive activity, which is still ongoing, has been characterized by a succession of lava dome growth-and-destruction episodes: pulses of effusive and moderately explosive activity alternating with periods of almost total quiescence. This pattern appears to be characteristic of all historical eruptions, several of which lasted for decades, with interspersed lull periods that in some cases make it difficult to identify the end of the eruptive episodes. In this chapter, we discuss the problems and challenges posed by a prolonged, low-level volcanic crisis (or "semi-crisis") of variable intensity that has lasted for more than 20 years, without showing any signs of coming to an end. Paradoxically, this still-continuing crisis has spawned two opposite developments: (1) during periods of little visible activity, people dwelling near the volcano become somewhat

S. De la Cruz-Reyna (✉)

Instituto de Geofísica, Universidad Nacional

Autónoma de México UNAM, CDMX, Mexico

e-mail: sdelacruzr@gmail.com

R.I. Tilling

Volcano Science Center, U.S. Geological Survey,

Menlo Park, USA

C. Valdés-González

Centro Nacional de Prevención de Desastres

CENAPRED, CDMX, Mexico

Advs in Volcanology (2018) 235–252

https://doi.org/10.1007/11157_2016_37

© The Author(s) 2017

Published Online: 15 March 2017
apathetic and indifferent; but (2) during times of easily observed visible activity, awareness of changes at the volcano—and their hazardous implications—is rapidly and greatly enhanced by the common use of social media by people.

# 1 Introduction

Disasters can occur when society fails to identify and foresee the potentially hazardous manifestations of a natural phenomenon. However, disasters may also occur if society fails to adopt adequate measures to reduce the risks—to people, property, and infrastructure—posed by hazardous phenomena even if recognized in advance. Responding effectively to hazards is a process as complex as is the fabric of society itself, as each hazardous phenomenon has a variety of destructive manifestations, and each may affect different sectors of society in particular ways. This is especially true when dealing with volcanic crises. As has been long recognized (e.g., Fiske 1984; Peterson 1986, 1988; Tilling 1989; Voight 1990; Peterson and Tilling 1993; Haynes et al. 2008; Solana et al. 2008; Fearnley 2013), the process of crisis response entails close interaction between three main entities: (1) The scientists studying the hazardous phenomena and their potential social outcomes; (2) the authorities in charge of public safety and infrastructure; and (3) the affected populace. The wide spectrum of backgrounds and attitudes of all the involved stakeholders during such interaction, together with the vague or imprecise information generally available during the crisis, often combine to hinder effective communications among the entities involved. Poor communications in turn complicate the perception of the risk, a factor likely to increase societal vulnerability. Thus, during an evolving crisis, it is critical to develop a perception of risk as uniform as possible among all stakeholders—no easy task when the affected population is measured in millions. To achieve this goal requires searching for communication tools that can describe—as

simply as possible—the relations between the level of threat posed by the volcano, and the level of response of the authorities and the affected public. In the case of Popocatépetl, the Civil Protection of Mexico addressed this challenge by developing and implementing the *Volcanic Traffic Light Alert System* (VTLAS). Distinct from other volcano alert systems (VALs)—typically referenced to the activity of the volcano—used to communicate warning information from scientists to civil authorities managing volcanic hazards (Fearnley 2013; Potter et al. 2014), the VTLAS scheme (discussed in detail below) additionally was intended to reduce the possibility of ambiguous interpretations of intermediate alert levels by the large populations at risk. This additional component marks a significant advance in the management of volcanic crises in Mexico (De la Cruz-Reyna and Tilling 2008).

Before proceeding further, we should summarize how risk is managed by the National Civil Protection System of México (SINAPROC), which was created in 1986 after the catastrophic disaster caused by a M 8.1 earthquake on September 19, 1985. The executive body of the SINAPROC at the federal level is the General Coordination, housed within the Ministry of the Interior (Secretaría de Gobernación). The General Coordination is supported by four agencies: (1) the National Direction of Civil Protection, an operational body in charge of implementing the preventive and relief actions; (2) the General Direction of Integral Risk Management, which provides the funding for prevention and emergency actions; (3) the General Direction of Interlinking and Regulations, which coordinates the different government levels involved with civil protection; and (4) the National Center for Disaster Prevention (CENAPRED), a technical
body created in September 19, 1988, with sub-
stantial technical and generous financial support
from the government of Japan. The mission of
CENAPRED is to promote the applications of
science and technology for the prevention and
mitigation of disasters, to train and inform pro-
fessionals and technicians on these subjects, and
to disseminate the necessary information for
preparedness and self-protection.

CENAPRED also acts as an active interface
between the operative, decision-making authori-
ties of the SINAPROC and the academic scien-
tific community. In conducting its work,
CENAPRED utilizes four advisory scientific
committees on topics relevant for disaster pre-
vention, composed of prominent, experienced
Mexican scientists in the areas of Earth sciences,
hydro-meteorological sciences, social sciences,
and chemical and industrial hazards. There are
also ad-hoc sub-committees, as is the case of the
Advisory Committee for Popocatépetl Volcano,
on which several international volcanologists—
especially from the U.S. Geological Survey

(USGS)—have actively participated. This advi-
sory sub-committee will be herein referred as the
Popocatépetl Scientific Committee (PSC).

Popocatépetl has remained persistently active
for over 20 years, thereby creating a
long-standing volcanic crisis that has imposed
additional difficulties in the management of the
volcanic risk. CENAPRED has been and remains
in charge of the monitoring of Popocatépetl
volcano, and it also continues to host and coor-
dinate PSC sessions as needed.

## 2 Popocatépetl Volcano: Geologic Setting and Eruptive History

Popocatépetl Volcano (19.02°N, 98.62°W), which lies within the central Trans-Mexican Volcanic Belt (Fig. 1), is located about 60 km SE of México City and 45 km W of Puebla City.

These two populations centers, combined with
other nearby cities within a 100-km radius
around the volcano, contain a total population of

<figure><img src="image_89.png" /><figcaption>**Fig. 1** Sketch map showing the location of Popocatépetl Volcano and other historically active volcanoes of Mexico: 1 Tres Virgenes; 2 Evermann (Socorro); 3 Ceboruco; 4 Colima; 5 Parícutin; 6 Xitle; 7 Popocatépetl; 8 Pico de Orizaba (Citlaltépetl); 9 San Martín Tuxtla;</figcaption></figure>

10 El Chichón; 11 Tacaná. The inset shows the distribu-
tion of cities and major towns (yellow) within 100 km of
Popocatépetl's active crater. The cities of Mexico City,
Puebla, Cuernavaca, Cuautla and others located within the
inset have a combined population over 20 million
<figure><img src="image_90.png" /><figcaption>**Fig. 2** Popocatépetl (right) and Iztaccíhuatl Volcanoes: Above, “Camino a Chalco con los volcanes” painting by José María Velasco, 1891 (Museo Nacional de Arte,</figcaption></figure>

MUNAL, Mexico). Below, view of these two prominent peaks through helicopter windshield (Photograph by S. De la Cruz-Reyna in September 2002)

over 20 million people. Rising 5454 meters above sea level, the conical volcanic edifice of Popocatépetl is topped by a 600 × 800 m elliptical summit crater. Popocatépetl is the youngest peak within the Sierra Nevada, a volcanic mountain range which extends in a roughly N–S direction. To the north of Popocatépetl, the volcanic complex Iztaccíhuatl complements the iconic volcanic landscape of the region (Fig. 2).

Geological evidence indicates that a large eruption about 23,000 years destroyed a pre-existing volcanic edifice, generating massive debris avalanches (Robin and Boudal 1987; Boudal and Robin 1989; Siebe et al. 1995). Since then, Popocatépetl's eruptive history has been characterized by at least seven major explosive eruptions and many smaller eruptions that have produced large volumes of ash and pumice. Three

of the most recent explosive eruptions (ca. 3000 B. C., between 800 and 200 B.C., and ca. A.D. 800) affected human settlements, as indicated by archaeological remains buried by ashfall deposits and pottery shards incorporated by mudflows (Siebe et al. 1996; Siebe and Macías 2004). After the last of these major eruptions, activity at Popocatépetl has remained moderate for nearly 1200 years. Batches of magma were extruded, producing lava domes and associated moderate explosions and ashfalls. Eyewitness reports since 1354 (in the native Nahuatl and Spanish language translations) describe episodes of activity, while more recent and detailed written reports since 1500 document that about 16 small and 3 moderate eruptive episodes have occurred within the past 500 years, some of them probably involving dome growth-and-destruction processes similar to those
of the current, ongoing activity (De la Cruz-Reyna et al. 1995; De la Cruz-Reyna and Tilling 2008).

## 3 Ongoing Unrest, Eruptive Activity, and Volcanic Crisis

Management of risk posed by volcanic unrest requires a comprehensive understanding of the natural phenomenon. In this regard, a sustained activity makes it particularly difficult to forecast the future activity and its consequences, because the commonly employed methodologies to recognize and assess the relevance of precursors of increased activity are obscured by the persistent low-to-medium level of activity. It is thus important to define and establish the context of the unrest to develop decision-making criteria.

Before the current activity, there was little public awareness outside the scientific community about Popocatépetl being an active and potentially hazardous volcano. The previous eruption of Popocatépetl was not a major event. It began in 1919, and the available descriptions of that activity (Friedländer 1921; Waitz 1921; Dr. Atl 1939) indicate that it consisted of a succession of dome emplacements and destructions, similar to the current eruptive episode, probably lasting until 1927. Then, after nearly 70 years of quiescence (except for a minor fumarolic event in 1947), Popocatépetl volcano reawakened in 1993 with increased fumarolic and seismic activity (De la Cruz-Reyna et al. 2008). By October 1994, this unrest further escalated, culminating with a series of moderately large phreatic explosions at the crater during the early hours of 21 December 1994. These explosions produced ashfalls on several towns to the east and northeast of the volcano, including the large city of Puebla.

At the time of the explosions, glaciers with an estimated total area of 0.54 km² blanketed the northern flank of the cone, below the crater (Delgado-Granados 1997; Huggel and Delgado-Granados 2000). With the vivid memories of the 1985 Nevado del Ruiz disaster in the minds of authorities and scientists, nearly 25,000 people living in some of the most vulnerable

towns located along the likely paths of pyroclastic flows and lahars were evacuated in the afternoon of 21 December as a precautionary measure. A week later, the eruptive activity decreased and its largely phreatic nature became better understood, the evacuated residents were allowed to return home. Ash emissions or projected explosions consisting mostly of gas and steam with relatively low concentrations of ash and a characteristic emerging seismic signal were referred to as “exhalations.” This type of relatively low-level activity persisted through 1995 and into early 1996, with decreasing intensity (De la Cruz-Reyna and Siebe 1997).

About a year later, seismicity and exhalation activity increased again, and on 26 March 1996 a lava dome was first observed growing on the crater floor. This dome was partially destroyed by an explosion on April 30, 1996, which propelled ejecta several kilometers into the sky and hot debris as far as 4 km and caused the only reported fatalities to date directly related to the Popocatépetl activity. Despite public warnings not to enter the 12 km-radius restricted area around the mountain, five members of a sports club climbed to the summit crater rim to obtain good images and videos of the activity. These climbers were struck and killed by incandescent fragments during their descent, a few hundreds of meters downslope from the crater, as evidenced by the images recovered from their cameras. Dome-building activity resumed mid-March 1997 (GVN 1996, 1998). These 1996–1997 events marked the beginning of a series of dome growth-and-destruction cycles that have continued up until this writing. Although most of the explosions have been moderate, some of them have been large enough to produce pyroclastic flows (in 2001) and lahars (in 1997 and 2001; Capra et al. 2004).

In summary, the current eruptive episode to date has consisted of a succession of moderate-size eruptions, characteristic of Popocatépetl's activity since the 14th century (De la Cruz-Reyna et al. 1995). Nonetheless, given the huge population in the region potentially at risk, together with concerns about possible escalation
of the ongoing eruptive activity to Plinian pha-
ses, or even worse a major sector collapse, the
management of the “volcanic crisis” at Popoca-
tépetl—already persisting for 20-plus years—
remains a major challenge for volcanologists,
national and local Civil Protection authorities,
and the threatened populations (De la
Cruz-Reyna and Siebe 1997). From experience
gained through 1997, involved scientists and
decision-making authorities became increasingly
convinced of the need to attain a more appro-
priate and uniform perception of the changing
risk among all the stakeholders. Accordingly, in
1998 the Volcanic Traffic Light Alert System
(VTLAS) was developed as a risk- communica-
tion protocol, hazards-warning system and
response scheme specifically designed to manage
the ongoing volcanic crisis at Popocatépetl more
effectively (Fig. 3). A detailed description of
VTLAS and its use are given by De la
Cruz-Reyna and Tilling (2008), but we summa-
rize below three salient points:

1. A level of activity of the volcano is defined by the PSC and translated into the most likely scenarios, describing them in specific terms, including time scales, names of threatened areas, etc. In general terms, these sets of scenarios may be grouped according to seven levels of response of the SINAPROC, which in turn are managed as phases within each of the Traffic light colors: two for Green, three for Yellow, and two for Red.

2. SINAPROC authorities translate the level of volcanic hazard defined by the PSC into one of three alert levels for the population (not of the volcano) that leave no room for uncertainty: **Green**, everything is fine; **Yellow**, you must be aware of the hazard and pay attention to any official announcements; and **Red**, you must leave the area according to the instructions given by the authorities.

3. All decisions involving mitigative actions are undertaken by the Civil Protection authorities according to the selected phase within the color level. It is important to emphasize that, in Mexico, the management of risks associated to natural phenomena is by law a

responsibility assumed by the three levels of
government: federal, state, and municipal.
The Scientific Committees are officially
appointed advisory groups of “more than 10
but less than 15 experts in the subject, which
can emit opinions and recommendations
about the origin, evolution and consequences
of hazardous phenomena, aimed to techni-
cally induce decision making for prevention
and mitigation to the population...”, as stated
by the Organization and Operation Manual of
the National System of Civil Protection
within the General Law of Civil Protection.

4 Evolution of the Activity Influences Public Perception of Hazards

Popocatépetl's historical volcanic activity has
been dominated by an irregular, yet sustained
dome-emplacement and destruction processes,
which in turn have resulted in some significant
fluctuations in the level of the summit crater
floor. The floor, which was about 277 m deep in
1906 (Friedländer 1921), was raised almost 100
meters because of the accumulation of
dome-forming products during the 1919–1927
eruptive episode (Gómez-Vázquez et al. 2016)
The episode beginning in 1994 further accumu-
lated enough material to raise the level of the
crater floor to near the crater rim after the peak of
activity in 2000–2001, thereby reducing the
capacity of the crater walls to contain or reduce
the range of potential pyroclastic flows. In fact,
under these conditions an explosion occurring on
January 22, 2001 produced pyroclastic flows that
traveled about 4.5 km from the crater, affecting
the glacier on its northeast slopes and triggering a
lahar that reached the outer limits of Xalitzintla,
one of the nearest villages (Sheridan et al. 2001;
Capra et al. 2004; Macías and Siebe 2005).
However, the general level of activity, as mea-
sured by exhalation-explosion events and
dome-emplacement rates, significantly decreased
after 2003, as the rate of dome destruction
exceeded the rate of lava emplacement, causing
<figure><img src="image_91.png" /><figcaption>**Fig. 3** A poster designed for public offices showing the VTLAS</figcaption></figure>

some reversals in the rate of volcanic material accumulation within the crater, and a significant increase of its maximum depth (Gómez-Vázquez et al. 2016). As of March 2015, the summit crater was again refilling with volcanic debris (Fig. 4), but so far, no other pyroclastic flows have been observed.

Figure 5 shows the evolution of seismicity during the period 1996–2014, as indicated by the fluctuations in total seismic energy release, a basic volcano-monitoring parameter expressed by RSAM values and their cumulative curve. RSAM (Real-time Seismic Amplitude Measurement) is a system that continuously samples the